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. 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

 

Tedd,

If you 4.030" dished pistons are the common conffiguration, they should have about a 14cc void, including what's left of the valve depressions and the outer taper.

If your specs are as listed, the static CR comes out to about 9.07:1 -

BORE SIZE - INCHES 4.030
STROKE LENGTH - INCHES 3.48
DOME VOLUME (CC) 0
DISH VOLUME (CC) 14.0
GASKET COMPRESSED 0.041
DECK HEIGHT / DEPTH -0.015
CHAMBER VOLUME (CC) 64
TOP RING DEPTH - 0.250
NUMBER OF CYLINDERS 8

Bore Area 12.756
Displacement 355.1
Free Area Volume (CC) 90.1
Static Compression Ratio 9.07

I would GUESS that it would be safe to advance the starting ignition timing several degrees at least, but make sure the detonation sensor is working and the fuel is up to the task. Every engine is different, and the ODB-II PCM is a lot more sophisticated and tolerant than the older ThirdGen OBD-I series ECMs.