Spark Plug Advice Needed!
 

I'm building a 2.7 from the ground up and would like a rec. for a spark plug. The set up is: 77 2.7, 9.5:1 CR, Weber 40 IDA. I was planning on higher octane gas just to be safe, 91+. I was told Bosch W5DS or W5DC would be a good start. However all the ref. books I have seem to indicate that higher CR cars with carbs. from the late 60s ran W3DC or W4DC plugs. Anybody out there run my set up or have experience with it? The car is far from done so the ol' guess and check is out right now. Thanks.

I have a similar motor in my '66 and I run NGK BP7ES Spark Plugs.

Thanks Scooter. How did you decide on that particular plug? Do you know what the Bosch eqiv. would be? I think the BP6 = W5D so is the BP7 = W4D?

I started with a hot plug and as carbs became more tuned in the plug selection became colder.

right now it's BPR8ES w/8.5:1 and E-cams

BP7 and W5 are suppose to be the same heat range. they are common at 6.

with carbs you may not get away with colder than W5, unless you do a lot of high way driving. it never hurts to try. i would run my own test with the 5' and the 4s without changing anything and just see how the plugs look after a few hundreed miles, that is if you dont have fouling problems with the 4's, especially at start up.

i ran W4's in my 2.7 CIS as a test. they were very black but did not foul. except the long trip i did, they came out clean. the W5's are much cleaner.

Similar motor. I started with Bosch WD's. They seem to suffer a lot of quality problems or so it seemed to me. Switched to NGK BP5's which did well with a rich motor. When I finally got the carbs better adjusted, I moved to cooler BP6's which are working well.

Hold on a sec. I was told that the Bosch scale and the NGK scale are inverse. So ex: W4s are hotter than W5s but on the NGK scale BP8s are hotter than BP7. So Bosch numbers go lower for hotter, NGK goes higher for hotter. Can anyone confirm? I was always told with carbs and higher the compression go with hotter plugs. The factory info. seems to support this. Thanks to all for the input.

Lower # is hotter for NGK.

The Bosch and NGK numbers do change in opposite directions.
I believe you want the coldest plug in your car that will not foul. The higher the state of tune, the more desirable this is as it tends to reduce detonation/pre-ignition possibilities. But there's way more analysis going on here than is necessary. Pick an NGK or Bosch plug and put it in. All of the plugs mentioned here are close to what you want. Take them out and check in 100 miles. NGK has a lovely site showing you how to read plugs... not rocket science. They are either cold, or hot, or spot on, and it's easy to tell. As you get the car tuned over time to where you like it, see if you can go one plug colder.

911s55 is correct. I just checked both web sites for Bosch and NGK (should have thought of that first ). For NGK lower # is hotter. For Bosch higher number is hotter. So the makers charts are inverse but my previous info. was backward. So this being correct the factory info. supports a cooler plug with higher compression. Bosch also rec. to err on the side of cooler. ie. fouled plugs being better than pre-ig. Thanks to all this is much more clear now and sorry for any misdirection earlier. I'll try the W4s and W5s and go from there.

You've got it.