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Old 10-05-2024, 12:11 AM   #1
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AFC tune / new trans

hey there.

my bus used to be an all mechanical 12v, but im the llucky new owner of a allison electronic conversion.

so... i want to tune it up a bit to run best with the new trans.

inthe few short drives i;ve been on... it seems to not downshift very well and luggs easy.

on another thread i was talking about how im set to roll a bit of coal with my smoke screw.

so tonight im sitting here and wondering if the 2 are related.

if i roll back the smoke screw , which to my understanding controls fuel from 0-5psi boost, i should reduce the lugging maybe?

im not sure what kind of boost im running when lugged out, but i would think 0. so if i back off that screw, maybe my luggin issue clears up?

does that sound valid?

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Old 10-05-2024, 10:32 AM   #2
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Quote:
Originally Posted by turf View Post
hey there.

my bus used to be an all mechanical 12v, but im the llucky new owner of a allison electronic conversion.

so... i want to tune it up a bit to run best with the new trans.

inthe few short drives i;ve been on... it seems to not downshift very well and luggs easy.

on another thread i was talking about how im set to roll a bit of coal with my smoke screw.

so tonight im sitting here and wondering if the 2 are related.

if i roll back the smoke screw , which to my understanding controls fuel from 0-5psi boost, i should reduce the lugging maybe?

im not sure what kind of boost im running when lugged out, but i would think 0. so if i back off that screw, maybe my luggin issue clears up?

does that sound valid?

if Lugging easy... I had issues with my red bus with lugging.. granted not a mechanical cummins.. But..


it was a 70 MPH maxx bus before my 6 speed allison conversion so after the 6 speed swap I was over-geared for 6th gar for sure and really at times i turned off 5th if I was up and down in the hills. not sure what your original top speed was.. I regeared and solved that issue



the TCM I got from CAC tended to want the transmission to be in 6th gear as quick as possible.. it shifted low for my liking.. since i am using J1939 electronic throttle, I didnt have any mechanical geometry that I could correct to raise the shift points of the transmission... I ended up getting a different TCM that I can tune..


in your case i think the TPS is still misadjusted or doesnt have the right curve for your engines pump profile.. im guessing the more the cable Pushes out, the higher the TPS value on yours? (im used to a pull mechanism and not a push).. you may eed to adjust the cable a bit so it shifts higher or uses a faster arc so that it reaches hgher shift point quicker... was hard for me to tell exactly how yours is built from the one pic I saw.. higher shift points will tend to reduce lugging... if you are changing your governer springs then you might want CAC to program you a TCM with the higher RPM profile on it which would raise your shiftpoints



in my case regearing to where I reached max RPM around 54-56 MPH in 4th gear allowed me to get great use out of 5th gear in that 50-64 range in low throttle conditions.. 65 and above 6th gear is he perfect gear to be in.. at 65 im taching in that 1900-1950 range.. you, being an I-6 might do fine with 100-200 RPM below that under light-moderate load and then have your 6th come-off if you crowd it.. thats the way it "should" work...


for me the only way I could get there was building my own shift tables....

CAC designs tend to favor the cummins 5.9's so you likely will have a much easier time getting it right when you get the TPS adjusted where it should be..
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Old 10-05-2024, 11:34 AM   #3
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What is your EGT? If you have a pyrometer and a boost gauge you can tune it. Without them you are shooting in the dark. Fuel drives boost. No fuel no boost. Too much fuel = black smoke and high egt. Driving style is also involved, that is where your tps is involved. I would start there and get your shift timing right.
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Old 10-05-2024, 12:04 PM   #4
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i havent driven it that much. so im not certain. i think my egts were high.

i had about 40 min test drive 2 weeks ago and then about an hour ride home from the mechanic.

since then, i've been instructed how to use the tap shift on the trans... the automatic downshifting isnt very good. i havent had a chance to try it out.

but back to your comment, i remember wondering why the egt was so high while it was lugging.... i was tapping the gauge to make sure it was reading right.

i have egt and boost gauges so going to tune... i just never know beforehand if it helps or hurts.
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Old 10-05-2024, 12:38 PM   #5
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Quote:
Originally Posted by turf View Post
i havent driven it that much. so im not certain. i think my egts were high.

i had about 40 min test drive 2 weeks ago and then about an hour ride home from the mechanic.

since then, i've been instructed how to use the tap shift on the trans... the automatic downshifting isnt very good. i havent had a chance to try it out.

but back to your comment, i remember wondering why the egt was so high while it was lugging.... i was tapping the gauge to make sure it was reading right.

i have egt and boost gauges so going to tune... i just never know beforehand if it helps or hurts.

EGT go high under lugging conditions because the cylinder pressures go through the roof.. the fuel is trying to combust but instead of being able to push the piston down and expand the gas charge in the cylinder, it simply raises the pressure which triggers a temperature increase both from the combustion and from the exponential pressure increase. the result is you are exhausting really hot gasses.. sometimes fuel is still burning in the cylinder during the exhaust stroke..


that will keep the air and the piston hot... normally your pistons get a reprieve from heat during the exhaust and intake stroke as the cylinder pressure goes down allowing the gasses to expand and attain heat from their surroundings (the piston, the cylinder walls)... and of course during the intake stroke where cool air is being pumped into the cylinder...


when you over-fuel and fuel still burns during the exhaust stroke you lessen the time the piston has to cool down..
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Old 10-05-2024, 01:32 PM   #6
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my ride to pick up the bus commented on how bad i smoked on the way home. from the drivers seat.. i dont see much.
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