I figured instead of disturbing other people's topics, I would start a new one here.  This will probably come off as a dumb question, here goes: Are mx dirt bike engines, from the factory, intended to use a blend of race fuel?

Modern or 1970's?  Yamaha or?  Two stroke?

Yes, 70's for sure, yamaha 2 strokes.  Also curious about more modern 2 stroke yz models as well.  I will try to broaden my thoughts process better.  I look at the early bikes, and see the mx and yz motors as being more or less actual competition race motors (that could very well be a flawed thought.) Where as more modern would get broke into consumer bike, vs the real deal high compression race bike.  If that makes any sense.

First thing to understand is that the octane ratings used back then on the pump here in the US (RON) were not the same as they are today ( (RON+MON)/2 aka (R+M)/2 )

UK and Europe still uses plain RON or "Research Octane Number".  MON is "Motor Octane Number".  What seems like a lifetime ago I did lab testing of gasoline octane with an official CFR Variable Compression engine.  More here if interested:  cfrengines.com/about/

The important thing to know is the MON test parameters have been judged to be much closer to "real life" conditions that automobiles face.  The RON number is "easier" than MON so in every test I ever did on gasoline the RON number was much higher than the MON number. 

So when people reminisce that they used to be able to buy premium (aka "ethyl" back then) 100 octane at the pump and modern 92 octane is crap in comparison, what they don't realize is that the "100" was strictly a RON rating. 

Modern gasoline octane in the USA is rated at an average of RON and MON.  If you could obtain just the RON number for modern premium, it would likely be from 98 to over 100 if the (R+M)/2 rating on the pump today is 92 or 93. 

To answer your question, if I remember correctly the RT2MX owner's manual in the Tech Library says to use a minimum of 90 octane.  The 1976 YZ175C manual recommended "90+" octane...  (and that 175 was pretty incredible for the day...  over 30 HP!)  The octane rating used back then was straight "RON".  Modern US 87 rated (R+M)/2 "cheap as you can buy regular" pump gas will have a RON of well over 90...  probably in the 93 to 95 range.  So no, you didn't need "race gas" then and you don't now. 

Keep in mind that one of the benefits of two strokes is they generally have low octane requirements compared to four strokes.

I have not owned a modern race bike but I'm sure they have fuel recommendations from the manufacturer that you can find.

Modern two strokes will generally tolerate premium pump fuel. A splash of race gas helps but by no means is it required that you spend eight bucks a gallon on 105 or 110 octane. You just have to keep an ear on the motor and make sure it's not pinging.

Thank you guys. I'll be honest, that throws a monkey wrench in my current train of thought. Lol. I do know of the ron/mon ratings, and it confuses me to think 87 ron+mon/2 is comparable to 90 ron. Do you think that particular yz175 would survive on 87? Not necessarily desert heat, but 80+ Fahrenheit and a spirited race?

Turbodan, did you ever try an mx360 head on a stock dt360 cylinder?  I'm the guy that sold ya that pipe w' yellow silencer, btw.

Yes, I would absolutely run US 87 in a YZ175 and not expect an octane issue. 

I took a look at an RAC guide on UK fuels and their "regular" is 95 RON.  Probably the same stuff we call 87 here. 

Good stuff to know.  I assume not, but could there ever be a scenario where the fuel has the same ron and mon value?  Or for example, 89 ron and 85 mon, equaling 87? 1 point less of the required 90.

It's been decades.  From memory the sensitivity of most fuels I tested (defined as RON-MON) was near 20.  You'd have to ask a fuel engineer and/or chemist if it was possible for a fuel to have a sensitivity of 0 or 1.  I never saw anything close to that.

Pump numbers are "minimums".  Pretty much every sample I ran from a gas station exceeded the minimum. Fresh "regular" 87 almost always tested with an average of MON and RON of around 90.   Octane can degrade with age...  so fuel at a small remote gas station in the boonies that only gets a delivery every 6 months could have an octane rating that has degraded below the minimum. 

I also did not say I would personally choose 87 for a YZ175.  I would buy the best pump fuel available to be safe.  But if I had to run fresh 87, I wouldn't worry about it.

A word on aviation fuel like 100 or 130...  it's tested under aviation test protocols.  When I tested aviation fuel using the RON and MON protocols, they came out being close to equivalent to regular and premium auto fuels respectively.  Plus running leaded fuel is illegal on public roads and in some places leaded is illegal to use except during a sanctioned racing event.