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post #1 of (permalink) Old 09-03-2006, 10:46 AM Thread Starter
 
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Timing....which is the better indicator?

4.2l with v-belts, howell conversion and pacesetter headers, tfi upgrade

i've set timing with a timing light at 8BTC per label and it runs good. and no, the dampner ring hasnt slipped....its a brand new one....had that problem before.

i've also set timing with a vacuum gauge (current setting) off of manifold vacuum.

the degree of split between light and gauge is ~6*. for some reason, this motor, and my previous motor always ran better mileage and performance-wise around 10-12*

what should i use to set timing, the light or the vacuum gauge and why? what is the difference and what is the cause. im not sure if its a bad thing or not having it run at 14BTC. the heep runs like a raped ape.

thanks guys
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post #2 of (permalink) Old 09-03-2006, 12:59 PM
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Re: Timing....which is the better indicator?

With that much advance it might run great, but the emissions are probably way out of tolerance.

A timing light is always the way to set the timing. When you set it by vacuum gauge you're optomizing it for idle only. It used to be that engine builders set the advance curve for best performance all the way through the range, so if it was set for best idle, it would be close for everything. But now with emission considerations there's no telling what the advance curve is doing.
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post #3 of (permalink) Old 09-04-2006, 08:31 AM
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Re: Timing....which is the better indicator?

More Information is always better!
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post #4 of (permalink) Old 09-04-2006, 11:13 AM Thread Starter
 
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Re: Timing....which is the better indicator?

leve, i read the article and noticed it prefers the vacuum source to the distributor to be manifold source.

lookin over my howell destructions, it has me tee into the cto vac line.

the cto vac source isnt manifold, and i made sure it had 0 vac with a gauge. the only time it draws is at higher rpms. im not up on tbi units to know which and why. i understand that the cto switch can be run off a demand vac source for things like cannister purge signals and egr, but why would you want 0 going to the distributor at idle?

anyways, i hooked distributor to manifold vac, and my total idle advance is around 34 before mechanical kicks in. the idle (21in mg) vacuum advances it another 24*. it doesnt have the accelerator lag like it used to, which is good. it responds best to throttle input with min preload on the diaphram (adj screw all the way in).

does this sound more correct than the way it was prior?

idle timing was 8btc with all lines connect as per howell. accelerator lag was there with and without the distributor vac line connected. didnt take a mechanical advance measurement, so i cant tell you what it is.

just tryin to get the most out of the 6 squirrels that i can, thanks guys.
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post #5 of (permalink) Old 09-04-2006, 12:22 PM
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Re: Timing....which is the better indicator?

The curve for manifold vacuum is strongest at idle, less as you increase load or R's.
Ported is just the opposite - 0 at idle, stronger the faster you go.
Measure them yourself! See for yourself!!!!!!!!!!!!!


The engine needs more timing advance the faster it goes.

Vacuum applied to the diaghram advances the timing.

Using the above - which makes more sense?

Todays gas still has not reversed the laws of physics.

Try it.

Usually ported is not near as strong as mainifold, use a timing light to make sure you are comparing apples with apples as far as the amount of advance, and when.
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post #6 of (permalink) Old 09-04-2006, 12:38 PM Thread Starter
 
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Re: Timing....which is the better indicator?

please dont take this as an opposing point of view, i dont kow how to not make appear that way.

i understand manifold will bleed off as rpms increase. i also understand that ported will not be as strong, but will also be delayed.

when connected to maifold, as manifold decreases with rpms, wouldnt centrifugal take up the slack? i know not as much, but will it not compensate some for the lack of manifold?

i dont totally understand the complete physics of the distributor, only because i havent committed myself to learning it. i understand most of it, and the portions of it, but i havent put all the parts together to see the biggerest picture.

thx for all your information so far, its not going to waste.
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post #7 of (permalink) Old 09-04-2006, 12:50 PM
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Re: Timing....which is the better indicator?

Since you've got a Howell system, have you considered using the GM ECM to control the distributor? That would eliminate the vacuum advance completely.
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post #8 of (permalink) Old 09-04-2006, 11:44 PM
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Re: Timing....which is the better indicator?

Actually there are 2 advance curves to consider. One is the Power curve. The faster the engine turns, the more advance needed. There is the "ideal" curve - easy to measure with a dyno. You simply experiment at RPM increments to find the most power output at each RPM.
Then you curve the "mechanical" part of the distributor to follow that curve - the springs and flyweights.
In reality, you want maximun cylinder pressure at about 37 - 39 degrees AFTER TDC for max power. Remember it takes awhile to get max pressure after you light it. To get that, the faster it turns, the earlier you have to light it.

But - max power is not necessarily max efficiency as far as mileage goes. You can get a bit more MILEAGE out of it if you light it slightly earlier. Thus the vacuum control - It ADDS to the mechanical curve. Usually lighting it sooner at speed by about 12-15 degrees really makes a difference. It's much harder to measure that on a dyno, so we usually shoot fr about 12 degrees.

At cruise - light or partial throttle ported vacuum is at it's greatest. Manifold is somwehat below it's peak, but not all that far below. Using ported will give full vacuum advance, manifold may not - depending.

As you increase throttle opening - obvioulsy you are wanting more power to accelerate - ported vacuum starts to drop, as the throttle blade is farther away from the port hole, venturi effect drops. Manifold does the same, it lowers a tad under load. Good! Either under that condition is fine too. The idea is get it closer to the power curve. Not dropping enough results in detonation - ping - death to an engine. Some ping is inaudable. If you hear it, believe it's doing damage!

Now lets give it a hard punch - ported goes away completely, manifold does too - OK, acceptable. But as the R's come up, manifold returns faster than ported - possible ping and damage. Who cares about mileage, you need to get out of the way of that truck! You want it on the power curve!

Now close the throttle - ported goes to 0, manifold goes sky high! Hmm, that often causes backfire. You want the advance to drop, not go up during decel!

Now, at idle, manifold is at max, so advance is too. At idle you are around 15 plus initial, say 8 - that equals 23 - too high to idle smoothly, causes overheating, etc.

As you accelerate, ported offers a smooth slowly increasing advance, where manifold stays fully advanced. Ping again. Way too much advance, and no curve if it's already at max. At max you are waaaaay above the preferred power curve.

Much above 1/2 throttle ported drops it back to the power curve, manifold does not.

The Howell - several guys I've worked with that had them switched from manifold to ported and eliminated their ping problem.

Use a timing light - look for this:
Idle - 8-10 initial
Vacuum disconnected, mechanical starts advancing about 1200 RPM.
2500 RPM mechanical is all the way in - about a 15 degree change above initial, but it came in SMOOTHLY AND EVENLY as the R's increased. Not jumped in!

Vacuum connected - idle - no change from initial.
vacuum starts advncing about 1200 RPM.
Remember, there is no way to disable the mechanical, so what you now see is CUMULATIVE - both vac and mechanical.
The advance should slowly go up, evenly, no jumping, until you reach about 2500-3000 RPM, where it no longer advances. The total when it stops going up should read about 35 degrees.
You don't want much more than 35 or you'll get excessive engine wear.

Remember - the TOTAL is the sum of the initial, plus mechanical, plus the vacuum.
8 Initial
15 mechanical
12 vacuum
35 total.


Close to that should give great performance without ping.

Remember, max power is curved by the mechanical, the vacuum is only to increase mileage SAFELY.

If you do use the EST - electronically controlled timing feature - and it's a very good way to go - be sure you use the knock sensor - it's an integral part of the EST.

If your distributor is vacuum advancing wrong, too fast, to little etc, then you'll need an adjustable diaphram. If it's advancig too much, make a little bump bracket inside the distributor to limit it.

The problem we encounter often - the distributor vacuum curve was designed for something else, a carb etc. The vacuum source's curve is different.
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post #9 of (permalink) Old 09-05-2006, 03:34 PM
 
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Re: Timing....which is the better indicator?

[ QUOTE ]
leve, i read the article and noticed it prefers the vacuum source to the distributor to be manifold source.

lookin over my howell destructions, it has me tee into the cto vac line.

the cto vac source isnt manifold, and i made sure it had 0 vac with a gauge. the only time it draws is at higher rpms. im not up on tbi units to know which and why. i understand that the cto switch can be run off a demand vac source for things like cannister purge signals and egr, but why would you want 0 going to the distributor at idle?

anyways, i hooked distributor to manifold vac, and my total idle advance is around 34 before mechanical kicks in. the idle (21in mg) vacuum advances it another 24*. it doesnt have the accelerator lag like it used to, which is good. it responds best to throttle input with min preload on the diaphram (adj screw all the way in).

does this sound more correct than the way it was prior?

idle timing was 8btc with all lines connect as per howell. accelerator lag was there with and without the distributor vac line connected. didnt take a mechanical advance measurement, so i cant tell you what it is.

just tryin to get the most out of the 6 squirrels that i can, thanks guys.

[/ QUOTE ]

I'd follow RRich's advice here. Even with the TBI, i don't think it should be adjusted with the vacuum line connected.
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post #10 of (permalink) Old 09-05-2006, 04:10 PM
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Re: Timing....which is the better indicator?

Think about it -

IF you set initial timing to 8 with manifold vacuum connected, then when you try to accelerate - manifold drops to 0 vacuum, timing retards -15 from initial, giving you about -7 RETARDED.

Stumble would be expected, as the advance is going the WRONG WAY!

If you set initial to 8 without the manifold vacuum connected, then once you connect it, it jumps to 23 - makes it very difficult to accelerate since the combustion pressure is trying to tur the engine backwards! Rings break, pistons melt - all kinds of neat things.
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