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MG Midget and Sprite Technical - Race Engine Spark Plugs

I am just starting to build a new race engine for my Frogeye. The block is 1275 +40thou. The head is an MED offset clubman 31/37 offset valves. I am currently running BP7ES plugs in my existing engine. The question is do I keep to the BP7ES or go to BP8ES and will this make starting from cold more difficult? Finally what about using B7ECS will this provide any advantage to avoid pre ignition? Thanks Peter
P B Chappell

Hi Peter

The plugs you need will be the hottest ones possible that will be cool enough to stave off detonation.

This is great advice for a road car where the plugs need to run in a variety of conditions but a Race engine is probably always flat out. So an error towards the cool side for a race engine is fine.

I personally would settle for the BP7ES unless the engine was on the limit in which case I would go for some 8s

s the engine on the limit?
Robert (Bob) Midget Turbo

B8ECS are my preferred NGK race plug

James Bilsland

Effective compression ratio is a key factor in determining plug heat range. What CR do you have, and what camshaft? The higher the effective CR the cooler the plug will need to be.

To follow from Bob's comments, "error towards the cool side for a race engine", go to the coolest running plug you can get away with without them fouling.

Paul Walbran

with NGK plugs the number is the heat rating. The P is protruded tip and the letters at the end are what the core and tip are made of.

So an NGK BP7ES is the same heat rating as the B7ECS only it doesn't have a protruding tip. The E is terminal nut or solid post. The CS is short angled ground electrode.

NGK 8s are colder than NGK7s. I know all about cold plugs fouling in traffic etc but I run NGK 8s in my road car and it will sit in traffic all day without bother. My car does have a high energy coil and pertronix Ignitor ignition.

S at the end is standard copper, P is Platinum and IX is fine wire Iridium which my car has in at present.

Go for the colder plugs and all that can go wrong is low rpm misfires. Go for hotter plugs and all that can go wrong is a hole in the pistons.
Daniel Thirteen-Twelve

Peter, on a similar spec engine I run 7's for hillclimbs, 8's for racing. Projected or not doesn't seem to make any difference. On another engine, the projected electrodes extended just far enough for the piston to hit the end of the plug and close up the plug gap.
Mike Allen

Thanks for the advise I know all about the hole in the piston as I started last year on 6 which performed fine for three races but at Cadwell pre ignition and hole in piston - an expensive lesson.Peter Balwin has suggested 8 but have some 7 to get the engine warm. I am testing in a couple of weeks so I will try both and see where it gets to. Thanks Peter
P B Chappell


There are a number of reasons for holed pistons (pre ignition) the heat range of the plug is only 1 of the controlling measures a more effective one might be the advance or perhaps grade of fuel.
It is unwise to control detonation by plug heat range alone and you ought to sort this out on a rolling road long before testing at a circuit.

Robert (Bob) Midget Turbo

Bob On Peter Baldwin's rolling road on Tuesday. All looks fine on 7s so I will follow the old adage - if it ain't broke don't fix it. Peter
P B Chappell

Just out of interest, The piston that got a hole in it
Where abouts on the piston did the hole happen
Usually - but not always (it's a car after all ) if you get a hole in the middle of a piston it tends to be the mixture running off lean
If it is caused more by detonation from ign. timing, too much compression for the fuel being used or too hot a spark plug it usually tends to be around the perimeter of the piston and down the side

Everyone seems to agree here 7's or maybee 8's

IF it is a full on race only car my advice would be to have a set of 5's or 6's to start it up on ,run it up to temperature then fit the 7's for hillclimbs or short sprint stuff and 8's for racing
If you have plugs that are too cold and have them in for hill climbing you won't get going properly till you're half way up the hill
and also if you start your car up cold on cold plugs they miss and carry on and it leaves a coating on the insulator of the plug which helps promote misfiring even after warmup
William Revit

My advice is not to bother with a set of NGK 5s or 6s to start it up on.

I have NGK8s in my road car and it will start first time on a frosty English Winter's morning. How many degrees below freezing is that?

Having started, it will run and warm up nicely and without a misfire. Once the engine is fully warmed up then it's ready for full rpm.

All that stuff about soft/hot plugs to start an engine goes back about 30-40 years when plugs had a narrower heat range and before hi-energy ignition systems were the norm even for a modified classics.

Daniel Thirteen-Twelve

I'm talking about a full on race engine not a road car
Thanks Willy
William Revit

Add to that the fact that you do not want a fould plug to ruin your race.
Just take the extra precaution of switching plugs
Onno Könemann

It doesn't make any difference if the engine is a full on race A-series or a highly modified road engine which mine is. If the grade of plug used for normal running or racing is an NGK8 or the warmer NGK7 it doesn't need anything warmer for a start up.

Supposing the race engine used an NGK 9 then there might be an argument for starting it on a 7 but who runs an NGK 9 in a race engine?

There is no useful precaution in warming up the engine on soft plugs so a the colder plugs don't foul and ruin your race because once the engine is warm the most likely to time to foul will be in the assembly area, green flag lap or standing on the grid - all places where you aren't going to be able to change the plugs.

I think it's far better to have an ignition system than can ensure no plug fouling even at tickover (even if that's 1500rpm+ plus) on a race engine than bother with a second set of plugs.

I can add that my road car has been calibrated on the rolling road with a full race mixture for maximum power. Yes that does mean the plugs are a tad sooty on idle but they never foul and they never miss.
Daniel Thirteen-Twelve

William - for information the hole was on the perimeter of the piston and the plug tip had melted. I will try both 8 to start but have a back up set of 7 when I go to Snetterton next week for practice. Thanks Peter
P B Chappell

This thread was discussed between 16/03/2011 and 20/03/2011

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