thats one way of doing it but you are steering him backwards suggesting he starts with the star wheel, arnt ya?
a) first of all the spring(s) (the springs you choose to run in your afc ) MUST have a free standing height of at least 36mm. that way once installed it/they will push the diaphragm/lever back to a spot where it creates a lean condition (pre boost of course). if the spring is too short (like the stock one is) once modded correctly the afc rod (which controls the racks starting position will not be far enough rearward to create the lean condition mentioned earlier which will let YOU bring the rod forward (with the smoke screw) to the exact position that your combination wants. but:
b) you are describing the tuning correctly BUT in the wrong order. it does not matter what the afc does thru the boost curve UNLESS you have the rod in the correct starting position first. to find that it is very simple. 1) once the afc is modded and has a spring with a free standing ht of 38mm or more the FIRST thing you work on is the levers starting position (controlled by the smoke screw). i pitch the stock screw and get a 35mm long shcs (allen bolt) (6mm x 1.0mm x 35mm) to make it easier to keep track of where it is. put the sealing nut on it and screw it in til it just hits the rod, then give it a 1.5 turns. i use a silver sharpie to then put a line on it, just for reference, and i put the line at 9 o'clock so i can see it easily, even if its dark adn i am using a flash lite.
i mod my afc's on a bench and set em up with regulated air pressure but if you do not have access its no biggie, just start off in the full lean adjustment (starwheel all the way back (in the middle of the hole for its plug). its better to set it up on a bench but this will work. from there it is just trial and error. get your junk to normal operating temp, pull over and give it 1/4 turn. get back in and mat the thing. watch in the passenger side mirror for smoke and use ur arse for tuning. keep doing that til the charger lites at the lowest rpm. you will know when you have passed the sweat spot cuz not only will it lite later but there will be more smoke. put it in its favorite spot, remark it, and start adjusting the star wheel
with the lite tst spring has a free standing ht of 38mm. if installed at a height of 32mm it will require 10 psi to just start to overcome the diaphragm and let the rack start to move. but there are two problems with this: 1) inst @ 32mm it only allows 18mm of rack travel before coil bind occurs. 2) that spring is really soft and i have yet to tune a truck that was such a mutt that it required such a soft rate. but here are the rates just fyi in the case that you are curious:
remember, this is installed at 32mm inst ht*
rack wont overcome diaphragm and start to move until 10 psi is reached. after that the rack moves as follows:
15# = 4mm
20# = another 3.5mm
25# = 3.5mm
30# = 2.5mm
35# = 2.25mm
40# = 2.25mm
at this point you are 1mm from coil bind
to take advantage of machine work you have did the rack plug and get another 3mm of rack travel you could increase the installed height (back off the star wheel) (move it forwards...towards radiator) 3mm and install the spring at 35mm. that will net a total of 21mm of rack travel (if you have machined ur plug or installed the one from mac but in doing so you also just changed the rate of the spring! now, instead of relying only on the preboost adjustment for correct fueling to get under the charger as quickly as possible (smoke screw adjustment) the afc is gonna start moving the rack at as soon as you reach 6 psi. most chargers are not 'lit' until approx 10 psi, some of the larger ones (64/71 etc) more like 12 or 13. and with the soft spring installed at 35mm your rack will have already moved forward almost .125" (3mm). 7mm by 15 psi ! iow its gonna smoke with this set up, not allowing you to get under the charger at the earliest possible rpm, and be detrimental to mileage and throttle response - what we are lookin for by tuning in the first place. the only thing positive from this tune up is that you will get full rack travel
therefore if using tst's springs i always install their heavy spring. the one with a free standing height of 37mm. i install it at 34mm. (that seems to work best for MOST apps) (for me). installed at 34mm the rack will not see any movement until 10 psi with the heavy spring
easy way to determine which spring is which is that the lite spring has a freestanding height of 38mm. the heavy one stands 37mm tall when in the spring tester and w/o a load on it (if you dont happen to have a long acre digital tester, they cost a grand so i imagine not many of you have them laying around your house or garage
you can always just use calipers to figure that part out
*an easy way to know your installed ht is to disassemble the afc, put it together leaving the spring out, count turns on star wheel and mark it, and use a t-gauge to measure the height. you can then reassemble the unit with the spring in it, and duplicate setting
stock gov springs free stand at 35.5mm. but it takes 10 psi to move em .030" (.75mm) so i am more concerned with their rate than i am with their length
yes, i made myself a chart for the stock gov spring set up. one for the heavy tst spring too. even made for the spring that i have a company back here custom wind for me
but time is money and if you knew how much time i have invested in just the afc part of my total truck time u would chit urself. if you knew how long it took me just to put this together for ya all you would prolly not believe it either!. hope it helps someone out.....
if you dont understand part of it (or any of it lol) feel free to pm me. i could email the other charts i made but there aint no way i am gonna spend the time typing something like this again - especially when i have 2 posts in the 911, both with 100+ views and not word one
peace
just a stupid ole hippi from mississippi
