Can anyone tell me how to determine my compression ration on my 350 TPI. I did a compression test today on it and averaged about 175psi to 180psi on each cylinder. Can anyone convert this or knows the formual to convert it? I need to get a custom chip for my car and need to know the ratio. Thanks, Jason

 

Jas,

To determine the actual static compression ratio, you'll need to know the cylinder volume, piston relief/dome volume, deck height, head gasket thickness, combution chamber volume, and top compression ring location.

If you engine is all stock, the numbers should be easy to acquire. Get the head casting numbers, block casting number, and calculate any changes you may have done in terms of deck height, gasket thickness, and chamber volums (valve sizes, milling, unshrouding, etc.).

Static ratio is simply the cylinder maximum volume compared to compressed volume. Since we should know the stroke length, the rest is in determining the free areas of the chamber.

------------------
Later,
Vader
------------------
"Make Me Bad"
Adobe Acrobat Reader 4.0

 

Compression ratio has nothing to do with compression test. A compression test simply tells you the amount of cylinder pressure that can be built up in the cylinder. Compression ratio is a comparisson figure of volume from the bottom of the cylinder to the top.

The most accurate way to determine compression ratio is to know a few variables.

Cylinder volume (area cover by the piston): Bore x bore x stroke x .7854
Deck Height (Area from top of piston to top of block at TDC: Bore x bore x .7854 x height
Head gasket thickness: Bore x bore x .7854 x gasket compressed thickness
Head volume: Volume cc x .061
Dome height of piston: Volume cc x .061

Once you have all these numbers you can get a very accurate compression ratio. The formula is:

Cylinder volume + deck height + head gasket + head size + dome volume / deck height + head gasket + head size + dome volume.

Without knowing the exact sizes i'll do some guessing.

Cylinder volume for 350 = 43.731
Deck height of .035 = .440
Head gasket of .041 = .515
Head size of 63cc = 3.843
Dome volume flat top pistons with valve reliefs = -.366

Putting all these numbers into the formula results in 9.468:1 compression ratio (9.5:1)

Your results may vary. Factory steel head gaskets can be as thin as .028 and Felpro composite gaskets can be .041. Head volume size can vary depending on head castings. Factory engine are never at zero deck height. Factory pistons are usually dished causing even lower compression ratios.

Vader typed all that while I was typing this response . I've never considered top compression ring location but yes it would be part of the formula since there is a very small area around the piston on top of the ring.


------------------
Stephen's racing page

87 IROC-Z Pro ET Bracket Race Car and knocking on the SuperPro ET class
383 stroker (carbed) with double hump cast iron heads and pump gas
LS6 Big Block buildup now in progress

Best results before the 383 blew up
Best ET on a time slip: 11.857
Best corrected ET: 11.163
Best MPH on a time slip: 117.87
Best corrected MPH: 126.10
Best 60 foot: 1.662

Racing at 3500 feet elevation but most race days it's over 5000 feet density altitude!
Member of the Calgary Drag Racing Association

87 IROC bracket car
91 454SS daily driver
95 Homebuilt Harley

[This message has been edited by Stephen 87 IROC (edited October 29, 2000).]

 

The casting number on the block is 14093638 and the heads are 14101083. The heads should be L98 cast iron and are 64cc the block is a L05 out of a 91 Cadillac Brougham The block was not decked and it was not bored out. I reused the crank which was the same as the LT1 engines and it came with Flat Top Pistons. The head gaskets are .040 thickness and 8.7cc according to felpro. Thats all I could come up with so far. I'm not sure what the piston volume is on the pistions. I do know thay have four relief valves. Does this help out any?

 

Valve releifs are about 2 cc each. Figure you have 8 cc and you should error on the conservative side.

Your piston will be .020-.025" down the hole if the deck height is stock.

I'm too lazy to use a calculator anymore. Try the on-line calculator at BRC piston's sight.
www.brcpistons.com

Mike

 

A recalculation with 64cc heads, .040 head gasket, -8cc valve reliefs and .025 deck height comes to 9.396:1 compression ratio (9.4:1)

 

OK I'm confused? I went to the www.brcpistons.com just to try it out and It gave me a 10.5:1 compression ratio. I inputed 4" bore, 3.48" stroke, 64cc heads, .025 deck height, .040 Head Gasket, and -8cc for piston volume. Did I input the correct info? Thats much higher than the 9.4:1 that was figured on here? Any more help will be appreciated. Thanks,

 

Try reversing the sign for the Valve reliefs. The "-8" may signify a "dome" of negative 8 cc, instead of the "+ 8cc" for the valve reliefs.

 

I did put a +8cc and it comes up to 13.1:1 ratio

 

Positive cc is a dome, negative is a dish or recession such as valve reliefs.

I'll do some research but I'm sure my formula calculations are correct. I worked them out years ago to determine compression ratio.

 

I did some searching and found web sites with the exact same formula as I use so I'll stick with my calculations.

 

Stephen 87 IROC, Thanks for your help. I'm going to go with your figure and get my chip pretty soon. I found a article in CHP that has a formula and it came up with what you have given. Thanks for everones help, Jason

 

Stephen,

You inspired me to break out the calculator and go threw a couple formulas of my own, plus yours. I’m glad you did too.

I’m getting almost a whole point lower C/R by doing the math than what the BRC calculator comes up with. To complicate the situation even worse, the Desktop Dyno 2000 compression calculator is off as well. It comes up with the same C/R that the BRC calculator does. I haven’t figured out why, but the DD 2000 computes piston at TDC volume lower than what I get doing the math. This is interesting, and I’m sure there’s a reason for the discrepancy. I just have to figure it out.

Thanks, Mike

 

Stephen, I am not sure about "adding" ccs when computing the compression ratio for a piston dome and "subtracting" ccs when you have a dish.

A dished piston increases the volume of a combustion chamber, whereas a dome decreases the volume of the combustion chamber. This is what I had to do for my formula which I have done on an Excel spreadsheet. It appears similar to yours except I am using actual Pi. I find rounding to 2 decimal places can cause a fair degree of inaccuracy.

Also, though you do mention deck height, others may not know what that means or how to calculate it. They acutally use "Block Deck Height - 9.00") to signify the difference between the height of the block (std 9.025 ) vs the height the piston travels up the cylinder (9.00").

But, using all of the specs Stephen gave, I too come up with 9.47:1, using a slightly different calculation using Pi as the factor and adding the 6cc for the valve reliefs in the flat-top pistons in Stephen's example.

 

Okay, I can't beleive I missed it after using these calculators all this time. I forgot to put a - sign for negitive piston to deck height in front of the deck height value.

The BRC calculator and the DD 2000 both come up with 9.394/1 when I "remember" to use the - sign.

 

When I mentioned deck height I'm refering to how far down the piston is in the cylinder when at top dead center. When someone mentions zero deck, they mean the top of the piston is right at the top of the cylinder. If there wasn't a head gasket it would touch the head. A typical factory engine has the piston down anywhere from .020 to .040 from the top of the cylinder. This allows some tolerance for piston and rod expansion and allows more piston to valve clearence.

What the formula is after is total area volume converted to cubic inches in the combustion chamber.

A 64cc head has about 3.904 cid's. Flat top pistons with the 6 cc notches becomes -6cc and -0.366 cid's volume. When you subtract the head volume from the dome volume you get 4.270 cid's (more than the original 64cc heads of 3.904 because of the valve reliefs).

Lets go the other way. Same heads but pop up pistons. Piston dome volume is 11.4 cc's and a CID of .695. Subtract the 3.904 head cid from the dome's .695 and the combustion chamber is now 3.209 cid. (getting pretty small) This would change the flat top pistons compression ratio of 9.39 now to 11.35 with the popup domed piston.

Oops. Just looked back at the formula. It's correct, sorta, but I can see the confusion.

Cylinder volume + deck height + head gasket + head size - dome volume / deck height + head gasket + head size - dome volume.

I had + dome volume instead of - dome volume. Sorry about that.

I also see the confusion with the brcpistons calculations. I was wondering myself how the came up with that compression ratio. They allow for positive deck height where the piston comes above the top of the deck. When most pistons are below it, they want a negative number to show it. If you piston is .025 down in the cylinder you need to put -0.025 into their formula as a deck height. From what you've said I have to assume dyno2000 uses the same logic. That's good to know.

[This message has been edited by Stephen 87 IROC (edited November 02, 2000).]