compression-octane

[351ford]

so is there some kind of general rule for compression to octane rating needed to run properly without detenating. in other words, can i use 93 or even 91 with 11.5:1 compression?

 

[brianj5600]

There is no rule for that. Quench, cam timing, rod length, bore size, weather.... One of the car mags had a good article on the subject recently, but I don't remember which one. It is a very complicated subject.

 

[5.0ina66]

DCR (dynamic compression ratio) is really what determines octane requirements. It factors in quench, cam tming, SCR (static compression ratio, the one we usually talk about) and all that good stuff. 8:1 DCR and under, your good on premium. 8.5:1 DCR is on the ragged edge of pump gas. Much below 8:1 DCR and your good on plus and maybe regualr
I have a free DCR calculator...I've seen the equation and it beats the hell out of doing it by hand....besides I'd screw it up

 

[D.Hearne]

11.5 to 1 is a stretch with 93 octane fuel. My 331 was fine with on it, with it's 10.4 to 1. Ignition timing is also a factor. More timing= more octane needed.

 

[69Rcode_Mach1]

Elevation is a factor as well, what is your elevation at fountain hills?

 

[351ford]

i think the elevation here is about 2400 ft but the nearest dragstrip is at like 3-4ooo ft. i would probaly expeiriment with different race fuels at the track so that probaly woulnt matter. im just hoping that i dont have to buy race fuel to drive in town every once in a while, and trust me that wouldnt be that often, its not really a good street car.

 

[Sicarius428]

I've seen as high as 12:1 on super. There are a lot of factors that go into pinging and such...
Kevin

 

[SQUEEZE&STROKE]

I ran 91 (good old california) with 10.8:1 compression at 2700 ft with no problems...but at 3-4000 ft and any timing over 35 total...I definately would have had problems!

 

[bnickel]

wow, our elevation here in lubbock is about 3200 feet. a lot more than i would have guessed. the offical elevation is 3256 ft. no wonder people from here who go race in dallas are usually a half to a full second quicker there. the elevation in dallas is about 600 ft.

 

[D.Hearne]

wow, our elevation here in lubbock is about 3200 feet. a lot more than i would have guessed. the offical elevation is 3256 ft. no wonder people from here who go race in dallas are usually a half to a full second quicker there. the elevation in dallas is about 600 ft.

You mean you didn't know it was UPHILL all the way from the big "D" to your house ? I always knew it when I got up to Amarillo, my 85 Pete would smoke like a steam locomotive when I took off up there and higher.

 

[bnickel]

You mean you didn't know it was UPHILL all the way from the big "D" to your house ? I always knew it when I got up to Amarillo, my 85 Pete would smoke like a steam locomotive when I took off up there and higher.

oh yeah, i knew it was uphill, i just didn't realize the hill was that steep

 

[thumper460]

11:1 is pushing pump gas !! Like above the ign timing will have to be conservative and the cam LSA tight. Then you loose all the advantage of the high compression engine!! If you can get the compression to a 10:1 and have the best of both worlds!! I run 11:1 in my BBF with a set of 4.56 gears and Big camshaft on pump gas.. but when it gets hot and I lug the engine.. there goes the Ping.. LOL
Just me..............................

Thumper