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Old 03-26-2012, 07:10 AM
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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?

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Old 03-26-2012, 11:35 AM
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Chevy small block? you have vacuum advance hooked up?
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Old 03-26-2012, 01:51 PM
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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.
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Old 03-26-2012, 02:49 PM
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Quote:
Originally Posted by v8hed
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.
If he took it up to 6000rpm and was not doing a WOT pass its possible the throttle plates weren't open enough to drop off all the vacuum in the manifold and the engine is still making vacuum and adding advance..... oops just read you said it wasn't hooked up my bad
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Old 03-27-2012, 05:03 PM
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What was it at when you went back to 3500-4000rpm? 34 again?
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Old 03-27-2012, 09:35 PM
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Was the timing measured w/an "analog" light and a timing tape or w/a dial back light?
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Old 03-28-2012, 02:41 PM
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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.
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Old 03-28-2012, 02:52 PM
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Quote:
Originally Posted by raceman14
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.
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Old 03-28-2012, 03:05 PM
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Quote:
Originally Posted by v8hed
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.
a bad module CANNOT advance timing, though it can slightly retard it.

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 34 at idle, at redline, and at every point in between.
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Old 03-28-2012, 03:48 PM
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Quote:
Originally Posted by ap72
a bad module CANNOT advance timing, though it can slightly retard it.

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 34 at idle, at redline, and at every point in between.
The timing wasn't locked-out when the car was on the chassis dyno. On the chassis dyno, the timing was set-up for around 14* initial + recurved (medium springs) mechanical advance, supposedly all-in by 3,000rpm. Vac advance was disconnected for all pulls. Prior to going on the chassis dyno, I had checked the timing with a xenon timing strobe and verified 34* total @ 3,000rpm and took it to around 3,500-4,000rpm to make sure it was all-in... the timing didn't advance any further, so I didn't take it any higher. The oddity is, on the chassis dyno, the operator of the timing light was saying it hit 50*+ at the top end of the RPM range, which I just don't understand. I'm starting to think that maybe he set-up his fancy timing light up wrong, or was just somehow mis-reading it. Having said that, the guy tried a different light and got the same result.

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?
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Old 03-28-2012, 04:26 PM
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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.
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Old 03-28-2012, 05:45 PM
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I'm looking forward to getting my exhausts back from the fabricator... proper merge collectors and a properly-sized straight through baffle should help with scavenging. I have a feeling the previous exhaust with its 2" baffle that had little half-moon shaped plates welded at regular intervals throughout its length and caused the exhaust gas to follow a snaking motion down each pipe may have been throwing the fuelling off and reducing burn time. The previous collectors weren't proper merge collectors, either. I'm pinning a lot on this new exhaust system I've designed (and it's costing a fair amount in stainless stock), so I hope it proves to be the magic bullet I'm hoping it will be.
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Old 03-28-2012, 06:01 PM
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I'll bet it's in the module! Try a stock GM module.

In the EFI tuning it has been found that differant modules produce differant timing readings even though timing table has not been changed. It is called latency timing if your into tuning EFI and can be as much as 11 degrees off just between 2 differant number GM EST/ICM modules that are both for same car. Aftermarket modules were not tested.

http://www.thirdgen.org/techboard/di...ncy-table.html
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Old 03-29-2012, 07:01 AM
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Quote:
Originally Posted by F-BIRD'88
From your description of what you had to work with there I think your new exhaust system is going to show significant gains.

Did you get to try it with open exhaust with just a extension pipe (collector extension) on the headers?
No, I was initially planning on doing that, but spoke to a couple of people who confirmed these exhausts are very restrictive. The design of the original system speaks for itself and it's easy to see how restrictive it is. So, I'm confident the new system should be a big leap forwards. It's the only item in the drivetrain that's not easily measurable in terms of losses and it's the only thing that can account for the large discrepency between the figures the motor achieved on the engine dyno and what it achieved on the chassis dyno. What was interesting to me was the shape of the curves from the chassis dyno... the chassis used to have a totally different motor installed (but same drivetrain and exhaust), yet the shape of the curves are remarkably similar between the old motor and the new motor... dips and peaks in the same areas, etc. The old motor was also a 383, but had a Performer RPM FT cam, RHS 230cc iron heads, a Performer RPM intake and an Edelbrock carb (quite a mismatch... I didn't build it)... nothing like the new motor that's in there now, yet the shape of the curves are almost identical. If you weren't paying attention to the numbers, you'd almost think you were looking at the same motor just from how the curves look. This is another huge indicator to me that the exhaust is/was a playing a huge part in things.
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Old 03-29-2012, 09:17 AM
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Quote:
Originally Posted by F-BIRD'88
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.
Manifold vac,,,just as a matter of debate for mild to mid duration cammed motors.

Every vacuum advance can I have ever tested and/or adjusted if that style of unit pegs out at 12 or 13 "hg vacuum. Most are all in by 11" hg. So if we use manifold vacuum on a lighter cammed engine making over 13" hg one will have it all in no matter how much one trys to adjust the setting. Limiting the total amount of vacuum advance is manditory in either case. What does this 383 pull for idle vacuum?

I disagree with not having it all in at idle in the first place, not only is it nearly impossible for most mild-mid builds due to adjustment restrictions I mentioned but in the case of high duration cam engines the vacuum level fluctuates too much at idle. The last thing you want is the vacuum advance canister actuator rod moving around when you try and tune the thing as it adds and subtracts advance at idle. Try it out, its no fun and when you drop it in gear things get worse. Best to have it set so that it is just all in with the tranny in gear. Under light load cruise the engine vacuum will be less and the vacuum advance will end up in the middle of the range which is what you want.

Basically engines making decent vacuum levels do not benefit from adjustable vacuum advance canisters because the can does not allow adjustment above the engine vacuum the motor is making anyhow.

If we tell someone not to use manifold vacuum and then say dont have it all in at idle it becomes important to know the engine vacuum levels at idle. Chances are that most engines talked about here for the average Joe build wont have a choice in the matter since there motors pull good vacuum and as soon as they hook up to manifold vac it is all in no matter what.
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