Thread: safe compression ratio

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Got to put another engine together this winter, probably just a 347 for the street. Wondering what you fellers thought a safe compression ratio would be for an aluminum head windsor stroker on E-85??? We've checked it numerous times, the octane rating stays right around 102.....

Dave, the motor won't be fuel-limited, it'll be cam limited. In other words, the fuel will support most any compression ratio you would want to run, but as you increase static compression ratio, you must also increase duration and extend the intake valve closing point. As you increase duration, you're moving the operating range of the camshaft away from streetability. It all depends on what you're willing to put up with, but most fellows I've talked to have found that a 3,000 stall converter is about all they want to contend with on the street.

Now, this is a Chevy cam, but the motor size is about the same as what you are planning, so should be pretty close just for gettin' you in the ballpark. The operating range is 3,000 to 6,800, lift is 0.491/0.497" (fewer problems with piston/valve clearance), advertised duration 286/294, 0.050" tappet lift duration 236/244, ground on 110 lobe separation angle and the intake closes at 43 degrees ABDC @ 0.050" tappet lift.

I'd build the motor at 12.0:1 (using E85) with a cam such as this and set the squish at 0.035" to 0.045". It should make phenominal cylinder pressure and embarass a few Chevies.

Others may have alternate observations on how to screw it together, but that's pretty close to the way I'd do it.

Oh yeah, sorry Tech.....should of mentioned... This will be backed by a 5 speed, and "streetability" is just a matter of interpretation.... Basic purpose will be a Saturday Night Special, good manners and mpg is not a consideration...... Cam will be a roller, to get the correct specs having to go mechanical roller is really a non-issue, I don't mind adjusting valves. No daily driver criteria applies, got my trusty-rusty Escort for that!!!!

Should have been a bit more specific, I guess....

Well, ok, but you'll still have to slip the clutch to get the motor from idle to the lower operating range of the cam. I guess it all depends on how much slipping you're willin' to put up with. I'll stick with my original combo and add a 4.30 or better rear gear. YOWZA.

Originally Posted by techinspector1

Well, ok, but you'll still have to slip the clutch to get the motor from idle to the lower operating range of the cam. I guess it all depends on how much slipping you're willin' to put up with. I'll stick with my original combo and add a 4.30 or better rear gear. YOWZA.

I'll keep the weight down in the 2400 to 2600 range, and of course with the five speed I can get quite deep on the low and second ratio, spread third a bit, 4th at 1:1, and probably a .72 or so overdrive..... Heck, clutch discs are cheap and I love to shift anyway!!!!!!!!

PS--I've been known to run a set of 6.00's on a spool for really "special occasions"!!!!