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Discussion Starter · #1 ·
Running a GM 2007 big block 502 with a new Holley 870 double pumper and HP-125 fuel pump in a 68 camaro.

Normally I run 93 octane pump gas which of course I can only find in 10% ethanol around me, but I know where I can get 89 octane Ethanol-free gas. (it's a hike, about 11 miles away from me)

Question - is it ok to mix 89 E-free with the 93 E-10? I assume this would just dilute the ethanol further, perhaps mitigating some of its detrimental effects. But would 89 octane in a 502 degrade engine performance too much?

(not sure if I made the right choice of forum for this topic...)
 

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Post up your cam specs and compression, or the engine part number if it's a straight GM crate. Your dynamic compression plays a big role in what octane you can run.
 

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Discussion Starter · #3 ·
It's an unmodified crate engine, so standard specs are:
9.6:1 compression
Cam specs - .527 in. intake lift, .544 in. exhaust lift
Duration at .050 in. for intake is 224 and 234 for exhaust
110-degree lope centerline
Forged aluminum pistons, forged steel crankshaft
 

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I think it would depend on what gm recommends and how aggressive you get with the ignition timing. I would think all 93 but I am not a gm engineer. Get the ethanol free 93 for a buck or two for a fill up I think it would be good insurance against detonation which kills engines.
 

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I couldn't find the full specs for the ZZ502 camshaft anywhere to be able to calculate the intake valve closing point. However, I've got a 276 advertised duration cam that is 224 degrees at 0.050" lift like the ZZ502 cam. If you use 276 as the advertised duration and the rest of the numbers from the ZZ502 cam and assume four degrees of ground in advance, you come up with an intake closing point of 64 degrees.

Now, plugging that into a dynamic compression ratio calculator along with all of the other specs of the ZZ502 you get a DCR of 7.8.

Here is a pretty good article that explains octane and dynamic compression.
http://www.popularhotrodding.com/tech/0311_phr_compression_ratio_tech/
The article includes a chart to figure what octane, temperature, and dynamic compression it is safe to run at. The chart is said to be conservitive and that is everything is right it should be considered safe to run a half point more compression than you come up with in the chart. Aluminum heads typically allow you to run more dynamic compression than iron heads as well.

If we assume 180 degrees and 89 octane, it looks like you are conservitively safe at about 7.5 DCR. Add in the aluminum heads and a proper tune and you can then assume you will be safe at up to 8.0 DCR with 89 octane. Move to 210 degrees and it looks like you'd still be OK.




 
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