Timing/mechanical advance conundrum....

[kawahonda]

Prepare for some black magic.

My distributor was re-curved. In crank degrees, it should be pulling 18 mechanical, and should allow me to set my initial at 16-17.

Well, since installing last year, at 17 it pings under medium load. 16-15 still pings. 14 it pings in very heavy load situations--faint and intermittent--but still means too much timing.

Checked my total timing today (yes, I disconnected vacuum advance). @ 13 initial, I was at 35 total. Um...that means the mechanical is adding 21-22. So OK...

I talked to my dizzy guy and he said nope, on his bench setup (sun machine) it was pulling 18. I know he's correct with his experience, but my car hasn't ever really agreed.

So off the distributor, and apart it goes.

The factory timing slot plate is marked "9", which means "18" degrees. Um...OK....but that's not what it's doing. This is the black magic part.

Yes, I am looking at the correct mark when checking timing on the front case which is the "0", between the "before" and "after" scripts and I am using a dynamic light.

I have an FBO plate and I confirmed the slot sizes with what the literature says for slot widths. So now I'm in this weird conundrum where I feel like I need to pick a slot size based on how my car acts, and not based on what is stamped or what the literature says.

My current slot size is measuring .383, and should be "18" but it's not. It acts like 21-22.

If I follow the literature and assume that for every .015 change = 2 degrees of crank change, then I chose to use the .349 slotted area, which FBO marks as "12". I want to end up at around 18 degrees mechanical.

I installed that tonight, and I'll fire it up tomorrow and measure it.

Was wondering if anyone else found something like this in their tuning days. It will be interesting to see what I find tomorrow. It's going to be very nice to pick up 3-4 more initial!

[RUNCHARGER]

It's advancing 5 degrees  from 0 Rpm to idle speed.

[kawahonda]

Hmm. Is that bad or indicative of a problem?

[RUNCHARGER]

No: I just think that would explain the discrepancy. The other plate should work for what you want I would think. FWIW, i struggled with my 71 Charger once when I moved 500 miles. Finally I had to jet it way up to what used to work and it ran great. I figured it was due to different gas in the two locations, in spite of it supposedly being the same gas. You're doing a great job tuning your car, You're seeing what it likes and responding to it.

[70 Challenger Lover]

Out of curiosity, what's your compression ratio?

I recently helped a buddy rebuild his 65 GTO motor. He wanted to go with the factory compression of 11.5 to 1 and use the stock cam profile. It pings like a bastard. We can't dial in more than 28 degrees total or else it starts pinging under a medium load. With racing gas, we dialed it up all the way and the car just screams. I'm trying to talk him into making a couple changes.

[kawahonda]

Thanks Sheldon! I'm kicking this cars ass and yep, it's responding very well!

The only thing I'm worried about is if I got the reluctor 180 degrees out. I'm not sure with this OE design if that's possible though. The more I think about it, the more I throw my brain in a loop. I marked the reluctor to the base timing plate, but you can still install the reluctor 180 out, and rotate it to where it lines up correctly. I know with the modern designs that's possible, but not sure with this design if it is.

If I did, oh well, it won't hurt anything, it would just run like garbage. I gotta get good at taking it out and apart.

I took out my point during disambly, then figured I didn't need to do that. Some time to set the dwell again. I had it at 32.5 degrees before. Is there an optimum range?

@70 Challenger Lover, it's a factory 340 with rebuilt J heads. Compression is really healthy. My guess is mid 9s based off what I've been reading on the "actual" 340 compression ratios.

28 degrees total....damn! That's some serious compression!

[Chryco Psycho]

Where is it pinging?
You may not be able to add extra timing at low RPM where it will normally ping so you might want to use heavier springs to slow the advance so it will not advance as fast

[kawahonda]

faint, intermediate, pinging only at WOT, heavy-load applications (think going 55 and flooring it in top gear) at 35 degrees total, around 13ish degrees initial. No sign of pings anywhere else, burnouts, aggressive accelerations/shifting, etc.

I've had this exact distributor setup installed at 17 initial....starts and runs easy....17"+ awesome vacuum, but yep...it will ping bad at medium loads at that initial. There's just too much mechanical in this distributor, even though 18 degrees is pretty decent (my car is treating it as if it were about 22 degrees). I'd like to be at 17 initial and 34 all-in for safe, high performance engine operation. 35 is "safe" for putin' and romping around if I don't take it on the highway and load it down like Kiwolsky, but a car should be timed appropriately to be able to do anything, and for this specific car that's 34 total, but that puts me at about 12 initial. Yuck...

So yea...probably going to be able to really do burnouts soon.

[Chryco Psycho]

Makes sense 
More fuel might kill the ping / cool the burn

[kawahonda]

Yep...still need to do WOT plug chops. Figure this timing thing that I finally looked deeper into required first attention.

[Chryco Psycho]

I would do it the other way round , adding fuel @ WOT should kill the ping

[kawahonda]

Perhaps, but I doubt it would kill 5 more degrees initial of ping! Mechanical advance is still not running at the spec of the recurve.

That's seems like a large order for fatter jetting to solve. I'd rather do honest WOT readings of the plugs before going there. Don't worry, I'll get there. Who knows, in a few months, the distributor may come out again and get a 1-slot larger.

[Bullitt-]

I'm wondering what a stretched timing chain might have on timing.... 

[mopar jack]

have you checked your balancer timing mark to make sure it is at true top dead center?

[73_Cuda_4_Me]

My step-up springs made a HUGE difference in the partial to full throttle acceleration pinging I had on my 340 last year... went to stronger springs to open a little sooner, and the pinging disappeared... no more marble rattling!

I ended up re-jetting and rod change to add more, and that did the trick as far as any hesitation or stumble on acceleration...