Why is advance curve cut off at lower engine revs?

[jb007]

Hi all.

With mechanical distributors, the mechanical advance is usually limited at a lower engine RPM, well below the engines redline e.g. with my distributor, the mechanical advance is all in by around 3,000 RPM. Why do they not keep on advancing to higher RPM's? Is it because there is a mechanical limit to the advance weights/springs? If so why do a lot of MS advance curves also emulate this limit of advance?

Jim

[slow_hemi6]

Not taking avoiding pinging into account which should be obvious, At some point engines just stop responding to timing increases.
The good old dissy had a 2 dimensional curve for acceleration. Nearly all of them were curved for standard engines, which became a problem when emissions regulations started to bite. They attacked idle advance and reduced it significantly to reduce emissions at idle but this meant longer physical curves were required to reach good acceleration advance. Therefore if you had this type of dissy and increased the initial for a modified engine then they over advanced under acceleration. This led to people recurving with weights and shortening the slots. Features that then made it to the aftermarket dissys.
So engines reach a point where they stop responding to an increase in timing, but every engine is different as to where and when that will occur. Stock engines with lower redlines really don't need to advance on into rpms they were not designed to reach. Some people just emulate the old school "all in by" curves mostly because they still work well. People who do a lot of tuning on dynos usually sweep a range of timing for a given rpm/load and find optimal timing points. I have seen engines that respond well to a timing dip around peak torque and then increase again. If you look at factory maps for performance models you can see this occur in some of them.
I saw in a magazine article years ago where they compared a standard ecm timing map, a performance hei distributor timing curve translated into a timing map and an actual dyno tuned timing map for the car which was a fairly mild V8. The surprise in the article was 5kw gain over std for the dissy curve but a further 4kw gain for the dyno derived curve for a total of 140rwkw.
I see your in Australia, if you wanted to chase up that article its in Zoom No11.

[jb007]

Thanks Luke,

I'll look at that article later. I'm wondering if cylinder head design has a lot to do with the limitation or diminishing returns on advance? I'm currently not running the MS, but will be going back to it soon, but instead will be using a MAF. I'll have to get some time maybe on a dyno to set up the ignition curve to suit the system properly. Interestingly, since writing the original post this morning, I have discovered the Lucas electronic dissy, has not been advancing at all, and was stuck at around 0-2 BTDC! No wonder it was flat spotting and running hot! Pulled it apart and found no problems, put it back and all's okay now... go figure!

[turbo conversion]

Something else you will notice on dyno tuned engines is smaller bore and stroke engines require less timing than
larger bore and stroke engines in the higher rpm range.

David

[rickb794]

Combustion chamber shape and size determines optimal spark advance. Many gas motors will make peak power around 38 degrees WOT. But at part throttle will like more as much as ~44 degrees. Turbo applications want less (significantly) than 38 due to increased cylinder pressures over 100kpa MAP.

[rickb794]

So at part throttle low kpa in your cruising rpm range you can play up the advance. Carefully!

[rickb794]

At lower rpm (especially manual trans) high advance #s will buck or surge. You can bump advance under the target idle rpm a degree or 2 to help stabilize idle.

[matthunter1448]

Thanks Luke,

I'll look at that article later. I'm wondering if cylinder head design has a lot to do with the limitation or diminishing returns on advance? I'm currently not running the MS, but will be going back to it soon, but instead will be using a MAF. I'll have to get some time maybe on a dyno to set up the ignition curve to suit the system properly. Interestingly, since writing the original post this morning, I have discovered the Lucas electronic dissy, has not been advancing at all, and was stuck at around 0-2 BTDC! No wonder it was flat spotting and running hot! Pulled it apart and found no problems, put it back and all's okay now... go figure!

Yay! The mini will live again on squirt.

[jb007]

Thanks Luke,

I'll look at that article later. I'm wondering if cylinder head design has a lot to do with the limitation or diminishing returns on advance? I'm currently not running the MS, but will be going back to it soon, but instead will be using a MAF. I'll have to get some time maybe on a dyno to set up the ignition curve to suit the system properly. Interestingly, since writing the original post this morning, I have discovered the Lucas electronic dissy, has not been advancing at all, and was stuck at around 0-2 BTDC! No wonder it was flat spotting and running hot! Pulled it apart and found no problems, put it back and all's okay now... go figure!

Yay! The mini will live again on squirt.

Yep Matt, hopefully I'll start soon on re-instating the squirt! This time I have to fabricate ducting etc for the MAF, so I'm trying to get some ideas... everything on a Mini is so small. I was going to anonouce this on my blog, but it appears it's down... can't see it from Oz... I'll check in the morning.

Jim

[Paul_VR6]

Combustion chamber shape and size determines optimal spark advance. Many gas motors will make peak power around 38 degrees WOT. But at part throttle will like more as much as ~44 degrees. Turbo applications want less (significantly) than 38 due to increased cylinder pressures over 100kpa MAP.

On small bore engines with high compression and efficient chambers, you will want a lot (a lot!!) less. I usually start tuning wot at 22-25deg and work from there on pump gas.

[jb007]

Combustion chamber shape and size determines optimal spark advance. Many gas motors will make peak power around 38 degrees WOT. But at part throttle will like more as much as ~44 degrees. Turbo applications want less (significantly) than 38 due to increased cylinder pressures over 100kpa MAP.

On small bore engines with high compression and efficient chambers, you will want a lot (a lot!!) less. I usually start tuning wot at 22-25deg and work from there on pump gas.

A book I have on ignition systems (Des Hamil?) states that the cylinder head combustion chamber shape used by the A series and B series (MGB etc) usually have max ignition advance of 32-34 BTDC and advance all in by 3,000 RPM. My problem with the current dizzy is the vacuum advance, it's to aggressive in the 2-3500 RPM range.

Jim

[Paul_VR6]

A perfect reason to start using spark control! I had similar problems in prepped vw race engines, but we would delete the vac advance. Not practical on a street car though.

[jb007]

A perfect reason to start using spark control! I had similar problems in prepped vw race engines, but we would delete the vac advance. Not practical on a street car though.

Your'e right... I might put the MS3 in just running ignition whilst I sort the MAF stuff out... at least I'll be able to use the car whilst pondering the MAF ducting!

[subwoofer]

Combustion chamber shape and size determines optimal spark advance. Many gas motors will make peak power around 38 degrees WOT. But at part throttle will like more as much as ~44 degrees. Turbo applications want less (significantly) than 38 due to increased cylinder pressures over 100kpa MAP.

On small bore engines with high compression and efficient chambers, you will want a lot (a lot!!) less. I usually start tuning wot at 22-25deg and work from there on pump gas.

Not just limited to small bore cylinders, the Lotus 907 2-litre has 95mm bore and 69mm stroke, max advance is 28°BTDC at WOT. I believe later multivalve chamber may require even less. The good thing about a fast burning chamber is that it usually leaves a greater margin against detonation than a slower burning one, you can actually over-advance and only get poor performance, not a meltdown.