LT1 Short Runners Affect On LSA

Just as it says, what do short runners (like the LT1 injection manifold) do to LSA requirements. I'm pretty sure I know but am curious to hear from the experts on this one. Yes, I know there are MANY things that affect LSA choice but I'm interested in knowing for sure what it does to a cam requirement. I know what an undervalved engine needs, and know what a low rod ratio does, what port velocity does, and what a tight vs. wide LSA does to a power curve. I really just want to know what someone who knows has to say about it.

Why be coy - why not post your opinion? The issue is intake tuning and rpm range. In general, short runners favor high rpm operation as does a slightly wider LSA. Widen the LSA too much though and it kills high rpm TINSTAAFL.

Rich

TINSTAAFL.

:confused:

:confused:

There Is No Such Thing As A Free Lunch

TINSTAAFL - Google it!

What kind of LSA numbers are high/low? The highest I've heard of is 112, lowest is about 105?

Cam specs:

1987 TPI system:
305 5spd 350 AT
(LB9-L98)
202 207, .404/.415 115 LSA

1995 LT1:
203 208 .450 /.460 116 LSA

A difference of about 16" in runner length didn't seem to affect that variable too much.

Just so I know, the LSA is the Lobe Separation Angle and having a smaller angle means the valves overlap more? So my V6 at 108 would have more overlap than a stock V8 as mentioned above.

If the LSA has little to do with port length, is it mainly a compromise between emissions and top end?

Does runner length change the LSA, yep. Is there a hard fast rule on the length and LSA connection, nope.

What you are dealing with is pressure waves and how to open and close the valves for a RPM range that works to get to the best cylinder filling for your given motor. Basically you need to have the most pressure difference between intake port and cylinder from TDC to IVC, for the entire RPM range you want to make power. That's a lot easier said than done.

Bret

Here's my take on it. . .

As we know, changing LSA moves around 4 different aspects of cam timing (EVO, EVC, IVO, IVC), so you can't just look at LSA alone. In fact, let's assume for just a second that the exhaust lobe stays in one spot since the runner length won't have much affect on exhaust lobe timing.

So, we're left with IVO and IVC.

IVO - in general, you want IVO to be as soon as possible. In other words, you want the intake valve to follow the piston as it leaves TDC. If you were to actually do this, the cam would have alot of overlap and not be streetable for most. The bottom line is that IVO will generally be chosen by P/V clearance or desired idle quality. Runner length will play a role in what pressure wave hits the port, when, and IVO can be chosen to optimize this, but the amount of analysis required to determine this exactly is beyond the scope of most.

IVC - I believe this is most affected by the short runner length. You see, stock IVC's are in the 30 deg ABDC range, while stout street cams will be in the upper 30's, lower 40's. Race cams get into the upper 40's, lower 50's. I've heard of numbers as high as 70, but that's in very high rpm engines beyond our scope. The point is that, when the piston is rising on the compression stroke, the intake valve remains open because the momentum of the air in the runner continues to fill the cylinder even though the piston is going in the opposite direction. With the LT1, there isn't much air in the short runner, so there's not much momentum to carry it in when the piston is rising. It could easily start going the wrong direction. Therefore, you want to close the intake valve directionally earlier than you would with a long runner intake manifold to achieve the same rpm range. For example, a while back, a reputable company swapped an LS1 over to a Victor intake manifold with shorter runners than stock. The rpm range went through the roof but it didn't gain any power. The closed the intake valve sooner. The result was more peak power at a lower rpm.

Closing the intake valve sooner results in tighter LSA, less intake duration, or more advance.

Mike

Mike,

IVO as soon as possible? Most performance cams always have the IVO BTDC. The key here is the pressure difference between intake port and cylinder at TDC and opening the valve earlier or later than optimum is going to lessen that difference.

IVC is related to runner length because longer runners working in the higher orders produce a larger pressure in the intake port, which allows the intake port to remain at a higher pressure for longer in the cycle to overcome the higher and higher pressure in the cylinder as the piston moves up the bore.

In the end it has to do with valve centerlines not LSA, because the exhaust event is tied to how well you fill the cylinder and how much to need to clean out of it. That can have a totally different need than the intake so you can't determine the LSA from runner length.

AGAIN THERE ARE NO HARD SET RULES HERE!!!!

Bret

IVO as soon as possible? Most performance cams always have the IVO BTDC.

I think all cams open the intake valves BTDC. Did I say otherwise?

The key here is the pressure difference between intake port and cylinder at TDC and opening the valve earlier or later than optimum is going to lessen that difference.

In every case I've seen, opening the intake valve earlier only improved power up until the point that P/V clearance became an issue. In one instance, they actually opened the intake valve so far before TDC that a special cam lobe had to be used to close it again to make room for the piston as it passed TDC. All this aside, pretty much all street cars wouldn't be happy with this much overlap due to idle quality, so it's a moot point anyway. Use as much overlap as you can stand.

IVC is related to runner length because longer runners working in the higher orders produce a larger pressure in the intake port, which allows the intake port to remain at a higher pressure for longer in the cycle to overcome the higher and higher pressure in the cylinder as the piston moves up the bore.

That's what I said, in so many words. Longer runners can take advantage of a later IVC. Conversely, shorter runners will require an earlier IVC.

In the end it has to do with valve centerlines not LSA, because the exhaust event is tied to how well you fill the cylinder and how much to need to clean out of it. That can have a totally different need than the intake so you can't determine the LSA from runner length.

That's why I started out by saying, "let's assume for just a second that the exhaust lobe stays in one spot since the runner length won't have much affect on exhaust lobe timing."

I'm having a hard time figuring out if some of your points agree or disagree with mine.

Mike

:shrug: I'm starting to remember why I don't visit forums as much anymore.

All cams don't open the valve BTDC, lots of small cams do but in this instance who gives a rats ass, just some more dumb **** to argue about that is moot.

I've had a lot of instances when a later IVO improved power throughout the RPM range of the motor, so that is a very bad rule to go by and people are not looking at the whole equation correctly.

Does runner length play a part in cylinder filling? (Easy answer is yes here)

So "let's assume for just a second that the exhaust lobe stays in one spot since the runner length won't have much affect on exhaust lobe timing."

That statment assumes that the parts of the motor are not interconnected, which is false or more appropritately, incomplete. If you can put more into the cylinder it's safe to say that you have to get more out, so the exhaust lobe timing WILL change when you are looking for max power.

"Use as much overlap as you can stand." I don't agree with this, use the overlap you need to get the most cylinder filling you can get for the RPM range you are working in. Again it has to do with pressure management around TDC, not a simple concept to explain or teach to a point anyone can do anything with it.

I've seen very little out there in terms of anything to do with getting valve events right by working on pressure differentials in a motor and until you look at it that way there are way to many gross instances where "rules" are very far off and unuseable. Not to mention most of the "examples" that guys use to draw conclusions on valve events are so skewed and the WRONG reasoning is behind the conclusions it's almost laughable.

BUT this is accurate "Longer runners can take advantage of a later IVC. Conversely, shorter runners will require an earlier IVC." I just explained the reason to WHY.

Bret

Why does it always have to be an argument, Brett?

As far as I can tell, you agreed with 3 out of 4 of my points, but got offended when I asked for clarification.

Most folks will only tolerate about 5 deg overlap @ .050 for street use. Lets assume for a second that it's a single pattern cam that's 4 deg advanced. That puts IVO at 6.5 deg BTDC. You think that's early enough to cause problems?

Mike

How would changing from a LT intake to a victor change things?

Would this call for a new camshaft to compensate? and why?

How would changing from a LT intake to a victor change things?

Would this call for a new camshaft to compensate? and why?

Oh you had to go and complicate the issue. . .

You see, the Victor has longer runners than the LTx intake. So, as Brett and I agree, could close the intake valve later to take advantage of the greater momentum of the charge to fill the cylinder ABDC. HOWEVER, dyno tests have shown that the Victor actually moves the rpm range up when compared to an LT4 intake. Soo. . . it depends on your goals. If you want to move the rpm range back down to where it was, you'd need to moved IVC earlier. If you want to take full advantage of the longer runners and make more peak hp, then move the IVC later, but you'll lose even more low-end than the manifold did itself.

Mike

Oh you had to go and complicate the issue. . .

You see, the Victor has longer runners than the LTx intake. So, as Brett and I agree, could close the intake valve later to take advantage of the greater momentum of the charge to fill the cylinder ABDC. HOWEVER, dyno tests have shown that the Victor actually moves the rpm range up when compared to an LT4 intake. Soo. . . it depends on your goals. If you want to move the rpm range back down to where it was, you'd need to moved IVC earlier. If you want to take full advantage of the longer runners and make more peak hp, then move the IVC later, but you'll lose even more low-end than the manifold did itself.

Mike

I want it to run its max. Its a drag car....

So this would be an issue when getting a custom grind cam? As in something would need to be done to get more power or the LT intake cam would just move my power to a higher rpm?

LT intake cam would just move my power to a higher rpm?

If you keep your LTx cam and swap to a Victor intake, you'll probably gain around 15 hp and it'll move the peak up by about 500 rpm, depending on the rest of the combo.

I think you should put IVC somewhere around 46 deg ABDC, maybe later depending on converter stall.

Mike

Most folks will only tolerate about 5 deg overlap @ .050 for street use. Lets assume for a second that it's a single pattern cam that's 4 deg advanced. That puts IVO at 6.5 deg BTDC. You think that's early enough to cause problems?

Mike

5° @ .050" hmmmm

GM847 = 14°
CC306 = 13°

MS3 = 13.5°
TRex = 25°
G5X3 = 14°

All popular cams that are well over 5°. I agree less can make the car more liveable but it's far from the only player in that game.

As for the IVO it depends on what works best. In the case of something like your favorite the TRex, 13 IVO is stupidly wrong in a RPM range that is used in any street car.

Bret

PS as for the arguement with everything, I just honestly don't think you see the forrest for the trees and you miss a lot of details trying to give a catch all answer that in reality is honestly impossible to answer definately.

I think you should put IVC somewhere around 46 deg ABDC, maybe later depending on converter stall.

Mike

How can you even give a general IVC with what info is given? Professionally I don't agree, and not knowing the compression, gear, max RPM, stall and vehicle mass you can't even get close.

Does a cam change due to the intake swap? Yep. Does everything else come into play? Yep. Could you run a later ICL with a single plane over a LT intake? Yep. Would you always? Nope.

Again I don't think there are hard fast rules to anything here. I don't want to say it's magic, because it's not but so many things come into play that a 400rpm change in RPM band etc... can change the optimum valve events.

Bret

5° @ .050" hmmmm

GM847 = 14°
CC306 = 13°

MS3 = 13.5°
TRex = 25°
G5X3 = 14°

All popular cams that are well over 5°.

Okay, I had an 847 in my last car. I thought the idle was too rough for a true street car. My current cam has 2 deg overlap and it's borderline for my taste.

In the case of something like your favorite the TRex, 13 IVO is stupidly wrong in a RPM range that is used in any street car.

All I can say is that lots of different IVO's were tried on that cam and the production version is what accomplished the goals. With the cam-only LS1 record, it's hard to argue with.

I noticed you avoided my question. Have you ever seen a power improvement from moving the IVC later than 6.5 deg BTDC?

Mike

You might want to say it that way the first time before you accuse someone of ignoring you.

Either way it's not just one valve event, and would you be starting at say 14 BTDC? Dumb question to just abitralily say that to entice a "discussion".

BUT to answer the question, yes I have seen that and in fact the cam would go in a LS1 and walk all over a TRex BTDT.

Oh yeah I forgot that the "cam only" super duper LS1tech record was something to brag about, mythical internet racing classes always are.

Bret

. . .I have seen that and in fact the cam would go in a LS1 and walk all over a TRex BTDT.

Oh yeah I forgot that the "cam only" super duper LS1tech record was something to brag about, mythical internet racing classes always are.

The TRex cam was specifically designed to give the maximum power possible when using stock or near-stock heads, intake, compression ratio, and cid. So, yes, it does what it was designed to do quite well. When your cam "walks all over the TRex" in these conditions, I'd like to see it. 450 rwhp and low 10's at 130+ with bone stock heads, intake, and bottom end won't be easy to "walk all over".

Mike

Either way Mike think what you want. Besides I don't consider a shop car getting good numbers worth much these days in LS1 land either on the dyno or track, what really matters is what customers miles and miles away get with the same parts. The "shop car" of today is a suckers bet to mindless consumers who follow the rest of the lemmings into the trick of the week.

The point here was that yes it's possible to make the same peak HP values (and make more average power thru the RPM range) with much later IVO and in fact have it earlier than 6.5° BTDC.

My point is that IF you do things correctly, and that means not moving the IVO too early you can have a larger difference in pressure between the intake port and cylinder when the piston moves away from TDC. This also tends to make the motor stronger in a wider RPM range. The problem is you can't get stuck in the stale thinking of "there is a certain split that is needed" or "we have to maintain a LSA of 112-114" blah blah. It goes much, much deeper than that and the only guy who I have ever, EVER, EVER even hear mention this was a R&D Engineer at GM Racing. Almost NOBODY talks about motors in terms of their pressure differences, but in reality that is all that counts. CFM, Velocity, LSA, Duration etc.... are all small players in comparison.

Bret

. . .I don't consider a shop car getting good numbers worth much these days in LS1 land either on the dyno or track,.

The car that made 450 rwhp stock long block was a shop car. However, the one that ran low 10's at 130+ was an ordinary customer like everyone else. His dyno pulls actually didn't get quite as much power.

I'm just curious. . . the cam that will "walk all over the TRex". How much power will it make? In order to "walk all over" a 450 rwhp setup, you'd have to be making 480 or better with a stock long block. Has this been done? If so, I'm sure a whole lot of people would be really interested. . .

. . .The problem is you can't get stuck in the stale thinking of "there is a certain split that is needed" or "we have to maintain a LSA of 112-114" blah blah.

No kidding. . . but everytime I point that out, you pop in and say that you can't look at individual valve timing events either without doing some Computational Fluid Dynamics and modelling each events' effect on the others, think about the effects of the Reynolds number, and blah blah. . . oh crap, save yourself some effort, don't bother trying to learn this stuff, and just buy a cam from me.

Mike

People can be interested all they want, I'm not into advertising and putting funny names on things unless the customer names them. FWIW a customer wanted his LS7 cam from me to be the "Chuck Norris" cam and the heads are "Bruce Lee" heads, it's what he wanted. Besides, racing a cam only LS1 car is about as stupid petty racing as you can get. If you want to impress me go build a car for a class and win it, I hear there is a real LS race in Memphis later this year.

Mike, I've come to the conclusion that you don't get it. Period. (See My Signature and then see yours) This isin't "Computational Fluid Dynamics" or "the effects of the Reynolds number" which don't play into this at all, but then again YOU DON'T GET IT ANYWAYS!

I'm not here hyping the new trick of the week, so you don't feel you can get anything from me. The reason is there is no new trick of the week other than LEARNING and hard work, seems you don't like doing any of that. I've already wasted enough time talking about this with you I'm not going to waste more to upload graphs and then try and explain something to you that very few people actually understand when I do show it to them in person. Frankly in the end, 1. I don't think you got the marbles to get it, 2. It's too much actual work to get anything out of it if you wanted to use it, and if you wanted all the forumulas and software to get the answers then go spend $1000 on it and buy it!

Then again my old man reminds me that I don't just post on here to explain things to the few choice morons, I explain things to the people who don't run their mouth to get attention and that really want to learn something.

So here is a simple REAL WORLD example of a setup I helped a engine builder who was building motors for Professional Competition (Multi-Million Dollar Sponsors) improve on what they had. My predictions of average power/TQ and peak values where verified and almost exactly shown on the engine dyno. (within 1.0hp) The cam I gave them made the most peak TQ, most average HP & TQ and the same peak HP that they have EVER MADE, and this was a well built and tweaked motor combination.

See PIC http://usera.imagecave.com/sstrokerace/Diagrams/ProTestExample.jpg

The data being displayed in the upper left of that picture is of what is happening at TDC. What you will notice is that there is a .7psi larger difference (+63.6%) from the orginal cam to my design, and this is just at peak HP RPM. You will also see that the IVO is LATER than with the old cam (2.5°). Reguardless who cares about the valve events, the pressure differences are key here, and the only big difference is at TDC. What really gets interesting is all the data at RPM points where there is a much larger difference in TQ/HP output, but this is a simple explaination.

Either way that's enough out of me for this thread.

Bret

Okay, I had an 847 in my last car. I thought the idle was too rough for a true street car. My current cam has 2 deg overlap and it's borderline for my taste.



Mike

Boy you should sit in/hear MY car....dude if you thought that 847 was too much for a streetcar.......WOW you should hear mine.


That beotch ROCKS DA HOUSE 'YO!!!


Bret any idea how much overlap mine has? It is your cam. I know, I CAN figure it out...but you KNOW I am lazy he he.


David

PS Bret I don't get it either....but I get some of it...vids coming soon.

What do you use to log that kind of data?

Besides, racing a cam only LS1 car is about as stupid petty racing as you can get.

Stupid or not, it's what people want and it's what people buy. I, personally, have never wanted to build a cam-only car, but there are plenty that do.

(See My Signature and then see yours)

If you think you can build a 900 - 1000 hp naturally aspirated motor that will run on pump gas, and do it for less than $20k using all new parts, and have it idle smooth and require no more maintenance than my stock pickup, then I challenge you to. I can and have built a couple of those. You can't do it without forced induction.

. . .so you don't feel you can get anything from me.

I never said that. In fact, every time I ask you a specific question, you say I need to buy my own dyno, flow bench, etc. . . and figure out for myself.


LEARNING and hard work, seems you don't like doing any of thatseems you don't like doing any of that. . . . 1. I don't think you got the marbles to get it, ...the few choice morons

Here we go with the personal attacks again. . . must you always resort to that???

So here is a simple REAL WORLD example of a . . . (within 1.0hp)

I'm surprised you finally posted some real numbers. A couple questions. . . what is so different about that particular engine that required the later IVO (of course, it looks like about 25 deg BTDC, which isn't that early)? Runner length? Plenum volume?

Also, predicting with 1 hp? All the engine (and chassis) dyno testing that I've done had a standard deviation well over 1 hp.

Oh yeah, and just how bad would your cam "walk all over the TRex" again? We talkin' like 5 hp? 30 hp?

Mike

This is going downhill fast..... and is about to be closed. Stick to the technical issues. Name calling and personal attacks are not productive.

Bret any idea how much overlap mine has?

You have over 15° @ .050" just on the exhaust side!!!!

38° Total @ .050", Hey you wanted all that power and 10" of idle vac, which is right where you should be.

A couple questions. . . what is so different about that particular engine that required the later IVO (of course, it looks like about 25 deg BTDC, which isn't that early)? Runner length? Plenum volume?

Nothing you wouldn't find in any other long runner street motor like a LS motor, Mod motor hell anything OEM today. Almost everything made today works in the 3rd and 4th harmonic wave.

Induction length is 15"+ total

Plenum Volume is 200cubes+

Yes it has a early IVO, but it's a RACE MOTOR!

Also, predicting with 1 hp? All the engine (and chassis) dyno testing that I've done had a standard deviation well over 1 hp.

1. You need to start working with better equipment.

2. Once you model the motor you are starting from accurately you can get things this close. There is as much time in getting accurate info to base everything on as there is doing the work to get the new designs.

You don't have to believe it but none of that **** is made up. I can't say every specific otherwise I would be giving out too much info.

I don't mean to pick on you but you really need to start looking outside of your box to find the answers you are looking for.

What do you use to log that kind of data?

If you want to setup motors to get this actual data the heads and headers need to be setup with pressure transducers, which run around $500K+, only place I know that has them is GM. Thank god for software and lots of good formulas to get you a accurate model of it without all that money invested.

Is there more in the cam on that motor in the example? MAYBE, but there are many other barriers to power that would have to be overcome first.

Bret

BUT this is accurate "Longer runners can take advantage of a later IVC. Conversely, shorter runners will require an earlier IVC."

Bret

Rod length would change the dynamics of this statement. Since a shorter rod moves slower at BTDC, there's more time before the piston starts back up the cylinder so a later IVC is possible I guess. Does that mean a short rod motor needs an earlier IVO with a short runner intake as well? I know this is breaking this stuff down to its simplest terms and there are tons of other factors to consider but during the exhaust cycle, it would seem then that a short rod motor could have a later EVO too but would require a good exhaust flow. That in itself would decrease LSA. I'm thinking then however that a short runner intake needs less time to get the air moving so would that mean a later IVO? Then we are back to the short rod engine which would need an earlier IVO because the piston moves down the bore sooner. It's a catch 22! Who wins the battle then between short rod and long/short runner intake when all things are considered?

Rod length would change the dynamics of this statement. Since a shorter rod moves slower at BTDC, there's more time before the piston starts back up the cylinder so a later IVC is possible I guess. Does that mean a short rod motor needs an earlier IVO with a short runner intake as well? I know this is breaking this stuff down to its simplest terms and there are tons of other factors to consider but during the exhaust cycle, it would seem then that a short rod motor could have a later EVO too but would require a good exhaust flow. That in itself would decrease LSA. I'm thinking then however that a short runner intake needs less time to get the air moving so would that mean a later IVO? Then we are back to the short rod engine which would need an earlier IVO because the piston moves down the bore sooner. It's a catch 22! Who wins the battle then between short rod and long/short runner intake when all things are considered?

Ok take an example of changing the rod length 1", around 20% in a small block motor. The shorter rod will dwell less, so you would think it should have an effect but it doesn't in most motors. We are talking 100-115% VE motors here, and the thing we are working on is the pressure waves and their correlation to RPM. In the end the valve opening and closing is tied to the pressure waves and RPM more than piston movement. I've said this a billion times but piston movement doesn't immediately = air movement or a change in pressure. This is the reason you don't see huge power changes when you change rod length.

The key is to open the valve when you can get the largest difference in pressure at TDC and close the valve when the pressure in the cylinder is higher than in the port. That all depends on the low lift characteristics, and one of the reasons why aggressive ramps and high seat angle VJ's are coming more and more into play. Most times that means the valve will be at .100-.030" lift when the pressures equal each other on valve closing. The real balancing act is the IVO, opening the valve enough to fill the cylinder with the big pressure difference at TDC and still keeping it closed enough to create the pressure difference.

In the end who cares what the rod length is, I want the runner that's making my total induction length work at the best 3rd or 4th harmonic for the system.

Bret

Thinking about this some more. Runner length needs to be a multiple of the harmonic for the induction system if it is to work effectively. The correct length and CSA is primarily a function of displacement and rpm for peak torque. IF a runner were sized correctly in the first place, the length and CSA would already depend on the LSA, to a small degree. LSA does not have a huge effect on rpm for peak torque, that is primarily a function of duration.

As far as I can figure, with a properly sized runner in the first place, and change will decrease power and fiddling with the LSA is unlikely to help much. The original question "what do short runners do to LSA requirements?" can therefore be answered as follows: "essentially nothing".

Rich

This is some GOOD stuff in this post, heated discussion seem to produce the best info.

Bret I gotta tell ya buddy, that cam WILL talk to ya. It aint NO joke.



David

Equipment??? Engines don't produce within 1 hp from one dyno run to the next. Heck, we've gained 15 hp within the first 3 dyno pulls without making any changes!

(see if you can respond in a mature manner)

Some shops can and do have dyno cells that can find 1 hp, and have engines that repeat within that window. Yeah, it's costly, and you need to be extremely anal about your testing, but it happens. Controlling the inlet air to 1-2°F, controlling fuel inlet temp to about the same, the coolant of a few °F, controlling room humidity, calibration of the dyno before and after a series of pulls, etc. is done to achieve these results. That info came directly from the guy who supervised the dyno shop @ a big Cup team, HMS. That was a few years ago @ AETC. They are even better now. Money buys technology and people if you know where to shop.

Good race shops see engines repeat within a few hp; accuracy good enough to find those few hp a change might make. The extreme in this country is probably Cup plate engines where a couple of hp is a BIG deal.

Perhaps your +15 hp in 3 pulls had something to do with the engine breaking in. At +5 average per pull, make another 50 pulls! ;)

Mike, what's your engineering discipline? I, too am an engineer, and very OLD...er, mature so I took the liberty of responding for Bret because the answer isn't about induction design/theory. BTW, Bret, does think OOTB (Out Of The Box) on a lot of things, and he gets me thinking there. Sometimes that gets one a leg up. It's always interesting to be doing that and then discovering that the same thing is being done by others who are having great success.

Jon, the OldSStroker.

Rich I don't know if I can say there is no correlation between short runners and a LSA, but you raise a good stance to think about and respond too.

Let's take a LT1 350 with a HP peak goal of 6000rpm with stock heads and stock intake.

All lengths for HP peak at 6000rpm

Ideal 2nd Harmonic 18.5
Ideal 3rd Harmonic 12.9
Ideal 4th Harmonic 10.1
Ideal 5th Harmonic 8.24

What's the real length the LT1 has? Basically the 5th Harmonic. It's a weak harmonic, but still an ideal one for the RPM range and peak.

Now with a LS motor those same basic lengths for the Harmonic apply, but the actual induction system falls into a hole between the 2nd and 3rd, which helps the area around TQ peak on LS motors. Usually good strong street motors do tune for the TQ peak rather than the HP peak.

The question at hand all depends are you designing the intake first and cam second, the cam first and the intake second or both of them at the same time? If you are in the first case, then you look where you are in terms of harmonic tuning and valve lift. That is going to give you a optimum IVC and IVO area for your RPM range, which gives you a ICL. I guess that leads to this.... we should be saying if you are designing a cam second to match a intake system, does the LSA depend on how fitting the runner length is to the RPM range the motor will be run in?

From what I have seen there are trends that do show something, mostly that you can't close the valve as late with a weaker harmonic that is tuned for the upper part of your RPM range. This usually moves the ICL lower, which can move the LSA lower, just depends on how well you fill the cylinder. Problem is a shorter runner with a weaker harmonic (higher) doesn't fill it as well so a later EVO can be used and therefore a lower ECL as well, which all leads to a lower LSA. That's just what I have found to be the case.

Ideally you get to design the intake length, CSA and taper angle along with the cam and headers at the same time. If you can work in the 3rd harmonic rather than the 5th there is a lot to be had there, mostly VE% that are unreal.

Either way this is a complex question trying to get a simple answer. Is it the runner length or the tuned RPM and it's impact on your RPM range that makes the big difference in valve events, in turn centerlines and LSA? I guess that's the real question here.

Bret

If you want to setup motors to get this actual data the heads and headers need to be setup with pressure transducers, which run around $500K+, only place I know that has them is GM. Thank god for software and lots of good formulas to get you a accurate model of it without all that money invested.

Is there more in the cam on that motor in the example? MAYBE, but there are many other barriers to power that would have to be overcome first.

Bret

$500K huh? They are probably quite a bit nicer than the pressure tranducers I have access to then. Could I do anything worth while with +/- .2% accuracy, or is that hunting with a stone spear? What software and formulas are you using, or could you recommend me a good book?

I'd be interested to know what those "barriers to power" are.

... Is there more in the cam on that motor in the example? MAYBE, but there
are many other barriers to power that would have to be overcome first....

Bret

IMO when working with the production LT1 intake, the plenum size and design
would be a primary barrier. With the TB in the front and the runners from one
end of the tiny plenum to the other end, no two cylinders will have the same
demand. Any cam you chose will be a compromise.

$500K huh? They are probably quite a bit nicer than the pressure tranducers I have access to then. Could I do anything worth while with +/- .2% accuracy, or is that hunting with a stone spear? What software and formulas are you using, or could you recommend me a good book?

I'd be interested to know what those "barriers to power" are.


The pressure transducers mentioned are inserted into the combustion chamber to measure cylinder pressure in realtime. To be effective they need to be tiny and able to stand the max heat of combustion. If you are running a Cup engine@ 9500 and around 850 hp and trying to see how each cylinder is doing its job, you need very good accuracy and a decent amount of life. That equipment ain't on everyone's shelf.

FWIW, measuring fwhp to ±1 hp accuracy in 850hp is ± 0.12%. That still amazes me.

IMO when working with the production LT1 intake, the plenum size and design
would be a primary barrier. With the TB in the front and the runners from one
end of the tiny plenum to the other end, no two cylinders will have the same
demand. Any cam you chose will be a compromise.

I was talking about my example in the graph I provided above. In that example there are rules issues that hold it back from making more power, but there are also more issues that would have to be tweaked.

What you are talking about does make sense in a general application, now think about cam twist etc... and different timing with different cylinders takes a lot of work.

Bret

The pressure transducers mentioned are inserted into the combustion chamber to measure cylinder pressure in realtime. To be effective they need to be tiny and able to stand the max heat of combustion. If you are running a Cup engine@ 9500 and around 850 hp and trying to see how each cylinder is doing its job, you need very good accuracy and a decent amount of life. That equipment ain't on everyone's shelf.

FWIW, measuring fwhp to ±1 hp accuracy in 850hp is ± 0.12%. That still amazes me.

Hmm, well these will probably not take that kind of abuse. I wonder if there is any benefit to reading pressures in the intake? Like maybe the slight pressure variance between the lower part of the runner and the plenum when the valve opens, or would the pressure be rising in the runner until the valve opens? I'm still not entirely sure how to log all of this information, or what I could do with it when I had it.

I'm with you though, accuracy to 1hp is amazing on those motors. I can't even imagine the instrumentation the big F1 teams get to use. Developing 900+ hp out of less than a three liter V8, now that amazes me.

Hmm, well these will probably not take that kind of abuse. I wonder if there is any benefit to reading pressures in the intake? Like maybe the slight pressure variance between the lower part of the runner and the plenum when the valve opens, or would the pressure be rising in the runner until the valve opens? I'm still not entirely sure how to log all of this information, or what I could do with it when I had it.

I'm with you though, accuracy to 1hp is amazing on those motors. I can't even imagine the instrumentation the big F1 teams get to use. Developing 900+ hp out of less than a three liter V8, now that amazes me.

It's ~750 hp from 2.4L with a 19,000 mandatory rev limit in F1. Power peak seems to be around 18750 from what I've read. The BMEP @ 18750 is great, but now where near a ProStock engine at half the rpm. Different horses for different courses.

Yeah, F1 teams have tons more money than any other form of racing. Some top F1 engine folks and others associated with basic engine research have moved to NASCAR in the past few years. The first example was at Hendricks for a few years. It seems to have helped.

I think if you wanted to really investigate the relationship between valve events and intake runner length, the best model to start with would be an individual runner system. Anywhere you can use one in pro racing, it's being done AFAIK. The common plenum design muddies the picture and probably serves to blunt the effect of any single change. With an IR intake system, there is ONE runner length and CSA and ONE LSA (for a given duration) that makes the most hp/torque. That's not to say that picking the ideal cam is easy because there may be other parameters besides peak power/torque that need to be optimized.

I am still trying to understand the question here. It seems to simple to me, I guess. IF you have a given intake, there is an optimal cam for that intake/engine combo. It is going to be harder to determine with a common plenum than for an IR system. For the CR system, I suspect there is a range of cams that would produce very similar results. That's because of the variation between cylinders in the "siamesed" type of ports Chevy blessed us with. IF you change the intake by JUST changing the runner length, you will get less power IF it were optimized in the first place. There will still be a best set of valve events with the caveats above. IF the runner length is moving towards optimum, power will increase and again there will be an optimal set of valve events for the new combo.

The general effect will be as follows, as I said in my first post and nothing I have read here has changed my mind, though I have tried to foolow what pthers have been saying.

1. Lengthen runners.
Power may go up or down. But the rpm range where you see peak power should be lower and this will favor a tighter LSA.
2. Shorten runners.
Same effects on power, up or down depending on moving towards/away from optimum. Peak power rpm moves up and a wider LSA is needed.

Of course, wider or narrower are relative to where you started. The above applies if the cam were optimized for the original runner length. To many posibilities to set them all out if the cam was not right in the first place, but they are not hard to figure out. There is a common misconseption about LSA. If the cam is near optimum in the first place, a wider LSA will move the torque peak UP the rpm range and vice versa. The common misconception is the opposite. I need to get ready to go to the track or I could expound on why I think the misconception exists. But briefly, you can figure it out if you consider that most off the shelf cams have too much LSA to start with.

Rich

One other thing, in the real world you can't change the runner length without changing the shape (except in a straight shot IR system). So, this is not a real world question. Similarly, you would be crazy to change the length without considering the CSA of the runner. They are interrelated and that is another reason this is just intellectual masturbation. Of course, masturbation IS sex with someone you love (hopefully).

http://www.seanhylandmotorsport.com/online/images/DSCN2365.jpg

Rich

They are interrelated and that is another reason this is just intellectual masturbation. Of course, masturbation IS sex with someone you love (hopefully).
Rich

Now I think we are getting someplace!!!!!

Here is another graph that can show you the difference in which harmonic you are working on in a motor.

http://usera.imagecave.com/sstrokerace/Diagrams/HarmonicPulses.jpg

The sheetmetal example is working on a lower order harmonic that has stronger pulses.

The LT1 intake as you can see has lower peaks on it's pulses and therefore can't overcome the cylinder pressure as the valve is closing so the IVC has to be earlier.

This really does get to be a mental "beating"

BTW the motors in those two examples are vastly different the sheetmetal intake motor is 2 times + the power of the LT1 motor.

Bret

Now I think we are getting someplace!!!!!

Here is another graph that can show you the difference in which harmonic you are working on in a motor.

http://usera.imagecave.com/sstrokerace/Diagrams/HarmonicPulses.jpg

The sheetmetal example is working on a lower order harmonic that has stronger pulses.

The LT1 intake as you can see has lower peaks on it's pulses and therefore can't overcome the cylinder pressure as the valve is closing so the IVC has to be earlier.

This really does get to be a mental "beating"

BTW the motors in those two examples are vastly different the sheetmetal intake motor is 2 times + the power of the LT1 motor.

Bret

Wow the LT1 intake pressure didn't vary much at all. Where do you get these graphs?