Engine compression ratio vs. Fuel octane

Just noticed that the Testarossa's engine is set at a compression ratio of 8.8:1, not trying to be cheap or anything like that but I have read somewhere that the type of fuel a engine should run on (87, 89, 92, 93 etc) depends on what the compression ratio is. Meaning, if it's ratio is low, you will get no benefits by running a higher octane fuel and it can also cause it to run not as smooth. Is this correct?

 

Don't know about the "smooth" part, but using a fuel with a much higher octane rating that spec'd by the vehicle manufacturer is unnecessary and of no operational benefit IMO.

 

What does the manual call for?

 

What ever you can get by with while avoiding the (ping), all aluminum motors can safely utilize one full compression ratio than iron blocks with alum. heads. It all has to do with heat removal from the combustion area, also keeping the motor reved to avoid lugging protects against pinging.

Gary

 

The US TR OM fuel spec is 91 RON minimum (which is about 86 AKI minimum as marked on US gas pumps) -- the 87 AKI minimum grade is plenty good.

 

I agree with Tim. Check the manual. It will tell you what octane you need. Running higher octane does not produce more power, despite what the TV ads from the oil companies seem to suggest. If you are running 93 octane now, try going to 89 for a tank or two. If the engine pings, go back to the 93. Chance are, it will run just fine.

 

The additives to increase octane are designed to prevent pre-ignition. The higher the compression ratio, the more chance of pre-ignition. This means that the heat generated by compressing the mixture in the cylinder gets the mixture hot enough to ignite before the spark plug ignites the mixture. The problem is that the mixture may start to burn before the piston has reached the top of its stroke, and so the explosion is fighting the engine, trying to push the piston down while its still coming up. VERY bad for the engine. You hear it as pinging or knocking. Bad enough and it will wreck your engine.

Using a high octane fuel with additives to decrease knocking in a low compression engine that doesn't need it will not harm anything, it's just a waste of money. Many people are under the false notion that high octane fuel is somehow more "powerful" or contains more ability to make power than low octane fuel. No. In fact, you could argue the opposite. High octane fuel has additives that make it harder to ignite so it won't pre-ignite on its own. Lower octane fuels actually burn easier. Therefore, you get the most bang for your buck (pun intended) by using the lowest octane that makes your engine happy. I use regular unleaded in my 308. The 8.8:1 compression ratio is considered pretty low compression by today's standards.

The gasoline companies actually add detergents to the more expensive fuels as well in an attempt to get people to buy them. You have heard the ad campaigns ("Drive your engine clean..."). The solution to that is to buy the cheap gas and run a bottle of techron injector cleaner through the car once a year.

Birdman

 

This is both right and wrong. High octane fuel IS harder to ignite; Fuel for the Blackbird spy plane is so high octane you can throw a lighted match in it at sea level and it will extinguish the flame. So why use it?

High octane fuel has more energy per gallon than low octane fuel; more oxygen. So with a higher compression ratio and correctly set engine you will get more HP. Put regular in a 10.5 Compression engine and even if it doesn't knock, it won't go as fast. If you retune your car that uses regular to be able to utilize the higher octane you will get more power, but of course putting premium in a car built for regular IS just a waste of money without modifications.

Ken

 

I also did a little research on my own on the net, and came across this site:
http://www.theserviceadvisor.com/octane.htm#1

I guess a potential harm by using a higher octane fuel than the engine is set for is on the Catalytic converters. Thank you all for answering, I know what I will be using the next time I am at the pump. I am trying not to think about how much money I had wasted since 1991.....

 

Regular Jet-A (or even gas) can extingish a match, that's an old parlor trick related to the fact that you have to have the vapor to ignite fuels. However, that fuel for the SR-71 WAS very special stuff, my Dad had some stories to tell about the requirements for it!

But as for high octane fuels having more energy and the implication that more Oxygen is responsible for that, well, that is, at best, a game in semantics. When we're comparing differences in heating values of ~1.5%, when variability in fuels of the same grade is half of that, it's quite a stretch. Adding Oxygen actually decreases the energy content of fuel; you can increase the quantity introduced to get the power back, but adding oxygenates by itself displaces hydrocarbons with Oxygen, so the engine tuned to run on "pure gas" with a stoichiometric ratio of 14.6:1 will run lean with oxygenated fuel.

 

There are a number of reasons to not run premium in a regular car, but worrying about it "collecting in the cat" shouldn't be one. #1 is simply economics. #2-41 are also economics. #42 would be driveability and it goes down from there. Because of the increased resistance to knocking that premium offers, as has been cited, premium is harder to ignite than regular. This has caused driveability problems in the past, but cars are tuned now to be sure they'll run well on "high DI" (you can do a google search on "driveability index" if you want to learn more) fuels.
I looked at the site referenced and, despite the fact that fuel makers have been distinguishing their premium blends for a long time (maybe not in 97? Not like I remember!) by their increased detergency, he claims there's no need to buy premium for added detergency.

 

When high octane meant literally a higher percentage of octane molecules (long-chain molecules that had a higher ratio of carbon to hydrogen atoms than the lighter hexane or pentane), then high-octane did indeed have slightly more energy. Diesel & Jet-A contain a high % of much longer chain molecules, thus have even higher energy density than octane

However, ever since the advent of cheaply boosting a gas's ping resistance (aka 'octane') by adding tetraethyl lead, and it's successors, including some alcolhols, the energy density of gasolene has had almost nothing to do with it's octane rating, and everything to do with it's resistance to prematurely detonating.

 

It is definitely correct that an 8.8:1 compression engine will run OK on low octane fuel.

However, being a) not confident of hearing pinging over all that er, music, b) having seen more than a few pistons burnt completely through and c) it's my baby, - I run my 308 on higher octane fuel which here is only a little more expensive.

(I also put in lead substitute which may prevent exhaust valve seat and guide wear).

Richard

 

Not trying to beat this topic to death, but just happen to be looking over the owner's manual and noticed on the inside of the front cover (US version) it states the following:

This automobile is equipped with catalyst: Use unleaded fuel only (Recommended: Premium unleaded)

 

Something else to think about is how they raise octane ratings by adding oxygenting agents like ethanol in quantities around 10% blended with gasoline. Ethanol only has about 50% of the available chemical energy per unit volume as does gasoline.

Therefore for a gallon of blended fuel:
.90 x 100% + .10 x 50% = 95% of the available energy compared to full 100% petroleum.

Depending on the combustion chamber design 8.8:1 may require higher octane fuel to prevent detonation, while for a different design 10:1 may run fine on 87 octane. There are too many variables to make a hard and fast rule. FWIW, I found my old Ducati (11.6:1) actually ran cleaner on 89 octane than the 93 I had used for years.

 

Great point, I guess we shouldn't just go by what the compression ratio is on the engine, will stick to what the factory suggested. Thanks.

 

Run 87.

You'll be fine.

You don't need 91 until you're at a static compression ratio of AT LEAST 12:1.