Timing Curve
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Cargo Master
Joined: Jan 2019
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From: Caraway, AR
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Cargo Master
Joined: Jan 2019
Posts: 3,369
Likes: 16
From: Caraway, AR
lol I haven't, mine's rattling around loose in a box somewhere out in the shop. I'd probably use a pick though. Haven't looked at it in a couple of weeks. Need to get it put back together. My breaker plate is set for 18 and 13, Which sounds perfect actually. Will see if I can't push it some and get it on the 18 one of these days. Lot of assembly though before I'm ready to recurve. In the mean time I've been drooling over some sun machines on ebay.
Posting Guru
Joined: Mar 2006
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and this is where it gets fun.
if you were to get it up to the 18L slot, that's 36 degrees of timing.
that's about all these old motors can handle...which means you'd have to run near-0 degrees of base timing.
and that would make it super-doggy coming up from low rpm.
i'm hoping Frenchtown Flyer is still reading this thread...if so, maybe he knows what scenario the 18L slot could be used for?
if you were to get it up to the 18L slot, that's 36 degrees of timing.
that's about all these old motors can handle...which means you'd have to run near-0 degrees of base timing.
and that would make it super-doggy coming up from low rpm.
i'm hoping Frenchtown Flyer is still reading this thread...if so, maybe he knows what scenario the 18L slot could be used for?
Thread Starter
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Cargo Master
Joined: Jan 2019
Posts: 3,369
Likes: 16
From: Caraway, AR
and this is where it gets fun.
if you were to get it up to the 18L slot, that's 36 degrees of timing.
that's about all these old motors can handle...which means you'd have to run near-0 degrees of base timing.
and that would make it super-doggy coming up from low rpm.
i'm hoping Frenchtown Flyer is still reading this thread...if so, maybe he knows what scenario the 18L slot could be used for?
if you were to get it up to the 18L slot, that's 36 degrees of timing.
that's about all these old motors can handle...which means you'd have to run near-0 degrees of base timing.
and that would make it super-doggy coming up from low rpm.
i'm hoping Frenchtown Flyer is still reading this thread...if so, maybe he knows what scenario the 18L slot could be used for?
Fleet Mechanic
Joined: Nov 2002
Posts: 1,643
Likes: 69
I am. And I don't have a clue. Maybe it would be useful for an application that uses a slower burning fuel - like alcohol or (gasp) nitromethane. But I have to agree that 18 degrees of centrifugal alone seems like too much mechanical advance to have at any RPM. That does not leave much room to add vacuum advance at part throttle for good fuel economy. And perish the thought of using that curve with a fast-burn EFI head. Detonation City
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Cargo Master
Joined: Jan 2019
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Likes: 16
From: Caraway, AR
I am. And I don't have a clue. Maybe it would be useful for an application that uses a slower burning fuel - like alcohol or (gasp) nitromethane. But I have to agree that 18 degrees of centrifugal alone seems like too much mechanical advance to have at any RPM. That does not leave much room to add vacuum advance at part throttle for good fuel economy. And perish the thought of using that curve with a fast-burn EFI head. Detonation City
Really I'd just like to see how much advance I can pull in, maybe play around with AFR and different fuels. I bet if I had a box of assorted springs I could play with that thing for weeks. I promise to stop once I break something(I can't fix or replace) or she starts singin' a tune o' 32mpg... You no, nothing crazy.
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Cargo Master
Joined: Jan 2019
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Likes: 16
From: Caraway, AR
I always remember the good stuff after I hit reply. I dunno if it's a banned topic or what, but I never hear any talk of propane.
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From: Boise, Idaho
The curiosity I've always had with mechanical advance is, why does it stop increasing?
The idea behind mechanical advance, as far as I know, is that the timing needs to spark the air/fuel mixture sooner and sooner due to the speed of the piston. The higher the RPM, the faster the piston is moving and the faster it's compressing the fuel.
So, why would you spark a piston travelling at 6000 RPMs at the same degrees of advance as a piston travelling at 3000 RPMs?
The idea behind mechanical advance, as far as I know, is that the timing needs to spark the air/fuel mixture sooner and sooner due to the speed of the piston. The higher the RPM, the faster the piston is moving and the faster it's compressing the fuel.
So, why would you spark a piston travelling at 6000 RPMs at the same degrees of advance as a piston travelling at 3000 RPMs?
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Cargo Master
Joined: Jan 2019
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From: Caraway, AR
AB I haven't researched it but I have thought about it some. I figure it's a mechanical limitation, like floating valves, sprigs are unable to keep up.
Does anyone know, or can we calculate combustion? Say pick whatever fuel is most readily available, low octane, with or without etheanol, whatever gets decided on. And use stock compression for whichever model is most requested. And calculate combustion time. I figure if we know that, how long the spark takes, we can apply that to rpm to get degrees of advance. And maybe we can get some time correspondence per degree of advance?
I may not have that right, but I think you guys can pick out what I was thinking and correct me. I have a feeling if we look at it that closely what we will see is insignificant as far as need for and ability to implement more advance.
But I'd love to be wrong. I mean what if we could reasonably apply 90-120* of advance before the laws of physics break down.
Does anyone know, or can we calculate combustion? Say pick whatever fuel is most readily available, low octane, with or without etheanol, whatever gets decided on. And use stock compression for whichever model is most requested. And calculate combustion time. I figure if we know that, how long the spark takes, we can apply that to rpm to get degrees of advance. And maybe we can get some time correspondence per degree of advance?
I may not have that right, but I think you guys can pick out what I was thinking and correct me. I have a feeling if we look at it that closely what we will see is insignificant as far as need for and ability to implement more advance.
But I'd love to be wrong. I mean what if we could reasonably apply 90-120* of advance before the laws of physics break down.
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From: Boise, Idaho
I thought it could be something to do with the mechanical nature of the distributor as well, but this theory also still stands for computer controlled timing.
I recently converted my timing system to be controlled by my Holley Sniper, so now I can input everything into a table, and the computer sets the timing.
I could see there being some sort of a limit with a spinning distributor since, if you advance too far back, the points on the stator might be pointing at the previous cylinder. But, it seems that that limit applies to pretty much any discussion about mechanical advance timing. I don't see anything about "if you have a distributorless ignition, the sky's the limit". They all seem to have the same limitations.
This is a timing curve for the Sniper, and even still, you can see that at 4000 RPMs it locks it out at 34° and holds (and then increases up to 44° for "vacuum")
I recently converted my timing system to be controlled by my Holley Sniper, so now I can input everything into a table, and the computer sets the timing.
I could see there being some sort of a limit with a spinning distributor since, if you advance too far back, the points on the stator might be pointing at the previous cylinder. But, it seems that that limit applies to pretty much any discussion about mechanical advance timing. I don't see anything about "if you have a distributorless ignition, the sky's the limit". They all seem to have the same limitations.
This is a timing curve for the Sniper, and even still, you can see that at 4000 RPMs it locks it out at 34° and holds (and then increases up to 44° for "vacuum")
Posting Guru
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AB, that's an interesting question. I would have said "mechanical limits of what can be done with springs and weights", but...computer-controlled ignition curves are pretty much the same, I think.
i'm looking at spark tables for the 4.6 mod motor, and even with computer control, it levels out after ~4000 rpm.
Kramer- more advance doesn't really mean more power, or mileage. fastest burn = most power. the fast-burn EFI heads that FF mentioned above requires significantly less timing, to do the same job.
if you were to tinker with the fuel, to slow the combustion to the point that you could utilize 90* of timing...you'd only be robbing Peter to pay Paul.
i'm looking at spark tables for the 4.6 mod motor, and even with computer control, it levels out after ~4000 rpm.
Kramer- more advance doesn't really mean more power, or mileage. fastest burn = most power. the fast-burn EFI heads that FF mentioned above requires significantly less timing, to do the same job.
if you were to tinker with the fuel, to slow the combustion to the point that you could utilize 90* of timing...you'd only be robbing Peter to pay Paul.
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Cargo Master
Joined: Jan 2019
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From: Caraway, AR
I figure it's all centered around spark right, that's a constant. I figure you can't time it and more than that. No clue how much time that event takes. What I'm wondering is at what rpm the engine has to spin to where the time from tdc exhaust to tdc compression is equal to spark timing. That would be 180* out right? Now what point would you have to worry about dieseling? Are other factors at play? Compression, afr, and is it thermal efficiency?
What's nagging at me right now though is, what if you sped up the spark?
What's nagging at me right now though is, what if you sped up the spark?
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no, spark is not a constant. Variables: amount of spark energy, width of spark plug gap, multi-spark ignition systems, optimal fuel mixture for fastest burn of the charge...all sorts of stuff to think about there.
Yes, compression and AFR are in play as well.
some people will say "all that is required is enough energy to jump the gap", but that leaves all sorts of things out, which can rob you of power.
you need the hottest ignition system that you can run, with the widest plug gap, and get your mixture right, in order to increase the speed of the flame front.
faster burn = more power
Yes, compression and AFR are in play as well.
some people will say "all that is required is enough energy to jump the gap", but that leaves all sorts of things out, which can rob you of power.
you need the hottest ignition system that you can run, with the widest plug gap, and get your mixture right, in order to increase the speed of the flame front.
faster burn = more power