I've read some posts where this was a common problem for the FR500.

If I recall, the typical fix was to open short runners at low rpm (idle & just off idle) with a transition to long runners at mid level rpms then back to short runner at upper rpms.

Sorry this is not the fix you need to hear for your situation. Hopefully some experts will chime in to solve this for you.

 

Thank you. I had no idea where to start looking. Searched online and found this on SVTP.com

That rattle is caused by the intake air resonance. Its the air at low speeds goes down and hits the intake valves it "bounces" back up-to the runners hitting the blades. This causes the noise.

The problem is easily fixed:

Set the Window Switch to have the blades open from idle to ~2000RPMs, then have the WS close the valves at 2000RPMs. This will allow you to run on the ("short") runners at idle and up to 2000 RPMs (no worries it will not hurt anything and not have any detrimental stuff to performance), then from 2000RPMs up to ~5000RPMs on the long runners, then have the WS open the runner again at ~5000RPM and transition to the short runners for sick top end performance.

Naz

 

Running only the long runner will cut hp by a "whole bunch" (technical term), probably more than is needed.

 

Only 1 way to find out. Atleast I'll gain low end torque over running just the short runners.

The torque rules are more liberal at 8.5:1 compared to the HP rules of 9.5:1.

The tracks around here are shorter tracks where more low end tq is better.

 

Only 1 way to find out. Atleast I'll gain low end torque over running just the short runners. The torque rules are more liberal at 8.5:1 compared to the HP rules of 9.5:1. The tracks around here are shorter tracks where more low end tq is better.

The FR long runners don't make a lot of torque.
Here's a comparison of the FR long and short runners and the stock intake.

 

Man, I'm really surprised that the long runners of the FR500 don't make good power down low. I'm sure that was the idea of designing it the way they did--the best of both worlds. How does the intake actually work? How are the runners controlled? Electronically somehow?

 

The long runners are very long and have a small cross section, too small to effectively feed the engine.

The runners are controlled by a box that is very similar to an IMRC controller. In fact, FR500 that fits the 96-98 cobras uses the stock IMRC controller. That intake is made to fit C head swapped 96-98s.