lowering compression ratio

[doubleogee]

hi there guys......welp, its summer and i got my project truck back out...this is a 90 chevy half ton, 350...when i was in highschool, like everyother guy i wanted more HP. I decided to build a lil motor........not for flat out racing or anything, its a lot like a 350 HO...lil hotter cam than stock, vortec heads, gmpp intake, msd ignition....well things didnt go so smooth and i got all into burning my own chips and self tuning....MY MAIN PROBLEM.....
i believe the stock 350 was about 8.5:1 compression ratio....didnt even think of this when i ordered the rebuild kit and so now its more like 9:1 maybe more...im getting a lot of knock, anybody have experience with stacking head gaskets...??other options??not looking to buy all new pistons...however.......im taking the heads off right now, all that knock im curious to see how the pistons look any advice would be much appreciated. Thanks guys

[chrisj6]

I dunno bout lowering it, why not just use a higher octane gas, it will be cheaper than buying all new parts.

Chris

[RedHeartbeat]

9 to 1 is low esp if you are using aluminum heads. those are good to 10 to 1 easily. better fuel, correct fuel mixture and timing curve are very important to prevent detonation.

[doubleogee]

the heads arent aluminum....they are the iron vortec heads....octane doesnt seem to make much difference....i know 9:1 is low but these things came 8.5:1 or less stock.....i have to pull so much timing out to get it to go away and that doesnt even completely solve the knock issue. my timing curve is very mild i assure you....its stock gm timing and they are very conservative, most people i have talked to have been able to add a few degrees here and there...i just wanna get it back to 8.5 so i can rule out that problem...

[doubleogee]

welp, disregard this whole thread, i was misinformed.......the stock compression ratio is 9.4:1 according to a chilton manual.......wonder where all that knock was comin from???pulled the motor apart yesterday.....everything inside looked good...been runnin a lil lean, all plugs were white as can be....maybe it was a lack of fuel issue HMMMMMMMM????somethin to do on my days off

[GMCwantsLS1]

A lean condition can cause knock. Maybe base timeing was off?

[NITROUS]

Yep, you're correct about the C/R. I just put a 97 vortec 350 in a 91 SWB truck and haven't had any problems. I used the stock V6 computer and a GMPP intake for TBI. What type of lift are you running on the cam? Anything over .475" will cause problems. You'll get coil bind from the valve springs and pinch the oil seals. The push rods will also hit the oil drain back holes and rub a hole in the push rods if you don't drill them out larger. Trust me i've experienced with a lot of these engines and that's why I'm converting to an LS1. I broke a valve spring with too much lift on the cam and roller rockers.

Is the knock in the upper or lower end of the motor?
Is the knock only during start up?
What type of MSD ignition do you have?
Did you use the vortec roller cam and roller lifters?
Did you keep the original 90 block?

I bet we can find the knock.