25 September 2018

Optional Wrenching

When I ordered The Precious I selected the optional NPP exhaust.

On a normal 'Vette there's four exhaust tips, but only one hole coming out of each muffler that 'Y's' into two tips.

If you got the NPP you get different mufflers.  The outboard exit goes through the muffler's baffles like normal.  The inboard exit bypasses the baffles.

The inboard tips have a butterfly valve on them that opens at around 3,500 rpm and gains a small amount of horsepower and torque over the normal arrangement.  SIX whole horsepower, in fact.

The butterfly valves are vacuum actuated.  There's a wee little controller box that senses the rpm and opens a switch, which cuts power to a solenoid which removes the vacuum holding the valves in the closed position.  They also open when there's no vacuum signal, but there's a reservoir to keep that from happening every time you hit wide-open-throttle.

You can also bypass the controller with a switch of your own so you get the louder exhaust note at low rpm / high vacuum conditions.


A couple of times when I started the car it was very slow to close the valves, but behaved normally once they had.

The other day, on the way home from Iowa, they decided to open up and stay open.

Assuming it was my switch, I replaced my fuse-tap with a fuse.  No change in function.

I put, my good friend, JT on the task because I was still driving.  He found that the most common failure mode was the solenoid, especially if you get a period of odd behavior prior to the complete failure.

I've ordered the part, but it was quite a fight to find someone who mentioned the part number.

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