Hi Guy’s, is anyone one using octane boost or mixing in a little race gas to bump up the pump gas? Here in NorCal premium pump gas is 91 octane and I would like to bump it up to 95 + - to see how it would perform. I have a 1976 DT400 with autolube and the manual states to run 90+ octane but I don’t have a lot of faith in the accuracy and shelf life of the calif. blends. I read the Lucas octane boost label and it said it was for Four Strokes Only. I read a Super Chunky article that stated premium gas back in the late 70’s was about 95-96 octane. Thanks

I mix VP Racing fuel with pump gas to get me in the 95ish octane ballpark. No experience with octane boosters.

I mix old race gas (110) with pump gas for the street bike.

The original Yamaha recommendation of 90+ is RON (Research Octane Number) . That's what used to be on the pump here and still is used in much of the world.

In the USA we've now gone to an average of the RON and MON (Motor Octane Number)... RON is almost always several points higher than MON so even modern US 87 (RON+MON)/2 fuel is plenty of octane. The Research method is not a "realistic" test when it comes to real life engine use. The Motor method is a harder test and many consider it closer to the "real life" conditions cars and motorcycles face..

Also note that "octane" is not equal to "power"... high octane fuel can sometimes make less power than a lower octane fuel... octane rating is just a scientific measure of the "knock and ping resistance" of a given fuel.

I had access to an official octane testing test engine years ago... All the octane boost additives did very little. Mixing race gas with pump gas... even just one gallon race with 4 gallons pump... made a big difference in octane. It's one of those "diminishing returns" things... One gallon in five (20/80) gets you a big boost... going up to 50/50 gives you more octane than 20/80 but not nearly the gain you get from goingfrom pure pump gas to just one gallon race in five gallons total.

P.S. Avgas has higher octane numbers BUT they are completely different tests Apples to oranges. 100 av gas tests about the same as 87 auto gas (regular) if you test it using the automotive test methods.

My DT400 pinged slightly when I rebuilt it and tried straight non-ethanol 91 pump fuel. Rejetted it down 5 and run it on 1/2 leaded Cam2...all is well now. PS I'm at sea level.

I've really never had any luck with octane boosters but Mark hits it on using high octane gas, and 4:1 is a great mix. 110 octane @ 1 gallon with 4 gallons 87 octane is 91.6 which is a good street # or 94 if you mix 90.
I mix mine a little different as I run 20:1 oil ratio on the race bike and when I get that mixed down to a 35:1 ratio for the street bike (haha sort of) I get about 95 octane.
Also pay attention the the AV gas ratings as it is not as high octane as you would think but it does last a long time!

In my experience with actual octane testing, you can't "average" the numbers when mixing race gas and pump gas... it's not that easy.

This is especially true with leaded race gas added to unleaded... the whole "diminishing returns" thing... just a little lead added to unleaded yields quite a significant increase in octane. That same amount of lead added to already leaded gas makes a much smaller difference... if the leaded gas already has a fair amount of lead, adding more lead makes almost no difference.

Of course, in the USA using leaded gas or mixing leaded gas with unleaded is only for racing and not highway use as it's against Federal law to use leaded gas in any vehicle used on public highways.

P.S. If mixing two fuels with similar composition and chemistry... then weighted averaging of the octane gets you pretty close... for example mixing the premium unleaded and regular unleaded gas that you get at the local station. Also keep in mind that the pump numbers are "minimums" and actual octane is often higher. With the exception of gas with ethanol added, modern gasoline is cleaner and better for engines... and it's the same or higher octane than the stuff we had decades ago. Remember, they used to advertise RON on the pumps... most of the modern premium pump gas brands likely have a RON of 95+

Thanks for all of the replies.