Well I've been looking all over the forums and I haven't found a good thread on this, so without further or do and unless someone can point me toward another thread, here's the thread we've all been looking for (even if we won't admit it).
I've seen people running a lowly 250 hp at 15 psi while others can hit well over 300 hp. So how does a power difference of around 100 hp happen at the same level of boost? It has to be small things that add up. I and many more want to know how does one squeeze every last drop of power and maximize efficiency out of any given level of boost. Even if the question is too vague to provide exact answers. a general outline would help. Thank you in advance

 

Im not sure how elementary of an answer you're looking for but...

In very general terms, CFM (flow) is what matters when it comes to horsepower. Boost (pressure) doesn't really mean much. Larger turbocharger, larger piping, less restriction, bigger ports, and so on, all allow for more flow. A 60mm turbocharger at 10psi will flow considerably less than an 80mm turbocharger at 10psi; meaning the hp per psi is different between the two. The trick is the find the happy medium in choosing a turbo that spools the way you want it and makes CFM you're happy with without having to push to much boost. A smaller turbo, running more boost to achieve the same CFM will have give a higher intake charge temperature which eventually is limited by your type of fuel. Also, besides running a larger turbo, if you can drop the intake temps, the air will be denser and provide more CFM. Intercooler is key for this.

Addtionally, the size of the ports and the displacement of the motor/compression ratio are going to affect hp per psi. A larger motor is also going to require a comparatively larger turbo to provide the same ratio of CFM to PSI as the engine is going to flow more (kinda like a bigger area to fill with air).

As a clear example, the last 13b made 525rwhp @ 25psi (maxing out a 67mm turbo. It wouldn't hold any more boost and was out of CFM efficiency. Any more CFM / Boost meant dangerously hot intake temperatures). The 20b has already made 550rwhp @ an easy 9psi, very low intake charge temps, and not even up to the efficieny hp/psi range yet! (80mm)

This stuff can be confusing and I agree with you, there's not really any straightforward easy to understand threads on this stuff. Moreso, when you start looking at turbochargers your mind can be blown with all the crazy terms, p-trim, inducer, exducer, A/R, inlet size, extended tip blah blah blah. Half the time, people will just buy a turbo because they know it works from other's success rather than actually understanding why

 

Actually it's not CFM. It's mass flow.

And that's the biggest difference. A larger/more efficient turbo will have a larger mass flow than a smaller/less efficient turbo at the same CFM.

 

As far as comparison between turbo's I've always seen the charts labeled as "CFM"

Ex.

Turbo---CFM----Racing HP-----Daily HP inducer
HX35----574------420----------330------ 56
HX40----763------560----------440------60
TP38-----767
T66------800-------560----------440-----66
GTP38R--897
B1/S300--900-------630----------495-----62
H2E------868------700----------550------63
HX50----1000-----700-----------550-----63,67
HX55----1050------735----------578-----67
HT3B----1085------770----------605-----76
GT42----1302-----------------------------75
T76-----1350------945----------743------76
HT60----1400------980----------770-----76
B2/S400-1500------1050---------825-----75
GT47---------------------------------------80
HX60-----1550------1085-------853----
TV81----1600-----------------------------85
T88------1750-------1225--------963-----88
HT4B-----1800------1260---------990-----88
Big Brother-1900---1330--------1045-----87
T91-------2000------1400--------1100-----91
HC5A/HX82-2450----1715--------1348----95
T100------2600-----1820--------1430-----100
T105-----2900------2030--------1595-----105

 

CFM alone doesn't tell you how much air is being moved. Changes in pressure and temperature will greatly affect just how much air mass the turbo is moving. If you look at compressor maps they tell you the mass flow in lbs/min as referenced by the pressure ratio. As well as efficiency at that point. This gives you the info you need to estimate fuel needs and power output.

 

I never understood how different turbos could flow different amounts of air at the same psi. If the intake system size stays the same, it should always take the same amount of cfm at any given rpm to compress the same amount of air to 10 psi. The only difference between turbo sizes I could see would be the heat the smaller turbo creates spreading out the air and in turn making less air take up the same volume

 

I would say another thing to consider is the type of dyno the car was tuned on. A mustang dyno roughly reads 11-12% lower then a dyno jet. I try to compare my tuned fd with others but comparisons always seem to be different.

Sammy

 

And that IS the difference

1 cubic foot of air at 15psi and 200degF

Has less mass than

1 cubic foot of air at 15psi and 100degF

Less air mass means less oxygen available to oxidize fuel. There for less energy output.

 

What are some small things people can do to maximize the hp at there current boost levels?

 

The CFM parameter is there to match the turbo to the engine flow rate, which is a function of the displacement, operating rpm, and volumetric efficiency. CFM is actually a pretty lame way to choose a turbo, so many of the newer maps use mass airflow instead.

If you think about it, you can't cram more air into a fixed-displacement container without compressing the air. Therefore, given a constant pressure, the engine is only going to flow whatever CFM that it is going to flow, and the CFM potential of the turbo will not change this. Faster rotaries need a higher-CFM turbo because they tend to have larger porting and/or operate at a higher rpm, both of which increase the CFM requirements of the engine. Therefore, you are correct that the higher-hp rotary engines do tend to have a high-CFM turbo, its just that slapping a high-CFM turbo on an engine will not change anything unless the other parameters are changed.

It is their thermal efficiency. The more efficient the compressor, the less the air is heated, therefore the more dense the air charge. Therefore, the CFM and PSI may be exactly the same, but a more efficient turbo will flow more MASS than a less efficient turbo. The turbine efficiency also makes some difference too.

You can learn more about this at this website (warning: lots of math, lol):
Turbocharger Compressor Calculations

The book "Maximum Boost" by Corky Bell is also pretty good, and I think it is written so that 'normal' people can understand most of it. The only bad part is that it is a bit dated and therefore uses the old CFM compressor maps. His newer book "Supercharged!" is better in this respect, although it covers the subject of engine-driven superchargers as opposed to exhaust-driven superchargers (aka turbos).

Unfortunately, it's the big things that make the most difference, such as an efficient (expensive) turbocharger, a good intercooler, well-engineered air path plumbing and exhaust system, engine porting, and a good standalone EMS tuned by a professional. I guess the best small things would be maintenance-related, such as a clean air filter, clean fuel injectors, clean fuel filter, engine seals in good shape, spark plugs and wires in good shape, intercooler inlet free of debris, etc. Also, note that ethanol fuel has a low energy content, so try to avoid gasoline with ethanol in it if you want maximum power.

 

A good tune is going to make a world of difference. Somebody already said that in order to maximize hp you are going to need a well built turbo. This is one thing you can't skimp on. The saying you get what you paid for needs to be taken to heart here. Also, I know there's a big difference between ball bearing and journal bearing in the way that they spool, but I'm not educated enough to really comment on it. Anyone else wanna weigh in on the difference in performance between the two?

 

Journal Bearings vs. Ball Bearings | Turbobygarrett