help With HEI timing on mild 305? questions?
03-31-2012, 12:44 AM
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help With HEI timing on mild 305? questions?
so i finally got a hold of a timing light and i started playing around with the initial timing.
First off the engine is a mild 305, holey 4160 carb, accel super coil HEi distributor with vaccum advance. Im not 100% of whats in the 305 but i know its not stock.
I set my timing to 12 degrees at idle, with the vac advanced plugged in and the car didnt seem to "pull" at all at WOT. I then set it to 8 degrees at idle and it seemed to help, it pulled better than at 12.
1)how do i calculate total timing? How would i measure that? I know i would need to rev up the engine until the timin doesnt advance anymore and that would be my total timing correct? But my balancer doesnt have any marks to tell me exactly how many degrees its at when it finishes advancing, only thing i have is the timing mark to go by.
2)Generally what would be a good initial timing for a mild 305? and am i doing it correctly with the vac advanced plugged in?
3)ive read some people said to advance it until i hear pinging then go down 2 degrees, but im having a hard time hearing any pinging over my exhaust, any tips on finding sweet spot?
Thanks for any help, i appreciate it.
First off the engine is a mild 305, holey 4160 carb, accel super coil HEi distributor with vaccum advance. Im not 100% of whats in the 305 but i know its not stock.
I set my timing to 12 degrees at idle, with the vac advanced plugged in and the car didnt seem to "pull" at all at WOT. I then set it to 8 degrees at idle and it seemed to help, it pulled better than at 12.
1)how do i calculate total timing? How would i measure that? I know i would need to rev up the engine until the timin doesnt advance anymore and that would be my total timing correct? But my balancer doesnt have any marks to tell me exactly how many degrees its at when it finishes advancing, only thing i have is the timing mark to go by.
2)Generally what would be a good initial timing for a mild 305? and am i doing it correctly with the vac advanced plugged in?
3)ive read some people said to advance it until i hear pinging then go down 2 degrees, but im having a hard time hearing any pinging over my exhaust, any tips on finding sweet spot?
Thanks for any help, i appreciate it.
03-31-2012, 04:43 AM
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Re: help With HEI timing on mild 305? questions?
8 deg before TDC at idle is good for initial. You should be set. As far as not having any "pull" at WOT you could possibly have carb issues... On a Holly carb I wouldn't try to achieve "ping" to set the timing... If you backfire you run a very real risk of blowing the power valve... 8 degrees is good.
03-31-2012, 07:01 AM
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Re: help With HEI timing on mild 305? questions?
"Total" timing is initial + maxium centrifugal advance. Most HEIs have about 20* of centrifugal advance built int them (the weights and centerplate determine this in a factory HEI). So.... 8* initial + 20* maximum centrifugal advance = 28* total timing.
The springs in the centrifugal advance mechanism determine how quickly it hits that maximum. Usually you want it "all in" by about 3000 RPMs. In other words, no more centrifugal advance above that RPM level. It isn't needed or wanted above that point.
Unless you own a distributor machine the only way to "map" your timing curve is to use a dial-back timing light and a tachometer.
Vacuum advance is IN ADDITION TO the centrifugal and initial. Most stock cans supply about 20* of vacuum advance, which is usually too much once you have the initial and centrifugal dialed in for best WOT performance. So you either have to use less initial timing to prevent part throttle detonation (pinging) or you have to find a way to limit the vacuum advance canister to supply less- typically 10-12* of maximum vacuum advance is what you shoot for.
If yu have a stock (GM) vacuum can it will have the amount of advance supplied stamped into it behind the canister head. 3 digits followed by 2 more. For instance, 624 22. Those last two are the maximum amount of advance it supplies (22* in this example).
The springs in the centrifugal advance mechanism determine how quickly it hits that maximum. Usually you want it "all in" by about 3000 RPMs. In other words, no more centrifugal advance above that RPM level. It isn't needed or wanted above that point.
Unless you own a distributor machine the only way to "map" your timing curve is to use a dial-back timing light and a tachometer.
Vacuum advance is IN ADDITION TO the centrifugal and initial. Most stock cans supply about 20* of vacuum advance, which is usually too much once you have the initial and centrifugal dialed in for best WOT performance. So you either have to use less initial timing to prevent part throttle detonation (pinging) or you have to find a way to limit the vacuum advance canister to supply less- typically 10-12* of maximum vacuum advance is what you shoot for.
If yu have a stock (GM) vacuum can it will have the amount of advance supplied stamped into it behind the canister head. 3 digits followed by 2 more. For instance, 624 22. Those last two are the maximum amount of advance it supplies (22* in this example).
03-31-2012, 09:45 AM
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Re: help With HEI timing on mild 305? questions?
"Total" timing is initial + maxium centrifugal advance. Most HEIs have about 20* of centrifugal advance built int them (the weights and centerplate determine this in a factory HEI). So.... 8* initial + 20* maximum centrifugal advance = 28* total timing.
The springs in the centrifugal advance mechanism determine how quickly it hits that maximum. Usually you want it "all in" by about 3000 RPMs. In other words, no more centrifugal advance above that RPM level. It isn't needed or wanted above that point.
Unless you own a distributor machine the only way to "map" your timing curve is to use a dial-back timing light and a tachometer.
Vacuum advance is IN ADDITION TO the centrifugal and initial. Most stock cans supply about 20* of vacuum advance, which is usually too much once you have the initial and centrifugal dialed in for best WOT performance. So you either have to use less initial timing to prevent part throttle detonation (pinging) or you have to find a way to limit the vacuum advance canister to supply less- typically 10-12* of maximum vacuum advance is what you shoot for.
If yu have a stock (GM) vacuum can it will have the amount of advance supplied stamped into it behind the canister head. 3 digits followed by 2 more. For instance, 624 22. Those last two are the maximum amount of advance it supplies (22* in this example).
The springs in the centrifugal advance mechanism determine how quickly it hits that maximum. Usually you want it "all in" by about 3000 RPMs. In other words, no more centrifugal advance above that RPM level. It isn't needed or wanted above that point.
Unless you own a distributor machine the only way to "map" your timing curve is to use a dial-back timing light and a tachometer.
Vacuum advance is IN ADDITION TO the centrifugal and initial. Most stock cans supply about 20* of vacuum advance, which is usually too much once you have the initial and centrifugal dialed in for best WOT performance. So you either have to use less initial timing to prevent part throttle detonation (pinging) or you have to find a way to limit the vacuum advance canister to supply less- typically 10-12* of maximum vacuum advance is what you shoot for.
If yu have a stock (GM) vacuum can it will have the amount of advance supplied stamped into it behind the canister head. 3 digits followed by 2 more. For instance, 624 22. Those last two are the maximum amount of advance it supplies (22* in this example).
Thanks for all the info this is great information.
The springs in the Accell Hei dizzy have not been messed with. Calculating everything you told me kinda makes me think im giving it too many degrees of timing
If my initial is 8, centrifugal 20, and vaccum adds another 20 max, wouldnt that put me at 48*?
which would be pretty high since most sbcs are supposed to get 32-36* max depending on year/heads no?
To adjust the advance to be in all the way by 3k RPM i would need the dial back correct? there is no other way?
i appreciate the help
EDIT: before i had to pull the distributor my car would "pull" very good at WOT, so im trying to get that back, so far no luck
03-31-2012, 10:11 PM
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Re: help With HEI timing on mild 305? questions?
The 32-36 degree magic number is at loaded WOT, at about 3000 rpm. At times of low load (= high vacuum), the motor needs more timing mainly for mpg purposes. If you're just revving the motor out of gear, the timing might easily approach the high 40's with vac advance connected, this is fine.
Install an adjustable type vac canister in your dizzy. Then get the mech advance sorted out first, while the vac advance is disconnected. Once that's done, add as much vac advance as the motor will tolerate without pinging in a low gear at part throttle.
Eg I've ended up with about 12 vac in my current configuration, on top of 15 initial and 33 at 3000 rpm with the appropriate selected springs sorting out the advance rate in between.
To measure timing past where the timing comb allows, you'll need a dial-back or timing tape stuck on the balancer (that's what I use).
"Performance" at WOT is going to depend more on the final mech timing at 3k rpm and the advance rate that gets there, rather than where it starts out at idle. You're going to have to work out how you want to measure timing up to 50 deg. The go out and make lots of WOT runs and make small timing changes, and write them down, so you can find what the motor like best.
Install an adjustable type vac canister in your dizzy. Then get the mech advance sorted out first, while the vac advance is disconnected. Once that's done, add as much vac advance as the motor will tolerate without pinging in a low gear at part throttle.
Eg I've ended up with about 12 vac in my current configuration, on top of 15 initial and 33 at 3000 rpm with the appropriate selected springs sorting out the advance rate in between.
To measure timing past where the timing comb allows, you'll need a dial-back or timing tape stuck on the balancer (that's what I use).
"Performance" at WOT is going to depend more on the final mech timing at 3k rpm and the advance rate that gets there, rather than where it starts out at idle. You're going to have to work out how you want to measure timing up to 50 deg. The go out and make lots of WOT runs and make small timing changes, and write them down, so you can find what the motor like best.
04-10-2012, 10:16 AM
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Re: help With HEI timing on mild 305? questions?
thanks tree fiddy,
Ive been playin around with timing and im having some problems. i started off by setting my timing to 13 degrees initial ( with vac advance plugged)BTDC. Noticed car didnt "pull" at WOT. So then i set it to 8 degrees BTDC Car felt a little better but it would bog down and try to die at stop lights. So i moved it up to 10 degrees BTDC which is what im running now, only problem is that im getting some horrible hard starts.
It litterally took me like 2 minutes to start my car this morning, it would crank for a second then shut off, i had to let it sit a bit to get it to start. I know im not flooding the carb and am about 80% positive its a timing issue.
Do the hard starts mean i have to go up or down? im at 10 btdc right now, should i move it up to like 14? or down to maybe 6BTDC?
I switched the vac advance to full manifold vaccum and the car idles much better but these hard starts are killing me. .
I dont have a timing tape yet so im not sure on my mechanical advance numbers or vac advance.
Generally what initial am i supposed to run? im thinking about dropping initial down to 6, but then i read people running 15 so im a bit lost. I know im supposed to listen for pinging but i cant really point out any pinging.
Ive been playin around with timing and im having some problems. i started off by setting my timing to 13 degrees initial ( with vac advance plugged)BTDC. Noticed car didnt "pull" at WOT. So then i set it to 8 degrees BTDC Car felt a little better but it would bog down and try to die at stop lights. So i moved it up to 10 degrees BTDC which is what im running now, only problem is that im getting some horrible hard starts.
It litterally took me like 2 minutes to start my car this morning, it would crank for a second then shut off, i had to let it sit a bit to get it to start. I know im not flooding the carb and am about 80% positive its a timing issue.
Do the hard starts mean i have to go up or down? im at 10 btdc right now, should i move it up to like 14? or down to maybe 6BTDC?
I switched the vac advance to full manifold vaccum and the car idles much better but these hard starts are killing me. .
I dont have a timing tape yet so im not sure on my mechanical advance numbers or vac advance.
Generally what initial am i supposed to run? im thinking about dropping initial down to 6, but then i read people running 15 so im a bit lost. I know im supposed to listen for pinging but i cant really point out any pinging.
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