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I own and use a 2005 Chrysler Crossfire SRT-6 as my daily commuter vehicle. I'm not interested in trying to eek out a few extra miles on a tank, I want to keep the vehicle and its engine running as long as possible. I do all the proper maintenance (e.g. change the oil every 3 to 6 months with a synthetic 0W-40W, proper air pressure, change the brakes, wash the exterior once every to one to two weeks).

I'm primarily concerned with the engine: it's quite expensive to replace, and I don't want to damage it. Is it absolutely necessary for me to keep filling it up with premium (octane >91) fuel?

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up vote 8 down vote accepted

The problem is, the crossfire isn't just high compression, I believe its supercharged as well. Forced induction motors like octane - the high pressure, high compression, and most importantly high combustion temperatures make you more prone to detonation.

There are enough electronics in the engine (knock sensor for one) to realize that something is amiss and it will usually pull (retard) engine timing to compensate as a safety measure. As a result, you will lose horsepower - a significant amount.

However, it can only reduce timing so much. If you're running low octane fuel and push the car hard enough, its conceivable that long term damage can still be done.

bottom line: manufacturers recommend high octane for a reason.

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You are correct. The SRT-6 is supercharged, along with other "racing" improvements. –  Tyler K. Apr 2 '11 at 5:12
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This is one of those eternal debates that have very few definite answers.

My view is:

Try lower octane gas, if you notice either knocking or decreased performance then you should stick with the higher octane fuel.

Try midgrade first, then if everything is good, try regular. My mother had a Mazda Miata that recommended Midgrade but would run fine on Regular.

As far as engine life goes, I believe the extra 'cleaners' in many premium gases amount to a negligible difference in engine life. Other maintenance makes a MUCH larger impact than fuel choice (ala regular oil changes, fuel system maintenance, etc).

I found a PDF article that says the 2005 Crossfire SRT-6 has a compression ratio of 9.0:1 which I believe must be when the supercharger is NOT running. Many of my naturally aspirated cars had that compression ratio or higher and ran on Regular fuel.

I believe that sometimes the recommendations by manufacturers (not just automobile makers) are a best-case scenario. For the absolute best maintenace/performance you should follow their guidelines, but oftentimes 90% of people can't tell any difference between the two maintenance practices.

As a rule of thumb, I put fuel system cleaner through my cars every 5,000 miles to make sure I have clean injectors, fuel pump, et al

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The 9.0:1 compression ratio is usually reported independent of the boost provided by the supercharger. The ratio will not change under forced induction, you will just be compressing a denser fuel-air mix. –  Ukko Apr 19 '11 at 21:27
    
@Ukko I agree that the ratio reported is generally not the boosted ratio, I believe the ratio will change when under boost since the amount of air in the cylinder will be greater before compression, hence higher compression ratio. I think :-) –  Patrick Apr 19 '11 at 22:41
    
I agree that the pressure when fully compressed will be higher since more air started in the cylinder under boost. But the compression ratio is measuring the volume of the cylinder when it is largest to when it is smallest. The pressure of the incoming gas and the relative pressure outside is not part of the calculation. There are engines in existence that do alter the geometry or retard the valve closing to alter the compression ratio, but that is not boost related. –  Ukko Apr 21 '11 at 15:46
    
@Ukko Thanks for the clarification, I was unaware of that! –  Patrick Apr 21 '11 at 16:01
    
Another interesting thing is even in an unboosted engine with a 10:1 compression ratio you can get more than 10 atmospheres of pressure at the top of the stroke. I always assumed that it would take so long for the air to fill the cylinder that it would not be able to get to 1 atmosphere before the compression stroke started. But apparently in a well designed intake the air will be rushing to the bottom of the cylinder and compressed by the air rushing after it. By closing the intake valve at the right time you can actually trap more air in the dynamic running configuration. Cool! –  Ukko Apr 22 '11 at 15:07
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Speaking to the asker:

From page 158 of the 2005 Crossfire Owner's Manual:

"DaimlerChrysler Corporation requires the use of 91 octane or higher premium fuel to minimize the potential for engine damage."

Spend the extra $4 per fill-up and get the 91 octane.

Speaking to the question itself:

Without specifics, and without defining "high" and "high compression" it isn't possible to determine an answer. However, all other variables being held constant, using a high octane fuel allows you to design a higher compression engine to take advantage of that octane number. The power then comes from the higher compression.

The compression ratio (CR) or induction type (turbo/super charged or normally aspirated) of the engine is immaterial unless you know the operating parameters of that particular motor! There are engines today that take 87 octane fuel at 13:1 compression, like Mazda's SKYACTIV, and some engines at 10:1 compression require 93 octane, such as the LS3 V8 in the Corvette. By the way, these are incredible compression numbers for unleaded gas, far higher than cars from the 1970's running 10:1 compression that used to require leaded fuel at up to 104 octane!

Different manufacturer's engines are designed around a specific octane number. Some can tolerate lower octane numbers for long periods by retarding engine timing or reducing turbocharger boost. This can be a designed operating parameter or a temporary measure to avoid engine damage. Depends on the manufacturer.

One owner's manual might specify premium fuel with no exceptions except in emergencies (Subaru WRX), while another owner's manual states lower octane merely reduces power with no long term harm (Nissan Maxima).

The only way to be sure is simply to follow the requirements of the manufacturer (who honors warranty work on the car) and engineers who designed and tested the engine.

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