MPS bench tuning, just for future reference Options

[emerygt350]

For those that play with these things, I thought having this around might be helpful. I wanted to see the effects of a little change on the MPS in an attempt to richen my cruise AFR.
73 2056 8.8ish compression
Stock bottom
Stock heads
Stock 037 djet components
123 with advance hooked up (non-Bluetooth setting 1)
timing 8.5 btdc
Proster LCR Meter https://www.amazon.com/dp/B07CYSV5LY
My car has very high vacuum. Idles around 20mmhg-17mmhg. Cruise 15-12mmhg

I was running 14.7 at 70 cruise. 31 mpg. Pretty high cht.
after changing to the 4th iteration below I am at 13.5ish at cruise. Maybe 26 mpg if I baby it. Temps dropped about 20-30 degrees on the cylinder heads during cruising 60 or less. 70 cruising cooled 10 degrees (my front end is something of a sail). Pretty rich during warm up. idle is 13.5 with my ecu pot turned all the way lean. I think this is likely the result of my extra high vacuum, to get the cruise afr lower I end up making idle extra low. Anyone else have a vacuum guage? I would be interested in knowing what mmhg you are at while cruising in 5th at 3200 rpm on a flat. I did turn the inner screw back a hair after startup since it was very rich. So I am currently running more like 3 than 4 in the figure.

As far as the figure goes, it is interesting to see how little of an inductance change results in very large differences in air fuel. Disclaimer: my lcr probably has different values than your lcr so just worry about the relative changes not the actual values. Altitude makes a difference (I am 800 feet) and even the condition of the battery affects my LCR greatly (this is with a fresh battery).





and has anyone ever played with setting 5 on the 123? Looks interesting...

[JamesM]

and has anyone ever played with setting 5 on the 123? Looks interesting...

If you use setting 5 you will still want to set your max timing to 27-28 at 3100+. This will result in dialing your timing back to -3 at idle. Your engine should be fine here and may actually help control idle speed however I am not sure how the MPS is going to handle the reduced vacuum caused by less timing. you will need to open up your air bypass as well.

two things I would mentioned about this curve not being ideal.

1. overall advance curve is a little sluggish as you don't hit max advance until 3100. This is going to reduce torque and increase temps below this RPM.

2. the advance curve starts ramping up below your idle RPM. this is going to increase idle instability /"runaway" potential




What port are you pulling vacuum from for your vacuum advance?

[emerygt350]

Ported. It's the switched version so kind of critical. Although it says there is a range till max vacuum advance in practice it appears to be on or off.

That ramping bit is what I was wondering about. It appears to me that if 5 is moderately linear it might be past or at 22 degrees by the time it hits 2700. My car has vacuum to spare so dropping the advance might actually benefit my mps. Come to think on it though, it shouldn't matter unless it is running less advance at every given rpm so probably no benefit for my mps. Nevermind....