Spark plug confusion

Trackrash · Trackrash's Garage

I'm hoping I can get some recommendations on which plug to use in my 911.

It's a '71 with a SS2,5 liter with 8.5 to 1 CR running webers with E cams. I track the car.

The last time I bought plugs I was sold NGK BP8ES. This is what is also specified in Pelican's parts catalog.

My searches indicate that this is a very cold plug, which may explain why I occasionally have a fouled plug.

I have two questions. First I would like to buy NGK BP7ES, which my research shows it correct for a tracked car. Does that sound like the correct plug to use?

Second, what is a NGK part#999-170-165-90-M220 which is listed here Pelican Parts - Automotive Parts and Accessories - Porsche & BMW ???

Thanks,

T77911S · 07-02-2013, 09:49 AM

if the 8 is fouling, try a hotter plug.
if you are tracking it, i would definately check the plugs after a WOT shut down. this is where it is most important.
if they look good, your idle circuit may be too rich and fouling the plug.
put 2 each in, bp 6, 7 and 8. run it and see how they are.
you can also look on the ground strap and you should see a heat line on there, that is suppose to be the timing mark.. if it is at the bend, it is correct.

Trackrash · Trackrash's Garage

Right now I have the carbs off and I believe that is where my problem lies. I dynoed the car a while back and knew I was rich at part throttle. I believe I have found the problem with the carb's progression circuits.

Once I get it running again I should be able to better evaluate how my new plugs look. I will start off with 8s.

I spent some more time researching. Most of the experts recomend 8s for the older cars up to '73. Then for the newer cars with CIS 7s are recommended. That seems counter-intuitive to me. I would think the leaner CIS motors would need cooler plugs.... Also it doesn't seem to matter what the CR ratio is when plugs are recommended.

theiceman · 07-02-2013, 12:45 PM

Bottom line is this .. run the coldest plus you can without fouling..

rsscotty · 02:08 PM

Quote:

Originally Posted by theiceman

Bottom line is this .. run the coldest plus you can without fouling..

That is the best one liner statement.

There are a lot of details we take into consideration when selecting the proper spark plug.

There are simply people out there that don't have a clue, we see this in engines all the time regarding plug selection. Do what Porsche recommends... in custom applications, do your homework. Again..go back to the one liner statement.

Bosch is a poor choice for plugs in our opinion. They have their place in stock engines, but fall short on selection for a custom engine. We prefer NGK or Denso....NGK has a very detailed page on the plug numbering system. It is very useful for custom applications.

T77911S · 07-03-2013, 03:43 AM

thanks for posting that. i had a chart like that but lost it.

CR does matter in heat range. but it is also how cold you can go without fouling. also, how is the car used. track cars spend less time idling, some even idle at 2-3k, so fouling is not that big of an issue. its about keeping the tip of the plug cool enough to prevent pre ignition and hot enough to clean itself. you just need to do some tests on YOUR car to see what it needs.