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ggevaert 02-18-2013 12:39 PM

Initial timing
 
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.

F-BIRD'88 02-18-2013 01:15 PM

If you aand the truck are happy with it idling using 22deg initial, then thats all you need.
You don;t need any more or any less.
When you get it all on the road you can dial in the vacuum advance system.

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:

F-BIRD'88 02-18-2013 06:34 PM

Its fine where it is. thats plenty for a 224deg cam and auto trans.
34 to 36 total.
if you want to check the idle T slot exposure pull the carb off and look.

there should be slight exposure. at idle
Pri and sec should be more or less even at idle.

what is the LSA on that cam? what is the manifold vacuum at idle? in gear and in neutral.... idle rpm.
are the idle mix screws active?

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

Quote:

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.

F-BIRD'88 02-19-2013 07:34 AM

Quote:

Originally Posted by MouseFink (Post 1647955)
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.

All you do is slow down the idle speed so its is idling on the base of the timing curve.
not that hard

And cars will start with 22deg initial timing.

When you are dealing with non stock cams you cannot use what is used on stock motors.
non stock long duration cams need more inital timing at idle.
If you are not idling on the base of the curve at idle, the timing will drop off when you are in gear.
You don;t want that on a cammed motor.
Especially cams that have a tight LSA .

every GM car with headers needs a starter motor heat shield.
and a heat wrap.
adding a Ford type starter solenoid to your GM car really helps too.
In extreme cases like with locked out timing, a simple spark power interupt toggle switch on the dash
makes it easy to start a hot motor with lots of initial timing.

MouseFink 02-19-2013 07:55 AM

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

F-BIRD'88 02-19-2013 08:03 AM

Quote:

Originally Posted by MouseFink (Post 1647965)
If you are seeing 22 degrees at idle, is is NOT the initial timing.

it is when the distributor cent curve has been modified and limited to 14deg travel.
This is not a stock distributor with a stock 20 to 24deg centr curve.

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.


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