1995 4wd auto E/OD GT4 Suburban

I have replaced the original 5.7 motor with a 355 c.i. split rear oil seal short block, a Crane cam [part # 113931] with intake lift at valve .440" and advertised duration of 266° and exhaust lift at valve .454" and advertised duration of 272°, a pair of 23° twisted wedge Trick Flow heads and Flow Tech shorty headers with O2 sensor coupled to 2 1/2" dual exhaust with dual cats and mufflers and no crossover tube. The motor is topped off with some 8mm ignition wires. The rest of the motor and car is bone stock.


This motor was originally purchased from Summit Racing as a "Summit Racing Tow Motor". The motor when installed had a bad knock from #7 cylinder. Summit took back the bottom end and I aquired a rebuilt short block with
(argh!) dish pistons. So now it seems as if I have a "low compression" tow motor

I have just run a compression test on all cylinders and the good news is there is less than 1 psi difference in all eight cylinders, the bad news is all cylinders read 120 psi. That seems a little low to me. Can a compression ratio be extrapolated from this figure? The heads are Trick Flow 23° twisted wedge aluminium with 64cc combustion cylinders with regular fel-pro head gaskets.

Also does this mean I can time the motor a little more aggresivley or not be as concerned about octane level ? Stock setting is 0° but that didn't work, right now i've bumped it to 8° BTDC, does low compression mean I could go more on the timing? 10°, 12° or maybe even 14°?

I'm running factory prom right now but that should change soon to a custom burned prom.

Thanks for any help you can give me.

Tedd Edmondson

 

i've heardf you can figure compression ratio from the psi, but it's sort of a generalized sort of thing and not sure i'd trust it. the 120 psi doesn't mean that much, the fact that all holes are the same does. you can run more timing and shouldn't have any problems.

 

www.smokemup.com has some nice automotive calculators on their site, Hp from quarter mile times, gear ratios, and even compression like you guys are talking about, dont know if this will help you out any or not.

 

I had a few replies to the compression ratio calculation question.

One used the volume at TDC and BDC to calculate his answer 9:1.

Another used my compression gauges reading (120psi) and divived by 1 atmoshere (14.7psi) to get 8.1:1.

The 120 reading seems quite low to me, I seem to remember readings around 150 or higher readings on past motors, So 8.1 seems feasible.

Does that mean I can worry less about gasoline grade and/or advance th timing a bit more? What do you think I could push base timing too?


Tedd