Compression Ratio & Octane - Ford Mustang Forum
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post #1 of 3 (permalink) Old 10-14-2019 Thread Starter
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Compression Ratio & Octane

They say higher octane is required when the compression ratio is raised. Well my truck has a compression ratio of 11.2:1 and the manufactures recommendation is 87 octane gas. The previous model had a compression ratio of 9.8:1 and it too ran on regular 87 octane. The 9.8:1 truck made 317 hp and the 11.2:1 makes 390. I was reading an article that says, “the addition of direct injection provides much of the extra power and reduces knock.”
So what’s the takeaway from this?
Do y’all think they “left a lot on the table” by tuning it for 87 octane? Do y’all think the timing is real retarded?
Quite a big difference between 11:1 and 9.8. Both using the same fuel.
https://www.wardsauto.com/engines/ni...gned-titan-v-8

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post #2 of 3 (permalink) Old 10-14-2019
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This is nothing new....Crower as an example back in the late 70's (IIRR) had a new cam grind that could run up to 11:1 CR and still run off 91 octane (which was the best you could get in SoCal). It's all has to do with cam/crank timing, the quench and materials used. There may be more left but each engine has to be carefully looked at as far as how they accomplished it.

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post #3 of 3 (permalink) Old 4 Weeks Ago
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Quote:
Originally Posted by 90lxwhite View Post
They say higher octane is required when the compression ratio is raised. Well my truck has a compression ratio of 11.2:1 and the manufactures recommendation is 87 octane gas. The previous model had a compression ratio of 9.8:1 and it too ran on regular 87 octane. The 9.8:1 truck made 317 hp and the 11.2:1 makes 390. I was reading an article that says, “the addition of direct injection provides much of the extra power and reduces knock.”
So what’s the takeaway from this?
Do y’all think they “left a lot on the table” by tuning it for 87 octane? Do y’all think the timing is real retarded?
Quite a big difference between 11:1 and 9.8. Both using the same fuel.
https://www.wardsauto.com/engines/ni...gned-titan-v-8
My '65 390 came from Ford with 10.5 CR. "Premium" required". But ran OK on "Regular" if you went easy. There was then no su. and lessening vacuum and centrifugal advance.

Today, we have instantaneous detection of and control over knock using electronic knock detection, and electronic control of spark advance.
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