Do you guys think running the NGK TR55IX plugs with its .055 gap will be fine with my LT4 build w/ 503 cam and 11.2:1? I keep reading that your not suppose to regap iridium plugs but i know the stock LT1 gap should be .050

 

I personally wouldn't run a 5 heat range on a 11.2 motor. Especially with a 503 cam.

 

According to the application info at Thunder Racing, the TR55-IX is pre-gapped to 0.050"

http://www.thunderracing.com/catalog...ories&pcid=102

Whether you can get by with a stock heat range plug will depend to some extent on the flywheel HP. You might get by the the elevated CR and 503 (DCR might not be a lot different than stock - have you checked it?), but if you have good heads (don't see any mentioned - stock LT4?) and are making 100HP more than stock, I would consider the TR6 heat range.

 

Could run a TR6IX ... Its 1 step colder...

 

Yea its a LT4 out of a 96 vette, rebuilt with stock heads and 503 cam in it. With the MR. G gaskets it gave it a compression of about 11.2. I wasn't too sure on when its necessary to switch to a colder plug, I always thought that was only if your were FI or using some spray. So you guys would recommend the TR6 plugs? And would you keep the gap at .050 also?

 

If you get a platinum or iridium plug, gap to 0.050". If you get a conventional plug, gap to 0.035". The platinum/iridium is there to minimize electrode erosion over time, so that the factory can recommend 100,000 mile plug changes. A conventional plug will start to erode the electrodes faster, and will open up the gap over time. You might be looking at a 20,000 mile replacement.

Heat range controls the temperature of the electrodes. Too hot, and the electrodes burn off, or become "glow plugs". Too cold, and the electrodes carbon up. The goal is to strike a balance that minimizes electrode wear and doesn't foul the plugs. Fuel injection engines are much more tolerant of a colder plug, because the computer is capable of controlling the A/F ratio much more accurately than a carb.

The rule of thumb used to be to drop one step on heat range for every 75HP you added. That seems to be fairly conservative. I run an Autolite 104 (1 step colder, although some feel its only 1/2 step) with about 500HP NA at the flywheel on my 381. For a small shot of nitrous, drop to the Autolite 103. For the 300-shot, an extremely cold Champion racing plug. A few minutes of idling with the Champion, and it covered with carbon.

 

Very good info Injuneer!