Thread: Compression and octane
05-13-2010, 12:41 PM #1
Compression and octane
I did a little reading but couldn't find a direct answer. If I switch over to a rec head from pro-tec, and have some 87 octane gas still in the tank, would there be anything bad happening?
The reason I ask is that pro-tec says:
"Pro-Tec recreational cylinder heads provide increased compression for better acceleration and improved throttle response. Recreational cylinder heads also offer additional cooling and have a more efficient combustion chamber design that allows for the use of 92 octane pump gas. Cast from aluminum and CNC machined for precise fit and function."I could drain it out, but was wondering what would happen if I didn't.
Also, 92 isn't available as easily as 91 - would using 91 be a bad thing?
05-13-2010, 03:49 PM #2
Depends on how much 87 you have left in the tank. If you have over a 1/4 tank left, you can add some race gas at 110 and get an average of over 92. There is a chart on the forum that shows you what ratio to mix for a desired octane amount...run a search...I know the Polaris section has one in the Tech. section. Also, there are a number of threads that talk about adding octane boost products...so check those out as well. Once you've gotten the 87 out of the system, straight 91 should be fine but just keep a watch on things at first.
05-13-2010, 04:00 PM #3
Here's the chart from the Polaris Tech section...they have a lot of good info. over there for everyone. Although it's not for 87 octane, you can at least get an idea of mixing ratio.
05-13-2010, 04:31 PM #4
Here's a cool mixture chart...just fill in the info and it tells you what octane you will get when mixing.
05-13-2010, 04:48 PM #5
x amount of gas + X amont of gas =
93 octane x 4 = 372 + 110 octane x 1 =110
total octane is = 482 / 5 total gallons of gas = 96.4 octane
very simple match ...
05-13-2010, 07:28 PM #6
Just fill it up with some 93 pump gas and run it. Just stay off the high speed runs for a bit. Vary the throttle for a bit til you burn 1/4 to 1/2 then fill it up again. No worries.
05-16-2010, 05:10 PM #7
Kris, he was just saying he can only get 91 octane. He should be fine with that. I only have a choice of 93 here at sea level. You will like the new head. Very snappy. Enjoy the new brapp!
Also, you do NOT need to adjust *JETTING* for compression. After hearing several arguments about it, I finally just called Dan Lamey, and was advised that compression has no effect on fuel requirement. I do not question his knowledge on this subject.
05-16-2010, 05:18 PM #8
05-16-2010, 05:27 PM #9
I am not sure, but I am guessing he can only get 91 because he is at higher elevation where 92-93 isn't really needed. In this case, he should be good to go.
05-16-2010, 06:08 PM #10
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