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Author Topic:   Spark Plug Gap
Dirt Full Roller

Total posts: 56
posted July 10, 2002 09:34 PM
What would be a good plug gap for a 12.8 to 1 406 with an MSD ignition. We run Champion Race plugs V55C or V57C on Alcohol.

Jim Cleaver Jr
Cleaver Racing Inc

Dirt Full Roller

Total posts: 57
posted July 10, 2002 10:27 PM
Try .035 I haven't tested alot of regular Ignition mostly magneto's. But on a friends modified engine I dynoed for him we tryed MSD-6al, Davis Unified Ignition HEI, and a mag. Plug gap was .040 on all tests except where noted. The engine was a 355 at around 12 to 1 (not sure I didn't build it). The engine peaked at 6500 hp and 5200 tq. The davis dist was even with the MSD up to peak at 6500 but was 35 hp better a 7600. I didn't try it but I expect if I would have closed the gap down on the MSD to .030 it may have stayed even with the davis. The mag was the same as both to 6500 But was 8 hp better the the davis and 43 better then the MSD at 7600. HEDRENS may be able to shed alot more light on this subject as I have limited test on non mag Ignitions.
I also tried Increasing the plug gap to .060 on the davis (they say you can run .080 w/ 18 volt battery) but it lost 40 hp at 6500. So my experience is to be a little conseritive on the gap better to be small than to big. If you're to big it losses power in a hurry and you won't notice it by the sound of the engine.
Needless to say I wasn't real impressed with the MSD I think the 6al box is to small. But alot of late models run them.

P.S. I did try 2 different boxes (one brand new) and two different msd coils to. No change in outcome.

Dirt Maniac

Total posts: 135
posted July 10, 2002 10:52 PM
I used to spend a whole lot of time diddling with plug gaps, until I asked a Winston Cup engine builder what he recommended. He said; "Take them out of the box, look at them to make sure they have not been dropped, and put them in. You will cause more trouble by trying to change the gap than any gain will be worth." My plug changes now take a whole lot less time. On the plugs you mention, out of the box gap is about .024 or so, and on most RACING plugs(Autolite and Champion) the gaps are very consistant. Others may howl, as did my crew chief at first, but he eventually came around and this way has worked quite well for us. I should also say that I don't keep plugs in an engine real long like some guys do(a holdover from my 2 stroke days!)

Dirt Full Roller

Total posts: 56
posted July 11, 2002 09:26 AM
Thanks for the replies guys.
Monster your right about the gap of the plugs out of the box. We have been gapping them at .025 and hardly any change is needed on out of the box plugs. I was just wondering if I was leaving anything on the table with the plug gap.

I think i will stick with what we have been doing it seems to be working.

Jim Cleaver
Cleaver Racing Inc

Dirt Roller

Total posts: 9
posted July 22, 2002 11:04 PM
Get yourself a medical inspection light like what Doctors use to check out your ears. You can get one at http:/// plus Moroso makes one too.
When you look at the plugs look deep down into the porcelain insulator and at the bottom you should see a tan ring around the porcelain. Now the higher it is up the porcelain the richer the engine is running.

Also look at the ground strap and it should be a light blue color. When this blue color gets close to the body the plug is running hotter. I hope this helps. Ron Melville

Dirt Freak

Total posts: 499
posted July 22, 2002 11:33 PM UIN: 23443678
For the plug gap we call it "box Gap" works for me .

Dirt Newbie

Total posts: 2
posted July 23, 2002 10:32 AM
always go on the narrow side for gap wide gaps are great in a perfect environment start adding,high cyl press.,wet dirt,and the worst thing glycol(a conductor)then things go wrong also carefully look at the ground electrode you will see an temp line if heat range is correct the line should be at the bend richer to center leaner to body try this it is much easyer than looking down insulators

Dirt Freak

Total posts: 295
posted July 31, 2002 04:11 PM
would too wide of a plug gap cause an engine to miss fire at high rpm?

Dirt Full Roller

Total posts: 57
posted August 01, 2002 12:54 AM
YES ,it can but you may not here it and power will drop off severely.

[This message has been edited by birdcage (edited August 01, 2002).]

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