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Compression...what is considered "High Compression"?

Mac_49

Mac_49

Member
With everyone talking about motors being all "high compression"...I'm curious at what point or ratio do you consider a motor "highly compressed" lol Also, is 87 Octane a "high compression" fuel? If so, A LOT of motors these days are considered high compression motors if they can run that without pre-igniting. Are there different levels of "High Compression"?

6:1?
9:1?
14:1?

Is it carved in stone where the idea of a "high compression" motor starts?
Would love to hear what everyone has to say
 
Personally I consider anything over that particular engines "stock" compression "high" compression. The point at which you need a higher octane fuel is generally slightly different for each engine, but I don't build engines. So maybe there is a better way for someone to explain at what point you should be considering a higher octane.

To me it's more relative to each engine's stock settings.
 
Interesting discussion here.

url]http://www.speedtalk.com/forum/viewtopic.php?p=86849[/url]

I feel a lot has to do with dynamic compression, which is related to cam timing, duration, overlap, and the rpm run at. If stay above peak torque, less octane is required to keep detonation at bay. Also able to run leaner mixture.

See restrictor plate cup engines before compression limits for an example.
 
The link doesn't work on my computer.

"High compression" is strictly a relative term. It's comparing one compression ratio, which some might feel is low, to a higher compression ratio. Let's say your American made car, which runs quite well on low octane fuel (87) with its 8 – 1 compression ratio is compared to a Mercedes-Benz, where some models have (I'm told) a 10 to 1 compression ratio and need high-octane fuel (92), the Mercedes-Benz could be referred to as a high compression motor, but only in comparison to a lower compression engine. With higher octane fuel (race gas) you can run even higher compression ratios. There is a limit. In my opinion, to be honest, that limit is only discovered through experimentation.

"Octane" rating is only (and I'm being quite explicit) a measure of a fuels ability to resist detonation. Detonation is sometimes "mistakenly" referred to as "pre-ignition". Detonation is caused by excessive heat and/or pressure. Sometimes referred to as "knock". With a two cycle, with a pipe, detonation (when it becomes severe) sometimes sounds like a rock rattling around in your pipe.

It's not well known, but detonation can start long before you can hear it. The best way to to detect detonation, in my opinion, is with the EGT (exhaust gas temperature) gauge. The exhaust gas temperature goes down when detonation occurs.
 
alvin l nunley said:
"High compression" is strictly a relative term. It's comparing one compression ratio, which some might feel is low, to a higher compression ratio.
Click to expand...

I know I'm taking just a portion of your response but basically it sounds like this is it in a nutshell.
So in my mind, after reading all of this, you can basically call every motor a high compression motor as long as you find one to compare it to with a less compression ratio.
 
Mac_49 said:
I know I'm taking just a portion of your response but basically it sounds like this is it in a nutshell.
So in my mind, after reading all of this, you can basically call every motor a high compression motor as long as you find one to compare it to with a less compression ratio.
Click to expand...
Whenever you say "high compression" you're always using that expression as a comparative reference. Meaning; "high compression, compared to what?" The term "high compression" has no specific value, it's only a reference. For instance; say an engine, in a stock form, has a stock 10 to 1 compression ratio, and you modify it so that the compression ratio is 12 to 1, you would now call that modified engine a "high compression" engine.

For instance; the Yamaha KT has an effective compression ratio of about 8.5 to 1. If you compare that to a Rotary valve engine, that compression ratio is low. "Effective" being, the compression ratio above the exhaust port. Two cycle people use that "effective" term because there is literally no (there is some) compression until the piston ring closes the exhaust port.
 
in my mind a "high compression" engine is a compression higher than pump gas can be safely run example 10.5-1 93 octane limit most engines so i guess over 10.5-1 is considered high compression also as octane goes up rate of burn slows so dont put race fuel in a low compression engine its doing no good
 
Mac_49 said:
With everyone talking about motors being all "high compression"...I'm curious at what point or ratio do you consider a motor "highly compressed" lol Also, is 87 Octane a "high compression" fuel? If so, A LOT of motors these days are considered high compression motors if they can run that without pre-igniting. Are there different levels of "High Compression"?

6:1?
9:1?
14:1?

Is it carved in stone where the idea of a "high compression" motor starts?
Would love to hear what everyone has to say
Click to expand...

Anything over 10:1 is a high compression ratio.
Anything over 12:1 is "highly compressed".
87 Octane is both a high and low compression fuel.
There are no different levels of high compression beyond calculated ratio and personal opinion.
You can either make highly compressed air and fuel burn properly or you can't.
The only limit to compression is when compression of liquid in the cylinder is involved or parts are stressed beyond the breaking point.
Octane is about how fuel burns and has nothing at all to do with compression ratio and is only one factor determining what compression ratio is used.

I drive a van which is designed to uses more then one type of fuel without making any change in compression ratio.
 
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