Where did my plug gap go?

Todd Simpson · 03-24-2016, 10:21 AM

At my last track event last year I had a missed 4-5 up shift (went into third) and a very brief mechanical over rev. Like, a half second and perhaps over the 7300 limit but not much. Motor is a 2.7 on carbs, 10.5 to 1 compression, S cams.

Idle was a little lumpy later in the day, but otherwise ran as normal through several sessions. Compression test says all is well. 170+ all around. Oil pressure also very high.

I had the carbs redone this year, installed them and lumpiness still there, and now worse. Pulled the #1 plug and gap was almost gone. The plug wasn't damaged at all. I regapped it, now motor runs fine.

What are my next steps?

Mick_D · 03-24-2016, 12:20 PM

The only thing that could cause that would be piston hitting the plug. In the old days, you could add a washer to the plug before you screwed it in but I don't think that's doable here.

I have no idea why the piston would hit the plug on an over-revved missed shift. Is there a way to pull that plug and use a bore scope to check the top of the piston for marks?

cgarr · cgarr's Garage

Were the heads twin plugged?

Steve@Rennsport · 03-24-2016, 05:19 PM

Are you using projected-nose plugs?

Todd Simpson · 03-24-2016, 06:17 PM

Single plug but I use race gas. Timing is 30 degrees BTDC.

Projected nose plugs...jeez, that might be it...I have a 3.2 and also a mild BMW M30 6 cylinder, maybe I put those plugs in? Will have to pull one and see later tonight which Bosch it is.

Todd Simpson · 03-25-2016, 04:48 AM

Plugs are Bosch WR6DC and measure ~22.5mm from washer to tip.

Car ran fine on these for two years, and gap got smaller, didn't disappear altogether. Still, I am guessing these are CIS or Motronic plugs and not right for my 10.5 pistons?

thanks

Flat6pac · 03-25-2016, 09:46 AM

Todd, I think your plugs need to be near the WR3 s
Bruce

cmcfaul · cmcfaul's Garage

Piston prob hit a valve on the over rev and knocked a chunk of carbon off that hit the plug and made the gap smaller.

Or, the high compression is causing detonation and the outer side of the exhaust valve relief in the piston broke off and caused the plug gap to get squished. I have experience with both scenarios.

Chris
73 E

PS, you should run colder than stock plugs.