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A Restrictor Plate Study

By Tom Carnegie

Cars in the Montana 500 with aluminum pistons have an 11/16" restrictor plate installed between the carburetor and the intake manifold. A lot of people are curious about the exact effect this has on the performance of the motor. I was curious myself, so I decided to do some semi-scientific tests.

The test
I picked a spot on highway 290 a few miles out of Spokane. Highway 290 is four-lane, fairly flat and straight. The conditions on test day were perfect. Temperature in the mid-seventies and almost no wind. I timed a flying mile both directions several times with and without a restrictor plate.

The test subject
The car I used for the test is known as "Tweety Bird". It is the yellow 1925 roadster that Nan Robison drove in 2000, 2001 and 2002. It is also the same car that I drove in 1975, 1976 and 1977. It placed first in 1977, fourth in 2000 and fifth in 2002. It is slightly detuned from its peak in 1977, but is still no slouch.

The results
Best time with restrictor plate - 1:10.55 or 51.03 mph. Best time without restrictor plate - 1:09.31 or 51.9 mph. If you continued this pace for 500 miles, the restrictor plated time would be 9 hours and 47 minutes. The non-restrictor plated time would be 9 hours and 37 minutes, a difference of ten minutes. If you were to throw out the worst run in each direction with and without the restrictor plates, then average the remaining times, it would be 50.23 mph with the restrictor plate, and 51.4 mph without. This would translate into 9 hours and 57 minutes for the restrictor plated car, and 9 hours and 43 minutes for the non-restrictor plated car, a difference of 14 minutes.

Conclusions
On this particular car it seems that the restrictor plate is about a ten to fifteen minute handicap over 500 miles. At some point in time I may do a study to see the effect that the restrictor plates have on the ability to climb hills. I have posted all of the test data on the Montana 500 website if you wish to study it a little more.

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