When You Start Modifying Spark Plugs...

[gene cooley]

When I use the correct 1" reach plugs in the Ecotec in the 600 the piston closes up the plug gap down to almost 0".
I have been using 3/4" reach plugs for a while but didn't like the partial shrouding of the plug.
I tried indexing washers but couldn't get enough clearance with the tapered spacers.
The first pic below shows a stock plug, the modified one, and the result of the piston contact.
2nd pic is the 1" plugs as purchased and modified for installation. ( The new plug in the pic isn't "bent", it's just lens distortion.

 

[MikeHynes]

I don't know about the Ecotech piston domes or combustion chamber but I had a similar problem when I used an extended reach sparkplug - Autolite AR3910 - in a SOHC engine. I had been using AR3932 plugs (not extended reach), but didn't like the way they seemed to be "hidden" in the corner of the SOHC's wedge shaped combustion chamber. So, I installed the AR3910s and found that by extending them into the combustion chamber just "that much" they made contact with the top of the pistons. The engine actually ran for a while before it shut down. Turns out the contact was so light that all that happened was the gaps were closed and the engine turned itself off. I was able to re-gap the plugs, index them so that the strap was pointing up to the roof of the chamber, and was then good to go.
Now I just have to remember that I can't use extended reach spark plugs in that engine without indexing them. I have no idea if using extended reach plugs, or if indexing them the way I did, makes more HP or not though. Who knows without extensive dyno work? But I do think you're better off NOT to have the threaded part of a sparkplug extend into the combustion chamber.

 

[gene cooley]

I don't know about the Ecotech piston domes or combustion chamber but I had a similar problem when I used an extended reach sparkplug - Autolite AR3910 - in a SOHC engine. I had been using AR3932 plugs (not extended reach), but didn't like the way they seemed to be "hidden" in the corner of the SOHC's wedge shaped combustion chamber. So, I installed the AR3910s and found that by extending them into the combustion chamber just "that much" they made contact with the top of the pistons. The engine actually ran for a while before it shut down. Turns out the contact was so light that all that happened was the gaps were closed and the engine turned itself off. I was able to re-gap the plugs, index them so that the strap was pointing up to the roof of the chamber, and was then good to go.
Now I just have to remember that I can't use extended reach spark plugs in that engine without indexing them. I have no idea if using extended reach plugs, or if indexing them the way I did, makes more HP or not though. Who knows without extensive dyno work? But I do think you're better off NOT to have the threaded part of a sparkplug extend into the combustion chamber.

My problem was that the 3/4" reach plugs
(the electrode and part of the ground strap) were partially shrouded inside the threaded spark plug hole. The stock 1" plugs match the plug with the hole. The modified plugs place the electrode correctly in the head, I just had to relocate the ground strap.
The pistons are 13.8 to 1 (nominal) Wisecos. I think my problems started when I replaced the stock MLS gasket with one about 1/2 thickness. The gap would close enough that you couldn't see it. The engine would start and run up to 7K and then foul the plug for about a second then clear up until we hit the limiter again.
Tomorrow is the test day, I'll either come back with a good report or a pile of parts.
Edit: One good sign, it does start and idle much better. Not sure yet about 7200.

 

[Dr.Jeff]

Your modified plug reminds me of the old days when we made "racing" plugs by cutting off the ground electrode so it was a 'rimfire' plug.

And your description of the shorter plug being shrouded into the plug hole makes me think of those "anti-foul" adapters...remember them?

Apparently you can still buy them.

 

[gene cooley]

Your modified plug reminds me of the old days when we made "racing" plugs by cutting off the ground electrode so it was a 'rimfire' plug.

And your description of the shorter plug being shrouded into the plug hole makes me think of those "anti-foul" adapters...remember them?
View attachment 48850
Apparently you can still buy them.

If you were oiling really bad or needed to get your O2 sensor out of the exhaust stream these were the "answer".
Unfortunately this is what I had built in before I modifed the plugs.
We did look at these to extend the life of the O2 sensor operating in the 110 octane leaded exhaust stream. In the end we decided to just use the O2 sensor for occasional datalogging, but we don't run the engine off of it. We just let it die when we finish a data log. I guess we could pull it out and put a plug in the bung, but we haven't done that or tried the extenders.

 

[Dr.Jeff]

They are quite pricey but wideband O2 sensors are available that are made for race fuels. I believe the Bosch version is referred to as their "motorsport" edition.

 

[gene cooley]

They are quite pricey but wideband O2 sensors are available that are made for race fuels. I believe the Bosch version is referred to as their "motorsport" edition.

We have been running off the Megasquirt "tables" for a couple of years.
Apparently even the race sensors are rather iffy when you are using leaded race gas and if they die in the middle of a run it can cause a problem.

We ran our modified plugs yesterday, there is a difference in power. One thing that I couldn't explain was that when the trans shifted from 1st to 2nd it would hang on the rev limiter for an instant and then shift. This happened leaving the line and after a downshift on the course. No other changes from an event 2 weeks ago with the 3/4" reach plugs. We have an old GM hydraulically controlled 3 speed trans with a mechanical shift governor and a TV cable. It took about a 1/4" added slack in the TV cable to cure the problem but I still don't know why it did this.