Side benefit of wiring cleanup

[grandfatherjim]

(This on a 72 type 4 engine)

Voltage at battery had been 13.9-14.0, while at +ve side of coil I was seeing 12.2. To get to the coil and all the other things that run from its little wiring tree, current has to flow to ignition switch and back, and having installed a relay earlier to deal with the hot starting issue (also due to the long run to ignition switch and back, and including its 40 year old contacts), I decided to supply all the engine electrics via a relay mounted back in the engine compartment.
I used to have a gas heater, so used its former always-live supply wire as the main power source, and used the former supply wire from the ignition switch just to energize the relay.
Things now being supplied more directly via the relay are: Coil, electric chokes, 3, valves on carbs, electric fuel pump, and electric in-line fuel shutoff valve.

This was meant as a general upgrade, just to remove the 30 feet or so of wire and the ignition switch contacts from the equation when troubleshooting.
I got some interesting changes:
1) My fuel gauge, instead of always bopping between two positions, is now rock steady. I'm glad I didn't get around to pulling the tank and replacing the sender as I have meant to do for the last three or four years.
2) Fuel economy has gone from 20 to 22.5 mpg. (Canadian gallons, if in US gallons it would be 5/6 of these numbers).

I'm in the habit of watching how many miles we've gone when the tank gets down to 3/4, then 1/2, then 1/4, to see if anything different is happening. I was consistently getting 70 miles from the first 1/4 tank, and was pretty happy if it was 75. This doesn't sound too good but I figured the gauge isn't too accurate, whatever. Then between 3/4 and half, I could expect to see it almost get down to 1/2, and could get all happy when it did its erratic thing and popped up to 5/8 before getting back down to half, at 150 miles. At about 200 miles I'd fill up, and by then the gauge might be anywhere between 1/4 and empty, bopping about.

Right after the wiring change, I filled up and we went for a trip of about 700 miles. Now I hit exactly 100 miles in the first quarter tank, and have gone 175 by the half way point, and I'm really liking the steadiness of the gauge. Sometimes, I also look at the road.

Why the improved fuel economy? Ideas? I can think of two possibilities:
a) My Crane electronic ignition is receiving nice steady (and higher) supply voltage.
b) Coil output is about 15% higher. I am less convinced this has an effect, unless I was occasionally missing a spark before the wiring change.

Jim

[Bleyseng]

Better spark voltage = better burn= slightly more power=you pushing less on the go petal=better mpg

[grandfatherjim]

I was thinking that, but then I thought that higher spark plug voltage should mean the spark would just jump the gap slightly sooner (but nanoseconds maybe).
Can't see that the spark would last longer...or would it?
Jim

[Bleyseng]

not sooner, but hotter or brighter however you want to picture it. Remember you are trying to light a fire in there so a hotter spark means a better ignition of the gas vapor to a point.

[SlowLane]

Here's a long shot: could be that your electric chokes were "lazy" with the lower voltage they were getting before and not opening fully. With the new higher voltage the heater elements will be reaching a higher tempurature, causing the bimetallic elements to exert a stronger spring pressure on the choke shaft.

Or it could be just the hotter spark, but a fuel consumption improvement of more than 10% seems unlikely from just better spark. Have you had a chance to inspect the plugs since the electrical re-routing?

Does the engine feel peppier now? If you had been missing sparks before, then the engine ought to feel smoother now.

[vdubyah73]

x2 on whats been said. hotter spark, low voltage and probably intermittent power fluctuations goofing on the coil, gas gauge, and electric chokes.

[grandfatherjim]

SlowLane: The chokes, interesting thought. I haven't looked at the plugs yet, but before this, they indicated I was running rich. I'll take a look.
I would not say the engine is any peppier, but I do think it's smoother. Cruising on level ground is delightful. Hills however, are still a struggle....
On the trip this weekend...I passed a car!
Bleyseng: I think that the high voltage is just needed to overcome the gap, and then the voltage drops once the current starts to flow and sustaining the current then becomes the issue. So I was saying sooner because if the voltage is going higher, presumably in the same amount of time, it would reach the threshold a little sooner. But again, not enough to matter I bet.
Often we see wider spark plug gaps along with higher coil voltages. But I have normal spark plug gaps - and now, maybe, normal voltages. So I suspect more that I was missing the occasional fire. In which case I would certainly have hotter fire in there now.
SlowLane: on the electric chokes - one day I got into really examining those things, and they work the opposite way from what I expected. Heating them up makes the coil contract rather than expand, which exerts less pressure against the spring which is always trying to open the choke.

Anyway, if I check the plugs and it looks like it's running leaner, I guess I won't actually know why. But if it's better, that's good.