Pump gas compression

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6pkScamp

6pkScamp

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Need input on what compression ratio your running on pump gas. I've gotten a range of 10:1 to 11.5:1. If you don't mind giving specifics on your combo I would appreciate it. I'm in the process of a small block stroker and would like to get the most out of I can. The only factor in the build that is a done deal is I have a set of Indybrocks with the larger valves and their port job. Thanks for any info.
 
Honestly - you can ignore the static ratio. IMO the first step after you know the stroke, rod, piston, and head package is the camshaft. After that comes the static ratio and that's only worked out after the dynamic compression is figured out.
I have 91 or 93 octane w/10% ethanol in the ground here so my self imposed guidelines: W/aluminum head, closed chamber, built with quench tighter than .035 - I use 8.75:1 as my max dynamic. You could do something with less dynamic if you want a cushion. Less dynamic can be from a larger cam or a reduced static ratio.
IMO you are much better served by using a mild dished piston rather than a flat top and milling the heads to reach the chamber size that makes it work. I ran a few numbers and a static somewhere just over 11:1 will give a dynamic around 8.-8.5:1 depending on the camshaft. I was using an intake closing event of around 70° ABDC.
Also IMO - if you're going with the 4.25 stroke and that top end I'd be looking for something in the 265-270° @ .050, lift in the .600-.650 area for it.
 
moper said:
Honestly - you can ignore the static ratio. IMO the first step after you know the stroke, rod, piston, and head package is the camshaft. After that comes the static ratio and that's only worked out after the dynamic compression is figured out.
I have 91 or 93 octane w/10% ethanol in the ground here so my self imposed guidelines: W/aluminum head, closed chamber, built with quench tighter than .035 - I use 8.75:1 as my max dynamic. You could do something with less dynamic if you want a cushion. Less dynamic can be from a larger cam or a reduced static ratio.
IMO you are much better served by using a mild dished piston rather than a flat top and milling the heads to reach the chamber size that makes it work. I ran a few numbers and a static somewhere just over 11:1 will give a dynamic around 8.-8.5:1 depending on the camshaft. I was using an intake closing event of around 70° ABDC.
Also IMO - if you're going with the 4.25 stroke and that top end I'd be looking for something in the 265-270° @ .050, lift in the .600-.650 area for it.
Click to expand...

Moper I think you might have stunned the crowd. I thought there would be more replies than just yours. Thanks for the info
 
You might try posting something similar in the small block thread... Some of the engine guys don't seem to look anywhere that they can't get business from...lol.
 
You also have to take into account intake design, Temp, elevation, fuel qaulity(and specific gravity or said fuel), VE, ect.

But if your going for ALL OUT power, aim for 10.5 static compression and 8.25 dynamic. KB has a great calculator to help you figure this out. Found here http://www.kb-silvolite.com/calc.php?action=comp2
 
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