spark plug gapping

[81280zx]

i know this has probably be talked about before and i have searched and saw a few recomendations on this. I just wanted to know what u guys are gapping at. I have a l28 n/a with headers port and polished intake mani cold air intake and muffler. The chiltons manual i beleve it say between .39 and .42. thanks all for ur opinions

[Tony D]

I gap for smoothest idle, and least dropout on the top end. Whereever that is, that's where it ends up being. May be .055, may be .035... wherever it runs the best.

 

The wider the gap you can run the better it will fire off more reliably at idle and keep emissions the lowest. But this means you need a good coil, wires, cap and rotor.

 

On a turbo, you have to gap for least dropout (misfire) under boost, in some cases this is as small as .015"

 

Gap by bending the ground electrode at the base---ideally you would use a gapping tool which keeps the gap even by keeping the strap parallell to the surface of the center electrode tip... but who has one of those besides me? (Had to gap 96 plugs really quick. I would gap them in sets of 500 each, so yeah, I bought a mechanical gapper!)

[katman]

Well that depends a lot on the ignition system you have. If stock then use FSM specs. If MSD, well you don't really need a gap because it will arc off the top of the piston. So, anywhere inbetween sorta depends on what you're running.

[TBK]

Katman, would you expound on the MSD thing, I just aquired a 77 that has MSD on it, and pulled the crappy autolite plugs out and put in NGK @ .0045. Dont know anything about the MSD, gotta find the rev limiter thing so I can change it, it is set @ 6000, NOT ENOUGH!!!!!!!!

[81280zx]

all i have is the blaster 2 coil and accel racing 8mm wires on it

[Zmanco]

Dont know anything about the MSD, gotta find the rev limiter thing so I can change it, it is set @ 6000, NOT ENOUGH!!!!!!!!

Pull the little white "plug" out from the MSD box and you'll remove the rev limiter entirely. Perhaps someone will have a spare plug for 7k? That's what I'm running and it works well.

[TBK]

so the rev limiters are preset at a certain rpm and you just swap them????

[Tony D]

They have pills that are white with little prongs on them, they usually are sold three to a pack, in a range covering 1500rpm. So 6000, 6500, and 7000 would be in one pack.

You just swap them for whatever range you want.

They also have a splitter, so you can put two "pills" in at one time, as swap between them with a switch. I used a keyswitch in the ashtray, and when I had to valet park, I have a 7000 and a 2000 rpm pill. The Valet gets 2000rpm... And the keyswitch key stays with me.

 

My brother parked cars, I don't trust any valet.

[TBK]

Thanks for the info Tony

[TimZ]

If MSD, well you don't really need a gap because it will arc off the top of the piston.

 

I agree with the rest, but I don't think it works this way. Even if you removed the ground electrode, the edge of the plug would be closer than the top of the piston at the point you would want to fire it.

[katman]

Yeah, I was sorta joking about the piston. With MSD we ran .060 gaps. But they'll arc about a foot outside of a combustion chamber. Yikes!.

 

Just a data point- In the ITS cars (200 crank hp, 9.5:1 CR, 7400 redline) we got a couple more ponies with the Jacobs unit over the MSD. However, the Jacobs has a switch you can close that gives it a little boost and actually does result in yet a couple more hp, but it is designed for low duty cycle apps like drag racing. On the engine dyno we noticed after several "switch on" pulls that hp dropped off. Probably overheating whatever circuit was involved, so for road racing we couldn't use it.

 

Like all aftermarket parts, Jacobs promised us 10 hp with our application and we got 2.