CR vs octane
Thread Starter
Drifting
Joined: Jul 2000
Posts: 1,266
Likes: 0
From: Allen, TX, USA
CR vs octane
At what compression ratio's is a specific octane gas required?
E.g. If I build a 9.0 CR engine, can I use 89 octane?
If I build a 9.5 CR can I use 89
if I build a 10.0 CR can I use 89?
When (what CR) requires 91? When (what CR) requires 93 octane?
Drifting
Joined: May 2001
Posts: 1,329
Likes: 12
Re: CR vs octane (adam)
I think it all depends on how much heat your heads build up. And this depends on the material (al. or iron) and your coolant system. Maybe I'm just wrong too... on a stock 350 I'd go with 89 at 9.0, 91 at 9.5 and 93+ at 10.0
Race Director
Joined: Dec 1999
Posts: 19,610
Likes: 778
From: Forked River NJ
Re: CR vs octane (Lohkay)
A lot depends on cam selection. Lots of duration can stand more compression. Cylinder pressure is the determining factor on the need for higher octane gas. There have been 11.1 motors that can run on pump gas. I have seen 9.1 motors rattle like crazy on 93 octane. There is more to it than just compression ratio.
Race Director
Joined: Nov 1999
Posts: 10,643
Likes: 8
From: Foxfield CO 1970 Convertible
Re: CR vs octane (adam)
Adam -
All the comments are correct, but there is another factor you can't ignore - timing. You can make an 11:1 engine run on 85 octane with no problems. It just will require the timing to be very retarded and you won't get the power you want. I believe to reach optimum performance you would need to factor in all aspects - CR, timing, cam, head material (iron/aluminum), intake type (NA, SC or turbo) and possibly altitude.
Good luck...
All the comments are correct, but there is another factor you can't ignore - timing. You can make an 11:1 engine run on 85 octane with no problems. It just will require the timing to be very retarded and you won't get the power you want. I believe to reach optimum performance you would need to factor in all aspects - CR, timing, cam, head material (iron/aluminum), intake type (NA, SC or turbo) and possibly altitude.
Good luck...
Safety Car
Joined: Jun 2001
Posts: 4,004
Likes: 2
From: Severna Park Md
Re: CR vs octane (adam)
I have 10.5 with the 268 on 93octane with full dist advance and never heard it rattle. You wanna make sure you can still get full advance without pinging.. Cam selection and compression ratio go hand in hand. On average pump gas i would recomend 10to1 for the 268 and 9to1 with the 260.
Drifting
Joined: Nov 2000
Posts: 1,361
Likes: 0
From: Honolulu Hawaii
Re: CR vs octane (adam)
The guys above pretty much said it all. I'd like to add one small thing though. Aftermarket heads with good combustion characteristics can allow a little higher static compression vs. a stock head.
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