Help with understanding exactly how an intake restrictor plate works

Ok, so here goes. Once again I am thinking of changing race classes next season. I am built to the limit of my class now and still just can't chase down all the mustangs in class in my miata(but I do get quite a few). Anyway, I want to drop two classes and need to cut my rear wheel horsepower number by 16hp. I don't want to unmod the car and figured a restrictor plate might do the trick. I have tried to find info on restrictor plate design etc... but no luck. All I have found is based on nascar and they say how they changed cams and timing and head desing to gain back quite a bit of what was lost. What I am trying to do is figure out a way to keep the most torque and hp from the low rpm and midrange then choke off the top end. Would something like a vertically oblonged hole promote good flow under part throttle but restrict the flow under WOT conditions? Or ??? I know trying different ideas on a dyno is the real only way to go, and I will, but I need some ideas/ guidance before spending the cash to hit the rollers. Thanks Paul

Would the restrictor plate that 1.8 litre SpecMiatas have to run do the trick?

Your question would have a very involved answer. It has a lot to do with fluid dynamics.

Most of the time people dealing with restrictors are looking to diminish the losses where you are looking to control the losses. Really the best way forward is going to involve a good bit of dyno time to play with it.

If you have the time/resources the best way to kill 16hp while gaining torque would be to lengthen the intake manifold runners.

Knock the ignition timing back a little.

Switch to a thicker oil?

pull 1 spark plug wire

Throttle stop.

+1 on the throttle stop. Easiest and fastest method.

But then you lose the low end as well, and he's looking to specifically address top-end horsepower.

A restrictor should work, and since you want it to be a restriction it doesn't need a lot of science. We've done this on a turbocharged Miata that had a HP cap in its class - the car had a lower horsepower number but we were able to really pump up the bottom end. Heck, just look at the power/torque numbers for a WRC car.

The flow at part throttle isn't critical, is it? If you're getting restriction from the restrictor plate, just open the throttle a bit more. I'd love to see a bellmouth on it but I'm not sure that's necessary.

Adjustable cam gears might help. You can move the power band around a lot. Think lobe seperation on a single cam.

I have heard that long tube headers are better for low-end torque than shorties.

iceracer wrote: Adjustable cam gears might help. You can move the power band around a lot. Think lobe seperation on a single cam.

This is my thought too, advance the cam timing to gain torque until you just hit your HP goal. You may have to drill the gears depending on what your class allows, but it should maintain your area under the curve.

perhaps a quieter muffler and a dirty stock air filter would do it?

DeadSkunk wrote: Would the restrictor plate that 1.8 litre SpecMiatas have to run do the trick?

It might but I think they still have a bit to much power for the class I want to run. Thats a funny statement about a miata

I do have adjustable cam gears, and it was very very suprising that changing them around on the dyno showed very little change overall. Maybe I need to move them more, I only went 3-4 degrees each way.

Keith was the restictor used in the turbo miata simply that, a plate with a smaller hole in it? Did you just play around with different sizes on the dyno untill you hit the numbers you were looking for?

Maybe a SM restrictor plate then play with my cam gears and ignition timing to maximize the setup would be a reasonable place to start!

Thanks for the ideas and help! Keep'em coming. Paul

Honestly, I never saw the plate on that restricted turbo car. Bill was the one working on it, and I saw the dyno charts with the fat bottom end without the corresponding peaks. I think it was just a plate with a hole.

iceracer wrote: Adjustable cam gears might help. You can move the power band around a lot. Think lobe seperation on a single cam.

+1, when i heard less top end more bottom, the first thing that came to mind was advancing the cam timing.

grimmelshanks wrote:

iceracer wrote: Adjustable cam gears might help. You can move the power band around a lot. Think lobe seperation on a single cam.

+1, when i heard less top end more bottom, the first thing that came to mind was advancing the cam timing.

+2

Hmmmmm...

what about a narrower air intake tube??? If yout T/B is 60mm, put in an intake tube of 45mm, or 50mm and see the effect

Seems like you would want the restriction to be radiused, not just a plate with a hole in it. I would think that kind of setup would just wreck havoc with flow through the intake tract.

I'm with OldSkool, why not try a smaller diameter intake tube? Breathing through a straw should keep the intake velocity up and might even improve torque down low.

Restrictors are used in rally cars (WRC and elsewhere). They work by making the airflow reach sonic velocity. Once that occurs you simply cannot increase airflow. They are not plates but small radiused cylinders that go ahead of the turbo.