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Initial timing

20K views 24 replies 6 participants last post by  ggevaert 
#1 · (Edited)
Hello,

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.
 
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#3 ·
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.
 
#4 ·
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:
 
#5 ·
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.
 
#6 ·
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.
 
#11 ·
"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.
 
#12 ·
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?
 
#13 ·
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. .
 
#23 ·
Funny, my timing is set at 22* at idle. Idle is 1000rpms. When I drop the idle to 700rpms the timing stays at 22* When I raise it back to 1000rpms and the rev it, it starts to build after about 1200rpms and is all in by 3000rpms, 36* total. How did I do that? :p Cam is .660" lift, 253*@0.050" 110LSA 106ICL

Skip White HEI distributor with tuned springs, plates and plate stop. Maybe $50 in it total. Pretty easy to prevent advance below 1200rpms with a spring change and new plates.
 
#15 ·
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:
 
#16 ·
Wow, lots of discussion.


Thanks for the continued assistance;

Cheers :thumbup:
Larger air bleed = leaner idle. If this is QF recommendations, I'd stay w/it unless the engine performance/plugs/a-f meter dictates otherwise.

As has been said already, use as much timing as the engine wants. The upper limit is determined by how it starts up, if it pings at throttle tip in (this will become more evident when the vacuum advance is brought on-line). But using the absolute maximum amount of initial timing isn't necessarily the best way to go about it. It is my opinion that using a medium amount of initial timing (which gives a broader amount of mechanical timing) should give better control of the spark timing over a broader range of engine rpm.

An example of this thinking is seen here, where Demon carbs recommend 14-16 degrees BTDC for your engine/cam/carb size as a safe starting point. I'd recommend you work upwards 2 degrees at a time from there.

When it comes time to set up the vacuum advance, if for some reason you end up using 24-plus degrees initial, you may find using ported vacuum better than manifold vacuum. Generally speaking, you won't need more than 10-12 degrees of vacuum advance.

More on timing is here. The page is on GM HEI, but the timing info is relevant regardless of the distributor.

DO NOT be hesitant to readjust the idle mixture screws! They should be readjusted after just about any change made to the engine perimeters (timing, idle speed, plug heat range, idle air bleeds, etc.), and as long as the engine is fully up to temperature (fully- as in after a drive or a long warm up if still undriveable), you cannot go wrong readjusting them as needed.
 
#17 · (Edited)
Timing/Carb

Hi,

F-BIRD'88, Idle timing is rock solid. Interesting about the vacuum, i thought that was odd but attributed it to the cam and low idle RPM; I will have to review the cylinders if any are missing; not sure. (i'm rather hard of hearing so I need to use alternate methods to investigate those types of things, any suggestions?). I did check for vacuum leaks with propane and none were found. I will hookup the wideband AFR this weekend.

Using a Quickfuel SS 680VS with pri and sec idle air bleeds changed from stock 70's to 74's. (as recommended by quickfuel when I provided them with the engine specs). The high speed air bleeds were left alone and are 28/28.
Everything else on Quickfuel was also left alone (primary main jet 68, Secondary jet 78, pri nozzle 31, needle and seat 110, power valve 6.5). As I got through my carb dialin I will be reviewing each of these to ensure they are the correct setup for my application.

I actually bought a wideband AFR electronic setup that I have not yet hooked up. Was planning to get basics on engine done then go back over the setup with it on. Never used one before so need to learn how.

cobalt327, I understand; At the beginning of this timing exercise I started at 18 degrees initial with an advance stop of 18 degrees (which was working well) but I decided to go higher to see how that would look. Hence my original question as I was trying to understand the reasoning for why we set initial timing where we do and why we stop where we do. I did increase the timing 2 degrees at a time from there, every time checking for starter issues or detonation and found none all the way to 24. I stopped there for no reason other than I was worried about going higher. Also, keep in mind that I have not been able to take the truck for a drive as I have no windshield in it yet (and its 20 below out here) so my only load testing has been with my foot on the brake and with a chock block on the wheels.

Vacuum can is the stock MSD one, it has a vacuum advance stop plate installed and set to 13 degrees; with the vacuum hooked up, at idle my timing was 30 (so +6 from initial) and at all in (3000 rpm) it ran at 42. Keep in mind that was at idle and not under load.

Mine is a GM HEI so the article you note is very relevant and, yes, I did read it thorougly.

I do plan on adjusting the idle mixture screws; that is the plan this weekend; I just wanted to get timing done before going there. (but I do understand it sometimes is not a linear exercise; you think everything is set, take it for a drive and end up re-adjusting everything :)) but that's the fun part.

Cheers :thumbup:
 
#18 ·
Also, keep in mind that I have not been able to take the truck for a drive as I have no windshield in it yet (and its 20 below out here) so my only load testing has been with my foot on the brake and with a chock block on the wheels.

Vacuum can is the stock MSD one, it has a vacuum advance stop plate installed and set to 13 degrees; with the vacuum hooked up, at idle my timing was 30 (so +6 from initial) and at all in (3000 rpm) it ran at 42. Keep in mind that was at idle and not under load.

Mine is a GM HEI so the article you note is very relevant and, yes, I did read it thorougly.

I do plan on adjusting the idle mixture screws; that is the plan this weekend; I just wanted to get timing done before going there. (but I do understand it sometimes is not a linear exercise; you think everything is set, take it for a drive and end up re-adjusting everything :)) but that's the fun part.

Cheers :thumbup:
Good deal. My only comment at this stage is that you cannot judge detonation (or the lack of it) w/o driving the vehicle. You can't duplicate the same load it'll see on the road using the brakes, unfortunately. What you'll want to do is go back to ~18 degrees initial and test it on the road after it's roadworthy. Then readjust from there. If you choose to leave it as-is, just be uber cautious and listen carefully for detonation.
 
#19 ·
Timing

Thanks Gents, I have some work ahead of myself.

F-BIRD'88, Quickfuel sells air bleeds in every size so I will just get one of their assortment kits at summit for $50, they come with every size from 36 up to 75. With items that small and precise I don't really trust myself to get them correct.

Don't know what cam brand is in there; it came with the motor and I'm trying to work with it.

My torque converter is 10", I purchased it new from a local reputable racing transmission shop because I wanted a good one.

How do you know a power valve is fluctuating at idle?

Also, I'm running 94 octane sunoco fuel so that will allow me some detonation headroom.

Cheers
 
#24 · (Edited)
Bad tidings

Ok, got some problems, something went south and not sure what that is yet. Need some help here.

Started up the engine today to verify my new tach and AFT gauge installation and to describe that it ran terrible is an understatement. :mad:

I have made NO changes (other than these gauges) since last time it ran, which was excellent.
* First could not start it at all; could only get it to run for a little bit using ether. tapping the accelerator to give it gas was having no effect and eventually was just causing it to flood.
* We finally got it going, roughly, but then she ran rough and at very high RPM's of 1600-2000 (compared to where I left which was 850). Playing with the idle has eratic results; it either goes really high (2000) or stalls out.
* when it was running really high I dialed down the idle to get it to come down and then I got lots of backfiring and engine would stall around 1100.
* a very heavy fuel smell around the engine
* and the oddest of all, when I look at my timing light, which was hooked up and in RPM mode, it would every once in a while go to all zeros; which means no signal. So I think that means I'm not getting a spark to plug 1 (I only see all zeroes when I turn off the engine so this is definitely odd).
* fuel pressure gauge stayed where expected; between 6 and 8.

The last point makes me think my distributor went south but maybe the cylinder is flooded to the point where the spark plug can't fire?

I verified everything just to be sure (wires correct, no vacuum leaks, etc) and I disconnected all newly installed gauges to make sure there was nothing they were doing to interfere. What gets me is that nothing was changed since the last run otherwise I would focus on that area.

I do have my old distributor so I will be putting that one in to eliminate any potential distributor problem.

Any help and/or troubleshooting tips are much appreciated

Gary :pain:
 
#25 ·
Bad dizzy

Looks like the Dizzy went bad; dropped in another one I had sitting around and it ran again. Had to clean plugs as they were absolutely black; likely from the constant stop/starts in testing the dizzy. jeez.

Ordered a pertronix unit to replace bad dizzy.
 
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