Restrictor plates - wow

[cstreit]

So I wanted to reduce the weight penalty I need to run in the GT3 Cup to meet the power to weight ratio a bit. Made a series of restrictor plates for the car and took it to the Dyno. Thought i'd share the results, it took a suprising level of restriction to drop the HP.

Stock Intake - 90mm 333 HP
Restrictor 1 - 80mm 331 HP
Restrictor 2 - 70mm 325HP
Restrictor 3 - 65mm 324HP
Restrictor 4 - 60mm 319HP

Amazes me that it took a 30% reduction in intake size to lose 14HP. HP curve is the same on all, Torque reduced at 1/2 the rate.

Thoughts?

[winders]

Wouldn't it be better to restrict HP by limiting RPM? Wouldn't torque be maintained that way? Also, wouldn't the engine last longer?

Scott

[cstreit]

Scott, theoretically yes, but there is no way to do this on the GT3 Cup without reprogramming the ECU. in a perfect world i would have an expert reprogram and remap but that takes a lot of time and $$$

[Flieger]

That is not a 30% reduction. You went to an area 4/9 of the stock one. What that shows is that the stock intake is nowhere near the limiting factor and there was probably no choked flow occurring with the stock setup. With the larger restrictor plates the air just sped up to make up for the lost area, but the greater speed and extra turbulence of the plate protruding into the airflow make the engine a bit less efficient. The restrictors on Le Mans type cars are so small that the air reaches Mach 1 in the throat of the restrictor and no more mass airflow can pass into the engine. It appears you are still getting near the same mass airflow but it just is being sped up. If you keep decreasing the restrictor size you should see the torque curve drop off a cliff at a certain rpm, with the max horsepower also falling in value and in rpm.

[winders]

Originally Posted by cstreit

Scott, theoretically yes, but there is no way to do this on the GT3 Cup without reprogramming the ECU. in a perfect world i would have an expert reprogram and remap but that takes a lot of time and $$$

I would think the extra engine life alone would make it a financial win. Not mention the driveability you would have compared to the restrictor setup. The biggest wear on those engines has got to be that last 1000RPMs.

Scott

[cstreit]

Winders, i'm sure you are right especially the last 1000 rpm comment. :-)

Not ruling out the idea.

Flieger, thanks for the explanation. the intake tract is so long i doubt the turbulence is a huge issue.

[Flieger]

Originally Posted by cstreit

Winders, i'm sure you are right especially the last 1000 rpm comment. :-)

Not ruling out the idea.

Flieger, thanks for the explanation. the intake tract is so long i doubt the turbulence is a huge issue.

I just mean that there are more energy losses due to the turbulence so the air has less stagnation pressure when it reaches the cylinder.

[cstreit]

Can you explain that in laymans terms?

[Flieger]

I will try to explain it but I don't think I can do layman's terms.

At any time, air has a certain amount of energy due to heat, pressure, and kinetic energy (motion). When it flows through a tube there is friction between the fluid (air) and on the walls of the tube and friction between the air molecules themselves bumping into each other. Turbulence increases the shear stress on the walls of the tube and also increases the collisions. This increased friction requires the fluid to do work to overcome and so it looses energy.

When the fluid stops moving (in the cylinder), the kinetic energy is converted to pressure energy but the total energy immediately before and immediately after the stopping are the same. This trade off between static and dynamic pressure should be familiar. It is how carburetor venturi tubes work, for example. Air speed up, pressure drops. Air slows down, pressure increases.

If the fluid has lost total energy along the way to the cylinder, then it will have a lower pressure when it stops moving (called stagnation pressure). Since engines suck a particular volume rather than a particular mass on each stroke, the lower pressure means fewer oxygen molecules in the chamber available to mix with fuel and make power.

So, basically the restriction causes a lower pressure in the combustion chamber. When you actually become limited by the restrictor, the pressure cannot get any lower in the combustion chamber and the power will start to level off and then drop because of greater frictional losses (bearings, pistons, that friction is what I mean) at higher speeds and because the air cannot flow into the engine any faster so therefore each power stroke is weaker even though you have more of them.

This is all assuming negligable heat transfer.

[Flieger]

You might like to read some wiki for a better explaination:

Stagnation pressure - Wikipedia, the free encyclopedia

Choked flow - Wikipedia, the free encyclopedia

de Laval nozzle - Wikipedia, the free encyclopedia

[cstreit]

So I think I get that a little. In essence could you say the smaller diameter thins the air out meaning less efficient combustion?

[Flieger]

That is the end effect. Thinner air in the chamber.

But the effect is non-linear with the area (pi*r^2) of the restrictor. At first the air will just speed up and the pressure won't be that different. The torque and power should look pretty similar, and you might pick up some at low rpm. But soon (at a certain rpm and restrictor area combination) compressibility effects will come into play and the air can't speed up as easily. The pressure will drop more with each change in area. Torque will drop at that point but you can still make more power by spinning faster. Finally there will be a point where the air reaches Mach 1 at the throat and at that point it can't speed up any more. At that point the power and torque will both decrease with rpm because of greater frictional losses.