Rec90 vs 91 Octane fuel
 

Guys,

Let's split some hairs for a moment....

As I stated in another post, with my engine requiring 91 octane or better fuel, I've been pumping a ~50/50 mix of 89 and 93, to try and save some coin, since nobody seems to carry 91 anywhere near my travels. I have, however, come across several sources of Rec90. I know the lack of alcohol is a good thing, for a number of reasons, but the octane rating is lower than my supposed minimum requirement. Just wondering what effect on octane does the alcohol have, if any, and if that single point of octane is really something to be concerned about. I know the Rec90 is actually more expensive than the 93 typically is, but the benefits of no alcohol has its value, so long as the sub-minimum octane rating isn't marching me to a blown out piston or eight.

Thanks. Brad.
(937)545-8991

Subscribing.

There’s a lot that goes into your octane rating. First, tell me about your motor and what kind of numbers your motor put out. Are you super charged, what is your compression of your pistons? Second, do you boat at sea level?
Octane rating is more than just used for detonation in advanced timing purposes.

Who and what determined your octane requirement? Is this a carb boat or EFI and what ECU? I'd go back to your engine builder or tuner with the question...tuneup may not be so far on the edge that you get knock from 90 octane.

Ethanol fuels are hard on carbs but most modern EFI systems deal with it no issue. The big gripe in boats is that the tanks are vented and if they sit a long time the ethanol absorbs water....well so does the methanol derived MTBE that the Ethanol replaced. They both absorb water in a vented marine environment if they sit and aren't used. I say skip the high cost of rec 90 and put regular 93 in it if you have access to it. Dump some Stabil marine stuff in with it if its going to sit on the trailer for a bit.

Circulate some 40:1-1-50:1 two stroke oil/fresh gas/ splash of stabilizer in your fuel water separator or from portable gas jug to your separator inlet when running for winterization. Helps alot !

Obviously get some stabilizer in the tank and well mixed up.

We’ve touched this many times now if you fo an oso search. Merc has even jumped in this bandwagon. :)

Drag,

Pretty sure you chimed in on my "Down On Power" thread, but...

496HO, M1 ProCharger, Whipple Stage 2 ECM tune. Both Procharger and Whipple stipulate 91 minimum octane fuel.

I'm fully aware of the hygroscopic nature of alcohol, and I maintain that aspect pretty well, as far as winterizing. My main concern was with the potential of hurting the engine with the slightly low octane rating. I am also curious as to how the alcohol affects the effective octane rating, if any.

Thanks. Brad.
(937)545-8991

My only point was that Rec 90 probably costs more than regular 93 if you have access to it easily off water. I did a terrible job of conveying that point.

Drag,

All good.

OH, I'm well aware of that. I paid $6/gal for Rec90 when 93 was ~$5, at least partially because the Rec90 pump was open and the others weren't.

I've heard that the alcohol affects the way the gas burns. Since my days running model boats on a fuel that is a mixture of methanol, nitromethane and oil has taught me that alcohol burns extremely fast, and it's the flame front speed that octane rating affects, I can definitely see how the Rec90 would behave differently than gas with alcohol in it. I'd just like someone who knows well more than me to explain it one way or the other.

Thanks. Brad.
(937)545-8991

Think your looking too far into things.
91 E10 has 91 octane
90 Rec has 90 octane

Yes, alchol itself has higher octane, but when the producer makes 91 E10 they mix the batch with 90% whatever octane fuel that mixed with 10% of their straight ethanol fuel, ends up with 91 octane.

Now, energy from 1 gallon straight fuel vs 1 gallon ethanol is a totally different thing.

If you are absolutely optimizing the fuel burn then you will run the E10 or E15 probably a little fatter on lambda....maybe. It can really depend on the individual fuel. I live on the engine dyno and can swap back and forth to different fuels easily between various pump gasolines...I hardly change anything but am also not stuck using primitive engine controls. The ECU I use does a lot of the ***** work itself so I'm a little spoiled. Someone with more experience in these engines and control units could offer you better advice.

Best advice from SB....regardless of ethanol, MTBE, or whatever chemicals were used, the Rec 90 is 90 octane and the 91 is 91 simple as that. You'll only know if you can truly run it if you try it and don't hurt anything. Does the engine have a knock sensor and can it make adjustments?