I'm soon going to order new cylinder heads for my Pontiac 389. I recently rebuild the engine, but a machine shop screwed up the rebuild of both my cilinder heads. Need to put way to much money in making my cast iron heads usable again.
So instead of rebuilding my cast iron heads, i'm going to order new aluminium heads. Problem is that i'm not sure what cylinder heads I should order due to the compression ratio.
Situation right now:
- Pontiac 389cui, bored .030
- KB flat top pistons -3,5cc
- Comp Cams XE256H cam
- Original cast heads, which are roughly 69cc
Static compression ratio is about 10.88:1 with these heads
Dynamic compression roughly 9.30:1, with a cranking pressure around 193 PSI
Had to retard the ignition sightly as it sounds like it wanted to ping under 1/3 to 1/2 throttle in the 2500-3000rpm. Something this engine always seemed to do, probably related to the cast iron 'bathtub' style heads. This engine originally didn't had much mechanical/total advance either.
Must admit that I haven't really checked the timing in that RPM range yet so don't know exactly how degrees it had in that RPM range. I was still breaking in the new engine, and didn't want to play around to much at this stage so retarted the ignition 2 degrees to be safe.
However, with going further with the performance aluminium heads route, I thought I might be able to increase the compression slightly.
Edelbrock and Kaufman have some nice aluminium cylinder heads for my engine, with 65cc or 72cc chambers. Using such heads should result in the following compression ratios:
72cc aluminium heads: 10.54:1 static comp // 9.1:1 dynamic comp // 186 PSI cracking pressure
65cc aluminium heads: 11.39:1 static comp // 9.73:1 dynamic comp // 206 PSI cracking pressure
From what i've read on the internet, LPG mixtures in NL/BE during the seasons appears to be;
Summer: 60% propane / 40% butane
Winter: 70% propane / 30% butane
The 65cc heads look very nice, but after reading multiple threads on this forum, the DCR might be a bit too high. Not sure what the LPG pro's on this forum think about this? Several threads on the forum where talking about cast iron heads, while i'll be running aluminium performance heads which (so far I know) should be capable of running a slightly higher compression ratio.
I'm building this engine with the intention to be a torque monster, with fuel economy on the second place and thus less important.
Currently running one Impco 425 mixer with EB converter, and considering to add and second 425 and converter in the future to give this engine some more breath.
I have AFR wideband meter (innovate MTX-L) to keep an eye on the mixture.
I have a brand new David Unified HEI ignition, and a recurve kit when needed ready. That shouldn't be a problem. I'm also going to run a Blackbox ignition controller to get the timing even more perfect - the HEI advance will be locked out that way.
Would be great if someone on this forum can help me with making the right decision
