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Tractor Pulling Discussion Forum

octane and compression

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Mopower

02-04-2004 07:38:57

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Hey fellas. I am working on a MM and am looking closely at compression ratios and wondering a couple things. Currently looking at about a 9.1:1 ratio and thinking 93 oct. pump gas would run ok. However I may go 11:1 or more if I wanna buy the expensive fuel. How high or low do you guys go out there?

1. How high can I go with regular 87 octane and still run ok? 8:1?

2. How about 93 octane from the pump?

3. Can anyone tell me about their puller machines and a if a general rule of thumb applies here?

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a puller

02-05-2004 07:20:00

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Re: octane and compression in reply to Mopower, 02-04-2004 07:38:57

I can't remember for sure, but I think that one rule that will get you close is say you have 9:1, 90 octane, 11:1, 110, and so on....its prolly figured high..but better too much then too little and ruin expensive parts.....maybe you can look in the archives and find the person that told me this rule...as i asked the same question....

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confusious

02-04-2004 08:12:02

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Re: octane and compression in reply to Mopower, 02-04-2004 07:38:57

I think a lot will depend on cam profile and how efficient an engine can breathe, a poor breathing eng can have a lot of comp. and not detonate on low oct. I run one at 13 to 1 on 112 - 114 gas, 290 psi at cranking, works well but don't lug the motor or it detonates. Sometimes to much oct. will decrease hp, blend it down with 92 and can gain hp. I don't think that there is a good rule of thumb but 8:1 on 87 up to 9 or 9.5 on 93 and anything higher use something better. Get good at reading plugs and experment a little.

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TimC

02-04-2004 08:10:56

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Re: octane and compression in reply to Mopower, 02-04-2004 07:38:57

11.5 to 1 is about the limit for 93 octane in a street motor with good carburation and everything flowing right (if i remember correctly. If the port velocity is poor and not enough swirl you may have a stoichiometric ratio of 12 to 14 to one mixture but with lousy mixing. i.e. if it washed down the runners and into the cyl you are going to have rich spots and lean spots. The lean spots are going to ping.

A good quench area will help give you an artificial octane benifet. You'd have to figure that out yourself but, long rods=more clearance. High rpms = more clearance plus other things.
You also have to figure in piston rock as it comes across TDC. I'd try to stay below .060. This includes deck clearance and compressed head gasket thickness.
Worked with a 600 ci IH motor at about 13.5 to 14. to one with 9.5 inch rods and a 4.75 bore and 5.625 stroke averaging 3500 rpm. We zero decked the block and ran a .035 gasket from ChadS and everything seems good. Should run fine on 115 octane with the 275 to 300 psi cranking.
But with heavy moline parts I'd give it a little more clearance.
I'd go with 11 to 1 CR. and mix 93 with something else. Cyl pressure and heat make power.
Hope this is of some use.

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