Megasquirt mpg test

[piledriver]

My car will start to surge if I push it too far.

Unless you have lean miss, it goes away with more advance.
(have ~lived here for years, still running 22:1 at cruise)

I get lean miss running batch fuel at ~18:1, sequential 22:1. injection timed for max residency time on back of valve, and lowest AFR reading.

Doesn't really help fuel economy that much, but knifes my CHT ~50F, -75 with E85.
I have to load another tune in late fall, target stoich when its below ~60F outside so as not to overcool.
(aircooled)

[slow_hemi6]

I kind of put it down to the 70 degrees of overlap on my cam coming into play. Surge turns up around 15.5:1. I will have to try and push it next time I take a trip.

[webb1624]

After some suggestions to increase the timing a little more, I was able to do a couple tests. First test timing was at 50 degrees, AFR 15.8-16.2, 22.20 mpg. Best so far.

Second test timing at 50 degrees, AFR 14.5-15.2, 18.528 mpg. That was a considerable drop and not consistent with what I've been getting so I'm going to do it again. The reason I say that is one of my last test was 45 degrees timing, and AFR 14.5-15.2 and my mpg were 20.897.

Next time I'm going to increase timing a little more and see what happens. At 50 degrees the engines runs great. No knock at all.

[Laminar]

Do you have a lockup converter? What RPMs are you running on the highway?

[webb1624]

Yes, I have a lockup converter. 2600 rpms at 70 mph

[subwoofer]

That your MPGs was falling at stoich and added timing would simply be that you have more timing than is most efficient for that mixture. I don't see an inconsistency in losing mileage when adding timing. You could try leaning out further to 17-18 and add even more timing. The weaker mixture burns slower so it needs to be lit off earlier.

[piledriver]

That your MPGs was falling at stoich and added timing would simply be that you have more timing than is most efficient for that mixture. I don't see an inconsistency in losing mileage when adding timing. You could try leaning out further to 17-18 and add even more timing. The weaker mixture burns slower so it needs to be lit off earlier.

^^^^ what he said.
The efficiency will fall off slower with too much timing vs too little IIRC.
(assuming safe low maps under ~80ish KPA for all this play)

Stoich wants the LEAST timing , and the leaner (or richer) you go from stoich, the more timing it needs.
It's all about flame propagation speed in the chamber at different mixes and pressures.

[Quagmire]

Update on my figures.

Running around without a TPS I was previously averaging 25-26mpg (UK).

TPS fitted and fuel cut enabled, no other changes made.

Last four tanks have been:

27.5
27.5
26.4
27.0

So a noticeable improvement!

This is on the factory fitted 3.5 in my 1973 Rover 3500s

[subwoofer]

If you experiment with lean burn operation you could probably get a lot better than that, even. Are you running sequential? There may be some small gains in the injection timing as well.

[Quagmire]

At cruise I am running around 16.2:1 - not got any lean surge so could go further with it but to be honest it scares me a bit!

Should also point out that the Rover has a four speed manual, and 60mph is around 2500rpm. I have a 5 speed to fit which should make a difference, dropping the revs a bit.