Timing advance

[Route666]

I've been reading up on ignitions, and find myself wondering some things.

For engines with distributors, when you set mechanical advance, everywhere I've read says to have it all in around 2500 - 3000 rpm. Is that just because it isn't reasonable to time it with the engine at 5000, or 6000 rpm or is it because the motor needs that much advance at 3000?

I ask because if it needs that much at 3000, surely it would be better to put more timing on at higher rpm, say by using a bigger arm and heavier springs in a distributor setup.

For discussion, the following motor is a 302 with EFI and EDIS.

I'm trying to work out what would be best to plot in an EFI spark table. Is it best to just have (for example) 34 degrees from 3000 up to max, or make it gradual? I do mean mechanical only for this.

As for load-based timing, (vacuum advance) if you are at idle, and punch the throttle to 100%, do you want to advance the timing immediately or leave it at 8 degrees or whatever idle advance is set at?

What about if you are idling and you slap on 1/2 throttle? How much advance?

I've read that manifold vacuum advance is good for off idle power and ported was used for stricter pollution control, more after-burning in the exhaust whilst idling. Does this mean more idle timing would be better? Note: I don't mean more initial timing, (which advances the whole curve) just more idle.

Anyways, the first questions are the most important, they will help me answer the last questions, but discussion about them is welcome.

I look forward to discussion.

 

[rbohm]

timing curves can be trickey to deal with. the reason you want the mech advance timing all in by 2500-3000 rpm is that you want maximum torque production at all rpm. same with the vacuum advance. to an extent the more timing an engine has and can use, the better for power and efficiency. that said timing curves and fuel curves need to coincide as closely as possible as well to prevent detonation, and maximize power and efficiency.

 

[HistoricMustang]

This explains timing a lot better than I can:

http://www.badasscars.com/techtips.html#Anchor15

You can also pick up a lot of other good "budget" stuff here for your early model.

HistoricMustang
www.historicmustang.com

 

[Route666]

Thanks guys! Historic, I've actually been to that page today, but I was looking at a totally different thing, how coincedental! I'll check it out. - Ok checked it, good info.

The thing I'd really like to know is: Is the max advance:

1) Perfect for when it just comes in (3000rpm) and is a low compromise at max rpm.

2) Perfect for max rpm (6000rpm or so) but too advanced for 3k, but is a compromise because there is no other way to do it. (I'm guessing this isn't right, as you'd have detonation at 3000rpm if it was too much, and you could use stronger springs to slow the advance)

3) Perfect for 3k and up because if you keep advancing it, even at high rpm it will pre-ignite, and detonate.

I'm guessing 3 is right, based on the reasoning in two and that number 1 would be detrimental to performance. I just wonder if there is any benefit to being able to adjust every load/rpm point in the advance table seperately, rather than as a function of springs, weights, and vacuum.

One benefit I've found with the electronic stuff is the ability to data log and replay your log and find out if it's detecting knock and pulling timing, and adjust the table to have max timing without the knock sensor needing to pull timing.

 

[WORTH]

Thanks guys! Historic, I've actually been to that page today, but I was looking at a totally different thing, how coincedental! I'll check it out. - Ok checked it, good info.

The thing I'd really like to know is: Is the max advance:

1) Perfect for when it just comes in (3000rpm) and is a low compromise at max rpm.

2) Perfect for max rpm (6000rpm or so) but too advanced for 3k, but is a compromise because there is no other way to do it. (I'm guessing this isn't right, as you'd have detonation at 3000rpm if it was too much, and you could use stronger springs to slow the advance)

3) Perfect for 3k and up because if you keep advancing it, even at high rpm it will pre-ignite, and detonate.

I'm guessing 3 is right, based on the reasoning in two and that number 1 would be detrimental to performance. I just wonder if there is any benefit to being able to adjust every load/rpm point in the advance table seperately, rather than as a function of springs, weights, and vacuum.

One benefit I've found with the electronic stuff is the ability to data log and replay your log and find out if it's detecting knock and pulling timing, and adjust the table to have max timing without the knock sensor needing to pull timing.

Satan, do you have a setup where you can totally control the advance electronically?

 

[VictorII]

I've been reading up on ignitions, and find myself wondering some things.

For engines with distributors, when you set mechanical advance, everywhere I've read says to have it all in around 2500 - 3000 rpm. Is that just because it isn't reasonable to time it with the engine at 5000, or 6000 rpm or is it because the motor needs that much advance at 3000?

I ask because if it needs that much at 3000, surely it would be better to put more timing on at higher rpm, say by using a bigger arm and heavier springs in a distributor setup.

For discussion, the following motor is a 302 with EFI and EDIS.

I'm trying to work out what would be best to plot in an EFI spark table. Is it best to just have (for example) 34 degrees from 3000 up to max, or make it gradual? I do mean mechanical only for this.

As for load-based timing, (vacuum advance) if you are at idle, and punch the throttle to 100%, do you want to advance the timing immediately or leave it at 8 degrees or whatever idle advance is set at?

What about if you are idling and you slap on 1/2 throttle? How much advance?

I've read that manifold vacuum advance is good for off idle power and ported was used for stricter pollution control, more after-burning in the exhaust whilst idling. Does this mean more idle timing would be better? Note: I don't mean more initial timing, (which advances the whole curve) just more idle.

Anyways, the first questions are the most important, they will help me answer the last questions, but discussion about them is welcome.

I look forward to discussion.

After getting a dial back timing light and learning how slow my stock timing came in (between 4,500 and 5,000 as I recall), I bought a Crane recurve kit for $25.00. I dinked around and found that for my setup (warmed up 302)/T5/3.40 Tractionlock rear combo), it ran best when timing, 36 degrees mechanical, was all in at 3,000 RPM. Really, all I did was replace the heavy spring with the lightest one in the kit. I also learned that an additional 10 degrees of vacuum advance boosts the gas mileage. Then I set it all on a country road with the help of a stop watch and my ear (to listen for pinging). I just advanced the timing until it pinged under hard acceleration and then dropped it back 2 degrees at a time while watching acceleration. It worked. As has been said, bringing timing in as soon as possible without pinging boosts torque. Vacuum advance will raise gas mileage. The kits give you adjustability for both. Oh, yeah, if you have a points-type distributor, lose the points and get a Pertronix drop in kit for consistancy. Between the two, you will have a lot less invested than a new distributor and it will look stock if that's what you want. Plus it was fun to install and set it all up. I think initial is at around 10 degrees but I don't worry too much about that so long as it idles smoothly.

 

[Route666]

Satan, do you have a setup where you can totally control the advance electronically?

I don't HAVE one, but I've got the software - Accel's DFI 7+. I love learning about stuff, motors mainly, because they are pretty complex machines.

 

[WORTH]

I don't HAVE one, but I've got the software - Accel's DFI 7+. I love learning about stuff, motors mainly, because they are pretty complex machines.

Do you have a way to dyno it on the computer if you make changes?

The theory is simple, the faster the engine spins the more advance you should need. The whole idea is to get the explosion right after TDC. Be interesting to see how it plays out.

 

[WORTH]

I don't HAVE one, but I've got the software - Accel's DFI 7+. I love learning about stuff, motors mainly, because they are pretty complex machines.

Do you have a way to dyno it on the computer if you make changes?

The theory is simple, the faster the engine spins the more advance you should need. The whole idea is to get the explosion right after TDC. Be interesting to see how it plays out.

But in reality you should be able to make more power if the fuel fired when the crank was horizontal because the piston has more leverage at that point. If you could figure out how to get a good bang with the piston half way down you'd be the MAN

 

[Route666]

Do you have a way to dyno it on the computer if you make changes?

The theory is simple, the faster the engine spins the more advance you should need. The whole idea is to get the explosion right after TDC. Be interesting to see how it plays out.

But in reality you should be able to make more power if the fuel fired when the crank was horizontal because the piston has more leverage at that point. If you could figure out how to get a good bang with the piston half way down you'd be the MAN

Well if I actually DID get to do the custom EFI and EDIS thing, I'd build a timing curve, do a lot of recording of knock sensor feedback and rpm and map sensor feedback, and tune the timing so the knock sensor ends up doing nothing. Then I'd take it to a dyno and see if I advanced or retarded the whole chart if I would gain or lose power in some rpm zones, and adjust the chart from there. I'm one who thinks that max advance isn't always the best, you might make more power having less negative torque, or at least use less fuel getting the same power.

I guess I'd have to think about introducing some nitro powder into the fuel to see if we could get the highest pressure spike right around 90 ATDC. I doubt I'd test that in a motor worthy of the fuel injection and electronic spark though hahaha.

Oh, one of the other neat things I worked out with this ECU is that on the fuel chart at say 5500rpm and 0% throttle, which is impossible unless you're coasting or coming down from a rev up, you can lean the fuel ratio right out, and save fuel, and also give the injectors a rest. Same with the ignition chart I guess, you could advance or retard it a lot in that zone where you're coasting, putting negative load on the motor. I'm not sure if you'd advance it or retard it though. I'm thinking retard it, as that goes with the emmissions thinking, and might even aid engine braking. I'm not sure though.

 

[WORTH]

Well if I actually DID get to do the custom EFI and EDIS thing, I'd build a timing curve, do a lot of recording of knock sensor feedback and rpm and map sensor feedback, and tune the timing so the knock sensor ends up doing nothing. Then I'd take it to a dyno and see if I advanced or retarded the whole chart if I would gain or lose power in some rpm zones, and adjust the chart from there. I'm one who thinks that max advance isn't always the best, you might make more power having less negative torque, or at least use less fuel getting the same power.

I guess I'd have to think about introducing some nitro powder into the fuel to see if we could get the highest pressure spike right around 90 ATDC. I doubt I'd test that in a motor worthy of the fuel injection and electronic spark though hahaha.

Oh, one of the other neat things I worked out with this ECU is that on the fuel chart at say 5500rpm and 0% throttle, which is impossible unless you're coasting or coming down from a rev up, you can lean the fuel ratio right out, and save fuel, and also give the injectors a rest. Same with the ignition chart I guess, you could advance or retard it a lot in that zone where you're coasting, putting negative load on the motor. I'm not sure if you'd advance it or retard it though. I'm thinking retard it, as that goes with the emmissions thinking, and might even aid engine braking. I'm not sure though.

If you shut off the fuel, you don't care about the spark because there is nothing to burn.

OH, and you better turn the fuel back on at low rpm or it'll die when you come to a stop.

 

[likesoldjunk]

2 Points that I didn't see addressed...

You use ported vacum, otherwise you'd get maximum advance at idle which would not
be good

You're on the right track by focusing on the
total advance rather than initial.
If you run into problems with not enough
vacume at idle, you may need to replace the
reluctor arms in the distributer. I wound up
ripping apart a dozen ot so distributers in the
junkyard to get an assortment.

 

[Ray@VSK]

Hey is there some better way of setting timing other than just pulling the plug & tapping the distibuter while @ idle to get it where it's supposed to be then put the chip back in or am I supposed to be adjusting the timing @ a higher rpm or something ?

 

[WORTH]

Hey is there some better way of setting timing other than just pulling the plug & tapping the distibuter while @ idle to get it where it's supposed to be then put the chip back in or am I supposed to be adjusting the timing @ a higher rpm or something ?

Maybe it's because I haven't had my Morgan & Coke yet, but that went right over my head

 

[Route666]

Ray, I'm not sure about stock computers, my guess is there isn't any way to do it short of getting a custom chip done. Turning the distributor advances or retards initial timing and thus overall timing, but it also moves the position of the rotor wrt the cap, and so if turned too much can make the gap between the rotor and cap electrode too big. Or so I would think.

That's what I love about electronic controllers, you can adjust it by pressing buttons, no physical changes.

 

[Route666]

If you shut off the fuel, you don't care about the spark because there is nothing to burn.

OH, and you better turn the fuel back on at low rpm or it'll die when you come to a stop.

Shutting off would be better, but I think you can only lean out the mixture with this computer, with a EUGO you'd be able to genuinely lean it out to 20:1, if it doesn't detonate. I think I was playing with either the Motec or Haltech computer and it actually had an option for shutting fuel off on coasting, very nifty. However, I think you'd have smoother transitions on and off the throttle with leaning as opposed to shutting it off totally as it is more gradual, less of an on/off thing. It'd probably be amplified by more surging in the fuel system too.