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Old 10-21-2008, 11:38 AM   #1 (permalink)
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Anyone experimented with a restrictor plate?

conventional wisdom says the bigger the TB opening the more power can be produced based on air inlet. Has anyone experimented with a restrictor plate in order to reduce the amount of inlet air? Curious if there was any value to MPG?

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Old 10-21-2008, 11:48 AM   #2 (permalink)
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Convention wisdom is not always correct. A larger TB opening will allow more power to be produced, but increase the rpm at which you achieve peak torque. Thats really not what we want to do. Nor do we want to restrict the engine from breathing. This is exactly what closing the throttle plate does. There is no need to add another plate! If you look at BSFC charts you'll see what I mean.

What you want is a properly sized port tuned for the RPM at which you'll be operating the engine.
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Old 10-21-2008, 11:51 AM   #3 (permalink)
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Old 10-21-2008, 12:06 PM   #4 (permalink)
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I have restrictor plate and reduced venturi size experience, but... in both instances, they were race applications.

I can offer a nugget of insight as to the effect on peak rpm and torque.

Again, one particular normally aspirated drag race engine with huge intake ports I had on a friend's dyno, a 50mm intake valve, and no regard to fuel consumption (2hp/cu in, if you're interested);
When we reduced the venturi diameter from 50mm to 48mm, peak torque was acheived at 300rpm lower with no reduction in torque at 8000rpm. Reduced again to 45mm, peak torque was acheived at about 100rpm lower, but the torque fell off more sharply at higher rpm, having a peak torque band narrowing effect. Throttle response was 'snappier' with the smaller venturis, not something you normally measure.

I'm with Daox, optimize the port runner diameter and length, thottle body diameter(s) for peak torque where you want it. The other thing to note is that when you start changing the place in which peak torque occurs, or alter the shape of the torque curve itself, you realy need to build a tranmission around the new combination in order to take advantage of the new charactaristics.
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Old 10-21-2008, 01:24 PM   #5 (permalink)
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A throttle stop that doesnt allow the computer to go into open loop would be easier, cheaper and obtain better results than adding a restrictor plate. Driving with discipline is free and it works too.
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Old 10-21-2008, 01:27 PM   #6 (permalink)
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Old 10-21-2008, 03:05 PM   #7 (permalink)
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Old 03-24-2009, 01:05 PM   #8 (permalink)
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I agree with Daox, but I'm going to experiment with my 2005 Focus ZX3 2.0L engine by putting a restrictor plate. I agree that the throttle body plate already acts like a restrictor plate, but this idea might be worth a look. If it doesn't work, I'll remove it and reset the computer!
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Old 03-25-2009, 02:07 PM   #9 (permalink)
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I can't help but think of Frank The Tank from the movie Old School -
"Took the restrictor plate off to give the Red Dragon a little more juice. But it's not exactly street legal, so keep it on the down low."
lol.
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Old 03-25-2009, 02:30 PM   #10 (permalink)
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Adding a restrictor plate might allow you to use more throttle to achieve the same result, which technically ups the VE, right?

Couldn't you put the hole in the restrictor somewhere so that the air wouldn't even come in contact w/ the throttle plate if you were over like 50% of normal throttle?

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