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Old 05-22-2005, 16:40   #1 (permalink)
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How important is spark plug gap?

How important is plug gap?
How do you know if it isn’t set correctly?

I run a 351 with flat tops and 302 heads (guessing close to 11:1 C/R) and straight gas thru a recurved XE electronic dissy.

NGK suggest that when using LPG to reduce gap from 0.9 mm to 0.8 mm and to use a plug that is one step colder then original.

How can I monitor what is the best plug gap for my setup?

I’ve purchased a Crane Hi-6 with matching coil. I would have thought that installing this hardware would totally move the goal posts again. Maybe NGK’s recommendations don’t matter any more?

Any suggestions?
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Old 05-22-2005, 17:03   #2 (permalink)
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Re: How important is spark plug gap?

Honestly? Very important as it impacts on -:

a) how big the spark is
b) the burn

I usually gap mine to .88 instead of .9 as when they get hot/worn they open slightly anyway.

The plug gap will also determine whether the engine will start easy or not. The optimum gap will allow it to start easy.
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Old 05-22-2005, 19:47   #3 (permalink)
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Re: How important is spark plug gap?

Fitting of a 6AL or Crane multiple discharge means you can run a bigger gap - both companies recommend approx an extra .18mm (converted from imperial).

I've tried it but found it made no difference to performance, though it didn't cause any problems.

Gas needs more energy to burn than petrol so the smaller gap is recommended - usually .1mm.

That leaves you with .9 + .18 - .1 = .98mm.

So you may as well just open up the NGK's slightly it won't do any harm.

Also make sure that the coil is a good one, even if it is "matching" it might not be a higher power than standard coil.

In MSD's case the blaster 2 coil (the one recommended) is nothing special... The 6AL unit works better with one of their HVC type coils.
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Old 05-22-2005, 21:52   #4 (permalink)
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Re: How important is spark plug gap?

I have never found any difference within reason. I run 1.1 mm gap, and it runs fine(with my MSD). Gapping them is hard to do accuratly anyhow.
If you have a std ignition, .8 or .9 and MSD up to 1.1.
brenx, do you have any dyno results showing results of different gaps?
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Old 05-22-2005, 22:00   #5 (permalink)
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Re: How important is spark plug gap?

Nah no dyno results. Just what I've experienced when playing with plug gaps. I doubt you'd gain power or loose power from incorrect gap as such. If you get technical an insufficient burn means less power which you can get from an incorrect gap so on the other hand power loss can happen. The difference between 1.1 and 0.9 is more than likely not going to gain anyone power as the gap is within spec.

The only thing I've noticed where gapping does make a difference is cranking rpm.

On my 279rwkw dyno I had plug gaps of .8->1.0. It varied throughout the 8 plugs and made no difference apart from starting.
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Old 05-22-2005, 23:42   #6 (permalink)
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Re: How important is spark plug gap?

My view is that plug gap is important.

High voltage ignitions can normally "crack" a spark across a very big gap and can cope with a lot of cylinder pressure.
We may not notice it on track or even on a dyno but a wider plug gap retards the spark. It takes a longer time to fire.

I have found from experimenting that changing plug gaps can make a difference to throttle response and the way a car launches. If it can do that it must have some bearing on engine performance.
By "engine performance" I don't mean outright horsepower or torque.

It matters just as much as having the accelerator pumps shooting correctly.

The fact that NGK have recommendations measured in 0.1mm increments says that small differences in gaps do matter.

The best way with plugs (if dyno runs are unavailable) is to put in a set that is recommended and try them but keeping a close eye/ear on how the engine performs. If it feels lacking in response or power, pull the plugs out and look at them. If the engine runs hotter have a look at the plugs. They show up problems with engines before you even know there's something wrong.

Find someone who knows how to read them and get them to show you - it can save losing engines.

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Old 05-23-2005, 02:36   #7 (permalink)
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Re: How important is spark plug gap?

Quote:
Originally Posted by jock
My view is that plug gap is important.

High voltage ignitions can normally "crack" a spark across a very big gap and can cope with a lot of cylinder pressure.
We may not notice it on track or even on a dyno but a wider plug gap retards the spark. It takes a longer time to fire.

I have found from experimenting that changing plug gaps can make a difference to throttle response and the way a car launches. If it can do that it must have some bearing on engine performance.
By "engine performance" I don't mean outright horsepower or torque.

It matters just as much as having the accelerator pumps shooting correctly.

The fact that NGK have recommendations measured in 0.1mm increments says that small differences in gaps do matter.

The best way with plugs (if dyno runs are unavailable) is to put in a set that is recommended and try them but keeping a close eye/ear on how the engine performs. If it feels lacking in response or power, pull the plugs out and look at them. If the engine runs hotter have a look at the plugs. They show up problems with engines before you even know there's something wrong.

Find someone who knows how to read them and get them to show you - it can save losing engines.

Jock
What gave you the crisper response - smaller or larger gaps?
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Old 05-23-2005, 05:02   #8 (permalink)
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Smile Re: How important is spark plug gap?

Quote:
Originally Posted by STROKEXD
What gave you the crisper response - smaller or larger gaps?
Smaller ones - I think they were .028"; larger were .038" with Mallory Super Mag.

But then using MSD 6A (same car/engine) going from approx. .028" up to .040" gave good results with that ignition.
I think the important part is getting a plug and gap settings that work well with a particular combination.

I have used differing plug gaps on a few types of engines to alter on track performance - drag racing, motorcross bikes and kart engines. All responded to gap changes at different times.

I never saw any increase in power doing this, only changes in engine response. It may be as simple as the wider gap creating a better spark and more engine heat therefore more grunt. I don't know!

I don't think it would matter such a lot in a manual car but with an auto - the better the throttle response the harder you can "hit" the convertor.

Lot of theory in there but it works for me.
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Old 05-24-2005, 01:44   #9 (permalink)
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Re: How important is spark plug gap?

Personally, on gas I run an NGK cold v-type plug at 0.9 with MSD and a bosch coil (the little black transformer type).

The gap makes a difference (in theory.....). The bigger the gap, the more power required to jump the gap, hence the spark ends up 'hotter' but shorter. Close the gap, and you get a longer 'cold' spark.

Now, all that was fine 20 years ago. We now have high energy coils, v-type (splitfire) plugs, MSD, etc, etc..... so the theory goes out the window.

How many people 'in the game' have pulled plugs out that are worn to a knife point with about a 1.5mm gap?? These cars are still going without any sign of a miss. Mind you, the coil is being pushed to the absolute limit to maintain a spark. Put plugs like this into an XW (or similar) with the original points ignition and a 12 volt coil and it will be lucky to start, let alone run smoothly.

My two cents.....
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Old 05-24-2005, 16:26   #10 (permalink)
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Smile Re: How important is spark plug gap?

Quote:
Originally Posted by RPO-083

Now, all that was fine 20 years ago. We now have high energy coils, v-type (splitfire) plugs, MSD, etc, etc..... so the theory goes out the window.

How many people 'in the game' have pulled plugs out that are worn to a knife point with about a 1.5mm gap?? These cars are still going without any sign of a miss. Mind you, the coil is being pushed to the absolute limit to maintain a spark. Put plugs like this into an XW (or similar) with the original points ignition and a 12 volt coil and it will be lucky to start, let alone run smoothly.
It's always been the case that we could get away with running engines that had plugs so fouled up that you hardly see a gap. Even with poor ignitions.

The point I'm making is that plugs should not be ignored and are still an important part of an engine. If they aren't set right or are wrong plug, straight away you have put a potential problem into your engine. The use of high powered ignitions only means we can get away with more for longer.

This site could be worth a look http://www.ngkspark.com.au/spark_plug_tech_tips.htm


Cheers Jock
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