Boost pressure vs air flow vs power
 

For the longest time ever I didn't quite understand why different turbos at same boost pressure produce different power.

Well, they don't. As long as engine restriction (and hence volumetric efficiency) doesn't change, same boost will make same power.

Boost pressure is a function of air flow vs restriction of the engine's intake path. The more restriction, the higher the pressure differential (boost pressure) needs to be to overcome the restriction.

The turbo's compressor is part of the intake path, and it has limits too. A T25, for example, maxes out at about 25lb/min. So if you put a T25 turbo on a V8 that flows 500cfm (~35lb/min), then you're effectively putting a fancy restrictor plate, since a T25 only flows 25lb/min.

Another simplistic example:
You have an engine/intake/turbo that can flow 30lb/min @ 20psi. If you port your head to flow better, you can now flow 30lb/min @ 16psi. If your turbo can flow 30lb/min @ 16psi, then you will make same power at 16psi as you could at 20 before.


So to recap:
The amount of power you make at a given PSI is the intersection point of your engine's intake restriction and your turbo's compressor map.

A really big turbo will not make 300whp @ 10psi unless your valves are size of hustler's anus.

Swapping a bigger turbo will make the intake path less restrictive so you will make a bit more power at same psi. Head porting will make the intake path less restrictive so you will, again, make more power on same psi.

A bigger turbo DOES NOT make more power at same psi simply because it "flows more". It flows more, if the restriction is less. If the restriction doesn't change, a bigger turbo will not make more power

not sure if trolling

I'm hoping it's copypasta from Miata.net.

I'm terrible at explaining my thoughts but no, not trolling.

bigger turbo doesn't automagically mean you make more power at same boost. A little bit, yes, because the turbo itself is less restrictive, but not 3x power at same boost.

Depends on the motor, and depends on the turbo, but possibru.

As long as we're doing copypasta:

In very basic terms, a "bigger" turbo will move more airflow with less heat generated during the compression stage. This is due to the compressor wheel being sized to move the appropriate amount of air within the "happy" compressor RPM range.

And, more importantly, once this air is combusted and turned into exhaust, a turbo with a "bigger" turbine wheel will spin at an appropriate RPM and will induce less power-robbing exhaust backpressure. The less backpressure on the exhaust side, the more air that can be pumped through the engine at any given boost pressure.

So you add up the effects of a cooler air charge with less back pressure and you can see why different turbos can make different power at different power levels.

It also explains why comparing turbos by comparing only compressor maps is meaningless. The turbine size/configuration is arguably more important.

Notice I put "bigger" in quotes because bigger isn't always better when talking about turbo tech/aero.

I agree and all I'm saying is unless you change the engine, the extra airflow will result in higher boost pressure - because boost pressure is a function of the engine's restriction.

That doesn't make sense either. When you put it like that, you make it sound like a GT40R would be forced to make more pressure than a 2554R simply because it's capable of more flow. This negates the point you made in your first post, which was negated immediately after by the 2nd point you made in your first post.

Thats exactly what I mean. Don't see how the original post negates anything. Want to quote the exact parts?

Sure, but it's not correct. It's CAPABLE of more pressure, but it's not being FORCED to create more pressure.


I missed a disclaimer in the first statement i was looking at. What's weird to me is that you're making a few true statements, then throwing a somewhat strange conclusion out of it.

Sky is blue, grass is green, therefor the moon is made of Swiss.

is that why your big turbo makes so little power?

No, its because I stuff 30psi of boost in a bone stock head @ 25* timing.

This is pretty much accurate I think.

Too often you hear claims that 5 psi on a 28r is "a lot" less power than 5 psi on a 35r, or something along those lines. Its just not true.
It comes from the understanding that at higher boost levels a larger turbo will make more power than a smaller turbo that is getting inefficient and has a small exhaust housing. This knowledge gets turned into a blanket statement thats used to explain to noobs that theyre stupid for asking "how many boosts psi can i haz on my stock native american?"

I don't think he is saying the opposite, just stating it the other way around.

He's not saying it the other way around, just backwards.

I have been thinking the same thing that Soviet is saying, IF we are talking about the MAP. If talking about the boost at the turbine outlet, then truely, there is no correlation between boost and power as there are so many restriction variables involved. But, in the manifold, there is pretty much just valves, turbine, and exhaust.

Yes, reversed cause and effect.

...didn't we already established in this thread that pressure is a measure of restriction? Oh, wait no.

The size of the valves being the same, the larger turbo WILL make more power per psi than the smaller.

If we are saying that a T25 can only push 25 lb/min of air at 30psi, but your large turbo can push 50 lb/min of air at 30psi;

then at 30psi, your turbo is compressing a lot more air, into the same space, than a t25 would. So at the same pressure, you have almost twice the available oxygen to burn and that right there should equal tons more torque.

The valves, rpms, cams only determine the mass rate of air--you're always displacing the same amount. But it's your turbo and the amount of air it compress that determines how many air molecules you can actually stuff into the cylinder at this rate.

All boost pressure is telling you is that you're stacking up a shit ton of air behind the valves/piston, but it cannot tell you how much air you're actually compressing; and it's not a function of the engine's restrictions. That would like be saying anytime you add pressure to a sealed vessel it's infinite pressure because the vessel has no openings.

https://www.miataturbo.net/attachmen...ine=1402341029

Theoretically, I think it would be like this for 2 different sized turbos at the same boost with similar efficiency at that boost level.

If the dyno were stopped at 6k rpm, the smaller turbo never becomes a restriction, and they make the same amount of power on the same amount of boost. But when the flow starts to move the small turbo off its efficiency island and its turbine housing becomes restrictive (6k+ rpm in this graph), the bigger turbo is making more power comparatively.
At 8k rpm, there is the same amount of boost in the IM, but the BMEP is lower for the smaller turbo because the air temps are higher and there is less exhaust scavenging due to high pressures in the exhaust manifold.

yes, heat efficiency is a big factor. i was trying to leave it out of it. obviously, the more colderer the charge, the more airer the boost. do not leave the island.

Above is also why I always say get the smallest turbo for the jarb.