Alternative Fuels

Last week I installed a 4-hole phenolic spacer between the 425 and Edelbrock Performer intake on my '73 318ci engine.
Also installed a large late '70s air-filterhousing and fabbed a cold air intake on it, feeding through a hole in the radiator frame.

I mainly had to install the 1" spacer to get the large airfilter housing to clear the distributor cap and wires.

Didn't expect much 'noticable' improvement, but I found engine torque was improved in the low rpms 800-2000rpm at part throttle.
Which is pretty much the same thing I've found with gasoline engines, where the engine's vacuum signal is enhanced/improved and the carb responds better/quicker.

In most circumstances a spacer will help an engine produce more torque. It is simply because the extra length helps to straighten and speed up airflow.

I agree mostly, but over the years I was let to believe that propane engines liked a single plane intake better, so I didn't think a 4-holed spacer would do much in this case. But I was wrong.

I did however put a slight chamfer on the holes on the underside of the 4-holed spacer to help airflow transition into the intake a bit more.

Yeah I was led to believe that to but after street testing many engines I find anything that reduces turbulence in the airflow will help an engine to run smoother and be more responsive, that makes them feel more powerful even if they aren't.

After the improved low end-surprise from the spacer, I started wondering if ignition timing changes would have any effect.
I have been driving for a good while with the current advance curve in the distributor.

I started off with checking the Vacuum Advance. It turned out the IGN-port on the 425 didn't not actuate the VA at all.
Decided to hook the VA to intake vacuum below the throttle plates and test&tune from here.
The VA made some changes in drivability, but no real noticable improvement.

Timing and curve was very fast. Mechanical Advance (MA) already came in at 700+ rpm and was full in around 1500rpm.
Also, total Mechanical Advance was limited to 16degrees.
15deg initial @ 600rpm (in Drive)
20deg @ 850rpm (in Neutral)
600rpm - init+VA: 20deg
Vacuum advance: 5deg 19" 600rpm)
850 - init+MA - 20deg
Total (init+MA): 31deg @ 1500rpm.

Decided to slow down the curve slightly and the change timing:
init: 18.5deg
init+VA: 25deg
MA: 16deg
Total init+MA: 34.6deg

This improved top-end acceleration, but not so much low-end.
Increased MA to 25 degrees and dropped timing down to 15 degrees initial.
init 15deg
Total 39deg
init + va: 21.5deg
This gave better performance all round, but felt not quite 'good' yet.

So I did some more changing, and this is what I'm currently running:
init: 10deg
MA: 26 deg (in @ 2200rpm)
Total: 36deg
VA @ idle: +6 (more at high rpm/no load)

Low-end performance as well as accelerating feels nice and strong again.
During steady (highway) driving, I'm noticing engine-vacuum is about 2 points higher at 19".

Highway mileage is 15mpg and 11.5mpg around town.

I think some further finetuning is possible but I want to continue this (or start over) when the 360 is in the car.
That would also be a good time to clean up the engine bay...