Please check my data for calculating compression ratio.
Thread Starter
Burning Brakes
Joined: Apr 1999
Posts: 966
Likes: 0
From: Mableton, GA, USA
Please check my data for calculating compression ratio.
Please help me in double checking my compression ratio calculation. The engine is an 383 LT4 that is getting a new 383 rotating assembly.
Crank: 3.75in stroke
Bore: 4.040
Rods: 5.7in
Heads: 52cc (I'm going with worst case since the heads were milled previously. I’ll get an exact number once the heads come back from being cc’ed. But they wont be more than 54ccs)
Deck clearance: .0001 (Block was zero decked with the prior 383 rotating assembly.)
Gasket Thickness .049in
Gasket Bore: 4.040 (I forget, is this always the same as the bore diameter or is it oversized by .010?)
Pistons with a 18cc dish.
So using the calculator at the following url: http://www.kb-silvolite.com/calc.php?action=comp
I get: 10.812:1 static compression ratio.
Anything blatantly wrong with the above assumptions? Is the compression ratio to high? This engine will be running an LT4 hotcam, will that bring down the effective compression ratio some? That same link has a option for figuring this out, but I don't have the cam specs needed.
Thanks,
Crank: 3.75in stroke
Bore: 4.040
Rods: 5.7in
Heads: 52cc (I'm going with worst case since the heads were milled previously. I’ll get an exact number once the heads come back from being cc’ed. But they wont be more than 54ccs)
Deck clearance: .0001 (Block was zero decked with the prior 383 rotating assembly.)
Gasket Thickness .049in
Gasket Bore: 4.040 (I forget, is this always the same as the bore diameter or is it oversized by .010?)
Pistons with a 18cc dish.
So using the calculator at the following url: http://www.kb-silvolite.com/calc.php?action=comp
I get: 10.812:1 static compression ratio.
Anything blatantly wrong with the above assumptions? Is the compression ratio to high? This engine will be running an LT4 hotcam, will that bring down the effective compression ratio some? That same link has a option for figuring this out, but I don't have the cam specs needed.
Thanks,
Last edited by 96CELT4; Mar 17, 2005 at 10:12 PM.
Melting Slicks
Joined: Oct 1999
Posts: 2,581
Likes: 14
From: Tolar Texas
For what it's worth, using the compression calculator in Desktop Dyno 2000 I came up with the following results.
52cc heads > 10.73:1
58cc heads > 10.06:1
64cc heads > 9.47:1
72cc heads > 8.8:1
Hopefully others will chime in for comparison of results.
Mike
52cc heads > 10.73:1
58cc heads > 10.06:1
64cc heads > 9.47:1
72cc heads > 8.8:1
Hopefully others will chime in for comparison of results.
Mike
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
From Smokemup.com calculator.
You really dont' want that thick of a head gasket!
Calculate Compression Ratio
Bore
(Inches)
Stroke
(Inches)
Connecting Rod Length
(Inches)
Cylinder Head Volume
(cc)
Deck Height
(Inches)
Head Gasket Bore
(Inches)
Head Gasket Thickness
(Inches)
Piston to Cylinder Wall Clearance
(Inches)
Top Ring Land Height
(Inches)
Piston Dome Volume
(cc)
Negative number is dished piston. Positive number is a domed piston.
You Entered:
Bore 4.04 (inches)
Stroke 3.75 (inches)
Rod Length 5.7 (inches)
Cyl. Head Vol. 52 (cc)
Deck Height 0.0 (inches)
Head Gasket Bore 4.060 (inches)
Head Gasket Thickness 0.049 (inches)
Piston to Cyl. Wall Clearance 0.003 (inches)
Top Ring Land Height 0.25 (inches)
Piston Dome Vol. -18 (cc)
Compression Ratio:
10.792 : 1
Bore / Stroke Ratio:
1.077 : 1
Rod / Stroke Ratio:
1.52 : 1
Total volume:
868.345 cc's
Compressed volume:
80.46 cc's
You really dont' want that thick of a head gasket!
Calculate Compression Ratio
Bore
(Inches)
Stroke
(Inches)
Connecting Rod Length
(Inches)
Cylinder Head Volume
(cc)
Deck Height
(Inches)
Head Gasket Bore
(Inches)
Head Gasket Thickness
(Inches)
Piston to Cylinder Wall Clearance
(Inches)
Top Ring Land Height
(Inches)
Piston Dome Volume
(cc)
Negative number is dished piston. Positive number is a domed piston.
You Entered:
Bore 4.04 (inches)
Stroke 3.75 (inches)
Rod Length 5.7 (inches)
Cyl. Head Vol. 52 (cc)
Deck Height 0.0 (inches)
Head Gasket Bore 4.060 (inches)
Head Gasket Thickness 0.049 (inches)
Piston to Cyl. Wall Clearance 0.003 (inches)
Top Ring Land Height 0.25 (inches)
Piston Dome Vol. -18 (cc)
Compression Ratio:
10.792 : 1
Bore / Stroke Ratio:
1.077 : 1
Rod / Stroke Ratio:
1.52 : 1
Total volume:
868.345 cc's
Compressed volume:
80.46 cc's
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
All the answers are within spitting distance of each other, including yours. The common head gaskets have a bore of 4.166 or 4.190, and a compressed thickness of .039" to .041". These minor variations in gasket dimensions won't make any meaningful difference in the compression ratio. I assume you decked the block to attain a good quench. As mentioned, a .049" gasket thickness starts to erode that advantage. Stick with the .039"-.041" compressed thickness gaskets and you'll be better off. A compression ratio of ~10.8:1 isn't too high with aluminum heads and that cam. Remember, too, it will only be less (slightly), depending on the actual head volume.
RACE ON!!!
Le Mans Master
Joined: Mar 2002
Posts: 5,889
Likes: 2
From: Castro Valley Calif.
St. Jude Donor '03 thru '26
An LT motor can tolerate a higher C/R, but I think you're running on the edge with a quench of 50 thou. I think I'd look at a gasket around .039 to .045 max, then adjust your valve relief/dome volume and deck height to achieve your final desired static C/R. Detonation also has a lot to do with the cam you're running as well......that's called dynamic C/R
