mgmuscari

So I've been tuning my new 347, and couldn't get it to rev past 2500 without breaking up. I had the timing table set up to be all in at 34 deg advance at 3000rpm, which seemed reasonable to me. I took a buddy out for a ride, and he suggested that I might be running too much timing (no pinging or anything, just backfires out of the exhaust and stumbling/breaking up). I pulled 4 degrees off, and lo and behold it revved all the way up to 5800 no problem. Sounds like a beast now!

We went for a ride and it was pulling hard, no misfires or anything. So all seems good. This has me wondering though. I hear a lot of guys talking about 34 degrees being a good number for their 347's. I have 5.4" rods, and a 3.4" stroke, and about 10.5:1 CR. I've got a set of CNC ported Twisted Wedge 170's on it, a Comp Cams XE274HR, and a Track Heat intake. Right now I'm all in by 3000rpm with 30 degrees advance, and it's running really well. I'm worried that I'm not getting as much out of it as I can, though (even though it hauls). I understand that the only way to be sure is a dyno tune, but I'm wondering if it's possible that my distributor could be set wrong. I have a MegaSquirt, and so I can arbitrarily set the timing offset in software to match what shows on the balancer with a timing light. Could I have the rotor too far out, so that when it advances too far the tip of the rotor isn't passing close enough to the cap point and it's not sparking? The backfires show extreme lean spikes in my data logs (using AEM wideband sensors).

wydopnthrtl

In my experience (OBD2 tuning) lean spikes are caused by pre-detonation / too much timing.

It's possible that your running more timing than you think. Assuming your pistons are not ceramic coated on top.. I'd pull a plug and look for spalling. (aluminum flakes spattered on the plug can sometimes indicate pre-detonation)

Generically speaking.. more stroke needs more timing because the piston speed is faster. But its really a balancing act of compression, cyl fill (cam), ect...

I'm not a fan of dyno tuning.. especially with dyno jets. But its a good way to get you into the ball park for a/f & EGTs. From there data logs of the a/f & reading plugs will help you fine tune timing and fuel.

mgmuscari

My plan is to take it to a steady state to dial in the optimal timing curve, but I want to make sure I'm not having a spark problem first. Unfortunately I have no way of determining that from my datalogs... all I can see is the lean spikes.