Plug gaps

[markmark2]

Hi all, NGK plugs, the question is...0.025 or 0.028 imperial? luminition ignition. Would 0.003 make that much difference? Book says 25, advice says 28. Was set at 28 when I got the car, I set to 25 and changed plugs and set to 28 to see if any difference was made. None really that I could notice. Opinions welcome please. Cheers Mark

[wilf]

Not enough difference to make a difference IMHO.

[marshman]

Leave it at the "normal" 25 thou. As a rule of thumb every 1 thou equates to around a 1000V increase in the HT voltage and hence an increase in the electrical stress on the rest of the HV system. So widening the plug gap places more electrical stress on the plug leads, rotor arm and distributor cap. LUCAS ( AKA the Prince of Darkness) designed for a breakdown voltage of around 26kV (26,000V) on their distributors, so yes you can increase the plug gap and yes you might get a bigger, fatter spark, but does it do much for the combustion process - not sure (as you have found out) - but it will as sure as eggs is eggs "stress" the H.V. distribution system more..

I did play with this a lot in my youth. I fitted a "Sparkrite" system to my car and opened the plug gap to 32 thou. To be honest I didn't notice or measure any difference to anything - power, acceleration or fuel consumption. The only thing I did notice was the plug electrodes wearing a bit faster.

So my advice is leave it at 25 thou as per most moderns that do have electronic ignition as standard.

Roger

[markmark2]

Leave it at the "normal" 25 thou. As a rule of thumb every 1 thou equates to around a 1000V increase in the HT voltage and hence an increase in the electrical stress on the rest of the HV system. So widening the plug gap places more electrical stress on the plug leads, rotor arm and distributor cap. LUCAS ( AKA the Prince of Darkness) designed for a breakdown voltage of around 26kV (26,000V) on their distributors, so yes you can increase the plug gap and yes you might get a bigger, fatter spark, but does it do much for the combustion process - not sure (as you have found out) - but it will as sure as eggs is eggs "stress" the H.V. distribution system more..

I did play with this a lot in my youth. I fitted a "Sparkrite" system to my car and opened the plug gap to 32 thou. To be honest I didn't notice or measure any difference to anything - power, acceleration or fuel consumption. The only thing I did notice was the plug electrodes wearing a bit faster.

So my advice is leave it at 25 thou as per most moderns that do have electronic ignition as standard.

Roger

Nice one Roger, great answer. Cheers Mark

[TR5convalescent]

Leave it at the "normal" 25 thou. As a rule of thumb every 1 thou equates to around a 1000V increase in the HT voltage and hence an increase in the electrical stress on the rest of the HV system. So widening the plug gap places more electrical stress on the plug leads, rotor arm and distributor cap. LUCAS ( AKA the Prince of Darkness) designed for a breakdown voltage of around 26kV (26,000V) on their distributors, so yes you can increase the plug gap and yes you might get a bigger, fatter spark, but does it do much for the combustion process - not sure (as you have found out) - but it will as sure as eggs is eggs "stress" the H.V. distribution system more..

I did play with this a lot in my youth. I fitted a "Sparkrite" system to my car and opened the plug gap to 32 thou. To be honest I didn't notice or measure any difference to anything - power, acceleration or fuel consumption. The only thing I did notice was the plug electrodes wearing a bit faster.

So my advice is leave it at 25 thou as per most moderns that do have electronic ignition as standard.

Roger

Interesting, Roger. My old TR5 never seemed to burn all its fuel. I suspect that there's a difference in combustion qualities of unleaded vs 4 star.

I fitted Pertronix ignition with a huge flamethrower coil and 10mm leads, opening up the plug gap to 35 thou. The difference was unbelievable. The car went like a scalded cat and all unburnt petrol smells were banished.

I guess that with carburettors and a head that is designed for lower octane fuel anyway (and doesn't have the TR over fuelling problem) such a step would make little difference. Anyway, your post has saved me needless expense so thank you.

[Guest]

My old TR5 never seemed to burn all its fuel. I suspect that there's a difference in combustion qualities of unleaded vs 4 star.

.. huge flamethrower coil and 10mm leads, opening up the plug gap to 35 thou. The difference was unbelievable. The car went like a scalded cat and all unburnt petrol smells were banished.

The effect above is purely imaginary.
I have tested the 6 cylinder Pi cars on all types of fuel from 95-100 octane and there is NO discernible or measurable change in power output whatsoever on a dyno no matter what you put in it.

There is also NO power increase whatsoever by increasing the plug gaps.

What I can tell you however is that there is a drop of 10bhp at the wheels for every 10C rise in inlet temperature, so a test at 30C July summer temp, will give you what is effectively a drop of 25bhp at the engine compared with a dry January day at 10C.
At 30C the TR5 will run rich and make barely 100bhp, ie barely 80bhp at the wheels.

Also for your interest there is NO power difference between the CP and CR engines.
They all produce roughly 117-125bhp DIN on a good day.

The only difference is the CR engine produces a lot more torque from 1000-2000rpm but peak power is earlier, while the CP engine has poor torque low down but reaches peak power some 1000rpm higher up.

I tested this extensively all at my cost 4-5yrs ago, and have all the torque curves archived if you don't believe me.