Compression vs Stroke

[Impalaguru]

I thought I'd post this here since we have such a wealth of knowledge!! A friend has a 383 stroker 350. He bought the engine already built as it was a good deal and it's the correct engine for his 70 corvette. All parts seem to be correct except for the heads. They are 69/70 low compression 327/307 heads. Casting number 3927188. According to Chevy by the numbers they have 1.72/1.50 valves and a 74.66 cc combustion chamber . Now, I have no idea if they have been reworked as the engine is assembled and he bought the engine from the ex-wife who could care less and wanted it out of her garage. The original heads would have had 64 cc combustion chambers and 2.02/1.60 valves.

The question is: Would an increase in stroke have any bearing on the compression ratio. I don't think it's a very exotic build. More than likely has standard flat-top pistons, block not decked, composite head gaskets, 5.7" rods (guess). I used some of the on-line compression ratio calculators and it seemed like increasing the stroke increased the compression ratio. Thinking maybe they used the larger cc heads to lower the comp. ratio to a livable level.

I really don't know. I'm trying to help him decide if he should find the original heads or go with the ones that are on there. I'm just going from experience as I had a 69 Olds 350 with mid-70s smog heads and it was running about 8.2:1.

Any guidance would be much appreciated!!

Ross

 

[61BISCAYNE]

In a short answer....YES....increasing stroke will increase the compression!
In fact, if the engine does indeed have flat-top pistons, a larger 76 cc combustion chamber may be necessary to limit compression. Smaller 64 cc iron heads could very likely cause detonation because of too much compression, especially if the engine has a mild cam.

 

[Kdurgin]

Compression ratio is just the amount of air in the cylinder and head chamber when the piston is at the bottom of stroke compared to when it is at the top.i.e. 10 times more at bottom of stroke than top, yields 10:1 So if you sweep a larger distance ( stroke ) so your beginning volume is now larger and the final volume is the same, so ratio goes up. Bore increase does the same thing.

 

[Impalaguru]

Compression ratio is just the amount of air in the cylinder and head chamber when the piston is at the bottom of stroke compared to when it is at the top.i.e. 10 times more at bottom of stroke than top, yields 10:1 So if you sweep a larger distance ( stroke ) so your beginning volume is now larger and the final volume is the same, so ratio goes up. Bore increase does the same thing.

Thanks, Guys!! When you think about the volume of air in the cylinder increasing it makes total sense!! Thank you for you setting me straight!!
Ross

 

[bignbad60]

Interesting....So if i'm running 11.25 : 1 pistopns in a stroker with a .40 overbore, my compression is greater than 11.25:1 ? Is there a simple math calculation to figure out the increase ?

Tim

 

[Dick MacKenzie]

To figure Compression ratio you need a little more information then bore, stroke and head volume, but as everyone has said, all other things being equal an increase in stroke should up the CR. Here's a link that may be some help and is fun to play with the numbers.

http://www.csgnetwork.com/compcalc.html

 

[models916]

Head volume example

Taken from Jegs site for piston in a 383. 11.88 Comp. Ratio with 64cc Head 10.22 Comp. Ratio with 76cc Head

 

[Ronnie Russell]

Tim, As far as I know, when you order pistons for a stroker application, the piston company figures the right compression height and dome volume for the piston to arrive at the desired compression ratio. And even then, it is not exact. Like Dick said, there are several factors.