This may be a very elementary question, so pardon my ignorance, but I don't totally understand the purpose of initial timing. I understand it' as being a starting point to get the engine to start and idle reasonably well, but as soon as you move on to total timing, isn't that number out the window as you are moving/advancing the distributor off the initial position set? If the car starts and idles well, why do I care about initial and can't I just go to total? I may be missing something obvious here...thanks for any clarification!
Initial timing doesn't go out the window. That base line stays and additional timing is added or taken away. If you want 32-34 degrees total most distributors don't have that much built in advance. So you start with 8-12 degrees on initial and then get maybe 22-24 degrees additional advance from the distributor to get where you want to be. It's all related.
Initial timing isn't nearly as important as total timing. But you do want it to run ok at idle and low rpm, so it's good to get it in the ballpark. With a different cam than stock, you probably want different initial timing, too. I usually set it to be in the 10 to 20 degree range, by adjusting the amount of mechanical advance, that still gives me the total timing I want.
with timing locked in at total timing numbers (too early for cranking rpm) it may be hard to start, it could ignite the charge before the intake valve is fully closed causing a backfire through the carburetor creating issues or even force the piston back down opposite of rotation.
Thanks Jim(s) for some clarification. I guess the gist of my question/confusion is that it sounds like you really have no choice on how much initial advance you can run with a given total advance? Is that essentially true? Given that if the distributor is set in a specific position at idle, but then I go and move it off that position for total timing, the distributor is no longer in that original idle position anymore so therefore initial timing is kind of pointless?
Mechanical and vacuum advance allows for varying timing dictated by engine RPM and engine loads. Lugging or loading the engine (early up-shifting, hauling heavy loads, trailer towing) with too much timing advance doesn't make for a happy engine. Prolonged knocking or detonation can do some serious damage. Anything street driven should provide less advance at low RPMs and heavy load conditions; more advance at higher RPMs and lower load. Want to get a better idea of when your engine is under a heavier load? Try driving it with a vacuum gauge connected to a source of full manifold vacuum.
The biggest issue with initial timing is fuel quality. If you look through old service manuals by decade, you'll see the progression. From 1930 to present, you'll see the average idle timing start at TDC to 16 BTDC. Its mostly to do with fuel quality/octane, cleaning additives, high pressure additives for EFI, etc... After that the number goes up with over-carburetion such as dual 4 bbl setups, 6-packs, or 900 cfm Holley Snipers. You may find a baseline of 20-24 is ideal in those cases? I know I'll get some backlash for these numbers, but this is from current dyno related tuning, and it changes every couple of years. The latest shift in fuel for direct injection has been crippling old cars that have never had a recurve!
This and what @squirrel says. You didn’t say what engine/cam etc you have. One thing I would add is to make sure your 0 degree or TDC on the harmonic damper and pointer is correct. This can be determined using a piston stop. Usually an extension of a spark plug when checking a fully assembled engine. Rotate full CW until rotation stops, mark the damper then rotate full CCW until stops. TDC is exactly between the two marks you made. It could be several degrees off the factory mark. Some distributors have to much mech advance (Mopars come to mind) causing the initial to be set to low. A stop can be made to limit the total. What I do is plug the vac advance, rev engine to max mechanical advance and set to the the 32 - 34 degrees stated above then check advance at idle hopefully it’s 10 12 or so degrees. Then I drive the car under a heavy load at various speeds with the vac advance functioning. If I have detonation I back off a couple degrees and repeat. Dan
When you set the initial to say 22 degrees where does the all in max advance fall? Do you have to limit the max with a stop? You’re one of the in house experts and I will pay attention to your suggestions. Thanks Dan
that's the thing, you usually have to limit mechanical advance with a stop, to get sufficient initial timing, but not too much total timing. It's a fun game. And some engines don't want too much initial, it mostly has to do with how well the engine works at low rpm. If it has more camshaft duration than stock, then it might want more initial timing, and less advance, but the same total timing that a stock engine would.
Say the initial only wants 6/8 BTDC Assume it gives you 30 BTDC at full advance. Couldn’t a person file open the stops on the limiting plate? I’d guess best to be done on a machine for ease. Just asking as I don’t know.
It depends on the distributor, some have a bushing that you can install a different OD to change travel. Others have a slot you might have to file or weld up.
To the OP...........the short answer is YES ! Slang-it-out-da-window.. Butt..... that's when you're tuning for total advance at whatever RPM. Initial advance number is important (like the guyz said) because you need a starting point with your timing. (Always establish TDC with a piston stop and properly calibrated damper/dampner/wheel hickey on the bottom!) Study and read and ask questions. TOTAL timing number will be higher when dealing with stock 'restrictive' heads. AFR heads (as an example) flow a LOT better than and are more efficient and don't need as much total timing. Like Squirrel said above...its a fun game.... Now....throw in vacuum advance in the equation annnnnnd......!!! 6sally6
With the sprint car, the timing was set to 45 degrees total. Since the mag was fixed with no advance, that was also the initial timing. We were running straight methanol. No problem with hard starting as a push truck has penty of torque to turn the engine over in high gear (the only gear it had).
Because of time and crank angle. Most spark ignited engines operate at varying speeds and loads, so the timing must also be variable. This is to keep peak pressure of the combustion process centered on the ideal crank angle for optimal power output. It's a moving target. Engines that operate under a constant rpm and load may have their timing locked out, but this is for simplicity and reliability.
I didn’t study all the responses, but one should really look at timing as a map over the full speed / load range of the engine operating range. for simplicity, we talk about initial and total timing, but that doesn’t really tell the story. Full load / wot points should be retarded from the timing at the same speed but light load for example. you’ll never have an optimized engine when you look only at initial or total timing. You Neto optimize the entire map, which most home tuners don’t do.
So far scratching 1/4 or so what can be Adjusted or miss Machine assembled. Cam plate. Point cam . Weight cam plate. Advance weights . Avance springs. Avance stops. Pick up . Adjustable rotor button. Adjustable cap / base . Rotor poll in relationship to polls on distributor cap. This just on Mechanical advance distributor. Large cap are more accurate to control then small caps . Almost Every engine like's / requires Different adjustments, In stock classes , all these Tricks / knowledge made other more successful than others. Back in Iroc class all engines where Dyno to make the same Hp , Torque Then the Crew chief & driver determined the winner. Thing's like this remind me of a few Conversations I had in late 80's early 90s with Scotty Cannon and Warren Johnson, While others are in bed sleeping I am in the shop working into the wee hours of the a.m..
If initial timing is retarded low speed engine heating may result. Rather than limiting total advance by using less initial advance it would probably be better to use bushings or modifying the advance plate.
I'd have to dig out an old motor manual and go to a tune up page as they show initial timing and advance at X rpm for STOCK engines that are expected to perform reasonably well in and under all conditions but are not by any means "performance" settings. Look on the tune up page for just about any small block Chevy from 55 to mid 70's and the initial timing is going to be 4 degrees and the advance at X rpm will vairy. Then with a stock engine and stock distributor we start tinkering with the initial timing and move it up to Six or Eight degrees initial and see how it does and maybe go 10 or 12. That is without touching the internals of the distributor. Then we go to reading that you can dial it in more if you buy this kit with bushings, springs and different weights that you can change the advance curve without even removing the distributor and hauling it down to "that guy". We either get it to run pretty damed good or take those pieces out and put the original stuff back in and go back to ground zero and start over. Change the cam and you usually find out one way or the other that that cam wants ZZ degrees of total timing at XX rpm to work the way you want it to. On the other end of the spectrum I have known guys who backed the initial timing off on what should have been some strong running engines because they didn't want to pay for premium high octaine gas but lost performance and lost a crap ton of gas mileage in the process. That happend a lot in the mid 70's when gas took that first huge price jump. Then guys down along the border like in Squirrel's hood were putting bigger tanks in their daily drivers and slipping over into Mexico to buy cheaper gas that was lower octaine yet.
@mhirscher You ask about why this number. It's due to optimizing the various adjustments for the engine speed. Then you ask how to adjust them. It requires changing the parts responsible for the changes. Please describe the engine, ignition, and vehicle you have to get more focused responses.
As a number, supplied by the manufacturer, (being conservative) however many years ago, yes base timing is irrelevant , but base timing is a relevant part of total timing, it’s just going to be different these days due to changes to your engine and fuel available today. If you’re changing things chasing a total timing number, do you know that it’s working for your engine or just an arbitrary number that should work? Do you know the real condition of your distributor , whole lot of things just in there that can affect how it works not to mention the rest of the drive train. Then the other thing is, if the vehicle is for the street, do you want it to start easily, sit and idle in traffic and not overheat. Sure we all want a vehicle that will sing when you put your foot in it but realistically how much of the time does that happen
I’ve been a mechanic my whole life but I’m just now learning how to tune a hotrod. First thing I learned is to keep my ears open and my mouth shut, right here on the HAMB.
@Adriatic Machine it's been going on for more than a year! https://www.ecosia.org/search?method=index&q=ignition+timing+HAMB+site:www.jalopyjournal.com
Going to a shop , seeing all those sleeve's with patches , Most Just took a written test & passed But do not have a clue on working of, Just what scan tool says
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