Engine Boost vs compression

[skiwesser11]

I suppose I pretty much “think” I know the answer here but at what static compression ratio would you not even consider trying any boost of any type or is that even a issue these days with tuning?? Anyway ... I built engine myself, for one thing, and have AFR 165 heads with the 58cc chambers “flat” top forged aluminum pistons, they do have slight eyebrows so are the FTop? and block has been decked so I honestly have no idea what my S compression is but I feel comfortable that it’s more than stock... certainly lower than 8-8.5:1 I normally see people talking about having when building for boost... I don’t know that I would do more than a mid sized supercharger if anything... should I shy away from that idea or can it be still be tuned and achieve maximum efficiency??
I’m sure there are other factors to consider as well that I’m not thinking of but just wanted a general idea of what you guys might think??
Thanks

 

[Blown88GT]

Compression is not the same as pressure.
Compression is a ratio of maximum volume to minimum volume of the space above the piston.
Pressure is a force measured in lbs per square inch.
Boost creates a higher density of air & therefore a higher cylinder pressure when compressed. Pressure rises dramatically when the fuel air mixture is burned.

Your statement: ...it’s more than stock... certainly lower than 8-8.5:1 ..is incorrect. Stock is 8-8.5:1
You can't be more than stock & lower than stock at the same time.

Another thing, boost means nothing, flow means everything. High boost with no flow gains no HP, no boost with high flow gains much HP.

 

[skiwesser11]

Compression is not the same as pressure.
Compression is a ratio of maximum volume to minimum volume of the space above the piston.
Pressure is a force measured in lbs per square inch.
Boost creates a higher density of air & therefore a higher cylinder pressure when compressed. Pressure rises dramatically when the fuel air mixture is burned.

Your statement: ...it’s more than stock... certainly lower than 8-8.5:1 ..is incorrect. Stock is 8-8.5:1
You can't be more than stock & lower than stock at the same time.

Another thing, boost means nothing, flow means everything. High boost with no flow gains no HP, no boost with high flow gains much HP.

I meant it is certainly higher than stock, which I thought was generally said to be around 9:1..? I always heard people drop to 8-8.5:1 when building an engine for boosted application but I have no real world experience with any type of boost.. stock 5.0 engines don’t have much “flow” yet 8 lbs of boost increases power quite a bit was my understanding... at any rate I’ve got no problems in the “flow” department but was thinking you did need to have a “low compression” set up to use much?
So are you saying static compression is irrelevant to a supercharged or turbocharged application that is properly supported/tuned?
Thanks for your time

 

[CarMichael Angelo]

Firstly, an AFR 165 head w/a 58 cc chamber is at least 2cc's smaller than stock, and the compression in the engine will go up. A true zero deck is also gonna bump the CR even more over stock. The head itself is fine for a N/A street engine, and certainly is better than stock, but is still far from optimal for a boosted engine.

Cylinder pressure is relative to compression ratio. A higher CR will certainly create more cylinder pressure as a byproduct. If you have a mild cam biased for a N/A engine w/o a lot of overlap, that'll add even more.

Next, Compression calculators are all over the web, all anyone has to do to find out what their CR is, is know the answers to the questions the calculator requires. And plugging in your stated specs, and guessing that your flat top Pistons w/2 valve reliefs add 6cc's to the total volume with a .040 head gasket thickness yields a potential "static" 9.5:1 CR.

For a 9.5:1 boosted engine on pump gas with the right cam and a "capable" tuner....perfect specs.
Static CR stays the same, but cam selection changes how it will spec out in a different build . more valve overlap, less cylinder pressure, less cylinder pressure, less detonation. If the cam in your engine is ground on a 110 CL or less (like 108 which is typical for a N/A engine) you will have to change that out.

With the right cam, and as long as you have good stuff in the engine..... i.e. Good bolts in a prepped rod, (or aftermarket) and forged pistons...you'll be fine.

My engine has almost 11:1 and I have ( and will go back to) somewhere around 10-12 psi. To do this, my timing is almost 20 degrees less in boost than what it would normally be w/o the turbo, and I'm injecting water/meth as soon as the engine sees 5psi. My cam has a 112 Degree CL

 

[skiwesser11]

Well, I was just curious.. my Ed Curtis cam isn’t going anywhere. although it’s like 110* and not much duration. .... I’ll stick with nitrous for now I suppose/ speak with tuner when I have $ ready.

 

[JD1964]

I kept my CC volume as close to stock as I could by ordering thicker head gaskets. I took the amount the deck and heads were shaved and added that to the thickness of stock head gaskets. I then ordered the thicker Cometic gaskets. They can make them what you want. Since my 92 was 9.1 stock, add a tad for GT40 smaller head cc. So I'm somewhere slightly above 9.1. With tuning and enough fuel I'm running 10psi no problem.

If you need to slack off on compression ratio, slap on some thicker head gaskets.

 

[FastDriver]

Stock compression on 87-93 is 8.8:1 (give or take over the years). When you're on pump gas, boost/timing/compression are all a balance to prevent detonation. Trading compression for boost is worth more power than vice-versa.

I have two cars that run higher levels of boost. I've got one car at 9.3:1 and another at 8.5:1 (possibly a couple tenths higher after some recent adjustments). I tune the cars myself, and definitely prefer the 8.5:1 setup. In addition to the ability to make more boost and power on pump gas, it also gives a wider window for a tune. I've also noticed I can cruise at lower RPM without adverse issues.