350 Chevy Engine with HEI- Spark Plug Gap?

[user-23911]

If you're running a turbo setup, you start off with the factory plug gaps.
As you progressively turn up the boost, it'll misfire approaching peak torque.
The usual fix is to close the gaps a bit.
Then you can turn up the boost a bit more, the cycle repeats.
There's all sorts of aftermarket solutions, none of which work very well in terms of percentage gains. Not only that but they're unreliable and overly expensive.

Smaller plug gaps mean a slower overall burn time, you get more knock so you need higher octane fuel.
Bigger plug gaps (as in factory gaps) with higher boost levels require an increase in coil energy.

The energy is stored in the coil, there's only so much energy that a coil can store.
These aftermarket solutions are like using a bigger hammer on the coil .
Bigger / more coils make far more sense.
A small lightweight coil can't store as much energy as a heavier , bigger coil.
The energy is stored in the iron core.


So yes, it's a very simple, cheap power advantage.

[jeff swisher]

10:1 is not getting up there high enough to really tax the stock GM HEI. unless you start reving above 6000 rpm.
Then voltage really drops off and jumping that gap gets harder.

I built many 350 street engines. I ran many GM HEI distributors.
One engine was .028 piston to head with 54cc chambers 11.9 compression 245 psi cranking pressure.
The HEI was not happy with this. I hat to tighten the gap to .035"
Even then my single point distributor with a 1982 Ford coil would turn in 2 tenths better ET. You could feel the HEI laying down sooner in the upper rpm range.

Both had the same curve in them.
I see no power or MPG gains with large gaps say .045" vs the .035" I have went up to .060" with the white module from the 1977 era of Oldsmobile stuff that called for .060" plug gaps. I gained NO power or MPG.

1995 corolla called for .032" gap and no matter what spark plug I would have those faint misfire lines at the base of the porcelain. Tried new factory coil and wires and nothing changed.
I tightened the gap to .025"
The cars idle smoothed out and got 35 MPG vs 33 and no more of those carbon lines on the spark plugs.

I changed spark plugs every 10,000 miles looking for one that would make a difference.. nothing changed.
Car has over 300,000 miles on it now.
Lot of testing.

I currently have 2 11 plus compression ratio daily driver small blocks and over 200 psi cranking pressure on both.. HEI is used and gaps are .035"

I will sometimes cut the ground strap back so to expose at least 3/4 of the center electrode.
Your plugs are fine center electrode..Never had any luck with those no better than a regular copper center for my stuff.

You just need to experiment for yourself and see if there is a measurable difference between wide and narrow gap.
If no difference then for less stress on the coil i would run the narrow gap.

Test and see what it likes.

[rally]

10:1 is not getting up there high enough to really tax the stock GM HEI. unless you start reving above 6000 rpm.
Then voltage really drops off and jumping that gap gets harder.

I built many 350 street engines. I ran many GM HEI distributors.
One engine was .028 piston to head with 54cc chambers 11.9 compression 245 psi cranking pressure.
The HEI was not happy with this. I hat to tighten the gap to .035"
Even then my single point distributor with a 1982 Ford coil would turn in 2 tenths better ET. You could feel the HEI laying down sooner in the upper rpm range.

Both had the same curve in them.
I see no power or MPG gains with large gaps say .045" vs the .035" I have went up to .060" with the white module from the 1977 era of Oldsmobile stuff that called for .060" plug gaps. I gained NO power or MPG.

1995 corolla called for .032" gap and no matter what spark plug I would have those faint misfire lines at the base of the porcelain. Tried new factory coil and wires and nothing changed.
I tightened the gap to .025"
The cars idle smoothed out and got 35 MPG vs 33 and no more of those carbon lines on the spark plugs.

I changed spark plugs every 10,000 miles looking for one that would make a difference.. nothing changed.
Car has over 300,000 miles on it now.
Lot of testing.

I currently have 2 11 plus compression ratio daily driver small blocks and over 200 psi cranking pressure on both.. HEI is used and gaps are .035"

I will sometimes cut the ground strap back so to expose at least 3/4 of the center electrode.
Your plugs are fine center electrode..Never had any luck with those no better than a regular copper center for my stuff.

You just need to experiment for yourself and see if there is a measurable difference between wide and narrow gap.
If no difference then for less stress on the coil i would run the narrow gap.

Test and see what it likes.

I decided on the 035 gap. Good advice, thanks.

[claysmoker]

I'm of the school that thinks a little less gap is better than a little more gap.