Hi Guys, this is a long post, but I need some help understanding what I'm dealing with in tuning my 340. I installed an AEM wide-band and have been watching the plugs. At the end of the info I'll add photos of plugs.
This is a converted race motor with low compression (8.5 pistons) X heads ported to the max, port matched LD340, port matched 1-5/8" mid-length headers, and a Lunati 284 Bracket Master (.218 @ .050 and .458" lift. (I've run 13.95 @ 96 mph, vehicle wt. 3450 w/ driver)
To get reasonable A/F #'s I've had to jet the Edelbrock 750 8% lean to .107 primaries (stock .113) and 4% fat .110's in the secondaries (stock .107) and use a rod that is a combo of the largest size made for cruise and the smallest size made for power. (75/37 rod) A/F about 13.5 idle and cruise, mid rpm 14-15, and WOT 11.5-12.5 range.
This jetting seemed unusual. And I'd read to NEVER jet the secondaries larger than the primaries!
The motor loves a lot of initial advance (happy at 24), but so far I can't run more than 33-34 total without evidence of detonation. (MSD distr. through a 6A box limited to 15* mechanical with home-made bushing.) So my timing now is about 17-32. (vac. adv. not hooked up)
Besides plug choice, the motor runs great at lower rpms, good response, pulls good WOT, but seems to be done at 5,000 and it used to pull good to 6,500 and happily pass 7,000. (good oiling mods) Also Holley red through 3/8" pumps a gallon in 40 sec.
So why does it no longer have the top end? And how do the plugs look to you? And is the detonation due to hot plugs? (saw silver flecks on porcelin at 36*, yikes)
Re plugs, Autolite 63's were looking very hot. I just switched to NGK BP8ES, put fresh plugs in #1 and #7 cylinders and made a WOT run, shut it down and coasted to a stop. Both were similar, Here are the photos:
