Racingjunk Forums - comp. ratio

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have a 350 sbc 3.48 stroke 4.030 bore heads 65cc with kb silverlite pistons 1/8 inch dome. the pistons are down .015 in the hole I used them with .016 steel shim gasket for total of .031 squench on alky don't remmber the comp. ratio. can somebody figure the ratio with 76ccheads and say .040 head gasket I know thats a pretty large squench. but it is under .060 Iwould like to put it on the street. with just with head an cam change becuase everything is pretty new.

Why not use the calculator on KB's site?

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

thanks but I don't know the numbers they ask for just hoping somebody might know

This should help you in the future if you want to figure it on your own?

First find your swept volume:
.7854 X BORE X BORE X STROKE X 16.39 = A
Measured Combustion Chamber Volume (cc's) = B
Measured Gasket Volume (cc's) = C
Figuring Gasket Volume: (.7854 X GSKT. BORE X GSKT. BORE X GSKT.
THICKNESS X 16.39 = C)
Measured deck clearance (cc's) = D
Figuring Deck Clearance: (.7854 X BORE X BORE X AMOUNT IN OR OUT OF THE HOLE X 16.39 = D) Note: Out of the hole is considered a negative (-) volume, meaning it subtracts volume from the cylinder like a dome piston would.
Measured Dome or Dish (cc's) = E
Again a dome piston is considered (-) volume, subtract dome volume from total swept volume.

A + B + C + D + (E +or-) = SWEPT VOLUME, DIVIDE SWEPT VOLUME BY: (B + C + D + (E +or-) = COMPRESSION RATIO

EXAMPLE: 355 SB Chevy .030" over (4.030" Bore X 3.480" Stroke)
64cc Chamber Dart Pro 1's
Fel-Pro 4.100" X .041" Head Gasket
.008" In the Hole
Wiseco Hollow Domes w/11.9cc Effective Dome

A = .7854 X 4.030 X 4.030 X 3.480 X 16.39 = 727.54
B = Combustion Chamber, measured: 64
C = Head Gasket .7854 X 4.100 X 4.100 X .041 X 16.39 = 8.87
D = Deck Clearance .7854 X 4.030 X 4.030 X .008 X 16.39 = 1.67
E = Piston Volume, measured: 11.9

"E" will be considered a subtraction from swept volume because it is a dome piston and it effectively takes volume away from the cylinder. If it was a dish piston it would effectively add volume so volume (E), would be added into swept volume.

(A)727.54 + (B)64 + (C)8.87 + (D)1.67 = 802.08
Subtract Volume (E): [802.08 - (E)11.9 = 790.18]
790.18 = Swept Volume

DIVIDED BY:
(B)64 + (C)8.87 + (D)1.67 = 74.54
Subtract Volume (E): [74.54 - 11.9 = 62.64]
62.64 = Clearance Volume/Unswept

790.18 DIVIDED BY 62.64 = 12.61:1 Static Compression Ratio

Looks complicated at first but if you walk yourself through it a couple of times you will find yourself using this math all the time, trust me you can do it, anyone can do it!

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motecarlo84 : Engine math bad......???.......???

WTH> is this math class?

I'd say somewhere around 10 to 1 give or take a couple small numbers.

You'll be ok if your wondering 'IF' you can run pump gas.

Cp

You can get a gauge at Autozone ........
http://forums.racingjunk.com/viewtopic.php?t=9864

Sorry about bringing the "math" into it. Just a thought. Who would ever think about actually checking their compression ratio and measuring cc volumes during engine assembly. Certainly not, hell just throw a guess out there. I love the "round-about" method, sure makes a whole hell of alot of sense to me. I mean 9.5:1 shouldn't make a difference, I mean I'm only planning to run alcohol..... :shock: DOH! Now lets turn the whole conversation into cylinder pressures and make a whole tutorial out of it. How informative you little stinkers are :idea: .