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MG MGB Technical - NGK BP7ES?

I have been seeing several mentions of this hotter NGK recently and even got the same suggestion from my cam manufacturer.

I mistakenly mentioned I was at 10:1 compression. Its actually 9:5:1

Is the 7 still recommended? At what compression ratio theashold is where the 7 is preferred over the 6?

TIA.
Max71

The 7 is a colder plug than the 6.
Dave O'Neill 2

Oh, thanks. That's even more interesting then. I would've thought with a higher performance it would be hotter.

Max71

The temp the plug runs at is dictated by the depth of the ceramic surrounding the centre electrode, the longer the ceramic the hotter the plug runs, so a 7 has less length/depth of ceramic than a 6 so the 7 cools faster. This helps prevent detonation/pre ignition even melting of the centre electrode. Run some 5s in a race engine that requires 8s and if it doesn't kill the engine in a couple of laps the plug centre will melt! Conversely, if the plug runs too cold it can foul up!
So, for a 'hotter' engine you need colder plugs.

Peter
Peter Burgess Tuning

https://www.ngk.com/learning-center/article/207/i-have-slightly-modified-my-motor---do-i-need

This article maybe of interest.
Mike
Mike Ellsmore

"A good rule of thumb is, one Heat Range colder for every 75–100hp added."

That's quite a big change in power for one step in plug temperature.

Other things that can cause hotter combustion chamber temperatures than would otherwise be the case are things like retarded ignition and weak mixture.

But never mind the theory, just monitor plug condition over time and see what happens.
paulh4

Thanks for the incredible information. With the amount I studied the engine I don't know why I never came across this. I'll give it a go.

Car runs great but if I don't open the butterflies immediately at shut down I'll get nasty pre-ignition. Since I'm good at the trick I just ignored it as a fact of a built out engine.

Being greedy -- I run at stock gap's. Is there a different gap preference?
Max71

Which distributor? 25D4 was 25 thou, 45D4 35 thou. The bigger they are the more spark you get, which may give a tiny boost somewhere, but the more it stresses the HT system as the bigger the gap the higher the peak voltage.

Running-on was so bad for me when we changed to unleaded I had to install an anti runon system. I say 'had to', but that was because I hated stalling it in 4th or flooring the accelerator.
paulh4

25D4 set to .025

I've set it to .030 in the past not seeing significant benefit performance wise and didnt want to stress the HT.

Sucks you had to do that. I've owned the car long enough to never recall it happening with 100+ octane. Now the best is 91. Unless I want to trek to a station that supplies the track and I can get 98 for a pretty penny.
Max71

Paul, where did the rule of thumb come from? Taking an MG midget from say 75 bhp to 120 requires going from NGK 5/6s to 8s and that is just +45 bhp. Run the 5s or 6s and we have seen holes in no 4 cyl after a few laps when they have been left in by mistake after warming up. Bs go from 6s at 85 bhp to 8s at 120 bhp, again up two grades with 35 bhp.

Peter
Peter Burgess Tuning

Peter,

That rule of thumb, that Paul was questioning, came from the article that Mike posted just above Paul's comment.

Charley
C R Huff

Hi Charley
A load of rubbish then, unless one is dealing with bigly 400 bhp V8s before tuning :)
When we prepare tiny A series engines which may go up from 38 at wheels to 65 we still have to go up from 5/6 to 8s!
Too complex an issue to merely talk about bhp increase in pure numbers, maybe they would be better talking % increases but that seems daft to me too.
We can also vary, on Bs from 6 to 7 depending on the use of the car. More town and slow driving and I would tend to runs 6s, more open road snarly stuff and I would run the 7s, that is with engines making the same power!
Horses for courses comes to mind. Getting the plug temp correct is a must.
Peter
Peter Burgess Tuning

I only posted the article!! I agree range is a pretty high but I do think they are talking about modern high performance engines not 60 year old B series clunkers. NGK do have a bit of experience with spark plugs though.

Mike
Mike Ellsmore

Not getting at you Mike, just that 75-100 is plucked from the sky in some manner which isn't apparently logical.We only use NGK plugs as we have found them to be the most reliable. However, these folk do not run engines to test which plug is best for our A series, B series, Rover V8s etc etc, people like myself who do maybe two or three Bs a week over a 30 year period gain knowledge of fuels and effects of states of tune on spark plug requirement. If you told the NGK rep an engine was in good order yet the plug tips were death white, once you explained timing and fuelling optimised they would say run a coooler grade of plug they wouldn't say all is ok cos you aren't 75-100 bhp in advance of the original bhp. Even modern engines 75-100 bhp is a good increase over standard. Unless turbo or SC when the plug grade can be very picky!

The same obtains with most cam grindeing companies, they do not build and test engines, they grind cams. To get the best cam buy one from the people building and having good reputation for your engine as they will know what the cams really do, not just on paper or a screen.

Peter
Peter Burgess Tuning

This thread was discussed between 22/05/2017 and 24/05/2017

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