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High compression auto

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Jst4Cyl

15+ Year Contributor
312
9
Nov 8, 2006
New Castle, Indiana
Looking into Wiseco "10.5:1" E85 HD pistons and curious as to if anyone has first hand experience. I'm new to high compression but plan to run E85 70-80% of the time. But leery of going that high of a CR on 93. Any input on that as well?

Will be on a 2.0 7 bolt HX40 T4 17CM. Uncertain about what converter would be most beneficial as this will see a lot of street time.

And a little billetness ;)

Thanks

Rich
 

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2.3- 10.5 cr here , running 94 oct with 3 nozzle 1500 cc pure meth injection @ 42 psi dd . The car is 2g auto, stock re stalled convertor ( like lock up ) . 10.5 CR is not high compression those days. Having an auto I would not go below that cr .
 
First off, I want to thank you for simply replying! For some odd reason I can never get results from threads but yet brand new guys wanting to build 4 second quarter mile cars get help LOL...

How does it do on straight pump? That's my fear currently LOL.

What turbo? Any numbers (spool, power level etc)?

How's the converter doing and who did you go through?
 
If you are getting off the shelf 10.5:1 pistons they will actually be closer to 9.5:1-10:1 since they are incorrectly labeled for our application/cylinder head CC. Figure out your true static CR with one of the calculators online.

With that said I am running a "real" domed custom 10.5:1 pistons from Force Engineering and have no issues. Its all in the tune and knowing the limitations of the fuel your using.
 
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On straight pump gas (94 oct) I can go 28 psi on very hot day (a good intercooler is the key) . The convertor is re stalled by IPT 3 years ago (no problems so far ) My turbo is pte 6466 dbb, full boost @5200rpm ,made 537hp@38psi . Now I'm in the process of building 2.4 long rod 12.5:1 cr engine (that's what I consider high compression engine)
 
For what it's worth, I don't think I've seen any power gains at all from 9.5:1 up to 11.5:1. On my combo at least, all it did was knock more and spool slightly worse.
 
Wait, knock more and spool worse on the higher CR setup? The knock makes sense, but I'd expect that the spool would happen faster/be better on higher CR.
 
3 things going on with the spool.

a. Higher compression hurts VE when drive pressure is less than manifold pressure, therefore less mass flow through the engine, which hurts spool.

b. Higher compression ratio = Higher expansion ratio, which means less energy in the exhaust left to drive the turbo.

c. The engine and turbo race to RPM, if the engine make more power out of the available airflow, it's gonna do better in the race to RPM, effectively hurting the spool (standing on it from a roll), all though that goes out the window trying to get it up on the stall it should help it, but it's an honest 1-3% hp gain per point, and once you get to where it needs a dome it's probably less as the dome impedes the flame progression making the combustion slower, and killing efficiency. Plus 1-3% of shit is still shit. So if it makes 100hp trying to get on the convertor, 103 isn't going to be much better.
 
3 things going on with the spool.

a. Higher compression hurts VE when drive pressure is less than manifold pressure, therefore less mass flow through the engine, which hurts spool.

b. Higher compression ratio = Higher expansion ratio, which means less energy in the exhaust left to drive the turbo.

c. The engine and turbo race to RPM, if the engine make more power out of the available airflow, it's gonna do better in the race to RPM, effectively hurting the spool (standing on it from a roll), all though that goes out the window trying to get it up on the stall it should help it, but it's an honest 1-3% hp gain per point, and once you get to where it needs a dome it's probably less as the dome impedes the flame progression making the combustion slower, and killing efficiency. Plus 1-3% of sh** is still sh**. So if it makes 100hp trying to get on the convertor, 103 isn't going to be much better.

Definitely some good info here! This is exactly why I was trying to do some research and not jump the gun and make an uneducated decision. I appreciate the input.

For what it's worth, I don't think I've seen any power gains at all from 9.5:1 up to 11.5:1. On my combo at least, all it did was knock more and spool slightly worse.

Not to kiss ass, but whatever you say I think it's worth anyone in the game's, time. I just wish I was on a closer level with people with first handeexperience with stuff like this such as yourself. I understand how simple it is to sit around and bullshit about hypothetical builds in the garage. But when it comes to forums, it's cumbersome to sit and write out a lengthy reply to a question when the OP probably won't appreciate the input to begin with.

So again, thank you both for your time! It's greatly appreciated. This auto game is new to me and I want to put together a setup that works and not something that doesn't mesh.
 
I should have prefaced my post, by saying in general. There's always an exception to every rule, but looking the science that governs these creatures, the above is how it is supposed to work.

I could suggest that you guys get on amazon an pick up a copy of "Fundamentals of Internal Combustion Engines" by John Heywood. It's the text I use in the course I teach. It's pretty dense, but it you work at it and get your "cousins buddy's brother" who is going to school to be an ME to explain some of the thermodynamics to you it will makes sense. It will give you a basic understanding of the underlying principles and help you know when someone is just blowing smoke up your ass.

As another comment on kiggly's statement, it's important to know that his setup is an extremely low backpressure setup, and that is a setup that will strongly benefit from the lower compression.

For example imagine he is running 40psi boost, and 30psi drive pressure. At the end of the exhaust stroke, the combustion chamber is at 30psi. Because the chamber is at lower pressure than the intake, it allows fresh air to flow in before the intake stroke even begins. So the lower the compression (greater clearance volume), the more air flows in before the intake stroke begins, and the higher the Volumetric Efficiency.

Now look at a setup like mine, at some cases I have up to 90psi of drive with 30psi boost. So at the begining of the intake stroke my chamber is at 90psi, and the intake is at 30. The exhaust gases expand with the intake stroke(maybe backflow into the intake). No fresh air flows into the chamber until it is below 30psi. The exhuast gasses might need to expand 3 times there initial volume to get down to 30psi...Hopefully you can see why a higher compression ratio would help here. The smaller clearance volume helps evacuate the exhaust gasses from the chamber.

This is why diesels get by with such high drive pressures. I've also seen high horsepower diesels (tractor pull) make more power with less compression. (think 9:1 LOL).

Also keep in mind this is an extremely simplified explanation, that is ignoring all the dynamic effects and such.
 
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