Someone tell me my damn compression ratio

**ThackerSS** · 06-01-2006, 09:13 PM

I've posted this on other "tech" websites, and no one fucking responded. Any of you tech guys let me know what my compression ratio is on this new short block?

347 CI/ 2000 LS1 Block

Diamond Flat Top Pistons, with 2cc Valve Reliefs
Lunati Pro Mod Rods
Stock 241 Heads
Final Square Decked / Deck Height = 9.230, Stock LS1 Deck Height is 9.24, so it was cut .010
Stock Crank

Thanks

**91Z28** · 06-01-2006, 09:31 PM

http://www.rbracing-rsr.com/compstaticcalc.html

I think this assumes that your gasket bore matches your cylinder bore though.

**91Z28** · 06-01-2006, 09:32 PM

Here try this one instead:

http://kb-silvolite.com/calc.php?action=comp

**ThackerSS** · 06-01-2006, 09:33 PM

Originally Posted by 91Z28

Here try this one instead:

http://kb-silvolite.com/calc.php?action=comp

That is the thing.. I don't know the deck height of the piston, I know its slightly above the top of the cylinder, but I dont know how much. I just have the number of 9.23, and it was originaly 9.24

**91Z28** · 06-01-2006, 09:36 PM

Originally Posted by ThackerSS

That is the thing.. I don't know the deck height of the piston, I know its slightly above the top of the cylinder, but I dont know how much. I just have the number of 9.23, and it was originaly 9.24

I feel your pain then, I've been unable to compute my CR either. My engine is using a stock LT1 piston, yet no one can accurately tell me what the valve reliefs volume are in cc. I guess unless you are an engine builder this shit is hard to determine.

**Hi-Po** · 06-01-2006, 10:32 PM

A burrette, vasoline, and a plastic square. Also a disassmbled engine. Swept volume and combustion chamber volume can be taken very accurately. If you really wanna know all this i will type it up tomorrow. But i understand this is only a way to know if you have a disassmled engine.

**ThackerSS** · 06-01-2006, 10:46 PM

Originally Posted by Hi-Po

A burrette, vasoline, and a plastic square. Also a disassmbled engine. Swept volume and combustion chamber volume can be taken very accurately. If you really wanna know all this i will type it up tomorrow. But i understand this is only a way to know if you have a disassmled engine.

Is anyone able to give me a rough estimate with the information i provided?

DisCrete · 06-01-2006, 11:59 PM

Originally Posted by ThackerSS

Is anyone able to give me a rough estimate with the information i provided?

Is it modded or stock 347?

I know the stock 347's are 9.6:1

specially recent years for the 347's I want to say 2000 and later or 2002 and later whichever year they implimented the aluminum block in the corvettes is when the ratio changed to 9.6:1

as all the camaros and GM cars ran the same compression since they were all 347 aluminum blocks with identical ratios... "But don't quote me on it" that's to the best of my knowledge... and the only reason I make such an assumption is because I follow corvette engines as often as I have time to learn.

DisCrete · 06-02-2006, 12:00 AM

my 350 LT1 is 9.8:1

but it's pretty much all stock as is... so 91Z28 for example although we run the exact same engine... I believe his is modded compared to mine... so his ratio might change slightly

but not by much

91Z28 if I had to guess I'd say 9.6 or 9.8 for you as well.

**91Z28** · 06-02-2006, 12:32 AM

Originally Posted by DisCrete

my 350 LT1 is 9.8:1

but it's pretty much all stock as is... so 91Z28 for example although we run the exact same engine... I believe his is modded compared to mine... so his ratio might change slightly

but not by much

91Z28 if I had to guess I'd say 9.6 or 9.8 for you as well.

Mine is actually 10:1 stock. I'm switching from a .051 to a .028 gasket though and wanted to know what that would bump me too. I've come to the same conclusion that Hi-Po just stated: only a disassembled engine will tell the whole story. I don't have that luxury though so I'll just use that calculator to guess, and that gives me 10.56:1. If the heads aren't on your car yet Thacker I'd do like Hi-Po says and measure everything. Hopefully you are around 11:1 since you are going to be spraying.

DisCrete · 06-02-2006, 12:36 AM

91Z I'm confuzzled about compressions

if you were charging/spraying

which would you want from compression

8:1 or 11:1?

Isn't 12:1 and up when you risk throwing a piston through your hood?

DisCrete · 06-02-2006, 12:37 AM

Originally Posted by 91Z28

Mine is actually 10:1 stock. I'm switching from a .051 to a .028 gasket though and wanted to know what that would bump me too. I've come to the same conclusion that Hi-Po just stated: only a disassembled engine will tell the whole story. I don't have that luxury though so I'll just use that calculator to guess, and that gives me 10.56:1. If the heads aren't on your car yet Thacker I'd do like Hi-Po says and measure everything. Hopefully you are around 11:1 since you are going to be spraying.

maybe mine is 10:1 then also?

I know we have the exact same engine stock.

I dunno

**91Z28** · 06-02-2006, 01:06 AM

Originally Posted by DisCrete

91Z I'm confuzzled about compressions

if you were charging/spraying

which would you want from compression

8:1 or 11:1?

Isn't 12:1 and up when you risk throwing a piston through your hood?

Nitrous likes compression.

**Hi-Po** · 06-02-2006, 09:26 PM

Originally Posted by ThackerSS

Is anyone able to give me a rough estimate with the information i provided?

Well Theoretical CR is determined by 2 things. Dispalcement and combustion chamber volume. In other words, its the volume above piston at BDC compared to the volume Above at TDC. CR= (SV+CV)/CV. -- SV= Swept volume and CV = combustion chamber volume or clearance volume. So no, with the info you provided I cant give you anything. You really need Combustion chamber volume. Displacment numbers are fine and dandy and easy to get obviuosly. But the problem is getting clearnace volume. Since all my experience is ONLY with a completely dissablmed engine i dont really know any other way. Actually there isnt any other way if you want real numbers. I dont know if anything i have said helped at all, but it was worth a shot. Good luck.

**91Z28** · 06-02-2006, 11:08 PM

Originally Posted by Hi-Po

Well Theoretical CR is determined by 2 things. Dispalcement and combustion chamber volume. In other words, its the volume above piston at BDC compared to the volume Above at TDC. CR= (SV+CV)/CV. -- SV= Swept volume and CV = combustion chamber volume or clearance volume. So no, with the info you provided I cant give you anything. You really need Combustion chamber volume. Displacment numbers are fine and dandy and easy to get obviuosly. But the problem is getting clearnace volume. Since all my experience is ONLY with a completely dissablmed engine i dont really know any other way. Actually there isnt any other way if you want real numbers. I dont know if anything i have said helped at all, but it was worth a shot. Good luck.

Combustion Chamber volume should be easy to find for a stock LS1 241 casting.

Rob 94Hawk 319 · 06-02-2006, 11:14 PM

LT1's varied from 10.2:1 to 10.8:1 compression ratio depending on year.

Thacker, I believe you should be in the area of 10.1:1 static compression ratio depending on which head gaskets you go with. The heads if they are dead stock combustion chambers, should be in the area of 68cc's.

91Z28 is right, Nitrous does love compression to a point.

**Hi-Po** · 06-02-2006, 11:43 PM

Originally Posted by 91Z28

Combustion Chamber volume should be easy to find for a stock LS1 241 casting.

well i kinda mis spoke i beleive. Lets take a 454 thats been bored .030-in oversize. Bore and stroke are now 4.280 in x 4.000. Displacment is now 460 CI. Swept volume is 1st, using this formula.

SV=0.7854 D2 = 0.7854 x (4.280 in)2 x (4.000

SV = 57.55 in 3

To convert CC to CC's (because its easier and more accurate to work in bigger numbers) by 16.39

Clearance volume is more difficult to determine because its a combination of calulations and actual measurements. Need to cc chambers(with burrette and Green rubbing alcohol) Again to be exact you will need to have heads off but slightly assembled. If anyone is going to ever do this i will explain how i did it in my classes. You will need to do this to each chamber. Then equalize each chamber to the largest chamber. done by removing material with cutter or grinder.

The next thing is to do, is find (Vc) here is the measuring and calulating part. damn im too tired to type the rest, besides im kinda goin too in depth. good night.