(long) hei/advance curve opinions needed 396 [Archive] - Chevelle Tech

: (long) hei/advance curve opinions needed 396

Feb 14th, 06, 10:34 PM
i previously posted on some timing problems i have been having, where my timing has changed on its own..its retarded itself, and most recently it has advanced itself to the point of hard starting. the distributor is a GM HEI unit, it was used when i got it, but was working ok as far as I could tell. when I had the motor done, I had the shop re-curve the HEI for my application on their distributor machine. motor was subsequently dynod well over 400hp, and the HEI has been on there for about a year now.

The info from the shop has this curve:

600 rpm 0 deg advance
800 4 advance
900 6 advance
1000 7 advance
1500 12 advance
1800 15 advance
2000 16 advance
2500 20 advance
2800 22 advance all in

shop told me to shoot for 36 degrees total as target. shop also kinda suggested that i would be better off with a newer distributor and/or ignition box, which i have ignored until now. based on the curve above, I am wondering if the springs (which are light ones i guess), may be wearing out, or something is going on in there to cause the timing to jump around to begin with.

Car seems to have lost something on the bottom, like its not snappy in 1st gear off idle, and it doesnt roast the tires from a slow roll in first without using clutch, which i think it should be fully capable of doing with no clutch. looking at some of the msd digital e curves, many are more aggressive (all in by 2000 or 2500)...I am thinking that this might help in my situation.

can anyone offer up some advice on my current advance curve for use with 396/4 speed/M22W/3.31 rear..running 14" BFG's. my seat of the pants dyno is starting to speak to me, but I am wondering if I am nuts.

as usual, any help is appreciated.

Feb 15th, 06, 12:14 AM
The setting you had a year ago ran well, but now something has degraded? I wouldn't start with changing the curve, I would try to figure out what changed. Return to the setting you had.

Start by ensuring you're initial timing hasn't shifted. You 22 mechanical and 36 total should result in 14 initial. Check this first. Also, make sure the rest of the tune up is good. Year old cap, rotor, wires, plugs are probably still good, but make sure they are perfect. If you had new springs installed when the distributor was curved last year, it'd be unlikely they would already have failed. Rather, look for sticking weights, carbon tracking or maybe even a module on it's way out (yeah, I know, they usually fail all at once) or a weak coil.

If you're set on the new MSD, then start by replicating the curve you had a year ago -- avoid the new distributor and a new timing curve at the same time so that you don't introduce too many new variables.

Feb 15th, 06, 12:37 AM
zeke..yeah it worked pretty good when i first got it all on the road, but the timing has been jumping around and acting a bit strange for about 6 months or so. i have had to reset the timing 3-4 times (runon/dieseling at shut off-found timing to be retarded), and I know it is sitting there too far advanced now(not by my doing), as the car has been hard to hot start the last few times I drove it. that happened out of the blue, all by itself.

the shop did advise me to use something else other than the HEI in question, I guess because it was obviously used..i think they did some grinding to gain more advance, and definitely lightened up the springs, I remember them saying they had a hard time finding parts on the fly. its hard for me to say how good it was originally running power/low end response-wise, as i babied it for a while after it was built..the more i drove it and got used to it, the more i started jumping on it and thinking that it seemed a bit soft down low.

plugs could probably at least use to be checked or replaced, I will say that. wires were brand new taylor spiro pros, entire hei was a mystery, took it off another car i had as it seemed to be ok. cap and rotor were new (replaced) when it was recurved though. coil was not replaced at that time. carb was new, and used/jetted on the dyno (but i think carb needs some fine tuning too..its fat). even though its been a year, car did not get driven much.. i doubt if i put 3000 miles on it over that time span.

if i get the e curve + blaster 2, it will be a while before it gets installed anyway, i will likely play around with what i have for right now..i agree that changing too many things at once is a bad thing!

Feb 15th, 06, 1:17 AM
Hmm...maybe worn bushings or worn advance plate, you've been though the rest of it --> time for a new distributor. Even so, if the shop set up your original curve and that worked, I'd stay close to that setting for starters. You can probably do that out of the box on the new dist considering you have the curve on the old one.

Feb 15th, 06, 1:48 AM
Just for the heck of it I would check the vacuum advance where it is screwed on I had one where the screws backed out and the timing was all over the place. My fault when I changed it I did not tighten it up I think.

Feb 15th, 06, 8:59 AM
right now it doesnt have vacuum advance hooked up, its all mechanical.

i think i might try running some vacuum advance on the new one, as it has a few curves to choose from, and the car has the stinky idle that a lot of people have noticed..I may need some carb fine tuning as well.

zeke your advice is well taken, and when i get to replace the distributor, i will try it out first with no vacuum advance and with a curve as close as i can get to the hei.

the thing i notice with the advance curves on the msd unit is that the "curve" is a straight line, with a steep slope up to max advance, whereas my current curve is more of a flatter, less aggressive curve, slightly less linear. I am hoping that this will lend itself to better responsiveness when the timing needs to come 'all in' like when i mash the gas to the floor from a slow speed roll..I hoping and expecting to see a change in performance in that area.

Feb 15th, 06, 3:16 PM
You say the timing has changed... but it might help to be more specific: are you talking about initial or total?

Total should not change and should never advance. It could retard if the mech adv sticks. It could also retard if the dist slips. If you rule out the latter through alignment marks, then you are left with the former. Establish this first.

Once you have total nailed then you can turn to initial. Your setup will exhibit some instability because you have advance coming in during idle conditions. Personally I would prefer a curve that did not start until 1000 RPM to avoid this. But that can be tough mechanically, so you get something like you have. On my motor I would be 5* up the curve at idle so effectively I would only have 17* total mech adv.

Be all that as it may, if your initial is varying more than a few degrees it would again suggest sticking issues with the mech adv.

You may want check into why this dist was suspect... for example had somebody previously replaced the mech advance with a hot rod curve kit? These are surprisingly (or not) poorly made and failure prone. This may be why a new dist was recommended.

Feb 15th, 06, 5:52 PM

I need to get out there and get my gun on it. When it has changed in the past, what I meant was that where I had set my timing at idle (factoring in that I am into the mech advance curve at idle speed)..it was different from where I had set it. When it gets too retarded, it tends to diesel. When it is too far advanced, its hard to start. So based on this, I would say my initial changed. when i have encountered this in the past, I simply re-set it to where i liked it at idle, locked it down and went with it..never got too far into checking the total every time, even though that would tell me more. on the couple times i did mess around with looking at total, it was not consistently done advancing at 2800rpm.

i really dont know what was in the hei before i had it hot rodded myself, your guess is as good as mine. you very well may be correct in your thinking that is why the shop just recommended a new distributor.

at any rate, i went ahead and ordered a new msd digital e curve, its on the way! hopefully it winds up helping me get the results i want..im sure a new coil isnt going to hurt either.

Feb 15th, 06, 7:11 PM
My suggestions were based on my own experience. I went to install a Spectre advance kit and found similiar junk parts already in there. These parts were much less robust than the advance in the new distributor I ended up buying. Of course the new dist does not have quite the right curve for my application, so still more work is required.

I advocate setting ignition timing based on total in all cases as it is much more reliable for what you get. It is less dependent on mechanical advance curves and maximum advance. It is also less dependent on insuring idle is low enough, so it can be performed on an engine that is not fully warmed up.

And one last point regarding vacuum advance. You mentioned you were running without one? This is not a good idea for all but drag racing applications. You want to see a maximum timing of around 50* with max mechanical advance plus vacuum advance for best cruise. You want the vacuum advance to put in full advance at 1-2" higher than idle vacuum. And you should hook it up to manifold vacuum. This will provide the extra advance you motor needs at idle eliminating the diesel problem. It also eliminates the advance during startup eliminating the hard to start problem.

Feb 15th, 06, 8:36 PM
well, the shop that did the distributor and motor work..the main man is an old school drag racer, so that probably explains the lack of vacuum advance. my car is far from a drag racer, although i like to blast through the gears like everyone else that owns a musclecar...so when i am driving, it sees a lot of stop and go, and also some cruising and punching around..seems like vacuum advance can do nothing but help my application, but i wasnt going to argue with the guy about it at the time..after driving the car around, i think i need to be doing some further road testing/tuning on the whole thing.

Feb 16th, 06, 5:46 AM
So...is there a vacuum advance screwed to the distributor housing, and hooked onto the pickup coil, just not connected to a vacuum hose? Or is there NO vacuum advance at all?

If you have nothing to secure the pickup coil except the wires going to the module, yeah, your timing is gonna be TOTALLY unpredictable.

Feb 16th, 06, 9:35 AM
yes, there is a vac. advance on the distributor..its a gm hei with the vac. advance can on the (passengers side) of the car. i have it capped off with one of those little black rubber caps. same with my carb and manifold..there is no vacuum signal going to the distributor.

the coil is in the top of the cap, secure as far as i know.

Mar 6th, 06, 6:46 AM
I'm dealing with similar issue now. Going to install the VC1862 vac advance can to get some vac advance at idle. Previously, I had lighter springs in the HEI, caused a very erratic idle due to timing jumping all around. For a motor that idles at about 850+ have to be careful with the advance curve. I think it either has to be very quick so the advance is well in at idle or delayed a bit. THe spring set up I have been most succesful with starts advance at 800RPM, full advance by 2600 (1-heavy / 1-light). It cam with 2 medium springs installed, but gave me a very erratic idle timing. Haven't tried going any lighter for a faster curve, although with a higher idle and a higher stall converter is may not be needed, unless you go to 2 light springs which will basically put it at close to full advance at start up, but I would be leary of detonation problems running it like that.