Octane minimum with 9.77:1 compression ratio?

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Yung Than

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I bought a set of AFR 1.90/1.60 , 60cc heads for my 94 GT. My car is also my daily driver and I use 87 octane during my daily 60 mile round trip from work everyday. I was told that the new compression ratio when I install the heads will be 9.77:1. Will I still be able to use 87 octane with 10 degree timing for my daily commute, or will I need to move up in octane and change the timing to prevent pinging and detonation?

 

G

getinaGTsoon

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#2

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Yung Than said:

I bought a set of AFR 1.90/1.60 , 60cc heads for my 94 GT. My car is also my daily driver and I use 87 octane during my daily 60 mile round trip from work everyday. I was told that the new compression ratio when I install the heads will be 9.77:1. Will I still be able to use 87 octane with 10 degree timing for my daily commute, or will I need to move up in octane and change the timing to prevent pinging and detonation?

Click to expand...

I have an all stock car and I run mid-octane which around here is 89-91 depending on the station. Whenever I get more high performance parts I'll run premium. Just a standard I go by.

 

Killer95Stang

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#3

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Well.. since you have stock pistons... and your engine isn't a 306.. (its stock 4.00 bore I presume)... You compression will be around 9.6:1... That is what I have with my GT40P heads.. I run 89 octane all day long, and then run 91 at the track to be safe... And thats with 15 degrees of Base timing, and no detonation... Since your heads are aluminum, and they disipate heater much better than cast iron, you should theoretically be able to run the same 87 octane on the street, and then probably 89 or 91 for the track.. without detonation... The reasoning, or at least what I've been told... is... "you can run almost 1 extra point of compression with aluminum heads"... and that makes sense, because if you compare to the mod motored GT's, they are almost running 10:1 compression, without problems...

 

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Yung Than

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Killer95Stang, I appreciate the info.

Does anyone know what my compression will be if I went with low comp pistons on a 306 block I plan on getting?

 

yellow5.0cobra

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9:3:1 is stock if im not mistaking.

As long as you are within the 9 range, its fine to use REGULAR.

Once you edge near the 10, upgrade to a better quality... but you just dont need SUPER(93) yet... thats mainly mandantory with 10:5:1 or BOOSTED applications.

 

2L8ULUZ2

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Your information is all wrong. A 60cc head is going to lower your compression not raise it. The stock heads are usually 58cc and with a increase in cc size the compression goes down. A stock motor is 9.0:1 compression so you will probably be more like 8.77 and not 9.77:1 when you change the heads. And if I am not mistaken the P heads have a 62cc or 64cc chamber which will also lower the compression even further. The only way that you will see that high of compression with that head is to change the pistons. So basically you will be fine with the lower octane gas just make sure that the timing is set down.

 

Killer95Stang

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#7

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2L8ULUZ2,

The only thing right about your post is the stock compression ratio of 9.0:1, and what you said about lowering and raising compression..... But....!!! Stock heads have a combustion of 64cc's, and GT40P's have a compression ratio of 59cc's (with tolerances most chambers end up being 60cc's) That drop on in 5 cc's, gives him approximately 9.5-9.6:1 with GT40P's...

 

GTJake

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I just read in a 5.0 Tech mag that the stock E7's have a 61cc Combustion Chamber. If you want me to scan it I can. I've always heard that stock is 61cc, but there are always production variances.

Yung Than: 1cc will not make that big of a difference to worry about.

Jake

 

Mrgreen94gt

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Stock heads vary greatly in combustion chamber size, even on the same head at times. Only sure way to know would be to test the stock heads in question and calculate from there.

 

GTJake

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Yung Than: Why did you buy the pedestal mount heads?

Jake

 

9

94DreamGT

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#11

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I have a set of milled AFR 165's that are 54cc going on my car this Spring. I asked around and checked out some forumla's and I don't think my comp. ratio will even be 9.77:1...definitely under 10.1.

You have nothing to worry about.

 

Y

Yung Than

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#12

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GTJake said:

Yung Than: Why did you buy the pedestal mount heads?

Jake

Click to expand...

Jake, there are several reasons I bought the pedestal heads, one to re-use the crane cobra 1.7 rockers I already have on my stock heads instead of dishing out $230+ for replacement stud mount rockers, and buying hardened pushrods. My application is for a daily driver AODE mustang that sees 50 miles daily so high performance, high rev action was not my main concern.

Since I'm getting the engine rebuilt at 155,000 miles I felt the stock heads are ready to retire. I'm hoping I could get a new feel for the car with the heads but not to the point where I can't use it for daily commute.

 

2L8ULUZ2

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#13

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Killer95Stang said:

2L8ULUZ2,

The only thing right about your post is the stock compression ratio of 9.0:1, and what you said about lowering and raising compression..... But....!!! Stock heads have a combustion of 64cc's, and GT40P's have a compression ratio of 59cc's (with tolerances most chambers end up being 60cc's) That drop on in 5 cc's, gives him approximately 9.5-9.6:1 with GT40P's...

Click to expand...

The stock heads that were on my car were 58cc's and all four sets that are lying around my garage were 58cc as well. My holley heads were 61cc's so that is what I am basing my information on. With a 60cc head he is not going to have close to 10 to 1 with a stock bottom end.