Ford Muscle Cars Tech Forum banner
1 - 7 of 7 Posts

·
Registered
Joined
·
146 Posts
Discussion Starter · #1 ·
An article called "Static vs Dynamic Compression" in the Feb 2002 edition of Car Craft said that a small-block engine with a cranking compression of no more than 160 psi could run on 87 octane. Does this method sound ok to you guys? The only reason I am questioning this is because my 302 has Windsor Jr heads w/ 58cc chambers and kb flat top pistons, according to kb's website this combo should produce 9.6:1 compression. On this information alone the motor has always gotten good quality gas of at least 91 octane but, after reading the article I did a compression test and my cranking compression was 135 psi, do you think I should try 87 octane gas? Not that gas prices are too bad these days but 87 is 20 cents cheaper per gallon and if I am running too much octane it is probably hurting power. Any input would be greatly appreciated.
 

·
Registered
Joined
·
660 Posts
Not totally sure on this, but I would think a compression test isn't the same as what the car craft magazine was talking about in terms of "static" compression. When you do a compression test the engine isn't turning very fast and the pressure has more time to leak out. It's a good way to "compare" cylinders and find problems but might not give you a realistic "running" cylinder pressure.

You have aluminum heads and 9.6:1 really isn't that much. Depending on the cam and timing I would guess that you could drop the octane down a little...

Just be carefull
 

·
Registered
Joined
·
359 Posts
although it might run on 87 just fine at an idle what happens when you start advancing the timeing and the motor is under a load

I think the internal cylinder pressures might be higher than you think, at 9.6:1 you might be ok with 87 just don't rod it to hard
 

·
Registered
Joined
·
109 Posts
im sure ur motor would run un 87 but ud be best off with 89 or 90 whatever your midgrade is, that way itll run perfect with that compression and aluminum heads and youll save your self 10-15 cents a gallon
 

·
Registered
Joined
·
146 Posts
Discussion Starter · #6 ·
This car is basically my daily driver and rarely sees over 3500rpm on my drive to work/school, I too was thinking midgrade might be a good starting point just to "test the water". The Car Craft article claimed that a compression test could guage octane sensitivity, 87 octane=no more than 160 psi, 92 octane=175-185 psi(big blocks should drop these #'s by 15-20 psi). Thanks for the replies so far.

<font size=-1>[ This Message was edited by: Jimbo on 1/16/02 7:42am ]</font>
 

·
Registered
Joined
·
5,551 Posts
I dont think theres any way to guage octane requirements with compression readings from a guage. Detonation comes from so many places, like sharp edges on combustion chambers, piston tops, spark plugs, etc. There is much more to it than just a "psi" reading from a guage. I've seen 460's with barely over 100 psi that will ping on today's cat-pee gas at 30 degrees of total timing. As another example, some of today's higher-performance engines on motorcycles use a static compression ratio of around 12-13:1, and will still run fine on 87-octane gas.
 
1 - 7 of 7 Posts
Top