Total Timing - 32 versus 35 degrees

[jtinpdx]

Hi All,
I just had a new crate motor put in my 71 C10. It's the 350 HO turnkey motor. I'm working out the kinks. The timing/idle/choke need fine tuning.

On this site and on timing videos I see online, most people say that 35 degrees total timing at 3000 or 3500 rpm is ideal for the engine. But the paperwork from GM says 32. Why the difference? Which to go with?

Thanks,
Jeff

[B. W.]

The newer GM small blocks with vortec heads like less total timing. Use the data that came with the engine.

[Gromit]

GM Performance recommends just setting at 32 max at 3000 RPM for their 290 HP version with the old style heads.

But that is with an HEI and with vacuum advance disconnected.

This no vacuum arrangement in their documentation really seems suboptimal to me and it is shocking that my naïve analysis could possibly be anywhere near that of the modern GM engineers.

Except perhaps they may be trying to make things easier for themselves and reduce outflows on their warranties rather than earnestly trying to make your engine run at its best.

Really GM? Ignore the vacuum advance altogether? And they call themselves GM "Performance"?

Sorry for the rant; please post your GM part number for the engine package maybe I or someone can say something at least honestly trying to be more accurate for your situation.

[jtinpdx]

Part number is 19210009. Here's a link to the engine.

http://www.chevrolet.com/performance...ck-350-ho.html

[Gromit]

Looked at GM's pdf installation guide for that motor and they show absolutely no mention of the installed distributor so I apologize but I have no way of knowing what they could have been thinking in terms of the timing.

Nice motor though - wowsers!

I guess I would just have to ask with whatever ignition they have installed - is as follows:

1.) How many crankshaft degrees centrifugal advance is designed in and at what RPM does it start to come in and then top out or stop advancing the timing?

and

2.) How much vacuum advance is designed in and at what vacuum does it start and then top out or come all in or stop advancing the timing?

[67 chevelle]

Originally Posted by B. W.

The newer GM small blocks with vortec heads like less total timing. Use the date that came with the engine.

This is correct , vortec heads are so efficient they create higher cylinder pressures , , and run best with a max 32 degrees total timing .

The older style heads , and aluminum are different and "MAY" run better with more timing .

Timing and cam selection are 2 things many get wrong

[demian5]

1. Advance timing till it pings.
2. Back it off 2 at a time till no more pinging.

[j_cst_10]

I would go with manufacturers recommendations if they give you some, but the only real way to time something and know if you did something positive or not is on a chassis dyno, or run it down the strip and go for max trap speed.

I built a pretty hot 383 with dart iron heads, ran it for 2 years with the basic 36 degrees of total timing, carb tuned to 12.4:1 at WOT with an oxygen sensor.

Then I strapped it to a chassis dyno, added 2 degrees of timing and that resulted in a 20 RWHP gain! Went 2 more and it did nothing so I put it back to 38. Then I removed 2 jet sizes out of the secondary side and gained another 20RWHP!

Completely different engine with 2 degrees of timing added and a leaner A/F. Timing is everything as they say.

Also, not sure on the distributor but you want to run vacuum advance on a street engine, it will always run better.

[jtinpdx]

Guys,
Here's what GM says about timing the engine. And there's a bit of info in there that could be part of the issue. The engine was built with the vacuum advance line between the distributor and carb. GM says to NOT do this. See below:

Set initial spark timing at 10º before top dead center (BTDC) at 650 rpm with the vacuum advance line to the distributor disconnected and plugged. This setting will produce 32º of total advance at wide open throttle (WOT). The HEI vacuum advance canister should remain disconnected. This engine is designed to operate using only the internal centrifugal advance to achieve the correct timing curve. Rotate the distributor counterclockwise to advance the timing. Rotate the distributor clockwise to retard the timing.

When possible, you should always allow the engine to warm up prior to driving. It is a good practice to allow the oil sump and water temperature to reach 180°F before towing heavy loads or performing hard acceleration runs.

Once the engine is warm, set the total advance timing to 32° at 4000 RPM.


So..... Should I follow this to the letter? 32 degrees total timing. And plug the vacuum lines?

-Jeff

[demian5]

Originally Posted by demian5

1. Advance timing till it pings.
2. Back it off 2 at a time till no more pinging.

Total advance is the max timing just before it detonates. If you can pinpoint that number with your particular engine, you will know what it's limits are. Then you can back it off to wherever it runs best and has a happy point. It will tell you when it doesn't like something.

Often we chase a carb or timing problem when we really only have a vacuum leak or find out we have a bad booster diaphragm or leaky master sucking brake fluid into the intake.

Every engine is different.

My timing may not work with your timing.

You may have a thermostatic air cleaner, that you don't know is stuck open, sucking the hot air from the exhaust manifold into the intake, raising combustion temps, causing you to retard the timing trying to get the ping out, taking it to 0 or retarded... thinking your engine is messed up or a slipped timing chain, or a bad center support bearing (forum diagnosis).

Set total advance (to the engine when as hot as it will get) using a timing light and running the RPM up until you no longer see advancement on the light and the timing marks, then adjust up or down depending on pinging - trial and error. If you time it cold, you won't know it was a bad deal till you are sitting in traffic on a very hot day and it sounds like someone is rattling marbles in a can as soon as you think about applying any pressure to the pedal.

Some people time to a vacuum gauge. Move distributor till the vacuum is at its highest, then drive and adjust from there.

Either way, its a starting point to see what kind of timing your engine likes.

Make sure your weights move freely and clean and lube anything that moves. Change the springs on the advance weights to get it there faster (total advance is built in to the distributor, unless you start messing with the shaft and installing other parts that came with the springs in the curve kit).

If you have to pass smog and require a XX +/- X spec with the XX unplugged or jumpered, then time it to that and leave it alone (or don't forget you changed it when you go back for the next smog, so you can put it back).

Its not rocket science.

[geezer#99]

Originally Posted by B. W.

The newer GM small blocks with vortec heads like less total timing. Use the data that came with the engine.

This is your best advice right here.
If you have a valid warranty and want to preserve it then set it according to the install instructions.

[Mike C]

Vacuum advance can be hooked up two different ways. One is under no or low load conditions it adds advance for cruising economy (manifold vac) or it can be used to add to total advantage (ported vac) which is ow most HEI setups came because they didn't have enough mechanical advance built in.

They each have their place but need to be used correctly. My preference in a driver is dial in your mechanical advance/initial total. Then hook vac advance to manifold and fix idle speed. This gives you complete control over total advance and WOT but allows for good street cruise manners.

Factory is just a recommendation that would work well enough for most motors. What you really need for your setup is a trial and error, also known as "tuning" exercise to find what works best.

[jtinpdx]

Guys, rookie question here. When timing the engine you go off the #1 cylinder right? Advance is based off top dead center of cylinder 1. On the old engine that meant that you hook the timing light up to the plug wire furthest forward on the driver's side. But with the engine's firing order, it looks like I should using the second furthest forward on the passenger side. Is that correct?

Thanks,
Jeff

[67 chevelle]

The picture you posted is for the torque sequence on the intake bolts , the furthest forward cylinder is number 1

Vortecs only have 8 bolts compared to the older style sbc heads and manifolds that had 12 , thats where you are getting confused

[geezer#99]

THose numbers in the circles are the torque sequence for the bolts that hold down the intake.
Forget them and go with #1 on the drivers side front. The cylinders are numbered on the intake runners.

[Gromit]

If you have a points distributor; the #1 spark plug wire on the distributor points to the #2 cylinder

If you have an HEI distributor, the #1 spark plug wire on the distributor points to the #1 cylinder

Please see attached diagrams from the manuals

(however be aware some people will still set up their points distributor so that #1 on distributor points to #1 cylinder - either because they are not aware there was a change way back in 1971 or because they ARE aware and just want to keep it simple.. in other words it can be a matter of choice). However if you have points I think you will find it is more convenient to position #1 on the distributor pointing to #2 cylinder because of where the vacuum can is and where the distributor hold-down fasteners are located.