darkhorse91

I heard that more powerful ignitions can handle a larger plug gap which promotes better burn. Is this true and if so how do you know how much to expand the gap?

AlkyIROC

Gap the recommended heat range plugs to the recommended gap. If you do a lot of modifications to the engine such as increased compression ratio, power adders etc, then you can play with heat ranges and plug gap.

Too wide a plug gap can be just as bad as too small a gap. I run an MSD HVC II coil and still only gap my plugs to .035". I also run very cold plugs.

305sbc

iTrader: (3)

I run a MSD and run cold non-resistor plugs gapped around 0.030".
The wider the gap the more prone to misfires.

serpentracer

I've often wondered why my S/10 4.3 V6 plugs are supposed to be at .060
and the camaro at .035.

matts86cam

iTrader: (1)

If too large a gap causes misfire, what does too small a gap do? lose power? I am trying to find out what to gap my plugs at with no success. 350 bored to 355 350 HP rough idle/cam any suggestions? Thanks

Apeiron

Around .045 should work fine.

Gapping too large means the coil secondary voltage might not be able to jump the gap and the plug may not fire under some conditions, like cold starts or high load. Gapping too small means that the spark might be too small to ignite the mixture.

Widening the gap might get you 0.5-2 more HP, if the ignition timing is already as advanced as it can be. It's something that might be worthwhile for that last little bit of power in an all-out racing application, along with things like indexing the plugs. In most normal cases though, it makes practically no difference.

There's also impact on the secondary coil current in both cases, which affects the coil temperature.

DBRjeep350TPI

would gapping the plugs to small cause a missfire?

305sbc

iTrader: (3)

While it is possible in some situations it is very unlikely.
Personally I wouldn't run smaller than 0.028".
Unless your ground-strap is short or clipped back, a very small gap shrouds the spark, which can cause delayed ignition or misfire.

Generally speaking, a low powered engine (close to stock) that isn't tuned near the lean-limit will be just fine with a 0.045" to 0.055" plug gap.
The wider gaps are harder on the secondary side of the ignition system, but some systems are capable of handling even wider gaps.

Most people on this site will be fine with stock heat-range plugs at the factory specified gap setting.

The more power your engine makes, and the leaner you run, the more you will need to close up the gaps to prevent misfires, and the more you need a powerful ignition system that can create some amperage.

By "power" I mean torque per cubic inch. Not horsepower.

DBRjeep350TPI

well this is my deal. this last weekend i was trying to figure out my timing issue and I found that i have two plug wires show zero resistance, so I bought a new set of MSD 8.5mm and threw them on along with a Accel ignition control module and a Accel super coil. i checked the cap and polished the brass terminals and the carbon bushing button with a wire brush and also replaced the plugs with a set of Accel 437SS silver tip projected racing plugs gapped at .035.
prior to that i had a stock set up with Bosch platinum +4 (4449) plugs and not to mention the 1 or 2 dead plug wires and it ran decent but backfired a bit at hire RPM. So long story short I put all this new ignition stuff in and started it up and is misfired worse... now out the intake (never did that before) and out the pipe when you rev it up and it starts to decelerate. I played with the distributor advancing it and retarding it but never got rid of the pop!!!
· I am thinking that I am going to gap the new plugs to .050 and try again.
· Also throw a timing gun on it and see where she is at and reset the computer.
· Pull the cap off and make sure she is point at #1 at TDC.
· Check the plug wire again (4th time).
· Check the fuel pressure, and maybe bump it up to 45psi. it is set at 39-41 psi right now
And whatever you can think of…
thanks

305sbc

iTrader: (3)

That is not a gap problem.
You have made some sort of mistake: wires out of order, bad ground on ignition, broken spark plug insulator, etc...

grumpyvette

Ive never seen any advantage to gaps over .045 and Ive seen plug gaps as wide as .060 or under .035 cause misfires with some ignitions in a FEW applications so after years of testing and racing I generally set plug gaps at .045, if your having issues with misses at .045 its not the plug gap thats the cause if your having issues with misses at .045 its not the plug gap thats the cause, its some other problem with your ignition or fuel/air ratio or ignition timing advance,high resistance ignition wire, moisture,a bad coil,loose connections, firing order or arcing ignition etc..

http://forum.grumpysperformance.com/...php?f=70&t=202

DBRjeep350TPI

that is what I am planning to gap them at and see what it dose. I was reserching and reading about the silver tip plugs by accel and I guess they take a lot less voltage to spark... and it i am throwing all the super this and that stuff at them, they might be needing to gap a bit more from .035 to .045. not sure but worth a shot and only cost me time. also I was looking at the application guide for the 437ss plug and in a FI 350 the gap is at .050.
My deal is that I put the TPI on a 67' 327 block that has gasket seal heads, so I have to find a plug in the same heat range of a HEI FI 350 tappered plug and that is a shorty to clear my headers and is gasket seal.