I have an 86 GT with the V6 engine. I used this product and the knocking sound (octane bottling sound as I describe it) was decreased by a factor of four or better. An excellent improvement. It was worth the $20.00 to me. I used to have to use 92 octane fuels prior to this treatment, and with the 87 octane I use now, I get a "MINOR" sound only upon extreme heavy/fast acceleration, which I understand is fine, actually good. I did immediately follow up though with the Slick-50 decarbonizing product, that is added to the gas tank. It was $10.00 at a local auto parts store.
The decarbonizing product is D-Karbonizer-4 from IRM 1-301-948-3301. I am a member of Fiero Owner's Association, and they give these members a 10% discount. According to them, this product is available to auto mechanics only, an not to the general public, but IRM being a catalog sales company, offers it to everyone.
Using the IRM decarb product is very simple. All you need is a set of pliers, a 3/16"(I think) rubber hose, and someone else. On my V6, I warmed up the engine, stopped it, then disconnected the hose just to the right of the distributor, comming out horizontally, with a right angle rubber molded hose connection. The EGR does NOT connect there. Something else does. The hose you use for decarboning must be of the same inside diameter as the right angle rubber molded piece's ID, to insure a good connection. Attach the new hose to the plenum, and put the other end into the bottle of decarb. Pinch the hose shut with the pliers, and tell your helper to start and run the engine at 2000 rpm, no matter how much gas to apply. Back off on the force of the pliers such that the entire contents of the bottle is sucked into the engine over a 5 minute period. A lot of slop is expected, regarding the 5 minutes. Immediately shut off the engine when all the decarb is gone, and wait a half hour. Then drive the car as normal.
Thats all! You should notice a difference almost immediately.
Don't follow these instructions, but rather the ones included with the decarb. This is written by memory, and since mine is shady, use it for getting the idea alone.
From: Ron Dittmer