Ignition timing oddity on chassis dyno
383 with GM HEI distributor (modded with aftermarket module, curve kit, coil and shimmed for end play). Before going to the chassis dyno, I'd set my timing up (with a good timing light) for 34* total. Checked it up to about 4,000rpm and I could see the advance curve adding more timing until around 3,000rpm then it remained static. As I said, I only went up to about 3,500-4,000rpm with the timing light... just enough to see that the advance curve had finished adding timing. On the chassis dyno, we were messing around with the timing (just adding/subtracting a couple of degrees to do some fine tuning). I asked the dyno operator to take it up to 6,000rpm and keep the timing light on the balancer. He said total timing went up to something like 50* by 5,500rpm! We checked with another light and same thing. Couldn't hear any pinging (although the car is v.loud). What I don't understand is how could this possibly be happening? I still don't believe everything is at it seems, since taking timing out resulted in less power. If is was really running up around 50* total timing, there's no way the motor would be making power. I've previously tried experimenting locking the mech advance out and didn't see over 36* (with about 14* initial). Since the holes in the base plate physically limit max advance, what's going on with this wacky timing reading? It had occurred to me that perhaps the ProStreet balancer I'm using could be slipping, but it's pretty new and looks perfectly fine to the naked eye.
Anyone got any other ideas or explanations? If not, my next step will be to try a brand new MSD HEI just to eliminate the distributor from the equation. What about spark scatter or some other kind of effect?
Chevy small block? you have vacuum advance hooked up?
When the mechanical advance is locked out the timing does not move at all.
How could you have locked out the timing and have 14deg initial and 36deg max.?
Thats not locked out .
Get someone who has a clue to set up the distributor curves before you damage this motor.
Its very easy to lock out the mechanical advance curve on a HEI.
Once its locked out set the fixed timing at 34 to 36deg BTDC at idle.
It does not advance with rpm, when "locked out"
Its also very easy to limit the travel of the mechanical advance system
for hi perf cams that want a lot of initial timing at idle.
Its possible you have the vacuum advance system set up to peg at a low manifold vacuum reading. This causes added advance when ever manifold vacuum is anything higher than atmospheric. This is not set up right.
Vaccuum advance is for cruising at part throttle. You likely have it set up so its all pegged in at idle. (vacuum advance diaphragm spring tension way too low)
I posted that in a hurry just as I was leaving work. The 14* part was a typo. But, I recall setting a low initial, THEN locking the timing out and the lock out added a sensible amount. I'll run this test again to be sure. My point is, the total mechanical advance wasn't in the 50* range.
Just checked my balancer with a known good unit and it hasn't slipped.
Re. Vac advance, it's a stock can and was disconnected for the chassis dyno anyway.
What was it at when you went back to 3500-4000rpm? 34 again?
Was the timing measured w/an "analog" light and a timing tape or w/a dial back light?
WOW T-Bird that pic is awesome clean and clear...
V8 in UK,
I doubt the timing light was bad because they don't usually advance. You could have a faulty module that is over advancing, if your dizzy is locked your springs are not losing control.
I guess I would change the module out, if that does not fix it get another dizzy and try it.
50* of timing will kill just about any SBC making decent power as it will burn pistons and kill ring lands.
Can a bad module affect timing? I'm still not sure if this supposed 50* total timing was actually there or not, since retarding the timing resulted in less power. Also, there was no audible spark knock with the hood up and standing right by the motor during the several pulls we did. Lastly, the motor still runs fine as far as I can tell, so I don't think anything was damaged as a result. Having said that, the motor does have forged pistons.
I'm wondering if you either have a bad advance mechanism, or just don't know how to set timing. you said "locked out" and locked out is locked out, that means 34º at idle, at redline, and at every point in between.
When I was talking about locking the timing out, I was referring to a time well before I went to the chassis dyno when I was experimenting with different set-ups.
So, as long as the initial is correct, how can a mechanical advance mech add timing beyond its design limits? The amount of mechanical advance is limited by the length of the slots in the base plate and this is all stock GM HEI. I believe stock HEI distributors add around 20* mechanical. To get into the 50* range, the distributor would need to be adding at least 36* to the 14* initial and I just don't see how this is even possible.
Finally, with 50*+ total timing on a WOT pull with the hood up and standing right by the motor, surely it would be rattling away and it would be very obvious. When I've had the timing too far advanced on the past, I can even hear rattling from the driver's seat when driving the car out on the road.
Unless anyone had any 'magic bullet' explanation, I can only assume the operator somehow misread the timing?
On a stock HEI distributor the meacnical advance is limited by the advance weights topping out. The problem with this is that the timing can and will creap up at higher rpms instead of truely toping out at around 3000 rpm.
This is why I recomend what I recomend. Physically limit the travel as shown in my pics or use a ( custom) 10° advance limit bushing on the pin in the slot.
Most stock HEI`s have no bushing on the pin and depend on the weights contacting the center piece under the rotor to stop the advance.
You want and need more than 14deg inital at idle. Install the screw to limit the travel so the initial can be around 24+/- deg at idle and the max advance is 34-36deg and does not creep up at hi rpm.
This is the easy way to get the desire curve and stop limit.
Use two medium tension springs or 1 light and 1 medium
Light springs result in inconsistant mechanical advance operation.
22-26deg base at idle +10 to 12 deg mechanical advance =`s 34 to 36deg at max advance. (approx 3000 rpm)
Fix the distributor. Its not rocket science..and 14 inital is not enough. Its like nobody reads my many many posts on this thou.
(if you were running a automatic trans with high stall converter you could simply just lock out the mechanical advance) but you`re not.
Fix the vacuum advance system too.
I already tried using more initial as well as limiting the mech advance and locking it out. I couldn't tell any real difference in how the car drove, so I reverted to the set-up I have now (with full manifold vacuum advance hooked-up for regular driving and crisp off-idle, part-throttle driving).
I just this second looked at the graphs and annotations from the chassis dyno and I must apologise as I got the figure wrong... it's not as bad as my memory suggested... total timing was measured on their timing light as 34* @ 3,000rpm and 43* by 5,800rpm. We tried retarding it to be 34* @ 5,800rpm and lost power (only a handful of HP, but still a loss). Whether this was due to my restrictive exhaust causing all sorts of issues, I won't know until the new system is back from the fabricators.
So, you're saying with a stock HEI (even one that's been recurved with lighter springs), the timing will never truly be all-in by 2,500-3,000rpm? This creep will always happen?
Yes, I tried using the lightest springs and found it was adding advance even at idle, so immediately went back to the medium weight springs, which made the timing APPEAR to be all-in by 3,000rpm.
Yes fix the distributor exactly as I posted, so many many times.
The way you got it now is not right at all.
Fix the mechanical and and vacuum advance systems.
24deg base at idle 34-36 at max mechanical
10 to 12deg vacuum advance at part throttle cruise hiway speeds and rpms
not all in at idle.
having the vacuum advance pegged all in at idle is not correct.
use ported vacuum.
if you use full manifold vacuum as a signal source, you should adjust the vacuum advance so little or no vacuum advance is actually added at idle.
and the vacuum advance does come in at hiway cruise speeds at part throttle light engine load Make sure it does not ever exceed 12-15deg at highest manifold vacuum (deceleration) by ensureing the vacuum advance travel is physically limited to 12-15deg.
if it pegs the vac advances at idle its not right, reguardless of what you have read.
Find the best rate of vacuum advance by adjusting the vac adv diaphram tension.
a tach and vacuum gauge helps get it dialed in right.
The way you got it set up, without specific limits on the advance travel of BOTH the mechanical and vacuum advance systems can and will result in possible-probable engine damage from excessive spark advance on WOT shifts and down shifts from high speed.
consider yourself for warned.
yup restrictive exhaust flow and incorrect carb jetting will make the car seem to run best with more than usual WOT timing. recycled residual Exhaust gas will not burn twice. over lean and over rich AFRs burn slower.
if it ain`t in the 34-36-38deg timing area at WOT something is off.
43 deg is too much at WOT on street premium unleaded pump gas
(some race only fuels have a slow burn speed for very high compression high rpm race engines)
If it makes x amount of ponies at 34-36deg and only makes x +2 horsepower at 38deg,,, use the lesser safer timing.
real world conditions on the street are never exactly the same all the time.
excessive spark advance costs money.
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