ls1290

iTrader: (20)

Ummmmm.... on paper yes, in the real world no.

quicksilverado

iTrader: (8)

Apples to oranges. The engine sizes are different.

cablebandit

iTrader: (37)

exactly, just like the turbos are different which is why 15psi doesnt equal 15psi.
But if you remember Dave I did run the bigger incons on my 408....21psi made 820....can make that now on 10 psi

MY99TAWS6

iTrader: (3)

Turbo potential power is best measured in air flow in pounds per minute usually. So a 50lb /min turbo can pretty much do around 500 engine hp. X10 is the factor. On a 2 litre it might require 30psi to hit 500 hp and 50lbs per minute. On a v8 it might require 15psi.

This is main reason people go to bigger turbos to get more potential power. Now its still meaningfull to talk about going up on boost on the same turbo and the same engine.
30psi will put out more power generally than 15psi on same engine if the turbo can boost that high and stay effiicent and all that other stuff.
Also better you get the engine flowing with heads, cams,exhaust, easier to make same power at lower boost. So on a great flowing or bigger engine 15psi might give that 500 hp but it miight take 20psi on the not as big or not as good flowing engine.

Dewey

iTrader: (1)

what he's saying is that "the more air and gas you have, the more power you get." bigger turbo = more air = more power. That better?

engineermike

No, because if 2 engines flow the same mass of air, the one with lower exhaust pressure will make more power - possibly MUCH more power.

A small turbo and a large turbo will both flow the same lb/min of air through any given engine (unless the small turbo is at sonic velocity in the inducer - or choke flow), but the small turbo will impose much more exhaust pressure on the engine.

MY99TAWS6

iTrader: (3)

So are you saying the lb/min flow rating of the turbo is the choke flow point or whatever you called it?

Obivously turbos are sized in terms of airflow with bigger ones giving more max hp on the same engine. I guess a really poor flowing engine might have trouble reaching the max potential airflow of the turbo. Maybe we are saying the same thing just in slightly different ways??

All I know is my big turbo on my 2.4 litre 97 talon puts out way more power at 15psi than my stock turbo at 15psi. And of course the two turbos have very different airflow specs and of course maps. Of course generally going bigger carries with it the penalty of slower spool up but the advantage of more top end power and more potential max power. And you can of course play with ar ratios which will affect spool up and top end and let you play with your possible powerband of course.

engineermike

I bet a dollar the backpressure is way lower on the big turbo, which is where most of the gain comes from.

Black01M6SS

iTrader: (3)

I think most of the info has been covered but is split up in the replies. I'll try to put it all together in simple terms.

What you should be concerned with is how much CFM of air you are flowing threw the engine. It may take 30psi to flow X amount of air threw a 4cyl and 15psi to flow the SAME amount of air threw a V8. Since you are flowing the same amount of air threw the two engines they should make about the same horsepower. Boost numbers are only relative when the engines and mods are the same.

That said you still could see a difference in horsepower between two turbo's at 15psi on the same engine. An efficient turbo flowing 15psi of 100*F air with minimal back pressure should make a good bit more horsepower than 15psi of 150*F air with high back pressure from a turbo that is outside of its efficiency range. As you can see, there is a lot more to think about than just the PSI in the intake.

98Camarod

iTrader: (43)

I think he might have figured it out 2.5 years ago

Silverado_13

iTrader: (5)

Black01M6SS

iTrader: (3)

He might have but what about the other people that have posted recently? The fact that this thread has come back from the dead suggest to me that the information was not clear enough for someone. I know the mods frown on bringing old post back from the dead but if you have something good to contribute to the post it only makes the search feature more useful for people who choose to use it.

98Camarod

iTrader: (43)

Search has to work first

z28adiction

iTrader: (1)

Even though your turbo is at 15 pounds, the wheel is very small, and cant move very much air compared to lets say an s88. both are pushing only 15 pounds of pressure into the intake, but the bigger wheel in the s88 will flow much more air at that 15 pounds compared to the little guy, due to its much bigger surface area.
Take a small and big fan put them both on high on a hot day next to each other. which one are you going to sit in front of. lol

engineermike

It doesn't work that way. For starters, consider that the little one will be spinning much faster than the big one to make the same psi. So, they will move roughly the same amount of air at 15 psi dP. Check out the fan laws. A small compressor will flow as much as a large one if you spin it faster. And (before you even bring it up), just because a small one is spinning faster doesn't necessarily mean its less efficient. The flow rate at 15 psi is determined by the engine.

I think you guys are failing to recognize that exhaust pressure is what it all boils down to.

Efficient compressor + large turbine = low exhaust pressure = more hp

Forget about flow, unless you're talking about choke flow in the compressor inducer.

Onemeanbird

Let me get this straight you and the guy that posted right above me are saying that on the same engine a smaller turbo pushing 15lbs of boost is not moving as much air as a bigger turbo pushing 15lbs on the same engine.

z28adiction

iTrader: (1)

" if you spin a wheel faster you will make more pounds of boost therefore, we are not talking about 15lbs to 15lbs As stated above. if a car is put together and there is too much exhaust back pressure in reference to the size turbo (s) used. the size of engine and what the heads/ intake and cam can flow both intake and exhaust. it will kill it in power. Also as everyone knows each turbo has a flow map of where it is most efficient in its operation as well.

ALTRDTA

iTrader: (69)

Look at it this way for those of you that don't understand. Boost is nothing but a measurement of restriction. The smaller turbo flows X amount of air through the turbine (hotside of the turbo) before it becomes inefficient . You take a bigger turbo, that puts the airflow up cause the compressor wheel is bigger and so is the turbine wheel which makes less restriction through the engine causing it to need less boost to make more power... It's really hard to explain how it works unless you can visualize it...

BTW. I know this thread was started 3 years ago. but obviously people don't understand the philosophy still....

crazymofo1986

iTrader: (3)

sorry i dont know how i dug it up. but theres good info in here.