so from what i understand you agree it makes less power. and to fix that you add more boost. so at what point does all this added boost become just as much of an issue. Plus if (i understand there are more examples but this is what is talked about.) when you leave of the line and at no boost you make x power with high compression and the same senerio you make the same x power with low compression and two psi. why cant you just add the 2psi to the high compression engine and make even more power? if you continue this trend then it would increase the power difference as you increase boost pressure correct? So by the time you hit 15psi you would have to run 18-19psi on the low compression engine for the same power. now both are at their peak but one was done with more boost and another with more compression.

now to go deeper you now have a harder working turbo with low compression creating higher iats. that will create a power loss itself and also make a smaller tuning window. on the same hand the higher compression wil have a less worked turbo and lower iats but have a higher cylinder pressure correct? so you are on the edge of detonation using the same pump gas.

if im correct then both can make power with a proper setup and have their perks. its just a matter of when you put an engine on edge. which problens do you want to.face.


im no engineer so forgive me if im way off, but its what ive always seen.

 

I've really stopped responding in this thread because I've said all I have to say in regards to theoretical power gains/losses from static compression ratios and how it's really not that big of a difference not in boost.

Obviously fuel type plays a much bigger role than anything else in how much timing, compression, boost you can get away with. When you are octane limited you will GENERALLY be capable of making more power on less compression / more boost and also have it be safer with a larger tuning window. OEMs don't choose lower compression ratios for boosted applications on accident when compared to their N/A brethren for both power and safety.

All things considered when not octane limited a higher compression motor will make more power, how much more is highly combo dependent. To a certain extent all it's really doing is raising the efficiency of the whole thing anyways.

 

Some modern turbo engines:
Ford F-150 Ecoboost: 10.0:1 CR
Ford Focus Ecoboost: 10.0:1
Ford Explorer Ecoboost: 9.3:1
Ford Fusion Ecoboost 10.1:1
Porsche turbo 911: 9.8:1
Porsche Cayenne twin turbo:10.5:1
Chevy malibu: 9.5:1
Dodge Dart 9.8:1
2013 Hyundai Veloster Turbo: 9.5:1

What were you saying about OEMs and compression ratio??

 

So wait, high cylinder pressure is now a bad thing? I thought that cylinder pressure is what generated torque?
No need to respond I know the answer. Sustained cylinder pressure is what generates torque and torque over time is power. Before anyone jumps on me, I also know that cylinder pressure is only benneficial when it's controlled and consistent, which means controlling flame front etc.

I think we need to stop referring to what the automanufacturers do as a reason to justify how to best create power. Manufacturers are all about making money, and if things break, they lose money. Also requiring people to run 93 octance isn't a good thing either. Also individually tuning every single engine is expensive. So they drop the compression, keep the fuel and timing conservative, and try to make the masses happy.

That is everything that any hotrodder isn't.

 

And you listed direct injected engines as your comparison? C'mon dude, not the same as what we have here and you know it. Why are you trying to start an argument knowing full well that those aren't directly comparable to what we have?

 

Really? I guarantee GM and every other OEM has tested to the Nth degree and didn't guess at this stuff. OF course it's about keeping it safe, emissions, etc but also power and efficiency given pump gas. A hotrodder is going to run higher octane, meth injection, whatever so of course things change and you can run things more on the edge and make it work. It isn't a 100k+ mile engine at that point either. I'm not debating that more compression is ultimately going to make more power when it's not octane limited.

How about a non-OEM comparison? We took a C5 with an APS kit that had 243 heads and a Z06 camshaft and could only really muster 7 psi (spring pressure) on 91 octane here in AZ and timing was in the single digits in the peak torque area creeping up to the low teens up top and it made right at 500 rwhp on the dyno unlocked. The graph was wavy because timing was in the gutter and it wouldn't take anymore boost or timing with our **** gas. So we decided to switch it to 317s to see if we can feed it a bit more and unsurprisingly we were able to run about 10 psi and it made 560 rwhp. That's the difference on that combo, even though it had a lower compression ratio it took more boost for our crappy gas.

 

Exactly, you are in AZ with high heat, high IAT's I bet, with only 91 octance, no doubt you had issues. I bet with a meth/water kit, you could have made more power with the higher compression with the same amount of boost.

 

Guaranteed with more octane or water/meth it could have made more power with the higher compression at the same boost level not to mention I probably could have turned it up too and I wouldn't think of arguing against that fact. Are you reading what I'm saying here because it doesn't seem like it?

All I was pointing out was that the combo was capable of making more horsepower even though it had less compression because it was octane limited. Unless you are running C16 or E85/E98 or Meth you will be octane limited to a certain extent, it just changes where you hit that wall.

 

And did you read what I said about the key to making the most power is maximizing cylinder pressure consistently and controlling flame front. You weren't able to do that with 91 octane and high IAT's, small camshaft (high dynamic compression), and high static compression.

 

I'm well aware, I'm the one that made the example remember? I never said anything contrary to that.

 

I got what you're say NicD...

 

If we limit ourselves to 87 octane this will continue to get lower and lower. but i think most every example in this thread is capable of running in 93 oct reliably without meth. yes tuning window will be smaller, but i dont want any car im driving to have a tune that is off no matter how safe it is...