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ggevaert 02-18-2013 12:39 PM

Initial timing

I'm working through retiming an engine with minimal engine information (i.e. don't know compression) and I need some assistance on determining initial timing.

Is there a way to determine how much an initial timing an engine can handle through some sort of procedure? Most articles and threads I read talk about trial and error if you don't know items such as compression and that is fine but how do you know when too much is too much? How do you know how much the engine 'wants'?

I have a 350 SBC (had been rebuild when I bought it 4 years ago, has about 10,000 miles on it) with 450 lift, 224 duration cam, 416 heads, dual plane edelbrock intake, quickfuel 680 carb (with slightly larger idle air bleeds), long tube headers, 2500 stall, MSD 8365 distributor,

I have run it at 18 degrees all the way up to 22 degrees (currently at 22) and it starts and idles fine at 800 rpm. I have a custom limiter installed in the 8365 that gives me 14 degrees of mechanical advance for a total of 36. I also have a custom 10 degree one and then the typical MSD advance kit 18 and up ones if needed; vacuum advance currently totally disconnected and plugged. Advance kit springs starts timing at 1200 and all in at 2800.

I was provided a procedure by an old timer as below but couldn't really see any of the conditions he indicated all the way up to 22 degrees so not sure if I was doing it wrong or if I need to keep going up:
* get engine up to operating temp.
* turn off and let the temp spike (about a minute)
* Start engine back up and if the starter has problems getting the engine to start or if you have air coming up the carb then decrease timing by a couple of degrees and that's your initial timing otherwise keep increasing the timing a couple of degrees at a time doing the above temp spike method each time.

Final note; truck does not have any windows in it so I can't take it to a road drive just yet so need to do most inital setup in the garage.

Before I move onto vacuum setup and carb tuning I want to get initial timing and total timing correct so any help is greatly appreciated. :D

Ps. Truck is a street vehicle but not a daily driver; more of a fun learning environment which may yet get to see some strip action just for the fun of it.

33Willys77 02-18-2013 01:23 PM

You are probably not far off. Initial at 18 (as long as it starts good) is ok with a total of 36. That is a good starting point and should be able to tune the carb in fairly well. Without being able to drive or have it on a dyno, not much more you can do.

MouseFink 02-18-2013 03:24 PM

To start the engine with 14-22 degrees initial timing advance, the engine must have between 9:1 to 9.5:1 compression ratio. If the engine has 9.6:1 to 10:1 static compression ratio, 10-13 degrees initial advance would be correct. More initial advance with 9.6:1 to 10:1 SCR with a short (224 deg.) duration camshaft and the engine would be cranking against the ignited mixture and would be difficult to start.

It is best to lock down the initial advance where the engine starts and runs best. Set the initial advance where it is difficult to start and then back in down two degrees. Set the initial advance with no vacuum advance and idling less than 700 RPM, if possible. If the engine idle speed cannot be turned down below 900 RPM you cannot set the initial advance.

ggevaert 02-18-2013 06:15 PM

Initial Timing
Thank You all for your assistance,

33Willys77, it is currently at 22 and I tried 24 this afternoon which is starting and idling fine.

F-BIRD'88, it is idling fine at 24 right now, so how high should I go? Do I keep going up by 2 degrees at a time until I cannot start it and then go down 2 degrees?

MouseFink, I have no idea what the compression is and I have no details to be able to calculate it so hence my question as to figuring this out using other means. The vaccum advance is disconnected and plugged so not a variable in this. Please clarify what you mean with your comment of 'If the engine idle speed cannot be turned down below 900 RPM you cannot set the initial advance'. Engine was running but barely holding in there at 700 rpm with 24 degrees so I turned idle up to 850 and it now runs fine (but it may be getting close, if not already, running on transition slot).

Thanks :thumbup:

MouseFink 02-19-2013 05:03 AM

The engine must be idling below 700 RPM to check the initial timing advance. That is because the centrifugal (mechanical) timing advance starts advancing the timing at 700 RPM and is fully advanced by 2300 RPM.

Definition of initial timing: The initial timing is where the crank timing mark is indicated by a timing light with the engine running below 700 RPM, without any centrifugal (mechanical) advance and the vacuum advance disconnected. You cannot check the initial timing on high performance engines because those engines idle too high which is due to the long duration camshafts. The factory idle setting is 650 - 700 RPM on regular production engines and the mechanical timing mechanism on those engines is designed not to function at that RPM. If you have a aftermarket distributor, check with the manufacturer of the distributor on how to check the initial timing advance.

If the engine will not idle below 700 RPM, forget about setting the initial timing advance. Some factory high performance engines had distributors that delayed the start of the centrifugal timing advance until 1100 - 1200 RPM because they have high performance camshafts and the engines will not idle below 900 RPM.

33Willys77 02-19-2013 06:10 AM

Mousefink, he does not have a stock distributor - its all in what he has put together in the MSD. That makes all the stock configurations void. F-Bird is right and this motor should run fine at 18-22. For that cam (without knowing much more), 22 should be plenty of initial.

TommyK 02-19-2013 06:47 AM

I think you may find that your starter has less tolerance for initial timing in August than it does in February. Might have to repeat this procedure on a really hot day after the engine is really heat soaked.

MouseFink 02-19-2013 06:55 AM

A automobile engine will not start with 22 degrees of initial advance.

You are confusing "initial" timing with "initial timing advance". Initial timing is when ignition occurs without any centrifugal advance..

You cannot set the initial timing when the engine is idling above 650 - 700 RPM because the centrifugal advance mechanism is advancing the initial timing. Stock production engines are the only engines that the "initial" timing can be set and only when the engine is idling below 650 - 700 RPM.

You must check with the distributor manufacturer if you want to set the initial timing when an aftermarket distributor is being used.

MouseFink 02-19-2013 07:00 AM


Originally Posted by TommyK (Post 1647954)
I think you may find that your starter has less tolerance for initial timing in August than it does in February. Might have to repeat this procedure on a really hot day after the engine is really heat soaked.

That is because excessive heat can lead to "pre-ignition" which is the same a over-advanced initial timing. Over advanced ignition causes the piston to compress the ignited mixture, making the engine difficult to start.

MouseFink 02-19-2013 07:55 AM

If you are seeing 22 degrees at idle, is is NOT the initial timing.

MouseFink 02-19-2013 10:34 AM

"Initial travel" and "initial advance" is a incorrect description. "Initial timing" is where the timing is before the distributor centrifugal (mechanical) advance starts. There is no such thing as "initial advance" or "initial travel". "Timing advance" starts between 700 and 1200 RPM, depending how it is built into the distributor. Aftermarket performance distributors are designed to have the "timing advance" start at a higher RPM for use with high performance camshafts that must idle at a higher RPM due to longer valve duration and increased valve overlap and low vacuum. .

GM did the same thing to their so-called "stock" distributors.
For example:
A "stock" Pontiac Ram Air IV distributor centrifugal advance mechanism and the mechanical advance on a "stock" Chevrolet K66 transistor ignition starts at 1100 RPM.

33Willys77 02-19-2013 10:59 AM

Just as F-Bird said, engines will start with 22 degrees without problems. I have mine set at 34 degrees (locked) and have no issues at all. Now, if the advance in a distributor does not start to move until 1200 RPM, why would it matter to have the engine idle at 'a must' 650-700? Would make sense that it would be fine at 1000 as well since the advance will not start until 1200. Right?

MouseFink 02-19-2013 11:42 AM

Engine Timing 101:

Regular production distributors have timing advance that starts at 700-900 RPM. The timing advance in distributors used in high performance engines starts at 1100-1200 RPM.

1.) Engines equipped with some high performance camshafts will not idle below 700 RPM.
2.) Since the centrifugal advance in a stock distributor starts at 700 RPM, you cannot set the initial timing.
3.) In order to lock down the initial timing in a engine e/w a high performance camshaft, you must have a distributor that starts the centrifugal (mechanical) advance at 1100-1200 RPM.

A distributor with a higher RPM centrifugal advance mechanism will allow you to turn the engine idle speed low enough so you can set the initial timing before the centrifugal advance starts. After the initial timing is locked by the distributor bolt, you can then turn the idle speed above 1100 RPM and observe the initial timing plus the centrifugal advance.

If your timing is 22 degrees BTDC, the initial timing is 12 degrees BTDC with 10 degrees timing advance in by 1200 RPM. .

33Willys77 02-19-2013 01:48 PM

I say, stop trying to confuse people. The one that started this topic noted he has an MSD distributor. He can very well have 22 degrees initial. Mine is set at 34 degrees weather its at 600 RPM or 6000 RPM. I'm done here.

ggevaert 02-19-2013 08:05 PM

Initial timing
Wow, lots of discussion.

Let me answer some comments/questions:

1. Mousefink, My initial timing is 24 degrees; Mechanical advance curve is set to come in at 1200 and be all in at 2800; I have verified this with a good timing light. Timing did not budge at all until we hit approx 1200 then started to go up. My idle is set at 800 and is stable. I use an MSD 8365 with custom stop advance bushing (14 degrees). And, yes, it starts just fine with 24 degrees initial timing, actually on the first turn of the key.

2. F'bird-88, don't know LSA for sure; tag on the engine said 450 lift, 224 duration, 206 lobe centre but I think the 206 is wrong? (maybe 106?, it was hard to read that part of the tag clearly)
Manifold vacuum at idle is 8-12 (bounces fast between that range) at 800 rpm; I could not find my notes with the readings in gear, will redo that next run, likely thursday night.
Have not got as far as idle screws yet, they are, however, set at 1.5 turns out on all 4 corners from the factory (I verified). I would hope they would be active as otherwise that means my transfer slot is too exposed. (which I initially set to 0.025)

3. TommyK, thanks, I'll put that on my list. I do have a mini starter with rather high torque capability and I have the headers closest to the starter wrapped. Went through 2 GM starters in 1 year before I learned that one; no issues now in 2 years.

4. I was told that the reason the recommendation for high performance engines to be set to 1100-1200 idle is to ensure proper oil splash and cooling on the cam. I prefer to keep mind a little lower at 800-900 but that is not yet set in stone; we'll see where she settles when I'm all done.

Also, keep in mind that I have larger idle air bleeds installed in the carb so that will also affect 'stock' thinking. This was a recommendation by Quickfuel to smooth out my idle and makes sense to me.

Thanks for the continued assistance;

Cheers :thumbup:

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