Displacement on demand

[NoDOHC]

I am not sure which type of spoiler you are speaking of, but i am referring to the type which reduces the aerodynamic drag on an automobile. A true spoiler is intended solely to improve fuel economy as it reduces the negative pressure behind the vehicle (similarly to a vortex generator. Downforce is generated by a wing (in the opposite direction as on an airplane).

Note that on the FC, the Aero package reduced drag (CD from 0.31 to 0.29) and included a spoiler.

Maybe they do something different on an airplane.

You don't have to take my word for it, please do some research:

This will get you started:
Spoiler (Automotive)

[vex]

Spoilers for cars are often incorrectly confused with, or the term used interchangeably with, wings. Automotive wings are devices whose intended design is to generate downforce as air passes around them, not simply disrupt existing airflow patterns

This is the only spoiler I am familiar with. Since no citation is given for this derivation I am hesitant to believe wiki on this.

[Libor]

Interesting discussion, but NoDOHC is right. Even diesel engines without throttle plates are giving lowest BSFC at almost full load. Load is essential.

Very late closing timing of intake ports could push quite large portion of fresh mixture back to intake allowing for more load and effectively make atkinson cycle with uneven compression - expansion, but we know that piston engines doing this are using high static CR, not option for rotary...

[NoDOHC]

Originally Posted by Libor

Very late closing timing of intake ports could push quite large portion of fresh mixture back to intake allowing for more load and effectively make atkinson cycle with uneven compression - expansion, but we know that piston engines doing this are using high static CR, not option for rotary...

The only option for this on a Rotary would be to add a roots-type supercharger. I thought about adding a supercharger and converting my rotary to diesel a while back, but then I realized that the fuel system development alone would break my budget.

I actually gave thought to the late port close while I was doing my 4-port, I was hoping that the later close that the 4-port has over the 6-port would work to my advantage in the mileage department, but I think that whatever I gained there I gave up in 35% intake runner size increase, polished intake runners, swirl-ported irons and polished rotors.

[j9fd3s]

Originally Posted by NoDOHC

I really should have done a better job of explaining my goals for this plan, I want my cake and I want to eat it too. I want a 250+ Hp rotary that will still give 35 or even 40 mpg.

Ted and i both know a guy who built a bridgeported Rx3, that did 230rwhp, and would get high 20's mpg on the freeway. with a carb! and an exhaust quiet enough that it was streetable. 10 more MPG would be tough though...

actually #2 thing i've noticed is that my other car, a 1958 Tr3, gets 35mpg. other british cars of the era, XK120, MGB, etc actually get decent mileage too, especially compared to "new" cars and they do this with carburated, tractor motors.

how? well the Tr2/3's are 2000lbs, with a 2L 4 cylinder. my S4 Fc weighs in @2880. that's a HUGE difference. and in 1952 the Tr2 was faster in the 1/4 mile than anything you could buy from an American car company, so its slow now, but it wasn't at the time.

oh and #3 back to the mazda rotary. we make the 1000mile round trip to sevenstock every year, and its almost the only drive i get that's almost 100% freeway, and having taken every car from a stock 79, to a 20B FC, to an FD, i've noticed that the FD and the SA both had an rpm range where they got better mileage. the SA gets the best mileage (23-25) in the 3500-4000rpm band. which in 4th puts you right around 65, and in 5th is like 80. driving @3000rpms, actually gets WORSE mileage, although not by a lot, and going over 4k, it gets bad quickly.

the FD also did the same thing. the FD pops out of closed loop @3200rpm, and best mileage is at 3199rpms, which is around 80mph. on that trip my stock FD got 23mpg, my friends ported car got 25. 23mpg + 22 gallon tank = you have to pee really bad.

the 3 rotor FC was plagued with ecu troubles, so it got tuned on the back of a trailer for 45 minutes and returned 19.9mpg, which is totally acceptable

i haven't checked the P port yet, i haven't put enough gas or miles on it to tell.

anyways, i hope it shows that i'm on board with better mileage too!
mike

[RETed]

Originally Posted by j9fd3s

oh and #3 back to the mazda rotary. we make the 1000mile round trip to sevenstock every year, and its almost the only drive i get that's almost 100% freeway, and having taken every car from a stock 79, to a 20B FC, to an FD, i've noticed that the FD and the SA both had an rpm range where they got better mileage. the SA gets the best mileage (23-25) in the 3500-4000rpm band. which in 4th puts you right around 65, and in 5th is like 80. driving @3000rpms, actually gets WORSE mileage, although not by a lot, and going over 4k, it gets bad quickly.

Another data point...

Several Sac to Vegas and back trips netted me 400 miles between gas fill-up's.
1987 Turbo II on a (reprogrammed) stock ECU
16.6 gallon full capacity fuel tank, but filling up at 14 gallons each time.
That makes it 28.5 miles per gallon.
Key is to drive run under the secondary injector crossover point at 3,800RPM, which is about 75mph - 80mph on my car.


-Ted

[FerociousP]

Originally Posted by RETed

Another data point...

Several Sac to Vegas and back trips netted me 400 miles between gas fill-up's.
1987 Turbo II on a (reprogrammed) stock ECU
16.6 gallon full capacity fuel tank, but filling up at 14 gallons each time.
That makes it 28.5 miles per gallon.
Key is to drive run under the secondary injector crossover point at 3,800RPM, which is about 75mph - 80mph on my car.


-Ted

I also noticed that with a Rtek ecu on my way to DGRR this year.... I would be around 15.0 afr until the secondaries kicked in, and then it dropped to 12.1 afr. I was alone so I couldn't tune the Rtek, but this is something that people with stock ecus are just stuck with

I got 23ish mpg while hovering around that point with probably spending most of my time about that

[j9fd3s]

Originally Posted by RETed

Another data point...

Several Sac to Vegas and back trips netted me 400 miles between gas fill-up's.
1987 Turbo II on a (reprogrammed) stock ECU
16.6 gallon full capacity fuel tank, but filling up at 14 gallons each time.
That makes it 28.5 miles per gallon.
Key is to drive run under the secondary injector crossover point at 3,800RPM, which is about 75mph - 80mph on my car.


-Ted

yes, it seems modded t2's get excellent mileage, because it runs 14.7:1 out of boost, and its easy to stay out of boost...

-mike

[PercentSevenC]

Has anyone here experimented with running negative split at low load? What are your observations? Has it allowed you to maintain stable combustion at leaner AFRs?

[88turboii]

I noticed quite a difference between +5 deg split and 0 split, about 1-2 mpg gain at 0 split. im running e6k, so i cant do negative split though

[PercentSevenC]

Sounds promising. Did you have to pull any leading timing when you reduced the split, or did you just leave leading as it was?

Sorry, I'm a noob to EFI tuning.

[88turboii]

i just left leading where it was, around 30 deg at cruise

[NoDOHC]

0 split definitely helps high vacuum stability. It also helps when running lean.

I want to justify driving my RX7 and it is hard to do when I get below 30 mpg around town with it and my Metro gives 50 mpg around town. If I could get DOD operating, I think that I could get 40 ish out of the RX7 which would make the difference livable.

RETed almost has be scared off because of his predictions of engine failure.

I don't run 14.7:1 while cruising, I run 16.2:1. I am running about 60 degrees of advance while cruising at -45kPA. You have to light the lean mix a lot earlier as it burns more slowly.

[Libor]

Originally Posted by NoDOHC

I don't run 14.7:1 while cruising, I run 16.2:1. I am running about 60 degrees of advance while cruising at -45kPA. You have to light the lean mix a lot earlier as it burns more slowly.

How is the driveability with such lean mixture?

There is interesting discussion over rx8club about negative split and lean burn during cruising but concensus backed by injector duty and actual gas mileage is, that best gas mileage, driveability and overal responsiveness is best with slightly rich mixture 0.92 - 0.93 Lambda. And this is with high CR, no overlap engine.

I agree with you that conventional rotary engine very same like piston engine has highest thermal efficiency at around 1.15 Lambda? But this is measured at WOT and practical parameters mentioned above aren´t investigated.

You should try it

[RETed]

My experience with negative split hasn't been very positive.
At idle, it does drop the pulsewidths just about 10%, which coincides with your mileage #'s.
When trying negative split at high vacuum, low load cruising driving, the engine is "unstable" - it's almost like lean surge but slightly different resonance...
I didn't further the experiment because, IMO, it's a waste of time trying to wring out that last 1% of gas mileage on a rotary engine.
I drive and build my rotary engines for power - not fuel economy.
Building and tuning an RX-7 to pump out 400hp and getting 25mpg is a perfectly fine compromise in my book.
I have a daily driver other than the RX-7 for fuel economy - that's how I solve the gas mileage issue.


-Ted