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

 

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 **** 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

 

$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.

 

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.

 

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.

 

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

 

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).



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/sstrokera...onicPulses.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