Max turbo output: psi vs. flow potential

[sutyak]

I need some turbo theory clarification, because I've apparently missed something.
First, the max flow potential of a turbo is defined by the cross-section at the compressor inlet. Yes/no?
Looking at a compressor map, this max flow is the "choke line" on the right side, sweeping upward.

In comparing my dyno chart to the compressor map it appears the max HP is seen when the max turbo flow is met.
HP will drastically drop after this flow is met, as does torque.
It also appears the max torque is met within the map's "island" and will gradually drop off outside that island.
(If you see this like I do, your turbo is either too small or best-case this happens right at redline)

These things being noted, I've read where people get more HP from the same turbo by increasing the psi, even though the turbo's flow potential was already met at a lower psi.

My question is: doesn't this seem to violate the first rule that the max flow potential is defined by the compressor inlet?

So if a compressor wheel is rated to flow 400CFM, how does someone get 450CFM worth of power out of it? Internet BS? (I'm serious)
Aside from that, if the max flow is met at a lower psi, then increasing the psi would only result in the max flow being met at a lower RPM and dropping off sooner, correct?

I started with a 350CFM (max) IHI RHB52 turbo, which saw it's max flow at just 5,300 RPM.
Recently I sent the turbo off to upgrade the compressor wheel to a TD0H 16T wheel, which flows 440 CFM.
The turbine side has not been opened up yet.

With that upgrade and a better tune it gained 5mph in the 1/4, and moved the max flow to appx 6,000 RPM.
There is still room though as it's clear I didn't get all of that 440CFM. More like 390-400CFM.
I'll definitely open up that turbine outlet some to help out, but until then I had ideas floating around.

One idea was to increase the boost a bit. (already at 10psi)
Well if it already sees max flow then there's no point right?
That would really just move my powerband down, which I don't want to do.

This thought took me back to some reading where I came across a Volvo owner making a supposed 282whp with that 16T, whereas average numbers were 240whp, which is what 440CFM should result to.
So this is why I actually posted. How can one get that much more power from a turbo supposedly restricted to 240whp?
I'm not wanting to, I just want some clarification.

[billyNOTnice]

I wish i could help you out, but i've never actually dyno'd my car to even compare. I'm interested to see what you find out, please post your findings because i'm curious. do you have a dyno graph i could look at. I understand your point and am curious now.

[sutyak]

I wish i could help you out, but i've never actually dyno'd my car to even compare. I'm interested to see what you find out, please post your findings because i'm curious. do you have a dyno graph i could look at. I understand your point and am curious now.
Certainly. I will post later tonight. Heading out the door now.

[Martin0660]

Mark ... i'm on the way out door too, but will check back in here.

I do nat agree the limit is met by the inlet (although it's a consideration). My junk was a classic example with north of 300 HP on a Garrett 60 A/R T3.

Also a good read from my friends at TurboFord ...

http://www.turboford.net/ubb/ultimatebb.php?ubb=get_topic;f=4;t=000050;p=1

The HP limit math starts pretty good on page 2 ... remeber we are very head limited, so out math is much different.

[sutyak]

Bob, I digress about the max flow being dictated by the compressor inlet. Not sure where I read that but I can't find it so my brain might have mixed info at some point.
For this discussion I'll use a map from Garrett's site.
Looking at the "Choke Line" we can see the "maximum" flow of the turbo represented.
At least this is where the turbo is way out of it's efficiency range.
This is the area I want to talk about - the "post-choke" area.
Is there any usable power beyond this area?
Does it require super-cooling like meth injection to make use of it?
Can the turbo actually flow more air in this area, or is it in fact "choked" and can literally not flow anymore air, regardless how fast the turbine is spun?
http://www.turbobygarrett.com/turbobygarrett/images/tech_center/tech_103/enlarged_Images/tech103_enlarges.gif

Take for example my car.
I had the IHI RHB52, with potential flow of 350 CFM (23.5 #/min) This translates to a max crank horsepower between 222hp and 245hp, approximately.
On the dyno my car made 204whp/224tq, with the peak hp at ~5300 RPM.
Assuming 14% drivetrain loss, this comes out to about 233bhp, right smack in the predicted range for a 350 CFM turbo.
So what happens after the turbo reaches 350 CFM? Look at the graph:

http://i229.photobucket.com/albums/ee320/sporadicmach/Focus/DynoSheet_Focus11012008.jpg

So what I'm saying is I don't see how a car can make more power by increasing boost if the flow potential of the turbo has already been met.

Here is the compressor map for this IHI RHB52 turbo, with the 350 CFM compressor wheel.
As for my torque statement, at 1.9 PR and 260CFM, the engine is at appx 4000 RPM.
This is why I say the max torque comes on in the center of the island.
Whether this holds true always, I'd be very interested to know!
When you look at 1.9 PR and 350 CFM on the map, there's nothing even mapped out there!
The question is what happens out there in the choke area?
I stick to my theory that nothing happens out there. The turbo is maxed out and the power suffers drastically.
http://www.njsr.org/pics/albums/userpics/10619/BRL4219%20-%20compressor%20map.jpg

[sutyak]

After analyzing my data logs I determined I need a significant upgrade to my intake cooling.
At about 10psi on the street I see up to 28 #/min of air at 6K RPM at 80* F intake temps.
Same psi at the track I would only see 26.5 #/min of air at 6K RPM, but at a whopping 125* F by the end of the 1/4.
Clearly I have some terrible heat soak going on, and the car is losing around 15-20hp due to that heat.

My air filter is under the hood, behind the passenger headlight, sharing air space with the engine.
Typical driving it doesn't seem to matter, but wow at the strip it makes a huge difference.
Once I get the air temps down where they should be 13's should come easy.

There are 2 things I'm doing for certain and some others I'm considering to help.
First I've already cut my bumper cover to get more air to the intercooler.
Second I'm going to route some ducting to the filter.
I'm also going to look into wrapping some piping, and also replacing the passenger headlight with a custom horn to route air straight to the filter (track only).
Lastly, if all that doesn't do the trick there's always meth injection.

I post this so others can see the drastic impact of heat soak on a turbo car and how much heat affects the flow potential.

Something else I realized is I'm probably running one or two psi too low to reach the full potential of this particular compressor wheel.
11-12psi should net that 29 #/min air I'm shooting for. (assuming around 80* F air temp)
I'll post results later.