What is the relationship between intake manifold pressure and horsepower? I've seen a few times that a "big" turbo can make 280 HP at lower boost levels (a T31/T04E at say, 10 PSI or so) than a "small" turbo (a T28 at say, 16PSI or so).

I know that a T04E compressor is going to be heating the intake charge a LOT less than a T28 would just by virtue of shaft speed, and I know that a T31 turbine would be inducing less backpressure than a T28 would. Other than that, though, is there another difference I'm not seeing?

I think it's just the fact that the bigger turbo flows more air with less work. More air = more HP.
The driving factor that bigger IS better!

that happens because even thought the small turbo is putting out more pressure, the bigger one is putting out more volume. Inturne meening more air at a lower pressure.

perfect example would be Frank O's car vs my car.

Frank made 250 whp @10 psi

i made 250whp @ 14-12 psi.

AND frank still had his exhaust on the car. my run was unbolted.

Originally posted by CowboyDren

I know that a T04E compressor is going to be heating the intake charge a LOT less than a T28 would just by virtue of shaft speed,


NO NO NO NO NO NO NO NO NO!!!!

I've seen this statement made several times and I want to kill this belief right here, right now!!

The lower efficiency of the smaller turbos had NOTHING to do with the fact that a smaller turbo spins faster and therefore 'whips up the air' more, or some other such nonsense.

Centrifugal compressors and turbines are all about tip speed. Tip speed is what compresses the air. A wheel of a given design will require a certain tip speed in order to create a given pressure ratio. As you go to a smaller diameter compressor wheel, the speed will go up. But, the speed goes up proportionately to the decrease in diameter, so that the tip speed (diameter times rotational speed) always stays the same. A small wheel doesn't 'whip up' the air any more than a large wheel does.
Air velocity at the wheel exit is the same for both wheels.

Smaller wheels tend to be less efficient than large wheels because: 1) Tip thicknesses don't scale, because they are largely a function of casting limitations. Therefore, smaller wheels tend to have more blockage (proportionately) than larger wheels. 2) Tip clearances are largely a function of machining limits, and therefore remain approximately constant across a large range of wheel sizes. Smaller wheels, therefore, have a proportionately larger clearance volume than larger wheels, resulting in more backflow, cross flow, and backside leakage. 3) Shaft size and therefore bearing journal area doesn't scale proportionately to wheel diameter (for packaging reasons), so bearing losses (which are baked into the turbine efficiency measurement) are proportionately higher for smaller wheels.

Although smaller wheels are generally lower in peak efficiency, there are all sorts of cases where that isn't true. The T25/T3/T4 line of turbochargers is pretty ancient, and Garrett has newer wheels that are significantly more efficient. A state of the art 52mm compressor will have significantly better peak efficiency than a 76mm T-04E wheel.

As a rough rule of thumb, the power that your car generates should scale with the intake manifold absolute pressure (boost pressure plus atmospheric pressure). Take Frank's car, for example:

250 hp @ 10 psi
380 hp @ 22 psi

250 hp * (22+14.7 psi)/(10+14.7 psi) = 372 hp at 22 psi. He made 380, pretty close to what you would expect.

at 14 psi, Frank should be making about 290 whp, versus Steve's 250.

Why did Steve make so much less power than Frank at the same boost level? Because at high flow rates (where you operate at 250-290 whp), Frank's compressor is much more efficient. Also, Frank's turbine flows more, so he has to wastegate a smaller percentage of his flow. The less flow you wastegate, the more flow goes through the turbine, and the lower the expansion ratio you have to operate at in order to produce a given amount of shaft power (which is used to drive the compressor). It all adds up to Frank's car having significantly less backpressure on the motor. Frank's car is working like a NA car with a nice 2.25" straight through exhaust, while Steve's car is working like a full bolt-on SR20 that has a stock GA16 exhaust with a crush bent mid pipe.

The different power levels that different turbos can support at a given boost level just comes down to a question of how much backpressure each one puts on the motor. The less, the better.

Originally posted by MarC
that happens because even thought the small turbo is putting out more pressure, the bigger one is putting out more volume. Inturne meening more air at a lower pressure.

NO.

The air flow rate through the compressor depends only on what the engine can swallow. A larger turbo will not flow more air at a given engine speed, boost level, and engine displacement unless the larger turbo can increase the engine's volumetric efficiency. If the larger turbo puts significantly less backpressure on the engine, then the engine's volumetric efficiency will increase, and the larger turbo will flow more air. Otherwise, it will not. It is bogus to think that a larger turbo will make more power because it can flow more air. The turbo is linked to the engine... the turbo will only flow what the engine can swallow.

I'm not quite wrapping my mind around all of this yet, but I feel much better. 'Preciate the correction, I'll try not to make statements like that in the future. :)

Hey Rob, i have a theory about my car though.

when looking at my dyno curve, i can see that right when the HP curve intersects 250 whp, it drops off instantly by 10 whp and about 10-12 lb-ft of torque. power peaks at a low 5500 rpm, where it then falls off, but stays right at 240 whp until redline. torque peaked at 4000, where the full 14 psi of boost was realized.

my theory is that my injectors, upon reaching above 80% duty cycle (or thereabouts) immediatly went to static, since they could no longer accurately control fuel delivery. this would also explain why my power peaked so low in the rev range, but never really fell off.

boost falling off would also contribute to my lower HP numbers, but when driving my car home, it fell to 12 psi at around 6500 rpm, so i dont think that was what caused my sudden drop in HP numbers.

I will be upgrading to 50 lb/hr injectors soon, as well as the turbo found on the disco potato, as soon as it becomes available. any ideas as to when garrett will release your concoction Rob? i like immediate boost. it is fun.