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Discussion Starter · #1 ·
Just got back going for a ride in friends 01 GT. He has the auto. We were talking about a SC for it. What will that do to the Mileage?

Last Q. As for the TC what does that do to the Compression ratio? Also, what to gas mileage...had a crazy What if thought run through my head for my Cleveland someday...


Jeff Given
 

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Who cares about mileage with a blower or turbo? Since either one is boost it will make the compression go up some.

<font size=-1>[ This Message was edited by: Hottarod on 1/7/02 7:13am ]</font>
 

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Discussion Starter · #4 ·
On 2002-01-06 05:41, MightyMach wrote:
Just got back going for a ride in friends 01 GT. He has the auto. We were talking about a SC for it. What will that do to the Mileage?

Last Q. As for the TC what does that do to the Compression ratio? Also, what to gas mileage...had a crazy What if thought run through my head for my Cleveland someday...


Jeff Given

Speak english? I think that you got a little "trigger happy" and wanted to make your first post.


T/C = Turbo Charger
S/C = Super Charger
Last Q = Last Question.

So again, what will a TURBOCHARGER do to compression ratio? If it works the way I think it does, it should not do anything to it. And about the gas mileage on a daily Driver 01 GT with a supercharger?

Doc...did not mean to offend either


Jeff Given
 

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On 2002-01-06 05:41, MightyMach wrote:
Just got back going for a ride in friends 01 GT. He has the auto. We were talking about a SC for it. What will that do to the Mileage?
I have a Powerdyne supercharger on my '88 GT. It didn't make a noticeable difference in my gas milage. In theory, you should get a little better gas milage if you keep your foot off the throttle (which can be hard to do sometimes).


My friend has a Paxton on his '91 Mustang and he also says the supercharger improved his fuel economy.
 

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Forced induction does nothing to static compression, that is a result of the hardware in the motor. It does however affect critical or total compression.
As a rough guide a 10:1 motor static with 7psi of boost will run at 15:1. This of course does not factor in valve overlap and few other things, but as a rough guide.
static compression is at atmospheric pressure = 14.7psi approx. If you boost by 7psi you operate at plus 50% of atmospheric ie 10:1 plus 5:1 = 15:1
The other thing to consider is that the higher the initial pressure the more "boost" it takes to increase it and eventually you get to a point where you cant efficiently make more power. There is a limit in other words. The higher the start compression the easier it is to make a big dynamic compression, so its better to start with a 10:1 and boost to 15 than to start with 8 and boost to 15. Static compression is also very cheap to make by comparison.


There are more things to consider but this is the simple way of thinking about it. In real life a normally aspirated 10:1 motor runs at around 11.5 to 12.5 anyway as you eliminate the valve overlap and increase the air speeds etc.

T/C and centri S/C dont really make boost at idle (at least not much).
In theory.. a S/C is a fixed multiple of atmosperic pressure as it is crank driven, while a T/C is an exponential booster in that when it add x cfm to the intake there is also x increase in exhaust. This then drives the T/C harder and it makes more pressure resulting in more exhaust etc.

There are lots of other factors to consider and generally speaking I doubt I can be relied on for anything (at least thats what the missus says)



<font size=-1>[ This Message was edited by: tony.k on 1/7/02 10:18am ]</font>
 

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Jeff ... To make sugnifficant use of a Supercharger you pretty much have to begin with a much lower compression ratio than you currently have in the Mach

If you are starting out at very high compression you won't be able to add much "boost" due to the fact that the engine can only withstand "so much" cylinder pressure. At these high static compression ratios you may only be able to utilize a pound of two of boost (hardly worth the $3,000.00 you put into the S/C system.

If you build the engine from the outset with S/C on mind, you shoot for maybe 7.5:1 up to 8.5:1 compression ... then when you add the "boost" you have "room to work with" before you crack the engine to pieces from excessive cylinder pressure.

Note: When we talk about "compression ratio" we are almost always talking about "Static Ratio" which is the calculation of when "total cylinder volume" is SMOOSHED down into "chamber volume" ... "Cylinder Pressure" is affected by several other things and adding "Boost" is a biggy.
 
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