Right now I'm at 0.044". Once I've reasonably optimized the carb and timing as far as MPG and AFR, I'm going to look into upgrading the ignition. Specifically I'm thinking a more energetic coil given the leaner mixtures which should also enable me opening up the plug gaps.

 

I still wonder what sort of role the power valve is playing here. As you are thinking, it's hard to believe there isn't an acceptable cam offering for a 600 CFM carb on a 351/400. As an experiment, I'm tempted to put in a power valve plug and grab a few logs with some different cams to separate the effects of the two systems.

 

With the vacuum mine pulls I can't see the power valve opening at much less than WOT. The rebuild kit came with a 6.5, I think I need an 8.5 or better, but I think I'll get better mileage if I keep a tighter power valve in there.

I wonder if my plug gap might not have something to do with the stumble. I don't know what they're set at, but I'm betting it is 35. I'll find out soon enough. I figured on going with 45, if it is already 45 I may try tighter first. I think I need stronger spark with the rich condition and maybe you need a longer spark to ignite the weaker lean mixture. Maybe.

I think I'm going to order a pink cam and see. Honestly I think I need a smaller discharge nozzle more than a smaller cam. I've been trying to come up with a way to do your wire trick on the discharge nozzle...

My 4100 has a spring loaded accelerator pump arm and tighter discharge nozzles, it shoots more of a mist than a monster stream. I've been thinking about trying the 4100 on here, see what it does.

 

Advancing the timing was a step in the right direction:



I've also been starting to look at MPG at a steady MPH versus the averages over a few days of driving I have in my table. At 40 MPH on level ground, I'm seeing about 13.5 MPG in 4th gear turning right at 1700 RPM.

 

I guess my 12.5 @ 55 with an additional 50 CID aint too shabby.

I'm still shooting for better, but that is good to know.

 

Advancing to 18 degrees initial (31 total with manifold vacuum) is too much. Not only did the mileage drop, but my previously smooth idle at 600 - 650 RPM degraded. I had my doubts at the beginning of this change, but wanted to see the impact on mileage. If I gained, that would've told me I needed a steeper advance curve - less at idle and more around 1500 - 1800 RPM). I'll turn it back to 14 degrees for now.



Next I'm going to experiment with the accelerator pump system and research ignition upgrades (for example, multi-spark) that may be able to squeeze more mileage out of the lean mixtures I'm running.

 

That is completely opposite of what I expected.

I'm going to dial back my vacuum advance a bit more and see how she does in town. It seems it may have made mine worse.

 

I've been thinking about this.

The leaner mixture takes longer to burn, longer to have peak explosive pressure. This is why we can run lean and push the timing high at cruising speed.

The richer mixture at idle and just above burns faster. Advancing the ignition timing with the richer mixture would make it hit peak combustion too early, which normally causes pinging or knocking, which *could* be masked by a rich condition. I don't think yours is nearly rich enough, but mine could be.

 

Just tinkering with my engine program, basic 351W, not exactly the same as what you have, but close enough for estimations.

For peak power running 89 octane the calculations show
19* @ 1000
22* @ 1500
24* @ 2000

not hitting 30* until 4500RPM. There isn't a setting to change the jets to lean it out or set timing for peak efficiency.

At higher RPM (2000+) running lean during cruise there is certainly mileage to be gained by pushing the timing beyond the peak power. Apparently there is a limit to that at lower RPM where the flame front is moving much faster than the piston in the bore. At higher RPM the piston speeds need earlier ignition timing because the fuel can only explode so fast, not that we'd really run into that on street driven engines, but...

 

Interesting . . . how are you calculating these?

 

Engine Analyzer Pro, just an engine simulation program. It is only as accurate as the information input, but it is pretty close if you get it all right.

 

Today I finished upgrading the ignition electronics to a MSD 6A and a MSD Blaster 2 coil. Pretty straightforward to do with most of the effort reworking wiring and making a new coil bracket to mount it vertical. Fortunately, the engine fired right up and the timing needed adjusted some to get back to the 14 degrees initial I had previously with the stock system.

As far as a difference between the stock system, it clearly idles better when the engine's cold and with no choke. Just for comparison sake, in this state I can idle reasonable well at 400 RPM (before I adjusted the timing and not where I want to run) which I've never seen before. The engine also seems to start better when cold. Now to see if it has a positive impact on the MPG. Here's a picture of the new hardware:

 

Nice.

I think I've figured out part of my stumble. With the vacuum advance pulling the timing so high it was shooting too much fuel to burn properly.

I connected the vacuum advance to the carb instead of the manifold and my hesitation improved dramatically.

I think I need smaller shooters and accelerator pump cam to run with such aggressive timing. Not sure if I can tune it for power and efficiency at the same time. I think the power valve might be a major part of it. I think I need the smaller accelerator cam and a power valve that kicks in earlier to make up the difference so I can run the timing very high.

Combine all three and I'm hoping to get the mileage up there. Add in what I've learned from your project about the carb...

 

Well I started to get some miles on the truck to determine the MPG, but the fuel flow meter quit on me today. Tomorrow I'll dig into it and see if at least the sensor is putting out pulses. Then I can finally bite the bullet and create something to connect into the data logger. Plus I'd like to collect real time measurements of the fuel flow so I can make a few changes in a day. Right now it takes me a few days to burn enough fuel to accurately discern differences with a change I make.

Qualitatively, it feels like the engine has more power. Today I found myself being able to release the clutch in 2nd at idle and move in stop and go traffic. That's something I could never do before - always had to give it a little pedal. I think this is also consistent with the richer AFR I'm seeing between idle and 1500. The only way this makes sense to me is that I have to open the throttle less to keep a given RPM. In this range I always saw AFRs of 12 - 13 in the driveway, but seldom saw them when cruising. Not the case now plus it feels like I'm barely touching the pedal cruising down the road under 35 MPH. I'll see if I can get some data on this with the logger - now I understand why people hook up throttle position sensors to these.

Finally, I think I'll need to do another timing scan. With multiple sparks over 20 degrees now, I imagine the sweet spot will be different than what I determined previously.

 

You still have the factory steel "flex" fan?

That is probably worth a mpg or two right there.

Clutch fan would be a huge improvement, electric would be better, but clutch to electric is not as drastic as going from the steel fan to a clutch fan.

One of the first things I did was swap my steel fan for a smaller flex fan, just a dinky little one I had lying around. I'm planning on going electric at some point, but my Comet isn't using it's clutch fan at the moment, might swap the racing-ish flex on my truck for it.

Just swapping the fan was probably worth 100rpm at idle. I had to idle it down afterwards.