Boost effects on low compression engines vs high compression engines

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Originally Posted by donniekak

If I recall, Chris hill was trapping 140 in the 1/4 with a fwd 10:1 setup to.
What it comes down to is tuning ability. The area between running great, and blowing up gets smaller the higher the compression is.

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Yeah, But I don't think he was on pump gas when he did those runs. He also didn't care if he broke anything as he would just slap in some new/used parts and do it again. I spoke with him a bunch about it when I first started my project.

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Originally Posted by NickS VR4

Just as the chart showed earlier. If you change the compression, and the boost levels stay the same, the internal cylinder pressure will go up. Pressure and Heat create knock. Unless you are able to change the heat, the increased pressure will, IMO, generate knock.

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Yeah was the problem I run into. EGT's too high. That was the reason to move to the FMIC and then Meth injection. I had to keep the intake charge cool to keep the EGT's down. I'm also running a little richer to help cool things down. The knock always seems to be around the 4k mark when it hits. So with the emanage in their now and more fine tuning and timing control, I hope to really be able to push things on this engine.

Pencil racers........

Greg, Ivan and I have shared a ton of information comparing a number of high compression and low compression setups here locally. If the turbo, heads, cam overlap, and timing is identical the major difference between the two is the effective cylinder pressure.

At 6000ft a 10.5 static compression engine generating 15psig generates an effective cylinder pressure of 20.01psi

At the same elevation an 8.5 static compression engine requires 22psig to generate an equivalent effective cylinder pressure. The resultant is 20.02 psi.

What this data illustrates is one setup produces more heat from the combustion process (high compression) and the other produces more by introducing more boost (low compression). You can cool the combustion chamber with fuels such as ethanol. The negative effects of the turbos external heat are greater than the internal effects from heat caused by equivalent effective cylinder pressure on a high compression setup.

If you can introduce an adder into the combustion chamber (meth, ethanol) to keep it cooler you will benefit more by going high compression becase you do not have the exponential thermal stress caused by turbo at higher boost pressures.

This is the benefit of high compression over low compression.

The fix is E85. And here's where this will all start to make sense. At 25psi the 10.5 engine has an effective cylinder pressure of 27psi. The low compression 8.5 engine requires a boost pressure of 35psi to generate the same effective cyliner pressure. Which one of these turbos is going to be generating more heat? There's your answer guys.

...and donniekak, quit talkin shit if you have no knowledge, experience, or results of your own to share.

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Originally Posted by J. Fast

...and donniekak, quit talkin shit if you have no knowledge, experience, or results of your own to share.

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I'm not the proven troll, kicked off another board.

In case you don't know, higher compression increases the thermal efficiency of an engine, requiring less timing to reach mbt. This is a good thing. The lower the timing advance you have to run to reach mbt, the less degrees of crankshaft rotation you are trying to puch a piston up against an already semi ignited charge, and the more efficient the engine is.
I'm also willing to bet I've written more complete timing maps than you from scratch. I've personally seen some timing maps written by some premier 3s tuners, and they scared the shit out of me.
I have a personal car right now at 9:1 compression that I run at about 26 psi, on straight 93 octane. I also know people that run almost 40 psi on straight pump, you should school them on how to tune.

...and this is on oem cams, oem overlap and lift, oem cam timing, oem heads, oem dsm intake, oem turbo? What style of piston crown? I think you need to give a little more information in order to validate your findings. Please elaborate on you setup.

Wiseco 9:1 dish pistons, no quench, cams are 212* @.050", and 111* lsa, stock head, stock intake, td05h20g turbo. More of an enemy than static compression ratio to pump gas, is exhaust gas reversion from a small turbine. I've seen people with gt40 turbines run way over 30 psi on straight pump gas. I have never seen anyone ever measure exhaust manifold pressure on a 6g7x setup. Lots of serious Dsm guys have a pressure sensor off the exhaust manifold, that is logged.
This platform is in the dark ages. If it wasn't for merkel bringing his gm v6 experience via pampena, and aem making a plug in unit, it would still be running afcs spinning bearings constantly, at 3 liters.

Don't get me wrong I love these cars, but most of the people in the online communitys for them have never built and tuned anything else, and have no idea about general building, and tuning concepts.

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Originally Posted by donniekak

Wiseco 9:1 dish pistons, no quench, cams are 212* @.050", and 111* lsa, stock head, stock intake, td05h20g turbo. More of an enemy than static compression ratio to pump gas, is exhaust gas reversion from a small turbine. I've seen people with gt40 turbines run way over 30 psi on straight pump gas. I have never seen anyone ever measure exhaust manifold pressure on a 6g7x setup. Lots of serious Dsm guys have a pressure sensor off the exhaust manifold, that is logged.
This platform is in the dark ages. If it wasn't for merkel bringing his gm v6 experience via pampena, and aem making a plug in unit, it would still be running afcs spinning bearings constantly...

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What generation of dsm is this setup on? what's your stock lobe separation angle and lift, and your current lift?

I agree with most of what you stated... Especially regarding the reversion window. Yes, the back pressure has never been measured to my knowledge, and there have been no published results.

Only logged ex pressure I have ever seen for a 3S...

http://shop.midwestturboconnection.c...0Stage%202.png
http://www.3sgto.org/f43/group-buy-m...olds-7580.html

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Originally Posted by familyMAN

Only logged ex pressure I have ever seen for a 3S...

http://shop.midwestturboconnection.c...0Stage%202.png
http://www.3sgto.org/f43/group-buy-m...olds-7580.html

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Nice find.

2:1 exhaust/intake pressure ratio is definetly not pump gas friendly. Most big time pump gas cars are right around 1:1.

Interesting that the MTC stage 4 (with bigger comp wheel, turbine wheel and "heavy duty" MHI housing) doesn't have a backpressure benefit over the stage 2 turbo. Not sure if the HD MHI housing flows better (or different AR) or if it just has more material to be able to machine for bigger wheel. Because of jrinks results, I wanted to do a stage 3 (bigger comp wheel but same turbine) on my 3.5, but MTC won't make any bigger than stage 2 anymore. I figured it could make the same (or really close) power, without sacrificing spool, since I interpret the data as housing restriction (or exhaust) and not turbine wheel restriction.

Ultimately, better housing is necessary. Would love to see what a FP or PTE turbine housing would do on Jrinks car.