Author Topic: Spark plug gap recommendations  (Read 396 times)

Offline 94touring

Spark plug gap recommendations
« on: September 10, 2018, 11:19:55 AM »
I know the book says .25 however that's bone stock with points.  Have any of you played with gaps on upgraded ignition systems?  I have my csi electric dizzy and tossing in a 40k volt flamethrower coil.  I have both 7 heat range plugs and 8. 

Offline MiniDave

Re: Spark plug gap recommendations
« Reply #1 on: September 10, 2018, 12:45:08 PM »
In theory if you're running a higher voltage you can run a bigger gap, what this gives you in terms of performance I don't know, but that's a good question for the Prathers on Dyno Day.
1989 Cooper Racing Green
2009 Clubman S
1969 Jaguar XK-E FHC
2004 Audi Allroad 2.7 TT

Offline 94touring

Re: Spark plug gap recommendations
« Reply #2 on: September 10, 2018, 04:09:24 PM »
I read the further gap takes advantage of the stronger spark.  Both performance and fuel economy.  How much seems to be a mystery

Offline John Gervais

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Re: Spark plug gap recommendations
« Reply #3 on: September 11, 2018, 07:39:10 AM »
I currently use 0.035" and have used 0.040" without issues, but haven't a way to measure performance differences.
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Offline 94touring

Re: Spark plug gap recommendations
« Reply #4 on: September 11, 2018, 08:11:07 AM »
I currently use 0.035" and have used 0.040" without issues, but haven't a way to measure performance differences.

What is your setup?

Offline 94touring

Re: Spark plug gap recommendations
« Reply #5 on: September 11, 2018, 08:29:36 AM »
Also on the subject of spark plugs, anyone side gapping?

Offline John Gervais

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Re: Spark plug gap recommendations
« Reply #6 on: September 11, 2018, 11:12:00 AM »
I currently use 0.035" and have used 0.040" without issues, but haven't a way to measure performance differences.

What is your setup?

It's a warmed up 998 with 9.5:1 CR, Lumenition Optronic ignition system, KC's small-bore stage-3 head, dual springs, roller tip rockers, 997 camshaft, HIF38, Maniflow exhaust, lightweight flywheel etc., etc., etc. -

EDIT:  Forgot to mention that I'm using NGK B6ES plugs.

Regarding 'side gapping', I haven't tried it.  As I see it, since the spark will jump the shortest distance between the electrodes (intersting reading in Vizard's TtASE 2nd edition, pages 443-445), regardless of where the negative electrode is physically situated.

Our A-series cylinder heads have good swirl/turbulent combustion chamber characteristics, so the likelyhood of the air/fuel mixture fogging up the air space between the electrodes is (in my opinion) minimal.  This is also why we generally don't index our spark plugs.  I also think that side gapping would wear the forward edges of the ground electrode rapidly, thus reducing any benefit by effectively increasing the electrode gap.

I think that, as noted on TtASE page 445, if anything, shortening the spark duration would yield greater benefits than side gapping and I would love a set of Nology wires to try this out on the MG shop's collaborator's (the engine builder) shop dyno. 
« Last Edit: September 11, 2018, 12:04:58 PM by John Gervais »
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Offline 94touring

Re: Spark plug gap recommendations
« Reply #7 on: September 11, 2018, 11:25:54 AM »
Maybe I put it to the test on the dyno.  I have been doing countless reading this morning and finally found a dynosheet on side vs regular... 3% increase.  As you say, may not matter on our engines.  Even if I make 100 crank that's only 3 ponies, but not insignificant on our little cars.

Offline John Gervais

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Re: Spark plug gap recommendations
« Reply #8 on: September 11, 2018, 11:55:29 AM »
Interesting dyno plots -  77.gif

I wouldn't mind an extra 2 or 3 horses; percentage wise, 2 or 3 on top of 65 - 70 would be significant.
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Offline MPlayle

Re: Spark plug gap recommendations
« Reply #9 on: September 11, 2018, 01:25:46 PM »
The "side gapping" reminds me of the Bosche W7DTC plugs I used to use in my old VW and the first few Minis I had.  They are a copper core, triple electrode plug and is a direct replacement for the plugs we typically use (NGK BP6ES or Champion N9YC).


Offline MiniDave

Re: Spark plug gap recommendations
« Reply #10 on: September 11, 2018, 01:34:01 PM »
Jesse said 30-35 would be good.
1989 Cooper Racing Green
2009 Clubman S
1969 Jaguar XK-E FHC
2004 Audi Allroad 2.7 TT

Offline 94touring

Re: Spark plug gap recommendations
« Reply #11 on: September 11, 2018, 01:42:11 PM »
Good to know and good to know.

Offline John Gervais

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Re: Spark plug gap recommendations
« Reply #12 on: December 06, 2018, 04:08:20 AM »
Dan, Have you seen the NGK B7ECS (and B8ECS, B9ECS...) spark plugs?

They tend to run just a wee-bit cooler than the B7ES plugs as the shorter ground electrode design provides a faster path to transfer heat away from the ground electrode, and have the 'oblique' ground electrode.  Originally designed for racing, they're more resistant to vibration as well. 

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Offline 94touring

Re: Spark plug gap recommendations
« Reply #13 on: December 06, 2018, 10:45:44 AM »
I went gapless with some 7 heat range brisk plugs.  Expensive suckers but those in conjunction with a 60k volt coil really made the car run smooth.