What Octane for cars with detonation-based spark retard system

I have an Acura with a turbo-charger and an 8.8:1 compression ratio. The car is also equipped with a gizmo that measures whether the engine is near detonation and retards the spark to prevent its occurrence. The manual recommends 91 octane fuel, but says it’s OK to use regular for short periods; however, knocking might occur and there will be a decrease in performance. It also says long term use of regular could lead to engine damage. My question is: providing the gizmo is working, why would use of a fuel with less than 91 octane lead to engine damage? When I asked the service guy, he said it would “carbon up” but couldn’t elaborate further. I don’t understand why you need 91 octane if you’re willing to accept a little less efficiency for the rare times that the gizmo would retard the spark to prevent detonation. I also don’t understand why the engine would carbon up.

It will only carbon up in theory; and that’s if it’s not running right. All cars have knock sensors, and have had for many years, and this retards the ignition timing in an attempt to offset any pinging that might occur. However, the ECM can only retard the timing to a certain point since it does have a limit.

It is not a given that pinging will automatically occur in your car with regular gasoline. It’s turbocharged and usually a forced induction engine will need higher octane gas but that is also dependent on a number of things; cam profile, if the EGR system is operative, etc.

If any pinging occurs while the engine is warmed up and under a load then more than likely either the EGR system has a fault or you need to step up the octane rating on the gasoline. It’s possible for a knock sensor to go bad but it’s rare.
And of course, one of the best ways of keeping on top of what is going on inside the engine is to pull the spark plugs and analyze how the tips are burning.

The disclaimer in the manual is put there by Acura to cover themselves, just in case. I think about 50% of the printed text in most owners manuals now consist of disclaimers and warnings.

First of all, you shouldn’t just rely on the knock box to constantly take care of any and all detonation problems. That’s like jumping out of a plane with JUST the backup chute and not the main. It’s intended as a preventative safety measure- not the norm.
Owners of turbocharged cars need to be particularly vigilant in taking care of their baby. You get into forced induction, going above and beyond the the typical engine vacuum means the proper operation of air/fuel/spark takes on xtra importance- or else things can go bad really quick. In fact, I’d recommend to anyone with a turbo car to invest in a Air/Fuel Ratio Gauge so they can at least make sure the fuel is keeping up w/ the incoming air.

Now, as for your exact question-- a lot of the answer depends on how much boost you’re running and how often you’re actually on boost.
A big concern for the fuel octane rating stems from these:

  1. Since turbos use the exhaust gas to spin, turbos get REALLY HOT when they’re used a lot- which in turn heats up the air being forced into the combustion chamber (though some of this is offset by the intercooler).

  2. Air heats up when it’s compressed. Therefore more boost= more compression= more heat.

1+2 = the chance of detonation increases with this higher temperature air-- especially with lower octane fuel because it has a lower combustion temperature than high octane fuel does.

So, if you’re like me, and usually not using the turbo- mainly just using a few lbs of boost to pass a truck or something- then you could probably get away with using a mid-grade fuel. If you’re lead footed though, you need to stick w/ supreme.

OK and VB did a great job of the details of your question. I might go a little further to something you did not ask.

When that gizmo kicks in, it protects your engine, but it also reduces performance and many times the mileage. Since you likely chose an Acura with a turbo for the performance, I suspect you still want performance.

Second is the mileage. You may not be saving much if anything due to the reduced mileage. The higher the fuel prices go the more likely you will not be saving anything and may even be paying more for fuel because you are buying more fuel than you are saving on the price per gallon.

All modern cars have this gizmo. It is called a knock sensor. With your car being turbo-charged, per-ignition and detonation WILL cause more damage to the engine than non-turbo, normally aspirated cars. The static compression ratio is 8.8:1, but many turbo and supercharged cars use lower-than-normal compression ratios because the system pre-charges the induction with the turbo or supercharger. The dynamic ratio, taking the forced induction into account, boosts the overall compression to 11:1 or better. Pre-ignition at this point could cause severe stress in the heads.

The knock sensor will sense a pre-ignition detonation, and retard the timing to prevent it from happening again. But, the spark will occur earlier than optimal, and lead to an incomplete burn of the air-fuel charger. The result is excess carbon being released, and coating the piston, valves, combustion chamber, and exhaust, including the turbo. Extended use of a lower grade fuel would cause a constant build-up of carbon that will only serve to increase the problem, and even if you return to premium grade, may not clear it out.

I suspect your desire to use regular is due to the increased price for gas. I also have a high-performance car that requires 91 octane. I use it, and just pay extra. In my area, premium is only 10 cents more that mid-grade, just like 5 years ago. That only adds $1.40 to a tank-full, which is only 3% more. 5 years ago, it was the same $1.40, but it was an 11% difference. Why risk your ride for the price of a convenience store soda?

Click here for more info: http://community.cartalk.com/posts/list/643810.page Especially see post of 12-15-07; 9:26:55 PM.

Whichever decision you make, consider the difference, in real money, if you lose the gamble. On the one hand, you gamble keeping a few pennies against loosing about $5,000 dollars for a replacement engine. Personally, I don’t like those differences.

it always gets me seeing people asking why they should put high octane in their engine(especially one with turbo/super chargers). You bought that particular vehicle/engine combination because you wanted it, right?

the price difference between 87 and 93 is almost always 20 cents per gallon. if you have a 20 gallon tank that you let run dry(not a good idea by the way), you’d only be spending $4 more a tank than if you bought 87 octane

How old is your Acura? Using regular fuel when premium is recommended could void the warranty. You are giving Honda the option of covering any engine problems, rather than forcing them to honor the warranty that you obeyed.

This reminds me of the post I linked above regarding the Lincoln. Nothing against the owners, but buying a late model luxury car and wanting to use regular or mid-range instead of the reccomended premium is like buying a fur coat and then wanting to wash it in the washing machine instead of having it properly laundered- just to save money. An Accura turbo has a high performance engine that is: 1.Designed 2.Manufactured 3.Tuned. Step 4 is for the owner to use the right octane fuel. Just do it.

The answer is in your post. Better use the 91. Just pretend that you didn’t talk to anybody and you don’t want to know about it. Short period of time means in an emergency.