The Fuel Truth

In reply to APEowner :

When racing I’ve always used a colder plug than the plugs I used on the street.  The MG T series tuning manual specifically recommends it and the Jaguar factory manual on racing is very clear. In fact I used a N3G Champion in My car when N12Y’s are called for on the street.   

There used to be a phone number where I could get plug recommendations from Champion but that number is no longer in service.  

frenchyd said:

In reply to APEowner :

When racing I’ve always used a colder plug than the plugs I used on the street.  The MG T series tuning manual specifically recommends it and the Jaguar factory manual on racing is very clear. In fact I used a N3G Champion in My car when N12Y’s are called for on the street.   

There used to be a phone number where I could get plug recommendations from Champion but that number is no longer in service.  

 

Heat range on older engines is much more critical.  With leaded fuel and older oil control technology (rings, valve seals, valve guides and crankcase ventilation systems...) plug fouling is a much bigger problem so it's important to run a hot enough plug to prevent deposit buildup.  The switch to colder plugs when those engines are subjected to the stress of racing and the accompanying higher combustion chamber temperatures is to prevent pre-ignition and if you left them in on the street they'd foul quickly.  Some vintage race engines require warming up with a hot plug and a switch to a cold plug before heading out to the grid but that has nothing to do with power output and everything to do with having the plugs work long enough at low loads to get the engine up to temperature and preventing pre-ignition under heavy loads.

In more modern stuff, for the most part, I'd just run the coldest plug you can without fouling issues.  If you're not running into pre-ignition or anything when running it hard, the plugs are cold enough.  For engines that aren't too hard on plugs, those same ones will often be fine on the street without fouling. 

In reply to APEowner :

Thanks, I forget just how far out of date I am. The newest engine I race was designed in the early 1960’s ( Jaguar V12 )  but built essentially the same way until 1997. My older Race car was designed during WW2 fire watching on the roof of the Jaguar factory. My other engine was a updated prewar design. MG T series. 

 So modern cars used in racing use whatever plug they use on the street?  Makes me wonder if I used modern plugs in a vintage car could I race using the stock street plugs? 

In reply to frenchyd :

It'll depend a bit on the engine, how it's tuned, how you drive it on the street, how cool the intake air stays when running hard on the track, etc.  If the intake temps don't go sky high on the track, the engine doesn't use much oil and doesn't run overly rich under light throttle and at idle, you've got a decent chance of being able to find 1 plug that'll work all the time. 

In reply to rslifkin :

Are you saying heat ranges are getting wider?  Is that brand dependent? I’ve got many decades of familiarity with Champions but maybe Champion isn’t as good as other brands in the heat range width department?  

I’m used to changing plugs, heck with 12 cylinders it’s a process you get used to pretty quickly, but since the heads are aluminum and the plugs aren’t,  a few less changes would  put the rare flatheads at less of a risk.

It's not so much that the heat ranges have gotten wider, as the engines, tuning, etc. has gotten better in a lot of cases.  And those improvements let you get away with running colder plugs without having them foul up on the street.  

There are some other tricks to apply too.  Look at plug with a cut back side electrode (like the Autolite racing plugs).  That gives less plug surface to retain heat and cause pre-ignition, so sometimes it'll let you get away with 1 heat range hotter for track use and such (meaning you're less likely to foul that same plug on the street).  

rslifkin said:

It's not so much that the heat ranges have gotten wider, as the engines, tuning, etc. has gotten better in a lot of cases.  And those improvements let you get away with running colder plugs without having them foul up on the street.  

There are some other tricks to apply too.  Look at plug with a cut back side electrode (like the Autolite racing plugs).  That gives less plug surface to retain heat and cause pre-ignition, so sometimes it'll let you get away with 1 heat range hotter for track use and such (meaning you're less likely to foul that same plug on the street).  

I can see with modern Fuel injection how it would be so much easier to get fuel mixture spot on  compared to how”crude” carburetors are. However mechanically  it’s pretty close to the same technology as 20-50 years ago  perhaps tolerances have tightened a bit with more automation. But the laws of thermodynamics and friction remain.  

Yeah, I think it's mostly a combination of tighter tolerances leading to a bit less oil consumption and more fuel mixture consistency plus emissions regulations leading to engines running leaner at idle and light load (even if they're carb-ed). 

In over 75K miles of abuse in my ZX2SR I never changed plugs from the OEM.

In reply to iceracer :

That’s the other thing that blows my mind. We used to change to a fresh set of plugs for every race and later when Chassis dyno’s became available you could see a power difference between new and one race old plugs.