compression+boost?

[FreshFRS]

Originally Posted by serialk11r

Whoa whoa whoa, you have ethanol free 94? Blend it up to 10% (or even 20%) ethanol and get some ~96!

yeah we have ethanol free 94. i prefer not to have ethanol in my gas. unless i'm running boost and i'd rather run 30% toluene and get ~102 octane.

[carbonBLUE]

Originally Posted by subatoy

I've been wondering this for a while now so I would like to hear an answer from the experts.
It has been said many times that high compression ratios are not good for boost but when you lower the compression then you have to run more boost to achieve the same power so whats the advantage of a lower compression?

Lets say you have the a 2.0 engine with 12.5:1 compression ratio and you run 5 psi and you achieve 250whp.
Then you take the same engine an lower the compression ratio to lets say 9.5:1 but you now have to run 10-12 psi to achiave the same 250 whp.

It seems like the strain you are causing to the engine is still the same so why not just stay with the high compression with lower boost?

all the numbers I used ARE NOT accurate i'm just using an example.

at sea level

air is at 1 bar at sea level = 14.5 psi

+5 psi = 20.5 psi + 12.5:1 compression = 256.25 psi after compression

256.25 / 10.5:1 compression = 24.4 psi - 14.5 (1 bar) = 9.9 psi

so.... at 12.5:1 compression you would need to run 9.9, so pretty much twice the psi to make the same power BUT heres the other factor, when you lower compression you INCREASE the amount of volume you can compress in the cylinder, = more air = more fuel can be used = more power

so with the same gas
12.5:1 + 5psi of boost = 250whp
10.5:1 + 8.5 psi = 250whp because you can use more gas

another advantage of using lower compression is there is less heat involved in the actual compressing processes, which means cooler combustion chamber temps before the spark...

[subatoy]

Originally Posted by carbonBLUE

at sea level

air is at 1 bar at sea level = 14.5 psi

+5 psi = 20.5 psi + 12.5:1 compression = 256.25 psi after compression

256.25 / 10.5:1 compression = 24.4 psi - 14.5 (1 bar) = 9.9 psi

so.... at 12.5:1 compression you would need to run 9.9, so pretty much twice the psi to make the same power BUT heres the other factor, when you lower compression you INCREASE the amount of volume you can compress in the cylinder, = more air = more fuel can be used = more power

so with the same gas
12.5:1 + 5psi of boost = 250whp
10.5:1 + 8.5 psi = 250whp because you can use more gas

another advantage of using lower compression is there is less heat involved in the actual compressing processes, which means cooler combustion chamber temps before the spark...

This is awesome but my concern is Torque, not whp, so my question is if there's a way to calculate/estimate torque based on compression ratio differences and what we know about this car.

[carbonBLUE]

Originally Posted by subatoy

This is awesome but my concern is Torque, not whp, so my question is if there's a way to calculate/estimate torque based on compression ratio differences and what we know about this car.

I'm not sure, you would have more tq but not by much, best way to get tq is modifying bore and stroke

If it were up to me, i would increase bore and stroke but keep compression

[gmookher]

try a different r&p ... for high TQ early on in the power band you want a turbo

Im not sure why you would want that type of power curve

[ngabdala]

Driving > 4500 rpm into redline is the best feeling ever.

[subatoy]

Originally Posted by gmookher

try a different r&p ... for high TQ early on in the power band you want a turbo

Im not sure why you would want that type of power curve

usable real world power, I don't want to need to down shift 2-3 gears just to get ahead on the highway.
I come from a WRX and GTI background which both have great instant torque, I also owned an integra GSR and 370z and I just couldn't live with them.

[gmookher]

Originally Posted by ngabdala

Driving > 4500 rpm into redline is the best feeling ever.

This is why I am supercharging!

[cf6mech]

Seems silly to me to dwell on torque from a four cylinder that lives for high revs to make HP,..... but my prior cars were 07 Shelby Mustang and then a Boss 302,.....

[carbonBLUE]

Originally Posted by cf6mech

Seems silly to me to dwell on torque from a four cylinder that lives for high revs to make HP,..... but my prior cars were 07 Shelby Mustang and then a Boss 302,.....

Well tq wins races, but weighing 1000 lbs less you only need 3/4 ths of the tq to create the same propulsion, so a turbo kit with 300 tq is just as good as a car 1/4 th more heavy that has 400 tq

[ngabdala]

+100000000000

Originally Posted by cf6mech

Seems silly to me to dwell on torque from a four cylinder that lives for high revs to make HP,..... but my prior cars were 07 Shelby Mustang and then a Boss 302,.....

[serialk11r]

Originally Posted by subatoy

usable real world power, I don't want to need to down shift 2-3 gears just to get ahead on the highway.
I come from a WRX and GTI background which both have great instant torque, I also owned an integra GSR and 370z and I just couldn't live with them.

Then you need a positive displacement blower. A low end focused turbo sucks balls IMO, you don't have high end power, efficiency is not great, and response can never be as good as a supercharger. Positive displacement blower can be thought of as roughly having the effect of adding more displacement.

Remember boost pressure only means something in the context of temperature. What matters is intake air density and quantity. Torque is roughly how much air mass the engine takes in per revolution, ignoring efficiency changes. 1 bar boost, room temperature is double the air, and thus double the torque, approximately.

As far as efficiency goes, at higher charge density, the engine sees more friction but typically less as a proportion of power, more energy lost to exhaust as a proportion of power, more energy lost to coolant as a proportion of power, more efficient combustion, possible lost efficiency due to changes in tuning needed to accomodate the boost, and various other factors. The balance between these determines how efficiency changes.

[cf6mech]

I love torque,....just being realistic about the capabilities of this engine and concentrating on it's attributes and not its inherant weaknesses.

[cf6mech]

Originally Posted by carbonBLUE

Well tq wins races,

Consider Formula One engines,....light cars granted,.....but so is the FT86 in the scheme of things. F1 engines develop very little torque but high horsepower.