Matt Gruber

CAR CRAFT Jan 2002 made 4 dyno runs changing the compression only:

Ratio, cranking comp, HP, TQ

8:1, 135, 440, 486
9:1, 163, 460, 506
10:1, 195, 484, 518
11:1, 232, 500, 531

440 mopar with 230, 238 110 cam

so you can see how little power is gained by running race gas in a street car, when going from 10 to 11:1

MikeC

Thanks for the info 61, I have always wanted to know exactly how much compression ratio affects HP.

For the little jump from 10:1 to 11:1, I'd rather lose that little bit of horsepower in trade for cheaper more easily accesible gas.

Nomad78SA

The article in Car Craft was pretty good. When I built my motor I was shooting for 10:1 to 10:5:1. I did a compression check a couple of nights ago and I have 240,240,240,240,240,240,240,215 compression on my motor, a 388ci. I have a .041 head gasket on it. the block was pretty close to a 0" deck. can't wait to get the body back on and see what she will do.
Neil in Tenn

MikeC

Does that 215 reading bother you? It seems a little low compared to the others.

mountainmotor

I have not read that article but it interesting they chose a Chrysler engine with little or no quench.Just an estimate of 60 degrees of overlap on the cam they used.I would like to see the same test run with a 454 Chevy with small combustion chambers with 75 degrees of over lap.

Nomad78SA

Mike,
To say no to the 215 that it does not bother me would be a lie, but from what I understand as long as each cylinder is within 10% of each other it is fine. 10% of 240 is 24+215=239 so I can live with that for the time being. Without finding my data from the piston distance in the hole, this cylinder reading may of been a little deeper that the others. After I break the motor in good I will check everything out again. I have not ran it much so far for the body is still off.
Neil in Tenn

nunus79

Good info. Thanks.

Neil
240, woohoo, thats stout! Go easy on the advance curve to start. Hope those aluminum heads do their job on controlling pre-detonation.

Matt Gruber

they were fully ported iron heads, with no quench.
they chose the mopar because they were lazy and did not want to change pistons and run up a bigger bill on the dyno time wise. They just stacked copper head gaskets.
But it was a valid comparison, and should provide insight into any V8.

427Hotrod

Man, if that dude really has 240 psi cranking compression, you are really going to be pushing the envelope on pump gas.

I've helped some friends with identical numbers... mild easy street driving is fine, very responsive, but once you start feeding it some throttle... detonation city!!

Did you advance the cam a bunch?

Timing control is going to be critical. You sure all you have is 10.5 compression? You may need some more cam to bleed off some of that. But even with that, eventually you get to an rpm point where that cylinder actually gets filled well and you're right back to detonation.

Good lucK!


Jim

Chuck Harmon

Neil,

I wonder if your 215 is because your lifters may be set a little tight on that cylinder. This would give it longer duration on that cylinder. All, in all, very good numbers. I think mine was around 195 at 12:1 compression with the L88 cam. 4 thousandths change in lifter clearance (only about 1/2 flat on the nut) made for about 10 pounds difference on the readings.

Chuck

nunus79

Neil
Which 268 cam are you using? I read in CompCams website that the 268H is recommended to be used in <9 to 1 engines. According to Vizard, 240 psi is racing engine territory.

Matt Gruber

Nomad
yes, that 240 psi will rise to maybe 250 once it is broke in. I figure u have 10.9-11.3 CR based on 64cc heads and 6-3cc valve reliefs!
Will need COLD AIR INTAKE, blocked heat riser, slow advance, maybe a richer mixture- o2 tuning a MUST!!! http://community.webtv.net/MATTGRU/ AFtesting
if it pings, a way longer cam will help it run on pump gas, either 268 won't do it


[Modified by drives61, 6:50 AM 12/3/2001]

mrvette

Thing is the cam changes accompanying the incresed compression will affect the performance much more than just the compression alone...and the obverse is of course true...
so it's the dynamic compression ratio that's critical, and the street driveability is hard to maintain with a wild cam, and high compression...

apparently there is a huge gray area where calculated compression, cam design, head flow , quench, all that...that gets crazy and it's so complicated it takes a good engine builder to sort it all out....
and that ain't easy.....no mistake about it....too many engines, too many years, too many goof ups witnessed.....you change the stock configuration and you are in for a whole lot of redesign work....all the bugs come out of the woodwork, overheating, pinging, sluggishness....timing,....one extreme to another...lowering compression on an engine out of the '60
s is just NOT that easy...dish pistons, I know....but then we get into cams and well the list just goes....

GENE

Turbo-Jet

is cranking compression directly related to what octane of fuel is required?

RatRacer

Yes, That's why a low compression engine with a short duration cam can ping on the same fuel that runs fine in a higher compression engine with a long duration cam

Techno

Where do you get street gas for 10:1?