Compression versus octane?

skipnrocks · Jul 30, 2015

I am getting to the point of starting my buggy for the first time. The motor I bought I don't have all the specs on. The guy suggested that I use avgas. Is there a way to test the compression to know which octane I should run? I would really rather run premium.

lowbudgetjunk · Jul 30, 2015

Re:

Run a few gallons of 87, 89, 91 etc. Within a few seconds, you'll know whats right for you.

Kel Lawrence

zukimaster · Jul 30, 2015

What engine do you have?, then take a plug out and look in with a flashlight to see the piston, look and see if it has a dome, or flat top. And if you have a compression gauge, take a reading. Report back

Elliott · Jul 30, 2015

My local airport had to stop selling av gas, the supplier was threatening to pull it due to their contract.

muddinmetal · Jul 30, 2015

zukimaster said:
What engine do you have?, then take a plug out and look in with a flashlight to see the piston, look and see if it has a dome, or flat top. And if you have a compression gauge, take a reading. Report back

x2

lowbudgetjunk · Jul 30, 2015

Re: Re: Compression versus octane?

muddinmetal said:
x2

Stop agreeing Tyler or I'll get mister Big G to Ban you.....too much agreement latelyy :flipoff:

Kel Lawrence

drycreekjeep · Jul 31, 2015

u could got the numbers off the heads and figure out what cc they are.

skipnrocks · Jul 31, 2015

I will see what I can do. I have a 383 and believe I have domed Pistons but I'll check. I also believe the deck has been machined but not for sure. I'll report back.

skipnrocks · Aug 5, 2015

Ok so the guy who sold me the motor says its a 12.5 or 13 to 1 compression. Locally I can get av100 av gas pretty easy will this work or to I need to sourch av110?

skipnrocks · Aug 5, 2015

I heard the cam overlap plays a roll in the octane used with high compression. I am 90 percent sure this is the cam I am using if that helps. I will run a compression check this weekend.

zukimaster · Aug 5, 2015

A lot of cam overlap helps bleed off compression at lower rpm's, AV fuel is cheaper, just according your riding style, if your going to be racing or hill killing, I would for sure run 110, but just trail riding with the occasional romp of the gas peddle, av gas will do. Or check on what it would take to run E 85

Craig_c · Aug 5, 2015

Compression is only a small factor in the octane equasion. Cam profile, cylinder heads (chamber, aluminum vs steel), tuning, etc.

skipnrocks · Aug 5, 2015

I have aluminum heads. I think I have flat pistons but possibly domed, I will find out. Is there any good way to know without pulling things apart? I don't want to run the risk of not running enough octane and having holes in my pistons. But I don't want to run too much since I hear that's not good either and its expensive.

Does the cam spec from the cam I linked to help you determine what I need? I don't fully understand all the numbers present there.

skipnrocks · Aug 5, 2015

Also while on the subject do I need to have my computer flashed for higher octane fuel. I was talking to a race shop in town and they said av gas is junk and to try an get tuned for e85 and that its cheaper. What are your thoughts.

skipnrocks · Aug 5, 2015

Ok so I did find the original sale add and it is supposed to be A 13/1 motor with camshaft gm performance 19210723.

CHASMAN9 · Aug 6, 2015

On my 388ci, I am running 14:5:1 with aluminum heads and burning 116 octane in it. I have had it tuned twice now and from what I have been told, the AFR (Air Flow Ratio) is the most important number to watch for in the tune. Mine started at 3 and was tuned to 12. I do not know what all that means, only that it brought the numbers up 287 hp and 254 ft#'s torque from the original pull, and that's on a chassis dyno tune running 44's. Not bad for a little mouse motor.

muddinmetal · Aug 6, 2015

CHASMAN9 said:
On my 388ci, I am running 14:5:1 with aluminum heads and burning 116 octane in it. I have had it tuned twice now and from what I have been told, the AFR (Air Flow Ratio) is the most important number to watch for in the tune. Mine started at 3 and was tuned to 12. I do not know what all that means, only that it brought the numbers up 287 hp and 254 ft#'s torque from the original pull, and that's on a chassis dyno tune running 44's. Not bad for a little mouse motor.

A 3 to 1 AFR is the richest I have ever heard of, she must have been weak as water & sucked the gas :****:

CHASMAN9 · Aug 6, 2015

muddinmetal said:
A 3 to 1 AFR is the richest I have ever heard of, she must have been weak as water & sucked the gas :****:

It was, the guy didn't even finish the pull in fear of making it a boat anchor. Didn't know how I didn't blow it up. SPeedSouth guys in Pelham did a great job tuning it, so good, I blew it up at RBD last year, but that was operator error.

Mortalis5509 · Aug 6, 2015

Re:

E85 will require bigger injectors. Up to 35% higher consumption.

skipnrocks · Aug 6, 2015

Re:

Mortalis5509 said:
E85 will require bigger injectors. Up to 35% higher consumption.

I scratched that thought. Its too much work for how often its used. I have been looking at race fuel. Its 500 a barrel. In looking at that boostane popped up in my search I cant tell if its legit but it seems to be backed by facts. If I could travel with that and add to pump gas That seems like it maybe a good alternative. I was told not to run av gas because of the lead and my sensors. But I can only get 100 anyways local.

Any thoughts on boostane and pump gas?