Looks like a couple weeks have gone by. The baseline of the experiment has changed with the new distributor. It is designed, according to Pertronix, for 7 degree initial timing advance. The vacuum advance adds 18 - 20 degrees. Mechanical advance adds up to 29 degrees, starting the curve ramp from zero at 1400 rpm up to 29 degrees at 4500 rpm. They say to use ported vacuum (probably for legal reasons to maintain emissions), but I am trying it out using manifold vacuum just like mentioned previously.

Only been driving it a couple days now, but happy with the results so far. Good power during acceleration, cool engine temperature, starts and idles well. Will monitor gas mileage for a tank or two.

Also installed a refurbished set of injectors. Unfortunately, one was bad, ran terrible, but tracked it down and replaced with known good one.

 

I had my intake plenum off for a few weeks while changing the fuel lines. Decided it was time to look into switching the vacuum timing advance while adjusting timing and base idle anyway. Before switching vacuum source from the intake nipple to a bigger vacuum source on the plenum I decided to test the vacuum (neg pressure) on each source. Interestingly I got 20psi off of one of the ports on the side of the plenum and zero (or very low) off the nipple. Vacuum raised with plenum port but no change on the nipple at least up to like 4k rpm. I cleaned all the passages out during the plenum rehab… maybe something got sucked up into that narrow passage? Any thoughts?

 

That is normal behavior for the ported vacuum (narrow passage). It lines up exactly with the throttle plate (the throttle plate blocks the passage), so no vacuum at idle, then about the same as manifold vacuum when the throttle is open a little, then both go low when throttle is wide open (WOT).

Most vacuum gauges read in Inches Mercury ("Hg) not PSI. If you got 20 "Hg at idle, that is a very good tight motor. Mine only manages 15 -16 "Hg at idle with 6 deg BTDC initial timing. I noticed my engine running very cool with the manifold vacuum advance, but it is difficult to tell the impact due to cool weather at the same time. When summer hits, then I will be able to tell if there is a significant difference.

Using a manifold vacuum source could cause a problem with emissions for folks that need to pass inspections because it will change the emissions gases profile at idle. For inspections, they should continue to use the ported vacuum.

The new Pertronix dizzy has an engineered advance curve that is quite a bit more aggressive than the Lucas dizzy was. I am in the process of checking out performance and fuel economy now.

 

Thanks for the info and benchmarking. Checked things over again today and I confirmed no vacuum at idle or 2-4k rpm via the intended vacuum advance nozzle. Strange… Plenty of vacuum from the rest of the inlets. Something must be stuck in there. Wish I hadn’t just taken the whole plenum off

 

Quick update - my vacuum advance nozzle is completely blocked at the plenum. Been trying to clean it out but it is something stubborn... Anyway running on the full manifold vacuum is going nicely! Engine feels great across the rpm range.

 

Be sure to remember the throttle plate lines up with that port, so don't go drilling into the throttle plate! That would be bad.

I am working on my first tank of gas with the new dizzy, manifold vacuum advance, and a new set of injectors to boot. Truck is running great! Earlier I found the vacuum line had become brittle and split at the fuel pressure regulator, and truck was running rough, so happy now.

 

So, here are the results for the '94 3.9L. I have now burned through 1/2 tank. Trip meter showed 165 miles. Mixed around town and highway miles. Compared to prior 125 - 150. At least 10% improvement (that's one free tank out of 10) due to the following:
1. Plug gap increase (0.055 vs 0.035 inch)
2. Manifold vs ported vacuum advance (rerouted vacuum tubing shown above)
3. New distributor and coil (Pertronix with advance curve matched to the Rover V8)
4. New (used from a '96 Disco) fuel rail with Schrader valve to check pressure
5. New injectors (Accel 19#/hr, after trying some cheap refurbed ebay 4-hole injectors that caused rough running)

Truck starts better, idles smooth, engine is cooler, accelerates better. The old V8 needed a little TLC. In addition, the CEL misfires at high RPM on the highway on-ramp have not returned. No pinging under load. I am happy with the results.

 

Yesterday marked 169 miles on 1/2 tank, mostly around town driving. Calculated 13.3 MPG vs 11 - 12 (mostly 11) prior to the work described above. Truck sounds and drives great.

Tweaked the CO trim on the MAF down to 1.6 V (was 1.8). Starting RPMs now a bit under 1500 (was over 1500 up to nearly 1700). I got a spare MAF model 20AM from my '96. Going to chart MAF voltage vs no-load RPM, then do a modification with the spare MAF for comparison. My original 5AM MAF style might be under reporting air flow. It has no label information, so might be an after market version. Been reading technical discussions on Aussie and Jaguar forums about MAF testing. I will start a new MAF Experiment thread when I get some results, so see you soon.

 

So, never mind about a new MAF thread. I gathered some data from both the 5AM and 20AM MAFs, and the signal outputs were nearly identical. Here are the readings (in Park, except #3. Signal ground to signal voltages):

1. Key On, jumps to 0.8 V then settles to 0.36 V after a minute or so
2. Start/idle, 1.21 V
3. Idle in reverse, 1.28 V (IACV opens)
4. 1000 rpm, 1.6 V
5. 1500 rpm, 1.8 V
6. 2000 rpm, 1.9 V
7. 2500 rpm, 2.1 V
8. 3000 rpm, 2.3 V