DanielRicany

Hello. I have a 1985 Corvette. Originally they came with iron heads with 9:1 compression. Now I have aluminum heads with 10:1 compression.

My car calls for 0.045" gap on the Catalyst sticker.

The later L98 cars have 10:1 compression with aluminum heads and call for 0.035" gap.

So should I use 0.035" or 0.045"?

Right now I am using 0.045" and haven't noticed any problems that I'm aware of, but I'm just wondering if going to a tighter gap would improve longetivity or all around performance.

Thanks.

John A. Marker

The HEI distributor has the power to fire the .45 gap without a problem.

BOOT77

I'd run .035, it'll be easier on your ignition components.

LongBeachC4

The 85's started out at a 45 gap but the factory went back to 35, I have the sticker on my 85. That said I run Pertronix ignitor and coil and was running AC 1 plugs. I ran into a problem of fouling #2 after about 5000 miles. I went back to standard R45TS plugs gapped at 35 and have no problems. I would run a 35 gap, I don't see a performance increase going to 45 but running higher compression I might look at a step cooler on the plugs.

DanielRicany

I am using the stock recommended plug for the later L98 cars.

Joe C

I'd run .035, but WTF do I know....

DanielRicany

Okay. Does an MSD 6AL2 ignition box have any effect on this?

LongBeachC4

Whole lot of time and money just to pick up a couple of ponies. A good coil, wires, cap, rotor, and igniter is plenty. As for the gap, it's better to have reliable spark than a longer spark.

Cliff Harris

Split the difference and make it 0.040".

Tom400CFI

The coil is the only thing that cares. The rest doesn't matter.



No. If it's not misfiring under load, the gap is fine.

BOOT77

I think if it becomes harder to jump it'll find the next easier spot and that can be anywhere between the coil and plug.

cardo0

Yes the MSD box will add a lot of power/energy but the spark interval is very short. Since u dont post a profile let alone if u use a MSD box or just the HEI im not sure what to tell u.

But ill gives this a try. The wider gap uses more energy and the HEI can be limited at high rpm with a weaker spark. But at low moderate rpm the HEI spark is longer duration hence the MSD uses multi spark when less than something like 2k rpm. I guess the term for the HEI is the spark is more robust at moderate rpm.

Yes an increase in compression sometimes can need greater spark energy - so an MSD can give u more spark engery and allow the wider gap. But from 9:1 to 10:1 compression increase i doubt u will see a need for the MSD. And an high energy system can create problems as over 50,000volts can exceed the insulation capabilites of the plug wires.

I guess if u have the MSD installed then why not make the most of it and use the wider gap? If still just HEI then use the smaller gap and monitor for misfire, knock counts, blackend spark plugs to see if the higher compression wants more spark. But widening the gap on the stock HEI is gonna drop the energy of the spark - more so at higher rpm. BTW the higher spark energy of the MSD box may use up the cap/rotor/spark plugs faster.

Hope this helps more than it confuses.

Tom400CFI

I doubt it did.

Increasing gap doesn't decrease spark energy. In fact, it increases firing voltage.

The MSD box in NO WAY, increases the "spark energy". What does the MSD box do? It does the same function as the stock ignition module; it is an on/off switch for the ground side of the primary circuit in the ignition coil. The ignition coil is what determines "spark energy", not the module. The only benefit that the MSD box provides are:
1. It "looks cool"
2. it could prvent a ignition module failure, as it presumably more robust than a stock module (although I have yet to see aftermarket ignition components out live stock ones
3. it has a rev limiter.

People think that the MSD box can do miraculous things b/c they don't understand what they actually do. They are an on/off switch for the ground side of the coil at that is it.

BOOT77

I've herd increased gaps can show more hp on a dyno, but with a real load they can miss. I've read a stock hei module is 4amps, a perf hei module is 5amps, a msd hei moduiel is 7.5amps and the 6al box says .7amp per 1000rpm. I really think the msd box does mostly what it's namesake is and like mentioned it's for the other features like rev, step, ect. I also know multi spark will fire a rich mixture easier sub 3000rpm and for example a drag car won't fine tune for cruise/low rpm since the motor sees maybe 4000rpm+ from launch to finish. Just like racers lock out a dist because the motor doesn't see the curve rpm's. MSD does say you can run more gap than stock, but most stock engines are low compression.

In other words every engine is diff and only trial & error in a real world measured test will give you the best results.

LongBeachC4

Will it run better, maybe but your really not going to notice it that much. Will you get more hp, maybe 2 or 3 with the 6AL. Were talking spark, there isn't much to it. It either fires or it misfires. The factory rolled back the gap on the 85 because some bad coils got thru production. Yes you can run .45 but your not going to see any noticeable difference from .35 and that is what the R45TS AC plugs come pre gaped at. I run .35 with a hotter coil and igniter and have no problems, it's reliable. Are you going to go faster or leave the line with a big advantage ? No