It's actually not that difficult. The hardest part is getting it through your head that it may actually not blow up! The general rule is negative pressure (anywhere in vacuum), negative split. Positive pressure (boost), positive split. Leading timing really isn't touched. Leave leading advance alone. You push trailing ahead of it. This sounds like it would be WAY advanced at low loads and rpms but you also have to remember each plug location. It would get very long and technical trying to explain why it works. So here goes...

A simple way to begin to understand it is to look at flame speed of gasoline. We advance timing because from the time we start to ignite the fuel to the time there is max chamber pressure, the engine has rotated. The only way we could ignite fuel at TDC and get good power would be if all of our fuel instantaneously exploded when ignited rather than through a travelling wave. This doesn't happen. In essence we let the flame front catch up to the engine.

There is a problem with this. Actually multiple. First is that flame speed is not constant. It varies with chamber pressure or as an easier way to look at it, load. However we must also play the balancing act of avoiding detonation. As chamber pressure rises, heat also does. You can reach autoignition temperature before your chamber pressure maxes out. Of course if this happens you will get spontaneous ignition locations. If this spontaneous ignition happens before the spark plug fires, it is preignition. If the plug has fired and the flame front is travelling through the chamber but other locations ahead or independent of the travelling flame front ignite, you get detonation. Both are bad.

To take care of this we back a bit of the total timing off. The controls those issues but we may still have the side effect that we weren't getting as much total timing as we needed to take full advantage of the expansion of the burning fuel. This is why higher octane gasoline can run more advance. It has nothing to do with flame speed. They burn at the same rate. The resistance to detonation is better with higher octane which allows us to get that little extra bit of time to use the flame travel to our benefit. It also gives us that little extra bit of time to burn it off cleaner in case anyone was wondering why so many cars today recommend higher octane fuel.

Here's the thing about negative split though. With conventional firing, we have the plug firing ahead of the bulk of the oncoming air/fuel mixture and it leaves a lot of unburned fuel behind it. This is why we typically fire the trailing later. We want to burn it off. This is also a reason for the higher emissions. However by firing the trailing plug first, we are firing the plug that is in the main concentration of the air/fuel mixture. This only works well at low loads though as due to lower pressure, flame speed is slower. It allows us to use more of what we put in which means we can pull fuel out without too many negative side effects. As rpm's and loads rise, we need to transition back to standard split due to the time available to ignite it all and for detonation resistance.

There are people that are scared about running lean but they shouldn't be. An a/f gauge actually doesn't tell you the whole story. It tells you what the a/f ratio is at the guage, not at the ignition source. I'm going to make up some arbitrary numbers here for example's sake so don't try to deduce the accuracy of them. It's just an example. Let's say that we have an amount of air represented by the number 10 entering the engine. Let's say we have 5 fuel entering. After conventional ignition we didn't quite burn off all of the fuel so now we have 2 fuel left to be read by the O2 sensor. We look at this number and think all is well and normal. Now lets say we ran a more efficient timing that better utilized what we put in. Let's say we still have 10 air entering and only 3 fuel entering but through better combustion we have 0 fuel left to reach the O2 sensor. Notice in both cases we burned the exact same amount of fuel and made the exact same amount of power. There was just less waste in one case which is going to throw off the O2 sensor readings. It doesn't matter though. The same energy was released, the same amount of power was made, and the same heat was rejected through the exhaust, water, and oil, even though less fuel was used and it reads lean.

An interesting thing is that going leaner and leaner doesn't always mean hotter and hotter. At some point you start to get cooler again. Don't believe me? What is the temperature of intake air before you add fuel? Conversely if you go richer and richer it won't keep getting cooler. The range that we run in is a temperature valley that is between two temperature peaks. However passing these peaks on either side reduces temperatures again. With negative split you can hit 17:1 or more af ratios in cruise and not worry about excessive exhaust temps for the above reasons.

Here's another nice side effect. If you look at a chart that shows emissions based on a/f ratios, you see that 14.7:1 is the balance point that gives the best average emissions among the 3 big contributors. Going richer or leaner may benefit one or 2 of them, but the 3rd will get higher. But is it really the best a/f ratio for emissions? No! 17.5:1 is a cleaner a/f ratio and negative split when done properly can get you to this range during cruise. The problem has always been controlling flame speed enough to get you this lean burn. The rotary possesses this ability now through negative split and has always had this ability. The piston engine is still attempting to catch up but it is doing so through very complex means such as higher and higher fuel pressures and direct injection. Keep in mind DI would also help the rotary too and if combined with negative split could really be what the engine needs to stay viable. Think 16X.

Hopefully that explains things a bit.

Cool!

One thing that I forgot to mention when tuning for negative split is that you really do need to think backwards. If you start to advance the trailing ahead of the leading and get to a point where power starts falling off or it starts running rougher, your natural inclination will be to add fuel. Don't. Remove some.

I remember reading this thread long ago while my car was being built, and totally forgot to try it out. I will be playing with this tonight. Unfortunately my m400 limits to -10* split. Hopefully it will be plenty to see some results.

That's an excellent post.

I'm really surprised Mazda never incorporated direct injection for the primary injectors on the RX8 engines. I have toyed with the idea of milling my housings to accept primary injectors. It's really simple to do and the fuel injectors would not get hot, like a typical direct injection setup on a piston engine would see. Once again, the R&D of the rotary has had a pinch of what the piston R&D has seen over the decades. :(

very neat info once again rotary god.

Negative split in vacuum works, folks. Been doing it for about 9 months now and should be part of all of my base maps. It ain't gonna blow nothin' up and it makes the low-end a lot more crisp. Plus it appears we can run leaner mixtures around town. Definite improvement over zero split.

B

They've been playing with direct injection in some form or function for the past 25 years. It's actually not quite as easy as it sounds. The farther around the engine you get towards the spark plugs, the more complicated things get. Once you pass the top of the housing, you really need to start getting into very high fuel pressures with specialized injectors. Timing and injector aim also gets very critical and the shape of the housing plays havoc with DI as it is really hard to take advantage of all the air in the chamber by injecting the fuel at a certain small spot. This is one reason why the 16X as well as the hydrogen engines have their injectors straight up top. It is a balance. There are other issues with that location though, namely blowby past the apex seals from one chamber to another as they cross that hole. That's why the trailing spark plug hole is so small btw.

Well I played with it a bit over the weekend, definitely some interesting stuff going on there. I was able to idle my 1/2 bridge motor cleanly at about 1.1 lambda with -10 split. However, being a bridgeport, I really can't take full advantage of it because it idles at ~75% efficiency, and I can't get good enough resolution to add enough fuel back in and get the split back up as load increases, so it wants to just fall on its face. I'll definitely keep screwing with it and hopefully see a decent increase in gas mileage..

I've always wondered if this effect is what causes detonation in rotaries when boosting. Except with the leading plug. Could the increased combustion pressures lead to pre-ingnition as the apex seal crosses the leading plug hole? Which is large. Leading to lighting the following camber?

Here's an arbitrary question. Why isn't the leading plug closer to flush to reduce the gap? I know why it's not flush but, just why not closer?

I guess the same could be asked about both plugs. Except I always thought the trailing needed the volume to blast through the hole. Kinda like a pre-chamber.

I would love to know the difference in mileage, and also with emissions.

BDC or anyone have any approximate figures?

No idea on either but theoretically it'd be measurable.

B

Im all for customers numbers. Can you ask anyone for mileage/gal at fillup numbers? Im really curious here. Theres a big difference between "good for an rx7" 20 mpg highway and a "good all around" 30 mpg.

On a personal note, I keep track of my average mileage every time I fill up, its just a matter of resetting the trip meter every tank of gas and seeing how much gas it took to refill the tank.

Can the pfc do negative split?

The location of the leading plug is at a place where as the apex seal crosses over the hole, the pressure from each chamber adjacent to it is roughly equal. This means there really isn't any blowby at that location. The trailing plug however has a very large pressure differential on either side of the seal at that point. If the trailing plug hole were the size of the leadings, there would be a very drastic amount of blowby.

It would be best to get the plug as close as possible to the apex seals. As close as possible withouth crashing into them. Why they didn't is anyone's guess. I run modified motorcycle plugs and mine do sit a bit closer.

Yep.

I forgot to mention that your static compression ratio ultimately affects how much total negative split you can run. The lower your compression, the more you can run. The higher the less. This is really no different than what is going on as you reduce negative split as vacuum decreases in the engine though as effective compression ratio is changing. As load increases, negative split needs to decrease.

So you guys would be idling it at negative split also ... didn't think you would be doing that also. So if I'm going from to try this out, all my high vaccum would be in say -15* split (range of about 45 kpa), then as I get to about 70kpa, have it down to around -5 and then by WOT I would be at +0.5 thats where I'm making the most power out of my engine. This is an NA engine by the way.

When do you think it would be obvious that you're running too much negative split (i.e. trailing advance) under vac? Does it feel like an obvious loss in power?

You can run negative at idle but it's not real necessary. If you can do it and can pull some fuel out then go for it. Provided of course everything isn't running too hot.

The only advice I can give for going negative is to go slowly. Don't try to push it too far until you really get used to it and know what you are doing. Depending on the compression ratio you have and the load/rpm you are at, it is possible to go up to -25* advance or so for high compression engines and over -40* for low compression. Again, don't just assume this. Take it slow and learn what it is doing and how to keep it happy.

So why is this only a good idea in vacuum? What about boost makes negative split a dangerous idea?

My guess is that the neg split in boost is bad because its like running with more advance. When boosted there is a much greater chance of detonation.


On a side note I tried this on my car today, my wideband crapped out recently so I only played around conservatively. I ran -5 deg at idle and tapered down to 0 split over load and RPM. Light throttle seemed better on the but dyno, no clue about my AFR though. I was, however, able to lean out my idle by a huge amount. Which is awesome as my car wanted to idle super rich before the negative split.

The higher the pressure, the faster the flame travel but the bigger problem is really that you are starting closer to the detonation threshold due to increased temperatures and pressures. You'll hit either detonation or preignition as a result. I wouldn't even run negative split on an n/a at full throttle at high rpms.

Okay I finally hooked up my wideband.

Pre neg split I was running around 11 afr. Post neg split its running around 12.5 afr. Again thats at -5.

The second thing I have noticed is my car no longer bucks at low rpm. This was a problem after I installed a lightweight flywheel, but its all but gone now. One hesitation at 1800 rpm and then smooth sailing.

are e6x's? capable of neg. split?