That's a good idea - I hadn't even thought about the fan from a MPG standpoint. An electric fan has been on my list, but more from a safety standpoint. I'll add a fan to the next pull-n-pay visit.

 

You're on to something about the timing at lower RPM.

I swapped the vacuum advance from the manifold to the ported carb and got closer to 9mpg in town. The problem is it did run a lot warmer. Barely gets to 1/3 of the temp gauge normally, it was 3/4 the gauge today, though it was pretty hot outside.

I did finally get the timing high enough to make it ping, then backed it down just a hair. I connected it back to manifold vacuum and I'll try to verify that lower timing helps mileage at lower RPM. If I lose mileage in town now then I'll tighten the vacuum advance some more.

 

Interesting you're seeing the engine temperature vary with timing. I see a similar effect. At my highest MPG, the engine looked to be running colder (~10-15 degrees) especially compared to some of my early data points. In hindsight, I wish I had been recording engine temps and the outside temperature. Nonetheless, in doing some research I think the effect is real with a simple argument that goes like: improved MPG is a consequence of the engine operating more efficiently meaning more energy is going into mechanical work rather than heat. Of course there's no free lunch here which is that my emissions are most likely increasing in my hunt for better MPG. I've found some interesting articles that show this which is typically why our stock motors have much lower timing.

 

Yep.

For all of those stories about a magic carburetor that does 100mpg but the car companies and the gas companies won't let us have it, it really comes down to the EPA.

Still wouldn't hit 100mpg, but we could do better.

 

To all those still following this thread, sorry for the delay in producing anything of substance. With the fuel flow meter going south, I had to come up with a new (and more economical) strategy moving forward. After digging back through lots of data and logs I've taken, I found a simpler approach and metric for tuning the carb at a high-level. All along the way, I've been taking test runs 'around the block' which is a 3 mile part of my daily commute. During these I've also been using the data logger capturing the RPM and AFR history. It turns out that averaging the AFR over the run (not counting data at idle) tracks the MPG trending reasonably well. So now I'm comparing data that looks like this:



The difference in these two runs are noted in the upper left where I've altered my notation to be cross-sectional area changes to the IFRs and IABs. Once I wrap up this thread, I'll pull all the data together including the wire and drill sizes in a large table.

So here's a revised 'output' table like I've been updating periodically (the MPG is the average of the 2 MPG data points I previously reported):



My next task is to settle on a method for measuring fuel consumption during my run. Since I have the plumbing for dual tanks, but only use one, I have a couple of ideas for putting a small fuel tank in the bed for which I can measure the fuel volume before and after. The trick here is coming up with an approach that allows me to determine the fuel consumed to ~5% which is the level that my carb tweaks have impacted the MPG. Hopefully more to come this weekend . . .

 

I just got another 1850, so I may just start playing with the carb more. Looks like this one may have the pink cam. I need to clean it up to be sure.

 

0ldman, if you start tinkering with the IABs or the IFRs, I'd be curious what size they are. In searching the internet, I've found some different numbers for 1850s. Any knowledge of its application or vintage?

 

The one on my truck is a R6291.

The one I just got is a 80457-1.

The 6291 has smaller main jets, 62 vs 64. The 80457-1 has hex head screws to hold the bowls on. 6291 has flat head screws.

I'm going to look up and see if I can find a rough manufacturing date on these. Printing out the respective pages on the Holley carb-number PDF so I can actually look at them side by side.

Edit: looks like the 80457-1, 2867 is Oct of 97 I think.
6291, 1397, so May of 77/87?

As far as I know they're both aftermarket. I thought originally that my first one came from an 84-85 Mustang, but that doesn't seem to be the case.

 

I was able to put together a system using a small fuel tank in the bed to measure gas consumption. Since it only holds 2 quarts, I can make a good measurement of the amount of gas before and after each run. I found a ~3.5 mile path that is part of my commute so I have a connection to all my other data before the fuel flow meter failed. Also, my average speed (~24.5 MPH) and max speed (~40 MPH) is very close to my previous runs. The advantage now is that I consume about a 1/20 of what I did before to get the same accuracy.

Besides determining the MPG in changing to the MSD system and further restricting the IFRs and enlarging the IABs, I was also able to scan across a couple of initial timings. Quite an improvement and it appears that 16 degrees initial is close to optimum:



Unfortunately, I have no way to connect the new MPG data (noted in italics) with the previous data. On my list was to verify the fuel flow meter accuracy, but no hope for that now. However, I did triple check my current method in both the distance covered (odometer, GPS, Google Earth) and fuel volume measured (borrowed a few things from the kitchen) so I have high confidence in these numbers being good to better than +- 1 MPG.

In looking at the AFR data for all the runs, I'm now running between 15 - 17 across the range from 800 - 1800 RPM at constant speed. I don't want to go any leaner at this point. Plus, I have to be even more conscious about using some pedal to add fuel for acceleration. But I'll trade this minor inconvenience for the mileage gain.

Today I also tried to keep an eye on the temperature. With my gauge in the thermostat housing, I could see a slight change with the timing variations. For both 13 and 19 degrees initial I was seeing 192, while for 16 degrees initial it was 188. This was on the same stretch of road at the same speed. I'm still looking into whether this makes sense from an efficiency standpoint.

Next up will be to tweak the accelerator pump and replace my stock fan with an electric.

 

That is pretty nice.

From that chart it looks like you gained more from the ignition upgrade than from the carb changes. Now the question is would those improvements have happened without the carb modifications?

I wonder what your highway mileage would be now?

 

Thanks. Since I'm running lean mixtures (16.5 - 17 at 1500 RPM cruise), I do believe the MSD system had a positive impact especially with the 20 degrees of multispark. Yes, in hindsight, I wished I had set myself up to be able to compare both ignition systems and compare my methods for determining the MPG. However, I don't think I'd seen the same gain without the carb modifications. I believe this from the AFR data in which I was still running rich off-idle and up to ~1200 RPM. Highway mileage will be interesting - last night I was thinking how to tweak my system for this.

 

Well, given our project's similarities, and now I have a couple of carbs to play with, and the gains shown, I'm thinking about grabbing an MSD now before I swap the carbs around.

Got a lot going on right now and limited play money, but the MSD could potentially pay for itself in a few months if I can get my city mileage up from 8-9mpg to 14. It would be encroaching on my Ranger's territory.

Worst case scenario that I see is I don't show much gain on my truck, I can always swap the MSD over to the Comet once I get it road worthy again.

Now I just need to go have a long heart to heart with my wallet.

 

0ldman - did you get an AFR gauge yet? Without it, I don't know how I would've known which direction to head along with ultimately trying to achieve lean AFRs across my typical RPM range. It helped me figure out whether to adjust the IABs or the IFRs.

Another point is to come up with an accurate method to determine MPG. Long ago, I tried to use the fuel gauge, but mine has so little needle swing with the auxiliary tank, I couldn't estimate the MPG to better than 30%. This wasn't good enough to see the impact of making reasonable changes to the jets, IFRs, IABs or timing.

 

I haven't yet. My fuel guage doesn't work, so I just fill it up every couple of days, check the mileage that way.

Need money and time. lol

 

This past weekend I spent some time tuning the accelerator pump system. Since my orange cam was sending the AFR too rich, I picked a few other cams similar in size and shape. These were black, pink and red. I also tried the different positions for each one and tried a size 25 shooter compared to the stock 31.

Here's an example of the type of data I was looking at. Typically I'd punch the accelerator to hit 3000 RPMs then close the throttle. For the case below, this is with the orange cam in position 2 and the size 25 shooter. Right after 24 on the bottom scale, one can see the AFR go way lean and the RPM stumble. So clearly the 25 is too small (I went back to my 31 - I'd like to try a 28 eventually).



In addition to the punch to 3000 RPM, I also tried a series of lesser taps on the pedal more similar to how I normally drive. The responses range from something like the black cam in position 2 which isn't providing enough fuel with lighter accelerations (way too lean)



to the pink cam using hole 2 in cam position 3 which is providing too much fuel with lighter accelerations (too rich):



After staring at all the data for awhile, I went with the red cam in position 2. With the lighter taps on the pedal, the AFR ranged from 12.5 to just under 15. It was also the 'best' as far as being the least rich when I stomp on the pedal to hit ~3000 RPM. In all cases, this 'rich' feature exists in general with the choice of cam and position having minimal impact. Since the much smaller shooter size also didn't affect this feature, I'm thinking it has nothing to do with the accelerator pump system. Rather, in this mode I suspect the power valve is opening briefly given the drop in vacuum when I so quickly open throttle. I don't think this is happening when I more lightly tap the pedal to hit lower RPMs as seen in the later part of the previous 2 plots. In general I'm not stomping on the pedal, so I'm not concerned about this fuel inefficiency. But I'll put it on the list to hook up the vacuum gauge again and maybe even try plugging the power valve to take it out of the question and test my hunch.