A few of you know from my other threads that I'm on a hunt for the best possible MPG with my 351C.

Today I went out and tuned the engine a bit more, I now have the initial timing set at about 20*btdc. (I was running 16* before.) it pulls higher vac at idle, about 19". And has more snap to it. However I know I'm pushing the limits with spark knock. I can't get any now, but I know it's right around the corner.

I'm only running on regular 87 right now, I can put In premium 91. But how far can I advance timing before it starts getting questionable?

 

Until it pings, then back it down 2 degrees.

 

That's what I've been thinking, but will that much timing have any negative effects? I used to run about 14* before it pinged, but I have a different cam now so that's why I've been moving up.


So just up until it pings, then back down two huh?

 

Well I went and tried 22*. It just didn't seem to run the same. So I guess 20* is the strong spot.

 

Kinda going through this procedure right now with my Y. The trick for me was figuring out that distributors aren't plug and play. Initial timing is just one of three or four components of ignition timing. Gearing, weight, anticipated loads, exhaust type, altitude, octane, engine compression, cam profile, everything affects the optimal timing curve. From what I'm reading MPG is usually improved ten percent or so by careful timing. Especially vacuum advance. Usually V8s are tuned to 36 to 38 degrees total. You can dial in whatever initial you want, but the total should not exceed this. Limiting the breaker plate advance to 15 or so crankshaft degrees would allow this?

You're gonna have to do all that, because the vacuum adjustments are always done after the total is dialed in, but the vacuum advance is where you will want to really experiment for mileage. Basically always running just short of ping or detonation is most efficient.

 

Timing is much more than the initial setting. Having it properly curved is the answer. Here is a nice read: FORDMUSCLE webmagazine: Timing is Everything - Distributor Curving for Maximum Power Deals mainly with the Duraspark, but the principles apply even with points.

 

Good article. Lots of info. It answered my questions. Now out for more tuning.

 

I went and checked total timing and vacuum advance.

Total is over 40*. Probably around 45. I can't tell for sure. But it isn't coming in till 3500

Vacuum shoots way up. 50+.


Do I need to adjust so that it never goes over 42-44? Vacuum, mechanical or otherwise?

 

Every engine is different but generally around 36-38° total is about right for a street vehicle. With a large, open chamber head and dished piston it may require more. Mechanical advance should start to happen just a little above idle speed and should reach full advance by 3,500 rpm (or earlier depending on the vehicle, cam, usage).

An ideal ignition system would fully ignite all the fuel/air instantly right at TDC - no energy wasted as the piston pushes up against the expanding F/A mixture. That ideal ignition system doesn't exist. With the ignition systems available advance is required to give the flame time to travel across the CC, and therefore more advance is needed as engine speed increases.

So from an economy standpoint less initial advance is better because the engine isn't working as much against combustion as the piston rises to TDC.

 

Hmm. Well I guess I'll run this tank at 20* and then back it down to like 12* or something.

 

Is the vacuum advance connected to manifold or ported vacuum?

 

Ported vac

 

. Ported vac is good, keeps the vacuum advance from adding in at idle...

. Whether or not it's a V8 makes no difference... although those are common ignition timing numbers thrown out by car magazines...

. Modern fast burn, small chamber, high compression ratio heads may give best power by only 28 degrees total at WOT... older, big bathtub chamber, low compression ratio heads/dished pistons/big cam may need 42-44 degree mechanical timing head start to make best power... most engines somewheres in between...

. As just mentioned, this truck should be mechanically advancing timing just off idle and prolly all in by 2500 RPMs... so there's good mechanical advance by cruising RPMs... plus vacuum advance... that you can run 22 degrees initial without pinging has me thinking again that this new engine has less compression ratio than the old one...

 

I guess it's possible I could have a lower compression ratio... Maybe it's time for my Aussie quench heads!

 

. Already got the Aussie heads? Ever seen the piston tops in this new engine?