on my rebuilt stock 340 with electronic ign, timed it to tdc ,mechanical 29 degrees without vaccum. total advance with vaccum hooked up 48 degrees does this sound right.this thing runs great but i thought it should be around 34 degrees total? there is no knock or pings and i shouldnt complain just was wondering what u all had total advance.
total advance ?
- Thread starter louie
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Burntorange70
Well-Known Member
It should like around 34-36. I would play around with it and see what happens. Allso it is going to want some more inital timing. Stock is 5-6 BTDC but it way like a little more mabby as much as 10-15 BTDC. But if you go up that high you will need to limit the mecanicl advance in the distributer. Hook up a vac guage and bump up your initial timing till you get the highest vacum reading then limit your mecanicle advance to give you some thing agound 34-36 total.
Welcome to FABO!
Welcome to FABO!
mopar head
Well-Known Member
I`ve read 50 total. but I don`t run Vac advance. Long as you ain`t pinging, your good:read2:
It's my guess that if you turn it back enough to get 34-36 with the vaccum hose hooked up, it's going to be way to far retarded, anless you have the vaccum hose hooked to a constant vaccum source. I like to run a total of 36-40 myself. But then I really not a fan of vaccum advances. I'd rather have a dist. set up to be "all in" at 2500 street, 2000 racecar. just my 2 pennys
moper
Well-Known Member
total is typically measured without the vacuum advance (hose disconnected and plugged), as it has no effect at full throttle. You should be aiming for around 36° total with a factory open chamber and stock cam. If it is not a full time drag car, a correctly set vacuum advance will only help in streetability, milage, and power.
dgc333
Well-Known Member
As moper stated the vacuum advance only operates at light throttle cruising. When connected to the timed port on the carb it does not operate at idle either.
The total everyone talks about is initial plus the timing in the distributor. Your distributor has 29 degrees in it (0 initial according to your post). If you bring the initial timing to 5-7 degrees before TDC you will have your 34-36 total.
The vacuum advance will have somewhere around 17 degrees in can. The sum of the total and vacuum is 53 but that much will seldom be added in. When cruising at light throttle the max vacuum advance will be added but the rpm's will most likely be lower than the max mechanical.
cudavert
DARTAHOLIC
I've never been a big fan of vacuum adv either.......it seems like that part throttle cruising (like up a gradual hill) is exactly when it will like to ping.....drives me crazy. I always thought most cars I've dealt with liked more initial than the factory recommended anyway. Somewhere I saw an article on how they showed people how to limit the total mechanical advance by welding on some tabs on the weights and then slowly grinding them down......it's sort of a foggy memory of it. Anybody see that?
Mopar73340
Well-Known Member
Exactly. I run my basically stock 340 with 18* inital and 15* in the dist for a total of 33* total mechanical in by 2500 rpm and another 17* of vac advance. There is a night and day difference in the part throttle response and driveability with the vac hooked up.
dgc333
Well-Known Member
Setting the timing at 2000 rpm to 34-36 would only be valid if the advance springs had been changed so all the timing is in at 2000 rpm. With stock springs the timing would continue to advance to almost 4000 rpm where you would have way to much. Doing it that way could very likely give you too much initial timing.
dgc333
Well-Known Member
If the distributor is set up properly you won't have part throttle detonation issues. When you start changing, initial, total and rate you will also need to adjust the vacuum advance.
Just cranking in more initial will give you too much total. The biggest contributor to how much initial timing an engine wants is the overlap in the cam (more overlap more timing). The head design is the biggest contributor to what the engine wants to see for total. Modern highly efficient cylinder head designs are using less than 30 degrees of total these days.
For max power and efficiency you want the spark to occur at the point that you get maximum cylinder pressure approximately 20 degrees ATDC.
Stock timing curves are a compromise to protect the engine against detonation by operates not using the proper gas or lugging the engine. So they can be dialed in for more performance and efficiency.
jeh
Well-Known Member
I did this to mine.. my stroker likes about 20 degrees of initial advance, and my distributor had 30 degrees built into it.. so I had to limit it to about 12. it's pretty easy to do, I should have taken pictures.. easier to explain that way.
anyway, there is a plate with a slot inside of the distributor that limits the travel of the centrifugal weights. You have to weld (I used jb weld, seemed to work ok...) the slot in the plate to limit the travel even more.. the plate will have a # on it ( mine said 15 ) that relates to the number of distributor degrees of advance ( multiply by 2 to get the number of degrees at the crank ).. if you measure the amount of travel that is possible that the plate is able to move, and you divide that by the amount of advance that you want, you can figure out how much of the slot you need to weld up...
for example, the amount of travel my plate allowed was somewhere near half an inch and it allows 30 degrees of advance. if you only wanted 15 degrees of advance, you'd close the slot by 1/4" .. not rocket science
-j
moper
Well-Known Member
I saw this idea before... The advance in the centrifical advance plate is not divided by two. It has two weights because one would not be balanced as it rotates. You could remove one entire weight, and if the plate is stamped "18", you will still have 18° of mechanical advance. The reality of stock distributors is they have a ton of advance in the plate (the centrifical advance) and limited at idle for emissions and starting reasons. So generally, to get the initial timing the engine likes (and we like) you have to limit the centrifical. This is done by looking at the bottom of the advance plate. It's stamped with a number that corresponds o the total number of degrees it will move. (dist. must be disassembled for this) Usually it's between 18-30°. I use a percentage. If it's 24° total, and I want to be able to run say 15-18° initial, then you need to limit it to 18° (36 total - 18 initial = 18 centrifical needed). 18 is 25% of 24, so I'll weld up 1/4 of the slots. It's important to make sure the weights can slide easilly thru the slot after the welding. You can do the equivilent of any MP performance distributor for about $25. $5 for the light spring kit, and $20 to a muffler shop guy to weld up the slot for you.
dgc333
Well-Known Member
moper, JEH is right, the distributor turns on 1/2 a revolution for each revolution of the crank. So if you measure 15 degrees of advance in the distributor that would be equal to 30 degrees on the crank. The mopar distributors are stamped with the advance in distributor degrees so you multiple by 2 for the advance when measured at the crank.
jeh
Well-Known Member
careful... if you weld up 1/4 of the slot, you will be reducing the amount of advance by more than 25%..
you have to take in account that the pin in the slot takes up room in the slot. you have to weld up the length of the slot minus the length of the pin, and take 1/4 of that...
-j
moper
Well-Known Member
No ****! You're both right of course... but can anyone tell me why my advance curves come out right?? I'd hate to think I was screwing up every time and I was getting lucky. Sorry about that...
I'd hate to think I was screwing up every time and I was getting lucky. Sorry about that...
cudaman51
Well-Known Member
I opened up my mopar distributor today to limit the advance and was pleased to se it has a built in limiter with torqs screws.It is one of those mopar performance convershion kit ones.surprise surprise.
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