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Discussion Starter · #1 ·
I have a question. Say I have two cars. Exactly the same engine displacement. One car has a 7:1 compression ratio, but it also has a turbocharger. The turbocharger is set at 14.5 psi of boost. The other car has a 14:1 compression ratio (I know, its pretty high, but it doesn't matter for this question). Will the engines put out the same power? The first engine puts twice the amount of air in double the space. The second engine puts the half the amount of engine 1s air in half the space. If that made any sense at all, and anyone understands me, I'd be interested to hear the answer. I bet there is some physics principle, something to do with heat and pressure, but I can't think that hard. Thanks.
 

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Thinking Man's Engine
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The first engine makes more power.

The compression is "static compression" usually measured at Top Dead Center (TDC).

In short.....more air, more fuel, more power, if the head and combustion chamber tops were the same.
 

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In order to get a higher compression ratio, you have to make the piston travel farther. A long stroke hurts your engines performance because the reciprocating mass (piston and conrod) are traveling further.

-Jim
 

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Discussion Starter · #4 ·
I see. That makes sense, thanks guys.
 

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Thinking Man's Engine
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PlatinumWRX said:
In order to get a higher compression ratio, you have to make the piston travel farther. A long stroke hurts your engines performance because the reciprocating mass (piston and conrod) are traveling further.

-Jim
It's one of the secrets of mid-range torque. Having a longer stroke.

You can also interpret JimmyNY's question, as the 14:1 engine having the pistons severly dished out, to effectively reduce the compression ratio, to 7:1.
 

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Discussion Starter · #6 ·
So, then why wouldn't companies make engines with compression ratios that are like 2:1, and put on turbochargers at like 30 PSI of boost? It seems like until the turbo spools, you won't have any power at all... So, say you use a supercharger instead. Wouldn't that be a way to make much more power?
 

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One I wish people would stop using theoretical psi levels or comparing cars with different psi levels. It all depends on what turbo you have, you could have a tiny turbo with say 14 psi or you could have a giant turbo with 9 psi and the giant turbo would probably have more power. Why you say don't they just do something like 2:1 because the fact that it literally would take forever for you to get to that 3000rpm or whenever your turbo spools, do you want to drop the clutch every time you take off just to get moving? With that compression down low the car would probably have like 10hp or something ridiculous and just literally wouldn't move.
 

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JimmyNY said:
So, then why wouldn't companies make engines with compression ratios that are like 2:1, and put on turbochargers at like 30 PSI of boost? It seems like until the turbo spools, you won't have any power at all... So, say you use a supercharger instead. Wouldn't that be a way to make much more power?
An engine 2:1 compression would not run, so the supercharger or turbo wouldn't be able to make any pressure.

~8 or 9:1 is perfect for forced induction.

-Jim
 

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Discussion Starter · #9 ·
It seems like you would want the shortest stroke for the most power, so wouldn't you want the compression ratio to be as low as it can be, and then use forced induction to add the real power? Or is around 8 just the magic number, and it works the best?
 

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JimmyNY said:
It seems like you would want the shortest stroke for the most power, so wouldn't you want the compression ratio to be as low as it can be, and then use forced induction to add the real power? Or is around 8 just the magic number, and it works the best?
Engine optimization is a complex science. Pickup Internal Combustion Engines by Ferguson and Kirkpatrick if you want to learn more about it.

Essentially, yes, 8:1 is perfect for a turbocharged car running 15-25 psi of boost and the octane to support the boost level.

-Jim
 

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Discussion Starter · #11 ·
Heh, okay, thanks for dealing with me.
 

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You have to look at it like this too. A 2:1 CR wouldn't put out enough exhaust gases to actually spin the turbine well enough, especially a big honkin' turbo. It would be like us having the car in neutral, our needle moves, but never goes to the positive side unless the engine is under load.

Also a supercharger probably won't work for a couple of reasons. One, the parasitic loss on the SC would probably be almost as much HP as the engine puts out. Two, in general, superchargers aren't as efficient at compressing air as turbos are, so they heat the air more. This is one reason why (pro racing aside) superchargers USUALLY don't get set much higher than 8-10psi if that even. Most factory SC are probably only running more like 6-8psi. Much higher and the air charge is being heated too much and predetonation starts to become a problem.

8:1 probably isn't the perfect ratio for ever single car either, but it's usually a good happy medium between Off boost power and On boost power.
 

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Thinking Man's Engine
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Most factory SC's are a Roots Type (eaton, positive displacement, etc)

The other type, popular in the aftermarket is a centrifugal type. This type requires high turbine speeds (unlike the Roots type which runs at engine speed), thus needing a geartrain to multiple the engine speed. At normal engineer, it's very inefficient.

A Turbo works similar to the centrifugal type, except for the fact, it's exhaust gas driven.
 

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Help the mechanical n00b

I've read this thread but I don't completely understand it, purely because I know jack about engines. Can someone give the long-and-short of what exactly the compression ratio has to do with your engine? Where is this compression ratio being measured and what exactly is being compressed? How does it affect pistons and such? :D
 

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Thinking Man's Engine
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Real quick (have to go technical)

compression ratio is the ratio of maximum cylinder volume to minimum cylinder volume;

Volume at BDC divided by Volume of TDC

BDC: Bottom Dead Center (where in the piston travel, it creates maximum cylinder volume; about the bottom of the piston travel)
TDC: Top Dead center (minimumvolume; about the top of the piston travel.

What is being compressed is the air-fuel mixture.

In general, the higher the compression ratio, the more power you get, as we compress the air-fuel, the act of compression adds heat. Too much compression, too much heat, too low of octane, you get pre-ignition (the air-fuel mixture ignites before the spark plug fires), knock.

Jim could fill in the missing holes....
 
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