[suprafly1986]

Dumb question: Looking for a yes/no and a why....

Will running 14 PSI on a large turbo (such as an SP61) be more effective than 14 PSI through a CT-26. I've heard yes but why so? The manifold pressure will be the same, right?

 

[jznatruck]

Short Answer; Yes.

The larger turbo, like the one talked about in your hypothetical situation would push MORE air at the same pressure. While the smaller, CT-26 turbo would push less obviously. The more air the turbo pushes the more power it will make. The larger turbo is going to pus MORE air at the same PSI, thus making more power. Basically, PSI isn't a meassure of how much air is going into your engine.

 

[Trent]

Short Answer; Yes.

The larger turbo, like the one talked about in your hypothetical situation would push MORE air at the same pressure. While the smaller, CT-26 turbo would push less obviously. The more air the turbo pushes the more power it will make. The larger turbo is going to pus MORE air at the same PSI, thus making more power. Basically, PSI isn't a meassure of how much air is going into your engine.

I agree and disagree with this statement. PSI is just a measure of the pressure of the air. It's not a measure of the VOLUME of the air. That's where the turbos differ. Both turbos can be spinning at 16psi, but one will make substantially more power based on its size.

Now what I don't agree with is going with a big turbo and running it on that low of boost. If you have ever looked at a compressor map, you will notice that every turbo has its "sweet spot". That is where the turbo is operating at its best and being the most effecient.

Yes it is better in that you will make more power at the same PSI with a bigger turbo, but at the expense of a higher cost investment, possibly more lag, and a turbo operating below it's "sweet spot".

What I'd do is decide on the power you want to make. Once you know what power you want to make, find a turbo that will make that power on pump gas at 18-20psi. That's where most big turbos wake up.

 

[jznatruck]

I agree and disagree with this statement. PSI is just a measure of the pressure of the air. It's not a measure of the VOLUME of the air. That's where the turbos differ. Both turbos can be spinning at 16psi, but one will make substantially more power based on its size.

Now what I don't agree with is going with a big turbo and running it on that low of boost. If you have ever looked at a compressor map, you will notice that every turbo has its "sweet spot". That is where the turbo is operating at its best and being the most effecient.

Yes it is better in that you will make more power at the same PSI with a bigger turbo, but at the expense of a higher cost investment, possibly more lag, and a turbo operating below it's "sweet spot".

What I'd do is decide on the power you want to make. Once you know what power you want to make, find a turbo that will make that power on pump gas at 18-20psi. That's where most big turbos wake up.

I'm speaking more of AIR/TEMPATURE ratio, not factoring in lag or any other sorts. The larger turbo is going to be more effecient at X psi then a smaller one, becuase it will be producing less heat. Less heat, less prone to detonation. That's the point I was making, but I completly understand what your saying. Thanks,
Matt

 

[Racefiend]

A larger turbo doesn't necessarily mean you'll get more performance at a given pressure, like 14psi. It all depends on the compressor map. With the SP61, 14psi is closer to the sweet spot on the compressor map than it is with the CT-26 (or so I've heard from everyone, I personally havent seen the maps myself). If you were looking at a larger turbocharger, it is possible that 14psi is far enough away on the map that you would actually lose performance over using a ct-26. Like I said, depends on the maps.

As far as the higher efficiency meaning it pushes more air volume , that is wrong. The volume that the engine can ingest is a constant, and is calculated by the VE and displacement, and there is only so much volume in your IC, pipes, etc. What being more efficient means is that the compressor can compress the air at a given volume and pressure more efficiently, as in it creates less heat during the process. Less heat means more air mass (ie density), whcih equals more oxygen molecules, therefor more fuel and more power. To see this lets look at the ideal gas law:

PV=nRT

P=pressure, V=volume, n=moles (mass), R=constant value (doesn't apply to this discussion), and T=Tempurature.

Since the pressure is a constant (14psi, well technically 28.7 because we use absolute pressure), and the volume is a constant, and R is a constant, then all we are concerned with here is n and T.

In the equation, if T is reduced (due to higher efficiency) then n must increase. So the mass of the air increases. And since air density = n/V, so does the air density. This is why you make more power with a more efficient turbo.

 

[suprafly1986]

Thanks guys. We will probably be running a tad more boost through it, now that we have fuel and SAFC, along with IC pipes and a 3" elbow/exhaust. I know it won't be as efficient until we get rid of the stock manifold but it should help.

 

[animantoric]

Now the one thing you guys were forgetting is the backpressure. With bigger turbos you generally have bigger turbines and generally less backpressure at a given flow. This definately adds to the efficiency/hp at a given psi. I can't quantify how much, but then again, I don't have lots o turbos and test equipment to verify...

[Edit] Something else I just saw in the faqs: (as written by Franzen)

This is why larger exhausts make such big gains on nearly all stock turbo cars-- the turbine compounds the downstream backpressure via its expansion ratio. This is also why bigger turbos make more power at a given boost level-- they improve engine VE by operating at lower turbine expansion ratios for a given boost level.

Original post

 

[figgie]

remember

pressure is the amount of air NOT being fed into the motor. Or we can call it resistance

turbos FLOW