How high of compression can you go on an LT1?

When it is time to rebuild my motor (396), what is the highest compression I can run with it? I would consider running race gas, or a mix of race gas/pump gas. I want to stay naturally aspirated with the 396, but was just wondering what kind of compression I could run on the street. If anyone would like to elaborate on what rods, pistons, heads, cam etc. would be good for the setup that would be awesome! I just thought of this 5 mins. ago so sorry if I am being vague. I just want a general idea of what can be done and with what parts. Thanks!

Jeff

well if you are willing to use race gas the sky is the limit on compression....on the other hand if you are careful about cam timing i have seen motors that have 13:1 static compression run on pump gas(87!) the secret is having enough overlap in the cam that at low rpm the cranking compression is much lower, due to the overlap bleeding off excess pressure. At higher rpm, due to faster valve events, cranking compression increases. so you can kinda get the best of both worlds.....for most people though arounf 11:1 is about the best you get on pump gas

13:1 on 87 octane.... i find that hard to beleive, maybe on 94 octane in a wildly tuned engine


pretty much highest i know of is a pretty mean lt1 (actually about all it has in common is lt1 block and reverse cooled) made by jim lab for an rx7, 12.2 compression i beleive
http://www.rx7club.com/forum/showthread.php?s=&threadid=37170&perpage=50&pagenumber=1

I am under the impression that CR above 13-14:1 do not produce any more power. Of course, you need to run race gas to use CR in this range. There are a number of points regarding combustion chamber and piston crown configuration that need to be taken into account to run a very high CR. If you want to use pump gas I would say not >11:1.

I would be interested to know why increasing CR beyond a point produces no further power gains. Is it a limitation of gasoline as a fuel, or something else? Or is this a myth?

Rich Krause

13:1 if you run a mix of race/pump gas. 87 octane with 13:1 is not gonna happen unless you pull lots of timing in which case the car would make actually less power than a 12:1 pump gas motor.

I would just go 12.0-12.2:1, run 93 octane, and call it good. You are not going to make that much more power with 13:1. FYI I run 12.2:1 on 93 pump gas and have no problems.

Jason

hey guys that car was in car craft about a year ago...i believe it was a malibu. anyhow it came with a fairly detailed discusssion about exactly how you can make that happen. The car ran 9's i think too.

Like i said before its very simple....your static CR really has doesnt have a whole lot of bearing on cranking compression, unless someone out there has a camshaft with square lobes on it. If a camshaft has a lot of overlap in the case of the motor with 13:1 compression, the cranking CR might only be 10 or 11, because at low RPM the exhaust valve is remaining open for a small time during the compression stroke...because of overlap. There is more to the cam timing than that, this guy used a custom grind which im sure he specified the LSA, intake opening/closing, exhaust opening/closing etc. as the motor revs (this thing was a high revver too if i remember correctly) because the opening and closing events are happening at an increased rate there is less and less intake charge lost through the exhaust valve before it shuts....thereby increasing the cranking CR.

i'll try to dig up the article for you guys

As some of the posts have pointed out, cam choice has a lot to do with maximum static CR. I always think "small cam" and hence my 11:1 comment. As Jason (and others) have done, if you have a big cam you can (and should) use a much higher static CR.

Rich Krause

If a camshaft has a lot of overlap in the case of the motor with 13:1 compression, the cranking CR might only be 10 or 11, because at low RPM the exhaust valve is remaining open for a small time during the compression stroke...because of overlap.


Just to avoid any likely confusion, that should read "the intake valve is remaining open...."
Also worth noting.... timing events, in non-variable valve-event engines are independent of RPM.

That said, this whole discussion is about dynamic compression ratio (DCR). Understanding DCR gives you a much better understanding in relation to, "How much compression can I run" type questions.
There are, as with everything, rudimentary calculations. I've seen a few posted on this board. One thing to consider though is that any calculation which doesn't take into account, altitude and temperature (at the very least) is not worth the crunching. A change in altitude alone... let's say, 2,000-3,000 feet is an approximate .4 points change in DCR.

In my research of this subject amd how it relates to reverse-cooled LT engines, I've found that they can generally tolerate DCR's in the range of 9.1:1 with proper tuning, MBT.

Let's take an example.... a 355 cid LTx with a Comp Cams 290 HR (CC306), running a static compression ratio of 12.5:1 . We all know people running a setup like this on 92 octane. With some calculations and assuming an altitude of 1000 ft, 6" rod length and 72º IVC we calculate a DCR of approximately 9.1:1. Too high for a conventionally cooled sbc (which should stay below 8.3:1) but not a problem in this case.
I've tested my old LT, running Evans NPG and a high volume water pump, at DCR's of 9.5:1 without any problems. The engine was extremely thermal efficient albeit at a loss to volumetric efficiency. There's always a trade off.
I've also run as high as 10:1 with water injection with excellent results.
All that aside, there is always a limit and a law of diminishing returns.... octane limit plays a big part as does proper tuning.

So, through quite a bit of rambling... I'm rather confident in saying that an LTx can run (NA) 9.0:1 DCR all day long without a hitch. Always safe to err on the safe side though.

It would be a good idea to do some research on dynamic compression as it will help in every aspect of planning an engine build.... cam selection, target static compression, etc.
I've purposefully grazed the surface (don't have much time) so if you'd like to get more into this we can.

Take care,
Chuck Riddeck
Progressive Race Engine Development

FWIW I did a little poking around and this is the best DCR calculator I've found so far:

http://www.rbracing-rsr.com/comprAdvHD.htm

By plugging in the numbers Chuck gave in his example I get a DCR of 9.09:1 so it appears to be fairly accurate. Unfortunately it doesnt take into account temperature but everything else is there including boost pressure for your FI guys.

I've tested my old LT, running Evans NPG and a high volume water pump, at DCR's of 9.5:1 without any problems. The engine was extremely thermal efficient albeit at a loss to volumetric efficiency.

Chuck, you have me a little confused here :confused: I realize the Evans NPG makes a more efficient cooling system but I fail to understand how it affects VE, could you elaborate?

Also I noticed that rod length plays a noticeable role in calculating the DCR. For instance in your example subsituting a 5.7" rod increased the DCR by .04 (from 9.09 to 9.13). Now we all know that the real world difference between a 5.7 and a 6.0" rod is almost non-existant but the DCR calculation seems to say otherwise. Am I missing something?

Thanks! :bow:

Edit: This (http://cochise.uia.net/pkelley2/DynamicCR.html) seems to be a good introduction to DCR for those who may not be familiar with it.

even with a dcr, the key term here is that is a ratio, its a volumetric quantity, and yes variations in altitude and in temperature will affect the given cylinder pressures, the compression ratio (including dcr) will be the same under all operating temperatures and pressures.

this is because the swept volume of the cylinder and combustion chamber size are not going to change. for example if the the DCR is XX.x at sea level and 60 deg......it will also, be XX.x at 4000' and 100 degrees. Compression ratios dynamic or not are a function of volume which doesnt change.....the atmoshperic conditions can only affect the actual cylinder pressures.

If you put 1psi into a cylinder with 10:1 CR (dynamic or not, this is an instantaneous event) and crank it over, the piston will compress the 1psi charge to 1/10 its original size....the same applies if it was 100 psi.

thus spake the laws of physics and the gas laws:bow:

Way to go Jason!
I guess some people are actually interested in learning. I'm impressed.
Looks like a good calculator for NA calculations. No parameters for compressor efficiency and or density change, so I wouldn't spaz out over the boost input. These are significant factors when considering knock limit in a force inducted engine.
Other than that, looks great.:)

Originally posted by Soma07
Chuck, you have me a little confused here :confused: I realize the Evans NPG makes a more efficient cooling system but I fail to understand how it affects VE, could you elaborate?

Sure thing.
It's important to understand that an increase in coolant temperature decreases loss of potential power to the cooling system.
The other side of this is the fact that increased charge temperatures reduce volumetric efficiency.

Generally speaking, increases in thermal efficiency are most beneficial to economy while increases in volumetric efficiecy are the key to maximum power output.
By the same token, inreased compression ratio can help both economy and power by improving thermodynamic efficiency.

In a little more detail...
An engine is more 'thermodynamically' efficent when it is hot due to the fact that there is less corresponding heat transfer between the burning charge and the combustion chamber. As in the perfect otto cycle, the combustion is adiabatic.

However in order for maximum power to be realised, maximum VE needs to be found, ie the charge needs to be as dense as is combustibly possible. Therefore the smaller the delta T between inlet tract and charge the better, with the enthalpy of the charge remaining as constant as is possible before it is intoduced into the combustion chamber.

Does that answer your question?

Also I noticed that rod length plays a noticeable role in calculating the DCR. For instance in your example subsituting a 5.7" rod increased the DCR by .04 (from 9.09 to 9.13). Now we all know that the real world difference between a 5.7 and a 6.0" rod is almost non-existant but the DCR calculation seems to say otherwise. Am I missing something?

You may have missed a little but that's easily remedied. In calculating DCR at the most basic level there are some basic relationships you need to understand.

1) static compression ratio, which is the ratio of swept cylinder volume in relation to combustion chamber volume.
2) dynamic compression ratio, is the swept volume at the IVC event.

So, rod length is a factor in that it dictates the amount of piston rise (from BDC) at a given IVC event. In essence, the volume will always be less than total swept volume.

If you'd like a math breakdown on this we can do that too.

HTH

Chuck

Originally posted by Mr. Horsepower
Does that answer your question?


I think so, I only had to read it over about four times before I completely understood, lol... ;)

So in laymans terms with the NPG system you were able to run higher coolant temps without detonation (resulting in increased thermal efficency, one of the benifits of NPG coolant) but at the same time you were sacrificing VE since the incoming charge was being heated more than usual. This also jives with something I read on the Rehr-Morrison website stating that the optimal coolant temp for a drag race motor was around 120F.

You may have missed a little but that's easily remedied. In calculating DCR at the most basic level there are some basic relationships you need to understand.

1) static compression ratio, which is the ratio of swept cylinder volume in relation to combustion chamber volume.
2) dynamic compression ratio, is the swept volume at the IVC event.

So, rod length is a factor in that it dictates the amount of piston rise (from BDC) at a given IVC event. In essence, the volume will always be less than total swept volume.

Got it, so am I correct in saying that while the rod length is a factor in determining DCR there are better ways to alter it if nessecary? Say by using a cam with a different IVC or by changing the SCR? For example lets say you built a motor that had a borderline DCR (say 9.14:1). If you wanted to lower it would you be better of subsitiuting a longer rod or selecting a different cam?

Thanks Chuck!

Originally posted by Soma07
I think so, I only had to read it over about four times before I completely understood, lol... ;)

So in laymans terms with the NPG system you were able to run higher coolant temps without detonation (resulting in increased thermal efficency, one of the benifits of NPG coolant) but at the same time you were sacrificing VE since the incoming charge was being heated more than usual. This also jives with something I read on the Rehr-Morrison website stating that the optimal coolant temp for a drag race motor was around 120F.

You've got it Jason. The NPG coolant ran at approximately 240ºF.



Got it, so am I correct in saying that while the rod length is a factor in determining DCR there are better ways to alter it if nessecary? Say by using a cam with a different IVC or by changing the SCR? For example lets say you built a motor that had a borderline DCR (say 9.14:1). If you wanted to lower it would you be better of subsitiuting a longer rod or selecting a different cam?

Depends on the goal. "More cam" (one with a later IVC event) will effectively reduce DCR but the engine makes power at a higher speed.
If that's not within the bounds of the intended design, then you simply alter SCR to get what you need.:)

Take care,
Chuck

Originally posted by Soma07
I realize the Evans NPG makes a more efficient cooling system but I fail to understand how it affects VE, could you elaborate?


OT, but everything I've read says that its worse at heat dissipation. This stuff is used in the airline industry at a much lower cost, I forget exactly for what though.

Originally posted by Mr. Horsepower
Depends on the goal. "More cam" (one with a later IVC event) will effectively reduce DCR but the engine makes power at a higher speed.
If that's not within the bounds of the intended design, then you simply alter SCR to get what you need.:)

Take care,
Chuck

I should have mentioned this before as I've seen a few posts that could benefit from a little understanding of DCR and the IVC event.
Why don't you try looking at the effect of running a cam (all things considered equal) with a shorter lobe displacement angle. What does that do to the DCR? mmm, homework.;)

Take care,
Chuck Riddeck

By "lobe displacement angle" you mean the LSA right? In other words the angle between the "peaks" of the intake and exhaust lobes.

If so then decreasing the LSA/LDA should cause the intake valve to close sooner increasing DCR correct?

For example take the CC306 you mentioned previously, on a 112LSA it has an IVC of 72°. Narrow the LSA to 110 and the IVC that drops to 70° (I think). Plugging that in to the calculator I found earlier the DCR jumps to 9.26:1 all other things equal.

Now I'm assuming that decreasing the LSA will also decrease the IVC on a 1:1 ratio. I couldnt find a formula to derive the IVC so thats what I deduced by looking at a few different cams that appeared to be identical save the LSA.

This is sorta the opposite effect I expected since a lower LSA increases overlap.

Am I right? Do I get a prize? :D

But if LDA != LSA then I've just wasted a whole lotta time :cry:

A narrower lsa advances IV closing so should lower DCR. That's similar to a "bigger" (more duration) cam: increases overlap.

Why though does a bigger cam, or a cam with more overlap, have a higher rev range? Is it because of a relationship between rpm and VE?

Thanks for the lesson, teacher Chuck ;)

Rich Krause

Chuck: sorry for a 10^6 questions, but here's another that goes back to the original subject of this thread. You stated that an LT1 should tolerate a DCR of ~9.1:1 on 92 octane. Is there a way to estimate the maximum DCR with a different (higher) octane?

Thanks again.

Rich Krause

So, I will be running a 242/248 duration solid roller cam with .570/.578 lift and 112 lsa. The CCs will be 52, impala ss head gaskets and stock shortblock. WILL this combo run well on 93 octane fuel (about 12:1 CR I think). For next year, I plan on running a similar combo only going to 6" rod, .030 over and forged pistons. Sound like it'll run well on pump gas?

Originally posted by rskrause
A narrower lsa advances IV closing so should lower DCR.

Rich,

Isn't that backwards? If the IVC is advanced (i.e. closes sooner) then shouldn't the DCR be higher since the valve is closeing with the piston is closer to BDC?

Originally posted by Soma07
Rich,

Isn't that backwards? If the IVC is advanced (i.e. closes sooner) then shouldn't the DCR be higher since the valve is closeing with the piston is closer to BDC?

DOH!

Now that I think about the question a little more, the way it's worded is confusing. Narrowing lsa can be done by either retarding the exhaust lobe, advancing the intake lobe, or both. The effects of each would be different, and would also vary with the intake center line.

So there is no simple answer. In general though, narrower lsa is usually achieved by moving the intake lobe and will increase dynamic compression. I am not sure what I was thnking when I gave my first answer though! Thanks for pointing that out.

Rich Krause

When you have high port velocity the cylinders keep packing with air after it starts the compression stroke correct? how do you compensate for the increased VE in this case? Also are there any particular places that you know of to research more into this? Hope I understood all this correctly.

From a different angle, are you asking basicaaly this? The DCR is altered with a larger overlap cam. IOW, if you increase overlap, ( advance IVO ) you are able to increase SCR. At higher rpms, when the VE of this cam timing increases, why doesn't the DCR exceed the capability of the fuel used?

If that question is directed towards me then basically yes thats what I am asking. Do you just set up the DCR so its about 9.1:1 assuming the VE is 100%? If the engine wasnt 100% volumetrically efficient then couldnt the number be higher than 9.1:1? How do you tell what the VE is? DCR is the compression after the intake valve closes correct?

I used the calculator in a previous post...
http://www.rbracing-rsr.com/comprAdvHD.htm

That's a pretty neat calculator, but I did notice a discrepancy. When it calls for IVC, it says "at .050 lift". Whoever came up with the calculator needs to delete that part because a valve doesn't close at .050 lift. Other than that, it seems like a very useful calculator and I wish I'd seen it sooner.
I will be using an XE cam 07-502-8 with an IVC at 62 degrees ABDC; Impala head gaskets (to improve quench) that will bring my static CR to 11.1 to 1; stock bore/stroke, NA and zero elevation; and came up with 9.04 dynamic CR. Is this too much? I also didn't figure for milling of my heads for better sealing; hopefully my head porter will take that same amount out of the combustion chambers to unshroud the valves.

Originally posted by onebadponcho

That's a pretty neat calculator, but I did notice a discrepancy. When it calls for IVC, it says "at .050 lift". Whoever came up with the calculator needs to delete that part because a valve doesn't close at .050 lift.

No, the valve does not close/is not closed at .050" lift. However, when you state cam duration specs for valve opening/closing, how are these degrees of duration derived? IOW, what parameters are used to establish the total number of degrees? Doesn't the cam manufacturer state the number of degrees at .050" lift? So, you start counting degrees when the valve has been lifted .050" off it's seat, and stop counting degrees when the valve gets to .050" from it's seat on the closing side of the lobe ramp. OK?

Originally posted by Mr. Horsepower
In my research of this subject amd how it relates to reverse-cooled LT engines, I've found that they can generally tolerate DCR's in the range of 9.1:1 with proper tuning, MBT.

Let's take an example.... a 355 cid LTx with a Comp Cams 290 HR (CC306), running a static compression ratio of 12.5:1 . We all know people running a setup like this on 92 octane. With some calculations and assuming an altitude of 1000 ft, 6" rod length and 72º IVC we calculate a DCR of approximately 9.1:1. Too high for a conventionally cooled sbc (which should stay below 8.3:1) but not a problem in this case.
I've tested my old LT, running Evans NPG and a high volume water pump, at DCR's of 9.5:1 without any problems. The engine was extremely thermal efficient albeit at a loss to volumetric efficiency. There's always a trade off.


That's odd, I put my stock Impala SS engine specs into the calculator everybody has linked to and it give me a DCR or 9.80:1 :eek:
4" bore, 3.48" stroke, 5.7" rods, 10.1CR, 22* ADBC, @ 339' altitude http://texas-on-line.com/graphic/longview.htm

And this engine was only available with iron heads...

Whooo………….. You guys are getting way too complicated for me. Guess I’m just plain not that bright as I favor the “been there..done that” school way over theory. Still say there are too many variables in the real world. Theory is a good place to start, but the real world doesn’t lie and is much more unforgiving.

With a 240/250-114 cam in a 396 with GTP stage II heads, I ran 13.3 to one compression without any problems. At the time, I didn’t realize the compression was that high and though it was 12.8 or so. Found out later I was running a .029 gasket instead of the .039.

The car ran fine on 93 octane (extensive dyno tuning) with no overheating or anything. Frankly if I’d hadn't known it was that high, I’d say you couldn’t of done it. I checked it with three different compression checking programs because it didn’t seem right when I heard it.

Had a huge cam (254/263-112) in a 398 running 12.8 to one with 227 AFR heads and for some reason had to pull timing out or add octane booster. Again, don’t ask me why, but the car ran hotter and required less timing with the AFR heads with less compression and more cam.

My current combo is is much milder being 11.94 compression 383 running a 229/234 custom ground cam. I’ve had to pull timing out compared to my 13.3 to one 396. I’d say in this case, the duration of the cam is causing higher cylinder pressure. Put in a 236/242 and I’d bet my timing would come back.

The jest…….. Really depends on heads, stroke, ci and duration as to how much compression you can run. The only way to find out is to do your homework and actually try the particular combo. The latest I hear is keep it under 12 to one for a street driven car and your probably OK.

Originally posted by Denny McLain
Whooo………….. You guys are getting way too complicated for me. Guess I’m just plain not that bright as I favor the “been there..done that” school way over theory. Still say there are too many variables in the real world. Theory is a good place to start, but the real world doesn’t lie and is much more unforgiving.

Some of the best engine builders I've met are also very well versed in theory... go figure.
I'd also say that Chuck and some of these other guys probably do quite a bit of dyno testing.... "applied theory" if you will. Very important to make that distinction.

-Mindgame

Anyone have an idea of what the ABDC on that calc would be for a GTP-IV cam? Specs are @.050 226/236 114 lob sep.

TTT for Prof. Chuck :D

Basically I was just wondering if I was on the right track with my last few posts regarding the relationship between LSA, IVC & DCR.

Thanks :p

Hmm...
Not to complicate things even further,but i for one believe that the whole topic of DCR is without relevance if stuff is omited from the discussion,and in this case it is.
Dynamic comp is VERY much a question of VE as well.
A VE that will be affected by intake or exhaust restrictions/alterations.
VE would actually just explain half of it too,as reversions as well as dilution will have a pronounced effect on DCR tolerance.Aso..

As those who have been in a dyno cell(-s) will attest there´s really only one thing that calls the day,and that´s the numbers.

For those running anything but pure WOT engines,please also be adviced that DCR is a variable with VE from throttle input.Something that can be of very high relevance for the roadracer and/or streeter alike.
Why?
Let´s just say..reason with the thought yourself for a minute.

There´s more to it then that,but hey..let´s start somewhere right Chuck?;)

Suppose you sort of take that variable out?

I am planning to build a powerdyne supercharged LT1 in a 95 caprice. (with reverse cooling it's almost like chevy WANTS me to add a blower)

I'm spending a LOT of my time trying to learn everything about how air moves through the engine.

Now, that calculator does allow you to calculate boost but when the question of intake & exhaust begins to factor into the VE of the engine the info I have starts getting shaky.

For instance, I will have "X" amount of boost on one side, with the stock intake valve size and on the exhaust side the biggest damn valve I can stuff in there and hogged out ports.

Now I know that on ordinary street engines, even hi-po ones, the exhaust flow, as a percentage of intake flow, doesn't get real far from 80% in either direction.

In fact, 80% is doing pretty good, since many street motors are at 70-75%.

So now I come along and screw that all up with a blower.

Let me throw out some fake numbers to explain what I mean.

Lets pretend that the intake/exhaust relationship puts the exhaust at 80%.

Lets also pretend that the numbers are 100cfm intake and 80 cfm exhaust.

OK, groovy.

So I come along and pop in a big exhaust valve, hog out the ports on the exhaust side and end up with 100/100.

So now I put in say, 7.25lbs of boost.

So that takes the intake flow from 100 to 150, because I'm adding 1/2 atmosphere.

Yes, I know, there is a lot more to it than that, but bear with me.

So now, my 100/100 engine has a 150/100 relationship.

In other words, the exhaust flow went from 100% of intake flow down to 66%.

With having to "push" against that exhaust "pressure" the DCR must be affected, right?

Perhaps not in a perfectly linear fashion like my "thought experiment" but it's got to happen.

So now how do I figure out how to set my static compression ratio so that once I factor in the relative VE of either side of the head I end up with a DCR of 9:1?

Originally posted by Soma07
FWIW I did a little poking around and this is the best DCR calculator I've found so far:

http://www.rbracing-rsr.com/comprAdvHD.htm

By plugging in the numbers Chuck gave in his example I get a DCR of 9.09:1 so it appears to be fairly accurate. Unfortunately it doesnt take into account temperature but everything else is there including boost pressure for your FI guys.



Chuck, you have me a little confused here :confused: I realize the Evans NPG makes a more efficient cooling system but I fail to understand how it affects VE, could you elaborate?

Also I noticed that rod length plays a noticeable role in calculating the DCR. For instance in your example subsituting a 5.7" rod increased the DCR by .04 (from 9.09 to 9.13). Now we all know that the real world difference between a 5.7 and a 6.0" rod is almost non-existant but the DCR calculation seems to say otherwise. Am I missing something?

Thanks! :bow:

Edit: This (http://cochise.uia.net/pkelley2/DynamicCR.html) seems to be a good introduction to DCR for those who may not be familiar with it.

I have found that calculator to be incorrect...

EAPro calcs 6.5 for my engine and this program calcs 8:1

I mean if my CR is 8.8:1 how can it be 8.09:1 with my 41 atdc IVC???

8:1 would not be good with 12psi boost from a ys
:eek:

Didn't see a link to this DCR calculator in this thread:

http://webpages.charter.net/darrell1/dynamiccr.htm

I like it because it's downloaded onto your computer. Seems to work fine on XP at least.

Originally posted by The Highlander
I have found that calculator to be incorrect...

EAPro calcs 6.5 for my engine and this program calcs 8:1

I mean if my CR is 8.8:1 how can it be 8.09:1 with my 41 atdc IVC???

8:1 would not be good with 12psi boost from a ys
:eek:

It's very incorrect. The author made the mistake of using 0.050" valve timing numbers instead of seat timing numbers. The calculator assumes (using the wrong numbers) that the intake valve is closing much sooner than it actually is...therefore, the dynamic stroke length is longer than it should be, resulting in a higher dynamic compression ratio.

If you want a better example, download the calculator from this site (http://webpages.charter.net/darrell1/dynamiccr.htm) and plug in the numbers it asks for. Now...instead of using the intake closing angle that the program comes up with, use the 0.050" intake closing angle from your cam card on the "dynamic stroke" tab. Then go back and recalculate your dynamic compression...you'll see that it's much higher now (and that it matches the number the "broken" calculator gives you).

To figure DC you HAVE to use advertised duration and not @50.You have to know the IVC at advertised not @50
9.0 DCR is ok WITH excellent COOLING.AND 92 octane.Some of the DCR numbers you hear floating around are NOT figured right.To get the advertised duration you have to call your cam maker,if it is not listed on your card(most are not) If anybody wants the correct number's give me the info on your engine and cam and I will tell you the CORRECT DCR you are running.

http://members.uia.net/pkelley2/DynamicCR.html

I just thought this link would be great to have in this thread for future reference ;)

Chuck: sorry for a 10^6 questions, but here's another that goes back to the original subject of this thread. You stated that an LT1 should tolerate a DCR of ~9.1:1 on 92 octane. Is there a way to estimate the maximum DCR with a different (higher) octane?

Thanks again.

Rich Krause

I'm bringing a hell of a thread back to the top hoping that somebody can help with Rich's question.

The fuel that I'm considering is E85. It's still relatively cheap and easy to get here and has about a 105 octane rating and much of the extra cooling properties that alcohol offers. With a small chamber aluminum head in a reverse flow motor, would it be safe to run DCR up to 9.5 or higher with E85?

I'm also wondering what DCR to shoot for with an iron head LT1. Maybe even when run on E85? Would the two cancel each other out?

Cheston,

It's more about the max cylinder pressure you can run AND how much compression you can get in there without slowing down the burn. The problem with the iron head is the large chamber already, the Al castings are going to help you get the compression to start.

Bret

Cheston,

It's more about the max cylinder pressure you can run AND how much compression you can get in there without slowing down the burn. The problem with the iron head is the large chamber already, the Al castings are going to help you get the compression to start.

Bret

Brett,
How does increased compression slow down the burn rate?

Throw a dome in there.......

Throw a dome in there.......

Yes, a dome doesn’t necessarily equate to slowing the burn rate, we learned that from Larry Widmer.

Well depends on the dome shape and the chamber shape..... most times it can cause a issue. There are a lot of things that can cause issues with how the burn works. The intake and exhaust port can both influence the igintion lead as well.

Bret

Well depends on the dome shape and the chamber shape..... Most times it can cause a issue. There are a lot of things that can cause issues with how the burn works. The intake and exhaust port can both influence the ignition lead as well. Bret

I prefer a small dome over a larger one, achieving short burn time is not a problem with either.

The key is development and distribution of TKE – turbulent kinetic energy or mixture movement. Careful delivery of high levels of mixture movement during combustion will increase flame intensity to allow short burn times regardless of piston shape.

Generating mixture movement is common practice in the ports but this has its disadvantages. Port generated turbulence normally comes at the expense of VE, not a good trade. In addition mixture movement generated in the port leaves plenty of time for the energy to be absorbed by the viscosity of the air/fuel mixture.

My preference is to increase mixture movement with enhanced squish action and to distribute the energy to areas where it can be of most benefit.

Piston dome masking squish flow is the major problem the must be resolved when using a large dome. Addressing this concern allows adequate flame velocity. This picture shows complete burn utilizing a large dome and a compact TopLine combustion chamber.

http://members.cox.net/raunch/fabian%20r11.jpg

It also shows a LOT of hard edges... the valve pockets also have a sharp corner, where a large radius in there would help dramatically.

Bret

It also shows a LOT of hard edges... the valve pockets also have a sharp corner, where a large radius in there would help dramatically.

Bret

Over the years I have spent countless hours carefully blending generous radiuses in to the sharp combustion chamber surfaces. The ugly piston top pictured is the result of a late night trash to make a race. Strangely the sharp edges didn’t appear to have an impact, since then they have been blended with no apparent benefit.

Notice how the burn left little behind, in some spots fuel contaminates (sulfur) can be seen. I have concluded when flame intensity is high enough, the chamber becomes less sensitive to normal requirements.