What does supercharging do to compression ratio


AZGT

AZGT

GT Owner
Mark IV Lifetime
Dec 20, 2005
1,354
Scottsdale, AZ.
Was just wondering, since the GT is a rather low compression engine, what is the relative effect on the compression ratio when the supercharger is engaged? Say it is at 12 pounds of boost, what does the effective compression ratio become? Is there a standard way to compute that?

Seems like some of you guys are pretty smart and could give me a little clarification just for my own knowledge building.

Thanks
 
AZGT said:
Was just wondering, since the GT is a rather low compression engine, what is the relative effect on the compression ratio when the supercharger is engaged? Say it is at 12 pounds of boost, what does the effective compression ratio become? Is there a standard way to compute that?

Seems like some of you guys are pretty smart and could give me a little clarification just for my own knowledge building.

Thanks
Click to expand...

compression ratio is unaffected......cylinder pressure might be what you are after.
 
Effective CR

AZGT said:
Was just wondering, since the GT is a rather low compression engine, what is the relative effect on the compression ratio when the supercharger is engaged? Say it is at 12 pounds of boost, what does the effective compression ratio become? Is there a standard way to compute that?

Seems like some of you guys are pretty smart and could give me a little clarification just for my own knowledge building.

Thanks
Click to expand...
Try this,

They did my supercharger for my '23 Ford T:

http://www.blowerdriveservice.com/techcharts.php

Jay
 
satx said:
compression ratio is unaffected......cylinder pressure might be what you are after.
Click to expand...

Or he could be asking what the effective compression ratio would be, relative to an NA motor with the same cylinder pressure. In other words, what would a NA engine's compression ratio be to have a cylinder pressue= The amount of cylinder pressure a ford GT has at 12 psi.

Jon
 
analogdesigner said:
Try this,

They did my supercharger for my '23 Ford T:

http://www.blowerdriveservice.com/techcharts.php

Jay
Click to expand...

according to this chart the compression ratio of the GT's engine is 15.4:1 under 12 psi of boost :eek
 
A formula for effective compression ratio comes about from the concept of mean-effective pressure, (which is optimistic in how power is scaled with comp ratio). The typical equation used is,

CReff = CRstatic*(1 + Pb/Patm)

where CRstatic is the static compression ratio, Pboost and Patm are the boost and atmospheric pressures, respectively. So for the Ford GT with CRstatic = 8.4 and Pb = 12 psi,

CReff = 8.4*(1 + 12/14.7) = 15.26
 
Thanks

That answers the question. The idea of the 15.2 - 15.4. I was just trying to get a number in my head for a little understanding.
 
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Pressure is pressure.

The blower increases pressure by pulley size and speed. Turbos increase pressure by speed of the hot side(exhast) of turbo which spins cold side of turbo which is a type of compressor. Both by size and design also. Both blowers and turbos are compressors. As is a piston and cylinder and rod.

Compression ratio, is the set up of the cylinders/rods/and head size. This is a set unchanging factor. The others are increased as speed or RPM increases.

The GT isn't a low compresson motor. Old blower motors had only 8cr. The blower had to work harder to get lots of pressure(HP). It's lower than say a 14 to 1 motor because if a 14 to 1 motor had a blower it would reach critical mass and blow up. It's already at maximum pressure.

The advantage of turbos and blowers is: the motor can "rest" at low and moderate RPMs, turbos more than blowers. Real high compression motors are always working hard(wearing out), even at low RPMs. The compression is always high.

The more compression the more expantion, more compression=more oxegen. More oxegen means more feul can be added. More feul and more oxegen means more HP. Add fuel and spark to lots of compression and WOW.

Allan :biggrin

PS That's a fact Jack.
 
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Pipelion said:
Pressure is pressure.
The more compression the more expantion, more compression=more oxegen. More oxegen means more feul can be added. More feul and more oxegen means more HP. Add fuel and spark to lots of compression and WOW.

Allan :biggrin

PS That's a fact Jack.
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Toooo much WOW=KABOOM! LOL!
 
You got me bud :biggrin :biggrin ,

Allan
 
Yea but you gotta admit its a fuuuuuuuuuun ride to get to BOOM on high compression. I have some background in Honda motors and we get compression up to 15.1 on race gas reeving up to 11 to 13k rpms. On my bikes that Ive had most are up to 12,500k rpms but my gsxr 750 went all the way up to 15,500 k rpms and that was fuuuuuuuuuuun on my little cruises around the city.
 
Yea,

and Formula 1 motors turn 19,500 RPMs . Try it :biggrin :biggrin :biggrin , at least once.

Allan
 
Actually, if we’re talking about compression within the cylinder, more compression ≠ more air. Within the cylinder, more compression = higher thermal-conversion efficiency, (which does also increase power). Look at it this way. While the intake valves are open, a given mass of air goes into the cylinders, (which depends on their volume, the pressure at the intake port, valve timing, blah blah blah). Except for blow by, after you close the valves, if you now increase the amount that air is compressed, it doesn’t change the amount of air at all. (This is just conservation of mass.) If anything, the higher residual mass fraction that results when the compression ratio is increased actually reduces the amount of fresh charge that can enter the cylinder, (i.e., a decrease in VE). The compression by the blower definitely increases the amount of air that enters the cylinders, for sure!

This is why I tend not to use the concept of an effective compression ratio…because it combines two completely different mechanisms. I prefer, if anything, to use the concept of an effective displacement, which has a very similar looking equation to the one posted earlier for CReff. But I know we all have our preferred ways of looking at things, and that’s cool. Hope this helps! :thumbsup
 
I'm sorry, confusion is now leaking from my feeble cranium.

Please clarify. I thought that more boost = more oxygen and if mixed with the proper amount of fuel makes more power. And, usually, to a point, higher compression makes for better VE= increased power. What did I miss?
 
Nah. You pretty much have it. Spinning the blower faster (increasing boost pressure) forces in more air, (which also means more fuel as given by the AFR), and results in more power. Increasing the compression ratio increases the thermal-conversion efficiency, (not the amount of air and fuel), which also increases power.
 
And, superchargers and turbos both force more air down the throat of the engine, but turbos don't rob the engine of hp to turn blower.
 
californiacuda said:
And, superchargers and turbos both force more air down the throat of the engine, but turbos don't rob the engine of hp to turn blower.
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But also typically do not have the low end "grunt" torque that the sc'er can produce........and they are huge heat radiating units.

Takes care

Shadowman
 
It is a myth that turbos don't rob the engine of power. Compressing air takes HP and it cannot be done for free! Turbo can be more efficient than blowers for the same boost levels, but it takes power away from the engine by increasing exhaust backpressure. Just think of the pistons having to push harder on the exhaust stoke to get the combusted gas out of the cylinder, that takes power.

BlackICE
 
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Positive displacement(roots or screw type) superchargers make massive torque in low rpm, but they usually can't keep going the way that turbos can. Centrifugals act more like turbos.

Very few things in life are free. Turbos do increase backpressue and in fact the amount of backpressure is an indication of a turbos effeciency and restrictiveness. More backpressure, usually, faster spool up and less top end.

In the book by Richard Holdner, BUILDING 4.6/5.4L FORD HORSEPOWER ON THE DYNO, page 161. Comparison of Eaton supercharger on a 4.6 Cobra engine at 11 psi=572 hp, twin turbo 11psi=750 hp. Difference of 183 hp. So, backpressure can affect performance, but if done right it is a pretty good trade off.

Oh, at 2500 rpm the eaton made 475lb tq and the turbos 350. The hp and tq of the sc compared to the turbos become equal at about 3700 rpm,
 
californiacuda said:
Positive displacement(roots or screw type) superchargers make massive torque in low rpm, but they usually can't keep going the way that turbos can. Centrifugals act more like turbos.

Very few things in life are free. Turbos do increase backpressue and in fact the amount of backpressure is an indication of a turbos effeciency and restrictiveness. More backpressure, usually, faster spool up and less top end.

In the book by Richard Holdner, BUILDING 4.6/5.4L FORD HORSEPOWER ON THE DYNO, page 161. Comparison of Eaton supercharger on a 4.6 Cobra engine at 11 psi=572 hp, twin turbo 11psi=750 hp. Difference of 183 hp. So, backpressure can affect performance, but if done right it is a pretty good trade off.

Oh, at 2500 rpm the eaton made 475lb tq and the turbos 350. The hp and tq of the sc compared to the turbos become equal at about 3700 rpm,
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You are very right

Good stuff; this is why turbo's typically work better on higher spinning motors whereas on a motor limited to 6500 RPM the superchargers work very well.

Heat and pressure = higher spooling turbo's the trick is to have one small enough to spool down low and not run out of breath on the top end....

Once again; good information

Thank you

Shadowman
 
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