100 octane unleaded. will it make a performance difference?

[Edward 96GTS]

I have a 96 GTS, completely stock. Calif has 91 octane gas.
-Will running a mixture of 91 and 100 octane unleaded increase performance?
-At what ratio, 50/50? Will that give 95.5 octane?
-Will it increase internal combustution pres too much and cause a head gasket to blow?
Thanks for any feedback. BTW, 100 unleaded at Laguna Seca Raceway is approx $6/gallon.

 

[Randy]

I'll defer to someone that is more of an expert, but until then, here's my thoughts on it:

I believe its no to all of those. Assuming you aren't already knocking at 91 octane (hopefully not), then increasing octane should not increase performance unless you advance timing - I don't believe Vipers have knock sensors, and thus won't change the timing based on sensing knock, thus no performance increase over 91 octane. Mixing 50/50 gives something like 91.1 octane (again, second hand, not sure). I would think it does not increase internal combustion pressure at all - it just slows down combustion.

I'll defer to someone who is an expert at this, the above is just based on reading/second-hand over the years.

 

[Chuck 98 RT/10]

Randy is correct. Unless your motor is currently knocking higher octane will not increase performance, in fact it could in theory reduce performance.

 

[Tom F&L GoR]

As you might guess by now, the answer to #1 is no. Unless the engine's appetite is higher than 91, you won't see a benefit.

For #2, the resulting octane is simply the calculation of the average octane based on the volume of each component. So 95.5 is reached with a 50/50 mix. It may be slightly higher if you actually measured it since a) posted octane is a required minimum, so the 91 is slightly higher and the 100 is slightly higher, and b) if the 100 has lead, the biggest improvement is with the first addition.

Not sure where the myth that generates #3 keeps coming from, but lower octane, causing knock, would be more likely to damage gaskets than high octane.

 

[ROGUE]

In all honesty, it's a low 9:1 compression all aluminum motor. It would run on 87 and never put up a fight. 91 won't hurt but going over that won't do a damn thing unless you're using some type of power adder.

 

[96GTS]

I heard the same thing at a Q&A tech seminar at VOI. The computer doesn't know the difference, so it's pretty much a waste of money.

 

[Big Medicine]

higher octane provides a more uniform combustion pattern in the cylinder. if you are burning all of your fuel per combustion cycle, there is no more power to be had from the fuel, regardless of octane.

 

[Big Medicine]

http://en.wikipedia.org/wiki/Octane

 

[Marv S]

The GTS has a 9.6:1 CR and some in So Cal (like Brad C.) have had a knock issue with some of the fancy blends they saddle people with

Anyway, a while back I put local Chevron 91 octane in the Viper and S2-M5 and went to the dyno. Pretty good numbers on the Viper and ok with the M5.

Filled the cars with 100 unleaded Union 76 gas and went to dyno next day.

Viper power was much less with the 100 octane. (even swapped out the ECU and still less power than with 91 Chevron)

The M5 power was much more on the 100 octane - )within 5 rwhp when both it and the GTS were on 100 octane)

(as a side note: another m5 was there the second day and he had 91 octane and went on the dyno right after me. My M5 was 60rwhp more that day with 100 octane than his was with 91 octane. I figure the 11:1 CR and adaptive spark control that pretty much advances timing until knock in the bmw is a factor that can make use of the higher octane. A Viper doesn't have those factors so a higher octane than it needs to avoid a ping or knock = less energy in that motor.

 

[kcobean]

Just to throw it in to the discussion....If you have an N/A car and say, a VEC-II installed, would there be benefit to increasing octane so that you could add timing and/or lean the mixture out for more power? The VEC-II doesn't really count as a power adder (does it?), but higher octane would give the VEC-II a wider envelope to work with. I guess in a way, you're just a "manual knock sensor" at that point.

 

[Chuck 98 RT/10]

A Viper doesn't have those factors so a higher octane than it needs to avoid a ping or knock = less energy in that motor.

That supports my theory. Thanks.

 

[Tom F&L GoR]

Just to get back to the fundamentals here...

Octane is the measure of resistance to knock. If it avoids knock by a little margin or a large margin, you won't know, care, or see or feel it. That's the big knob.

The little knob is how you get high octane. Sometimes it's with less volatile fuel components, so the small difference in evaporation rates means you have more droplets and not as much vapor, so it won't burn as well (less power.) Sometimes it's with an alcohol, which is less energy density. Sometimes it's higher octane because it's a heavier component, and then it gets harder to start when cold. By itself, higher octane fuel doesn't help and usually hurts.

kcobean is on the right track, though. If you have engine changes that can take advantage of the fuel changes, you may find the benefits. Although 60 hp seems a bit high just from spark advance!

 

[Marv S]

Tom,

The delta 60 rwhp was my M5 with some mods (s2) and 100 oct unleaded compared to another person's stock M5 on 91 octane. I think the hp change from octane alone on that car was about 20.

Too bad one of the cheapest ways to bump octane, tel, had such bad sides.

Although 60 hp seems a bit high just from spark advance!