Compression of SC engines

ChrisPearl95

Registered User
Good morning,

I was talking to a co-worker yesterday about having to use premium gas and gas prices. He asked why we had to use premium and I did not know the technical answer. I only heard that the cars do not run well on lower grade gas.

He is a skilled car guy and said that it must have something to do with compression. If its less than 9 or so, you can use less than premium depending on cam and piston set up....but if compression is higher, premium is required.

Look forward to your responses and thanks,

Chris
 
Hey,

Well Ive read on here a while ago that the normal compression is like 8:2:1 I believe. But somebody said that the compression is raised while under boost. I dont know the technical side of it, but it makes sense to me :D . There was a post about using less than premium gas, and was responded by several people who had mixed opinions, but the majority said that if you keep it out of boost that using less than stellar gas would be ok. Now it makes more sense to me :eek: Sooooo now you can tell your co-worker why you use premium :D
 
my memories a little shotty these days
but I think its like

3lbs. boost == 1 point of compression

so say a guy pegging 15lbs. of boost is theoretically
increasing his compression ratio to to 13.2 : 1

higher octane fuel must ignite at a higher compression or temp than a lower grade fuel--to prevent detonation

some body correct me if im wrong please
g/l
 
Brianas said:
higher octane fuel must ignite at a higher compression or temp than a lower grade fuel--to prevent detonation

youre close to the right idea. lower octane will ignite too quickly under high compression/temps. Usually causing detonation, an uncontrolled burn, from what I have read. This will eat pistons fast, like it did to mine when I was making a lot of boost and filled up with 91. About $3k later I learned my lesson. Don't make my mistake. I could have been making too much power for my injectors as well, but it was probably a combination of the 2.

Higher octane can withstand higher compression, it does not Need it to burn though.

I personally would NEVER run lower than 91, and if I had to I would pull my SC belt. Too much damn money in this motor to worry about it.

Edit, the 3 pound of boost-1 compression comment. Think about it this way. Atmospheric pressure is ~15psi at sea level, so when you give it an extra 15psi you just doubled your compression ratio. 16.4. This is no where near exact though, and 13.2 sounds much more realistic. The only way we get away with this high of ratios is from the intercooler, chilling the intake air after the compression of the SC. This lowers the temp. If you have a 16:1comp. NA race motor there is no way to cool the temp midway though the compression cycle, so some kind of HIGH octane is needed. I have no idea what it would be.

I hope this helps, and have said everything correctly. I studied this stuff after killing my motor.
cheers
 
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I beleave 89-93 is 8.2 to 1 and the 94-95 is 8.5 to 1. So you you want to kept the car out of boost I guess you can get away with it (What fun is that :D ) Other wise you need the higher end gas. I dont know what the gas in the US is like but here in canada the low stuff not only has less octane but it's crapy gas in the summer more so.

This is how I look at it you can pay the extra now or wate till you blow it up and then spend the money.
 
Jason Wild said:
I beleave 89-93 is 8.2 to 1 and the 94-95 is 8.5 to 1. So you you want to kept the car out of boost I guess you can get away with it (What fun is that :D ) Other wise you need the higher end gas. I dont know what the gas in the US is like but here in canada the low stuff not only has less octane but it's crapy gas in the summer more so.

This is how I look at it you can pay the extra now or wate till you blow it up and then spend the money.

In Connecticut octane prices are 87=$1.7199 89=$1.8199 93=$1.9199. The price difference between a tank full (19 gal) of low and high is only $3.80 so why even risk it. :eek:
 
It's just like Julian said...the effective compression ratio on a 8.2:1 engine with 15 pounds of boost is 16.56:1. Pretty much anything over 10.5:1 requires a higher octane fuel to resist preignition.

Preignition/detionation/pinging are all the same thing and can cause severe engine damage. The damage occurs because the piston is still traveling up on the compression stroke while the expanding gas from early ignition of the air/fuel mixture is trying to force the piston down. Results are, blown headgaskets, broken ring lands, broken rings, broken pistons, bent rods or all of the above.

Running to lean or with excessive oil mist or heat in the intake air charge can cause the same thing. That's why these cars are equipped with a knock sensor, but it can only retard ignition timing so much.

David
 
What does octane mean?


If you've read How Car Engines Work, you know that almost all cars use four-stroke gasoline engines. One of the strokes is the compression stroke, where the engine compresses a cylinder-full of air and gas into a much smaller volume before igniting it with a spark plug. The amount of compression is called the compression ratio of the engine. A typical engine might have a compression ratio of 8-to-1. (See How Car Engines Work for details.)

The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.

The compression ratio of your engine determines the octane rating of the gas you must use in the car. One way to increase the horsepower of an engine of a given displacement is to increase its compression ratio. So a "high-performance engine" has a higher compression ratio and requires higher-octane fuel. The advantage of a high compression ratio is that it gives your engine a higher horsepower rating for a given engine weight -- that is what makes the engine "high performance." The disadvantage is that the gasoline for your engine costs more.

The name "octane" comes from the following fact: When you take crude oil and "crack" it in a refinery, you end up getting hydrocarbon chains of different lengths. These different chain lengths can then be separated from each other and blended to form different fuels. For example, you may have heard of methane, propane and butane. All three of them are hydrocarbons. Methane has just a single carbon atom. Propane has three carbon atoms chained together. Butane has four carbon atoms chained together. Pentane has five, hexane has six, heptane has seven and octane has eight carbons chained together.

It turns out that heptane handles compression very poorly. Compress it just a little and it ignites spontaneously. Octane handles compression very well -- you can compress it a lot and nothing happens. Eighty-seven-octane gasoline is gasoline that contains 87-percent octane and 13-percent heptane (or some other combination of fuels that has the same performance of the 87/13 combination of octane/heptane). It spontaneously ignites at a given compression level, and can only be used in engines that do not exceed that compression ratio.

During WWI, it was discovered that you can add a chemical called tetraethyl lead to gasoline and significantly improve its octane rating. Cheaper grades of gasoline could be made usable by adding this chemical. This led to the widespread use of "ethyl" or "leaded" gasoline. Unfortunately, the side effects of adding lead to gasoline are:

Lead clogs a catalytic converter and renders it inoperable within minutes.
The Earth became covered in a thin layer of lead, and lead is toxic to many living things (including humans).
When lead was banned, gasoline got more expensive because refineries could not boost the octane ratings of cheaper grades any more. Airplanes are still allowed to use leaded gasoline, and octane ratings of 115 are commonly used in super-high-performance piston airplane engines (jet engines burn kerosene, by the way).

Pulled from the Howstuffworks website.
 
the higher octane gas, the more pressure it can withstand before it spontaneously detonates. since we run boost, which brings up the compression, there is higher pressure in there on a compression stroke. too low octane and you got detonation, thats not a good thing. so thats why we have to run high octane
 
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