negative split
 

ok i had negative split in my head awhile back i thought it was just silly, but then i heard of a forum member actually running it under vacuum on his FD. i won't mention his name, he can chime in if he wants.

anyways, someone want to explain to me any POSSIBLE reason you would want to run negative split, and even how the hell that works, lol.

i mean, what EMS even allows you to purposely run negative split?


- Aaron

https://www.rx7club.com/search.php?searchid=956921

Come on...you've been around long enough to search.
Oh wait....


http://railgun69.homeip.net/menu-search.jpg

what the hell are you talking about??
give me one thread in your search that talks about NEGATIVE split, other than the one thread on the guy who running his FD with it under vacuum.

get that god damn search pic out of my thread.

You never miss a chance with that button do you Mr.


http://railgun69.homeip.net/menu-search.jpg

lol, this is the "rotary performance" section? i feel like i'm in the freaking lounge.

come on guys, just because you don't know anything about the topic doesn't let you post that stupid picture.

https://www.rx7club.com/showthread.p...negative+split

Post # 3. There's 1 reference. Ask him perhaps?

Maybe YOU should take the hint and not be so defensive as I don't blindly post this just for my own amusement and health.

You only asked for one...shall I go on? :rolleyes: And you didn't specify which thread you were talking about. But since it was there....oh, I dunno. BRING IT UP INSTEAD OF MAKING A NEW THREAD? :p:

Sean...yeah. Can't get enough. :D

PFC can run neg split. You can still detonate under vac, so I can't see why anyone would want to.

hey, i just take offense to seeing that search button! lol.

honestly though, it's not like i'm asking "will these tires fit on my rims".
there has been not been much discussion on this issue at all, and i would like to get a thread on it. so it's not just for MY benefit, but for others. after all, we are supposed to learn here, not just tell everyone else to figure it out on their own...
besides posting it in any of those threads would be going off topic really...

sure, there has been plenty of discussion on running ZERO split, but no one ever talks about NEGATIVE. all i ever hear is that a lot of pro tuners are running it in vacuum, i just want to understand why...

well, thanks for the info on the Power FC running neg. split. i know the Haltech E6 can't, don't know about the e11...

I'll chime in :-) The "short" version of it is the creation of a pressure wave which pushes a majority of the unburned fuel towards the leading spark creating a bigger, hotter (your EGT's will go up) better bang. Think of the way the rotary "sweeps" the air and fuel and where that air/fuel is residing when the leading plug fires. Yep, at the wrooonng end of the combustion chamber. By firing the trailing first, you're pushing the mixture to the other end before the chamber quench becomes so small it inhibits movement. And yes, I run negative split, daily driven on pump gas ONLY under vacuum.
Michel

Is there a noticable difference in fuel economy or off boost power?

Why is everyone on this forum so intent on shutting down threads with a massive search button. Sure this topic has been covered before, and i searched for more information on timing split, but you get a huge list of threads, maybe if you sifted through all of them you could get closer to understanding how it works. But whats wrong with having a thread all about this one topic?
I have to admit that when i first got into rotaries i was puzzled why the leading plug was on the bottom and fired first, because it would send the flamefront the opposite direction from the spinning motion of the rotor.
What your saying certainly makes sense, but when the trailing plug fires on the front rotor early, wouldnt that pre-ignite the mixture on the rear rotor?

Yes it does, partially....and that's how you get the push in the other direction. I do get excellent gas mileage during steady state cruising. I do run her fairly lean on the highway, in the 15's. The whole point of running neg split is to increase the EGT's and get INTO boost sooner than normal. I do this partially because I run lower compression rotors than the stock 93+ FD rotors. This helps eliminate any of the laggy feel provided by the low comp rotors and large ports. I haven't hooked up the prototype secondary butterfly controller yet, keeps the secondaries closed until a preset boost level, say 3psi, then slams them open. Both are designed to boost the bottom end. Currently, I have the secondary (double throttle) hooked up but it opens fully at WOT even under vacuum. Once I get the solenoid hooked up and working right, I'll be able to set it at a predetermined point.

On the search note, there's never been a thread specifically about neg split. It's buried in my CLR Motorsports engine post which is pages long. Not easy to find! So it's good we get a "negative split" thread going.

I think i see where you are going with this. You keep the neg. split to get some good exhaust velocity before spool up on a turbo car. I know the F-con v-pro has individual timing maps for leading and trailing, also the emanage ultimate is supposed to have this capability as well. Where would the benefits be (if any) for an n/a car?

Dude,

If You detonate while in vacum, ?? ........You have OTHER MAJOR ISSUES.....

This is true I know nothing of rotaries or how they work. Now due to me being late to the party I have no need to go into it since Michel is here to cover for me :D

Michel why not run a simple pressure switch type thingy on the secondaries very cheap and see how you like it first. If in need let me know.

thanks a lot for the input Michel!

now finally this thread is getting some good tech info :)

Michel, what EGTs are we talking about while you are running neg split? just wonder how much hotter temps you are achieving through this.

I believe there are benefits to running neg split in vacuum too but I'm not well versed on n/a rotary engines (we're talking pro-level racing engines). It may help with torque production low in the rev range.

I haven't done too much experimentation back and forth between neg and positive split to be honest...I broke the engine in with a "normal" map and then switched over to the neg split. Ran into a few issues and lately I've been waiting for the local dyno to get up and running again so I haven't done any real back to back testing between the two. I've mostly been preoccupied with gas mileage and messing with a/f ratio. I do have to mess with cruising timing and fuel, especially in the 70-85mph range as my egt's are a bit too hot (800C) for my liking. It's not causing any cooling issues however. I'm still waiting on the new front bumper and thus am running no splash guard. That's usually a big no no but the car runs below 93C on the highway no matter what the speed. And it's friggin hot down here at the moment. I'll guestimate I'm seeing 100C hotter EGT's at steady state cruise. Acclerating, it's too hard to tell as the car gets into boost very quickly.

Sean, got your back! Anyway, I do have a simple Hobbs switch in there now which is apparently adjustable. Comes preset at 4psi. Problem is, when you "adjust" it, there's no way of knowing what you've adjusted it to. Another forum member who lives in Miami gave me a head's up on a small digital pressure switch and was even going loan me one. It's remote, with the solenoid in the engine bay and the digital switch run into the cabin. You can adjust in .2psi increments and can do so on the fly. Apparently they're only $50 or so. I'm just so busy as of late I haven't had much time for anything besides work, home and family. I'll get my act together and start getting some test data and dyno numbers.

a hobbs switch huh, maybe i should sue for using my last name in that manner.lol
I was thinking about this some more. Does your split get larger as load increases? Or is there a certain point where the difference becomes unnoticeable? Also, for n/a engines having a close split at low rpms might help torque production on ported engines to get the exhaust out quicker and get the charge air in. Of course you would be throwin quite a bit of raw fuel out the exhaust, but im in the mood for a light show.lol

There is a simple way to test it, as I did this long ago.

Hook up any adjustable low pressure souce to a "T" splitter. One outlet goes to the Hobbs which will have an ohm meter across it's switch to detect when it makes contact (closes), the other outlet goes to your boost gauge.

I'll let you figure out the rest.

LOL, dunno maybe there are some royalties in there somewhere if you're related in the right way. As for the split becoming larger, it starts out large and as you approach atmosphere, it zeros out before becoming positive under boost. Chuck, I'll try your experiment.

Had a talk with Carlos about neg split so I understand a bit better. I'll paraphrase a bit as he always has to put things in layman's terms with me :) Carlos, you need to hold rotary seminars in your free time (big pun there!).

Negative split and EGTs.

The Higher EGT's don't come fron the advanced trailing timing, but from leaner A/F ratios.

One can run much more complete combustion cycle with negative egt's and the leaner a/f ratios are a direct result of negative split. Negative split helps maintain the highly efficient conbustion and it is capable of sustaining super lean A/F's.

Without this negative split advanced (t) ignition timing, the car would simply buck and miss, while in trailing throttle or cruse; all due to not been able to maintain a stable combustion with these leaner than normal A/F's

The fact that the A/F is near stratification, it in is self, requires extremly early ignition, so that the flame kernel can propogate with enough time to catch up the leading portion of the chamber. ... Which in it self is always and constantly runing away from the flame front.

Therefore, the earlier one lights the fire, the more mixture the flame can consume before it is extinguished by the many factors that affect a Rotary engines combustion cycle.

These factors include but not limited to a) a very HI surface to volume ratio, b) cold regions of the "cylinder" walls, or side housings that are rarely exposed to high temperatures c) very hi "Gas Transfer Velocity " that takes place only on the trailing side of the combustion chamber just at BTDC. which in it self, poseses inherent characteristics of blowing out the spark, like blowing out a candle, d) the nesesity of running super cold spark plugs, due to a very low "Water jacket to surface contact ratio.
So on and so on. !!! .......I could go on for ever on this subject, but .........

Advancing the trailing timing will actually reduced EGT.
If You care to experiment. simply retard the (t) timing say 20 degrees behind the Leading (w/ out changing You'r A/F's) and whatch what happens. (I'm not going to try!)

Guaranteed, you will see your turbine housing glow in the dark while in the middle of the day ! LOL ! Try it.
You will also see Your turbocharger come apart in no time if You continue to drive for extended periods in this condition. (Ok, Carlos, I'll try it, but only if you agree to replace my turbo!) :)

This DOES promote a cleaner burn of spent unburned HC's .
But we are talking performance, not emissions here.

On the other hand, a less harmfull experiment WILL prove that the high EGT is mainly fuel related:
Keep Your ignition timing maps as they are, and simply adjust the fuel maps to a more traditional tune, say just below stoch. and only in tha Vac. side of map.
Guarantee You WILL see the EGt's DROP like a bomb, and fuel economy fall off as well.

Now with that in mind, a couple of notes from me. First, I get AMAZING fuel mileage for my setup on the highway. It's almost unbelieveable. My a/f ratios on cruise are in the high 15's, low 16's. I did add a bit of fuel the other day to bring me near the low 15's and my egt's dropped by about 50+ degrees.

I won't say I get very good gas mileage in town. Part of the problem is my excessively rich idle and part of the problem is my right foot. There are days I really try to baby the car, and it's tough, and the gas mileage seems to be acceptable. I have no overheating issues with the higher egt's on the highway. I've driven the car for hours straight without issue. Part of it is due to the "total package" philosophy of the motor and cooling system setup per Carlos' experience and recommendations.

I do run a fairly large turbo, large hot side, with a poorly designed manifold. I'm hoping to switch to an A-spec manifold here shortly which I'm sure will improve things a bit. It's not as laggy off boost as say a T78 and it's fairly close to a 35R in spool time (GT4088/82 hybrid plain bearing, 88 compressor with the 82's turbine on a .94 hot side housing) but I feel like I'm giving something away due to the manifold, midpipe muffler and the GReddy Power Extreme. I have a feeling switching to a higher flowing exhaust system and better manifold will make a world of difference. BUT, the good news is that if you're cruising and nail the throttle, there is no discernable lag. Granted I'm running around .95kg at the moment but it'll go from vacuum to full boost in less than a second if you're at 3K or above. The higher egt's already have that turbine spinning so when you nail it, the car just goes. Now if I just had traction...

I run B10EGV's, no fouling as of yet. I did just switch to a higher amp alternator and my idle quality has never been better. One would never know I have the phased rotors and a very large street port.

I'm going to double post this under the CLR Motorsports engine thread I started just to have it there....

Forgot to respond to this...I'm using port phased 1988 Turbo II rotors. What that means is they're of a lower compression ratio and heavier than the 93+ rotors. The benefits of using the rotors alone are reliability, ability to run more boost on pump gas and the castings are of a much higher quality. Now each rotor has been lightened to 93+ spec, balanced, and the port phasing (scallops) were added.

Normally, as I'm sure you can imagine, the lower comp rotors would hurt the engine's response to throttle inputs off boost, hurt acceleration due to the higher weight, etc...the negative split helps to undo that and I don't notice a bit of difference between the low comp rotors vs. the high comp 93 rotors. The car is very hard to stall and pulls cleanly from just off idle and revs with abandon. I have lots of tuning left to do on her but most people would be very, very satisfied with the car's response and power as it is now.

wow, some great info your last two posts! i think it has proved this thread worthwhile :)

it's good to see that some of these lesser known methods of rotary performance, including running those awesome "phased" rotors, is working well for you on the street.

most people are too busy trying to see how big of a port, or how big of a turbo/how much boost, they can run. there are a lot of other small details about these cars, and i think paying attention to those is worth a lot more.

- Aaron

I completely agree with you here...as they say, god is in the details. I will admit that I have very large street ports (heck they started out as Cosmo ports to begin with!) but the engine works fine, daily driven. One of the reasons I went with CLR was their reputation, their professionalism and overall knowledge. I mean they build pro-level race engines and really take the time to do things right. The knowledge alone that I've gained in knowing Carlos for a short few months has been invaluable. I've said it before and I'll say it again, if everyone paid attention to the details like CLR did in building rotary engines, I don't think turbocharged rotaries would have nearly the reputation they do for reliability issues. Certainly CLR isn't the only company out there but they're good. It also helps when you have an owner that takes things slowly. I'd like to be a bit further along in tuning and if I had more time to devote to it I would be. As is, I'm keeping things on the low until I can do them right. Carlos doesn't even think the engine will "come alive" until 15psi going by the internals :) It's fast enough now at .9kg to easily trounce a Z06 or any rice that comes my way. I'm not even in the engine's (or the turbo's) sweet spot yet. At 20psi, and assuming I can actually get traction, she'll be a complete beast. And reliable.

^ Mich's car is a beast, he got his car tuned at the last tuning session last spring when we brought down steve kan. I actually went off alot of his inspiration when building my new motor. If you ever have questions hes full of some great info.

btw, i am running a good amount of negative split on my car, runs awsome both street/vacuum driving as well as in boost.

That was the old motor...it's nothing compared to what I have now :) Same Cosmo block (new rear plate) but new housings, lapped plates, pretty much everything you can do. My receipt is around four pages long!

You aren't running neg split in boost are you? That's a big no no.
Michel

depends on where in the map you are, around 0-150 mmhg there is a small mount of neg split, after that there is none. Ya my new motor is pretty stout, pinned the block etc. Next rebuild im gona do some grinding on my rotors simular to clr has done to theirs.

At high vacuum, you should have a high negative split. As you trail towards atmospheric, (zero vacuum, zero boost) you should be at no split or very slight positive split. At no point, under boost, should you be running negative split. As soon as you're into boost, go to positive split.

FYI, it took years of experimentation to get the CLR rotors correct. He had an "imitator's" rotor, someone who tried to duplicate what he did to his rotors. It came out of a granaded motor. They made a very big and easy to do error in machining the rotors. I'd have CLR do your rotors and not do them yourself!

sounds like these guys at clr really know their shit. but if you have lower comp rotors, and have your ignition so finely tuned that you can hit full boost that quick, couldnt you benefit from a larger turbine housing? Great thread guys, learned a lot.

If you lug the engine at say 2000 rpm in third and just nail the throttle, full boost (I'm guessing 18-20psi) doesn't come on until around 4K rpm. It's when the engine is spinning and the egt's are already hot. Then there's virtually no lag. Doesn't feel turbocharged at all. Carlos thinks my hot side is too large now actually...I haven't even begun to discuss turbocharging with him yet, LOL. I think he believes in the small hot side, large wastegate theory, something I haven't really explored in depth. It seems like there are lots of people I've come into contact with in Florida that are using itsy bitsy hotsides and making 500 plus at the rear wheels.


CLR really does know their stuff...only recently have they taken on the lowly street guys like myself. In the past it was pro stuff only. In fact, they're working on a super top double secret project now! Of course I am sworn to secrecy! But Carlos has vast road racing experience including IMSA GTU stuff and most recently SCCA with his own car.

^hmm, i guess the neg. split wouldnt do much for a drag car then. since you dont really spend any time in vacuum, but for a street car its definately something i would look into.

Grinding ?? :confused:

So in effect you are using controlled detonation in the form of two flame fronts coliding in order to create enough cylinder pressure so that the engine can run under extreme lean conditions? How would firing the trailing plug early pre-ignite the other rotor? The leadings are waste spark not the trailing.

Two flame fronts /= detonation.

A normal burn cycle is smooth and controlled *burning*, propagating from the spark plug outwards. A common analogy is a grass fire set with a match in the center of a field - it more or less travels smoothly outwards. The flame heats up the fuel and oxygen immediately next to it, which burns, heats up the fuel and oxygen immediately next to that, etc.

Detonation is when everything *explodes* instead of burns. It's caused by too much heat and pressure, and everything goes off all at once. BANG. Your flame front is also expanding, and it is compressing the unburned air and fuel in the chamber... if it compresses it too much, it gets heated to the autoignition point (compressing a gas raises its temperature) and it all go boom.

You can have two flame fronts without detonation. Hell, all Mazda rotaries, technically, have two flame fronts, when leading and trailing ignitions are both active. Having two flame fronts just makes detonation more *possible* because the burn can happen too quickly and cause the excess pressure increase that results in detonation.

Rotary's main achilles heel is the burn happens too slowly, so anything we can do to make the burn occur more quickly is GOOD. Hence two or even three plugs per chamber...

It won't.

BTW - Rotaries can run absurdly lean. I would consider 16:1 to be an excessively rich cruise mixture, even with grotty standard ignition timing. EGT is *not* a consideration at low power. Run lean enough and EGT's drop again, peak EGT is just lean of stoich and anything richer *or elaner* drops them off again. Probably not a good idea for runnign under boost, but it's great for getting highway fuel economy that makes people in econoboxes envious.

rx7tt95, what kind of gas milage are you talking about, in mpg :)

Dunno exactly...I usually gauge it by how much gas I use to make a run that occurs often. Usually jetting up to Tampa leaves me with just over half a tank at an 85mph cruise with the air on, windows closed. I was seeing just OVER 3/4 of a tank on my last run. So I was using about 1/4 of a tank to get to Tampa.

I did notice when I leaned out my a/f ratios initially I dropped my egt's slightly. Perhaps I should lean it out more (if the car is fixed, see my other CLR post for the bad news). I was initially worried about heat related wear to the turbo, etc...cruising with higher egt's. I guess it's really not an issue.

I have lots of questions so bear with me here. When you say run larger negative split the farther you are in vacuum, exactly how much split are you referring to, -5 degrees, -10 degrees, more? Also when running this negative split, how much total advance are you running on the leading side? Also what rpm's do you do this at? Is it in vacuum at any rpm such as cruising speeds are is there an rpm cutoff point where you wouldn't want to do this above?

The RX-8 runs 5 degrees negative split at idle only. They have said this is for emissions and apparently they can run leaner because of this according to what you have said. Are you saying that negative split with a lean a/f ratio can give you great economy for cruising compared to traditional split timing and leaner a/f ratios? The key is in how much. I am curious as to trying this on a naturally aspirated car. Thanks for all the great info. If you can't give out specifics I understand but any neighborhood guidelines would be appreciated.

Curious as well. I run up to 35 degrees on the leading side, trailing would have to go right on up there to be negative split...

Its more like pre-ignition really. detonation is when the mixture becomes so hot that it ignites and collides with the flamefront provided by the spark. Here, we are intending to igntite the mixture earlier.

SGP: Off topic, is Kyle Puckett still around? I used to work with him at Envirotech Electronics about 10 years ago.

The only way the trailing plug can ignite the next chamber is if the timing is so far retarded that the apex seal is between the leading and trailing plugs when they fire. The leading plug will fire into one chamber, the trailing plug will fire into the following chamber.

No, it's not a huge secret. I'll do a screen shot of my trailing and split maps and post them. I "think" I have my maps on the laptop. If not, I'll have to swing by the shop and download them. I've been waiting to tune on the local dyno which is/was waiting on parts for a rebuild (coils had to be rewound) so I have not dialed in timing as of yet so everything's conservative. I've mostly played around with logging fuel at WOT and various areas of cruise for better gas mileage as I drive the car daily. For those that do not know or have not read the last few CLR engine posts, I, more than likely, spun a bearing as a result of the oil pan incident a few weeks back. Anyway, wont' be working on the maps for a bit. Take a look at them when I post them and use as a rough guideline. My setup is very different from yours. If I remember off the top of my head, split starts out at about neg 25 and goes down from there. I'm pretty sure it's 25 at idle. I think I could probably go higher in areas but I haven't had time to mess with it nor bring the car to CLR for tuning. If/when the engine goes back together, it'll be one of my top priorities.

wow, this is EXTREMEMEMELY interesting. this negative split thing really does work eh? is it something that should only be tried on "scalloped" rotors? low comp rotors? or can it be done across the board? im in the midst of tuning so this is all VERY interesting. id love to check into it.

also, about the double throttle blades. . . michel. . . i totally forgot about that. i figured you had decided to give it up. i STARTED working on something and put it up. now, i wont be able to do anything for a good while. . . due to my untimely trip to afghanistan by the end of the year.

still. . . this is great. . . im really wanting to start doing this on my car. . .

now that i think about it. i had something happen while i was at a steve kan tuning event in alabama about a year ago. i hooked up my spark plugs BACKWARDS. negative split at 20ish psi was not cool (i had a wastegate problem as well). im STILL on the same engine w/no problems. i always thought that was funny. i even thought the kdr anti det device had something to do with negative split. . . . man, this is nice. geh. . . im excited.

paul

LOL, paul. You watch yourself over there in Afghanistan. I had a chance to go there and to Iraq and turned both down (photojournalist).

Negative split really does work. I think one could do it without issue if you do not have phase ported rotors (scallops). I'd think one could do it for any rotor but the split and timing would probably have to be optimized for your engine, just like any tuning.

Haven't given up on the double throttle. I have them hooked up now but they go open at WOT, even in vacuum. Will work on that a bit if I managed to come out unscathed with the whole oil pan incident.

haha, ill watch myself. . . or try anyways. . . ill have my first son to come home to. . . so yeah, ill be careful. helicopter mechanics are pretty much on the safe list as it is anyways.

is there any way you could get a shot of your timing maps on the forum? id really like to see it in comparison. im having trouble figuring out what does what. trailing advanced? leading retarded? both moved in conjunction? does trailing take the place of the leadings original timing? im just a little confused on the placement of such timing. :D

about the double throttle. i had plans to modify a throttle body because i figured the double secondaries might cause a little bit of turbulence. . . maybe i thought into it a little too much!!!!

very very exciting

Since we know that Mazda uses the negative split during idle on my RX8 for economy benefits, I began to wonder why they didn't run negative split accross the board since the car is NA if it really helps the fuel economy.

My guess would be that the lean condition heats the EG up so much that at high RPM's it would burn the cat out to early, and Mazda has already had an issue with cat temps, hence the 11th 1/2 hour ECU flash port campaign as the cars came off the boat.

Kinda ironic that the car could get better economy if emissions weren't holding it back. Not to worry, if RG and I can get the Megasquirt working, we'll try a negative split and and see what happens.

I decided to have a little fun and play with a crude form of negative split on my 1st gen using the distributer. The car is still nonturbo. I adjusted the split to 0. Total timing advance is set to 25 degrees. Then I hooked up the trailing vacuum advance to the manifold but left the leading vacuum advance disconnected. This gives me 15 degrees of negative split at idle and then it tapers off to 0 as load increases. Probably not as smoothly as rx7tt95's but it still does it nevertheless just to see what happens. It works. The car runs great. I no longer have the factory ecu so I can play with fuel trims. Because I no longer have the stock ecu, I also no longer have the vacuum actuators that limit vacuum to the advance below 1000 rpm. Without these you will have full advance at idle. Since I just have the trailing vacuum advance running straight to the manifold, I have 15 degrees of advance at idle on the trailing side. It does idle a little high and a little rough. If I were to use the vacuum switch, I could take care of the idle issue.

Overall the car still drives nice. I will have to see how well it performs in the long run compared to my standard 10 degree conventional split. I know this isn't a finely tuned and I have less control but at least I can see to a point if it works. Anyone with a distributer can do this. Disconnect leading vacuum advance. Set split to 0. That's it. Crude but testable.

rx7tt95,


We are still waiting for you to post your negative split timing maps.


Chuck

I was thinking just the other day of how to do this with my distributer, I had pretty much the same idea. I'm gonna try and combine with with my Megasquirt and WB02 to see what kind of MPG I can get out of it!
Grant

I'll take a peek on my laptop today to see if I have my current map saved. I make it sound like it'll take a lot of effort because I use the Windows-based laptop for tuning only. Everything else I do computer wise is on a Mac. I literally have to go dig the laptop out, LOL. It's the wife's B-Day today so it's her day but I'll try to get to it. I know I have "something" on the computer, maybe a slightly older map. Won't be able to get to my car for a few days as it's at the shop.

Oh yeah, forgot to mention...That's one of the things Carlos mentioned. Try it on a 1st gen by hooking the trailing side up to vacuum. Very simple to do.