KirkF

Where I live I have a choice between 91 Octane gas, or 94 Octane 10% ethanol mix.

Which is the better choice for power?


Kirk

Kevin

Go with the 94

997inTO

91 is better without the ethanol! The ethanol will cause Pre-mature failure of rubber parts and seals. There are some other threads on this!

jcnesq

94 causes premature ejaculation.

I don't have the choice in Kalifornia, but I do know from my 356 that the ethanol does cause premature failure of rubber parts - however, not sure the modern P-cars don't have synthetic seals, etc. that avoid that risk.

997inTO

It's an old question! I like the umpth ! Of the 94, but I want to do what's right for the car!

Macster

The power between the two gasolines is likely not very different.

When the driver makes a large enough torque demand of the engine the engine controller is free to go open loop and deliver to the engine a much richer than usual air:fuel mixture. This does a couple of things: It produces more power and satisfies the driver's torque demand and the richer mixture is less likely to detonate.

Now torque can vary some.

In an EVO (UK car mag) gasoline test a few years ago the power difference between the best and worst gasoline was 218hp (@ 5200 rpms) to 204 (@ 5600 rpms). (Test car was a stock late model VW GTI with the 1.8l turbo-charged engine.)

Torque varied much more. The best gas delivered 252 lbft between 2400 - 2500 rpms. The worst gas delivered 234 lbft at 2650 rpms. This one might notice. We 'buy' hp but we 'drive' torque.

The results found the best overall gasoline was not the one that delivered the most power or even the most torque. Some gasolines, at least one in the middle in terms of hp and torque, affected the engine to the point the professional test driver said he wanted to run the engine right to redline. The car on this fuel was the 'crispest' and the most fun to drive.

Which strongly suggests one needs to experiment with the various gasolines in his area to find the one the engine likes best.

But if one can -- those of us in CA have to make do with 91 octane -- he should feed the engine the proper octane of gasoline. This is most critical to extracting the best overall performance/economy from the engine.

So, in your case feed the engine 94 octane. Be sure you buy from a busy station to get the freshest gas. High octane gasolines do go stale. Which brings me to this: If the car sits a long time (weeks) between uses and fillups likely the gas will go a bit stale over this span of time so consider this when you take the car out after a long between uses.

Sincerely,

Macster.

KirkF

If ethanol has about 34% less energy per unit volume than gasoline, I assume the ethanol blend will have about 3% less power.

However at 91 Octane, the pure gas is causing the turbo engine to retard its ignition to avoid detonation.

So the question is do you get more power by not retarding the ignition or by the 3% extra energy in the fuel?


Kirk

Macster

Like Kevin said "go with the 94".

All other things being equal the engine controller *not* having to retard the timing any due to use of 94 octane gasoline will deliver better all 'round performance.

The sooner the engine controller can start the combustion process the more power is derived from the gasoline, the car's economy is better, and the car will be a better driver.

Besides the downside of 91 octane gas is when the engine controller has to retard ignition timing this raises the temperature of the exhaust gases which subjects all engine components downstream to higher temperatures than they would otherwise experience.

Retarded ignition timing is bad. 'bout the only thing worse is what is it retarded to avoid: detonation.

As an aside, my info is ethanol blended gasolines can have up to 10% ethanol but gasoline refiners have balked at delivering this 10%. The level of ethanol is in the 7% to 8% range.

Go with the 94 octane gas.

Sincerely,

Macster.