Real Compression Ratio Daily driver

[Orr89rocz]

For a DD it's all good pushing compression until you get into an area where the premium fuel maybe doesn't have a decent turnover or is adulterated.

Or until a buddy borrows your vehicle and tanks it up with low-test swill...

Highly unlikely......but maybe wifey

This is a good subject as I have a 383 TPI that was tuned on 90 octane, 9.5:1/alum heads it's in a truck which I occasionally use for towing. It will be retuned for 87 shortly as last year was travelling in northern Canada and filled up with poor fuel. When driving sometimes you can nurse things along but this was really bad shit- lesson, carry a jerry can of 94.

If you have the tpi ecm and knock sensor you can set it up to pull additional timing when it detects issues on low oct. factory settings do pull alot of timing

[jeff swisher]

I have ran what most consider too much compression for pump gas and it is not a light car situation I pull loaded trailers and a 20 foot pontoon.

Someone stated maybe the math was wrong maybe the ring seal will not last I have not had any ring seal issues but My rings are thick the old cast flat tops in my 350" stuff and 383 stuff with E251K rings 91 octane fuel is used.

The way i do it is small chambers under 62cc and my current 350 is 57 cc and I have ran the 268H in it .013 deck clearance 5cc reliefs in the flat tops and I have ran as tight as .015" head gasket with it. with 268H and the 270H and now have the 280H cam.
well over 200,000 miles that I put on it myself.

35 total timing with my ported 601 heads and 41 total with 186 heads milled to 54cc and the same .028 piston to head clearances.

Intake runners are finished very rough . All sharp edges are blended and smoothed in the combustion space.
water temps kept under 195.
If pulling the 5000+ lb loads I do not drive it like a race car. It is a daily driver that is respectable. I pulled many loads with 3.00 gears and 3.70 gears.

Timing stays the same.
Never have issues..I do not run any vacuum advance.

Another is a 396" with 11: 1 compression and 280H cam in that also.
My 350 cranking pressure with 268H was 225 PSI and the 396" was 210.. that is also dependent on starter cranking speed.. all are ran on 7-11 91 octane fuel. for over a decade of daily driving.

[RevTheory]

Regarding compression ratio and iron heads, I've read a couple articles claiming that AMC's chamber was quite resistant to detonation and they regularly build (apparently) up to 11.5:1 engines on pump fuel. On the other hand, I've read at least that many claiming that "street friendly" compression should be kept around 9.5:1.

That's quite a split and am hoping some of you AMC guys can shed some light on it.


Shawn

[Ken_Parkman]

I'm an AMC guy and would not do that. IMO the AMC chamber is quite good - even excellent - for 1966. I'm not a believer in pushing compression for street use.

Have chased detonation at less than 10 to 1, iron head, good quench. Current pump gas street car is 10.2 compression, 232 degree HR cam, cold air induction and so far has been fine on 91 octane. Engine is pretty decent, 1.31 ft-lb/in, 567 hp, 11.0 @ 126 in the Rambler, street trim.

[RevTheory]

Thanks, Ken.

[Belgian1979]

Q : I always have the impression that when my car sits after a winter and right before I fill here up again, that she kind of runs 'rougher'. Does that indicate detonation ? I always wondered.

[Biteme]

10.5 is fine for pump gas with Ali heads.
I run 10.8 and have zero problems.
Some guys run quite a bit more than that.
I think it all comes down to valve timing.

[RevTheory]

Rather than static, are any of you using dynamic compression ratios as a guideline for iron heads on pump gas? If you do, where's your limit? Have you seen a dynamic compression ratio that seems to crank around 190 to 200 psi? I'm not too sure about using calculators to predict cranking pressure because you can always seem to find one that agrees with you.

[DCal]

Rather than static, are any of you using dynamic compression ratios as a guideline for iron heads on pump gas? If you do, where's your limit? Have you seen a dynamic compression ratio that seems to crank around 190 to 200 psi? I'm not too sure about using calculators to predict cranking pressure because you can always seem to find one that agrees with you.

My cranking pressure is 185, timing is 36. Aluminum heads. CR is 10.5 with no problems at all. If I have adequate CR I see no point in pushing it into a narrower tuning range.