[Fot] Spark plug suggestions

[Bill Babcock]

It's also a common motorcycle plug for two stroke Yamahas (which  
aren't that common anymore--I don't remember the last time I saw a  
ring-ding on the street) though it's usually in B7HS guise. BP is  
projected tip, which runs a little hotter than standard tip. Each  
full step down in heat range changes the tip temperature about 100  
degrees C. Half a heat range step is about 50 degrees C (gee,  
really?) and projected tip is about 20 degrees hotter than non- 
projected. The ideal temperature band is about 100 degrees wide,  
though permissible operation is from 500 to 850 degree C.  No way for  
us amateurs to measure tip temperature other than staring at plugs,  
but you can tell a lot from that if you do a clean plug chop. The  
Champion spark plug guru, Bobby whatshisname could tell you what you  
had for dinner from your plugs. We motorcycle racers used to run  
Champions just so he would look at our plugs. Pulling plugs from a  
motor that made it's way back to the paddock is OK for determining  
you're in the ballpark for heat range, but ineffective for  
determining mixture.

You want to se a clean white tip with just a little coloration of the  
porcelain, no blistering, and the metal bits need to have retained  
their sharpness--that happens at about 750-800C. The porcelain starts  
looking blistered at 850C and the metal starts losing it's sharp  
edges (melting) at 900C. Unless you have an Iridium plug, which is  
just fine at over 1000C.

If your engine is pumping oil or your mixture is very rich you'll  
never see that temperature regardless of the heat range. Assuming  
everything is in the ballpark then once you have established the heat  
range that's best for you, you can start adjusting your mixture by  
reading the base of the insulator. That's why the serious tuners use  
lighted magnifying glasses to look at plugs. You want to look at the  
circle of insulator at the bottom, and what you want is light brown  
color after you have driven the car with the throttle setting you're  
testing (usually WFO) as much as you can. When you get close to the  
hot pit entrance, you flip off the ignition at full throttle, push in  
the clutch and coast to the pit wall where your expert crew chief  
awaits. Or if you're like me, you crawl out and look at the plugs  
yourself, my crew consists of Sam the Gay Dog--Diane now signs in as  
"Personal Chef" on my crew sheet.

Oh, incidentally, the Iridium plugs for TR3/4 race cars are BPR7HIX.  
You should be able to order these. Expensive, but they have the  
interesting virtue of lasting almost forever. You can probably use  
them for ten years of racing with no change in performance. Just  
don't put them in a motor that's way too rich or oiling heavily. They  
are magic for weak ignitions--you can run them with a dinky gap and  
they won't foul because the electrodes are so skinny.

On Sep 24, 2007, at 8:37 AM, N197TR4 at cs.com wrote:

>> A bunch of the TRactor motor folks run the NGK BP7HS with good
>> success.  Normally gapped to something like 0.030" but may work OK at
>> 0.050" as MSD suggests.  I have similar compression (different cam,
>> carbs, and ignition though) and they work well for me.  We've used
>> them with many different cams and several carbs - Strombergs, SU's  
>> and
>> Webers.
>>
>> - Tony Drews
>>
> I went to a NAPA store expecting to order these plugs. It turned  
> out that it
> is a common plug used in GOLF CARS.         FORE!
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