454 Compression and Octane? - Chevelle Tech
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post #1 of 13 (permalink) Old Yesterday, 9:46 PM Thread Starter
 
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454 Compression and Octane?

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

Thank in advance
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post #2 of 13 (permalink) Old Yesterday, 11:37 PM
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Re: 454 Compression and Octane?

Cylinder bore diameter
Crankshaft stroke length
Head gasket bore diameter
Head gasket compressed thickness
Combustion chamber volume
Piston dome volume
Piston deck clearance volume

I would need the above information to determine the actual compression ratio.

I can guess by the head casting number, piston choice, and cam selection that you are in the mid to low 9's compression wise and you should be running premium fuel.

Ask your builder or machine shop, he should be able to answer your questions and also supply the above information.
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post #3 of 13 (permalink) Old Today, 8:08 AM
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Re: 454 Compression and Octane?

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


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post #4 of 13 (permalink) Old Today, 8:17 AM
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Re: 454 Compression and Octane?

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; Today at 8:18 AM. Reason: adding stuff
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post #5 of 13 (permalink) Old Today, 9:41 AM Thread Starter
 
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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.
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post #6 of 13 (permalink) Old Today, 10:16 AM
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Re: 454 Compression and Octane?

Quote:
Originally Posted by ScrubbyUpDoggie View Post
I donít think With my specs Iíll need an additive to the gas station fuel...
Iíll find the specs and post the compression today or tomorrow.
You are correct,you certainly won't need any additives with what you have.
FWIW,My 11 second LS6 Chevelle that has an honest 10.7:1 compression runs fine on the street or strip on pump gas 93 octane.


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post #7 of 13 (permalink) Old Today, 10:26 AM
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Re: 454 Compression and Octane?

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.
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post #8 of 13 (permalink) Old Today, 10:40 AM
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Re: 454 Compression and Octane?

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.

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post #9 of 13 (permalink) Old Today, 11:54 AM
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Re: 454 Compression and Octane?

Quote:
Originally Posted by ScrubbyUpDoggie View Post
I will try and get the specs from the builder.
I Will also remove a plug and do a cylinder compression.
Be sure to remove all the plugs when doing that test.

67 Chevelle convertible, 138 vin, original 4 speed, bucket seats, 12 bolt, Marina Blue with black interior
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post #10 of 13 (permalink) Old Today, 5:12 PM
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Re: 454 Compression and Octane?

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.

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post #11 of 13 (permalink) Old Today, 5:18 PM
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Re: 454 Compression and Octane?

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?

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post #12 of 13 (permalink) Old Today, 7:00 PM
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Re: 454 Compression and Octane?

If they're true flat tops with a single 3cc intake valve relief and heads are 98cc's, it's going to be very close to 9:1. Cam won't matter, you can run pump piss even with small cam.

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post #13 of 13 (permalink) Old Today, 8:02 PM
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Re: 454 Compression and Octane?

9:1 real compression is good IMO. especially if you have a reasonable cam.

Tom Terrific or Terrible Tom, depending on the phase of the moon, passing cosmic rays or other factors not fully understood except by my wife.
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