Syonyk

How many people have played around with negative split under vacuum? Looking at the various threads (both here & rx7club) on the CLR engine setup, he's running negative split in vacuum, idling at insanely lean AFRs, and getting rather absurd fuel economy on the highway (apparently... I didn't see any numbers).



It seems that people find that the closer to zero split they get under load, the more power the engine makes. The 787B ran no split at all, and was making a good deal of power as well.



There's little to no risk of detonation under vacuum, and the fact that people are doing it would tend to indicate that it doesn't immediately destroy the engine. Lean AFRs in a rotary? That would be nice...



Finally, what ECUs are capable of doing this? I'm fairly sure Haltechs can't do it (or so I've heard... haven't looked at the manual that intensely).



-=Russ=-

Lynn E. Hanover

[quote name='Syonyk' date='Jul 21 2005, 06:05 PM']How many people have played around with negative split under vacuum? Looking at the various threads (both here & rx7club) on the CLR engine setup, he's running negative split in vacuum, idling at insanely lean AFRs, and getting rather absurd fuel economy on the highway (apparently... I didn't see any numbers).



It seems that people find that the closer to zero split they get under load, the more power the engine makes. The 787B ran no split at all, and was making a good deal of power as well.



There's little to no risk of detonation under vacuum, and the fact that people are doing it would tend to indicate that it doesn't immediately destroy the engine. Lean AFRs in a rotary? That would be nice...



Finally, what ECUs are capable of doing this? I'm fairly sure Haltechs can't do it (or so I've heard... haven't looked at the manual that intensely).



-=Russ=-


[/quote]





The rotary can be run very lean or very rich, at high throttle settings without damage.



The most ignition advance can be seen in stock mapping (RX-8) at zero throttle high RPM, high vacuum.



If the trailing is fired first I am not aware of that. But the list of things I am unaware of is huge.



Very lean and very rich mixtures burn very slowly. So, they mimic very high octane fuel. For NA engines detonation is not even a factor.



In aircraft use, overlean mixtures are common to extend range. Even in piston engines as well as rotaries. Much leaner and much richer mixtures respond well to high energy ignition systems.



Leaning is based on a figure in the EGT. Peak EGT is identified. Best power is just rich of peak EGT. The peak number is different for each altitude and outside air temp.



So at about 50 degrees rich of peak EGT for any value peak may be is best power for that situation. This figure results in the highest oil temps, water temps, plug temps and so on. In piston engines this is where the pistons melt.



In the rotary, this is where the highest stress on the apex seals, and in turbo rotaries where detonation lives and waits to take out the apex seals.



So way lean of peak and way rich of peak is common ground for long life and cool running. Just to be safe when high power is required, full rich in the turbo is the safe place to be. Both reduce total output. If you are lean of peak at wide open throttle, and you loose a fuel pump, then engine leans further (EGTs go down) and the engine enters lean stumble (lean misfire). If you are well rich of peak EGT at wide open throttle, and loose fuel pump, the engine will pass through the very highest stress temps (about 50 degrees rich of peak EGT) as it leans out.



So long as best power EGT is not continous, there will be no damage.



In aircraft, the highest (take off) power requres the richest mixture. Often not an EGT setting, but a measured fuel flow rate per HP. The pilots handbook might say 22 GPM fuel flow. Good slow burn like higher octane fuel, and some extra fuel cooling.



Running as lean as is possible at cruise is great for max mileage. You only need about 30 HP and less than half throttle so there is no stress at any mixture. So why not lean it out and get some good mileage? Lean it until it stumbles and richen it back up just a hair. Just like airplanes do every day.



I have never run the trailing ahead of the leading, so I have no data on that.



I do know that if you a dumping in a bunch of nitrous shutting off the trailing ignition will eliminate detonation in most applications.



Remember the definition of detonation. Take away the second source of ignition

that is built into the engine and you have most of it cured.





Lynn E. Hanover

Syonyk

What I was seeing in the other thread (I'll dig up the link shortly) was a reference to full negative split - the trailing firing before the leading. The theory is that it kicks the fuel mixture up to the front of the chamber, where the leading plug can do the work.



Thanks for all the info on the tuning! How lean can one go with the stock ignition before getting misfires?



-=Russ=-

wanksta

I like the idea of running a leaner mixture at idle...



It seems to be happiest at 13:1 at idle...but if you could run leaner that would yeild a hotter egt (to a point) at idle. And hotter exhaust might translate to keeping the spark plugs hotter...and less chance of fouling them out at idle.



Just a thought...not based on fact. Except it being happy at 13:1 at idle.



James

Lynn E. Hanover

[quote name='wanksta' date='Jul 21 2005, 11:57 PM']I like the idea of running a leaner mixture at idle...



It seems to be happiest at 13:1 at idle...but if you could run leaner that would yeild a hotter egt (to a point) at idle. And hotter exhaust might translate to keeping the spark plugs hotter...and less chance of fouling them out at idle.



Just a thought...not based on fact. Except it being happy at 13:1 at idle.



James


[/quote]







At idle, the EGTs are so low as to be of no value in tuning. Also so little fuel is being burned that no heat damage need be feared.



Try leaning until you get stumble, then add some advance to see if you can get it smooth again. Try a high energy system on the leading only and retune. Then on the trailing only and retune. Record the result and publish them here.





Lynn E. Hanover

setzep

I wonder if running the trailing a tad before the leading in the low load ranges would get rid of my "bucking" problem? With very light and no throttle in gear at ~ 2400rpm and lower she will start to buck. Lead timing is ~30 and trail split is 10 in that range.



If I could get rid of that problem I'd do a back flip.

heretic

Have you tried retarding the timing in that area?



The "bucking" at high vacuum is a rotary characteristic.

heretic

(will not let me edit)



The "bucking" at high vacuum is a rotary characteristic. Mazda went so far as to devise a way to introcduce a lot of air to one rotor, and shut off airflow from the carburetor to that rotor, in an attempt to reduce manifold vacuum and kill fuel flow, all in order to stop that bucking in the pre-EFI days. And thus the "shutter valve" was born.

setzep

I've tried retarding the timing, advancing it, making it rich and lean. Nothing seems to help. The only way it will go away all the way is if I cut the fuel in those load ranges but then it will be a little grumpy once I get back on the gas.

j9fd3s

i hooked up some guages to my stock 83 fb. the factory retards the timing as much as possible @zero throttle. they turn off the vacuum advance solenoids