What compression ratio would you recommend for a naturally aspirated LS running 92 octane pump gas?

No E85, no octane boost additives. The car is used primarily for HPDE track days.

Asking because the engine was accidentally made with too high a compression ratio (11.8:1) and the builder has offered me a few options to set things right.

Dart aluminum block, 4.185 bore, 4.125 stroke, so 454ci, in a C5.

Thanks in advance for any insights...

 

What's the dynamic compression ratio? You can always retard cam timing.

 

9.51:1 for the DCR.

But, does DCR really matter at high throttle + high RPM? I mean, I thought that the whole point of keeping the intake valve open into the early part of the compression stroke was to use charge momentum to squeeze even more air into the chamber. What would be the point of leaving it open past BDC, if that was just pushing the intake charge right back into the manifold?

 

11.8 would be ragged edge of 92 octane but it should work with the right tune. I’m 12.42 and run 93 exclusively in the Chevelle. No octane boost here. A good tune…a great tune actually…is mandatory.

 

My tuner experimented with AFR as rich as 11:1 and at full throttle he still couldn't get it to take more than 20 degrees before knock set in. It runs fine, but it should have have another 5-6 degrees of advance, and another 50-75 horses or so.

The tuner was pretty sure I had a compression ratio problem, and it turns out he was right. At the time, the builder insisted it was 11.5:1, which seemed reasonable to me since that's what I'm told my LS6 had. (A previous owner did the heads, but the shop that upgraded the cam for me measured the volume and did the math while the heads were off.)

The tuner at the shop that has the car now thinks that even 11.5 is excessive. We're going to talk a bit more before I commit, but I wanted to get some more data points in the meantime. And this is all I can think about this weekend anyway.

I'm amazed that 12.42:1 works for you on pump gas... what AFR do you run and what's your max timing at full throttle / high RPM?

It might also be relevant that I'm running a P59 PCM and (of course) a 24x trigger wheel. I gather that more resolution and more CPU power makes for more accurate spark timing, but I have no idea how much that matters in practice.

 

11.9:1cr 376" here on pump 91-93 I agree your problem is dcr and not static cr I have a baby cam you can try 220/224 115 -4 for ivc of 49 @ 0.050 and 76 @ 0.006 it worked in my setup very mild idle I'm just running a much larger cam now for more top albeit with a slightly earlier ivc event, though it's got far more overlap and that helps too, I can share my timing map the higher compression stuff can't tolerate nearly as much advance at lower RPMs hell mine only wants 5.5° to idle nicely, I't only wanted 10° idle advance for the 2600 stalled automatic, come to think of it be automatic made a lot more heat and the total timing wanted to be less due to that so other factors at Play still total advance above 3k is still fine 26-30° depending on specific rpm

 

Agree 100% here. An internal combustion engine doesn’t know or care what the static compression is. Dynamic is everything, and your IVC controls the majority of that. A taller static does make your tuning window smaller fwiw. Like stockA4, my 434 only wants 5 degrees at idle. I’ve sent a bunch of high end max effort builds out the door here that are between 12:1 to 12.4 and all run well on pump 93. My 434 wants 26 degrees total at rpm. My limiter is set at 7800 fwiw. We run them hard…

 

Please help me out here...

It's my understanding that the whole principle of DCR stems from the idea that when the piston rises from BDC to IVC, it is pushing air out of the cylinder, so it only compresses the air that remains in the cylinder after IVC.

And yet the whole reason for trading a stock cam for one with a later IVC is to get more air into the cylinder, especially at wide open throttle and high RPM.

So, I don't see how DCR is relevant to what's going on at (or even anywhere near) peak power.

What am I missing?

 

Mass and momentum. Even once the piston hits BDC on the intake stroke, the cylinder isn't yet full of air. It is usually under slight vacuum.
Even while the pistons is starting upward on the compression stroke, air is still rushing in to fill the vacuum. Holding the valve open longer after BDC will help.
The key being to find that sweet spot where the cylinder is full, the intake valve can close, and the piston hasn't made it too far up to lose too much compression.

 

Yeah, I get that part. It's why a later-than-stock IVC tends to improve peak power. No question about that.

The question is, what's the point of DCR? The DCR is based on the idea that the true compression ratio is determined by the IVC, and therefore is lower than the static compression ratio - which just means that it doesn't take into account any of the facts that you just covered.

If anything the "real" compression ratio is larger than the static compression ratio because the air mass is great than that what the static compression ratio alone would indicate. Whereas DCR gives you a ratio that's smaller.

 

Physics boys it's physics. The DCR determine your starting cylinder pressure IE cranking pressure,the scr determine your compression or pressure potential. Think of the IVC more as an ore lapping a wave then a door trying to open and close

The rest of the intake and exhaust track are going to "like" the later ivc as well as early evo regardless of overlap when talking increases in scr over stock

 

I get that. All of that.

What I don't get is why so many people insist that DCR is important when tuning for peak power - full throttle and high RPM.

 

Cylinder pressure. That’s the basis for power. Cylinder pressure is torque. The DCR determines actual cylinder pressure. I can set up an engine with 15:1 static and have it run on pump gas with the right camshaft choice. It would run ok, but nothing to write home about. Valve events dictate everything about how an internal engine runs, responds, acts, etc. Cylinder pressure is variable of course, depending on rpm, load and the overall efficiency of the combination. Having said all of that, because of simple math, torque peaks around 5250 in an internal combustion engine and as RPM’s climb, horsepower takes over…which isn’t as dependent on the DCR as torque is at lower RPM, but relies on the entire combination to shine. But because HP is a multiplier of torque production, having the highest SAFE DCR possible that will boost torque numbers is key on making big HP numbers…which is all why so many people insist that DCR is important when tuning for park numbers. You won’t have good HP numbers without good torque numbers unless you’re building a big RPM build such as endurance racing, NASCAR, etc. They all rely on RPM to carry the power. Combination and application is everything. Everything!

 

It's my understanding that the whole principle of DCR stems from the idea that when the piston rises from BDC to IVC, it is pushing air out of the cylinder, so it only compresses the air that remains in the cylinder after IVC.

And yet the whole reason for trading a stock cam for one with a later IVC is to get more air into the cylinder, especially at wide open throttle and high RPM.

So, I don't see how DCR is relevant to what's going on at (or even anywhere near) peak power.

What am I missing?

 

Wouldn't more overlap reduce cylinder pressure?

 

Read the second half of my post again…

 

No. overlap occurs at the end of the exhaust stroke, and the beginning of the intake stroke for a few degrees while the piston is at TDC.

 

I'm sure you know this and just worded it poorly, but to be clear, the 5252 has absolutely nothing to do with where an engine makes peak torque. It is simply the mathematical calculation that relates horsepower and torque at any rpm. As for DCR, I believe most people are greatly underestimating the level of question that NSFW is asking, based on the simplistic responses given.

 

The stock Ls1 in the f body is a good teacher 99' vs 01 to be specific compare the stock camshaft specs and the stock timing maps from each, I've degreed most of the stock camshafts for the LS series and the major difference is the 99 closes the intake valve significantly later and produces less cranking compression, the 99 has a lower DCR than the 01! the 99 timing map is also more aggressive with more spark in most areas including loaded and wot areas if you've driven both as I have as well you'll certainly feel and tell a difference between the two programs and curves they produce

 

I would add that the overlap doesn't change the cylinder pressure so much as the composition of it, remember the 99 LS1 cam had less overlap than the 01 and that engine utilize an EGR system so it could re-breath its own exhaust