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Super Moderator
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Discussion Starter #1
can someone please explain "compression ratio" and how it affects milegage, fuel type, power, torque?

i'm an undergrad physics major, so i'm not looking for a layman's explanation, but rather a scientific one
 

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Refrigerator Raider Hater
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guitar maestro said:
can someone please explain "compression ratio" and how it affects milegage, fuel type, power, torque?

i'm an undergrad physics major, so i'm not looking for a layman's explanation, but rather a scientific one
The compression ratio equals the ratio between volume of the cylinder at TDC and BDC.

compression ratio has nothing to do with mileage.

the higher the compression ratio, the higher the octane you need. 87 is for 9:1 engines, and 93 (or 92 or 91) is for 10:1 engines. There are some exceptions, but not many. Octane is also dependant on ignition timing. you can run 87 in a car designed for 93, but you will need to remove timing (bring the ingition point closer to TDC.

The higher the compression ratio, the more torque. this is a small change, and torque is more effected by stroke.


overall, a higher compression ratio increases efficiency. however, you must have better fuel, and if high enough, better parts.
 

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AED Specialist
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Hmmm...

Greenbird,

The higher the C/R, the more torque? That isn't always true.

The higher the C/R, the more throttle response you'll have. That is a true statement. Also, the higher the C/R the higher the cylinder pressures.

I mean a 8.5:1 C/R '03 Cobra makes 390 lb.-ft. of torque while a 9.85:1 C/R '01 Cobra makes 318 lb.-ft. of torque. Or so Ford advertises.

A-Train
 

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Super Moderator
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Discussion Starter #5
thanks for the response......i'm not too keep on exact details on engines, so i'm learning something new everyday.......with that said, what does TDC and BDC stand for?

i found the listing for the compression ratio at carpoint.msn.com for my 97 cougar 4.6L........so i'm guessing that the 10:1 compression ratio for my car is relatively good....based on what you guys have told me
 

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Sheepish
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4,085 Posts
Re: Hmmm...

A-Train said:
Greenbird,

The higher the C/R, the more torque? That isn't always true.

The higher the C/R, the more throttle response you'll have. That is a true statement. Also, the higher the C/R the higher the cylinder pressures.

I mean a 8.5:1 C/R '03 Cobra makes 390 lb.-ft. of torque while a 9.85:1 C/R '01 Cobra makes 318 lb.-ft. of torque. Or so Ford advertises.

A-Train
Hey A-Train, did you forget to mention that little minor change between the 01 and 03 Cobras? ;) You know what I'm talkin about, that little supercharger sitting up on top> ;) (Just ribbin ya man, you know more about Fords than I could hope to right now)

I know that 5.0 Mustang magazine dyno'd a bone stock 03 Cobra and got 360hp at the wheels and they calculated it to make roughly 425 at the crank with those numbers. Also, after a cat-back exhaust and a K&N filter, they got an even 400hp at the rear wheels! And they got it into the 11s on a stock car that would still be under warranty!!!

(Gotta love the a$$-whoopin Ford just unleashed on GM and Chrysler!) :)
 

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Gauge master
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884 Posts
As a rule of thumb, the higher the compression ratio, the more efficient the engine will run. But this is limited by pre-ignition, which is the tendency of the fuel/air mixture to ignite by itself due to high combustion chamber temperatures before teh spark plug fires. The higher the compression ratio, the higher the fuel octane must be.

This is why Diesel engines are slightly more efficient than gasoline engines. They run a higher compression ratio. They can get away with this because the fuel that they run has to be heated up hotter before it ignites, and also due to the fact that the fuel doen't get injected until the piston is close to TDC (top dead center). As the piston travels up the cylinder, the only thing that is being compressed is air.

The future of gasoline engines might make them more similar to Diesel engines. Gasoline direct injection technology sprays the fuel into the combustion chamber as the piston is approaching TDC. Please see the attached link if you would like to learn more. http://www.mitsubishi-motors.co.jp/inter/technology/GDI/page2.html
This would allow for a higher compression ratio, which would yield higher effiecency.
 
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