one cylinder running colder than the rest.....

[Verne_Frantz]

[ QUOTE ]
Yep..forgot about plug heat ranges affecting things. I learned that with single cylinder 4 stroke go-kart racing.

[/ QUOTE ]

Tom,
I doubt the plug heat rating will affect the combustion temp. It will affect how long it lasts before melting or fouling. I think that changing a plug heat rating in a cylinder is just a compensation for the heat of that cylinder; not a method of changing the heat in that cylinder.
JMO
Verne

[sean70ss]

Usually if a motor is running cooler on a cylinder it is fat I.E. blow by or to much fuel. I would still do a leakdown better safe then sorry but I agree 100 degree temp is not alarming. On a bbc it is hard to tune each specific cylinder eqaully. 8-5 are the most difficult.

Sean

[Chevy454]

[ QUOTE ]

Tom,
I doubt the plug heat rating will affect the combustion temp. It will affect how long it lasts before melting or fouling. I think that changing a plug heat rating in a cylinder is just a compensation for the heat of that cylinder; not a method of changing the heat in that cylinder.
JMO
Verne

[/ QUOTE ]
Verne: a spark plug is doing exactly that, ie, helping regulate chamber (combustion) temperature by dispersing (or not) heat...the amount dissipated has to do with insulator length. I can't remember the exact numbers off the top of my head, but the % of cylinder temp that is transfered via the spark plug is surprisingly (alarmingly!) high...over twice the amount cooled by the charge, I believe?

Stagger jetting and plenum design/alteration can help cover up the distribution problem, but I'm gonna bet Eric has some great info/numbers on this subject...

[VintageMusclecar]

Actually Rob, you guys already nailed it.

[Verne_Frantz]

Rob,

I know that I'm paddling against the current with my perceptions, but I still believe that the heat range of the plug should be selected based on the combustion temp that is is present so that it does not foul or heat to the point of melting or causing a "hot spot" that will cause pre-ignition. Race cars make more power with more heat in the combustion chamber (heat is energy), but race cars always require colder plugs than a street car. At low rpm a hotter plug is better because it will keep the electrode cleaner (not fouling) but race cars operate at high rpm which brings in more cool fuel which produces more energy when it's ignited. The transition from cold to hot is what produces more power. Cold fuel mixture is denser, thereby allowing more molecules of fuel in a given mass of air. Power is derived when that fuel-air mixture is ignited by a spark. I don't think that if the electrode of the plug is hotter or colder, it is going to increase the heat produced by the ignition of that mixture. I believe the spark itself ignites the mixture and the mixture produces it's own heat, regardless of how hot the plug electrode is.
If a hotter plug produced more combustion heat (which equals power) everyone would be running hotter plugs, but race cars require colder plugs. The best way to tune a race car's plugs is to start with the coldest plug you can find, then gradually increase the heat rating until the porclain is nearly white, without a trace of aluminum spots on it.
As I said, I know I'm rowing upstream on this debate, but that's how I see it.

Verne

[bad1racing]

Hey guys,My brother used to work at a engine shop and I watched them run a alcohol sprint car motor on the dyno.When they were warming it up he had it running at about 1500rpm and a couple of the cylinders were running cherry red.So I said something to him and he buzzed the motor a couple of times and then set it at a different rpm and then all the pipes were the same.Like a couple of the cylinders were running a little lean at the first rpm and fattened up at the other rpm.Just an observation.

Kendall