Distributor timing

[81GMCSierra1500]

Hello again, all,

I just looked at my timing, and I'm running about 9 degrees BTDC, by the chart on the radiator support, it recommends 4 degrees BTDC, but what is the most economical setting for power and efficiency? I understand the theory on adjusting it, but where is the most ideal setting for timing on a 305 4BBL? Keep in mind that as of now, the engine has absolutely ZERO mods, except having the cats and mufflers removed, and glasspacks were put on... I don't have a big ol' beast of a motor :P

Cheers,

'81

[rich weyand]

Two degrees short of detonation under the worst circumstance, such as low gear from a stop up a steep hill.

http://67-72chevytrucks.com/vboard/s...d.php?t=646219

[81GMCSierra1500]

Originally Posted by rich weyand

Two degrees short of detonation under the worst circumstance, such as low gear from a stop up a steep hill.

http://67-72chevytrucks.com/vboard/s...d.php?t=646219

How do you mean "worst Circumstance?" I'm just looking for power and efficiency whilst not causing damage...

[rich weyand]

Your best power and efficiency occurs at the most advance the engine will take without detonation (knocking). The "worst circumstance" for knocking (the point where the engine will knock first, where it may not knock anywhere else) is when the engine is at low rpm (being "lugged") against a heavy load, such as starting from stop up a steep hill.

With a stock 305, and normal low compression heads, that shouldn't occur until 18-22 degrees BTDC. Also, the standard HEI has 20 degrees of mechanical advance, and 36 degrees of total advance (base timing + mechanical advance) is a sweet spot for Chevy V8s. So I recomend 16* BTDC as a starting point. If you get knocking in the worst circumstance, back off 2 degrees, and keep backing off 2 degrees at a time until you get no knocking at any circumstance. If you get no knocking at 16* BTDC, leave it there. With a stock engine and heads, you should not get knocking at 16* BTDC.

Or you could just read the link I gave you, and you would already know all of that.

[donut]

I agree with everything Rich posted, except the starting point. Personally I like 10-12*.
Reason, my last smallblock loved 16*, except when it was warmed up. 16* would "backlash" against the starter. Not readily apparent until I was 5 miles up the road and shut the truck off.

Finding the sweet spot is really finding out what your truck likes. 10* may be the sweet spot on this one, another smallblock in the driveway may tolerate 20*...

[rich weyand]

Starter kickback means you aren't turning over fast enough on the starter. New battery cables, clean connections, and a gear reduction starter will get rid of that.

[donut]

Had new cables, 900 cca battery. Didn't go with the gear reduction, though it was a higher torque starter off a BBC. To me, the cost of the gear reduction wasn't worth the cost. Not for 2*. If I was too worried about it, the power to the HEI was on a toggle. Get the motor spinning then power the ignition.

[Ziegelsteinfaust]

I did 12-16 tends to be where the sweet spot lies. I never had a stock 305 so I really only know 350's. With a stock starter, and a fresh motor 16* can be rough. 14* usually is the most. Toss in a HD starter like I have on my K20 I can run it up to 20* initial.

[greg64]

Hey donut, is your vacuum advance connected to full time manifold vacuum or to ported vacuum on the carb?

[donut]

Full time, ran better than when I tried ported. This was on my '73 which I no longer have.
My 454 is on 16* currently, more to be had I think. Curve in it seems lazy. One of these days...

[motornut]

Originally Posted by 81GMCSierra1500

Hello again, all,

I just looked at my timing, and I'm running about 9 degrees BTDC, by the chart on the radiator support, it recommends 4 degrees BTDC, but what is the most economical setting for power and efficiency? I understand the theory on adjusting it, but where is the most ideal setting for timing on a 305 4BBL? Keep in mind that as of now, the engine has absolutely ZERO mods, except having the cats and mufflers removed, and glasspacks were put on... I don't have a big ol' beast of a motor :P

Cheers,

'81

My 79 305 is at 12,
I can't wait to loose the stock 2 barrel.
with short 2.25 pipes to flow-masters 40s.
how is the rest of your timing parts?

[greg64]

Donut, I agree with you that full manifold vacuum gives better idle. However, ported vacuum will make starting easier, because the timing isn't as advanced while cranking.

[K5owner]

Mine from the factory uses ported vac advance. Here is a pic of the engine decal.


BTW this is a Federal Emissions 1986 K10 4x4 that came with a 305 (now a GM 260 HP crate) I am going to try manifold vac tomorrow and see how it runs. Mine as well says 4 degrees BTDC. I currently have it set at 16 degrees BTDC and it runs well. 4 degrees runs like crap.

[rich weyand]

Originally Posted by greg64

Donut, I agree with you that full manifold vacuum gives better idle. However, ported vacuum will make starting easier, because the timing isn't as advanced while cranking.

That's incorrect. The vacuum during cranking isn't anywhere near enough to advance the timing. The engine is not yet running, after all.

[rich weyand]

Originally Posted by K5owner

Mine from the factory uses ported vac advance. Here is a pic of the engine decal.


BTW this is a Federal Emissions 1986 K10 4x4 that came with a 305 (now a GM 260 HP crate) I am going to try manifold vac tomorrow and see how it runs. Mine as well says 4 degrees BTDC. I currently have it set at 16 degrees BTDC and it runs well. 4 degrees runs like crap.

Correct. Vacuum advance connected to ported vacuum is meant to accommodate emissions equipment, and was introduced in 1968. Engines without emissions equipment (such as AIR pumps) are run on manifold vacuum, and have been since 1930.

[K5owner]

Rich do you think driveability would benefit if I switched it from ported to manifold vacuum for my vacuum advance? I would have to switch it back for emissions testing like I do with my timing but not a big deal.

[rich weyand]

Yes. The primary benefits are a smoother idle and snappier off-idle response. The engine will also idle cooler.

The idle speed will need to be readjusted when you change it, because at the same idle throttle setting it will run at higher rpm. So you will need to dial it back to the correct idle rpm.

[geezer#99]

Just make sure you listen for detonation at idle with it hooked to manifold vac with 16 initial.
It'll sound a bit like a deisel.

[rich weyand]

Due to the high manifold vacuum, cylinder pressures will be too low to detonate at idle. If base timing is too advanced, detonation would occur when lugged at low rpm, such as getting on it going up a steep hill.

[greg64]

Originally Posted by rich weyand

That's incorrect. The vacuum during cranking isn't anywhere near enough to advance the timing. The engine is not yet running, after all.

Rich, you're right, of course. I think I must have been thinking when the engine is actually running when I posted that. As far as I've been able to tell, ported vacuum only exists for emissions reasons. I'm thinking that at idle, the retarded timing lessens the NOx emissions.

[rich weyand]

At idle, with the intake manifold vacuum high, there is only about half a bar in the manifold, so an 8:1 compression ratio gives you at most 4 bar cylinder pressure at TDC. This low-pressure mixture burns much slower than the 8-bar mixture one gets at zero manifold vacuum, so it must be lighted earlier for the burning pressure wavefront to get to the piston face at the right time. Hence, vacuum advance.

To get rid of unburned hydrocarbons in the exhaust, the AIR pump pumps air into the exhaust manifold. Ported vacuum lights the mixture at idle late, so the mixture is still burning when it exits the cylinder into the exhaust manifold. The burning mixture plus the injected air continues the burning process inside the exhaust manifold, reducing unburned hydrocarbons. But the burning wavefront is too late to develop best power, so when you open the throttle, the vacuum is re-established to the vacuum advance and the timing is set back to optimum.

Basically, it's like, Well, since you're not using the engine for power at the moment, we're going to reconfigure it for lower emissions. This is important in place like LA and NYC, where lots of cars sit idling while stuck in traffic.

The issues one gets from ported vacuum are: rougher idle due to the pressure wave getting to the piston face late; a hesitation in off-idle response until the vacuum advance can re-establish proper timing; and higher engine heating during idle due to the mixture still burning as it passes out through the exhaust runners in the heads.

[greg64]

Great explanation, Rich, thanks! There's been so much misinformation over the years about vacuum advance.

Want a brain buster? If we advance the timing centrifugally with RPM, which is to compensate for the fixed burn time, but non-fixed rotation speed (that's a given) then why does mechanical advance top out at a specific RPM? Shouldn't we start out at idle with no mech advance, and continue to increase the advance as RPM rises... It seems to me like the physical property we're compensating for doesn't magically run out at 3000 RPM.

[donut]

They don't.
In the case of my '73, changing springs on the distributor weights could control when it would start to advance and how quick the advance was. You can fabricate a stop to ensure the weights don't keep advancing. Or, in my case, just not wind the engine up.
It was an old 4x4 so it wasn't built for acceleration.

[rich weyand]

The burn rate changes as the engine winds up, because the faster the engine goes, the more air it moves, and the induction drag causes the vacuum to increase. So the required advance (in msecs) actually flattens out above 3000 rpm.