I've been using negative split and about 16:1 afr when cruising for a while now; since I switched to MS2 about 8 months ago. No problems with temps creeping up, smoothness or drivability, and I've seen up to 27 mpg (S4 turbo-swapped 'vert, so heavy, less aerodynamic car with lower compression ratio) on the highway over a tank, with avgs about 22-24 mpg with mostly highway.

Using negative split allows you to fire the trailing plug first, and in situations where timing needs to be very advanced (such as low load, lean, high engine speed since the mixture is less reactive), the leading spark will not be within the bowl in the rotor, so any flame it starts will be largely quenched by the rotor face and housing, having no room to expand into the chamber. However, the trailing plug will be within the bowl, so the flame front from it's spark can travel further and begin to engulf more of the mixture. Firing the leading spark later (so negative split, once it is also within the bowl) will allow it to be most effective, while still getting the more advanced timing necessary for the aforementioned conditions.

 

This is good stuff - what kind of workable numbers are we looking at here?

 

I'm running up to -18 at very low load in my table...

There are parts of my table I still need to run a bit rich even with negative split such as low revs low load for freerevving without misfiring, but even there using negative split has helped. Previously those parts of the table (below about 1500 rpm freerevving) would misfire occasionally even with everything tuned the best I could get it. I no longer get any misfires there.

I'm also running a lot leaner on cruise without any misfires or lean surges like I would've previously had.

Finally, I'm even running about -5 degrees split on idle, and it helped smooth out idle a lot.

I plan on allowing a lot more negative split than currently in the very near future as well. Right now the limit is pretty arbitrary and very easy to change, and as far as I can tell making the limit allow more negative split would not cause any problems.

Ken

 

It's good to see more people using it and learning what can be done! This makes me happy! I'd love to see the maps you guys have come up with to this point. It would be interesting to see how they differ and who is trying things that others aren't and to what extent. You guys will be shocked to learn just how negative you can go! Of course those limits are also determined by compression ratio.

 

god, can you expand on the effect of compression on split? Is it the smaller/shallower rotor bowl on the higher comp setups that affects this?

 

Higher compression can't run as much total negative split as lower compression can. A 10:1 compression motor may only be able to get away with 25 degrees or so of max negative split which an 8.5:1 motor could run upwards of 45 degrees negative. Keep in mind these are at the lowest loads and as load increases, nagative split decreases. My philosophy towards tuning is basically nagative pressure (vacuum), negative split. Positive pressure (boost), positive split.

I'm curious to see how those who are trying it have done their base leading timing. Have you guys backed it down at all? Basically you should start tuning by setting leading timing only as you always would. Right down to total advance. Then go back and set split.

 

I more or less left my base timing table alone except for below about 1100 rpm, where I reduced the timing a bit.

I'm not sure my base table is great though since I've never gotten it on a dyno and tried to tune it in any kind of scientific manner. It probably isn't horrible, but it could probably be better.

There were times where I was running up to 17:1 at cruise recently with no difference in the way the engine ran compared to 15:1 a couple of days ago. I'm pretty impressed.

Once I get ms3's sequential staging and rotary sequential staging code done, I'm looking forward to seeing what negative split plus properly timed injection will do for my fuel economy and low-rev response.

Ken

 

Thanks for posting the info about negative split, gentlemen.


Until recently, leading timing was set to about 10 degrees advance at idle, with the trailing injectors firing about 10-15 degrees after. After changing the split only (trailing plugs firing about 5 degrees before the leading plugs), the idle seems smoother. Removing a little fuel didn't have the adverse effects it usually does, but I put it back in (change only one thing at a time for now). After a few more days of testing I'll see if I can remove some fuel without upsetting the idle.

 

That's a step in the right direction.

On a 2nd gen I'm in the middle of tuning, I decided to try negative split in vacuum. At idle, it's running a lead advance of about 13*BTDC (it's an 8.5:1 4-port turbo motor) with a trail split of about -5 to -6* in that spot. It works fine and seems to have smoothened the low-end out somewhat. Basically the way I've got the negative split setup is at 30" of vac it's set to -13* across the board tapering down to zero split by 0" vac. I'd advise folks to give his a whirl. It's also able to run a bit leaner mixtures and therefore have a marginally better fuel mileage while cruising around town.

B

 

BDC again Thanks! I'm JC wrote on your wall last week.

Idle is higher now using the same old fuel maps.

I'll be going to the dyno soon. My car is running your split and advance timing maps as of one week now. Needless to say it is running great, just need to tune at the dyno for fuel. 690cc and 1600cc combo with a 60-1 turbo set at only 8psi for the moment.

 

This was a great thread, any updated information on this subject? also, the links aren't working in the first post.

 

I agree Keith, This is interesting. I would like to know in addtion to the OG posters whom may no longer even be members, has others has any contributing split timing info to add?

 

I ran negative split in vacuum for a few years and liked it. I had a pretty large street port that had a lot of overlap. The negative split seemed to help drivability under light throttle/high vacuum. More fuel also helped with that too, but it never went away completely. When cresting a hill and backing off the throttle it would still get a little jerky. I guess that's the price you pay to have a cool sounding idle and more power :/

 

What kind of table were you guys running for negative split? I attempted to slowly road tune mine and get it working but I always seemed to choke the motor before it would be happy even at negative split numbers.