Advance limits?

[rmaddock]

Brent,

I've been contacted by a fellow Land Rover owner who's installing Megajolt and he's spotted a problem....not a bug as such.

It's that the upper limit for advance values has dropped from 59 to 49. The map which I devised for the 2.25l includes advances up to 60. I was happy to settle for the max 59 but 49's a long way short.

Is 59/60 just a silly number? Am I wrong? You can see my workings <a href="http://otly.mysite.orange.co.uk/Megajol ... l">here</a>.

Cheers!

Robert.

[MartinM]

Hi Robert

A few thoughts from your web page (thanks for mentioning me!)

Vacuum advance starts at 102mm HG is certainly a bit confusing, and must mean starts at 102mm Hg <b>below</b> atmospheric - so about 86kPA, as you derive.

But I wonder if this is what the engine designers really wanted it to be, or just what a typical low-tech vacuum advance physical design (ie a diaphragm pulling on a sprung plate) actually gives in practice due to stiction, not being able to get springs that give the right advance a high kPA as well as low kPA etc etc - and thus the manuals just indicate what a technician might measure in the workshop.

I'm no engine designer by a long, long way, but I can see why an engine shouldn't start to benefit from a vacuum advance as soon as the manifold pressure drops below 100kPA - why wait until 86kPA? So I don't know if you drew a straight line on a graph to get the intermediate vacuum advance points and had 0 degrees at 86kPA - but I'd have it at 100 (OK 101.3!)kPA. That will change your map a bit.

The basic equations for the SAW pulse (http://picasso.org/mjlj/?q=node/2) show 57.5 as the max advance - I too spotted that Configurator V3.0.0 stops at 49, but forgot to mention it.

However......I don't think you can get max vacuum advance at high rpm (the latter implies full throttle, which means butterfly rending to fully open, which means not much vacuum in the manifold) so your advances in the 50's are probably in an area of the map that never gets visited. It would be good to know if anyone's observed these parts of the maps being used.

My RR engine tuner (competition engines mind, where we're talking 2000cc engines producing 250bhp at 8500rpm - runs very nicely on the MJLJ!) tells me that vacuum advance tails off to zero around 3500rpm, so his MJLJ maps have the same figures on every row above that rpm. Don't know what the equivalent figures for a Land Rover are though

Shame you can't find any info on the absolute maximum advance that's ever likely to be encountered under real operating conditions...

Finally, the "Maximum vacuum advance is 12deg at 457mmHg" is open to interpretation. Is it 457 absolute or 457 below atmospheric? Either way I think your sums are wrong...
"Thus for the map, advance starts at 86 and maxes out at 29."
should surely be
"Thus for the map, advance starts at 86 and maxes out at 39." (since 100-61 is 39!)
...but then taking...
"Test runs show that MAP pressure ranges between 21 and 101"
...into account makes my head hurt!!!!

Regards
Martin

[brentp]

Robert,

The 49 degree max value was an arbitrary limit in the software, which was further exposed when I added (or rather, re-enabled) enabled some data validation code. The controller does indeed support advances from 0 to 59.

I'm posting a new version of the software which fixes this!

Regards,

[MartinM]

However......I don't think you can get max vacuum advance at high rpm (the latter implies full throttle, which means butterfly rending to fully open, which means not much vacuum in the manifold) so your advances in the 50's are probably in an area of the map that never gets visited. It would be good to know if anyone's observed these parts of the maps being used.

I suppose lifting right off the throttle when it's buried in the carpet and the engine is close to the rev limit might get you somewhere near this part of the map...

[4600cc]

Either that, or upshifting, such as with auto tranny. You see the "tracker" on runtime display go all the way down and to the right as RPM pick up, but then as you go through the gear box, "tracker" goes up towards MAP bin #1, vacuum, so those values get used at some point, especially those bellow 4k rpms.

Id like to know what is the full range of timing EDIS unit can support. Im sure there is negative timing, positive timing even past 59*, and perhaps tenth and hundredth of a degree? I saw timing expressed in ##.#### in OEM ford maps. Id like to see if this is possible to implement, as it will be usedful for traction control, launch control, and rapid acceleration features, along with a bigger map.

[brentp]

Robert,

A new version is available which corrects the 49 degree limit. You can download it in the usual place.

Thanks,

[rmaddock]

Thanks for the new version Brent.

What service eh! Cool.

Thanks also to MartinM for finding the intentional mistake in my workings. I've been waiting months to see if you guys would spot it I'll have to mend it now. As to your other comments, I was only trying to simulate the dizzy with this first map as that seemed like a safe place to start. I agree completely that this is probably not what the engine designers would have liked in a perfect world. When I have time I'll get it sorted properly. I believe that a new 4x4 dyno facility has opened up recently in Carnforth...not a million miles from here.

Cheers all!

Robert.
________________________________

http://otly.mysite.orange.co.uk/

[4600cc]

Chapette??

[rmaddock]

Pardon?
________________________________

http://otly.mysite.orange.co.uk/

[4600cc]

On your site you calling me chapette. Check your mail.

[rmaddock]

Dear <b>MR</b> 4600cc

I have corrected the offending page. I apologise unreservedly and abjectly for any offence caused. Grovel, grovel.

When I wrote that page you were just an anonymous user name and I'm not one to jump to sexist assumptions. Just because you've a big capacity doesn't necessarily make you a chap.

Merry Christmas!

Roberta. [sic]
________________________________

http://otly.mysite.orange.co.uk/

[4600cc]

Hahaha, just when I was going to say find me a girl who does this stuff, your name turns out to be Roberta. ) Ok, I forgive you this time. Look for a brand new tuner early next year! Guaranteed to make Megajolt even more popular.

[Edwardatherton]

But I wonder if this is what the engine designers really wanted it to be, or just what a typical low-tech vacuum advance physical design (ie a diaphragm pulling on a sprung plate) actually gives in practice due to stiction

"Thus for the map, advance starts at 86 and maxes out at 39." (since 100-61 is 39!)
...but then taking...
"Test runs show that MAP pressure ranges between 21 and 101"

So the manual says advance is (in theory? or practice? we don't actually know) between 86-39.
Testing with the engine running shows between 101-21.

Therefore Martin's quote at the top is right - we should go with the practice rather than the theory and map between 101-21. Also, mapping up to a limit of, say, 3500 rpm (for arguments sake) the levelling off - this would give more lower-end and less top-end advance.

[Edwardatherton]

I have just remapped at work with my ideas from above, and it is 100% better. Less popping on overrun, no frightening old ladies when backfiring, and pulls much better.
Just got home and tweaked a few figures, so we'll see how she goes in the morning. Only trouble with a Land Rover is that the weather makes such a huge difference - windy weather can knock 5-10mph off the speed easily!