Aviator 902S

Actually those figures were calculated by the rotary engine gurus who are using the rotary to power their experimental-category, ie: home-built aircraft. They compare the known power requirement for a particular aircraft at a particular airspeed and gross weight, calculate hp produced at a given airspeed, then measure fuel consumed.

It's amazingly accurate. Auto manufacturers can stretch the truth all they like. But in aviation, claims always end up having to stand the test of scrutiny, and bogus claims are quickly exposed. This lends much credibility to the findings of these rotary/home-built aircraft guys.

BTW, no offense taken--- How can we ever learn anything if we're not willing to listen to opposing points of view? Those who won't end up thinking they know it all, and are most annoying to those of us who do.

It's absolutely true that most newer (small) cars only require about 15 hp to travel at a constant speed of 50 mph on level pavement. But fuel economy is also dictated by gearing. This is because, as you no doubt already know, if the transmission and final drive ratios are not tall enough to keep the rpms low enough to take advantage of that low 15 hp setting, you'll be producing more hp than you actually need at that setting.

For example, the final drive ratio of my 1st gen is around 3.9: 1 (The GSL-SE diff is over 4: 1). These low ratios are to partially offset the inherent weak torque at low rpm for the non-turbo'd rotaries. The result is that at 60 mph ( which is what I based the 35% hp figures on) in top gear the 1st gens are turning about 3000 rpm and producing about 35% of their rated hp. Sure, Mazda could have put a 3: 1 ratio final drive into these cars and fuel economy at cruise would have been better. But it would have been no fun dropping from 5th gear to 2nd at 60 mph to pass that semi up ahead, and then finding that the acceleration still wasn't adequate.

When Mazda was still producing the 1st gen, mags like Car&Driver were comparing their observed fuel economy claims to those posted by Mazda. Mazda claimed around 27 mpg hwy and 18 city, while C&D observed 21 mpg combined hwy/ city driving. Of course, you gotta know C&D staff were burning the **** out of it.

With the four well-maintained stock 12A 1st gens I've owned, a very common trip was the 600-mile drive between Vancouver B.C. and Calgary, Alberta. This trip requires a shade over two full tanks of gas travelling at a typical highway speed of 60 mph, only slowing down through cities and towns.

The 1st gen 12A RX7's fuel capacity is 12.1 Imperial gallons, which is equivalent to about 14.5 U.S. gallons. This gives us a fuel requirement of about 30 U.S. gallons for the 600-mile trip. 600 miles divided by 30 gallons = 20 miles per U.S. gallon consumed, or just over 24 miles per Imperial gallon. These figures support the SFC figures I quoted earlier.

Of course, this type of fuel economy from a 101-hp engine sounds pretty bad considering that my wife's '91 Nissan Sentra (which is rated at 110 hp) does the same trip getting over 40 mpg, and that car weighs about the same if not more than my GSL. But her final-drive ratio is much higher than mine, so she's only turning about 2200 rpm (much less than 35% power) at 60 mph. Considering also that piston engines produce more torque per hp and have a lower SFC at low power settings, it's not hard to see why small econo cars get the fuel economy that they do.

But they're no way near as much fun. The 7 produces less peak hp, but once it is revving over 3500 rpm it produces near-peak hp over a much wider rpm range that the boinger does. All the better to pass semis with.

Those economy figures you're getting sound impressive. I'm wondering if your set-up is running leaner than stock, at least on your primaries. (Smaller main metering jet?) If so, is it still free from detonation?

Also, 600 CFM sounds like an awful lot for a street-ported rotary, kinda like chugging beer from a 45-gallon drum. I'm thinking 450 CFM would be a typical mod. If that engine is actually pulling in 600 CFM at wide-open throttle it has to be also pulling in at the very leanest (about an 18: 1 air/ fuel ratio by weight, but 16: 1 is more suitable) considerably more fuel than a typical street-ported engine.

Sounds like I'm missing something here. Need more info.

Sanspistons

Oops! -spotted an obvious screwup in my last post on the way to work; that's what I get for looking at this site with a time limit!

If you think about it, 15hp IS about 35% of the stock engine's power; AT THE REAR WHEELS!!! I seem to remember somebody dynoing their stock 12A at around 75rwhp so that makes pretty good sense! Sorry about that Aviator!!

However, I still think my basic idea stands; even if you modify the stock engine to 160hp most of the mods only make more power in the UPPER rpm range with little or no effect at the 3000rpm cruise we're discussing. So I can't agree with your premise that modding automatically drops your fuel mileage.

I think that the reason that Weber carb set-ups get poor mileage is the basic design of the manifolds: to wit, the combining of each rotor's primary and secondary runners together. This tends to kill low-end power (while boosting top-end) b/c the rotor "sees" the entire 40+mm throttle all the time while the stock (and RB Holley) manifolds let each rotor see only one small primary. This is why my 600 Edelbrock runs so well: most of the time it's just a small ~230cfm two barrel. And at WFO the counter-weighted air-valves let in air ONLY when the engine can take it. Yes, it's getting the full 600cfm; I've tied the air-valves almost open to get better mid-range....

Running leaner than stock? I hope so!! Those old emission controlled carbs were set quite rich to feed the dreaded cat-cons! Can't say if I'm too lean, though; the pipe is still very black and if I go one step leaner on the primary step it misses and protests. Never any sign of detonation that I know of; it'll pull from 1500rpm in fifth (fairly) cleanly -I don't do that too often! I'll recheck those old plugs for glass beads (I'm assuming that rotories have the same symptoms?!). I soon will be investigating ignition upgrades to see if I can run the leaner needles for even better mileage; hope this won't run it TOO lean. I seem to remember RB saying that rotaries are NOT affected very much by too rich or too lean mixtures. Maybe the exhaust temp will climb all the way to blue-hot at the collector....! Anyway at highway loads this should not be a large factor.

I tend to shift at 3000rpm in every gear (except 4th) when warm; the rpm stays between 3000 to 2000. I'll admit I normally just drive the damn thing like a good little citizen (maybe the wife and kid has something to do with it...) except for an occasional Honda Hit. Sometimes I skip-shift; 5000rpm in first, then 4000rpm in third into fifth. Doesn't seem to affect mileage that I'm aware of; just getting tired of that drag-race gearing!

I have nothing against the fine Weber carbs it's just that I've seen 2.3 liter Pinto engines (the same size as the 12A) with dual 45mm sidedrafts run very well while getting stock or better mileage; it bothers me that the same setup should not work right on my beloved rotories. Again I think the (cheaper) single combined runner Weber manifold is the main culprit. Maybe Alien rx7's dual Weber's are NOT such a bad idea after all!

Happy Rotoring!

Sanspistons (and Sanscash)

Aviator 902S

Some good points here. You're right--- if your set-up is as described (secondaries that aren't open at the lower revs) there's no reason you'd be paying a fuel penalty driving at sane speeds and shifting early. And too many CFMs at the low end does nothing for torque to begin with on a rotary.

It's good that detonation isn't a problem, and thinking about it, a 9.4: 1 compression ratio still leaves some room at the top even running 87- octane. (BTW, do you run the 87 or premium fuel?) The biggest benefit over boingers with regard to high exhaust gas temps (EGT) is that we don't have to worry about burning out exhaust valves.

And come to think of it, the rear-wheel hp consideration makes sense--- publications and dealers usually quote brake hp (potential hp minus friction hp, measured at the flywheel)--- it sounds better and sells more cars. If the car were truly geared to run producing only 15% of RWHP at 50 mph there would be nothing left with which to accelerate or climb hills without dropping it from 5th gear into 2nd.

Emission systems are another power and fuel thief. If you've gotten rid of most of them you'd likely see better SFC.

alien_rx7

Dual Mikuni's but who's counting All this discussion is very interesting and I cannot wait to get them rebuilt, installed, and tuned. From what I hear, everyone that has run a dual setup, especially with the TWM intake, sees more low end torque and a faster revving engine due to the straight through nature of the TWM intake. (I can look down the barrels of the carbs and see right through the intake to the ports)

Jon_Valjean

Too rich and you pay for it at the gas pumps, too lean and you pay for it at the engine reconditioners. Apart from running your 12A without oil or water, the worst thing you can do for a rotary engine is prolonged detonation.

Sanspistons

Aviator I'm running with the air-valves "wired" (O.K.: weedeater stringed!) almost wide open with just a little free play. Since the Edelbrock is a mechanical secondary carb this gives the full air flow almost at will. Only if I WFO below 2500rpm in 4th and 5th will it protest. Otherwise it'll take everthing Mr. Vic can give it.... Thanks, Edelbrock/Carter! The weedeater string is a temporary mod till I can drill the counterweights as per Edelbrock. Before I this mod it didn't generate sufficient flow below 6000rpm to open the air-valves. Now it has killer midrange.

Sorry Alien RX; Mikuni CV's are one of my favorite carbs (I'd like to run 4 on my RX....!) Haven't seen the TWM intake, sidedraft I guess. Kick ***!

You're right Jon Valjean, but I show no signs of detonation at any rpm and I'm running the cheapest gas I can find. Also lack of glowing hot exhaust valves is a rotary plus for octane insensitivity.

Sanspistons and SansSouci

Rotofire

uhm i have 45 idle jets in my weber dcoe......my head hurts

bliffle

DriveFast,

Nice looking REPU. I just picked up a nice red 74 with 120k. Always garaged.

B

YapaKanichi

my car has a dellorto intake manifodl with a mikuni on it and I get horrible gas mileage. Like in 75 miles I have to refuel, that's crusing at 80 or 70.

rxAustin

Way way way off the mark. It takes X HP to maintain 60 mph cruise. Now mod the engine to produce twice the power. It still takes X HP to maintain 60 MPH. Max power you can make has nothing whatsoever to do with fuel economy (that's what the throttle is for!).
The only way you can get fuel consumption figures from BSFC is to know how much power you are making at cruise, and what the BSFC is at that point. Note that BSFC is higher at light loads - 0.47 is way off for cruise.

Just because the numbers appear to work out in your case doesn't mean you arrived at them correctly (other wise you'll have to explain how I consistently got 22 mpg from a street ported 12A that ran high 13's).

DriveFast7

iTrader: (1)

gas mileage is greatly influenced by how well the carb and engine is tuned; and how much time is spent at full throttle!