What do you all think should be the max CR for a mid 80s iron head 360 in a heavy vehicle with small cam (.427 lift 204 deg@ .050 I think). I am thinking no more than 9:1 IMO.

Hello everyone! I am back on Moparts! Been a while.

We have a '86 W-150 4x4, 4-sp, with a 360 factory Q-jet 4bbl that my brother and I re-built and has been squared, decked, bored .030 over with torque plates, aligned honed, shot-peened, balanced, and blue-printed, etc. It has a "true" 8.89CR with a Scott Brown custom cam that measures .201/.206 @ 050 with .432 lift on both the intake and exhaust on a 113 centerline (I think)installed and degreed "straight-up"! We put 308 heads on it, port-matched them and ported them mildly. Timing is set a factory specs of 16 degrees minus the advance. I can tell you under medium to heavy acceration, when towing it pings on 87 octane. 92 octane works fine. I know this engine begs for a cam with more duration but this is a smog engine and passes just fine. The torque is very strong, especially with 32" tires and stock 3.55 gears. If you don't have to worry about smogging it, go for 9.5 CR in a light car and gears with much more overlap on the cam and you'll be fine on 92 octane gas.

Good luck!

Bret

9:1 is a max I think.

Well, I can't agree more with your cam's duration specs! I would say, with a heavy vehicle, absolutely no more than that but you're pushing it! Like I indicated, our 360 is a very fine running engine and has been measured and re-measured at 8.89:1CR and with 92 octane your safe, 89 octane (I've monitored it) it's close but safe but with 92 your certainly good.

Keep us posted!

Bret

9.0-9.2-1 without quench and iron cyl heads.9.5 with quench and iron cyl heads.Your set up fits the 9.0-9.2 without quench.92-93 octane of course.

I sorta agree with Mike S. I was thinking 9.2:1 but I was comparing your combo to mine. I have the magnum heads on a stock shortblock though. Not sure how much advantage this may be. They say in a "no considerable quench" setup, a safe DCR is 8.0:1 compression. With your cam it should be really close to this, within a couple tenths but I don't have your valve events.

if you are willing to suffer a loss of torque and horsepower at full throttle,
you can retard the ignition timing to prevent detonation
and (possibly) decrease the coolant temperature
and get up to a 6% fuel economy increase:

sorry about the overly large attachment,
but it did that when used the stored image here at Moparts.

More in this older post
on another option of using water injection:

https://board.moparts.org/ubbthreads/show...rue#Post5594673