Compression ratio?

monkeyman08880 · Oct 27, 2004

I have a 289 v8 that has a stock compression ratio of 8.71 to 1. It was recently rebuilt and it was bored out .030. Does anyone now what the compression ratio would be with this new configuration. I can't do a compression test because we have the engine pulled out, and i need to know in order to buy an appropriate camshaft. Thanks....

jerry S · Oct 27, 2004

Give a man a fish and he will eat for a day

monkeyman08880 said:
I have a 289 v8 that has a stock compression ratio of 8.71 to 1. It was recently rebuilt and it was bored out .030. Does anyone now what the compression ratio would be with this new configuration. I can't do a compression test because we have the engine pulled out, and i need to know in order to buy an appropriate camshaft. Thanks....

teach a man to fish and he will eat for a lifetime.

http://www.turbofast.com.au/TFcomp.html

using this calculator and making some assumptions, I got 9.25:1

monkeyman08880 · Oct 27, 2004

thanks?

Thanks, but since i dont kno any of the stuff it asks for (im fairly new to this), its not all that helpful. Does anyone else have an estimate?

jerry S · Oct 27, 2004

monkeyman08880 said:
Thanks, but since i dont kno any of the stuff it asks for (im fairly new to this), its not all that helpful. Does anyone else have an estimate?

Nobody can tell you without more info. You have to know what gaskets you are using (.030, .040, .80 thickness), what pistons you are using (stock or aftermarket) and whether you have stock heads or not (you could have between 58cc to 70 cc, and that changes things a lot). Verify that, do some googling, plug in the numbers and you will have your answer. It might be close to the 9.25:1 guestimate.

monkeyman08880 · Oct 27, 2004

I have stock pistons, and 60cc performer rpm heads (aluminum). I also have a performer rpm intake and 600 cfm edelbrock carb. Now do u have an estimate? (sorry but i dont know the gasket thickness - probably stock or whatever regularly came on 289's)

charlies · Oct 27, 2004

are you kidding?

i estimate 9.5:1

monkeyman08880 · Oct 27, 2004

Am I kidding?

D.Hearne · Oct 27, 2004

I doubt he'll have any more compression than it had to start with, seeing as he's now got Ed heads on it with larger ( by 6 cc) combustion chambers. Also depends heavily on what pistons he's using ( as the others have already said. ALL this NEEDS to be found out before making ANY assumptions on his comp ratio, a 1 point mistake can make the difference between a screamer of a motor and one that's lazy as a hound dog on a hot summer day. DO your homework BEFORE ordering a cam, otherwise you'll be right back here wondering why your motor is so lazy. ( and blaming us for your troubles) I don't mean to come off sounding mean, but the fact's are the facts. When you ASSume something or get someone else to ASSume something, you're in for trouble.

Great68 · Oct 27, 2004

D.Hearne said:
I doubt he'll have any more compression than it had to start with, seeing as he's now got Ed heads on it with larger ( by 6 cc) combustion chambers. Also depends heavily on what pistons he's using ( as the others have already said. ALL this NEEDS to be found out before making ANY assumptions on his comp ratio, a 1 point mistake can make the difference between a screamer of a motor and one that's lazy as a hound dog on a hot summer day. DO your homework BEFORE ordering a cam, otherwise you'll be right back here wondering why your motor is so lazy. ( and blaming us for your troubles) I don't mean to come off sounding mean, but the fact's are the facts. When you ASSume something or get someone else to ASSume something, you're in for trouble.

Doesn't it depend on what year 289 he has?

For example: A '68 289 has 63cc chambers stock, so if he put edelbrock heads on that he would have gained compression, no?

SadbutTrue · Oct 28, 2004

larger combustion chamber = lower CR

jerry S · Oct 28, 2004

monkeyman08880 said:
I have stock pistons, and 60cc performer rpm heads (aluminum). I also have a performer rpm intake and 600 cfm edelbrock carb. Now do u have an estimate? (sorry but i dont know the gasket thickness - probably stock or whatever regularly came on 289's)

If you don't know the compressed gasket thickness, then buy new gaskets. Thinner gaskets will up your CR and thicker gaskets will drop them.

I cannot find the specs anywhere on your pistons so I had to guess. I found out that the deck height on a 289 was 8.206 inches. I found some pistons for a 289 that get to 8.190, or .016 from the top of the deck. I plugged your bore and stroke and your head vol into the calculator, assumed .030 compressed gasket thickness, and entered .016 deck height in. This gives you a CR of 10.0:1 If the stock heads had 63 cc of vol, and you were bored .030 over back in the day, then your CR was 9.6:1 according to the calculator. But we really don't know what pistons you have in there and that can change everything.

The CRs on the 289's varied widely according to the year. For example, the 1968 mustang with a 289 had 8.7:1 CR, the 67s had either 9.3:1, 9.8:1, or even 10.5:1 !!!! a 1964 mustang with a 289 had a stock CR between 9.0:1 and 10.5:1. As others have noted here, it depends a lot on what year your car was and the engine code was. Do your homework first before ordering that new cam. Talk to the guy who rebuilt it and talk to him about what parts he used and what his observations were.

D.Hearne · Oct 28, 2004

Great68 said:
Doesn't it depend on what year 289 he has?

For example: A '68 289 has 63cc chambers stock, so if he put edelbrock heads on that he would have gained compression, no?

Yes you're correct, but he hasn't a clue as to what he's got , so that throws that out the window.

monkeyman08880 · Oct 28, 2004

Your all right, i get it, i have no clue what ive got. But I do know that it is a C code 68 coupe. Thanks for your help...

jerry S · Oct 28, 2004

monkeyman08880 said:
Your all right, i get it, i have no clue what ive got. But I do know that it is a C code 68 coupe. Thanks for your help...

that makes it easy. I found this info on google.

http://www.geocities.com/~mustangs68/eng.html

Hopefully it is correct. The best advice I can give you is to call Edelbrock and have them select a cam for you based on one of the Performer packages they have since you already have Performer heads, intake, and carb. You will be ok. Good luck and report back with the outcome:

289 "C" Code V-8

spark plugs
BF-42

Gap
0.034(in.)

point dwell
27(deg.)

point gap
0.021(in.)

timingauto/man.tans.
6deg.B

man.idle speed
625 rpm

auto idle speed
550 rpm

oil pressure
35-60 lbs/sq.in

carburetor
2 barrelAutolite 2100

FUEL
Regular

Cam type
HYD.

Bore & Stroke
4.00x2.87

Compression Ratio
8.7 to 1

Compression ratio?

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