Ok guys, I've just been curious recently about how to tune cruise parts of the map for best mpg?

There is always talk about tuning for WOT and boost.
KPa, manifold pressure and cruise:
So, what is the best KPA to cruise at? if I make it richer I can cruise at 70 ish kpa, and if I lean it out i can cruise at 95 kpa. which is more efficant MPG wise?

Timing:
For a stock to street ported motor, what is the most effecant timing to run for cruise? is more advanced and richer better? or more retarded an leaner?

ARF's:

is 14 the magic number, or would you lean it out till it starts bucking at 16.5 or so. Also the ARF's you can run before it startes bucking seems to depend on the timing advance.

Compression and MPG:
How much does T2 vs NA rotors make in tearms of MPG? (assuming both are s5)


Thanks for you help guys, i've been working and playing with this for some time now and my car has always gotten **** MPG

 

Usually around 16:1 AFR's seems to be the most efficient during cruise, unless you're having misfires at that point. anything richer than 14.7 and you're wasting fuel. If you have a cat in the exhaust, you pretty much need to stay around 14.7 to keep it alive and working properly.

Tuning my S4 TII 'vert to about 16:1 on the highway gives me about 22-24 mpg at 70-75 mph. I dont know how that compares to a stock turbo-vert, but thats with the lowest compression rotors.

 

do you have a t2 drivetrain? I think that hurts mpg quite a bit because of the higher rpm's at cruise.

and I have s5 t2 rotors, don't know if that matters.

What manifold pressure (kpa) do you cruise at? and what is the timing advance? Are you on a stock ported motor?

Thanks

 

eage8 and I are running his at 13:1, and at that AFR, we got 22 MPG on a 650 mile trip to GA at 70-75 MPH.

I usually don't recommend running all that lean on the rotary. I don't know if this happens on every engine, but when I run mine leaner at cruise, my mileage doesn't go up, but the cooling system has a harder time keeping the engine cool, and the engine idles rougher for a few minutes after cruising than when I run it a little rich.

The only exception is when you're using negative split.

Ken

 

hmm, ok... but what about the other 2 parts of my question?

What kpa and timing advance do you cruise at?

 

I cruise rich too, I think you will find that a lot of the australian tuners tune to 12:1 across the board off boost coming down to about 10.8:1 at full boost.

I too get stalls if I cruise lean and then try to return to idle (IT DOESN'T IDLE) also I find cruise is nicer and much nicer between off throttle and a tiny creaked open throttle.

 

I cruise at about 30-50kpa depending on load, with timing in the region of 15 to 35 degrees depending on load and RPM. I use very conservative timing below 3000rpm for cruise because the car will misfire if I don't.

And finally no, changing compression of the rotors from 8.5 to 9.7:1 won't make 'much difference'.

Fuel economy will be best at peak torque, which is where BSFC is lowest, but remember wind resistance increases as RPM increases, and you should use your highest gear, so best fuel economy should be around 55-65mph in my opinion

 

I'll have to check my logs, which are on the laptop at home.

Ken

 

I don't get stalls because I idle at 12:1 even if I cruise higher.

It's just when I cruise higher, even idling at 12:1, the engine runs rougher at idle just after cruising.

Ken

 

You are using port injection with stagged throttles, I'm using downdraft throttles with injectors further from the engine, its still the same symptom, the injected fuel doesn't make it into the engine until the walls are re-wet. Your idle will steady out as the equilibrium is reached. Mine doesn't get a chance between there is lots of manifold surface area and I get a lean stall, rather than rough running.

 

Are you running a 2-injector setup? If so, try using EAE. It works pretty well with those setups at this point (and I have an idea that should make it play nicely with staged injection as well).

Ken

 

I tried before and it was too slow, throttle pump and rich tuning is working best for me.

 

I didn't think it'd work well on a rotary as-is either, but Mike Robert was able to get it to work as well as his "throttle pump" setup had. If I remember correctly, he said it took him about 20 minutes to get EAE right, but about 2 weeks to get MAPdot right.

EAE is a wall-wetting algorithm, so tuned properly, it'll account fairly well for your stall issue.

Ken

 

yea, i have the T2 drivetrain, and run at approx 16:1 AFR and 60-70 kPa at 75 mph with S4 T2 rotors with a little bit of exhaust porting. With this, i've gotten a best of 27 mpg, but that was really driving for efficiency, with 24 much more achievable.

I've played with richer and leaner, and 16 seems to be the best for economy but she'll run without misfiring out to 17. She idles nicely at 13:1 (gets a bit rough out at 14:1 tho) with the closed loop idle. Running the engine at 12:1 to 13:1 at cruise is just throwing away fuel, and i'm actually pretty surprised you got 22 mpg this rich.

 

My brother has the same setup with a 48 IDF throttle body w/ 4 injectors. He's running the older 13B block, so there is no place for the primary injectors like the later models. I know EAE would help him out alot, but I never got ms2 to fully work right without constant corruption. It's very annoying to cruise around in.
In other words, he has the same problems you have, so I know what you mean when you say you get a lean stall.

 

Like I said, I've seen at least 2 engines idle a lot rougher after cruising any higher than ~14:1. I can watch the CLT sit higher when cruising that lean on my engine (the other one has a much better radiator than mine). They idle rough like that until you've been idling at 12:1-13:1 for a while and things cool off. Unless you're using negative split I'd not go any leaner as the combustion temperatures go pretty high when you run lean.

I've also seen someone (the original 1st rx7 to run stock ignition on an MS in fact) try to get too greedy with fuel economy, and run 16:1-17:1. He was running fine like that for a while, tuning and tweaking to get better and better fuel economy. Finally, one day the engine just started knocking any time he got on the throttle. He tried switching to 93 octane (was running 87 on an NA), which helped a little, but didn't get rid of the knock completely. This same engine had run fine ~14:1 for months. The issues only started when he started trying to run leaner. We double and triple checked his MS without finding a single problem. Everything looked right on the scope. I can only blame it on running too lean for too long.

I figure Mazda had a reason for running AFR where they did with the ignition timing they were using (and no negative split), and my first guess would be for engine longevity. I think they ran excessively rich but not by as much as you might think.

I intend to tune to about 17:1 once I switch to using negative split (essentially turning the engine into a stratified charge engine). I just need to install some EGT probes.

Ken

 

Did you ever try beefing up the +12v input filtering? There are suggestions on doing this on msextra.com and msefi.com I believe. Most likely these, and/or bad grounds are the causes of resets and corruption.

Did you ever try a different ms2 daughtercard?

Ken

 

After cruising, the idle was a little bit rougher until i started experimenting with coolant temp based enrichments and closed-loop idle. A steady highway cruise at 16:1 on the MS1 tune (i'm still working on the MS2) would have coolant temp at approx 86-88*C, and after coming down from cruising, the manifold and IAT seemed to be heat soaked, which I think was causing the rough idle. The clt/iat enrichment really helped this. With MS1 i couldnt run any negative split, but the engine's held up to about 10k miles and 2 years along with a number of autocrosses so far without problems. Even with negative split, i'll probably be richening it up a little since the 7 is no longer my only car, and it's fuel economy isnt as important anymore.

I dont think that running negative split will allow you to run stratified though; at least not without direct injection. The fuel will be pretty well mixed by the time it sparks. The negative split allows you to fire the plug closer to the center of the chamber at the early timings necessary for low load/lean operation. This most likely works better than regular split or no split because the leading plug would be fairly close to the apex seal where the rotor face and housing are very close together. If the leading were to fire first, the initial flame kernel from the leading spark would hit the chamber walls sooner and quench, leaving the trailing spark to cause most of the mixture to burn, but later in the process. Negative split allows the mixture to begin to burn at the proper time, but fires the plug with the greatest potential to cause the mixture to begin burning first.

With a conventional, positive-split-only system you'd either need to have fairly retarded timing to avoid leading spark quench, rely only on the trailing spark, or run excessively rich so that the mixture is more reactive and will burn more quickly, which is what the mazda engineers originally did.

Either way, on a non-direct-injection engine, you really arent going to have any stratified charge combustion unless you're running a rich pre-chamber and lean main chamber, which isnt really possible a rotary.

 

That's not really how it was explained to me.

The idea being that the trailing plug firing first causes an "incomplete" combustion, that pushes the rest of the mixture up to the leading plug, which then fires in a "locally rich but globally lean" mixture. That can allow you to run very lean safely, without worrying about temperature issues. The trailing plug firing is essentially used to "stratify" the charge, at which point the leading plug causes full combustion.

That's the way that RotaryGod explained it to me in any case, and it makes sense based on the reading I've done (Heywood, and others).

In any case, the increased temperatures I observed did not cause any change in the AFR (as heat soak to the MAT sensor would, and my MATs were basically the same either way, only my CLT went up significantly). This was rougher running on idle after cruise with the AFR staying the same.

My recommendation still stands, we've seen 22-24 MPG (22 turbo, 24 NA) with AFRs of around 13.2-14:1 and cruise speeds below 75 MPH. That's better than stock, and in my opinion safer. I won't be leaning my tune out any more than that until I get EGT probes and start tuning with negative split.

Ken

 

well, i think that the most any of us (outside of Mazda or anyone else that has the capability to install multiple pressure sensors in a rotary housing and interpret the data) have is a justification for an observed phenomenon, but I dont see how firing the trailing spark will force more of the fuel than the air towards the leading plug to result in a locally rich region when the mixture started out as mostly homogeneous.

As for temperatures, the max in-cyl temperatures should be decreasing as you get leaner from 13:1, since at an equivalence ratio of 1.1 the adiabatic flame temp is the highest. Maybe igniting some of the mixture earlier than TDC causes that portion of the charge to get compressed a bit more after it burns, creating a locally higher temperature, but overall if you have correct timing for the equivalence ratio (which may be difficult to do without negative split), the avg gas temperature, energy imparted to the walls, and therefore coolant temperature should be lower.

Either way, cruising at 14:1 is a good safe recommendation to start at

 

I'm just going based on observations. Without negative split, the leaner you go the hotter combustion is, at least until combustion can no longer happen.

Ken

 

both you guys write like you both know whats going on in a rotary engine.
seems as tho,information on these little magic engines, just keeps getting better.
just when i think i know whats happenin, everything changes.

anyway keep at it, it is interesting knowledge.

THX Ron

 

Actually temps rise going richer or leaner from a certain point but they don't do so indefinitely. You can lean it out and see temps go up but after a certain point, going more lean will start to lower temps again. This is the advantage of negative split. It puts the initial firing plug in a more suitable place to light off a mixture as the mixture is richer on the trailing side than the leading side at this point due to the air being forced backwards through the rotor dish. If you run too lean you can't light it off. Where the a/f is at the point of ignition and not out the tailpipe is what counts. You could have 2 different readings on an O2 sensor but at the point of combustion could have the same air to fuel ratio due to differences in internal flow characteristics and efficiency differences.

If you have an engine torn apart, take a rotor and place it in a housing. Figure out where the rotor would be at a nice low rpm at a general cruising rpm for lets say 35* of advance. Look at where most of the area in the chamber is at this point and then look at where the leading and trailing plugs are in relation to it. This should explain alot about why negative split under light loads works so nicely when done properly. To really take advantage of it, you really need a strong ignition system. That trailing plug is really shrouded and there's nothing that can be done about it.

Interestingly enough I was looking over the stock Mazda timing maps for the S4 turbo and nonturbo. The TII runs some negative split (up to -12*) at low loads while the n/a never runs negative. Also of interest was that the TII uses more total timing advance at full load and high rpm than the n/a does!

 

Yeah, I think my point was that once the temps start going back down, you really need negative split to keep from misfiring. That and I've seen the temps keep climbing all the way up to the high 15's for AFR, though I guess that also depends on ignition advance as well.

Running negative split + higher total advance (I'm assuming off boost) makes sense since the compression is lower on the TII. It seems like they were pretty conservative with it though.

Funny that your explanation of negative split's advantages has changed since we talked on the phone a couple years ago... Back then you were saying what I said in this thread about the trailing plugs causing a partial ignition which blows the rest of the mixture up to the leading plugs.

Ken

 

That's how it was first explained to me. I've since learned a bit more about it though.