hp output for GMC 4-71 on a 235

[Kev Nemo]

Any ideas?

[porknbeaner]

You have a lot of unkowns there. Boost, compression, carbs or injection, cam, headwork?
It probably won't help much to let everyone know, but it may get the juices flowing.
Hell just do it and tell us what happened after you're done. http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif

[Kev Nemo]

I'm looking at headers (split-maybe fenton), daily driver boost setup, maybe mild grind cam due to reliability issues...
I've heard porting and polishing won't really help out much, so I'll probably leave the heads alone.
What kind of bottom end build do you think to keep it from kablooeyville?

[porknbeaner]

[ QUOTE ]
What kind of bottom end build do you think to keep it from kablooeyville?

[/ QUOTE ]
That'll take a lot of answer. But on the simple, good solid crank, steel if you got it. Stud mount the mains and a girdle won't hurt a thing. Forged pistons, hypereutic at the very least. Gapless rings, and O-ring the block, I'd probably run about 8:1 compression. But some will say less and others more.
Someone else will get something that I've missed, no doubt.

[squirrel]

The 235 has a steel crank, but it has only 4 mains, so it can flex a bit. Probably the best approach to this engine is to keep boost down around 6 psi, and rpm below 5000. Forged pistons would be the way to go, and compression between 7:1 and 8:1 would be about right. Rods should be stronger than the stock 235 units, maybe 261? or if you could use rods (GMC?) with a bigger wrist pin it would add some insurance. The head should be ported because it has to move more air than even a high rpm engine.

HP estimate? probably 200-250 with 6 psi boost, more if you crank up the boost. This is with a relatively mild cam, to keep the torque band down in the 2000-5000 range.

[Unkl Ian]

A "good" porting job will help,especially on the bowls.



Polishing is mostly Eye Candy.

[48_HEMI]

If you plan on dropping some serious cash, you should start with a good base motor. GMC's had 7 mains made strong blower motors. cubic inches alway help. the later 292 makes a good base motor that will be cheaper to build and more reliable than the old 235
all this info is from someone that is fumbling with a couple of 235's in a losing battle. I'd suggest logging on to the inliners board and get some real experts before putting out the cash http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif