The Lotus Grand Tourers

Shock horror this week, fuel consumption dropped to about 8 mpg, car still running like a scalded cat but sounded a bit flat. So prior to MOT got her compression tested to see if the valves clearances had tightened post the rebuild especially as she has had two track days and rolling roads sessions but much to my pleasure the test came back as 235 on all cylinders. So what was the fuelling issue? Turns out that changing the spark plugs solves the problem.

Hey Jerry,

So with my team, I beat the snot out of them to not just say "fixed" but to give a detailed and considered analyse as to why it's fixed. They hate it and other things but.... why do you think? I can imagine but would appreciate your view...


M

muphy wrote:
why it's fixed.

I thought the same so I had to google it - here is one answer:

In a nutshell, as spark plugs wear, the gap between the plugs electrodes closes up. This in turn can effect your cars fuel consumption and increases your cars emissions due to a poor spark.

So, what does this mean for you and your car? The finer point equates to a more complete burn that delivers:
Improved acceleration
Improved fuel consumption
Smoother idling
All this means decreased emissions and improved fuel consumption from your car.

NGK have developed a range of Iridium spark plugs to meet and surpass the modern requirements of vehicle manufacturers. Precious metals such as Iridium have much higher melting points over traditional metals such as Nickel. This allows centre electrode diameters to reduce from 2.5mm using Nickel, to 0.6mm using Iridium.

Not quite...

As a spark plug wears the gap actually increases due to material being electrically eroded away.
If you have a weak coil or in Jerry's case a very high compression engine - the extra threshold voltage required to jump the gap becomes critical.... Potentially being blown out with the high compression.

If it is erosion and wear related then yes iridium would maintain the specified gap for longer and the current required to jump the gap would be less due to the finer point focusing the energy more effectively for the given gap.

My question would actually relate to how many miles Gerry's plugs have done?
If they are covered in carbon - perhaps running too rich, which would cause shorting from the carbon and effectively reduce plug gap size.
Overheating....this is an important question for me as I am contemplating moving to number 8 iridiums. Maybe Gerry's driving style has found that the current heat range isn't cool enough to cope with the power.

My suspicion is the wear factor of the plug has come into this and a weakness in the coil voltage and saturation time has been unearthed due to being a race engine. It is cheaper to keep replacing the standard bpr7es plugs than it is to get a better coil and more expensive plugs. Just replace sooner I guess as the longer you keep the plugs in the more fuel is left unburnt. Eventually they will pay for themselves.

I am using Ngk iridium plugs but they had done two track days and a rolling road session, to say all was sorted by changing the plugs was a little glib, it went a long way to sorting it but probably due to the engine settling she was running rich so back on the rolling road today and all now sorted with a few more ponies thrown in. Initially after a run to get the map corect we did the second power run and got 273hp and all was very excited, but only got that because she jumped on the rollers pooooo. Final run got us 189lb ft. and 253hp which considering my exhaust is restrictive and I am only running 45 throtle boddies I was chuffed with. I wasn't there to get the prints but should pick them up tomorrow so will find out what the peek power revs were I think the limiter is 8400 soft and 8700 hard so would expect it somewhere under that.

With regards to the plugs I will see how this set of iridiums go and how many miles I get before I get a significant drop off

Wow! that's a big figure. Congratulations

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Rolling road results.

MrCoolA wrote:[/url][/img]

Rolling road results.

Did they pull the engine out of the car or is it an estimated engine power figure?

Don't understand your question, it was in the rolling road.

MrCoolA wrote:Don't understand your question, it was in the rolling road.

I think he's asking how accurate is the conversion from wheel hp to flywheel hp ? (i.e. you can't get 100% accurate flywheel figures with the engine in the car

amarshall wrote:

MrCoolA wrote:Don't understand your question, it was in the rolling road.

I think he's asking how accurate is the conversion from wheel hp to flywheel hp ? (i.e. you can't get 100% accurate flywheel figures with the engine in the car

Yes, I just didn't express myself quite clear

Sorry. Didn't mean to sound so blunt. To answer your question, I have no idea. Never really thought about the flywheel just pleased together the hp at the wheels. Presume they have some kind of conversion factor, I'll try and find out.