i ordered a hpr 12 to1 piston and on the web site it says 98 pump gas????? we dont have 98 pump gas here in
wisconsin, we have 104 race gas... but i run our 92 pump gas in my trail bike and thats 12to1 comp. i think
i should be ok... what do you guys think?????????

 

If you attempt to run the 92 your fourwheeler will overheat and bog out with no power. I had to run 89 in my 11.5:1 piston for only a few miles once, and it kept overheating and bogging with no power. so i would say just pay the extra couple dollars a gallon and buy the 104

 

I ran 93 pump with 11.5:1 for 4 years, but supposedly that 11.5:1 I had was actually 11.2:1. I was supposedly at the threadhold of preignition.

I would not run 93 octane with a 12:1. But 104 is not necesary.
I only need 106 with my 14:1 I run now, but stay on safe side and run straight up 110 leaded race.

You would do well to mix the 104 with 92 pump 50/50 I think.


when you say you run 92 in your 12:1 trail bike, what bike are you refering to? something other than a DS? the 450's can run lower octane than us with our ds's, cause the stroke is so much shorter.

 

Cylinder head temperature is generated by stroke and dynamic compression ratio.
Longer the stroke, the more time piston dwells at top dead center.
Bigger cams with longer overlap result in lowering the final static compression.
Thus smaller motors with short stroke cranks and big cams can run lower octane fuel.

To run street gas you are limited to what Hightower said: 11.25 to be safe with aftermarket ignition and 11.5 with stock ignition.

Add an extra .020 base gasket when installing your new piston and it will lower your compression from 12 to 11.3. My recommendation.

OMR

 

The only thing I would add, to the very good comments so far, is that you will want to consider your elevation. There is more oxygen at lower elevations than higher elevations. When I was putting the sled turbo project together for BD, the octane levels were very important as they relate to the turbo. In other words the avaiable air is more dense at lower elevations thus higher octane levels are needed to couteract that. Most rated octane levels on pistons take that into account and rate octane levels for sea level. So the higher you are, the less octane is needed. What those actual numbers are still a mystery and where professional tuners use thier experience and tools to determine exactly what octane you should run. In my book, I always run a bit more than I think to be safe.

 

What Hightower said.
What ODP said.

In all my life I've never seen a motor damaged by running too much octane. On the other hand, I've popped one by rattling it to death, and I've seen MANY more die the same way.
If your a good tuner you can play around with this stuff. It's like a bag of snakes though, one of these days your going to get bit. Just put some good fuel in it and go have fun. Life is to short to worry about that 1 horse power your missing out on by running too high of an octane fuel.

 

In Wisconsin elevation is not a major issue.
Atmospheric pressure is not a formula that is used.
Remember that air is not sucked into a motor but is drawn by the changes in pressure between the internal cylinder and the rated atmospheric pressure.
If you ride at very high elevations then the atmospheric pressure causes a reduced pressure that results in lower vacuum signal. To increase this signal you either can advance your ignition or run lower octane fuel. Since most ignition timing is preset, then the need to run lower octane becomes the most used method.