I have read that the AR3933 racing plugs from Autolite are to be used as is out of the box. It actually measured a .025 gap out of the box. I read that because it has a cut away strap/side electrode that it DOES NOT need to be gapped and is ready out of the box. Is this true? Thx, DeAngelo

I go as wide at some .055 or larger as long as I dont have starting problems. I run an MSD ignition with 3924 plugs.

Thx, but the 3924 plug is not a racing plug with the cut electrode. Does that mean that I can gap my racing plug with the cut electrode. See the link: http://store.summitracing.com/partde...5&autoview=sku


Thx, DeAngelo

They are listed as a racing plug. Listed in your link. What are you doing, Weekend track? Change them out every race weekend?

No, it is a street car with a 440 HP 383. When built, it had AR3933's and I had no problems. It ran great! I really get on it. I take it to 4 grand frequently so I figured that any carbon would be burnt off. Still, a mechanic told me to go with hotter plugs and I went to a regular Autolite 3924 and had detonation. I then used a regular 3933 Autolite and got detonation at higher RPM's. I even used Rapidfire plugs and they were HORRIBLE!!! It was pinging at 2500!!! Now, I want to go back to what was in the engine when I bought it: AR3933's. I just need to know if I should put them in straight out of the box or attempt to gap them. I heard that you shouldn't. I don't want to do something wrong. if I can't get an answer, I am going to call them Monday. thx! D

I don't think the plug is carboning up due to gap setting. You may have an over rich condition or oil going past rings or valves, causing carbon to stay hot, causes hot spots and preignition. Too much quench does not help, too much timing initial or total .. Need to find out what you have . If you can get by with a cold plug to prevent detonation, then do it but I dont think thats all of your problem.

What is your quench, Compression? Timing? Fuel? Heads?

Thx. The plugs I use don't seem to have been fouled. I just want to go back to what I had that worked. My compression is "supposedly" 9.5 to 1. I have Holley Alum heads and only 2 thousand miles on the engine at most. The cam is a Lunati 235/240 112 center and 490/508 lift. I always use 93 octane. After changing the plugs, I will fiddle with the air mixture screws of my Holley 770 Avenger carb and use a vacuum gauge to adjust it. Should I bottom them out and start fresh? or should I just adjust them 1/4 turn at a time and go back and forth based on the vacuum gauge attached to the front of the carb? It could be that it is running lean. I don't know. Thx!

OK, You have about the same head set up as I have but with less compression, less quench I think. I see nothing wrong with the way you are adjusting the carb, at idle with vac guage, basic curb idle . If you want to get serious about getting air/fuel mixtures on the money, you will need a tool for just that. This tool will tell you where your mixtures are at idle and cruise under hiway conditions. From there you might find other problems that may be causing lean or over rich conditions.
If the vac gauge is all you have , then thats the best your can do there. You can do some WOT testing if you know how to read plugs but when this is done, you need to cut motor clean on a wide open run, no idleing. Maybe you can get a run at the tracks and get someone to tow you back to pit, pull plugs and see what they are like, down inside the projector tips or insulators. You need to make sure you don't have too much timing too.

Your cam is not far from mine, 231/239° duration. I run a 3310, 750, with 65 pv and 70 jets in pri, 74 in rear. I did my fuel tuning with an LM-1 air /fuel tester at 3500 ft elevation. Unless you have some leaking needle/seats or leaking power valves or even plugged air bleeds, I don't see any reason why you should be running rich. You jus need to get down to basics and tune the best you can with what you have.
Here is a compression calculator, http://www.csgnetwork.com/compcalc.html
Feed it the information you have on your motor, it will tell you where your compression is .
Hope this helps.

Thx, Don! That was very helpful. I do know that the idle mixture screws were tinkered with a bit when I had it dynotuned. Now that I have different plugs, I need to use the vacuum gauge and re-adjust. it may be a bit lean and that could be why it is detonating. I would LOVE to have one of those LM-1 A/F devices but they are a little expensive. They also require an 02 sensor don't they? I did see a machine online that lets you put a probe in the exhaust pipe. It was around 200 bucks but i wasn't sure how well it worked. It is called a Gunson. Heard of it? Thanks again. DeAngelo

The LM-1, with logger is about $350, http://www.innovatemotorsports.com/products/lm1.php
I was thru with 6 month old unit and was thinking of putting on a full time unit that does not have the logging program. This is the program that allows you to record about 45 min of running, then put the plot on the pc to see exactly what is happing and at what rpm. There are cheaper models but I find this the best for the bucks. Yes, it as well as all others require the 02 sensor welded in the collector but can all be removed after you are tuned in.
If you had car dyno tuned, they should have run this tail pipe sensor test, thats the way I found out I was leaning out bad during the power tour, DynoMax put my car on the drum dyno. I found out I was leaning out big time, found out my fuel pump was checking out.
Send me a pm if you might be interested in the LM 1 , still have everything including the program disc.

Thx, Don. And you are right. They did do an A/F test. It has been changed since then due to a shop fouling my plugs and then using those crappy rapid fire plugs. Can I start over by screwing the mixture screws all the way in and starting over by backing them both out the same degree and suing the vacuum gauge? I will pass on the LM 1. I don't want to worry about welding. thx. D

Get a vacuum gauge, plug into the manifold and adjust for a max reading, whatever direction needed, the gauge will show you, then maybe 1/4 turn richer. Run as much initial timing as you can, readjust idle.

Do the header adapters have an O2 bung welded in? LM1 comes with a wideband sensor.
AR3933 gap is set by design.

Thx, Everett, again. That is where I learned the vacuum gauge trick. From you. No, I do not have an 02 port welded in place. I am thinking about buying the Gunson A/F machine though. And thx for the answer about the plug gap! DeAngelo

I run a 7 second dragster and set the plug gap to .038, but .035-.040 will work just find on any engine.

Why not call the engine builder and ask them? They had those plugs in there for a reason. You said you had it dyno tuned so I do not understand why you would want to change anything. An afr meter costs $350 but how much does a burnt piston cost? I got an lm2 because it was less than a dyno tune. Good luck be careful with detonation it will kill that engine.. Definitely call the engine builder he should have your answer.