Spark plug variations

[mongoose]

Yesterday i went to my local motorfactors for some plugs for my Griff 500.Instead of wanting the normal NGK B7ECS type,i was after a resistive version(i'm now running coil packs, not a dizzy).The nearest equivalent that NGK had in their range was a BPR 7ES type.The plugs were different in that the latter plug had a central electrode that protrudes past the thread length,and also a longer earth electrode.So i got to thinking,whats this all about then,why do TVR reccomend the shorter central electrode that finishes level with the end of the threads,and has a shorter earth electrode.And also what are the differences in design for?
What was interesting,is that Champion reccomend their equivalent of the BPR 7ES!
I was worried that the BPR7ES might protrude too far into the combustion chamber,but if Champion reccomend the same design,then surely this must be ok?

[Pigeon]

I generally find the protruding-gap BPxxx plugs give slightly better ignition than the flush-gap Bxxx ones. The protruding gap gives a slightly larger area for the flame front to spread from, and the mixture is likely to be moving faster further away from the wall. However, I'm talking of fairly crude two-strokes with ignition systems that are suboptimal to start with. Whether you'd notice anything on a four-stroke with a high-power ignition system is debatable, but it doesn't cost anything to try.

It's easy enough to check the available clearance with minimal risk of damage to the engine. Take an old plug and wind some solder wire round the earth electrode, with the end sticking down 1cm or so along the axis of the plug, screw it into the head and turn the engine over with a spanner on the crank nut. Then take the plug out and see what length the protruding solder has been squashed to. Chances are it won't have been touched, but it doesn't hurt to check

[v8 racing]

Yes they are fine without going into detail!

[mongoose]

v8 racing said:

Yes they are fine without going into detail!

Thanks for the replys both of you.
I'd be more than happy for the detail,you certainly wouldn't bore me Rob!.I enjoy learning about things like this,as the Griff is my great hobby

[GreenV8S]

I'm running the longer plugs too bcause I couldn't get the right combination of length, resistance and heat rating. Must admit I did wonder whether the length would make any difference, but I didn't notice any.

[stevieturbo]

You would need to run extremely protruded plugs, or some very odd pistons for them to ever make contact.

I used to run PFR7B platinums on my RV8, and they worked well. DId try all sorts though, although never the short types like TVR reccomend.

[Hasbeen]

A few years ago, I built a new full race engine. It had a different combustion chamber shape to the old engine. On the engine dyno we tried 3 different plugs, with different reach, & electrode position/shape, all recommended by Bosch, as suitable for the engine. One was the same as used in the old engine. The difference between the best & worst was 5% increas in BHP. I have never got so much extra BHP so cheaply. With a modified engine it can make a big difference, as the position of the spark will change the flame front in the cylinder. With a stock engine I doubt you will improve on the manufactures recommendation, as they will have spent a lot of time dyno testing, in developement,including plugs. Where you may be able to make an improvement is if a new type of plug was developed after the engine was built, or you have improved the ignition system.