heads

[1 fast z]

Only way you get above 100% ve, is with a properly designed intake. The sound waves, need to be timed to where you PACK that sound force into the chamber when the intake valve opens. Idealy you want only one sound wave, BUT this calculates on a z, with like a 1.75" id runner, and a 7000 rpm peak power, the runner needs to be like 26 inches long, obviously not doable very easy, so you take it in multiples, say divide that by 2, 3, or 4. So when that wave is going back and fourth, and its coming back to the valve, and that valve opens at the exact same time, it will create a supercharger effect, and pack the cylinder with above 100% VE, but the cam and the intake need to be tuned for the same RPM for this effect to happen. Hope that made sense.

[JMortensen]

Been absent awhile, but I gotta say, I do not think big cams "bleed off compression at ALL rpms". My understanding is that volumetric efficiencies of greater than 100% are possible with big cams at elevated rpm. My 3.1 went from 189 lb-ft of torque with the stock cam (at 10.35:1 CR) to 210 lb-ft (same CR) with a 290/.490" Schneider cam, to 220 lb-ft with a Sunbelt 310/.550" cam (at 11:1).

 

When you "get on the cam" with high lift cam profiles, you go from losing a ton of v.e. at lower rpm to GAINING v.e. at higher rpm. The engine will aspirate its actual displacement (even more!). Which means you are not "bleeding off compression" at higher rpm. The risk of detonation isn't as high at elevated rpm because detonation requires 1) load and 2) TIME. At high rpm, there simply isn't enough time for detonation to occur.

 

This is not to say that high-rpm detonation (which is more difficult to detect) can't be a problem. It has wrecked plenty of engines!

 

But I disagree with the idea that big cams in a well-developed engine build will "bleed off compression" at all rpm. They sacrifice low-rpm v.e. for high-rpm v.e. Which also allows for higher compression ratios. Win/win, if higher-rpm power is your goal!

Good to see you here again Dan.

 

I think when I used the term "bleeding compression" it was in an attempt to convince Mack using his own terminology. I think that's a really bad way to think about what is going on when you install a larger cam. I think "dynamic compression ratio" is much more accurate, but Mack seemed to be saying that a larger cam was a sacrifice because it "lowered compression", and that post was an attempt to try and convince him he was wrong.

 

http://www.empirenet.com/pkelley2/DynamicCR.html

 

As stated in the above article dynamic compression ratio is effective over the entire rpm range, just as static compression ratio is. In that sense the cylinder is again "bleeding" compression all the way across. The difference is that at high rpms VE goes up due a combination of air flow in the intake and the later closing of the intake valve and the resultant cylinder pressure is higher, even though the dynamic compression ratio is still lower. That's when you're "on the cam" as you said. I agree with your last point about sacrificing the lower rpm power for upper rpm gain, although I'd say that a medium sized cam might do better than stock over a very large portion of the rpm range than the stocker. The stock cam is designed for emissions and efficiency at low rpms where most engines spend their lives.

 

As it ended up he was arguing that he shouldn't be increasing the size of the cam because he was using stock FI. Later in the thread he conceded that the stock EFI does suck. Not to pick on Mack any more here but there are actually a group of people here who are fascinated by what they can do with stock compression, or a stock cam, or a stock head, or a stock intake, etc. I just don't get the fascination with this. I see no point in hamstringing yourself so you can claim that you've made the most of a shitty situation. Make the situation better and make the most of THAT situation. I just don't get the "Look at the shine I put on this turd" contingent.

[Dan Baldwin]

Welcome back Dan!

Good to see you here again Dan.

 

Thanks, fellers! I plan to run the Z again this year (ran my stock S2000 in '07), so I should be spending a bit more time here.

 

I think when I used the term "bleeding compression" it was in an attempt to convince Mack using his own terminology. I think that's a really bad way to think about what is going on when you install a larger cam. I think "dynamic compression ratio" is much more accurate' date=' but Mack seemed to be saying that a larger cam was a sacrifice because it "lowered compression", and that post was an attempt to try and convince him he was wrong. [/quote']

 

If the intent was to say that the larger cam does NOT "lower compression", then I agree 100%.

 

As stated in the above article dynamic compression ratio is effective over the entire rpm range, just as static compression ratio is. In that sense the cylinder is again "bleeding" compression all the way across.

I am not a fan of "dynamic compression ratio". It *implies* a more accurate effective "compression ratio" for an engine that is actually running, but in fact it is only accurate at zero rpm. It gives the impression that with a big enough cam you can run much higher static CR because of this "bleeding off", which *is* ocurring at lower rpm, but it is NOT ocurring at higher rpm. Fortunately, at higher rpm the time available for detonation to occur is greatly reduced. That is the *real* reason a higher CR can be run. At low-rpm high load, the big cam does reduce effective compression ratio.

 

The difference is that at high rpms VE goes up due a combination of air flow in the intake and the later closing of the intake valve and the resultant cylinder pressure is higher, even though the dynamic compression ratio is still lower. That's when you're "on the cam" as you said.

 

That's why it's a piss-poor concept. The lower "dynamic compression ratio" is meaningless at rpm where the cam is designed to work. The engine is aspirating as well (better even) than a smaller cam in its rpm range.

 

Better to find out what the top builders are able to run for CR for similar builds than to go by "DCR", which could get one into trouble IMO. You think you're calculating "effective compression ratio" at rpm, but that's not at all the case. At 13:1 with a big cam, you are STILL squishing the cylinder's displacement by ~13:1 in the cam's operating range, even if you are aspirating enough less than that to get away with it at lower rpm.

 

I agree with your last point about sacrificing the lower rpm power for upper rpm gain, although I'd say that a medium sized cam might do better than stock over a very large portion of the rpm range than the stocker. The stock cam is designed for emissions and efficiency at low rpms where most engines spend their lives.

Yup, all depends on intended usage. Unfortunately, most people get a cam just to "wake up" the engine a little, and end up disappointed that the engine doesn't make any more torque, and they have to rev it higher to get that!

 

I see no point in hamstringing yourself so you can claim that you've made the most of a shitty situation. Make the situation better and make the most of THAT situation. I just don't get the "Look at the shine I put on this turd" contingent.

 

Har har har! Agree 100%. Though I gotta say I've always been impressed with what Norm has been able to do breathing through SU carbs. Me, I wasn't up to screwing around with them to maximize their performance, so I just bought 45mm 3x2's. Don't have them anything like optimized I'm sure, but less work required to get more power!

[JMortensen]

I am not a fan of "dynamic compression ratio". It *implies* a more accurate effective "compression ratio" for an engine that is actually running, but in fact it is only accurate at zero rpm. It gives the impression that with a big enough cam you can run much higher static CR because of this "bleeding off", which *is* ocurring at lower rpm, but it is NOT ocurring at higher rpm. Fortunately, at higher rpm the time available for detonation to occur is greatly reduced. That is the *real* reason a higher CR can be run. At low-rpm high load, the big cam does reduce effective compression ratio.

 

That's why it's a piss-poor concept. The lower "dynamic compression ratio" is meaningless at rpm where the cam is designed to work. The engine is aspirating as well (better even) than a smaller cam in its rpm range.

I agree with everything in the first paragraph. DCR only works at 0 rpm. But in a sense, so does CR, because you don't get a perfect full fill of the cylinder (no more no less than than the area of the cylinder and combustion chamber) when the engine is actually running. Relating the CR to the DCR does provide more useful information than either number on its own. The more thorough a person's understanding of the concepts in their own rights, and how they relate dynamically to the engine's performance at different rpm ranges the better, and the better choices they should be able to make.

 

Better to find out what the top builders are able to run for CR for similar builds than to go by "DCR", which could get one into trouble IMO.

I'm a bit split on this part. For sure emulating the best out there will get you a good result. But if you just use whatever they feed you then you're relying on them to improve things. I don't think it's beyond the scope of people here to improve on something that the "best" suppliers currently have to offer whether your talking about engines, suspension components, etc.

 

Har har har! Agree 100%. Though I gotta say I've always been impressed with what Norm has been able to do breathing through SU carbs. Me, I wasn't up to screwing around with them to maximize their performance, so I just bought 45mm 3x2's. Don't have them anything like optimized I'm sure, but less work required to get more power!

Yeah, Norm certainly was wringing every last drop out of those SU's. I wish he would get a hold of a set of 44's or 45's and put as much effort into tuning those.