Ok I understand that a larger turbo will likely produce a cooler charge air because for the high'ish boost we all run it will generally be more efficient at producing that boost pressure. And also that a cooler charge means it is more dense which means more oxygen for the combustion process.

But what I'm wondering is, all things being equal, we have a fixed space the charge air flows through (IC pipes, IC, TB, intake manifold). And given identical boost pressures... will a larger turbo likely flow more air than a smaller turbo?

I'm assuming it must just because I see a lot of great results out there for people running stock turbo like boost pressures (22 psi) and making a lot of power 500+ at the crank.

However, I'm also thinking an engine has a certain VE which in my simple thinking is an expression of how well it breathes. And it is this VE which would dictate how much charge air the engine can consume. So larger or smaller turbo flowing more or less is sort of irrelevant provided the turbo can produce the target boost pressure.

I'm rambling! I look forward to your replies.

 

Yes, a larger turbo will flow more air at the same psi as long as that boost level is within the larger turbo's efficiency range. You can actually be TOO low. A GT35R will make way more power at 22psi than a stock 16G, but probably not at 15psi.

 

Exactly what I was going to say. Actually if the turbo is to big it might make less power.

 

The turbo affects the engine's VE. A larger turbo will generate less backpressure at a given boost than a smaller turbo. The reduced backpressure improves the engine's VE.

Improved VE = more flow through engine = more power.

This is why larger turbos "flow more" at a given boost.

 

Yes thank you! I was actually reading some tech docs at Garrett and realized just now as well, back pressure is so important yet is something I've hardly ever given any real though to.

Back pressure also answers another question that I always had in my mind. I always knew a small turbo like a 16g loses efficiency at high boost levels and also starts producing a lot of heat. But I never thought why... but again, back pressure is the answer to this question.

 

you forget that the turbos are not under the constant volume constraint... they are the variable and being such they move more air internally...

if you have a 1 gallon can with air in it... and you have a 2 gallon can with air in it... the 2 gallon can has more air. but they're both at atmospheric pressure 14.7 psi or whatever.

https://www.evolutionm.net/forums/sh...orque+dropping

 

But isn't the volume constraight the intake manifold, i/c piping, etc?

So all things being equal, and you bolt up a bigger turbo... the only gains you make are a possibly cooler charge, and an engine that breathes better due to less back pressure?

It's interesting to me because you take a stock'ish evo at 23 psi, and let us say it makes 360 hp at the flywheel. Then you bolt on a GT35R, and run 23 psi. Theoretically the only reason the 35R makes more power is because of the possibly cooler charge and increased VE from less back pressure.

What type of gain would you expect to see from the above? Could you really see in excess of 100 hp more just from the above change?

 

Yes it does.

If you have a 1" diameter pipe, and a 2" diameter pipe, both flowing air at the same pressure, which is going to be pushing out more air in terms of CFM?

The same rule applies to turbos. You can essentially push more air at the same given pressure without compressing it (heating it up) and going out of the turbos efficiency.

Tis why you will see much flatter torque curves at elevated boost pressures with larger units

 

Ok I re-read the first couple posts in the thread you linked to Triny. Thanks for that thread by the way it helped me understand some things.

I guess another answer to my question is, that as the larger turbo increases VE because of back pressure, intake manifold air speeds increase even at same boost pressures and same fixed manifold/TB volume size.

Maybe what you are saying to me in your post above, is that if you think of the system as whole, and of my question in changing turbos. The turbo becomes the variable in the volume constaint in the system as a whole?

 

no... this is not right.

what moves more air... a big fan or a small fan? you get more wind from the big fan not because of increased ve cuz you're blowing into open air, there is no effective ve. you move more air cuz the blades are bigger and they swat more air molecules per revolution. this moves the air, giong bigger moves just more of it.

 

Wow I'm confused

Ok so all things being equal, moving to a larger turbo does the following:

- decreases back-pressure
- increases air flow as measured in the throttle body for example
- decreases charge temp in some cases

I can sort of understand all of this... except if I think for example of a hose with water flowing through it. The water is being pumped by a turbine. The outlet of the hose has a small tip which provides resistance and therefore pressure builds in the hose.

If you swap a larger pump supplying the water for this hose, and keep hose pressure the same, I'm just wondering if you see more flow out of the hose end?

 

Thinking of a turbo as 'pushing air' is potentially confusing. That isn't what's really happening, simply because there is always pressure pushing in the other direction on the exhaust side. Separate the concepts of pressure and volume in your mind, as they are independent.

The turbo is increasing air mass by increasing air density. The engine must still draw air into the cylinders as it does in normally aspirated configuration. The volume of the engine, pipes, etc., remains static. The air mass ingested increases simply because the turbo increases the air pressure, not volume. The only areas that see increased volume are depressurized areas (e.g. the intake tube and exhaust system).

The VE of the engine is indirectly affected by backpressure, which is in turn affected by the turbine size (and other variables).

The compressor size (and efficiency) largely determines the base temp of the compressed air charge. The higher the base temp, the more we are dependent upon the IC to dissipate the heat.

Indicated manifold pressure doesn't tell us anything about VE. A MAF signal is a much better source of information. If we have no MAF, we can monitor pressure in both intake and exhaust manifolds to get a better idea of what's really happening.

 

No.

 

Agreed, with this example the back pressure is supplied by the small hose itself. In terms of a turbo, the back pressure I speak of is from the turbo/turbo housing itself.

And yes, trindex is correct as well in his small fan vs large fan analogy. You'd need to spin a smaller fan much faster than a larger fan to produce an equal amount of flow, in general, if something is spinning very fast, its efficiency/duty cycle will wain, which then degrades performance

Turbo systems are not trivial, so to come up with a simple explanation of these things is not always easy.

 

Thanks for walking me though this guys.

I think I've absorbed most of it.