Spark Plug Gap

SydneyRoverP6B

Well-Known Member
Staff member
Following the initial change by Rover of the spark plug specification to Champion L92Y (now L92YC) the specified gap was 25 thou. This figure was quoted regardless of the compression ratio, so engines running the 10.5 : 1 CR would run the same plug and gap as those with the 8.5 : 1 CR.

I have been giving thought to the subject of spark plug gap and whether cylinder head changes or engine capacity changes or the addition of electronic ignition would warrant a variation.

The MGB GTV8 which also used the 3.5 litre Rover V8 with a compression ratio of 8.26 : 1, fitted with the Lucas 35D8 points distributor and 16C6 6 volt coil all as used in the 3500 and 3500S also used the same spark plugs, Champion L92Y, but in this case the factory specification called for a gap of 35 thou, a full 10 thou more. Clearly the coil and standard components were more than capable of dealing with the additional voltage required to bridge the plug gap, although I don't know at this stage whether or not any component reliablity issues surfaced as a result of the increase.

The SD1 saw the introduction of a new and improved cylinder head which featured larger valves and long reach spark plugs. In 9.35 : 1 CR form, the spark plugs were Champion N12Y and the gap until the end of 1981 was set at 30 thou. From 1982 onwards the gap increased to 35 thou.

So to question time. If your 3.5 litre engine is fitted with later cylinder heads or your lovely Rover has a nice new engine of increased capacity running under the bonnet, what spark plugs are you running, and what gap have you set them to? Have you experimented with different gaps and found any improvement with one setting over another? If you are now running an electronic ignition system or your engine is fuel injected, did you make a change in plug or gap?

In my case, my 4.6 runs Champion N9YC spark plugs, which I have set to 25 thou. I use Lumenition in a new Lucas 35D8 distributor that I purchased 18 years ago. The original Lucas 16C6 ignition coil manufactured in 1973 continues to provide outstanding service. Being a "Thor" engine, in original form my 4.6 would have been distibutor-less, powered by a Bosch Motronic engine management system and running platinum plugs.

I value and appreciate your thoughts.
Thanks in advance,
Ron.
 
As you say early V8's ran with L87Y's until the intro of the L92Y's which were then retro-fitted to all engines at that point, and gapped at 25thou. Increasing the gap will give a more powerful spark hence the bigger gap on the SD1's, their electronic ignition being, supposedly more capable of providing the extra higher voltage needed to bridge the larger gap. The more you increase the gap however, the longer it takes to build up the required voltages, so at higher RPM problems can occur, so it becomes a case of diminishing returns. On a standard P6B you could try to see any improvements brought about by increasing the gaps, and with later heads and electronic ign I would definately run more than 25thou, 35 being a starting point and see what happens from there. I wouldn't think there would be any real advantage of opening it up much more than that.

Remember that all comparisons would have to be done on a "like for like" basis, without any other alterations done at the same time. There's also the matter of different makes of plugs, and heat ranges to consider as well.
 
Well as I mentioned over at "My Rover" thread in the Member's Project section, I am currently experiementing with spark plug gap. The original P6B engine called for a 25 thou gap, later Rover V8 engines without points up to 35 thou and possibly even 38 thou with the P38 Range Rovers.

What gap do you run on your engine? Do you have SD1 or even later heads and/or electronic ignition? If so, have you increased the gap and if so what have been your observations?

Ron.
 
I should add that I started this thread back in June 2009. So far Harvey is the only person to kindly reply. Given we all have spark plugs in our engines, and the size of the gap is of importance, what gap do you run and why? Have you made allowances for changes to your iginition system and / or cylinder heads? Why do you run the gap that you do? Do you check the gap of the spark plugs when you buy them prior to installation?

Ron.
 
Hi Ron, I prefer the lazy side of maintanence and havent changed my plugs since about 2005 :LOL: have changed to electronic ignition with an ignitor II and ignitor II coil so probably could run a 50 + thou gap so when I change the plugs in about 2015 I will give it some thought :mrgreen:
Seriously though I would love to know other people results to see if it would be worth pulling the plugs to regap.

Graeme
 
Hi Ron.
When I recently changed the plugs, I asked a mate of mine who owns a car yard, about gapping the plugs.
He said..."dont bother, nobody does that anymore."
So I just put them in...
Car starts beautifully...but then starts to run rough after about 10 seconds.
Presently though, radiator is not fitted, so cant start it.
So basically, I have no idea what gap it has, but I will.
 
Hello Brenten and Rich,

I have a feeling that is the case with nearly everyone on the forum. I have always been particular about setting it correctly, as it most certainly does matter.

Ron.
 
I can believe it but my experience with the car so far has been more about getting it to work and be roadworthy and not very detailed in these things I'll admit.

Hopefully once MOT has come and been passed then I'll probably spend more time getting some of the niceties done like this, better leads and get rid of the mechanical fan, the rear windscreen rubber etc...
 
Maybe new plugs for old cars come ready gapped at 25thou but do these fancy plugs for modern cars come with bigger gaps ?
I can't see many new cars using N5's and N9Y's !
 
DaveHerns wrote,...
Maybe new plugs for old cars come ready gapped at 25thou but do these fancy plugs for modern cars come with bigger gaps ?
I can't see many new cars using N5's and N9Y's !

Hello Dave,

Modern fuel injection and engine management systems call for different plugs. The P38 Range Rovers for example running the 4.6 like mine would use platinum plugs gapped to 35 thou.

Gaps can't be assumed, they need to be checked.

Ron.
 
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