I have a newly rebuilt 1973 454 in my 66 Chevelle. I’m trying to figure out what my compression is to see what Octane I should be using. I’m currently running regular and I know it’s to low. I had a local machine shop build my engine but I’m not sure what the compression is. Can we tell from the specs below what my compression would be?
-Comp Cams XE268H-10, 11-242-3
-GM heads casting 3904390
-Keith Black Pistons KB202 .040
-Keith Black piston rings 4250AM8.040
-Clevite main and rod bearings .10 under
-12 bolt posi rear 3.31 gears
-Holley 750 street HP carb
A Keith Black KB202 is flat top piston. Your head castings are closed chambered,98 cc
Your back of the envelope compression using standard piston to deck height,.039 head gasket and the info above is probably about 9:1 max.
We have a 454 that is about the same (flat tops with 101 cc chamber heads) with a 238/[email protected] .540/.540 114 LSA and it runs mid 12's at the drag strip in a 69 Chevelle SS on 87 octane. Doesn't run any faster or better with 93 octane. Cylinder pressure is 145-150 psi
Mr 4 speed touched on a point you could try also. Pull all the plugs and do a cranking compression test (look on you-tube for details). I think hi-test pump gas the limit is about 180-190 PSI. I takes 3 continuous turns of the motor may hit like 80 the first turn then 140 and then the third it may get to like 170 after that it stops climbing. With his final pressure of 150 he can run 87.
Last edited by Lew540; Sep 9th, 19 at 9:18 AM.
Reason: adding stuff
I will try and get the specs from the builder. I Will also remove a plug and do a cylinder compression. I’m feel pretty confident from the above posts that I’m in the safe range for fuel from the gas station 87-93 octane. I’ll probably start using 93 but I don’t think With my specs I’ll need an additive to the gas station fuel...
I’ll find the specs and post the compression today or tomorrow.
All I know is my 454 had a 12.5 :1 compression ratio and detonated badly even with the best and highest octane pump fuel available (Shell) in these parts. I had the pistons shaved and brought it down to 11.5:1 and now it runs good on the Shell fuel but you can notice the difference if you try other brands. It really likes VP Racing Fuel but its so damn expensive I save it for special occasions.
And while I am not questioning mr 4 speed, my 10.2:1 427 sometimes had issue with pump fuel running iron heads. 205 PSI cranking compression will do that. Cranking PSI is only a guide, but for this, not a bad one.
ACES 3112/Team Chevelle Gold #62 Be big, be a 'builder'!
Cam selection plays a key role on what static compression ratio will run on "pump gas". Dynamic compression ratio is a better indicator which takes cam timing into account. With iron heads you need to have a dynamic compression ratio of 8:1 or less and with aluminum heads you need to have a dynamic compression ratio of 8.5:1 or less.
I run a 396/375 with 11:1 static compression, aluminum L89 heads, and a compcams factory muscle 396/375 cam and it runs fine on 91 octane pump gas. The intake valve doesn't close until 80* after bottom dead center so it bleeds off alot of compression.
what Beth said. The engine needs to spin free to give an accurate result. often there's a couple plugs that are challenging to get it and out. be prepared and have all the tools you have that might help at hand.
IMO, skip all the baloney and just run pump premium. Do you know how your spark advance and timing are set up?