Spark Plug Gap

[RoverP6B]

I am just wondering what gap you set your plugs too and why? Have you tried a different gap, with all else being unchanged, and what was the outcome?

I understand that a higher compression calls for a smaller gap compared to the same engine running a lower compression where a wider gap can be used.

Thanks in advance,
Ron.

[sidecar]

I am just wondering what gap you set your plugs too and why? Have you tried a different gap, with all else being unchanged, and what was the outcome?

I understand that a higher compression calls for a smaller gap compared to the same engine running a lower compression where a wider gap can be used.

Thanks in advance,
Ron.

I run a standard ignition setup and gap the plugs to 32 thou.

I've read a bit about plug gaps in A. G. Bells book on 4 stroke lumps, basically he thinks that big gaps do not make more power, they load up the ignition system and may cause mis-fires. I guess it depends on the HT voltage at the end of the day. If you are running around 25 - 30 kV then you can not run huge gaps. If you run 50 kV then you are going to need very good leads and dizzy cap. Just look at what the power supply companies use on their pylons to stop 50 kV leaking away.

JMHO!

Pete

[RoverP6B]

Hello Pete,

I appreciate your reply!

The reason I asked is that I wondered whether or not a slightly wider gap, all things being equal would be beneficial.

The gaps specified for the 3.5 litre as fitted into the P6B was 25 thou. That covered all compression ratios from the 10.5 : 1 to the 8.5 : 1. The ignition system was essentially the same for all.

With the MGB GTV8 which ran the same engine, CR of 8.26 : 1, same distributor and ignition coil, the plug gap was set to 35 thou. For both the P6B and the MGB, the spark plugs were Champion L92Y.

I understand that for higher compression engines, the gap needs to be reduced, so maybe Rover employed a blanket approach with the P6B.

For lower compression engines a wider gap can be used, so maybe this is why the 10 thou increase applied to the MGB GTV8. In both cases the ignition coil used was a 6 volt Lucas, connected through a ballast resistor. The only time full available battery voltage was applied to the coil was during starting.

With my 4.6, compression ratio 8.37 : 1,.. I have the plug gap set to 25 thou, but was wondering if when I next change them whether an increase to 32 thou might be beneficial? Would I notice any difference at all?

Thanks,
Ron.

[sidecar]

I doubt that you would notice a difference if you opened up the gaps but it's worth a try! In the book that I mentioned I think that it stated that the larger gaps can help at very low RPM but not when the revs are high.

You may notice that the engine seems a little better just due to the fact that you will have fitted new plugs, I guess a better test would be to re-gap the old plugs and re-fit them.

BTW my 4.6 runs a 32 thou gap and a CR of 10:1

Pete

[ian.stewart]

I run a stupidly high comp engine on the street, admitidly with a huge overlap cam,
When I fitted my Jacobs super amplified coil it advised to open the plug gap to 45thou, started wonderfully every time, BUT every time I raced it I had a horrible top end misfire, which was eventually was cured by returning plugs back to stock dimensions,

[r2d2hp]

Quite a good article here >> http://www.centuryperformance.com/spark ... pg-26.html

[JSF55]

I tried mine at 32 thou and ended up with a misfire so swopped them back to 28 thou, pertronix set up in a stockish rover

[CastleMGBV8]

Good subject, and something which can be overlooked when problem solving.

Reg, Interesting article. Didn't know the heat range of the plug was to remove heat from the cylinder!

I've always been and advocate of not using mega coils on otherwise standard ignition systems as this can give more problems than benefits.

Kevin

[mgbv8]

I run mine at 25 thou with a mallory 6AL and a blaster coil + nitrous. No probs for me with stock gaps.