High compression motors

[250Renvy]

Just a couple questions on High compression setups.

Is there a formula for figuring out octane requirements? 220-250 psi should use what octane?

Is it as simple as changing out the dome to run pump gas?

[GPracer2500]

Originally posted by 250Renvy
....Is there a formula for figuring out octane requirements? 220-250 psi should use what octane?....

There's no reliable formula I've ever seen. If someone thinks they have one I'd love to know what variables are inputed. Maybe a detailed formula exists. But I wouldn't trust a simple XXX psi = XXX required octane rating as being a gold standard. There are too many factors independent of cranking compression that go into figuring an engines propensity to knock/ping.

Simple cranking compression is probably the largest single factor though. So, that's a place to start. I might guess something like:

Under 180psi pump gas is probably ok.
180psi to 210psi, ~100 MON is probably sufficient (100LL avgas, for example, is a little over 100 MON)
210psi to 230psi, ~105 MON is probably sufficient.
230psi or more, 105 MON or higher.

Note there aren't many fuels that are even available with octane ratings at or over 110 MON. When looking at race fuels, always pay attention to the MON rating. MON+RON/2 (the formula pump gas uses), RON, or unlabeled descriptions like "110" should be ignored. Look for the MON rating listed on the spec sheet.

Others may have their own guidelines. But I would only use something like that ^ as a starting point. The specs of the engine beyond cranking compression matter and could push your required octane rating up or down. In my view, the best way to finalize a fuel choice is through actual testing of a potential fuel in your engine. Find conditions that favor deto and try and make the engine ping. Give the fuel a pass/fail grade based on the results.


FWIW, I've been running 230psi cranking compression, tightish squish, close to stock exhaust port timing (I think, I haven't actually measured it--just looked at it), and a PT High Rev on VP110 (107 MON) with zero knock/ping.

$0.02




Originally posted by 250Renvy
....Is it as simple as changing out the dome to run pump gas?....

Generally yes, I believe so.

[wilkin250r]

Originally posted by 250Renvy
Is it as simple as changing out the dome to run pump gas?

Yes and no.

Pump gas can be a gamble. Every so often, you might run across a tank of gas that just doesn't seem to run right, runs like it's rich, or lean, or poor throttle response. Sometimes, an engine just isn't happy on pump gas at all, although you can't find any rhyme or reason.

Sometimes, you find yourself with a custom-cut dome in your engine for some reason or another, and the off-the-shelf domes don't work or don't fit properly.

Complications are fairly rare, most often it IS just as simple as changing out the dome to run pump gas. Just be aware that there are possible issues out there so if you do run into something, you know what you're looking at.

[bkconwayls1]

????? Is it worth the hassle of dealing with race gas ?????

I ask because I have a rebuilt 275PV on it's way back from Duncan Racing next week. I will have to use race gas now (210-215psi), and did not realize this before geting this deep into the build. I'm not even sure where to get it around here.

[Aceman]

Call your local fuel/oil distributor. You may luck out and be able to get it straight out of the pump, otherwise it usually comes in drums or barrels.

[bkconwayls1]

Any opinion on race gas? I'm just a rec rider, but I love acceleration!!!! I might end up selling the cyl. before I even use it.....



Originally posted by Aceman
Call your local fuel/oil distributor. You may luck out and be able to get it straight out of the pump, otherwise it usually comes in drums or barrels.

[Aceman]

I just buy whatever brand of 110 leaded they have. I'm at 222psi and run it straight.

[bkconwayls1]

What I mean, is do you find the added power worth the price of race gas and the inconvienence to get it?

[Aceman]

Sorry I can't help ya. I've never ran just "pump." When I first bought my R a couple years back it was at 180psi(I used 50/50). Rebuilt it to where it's at now, which is race fuel only. Plenty fast for what I do, so I think the race fuel is worth it in my opinion. They say it adds bottom end, but I'm not willing to drop my compression just to see if it's true. Maybe someone with more experience can chime in to answer your question...

[bushwesl]

C'mon, who are we kidding here guys? Leaded 110 is worth the extra $$ just for that sweet, sweet smell :)

[312R1]

Originally posted by bkconwayls1
What I mean, is do you find the added power worth the price of race gas and the inconvienence to get it?


The use of a high quality fuel such as VP C-12 will enhance the reliability of your machine; lower operating temperatures and increase jetting consistency. I don't know about you, but I would rather be safe then sorry.;)

[mfr422]

Here's my 02.
I had a motor in my R that was an lrd woods setup requiring race gas. I liked the bottom end power of the higher compression but like you fought over the decision of whether it was really worth the extra expense. And yes it does smell AWESOME!! I finally decided to change to lower comp. to run pump gas and have been happy ever since. I think for joe average trail rider it is a hassle and probably not worth the expense. In a race machine it's a different story. You really need to figure out whether 5% more power will make your ride that much better. At the time I was using it, i was riding ALOT and it was getting really expensive, now it might not be as much of an issue as regular gas is more expensive and i'm not riding as much. Your engine will definetily run better on a race gas setup but you are the only person who can decide whether it's worth it or not. Maybe you should try it both ways and decide then, domes aren't that expensive or hard to change.
I had a similar problem when I started as you, I wanted a pump gas motor, but nobody would build me one, all the engine builders I talked to said you MUST have race gas or our motor won't make good power, but that's just not true. So, I bought what THEY wanted to sell me and then modified it to suit me. Worked for me, Roost on!!

[312R1]

Originally posted by bushwesl
C'mon, who are we kidding here guys? Leaded 110 is worth the extra $$ just for that sweet, sweet smell :)


Gotta love it...The wifes just shakes her head when I say oh that smells so good..LOL

[my88r]

Originally posted by 312R1
The use of a high quality fuel such as VP C-12 will enhance the reliability of your machine; lower operating temperatures and increase jetting consistency.

short and straight to the point about race gas.lol

[bkconwayls1]

I doubt this R will see much more than 30 hours a month, and I have reason to believe there are some local places that sell 110. I'll continue with this build, Thanks!!!

[oldschoolr]

Originally posted by 312R1
Gotta love it...The wifes just shakes her head when I say oh that smells so good..LOL Thats funny 312 b/c i do the same thing.

[bkconwayls1]

Originally posted by 312R1
The use of a high quality fuel such as VP C-12 will enhance the reliability of your machine; lower operating temperatures and increase jetting consistency. I don't know about you, but I would rather be safe then sorry.;)

This alone is worth it, now that you have pointed it out

[312R1]

Originally posted by oldschoolr
Thats funny 312 b/c i do the same thing.


LOL..good it's not just me. :)