Gas milage with weber?
 

What kind of gas mileage are you guys with DCOE webers getting out there? Mine SUCKS. Like 10 MPG. I'm assuming once I get the rest of my RB exhaust on, I can put in my other jets and that should help. But none the less 10 is pretty damn bad.

not a weber but on my LARGE s.p. 12a i had a 48dhla dellorto. with the stock 12a jetting in it from Racing Beat i got 28 mpg on the open road... i also had the fuel set at 3 psi. and when i rejetted for power i upped the fuel to 5 psi ...and got 165@ the wheels and 12 mpg... so like they say speed cost. what i wound up doing was having 2 jets of jets at all times... in the glovebox so i could drive on a long drive and get great milage and have NO balls or have major nuts and fill it up on an hourly bases.
now on a diffrent beast... i just rebuilt the b2200 and also installed a header and a 32/36 dgav weber carb 1st tank of gas on this carb and motor i got 20.7 intown factory claimed 21 intown and 28 on the road so i cant wait to see if i can get that kinda #'s my stock carb was PHuckedi went from 19 mpg to 12 in 3 weeks

My SP 12A with weber 48 will get right about 18 on the Hy. in town and having fun it gets like 10. I have to fill up at between 100 - 120 miles on the trip.

My REPU weighs 2750lbs with a mild streetport on the 13b 4-port. Dellorto 48DHLA and 41mm chokes. It is jetted for power and with occasional trips to redline mixed in with normal combined city/highway driving it gets 15-16 MPG.

When I floor it all the time it gets 10-11 MPG.

1MPG freeway/0MPG street

lol sorry to much simpsons

when i was 48IDA it was like 10-15mpg on a 13b

I get a wicket 8mpg with mine :/. I really need to get smaller jets. I'm driving the car from California to Texas in March; I'm bringing along a 10 gallon thing of gas with me just in case.

Is there any other way to drive. :D


STOP = Spin tires on Pavement.

hi prefix,

the idle fuel jets play a large role in fuel economy. since the majority of driving is light throttle, having idle fuel jets the wrong size will make the motor misfire and get bad MPG. might want to ask around for the right size.

and you are right, the main circuit needs to be jetted properly too for better economy above light throttle.

8 gallons per mile, at least thats what it feels like:). still havent had it running long enought to get good numbers.

Never thought of fuel pressure effecting MPG. I dont even know what mine is set at, because my gauge is broken and never worked in the first place. But none the less I just randomly set it and I'm not flodding or having problems at high RPM, so I dont think it'll be a problem.

The problem may fix itself once I get the RB exhaust, as I'm sure my idle jets or probably desined to work with that. Not to worry though, the presilencer is in the mail. Just need a gsl-se connecting pipe.

However, what are you guys that are getting good gas milage running for idle jets? I'm running 65F9's which for a weber 48 seems to be the norm. Are you guys running a different jet?

You guys are freakin crazy bitchin about fuel economy. If thats what you want but a God damn Toyota Prius tree huggin Birkinstock wearin peace of shit.
Whew glad I got that out!

I aint a tree hugger man, just when I when my exhuast pipe is blacker than hell, and my gas milage is shit, I know I got some major tuning to do.

Anyway, back to those jet sizes...

Even if you have the carb set up correctly so that it's not burning too rich at either idle or cruise, fuel consumption is still gonna suck--- Fuel burn is directly related to hp produced.

Specific fuel consumption (SFC) for the rotary is on the order of 0.47 lbs per hp per hour at about 75% power setting. Since gasoline weighs 6 lbs per U.S. gallon this translates into about 0.078 gallons per hour per hp produced.

For a 101 hp bone stock 12A this gives us a consumption rate of (101 x 0.75) x 0.078 = 5.91 gallons of fuel per hour at 75% power, which comes in at about 5200- 5400 rpm, which in fifth gear on the highway translates into around 100mph. Divide this 100mph by 5.91 and you get about 17 miles per gallon.

But most of us don't cruise at anywhere near that speed and power setting on a regular basis. (I hope). At 60mph the engine is only doing about 3000rpm, or approximately 35% power. But with a rotary SFC is slightly lower at this rpm, about 0.45 lbs per hp per hour. (But below about 2500 rpm and above 5500 rpm SFC goes up exponentially, which is why city stop-and-go driving and all-out revs on a racetrack produce such horrendous fuel consumption figures).

Doing the math for 60 mph at 3000 rpm and 35% power with a SFC of 0.45 looks like this:

0.45lbs/hp/hr divided by 6 lbs per gallon = 0.075 gallons per hp per hour. Multiply this by 35.35 (35% 0f 101hp) gives us a fuel consumption of 2.65 gallons per hour. 60 mph divided by 2.65 gallons per hour = just under 23 mpg, about what we've come to expect with the bone stock 101hp 12A-powered 1st gen 7. Throttling back to 55mph gives us over 25mpg, but that's no fun by anybody's standards, is it?

Now using those same formulas, let's look at what happens when we mod this engine and exhaust to produce 165 hp.

35% of 165hp= about 58hp at 60mph. SFC at this setting is still 0.45 lbs per hp per hour, or 0.075 gallons per hp per hour. 58 x 0.075 = 4.35 gallons per hour. 60 mph divided by 4.35 gallons per hour = just under 14 miles per gallon!

The very best fuel economy you'll get with this engine will be in the neighborhood of 15mpg at around 55mph. In stop-and-go city driving it will do much worse--- the 10 to 11 mpg figures quoted in previous posts are quite believable.

Piston engines may get marginally better fuel economy at the same settings given here, but who the f%#k wants to drive a car with pistons?
:cool:

can i get a summary? j/k

The real question is whether modifying a 12a engine leads it into an inefficient situation. Add a Weber to a streetported 12a and you get a 12-14 mpg result according to all the comment, while having some 200 hp to play with.

However, take a 1993 gen 3 producing 255hp, its fuel consumption is officially rated at 17 mpg city and 25mpg highway. I drove one for a weekend and got 22mpg driving the same way as my gen 1.

Does this mean if you drop in a 13B REW and get 255hp you get better than this due to the gen 1s lighter weight?

It's also interesting that when there were NA models producing 160hp and the turbo version produced 200 hp, the difference between them in fuel consumption was only 1mpg..

All true. While the 12A-powered 1st gens put out comparable performance and fuel consumption figures to their direct competitors of the day ('79- '82 Datsun 280ZX, '82- '83 Toyota Supra), their fuel burn per hp produced was still pretty bad.

But by the time the fuel-injected GSL-SE came around SFC had improved--- about 35% more hp for a fuel-burn penalty of only a couple of mpg less. (This even with a lower final drive ratio).

Fuel economy took a dive with the heavier 146hp 2nd gen, but was still not unbearable. Actual fuel burn per hp produced was no worse than that of the 12A, but the heavier weight of the FC kept the SFC from being as low as that of the lighter GSL-SE.

But the real improvement came with the arrival of both boost and fuel-injection in the same package. Fuel burn per hp actually went down--- less mpg than N/A rx7s, but way more peak hp being produced.

This can be further explained when you consider that it takes a specific hp to propel a car of a given weight, roll resistance, and drag coefficient down the road at a given velocity. In top gear at 60 mph (especially with taller gearing) that boosted 7 isn't revving high enough to produce anywhere near peak hp and peak boost--- or peak fuel burn. It doesn't have to at a relatiely low constant speed of 60 mph.

But its fuel economy on the track at peak hp is going to go straight down the toilet. But that's ok, because the 150+ mph is worth the price of admission. :D

What your saying makes sence, but that all assumes the engine is PERFECTLY in tune.

When I still had my fuel injection, I got piss poor power and about 50 miles to the TANK. I was running incredibly rich.

Other variables come into play, such as ignition, exhaust etc. I direct fire ignition will yeild better gas milage and more power, as will a free flowing exhaust. Ignition timing is another big one.

I dont know if you could put an explanation to this, but could you perhaps but your reasoning as to why some of the RX-8's get really bad gas milage? Some are good, and some are bad, and they all use the same setup. I'm guessing driving style is probably it.

What your saying is in a perfect world, but unfortunatly we never get absolutely perfect conditions.

However, your explanation of how boost comes to play in this makes a whole bunch of sence, as did everything else you said.

Dont forget about those jets, 65F9 too big? I just got my presilencer in the mail today, so I just need the connecting pipe and split air pipe. So I should be able to see how much an effect the idle jetting has on exhaust setup.

You think you're getting bad gas mileage with one 48IDA or 45DCOE, I can't wait to see what my mileage will be like with a dual 40PHH setup :D

BTW, the Nikki I have on the 12a right now is getting 17mpg in the city and 23-25 on the hwy so Aviator 902S isn't far off at all.

Some very good observations here. Ignition timing, exhaust, direct fire, and anything else that can effect fuel efficiency will most definitely make a difference. And yes, those figures I covered do depend on having everything in top tune and working order.

As for the RX8--- yes, I've heard the rumors. The only thing that makes sense is exactly what you suspect: Differences in driving habits.

It all comes back to my point in my previous post: car capable of 230+ hp can still get decent fuel economy--- as long as you don't USE that extra hp on a regular basis.

The guy travelling at a constant (legal) speed on level terrain, and shifts at between 4000 and 5000 rpm (except in situations where it may be necessary to open it up) will get good gas mileage because it only takes about 30-35% of the car's hp to accomplish this.

But the RX8 driver who consistently spools up to 9000 rpm before shifting, does lots of city driving, and cruises on the highway at an average speed in the go-directly-to-jail range will be spending almost as much to keep the car gassed up as he does on car payments.

excellent explanation man. Some more things to think about, unlike piston engines, I find that if I drive a rotary like a grandma, I get WORSE gas milage. I get my best milage driving the car how its ment to be driven.

Some of the RX-8 people are saying this as well, allthough the more I read about it the more I'm thinking people just dont know how to drive a 6 speed, 4th for around town not 3rd!

Anyway about those jets, 65f9 is what everyone else is running?

Sorry, can't help you with the jets--- My area of "expertise" (for lack of a better word) is with aircraft applications for the 13B and with bone-stock 1st gens, which I've never had any major carb problems and therefore no prior need to strip one down.

As for poor fuel economy from driving like a grandmother, that's possible too. The rotary's SFC is at it's worst at revs above 5500rpm, and also at rpms below 2000. The below-2000 range SFC is one area where piston engines fare much better than rotaries.

So over-revving sucks fuel, but so does lugging it. Not only that, but lower-than-normal rpms also provoke detonation.

The range for best economy is therefore any constant setting between about 2500 and 3500 rpm, in the highest gear possible.

So putting it in netrual and coasting wouldnt be as good as keeping it in gear at around 2500rpms? No throttle and 700rpm would seem not to suck as much, but this is my first rotary.

Actually that would be worse. With a carburetor, the idle system supplies fuel to the engine (a rich mixture, btw) to keep it running at very low rpm.

An idle system is necessary because there's not enough incoming air past the (almost closed) throttle plate at idle. Without an idle system the engine would just quit whenever you took your foot off the gas with the clutch disengaged.

Fuel burn at idle is in the same ballpark as fuel burn at cruise speeds--- which means far worse fuel economy because at idle your not covering anywhere near as much territory.

Fuel-injected engines are more efficient at idle than engines equipped with carbs.

Some interesting stuff on this website!

Aviator 902S; interesting info about SFC for rotaries (I assume tested on a dyno or a whirl-stand?!) but I have to disagree about your steady-state fuel consumption figures. Mainly the fact that my street-port, exhausted, 600cfm Edelbrock-ed stock ignition-ed 85 RX gets around 20mpg in town (21 last tank, love those new spark plugs and cap!) and around 25 at 78 mph, 4000rpm cruise. I think the bad figures seen by the Weber/Dellorto crowd is because they are set too rich for good throttle response. As far as performance comparisons I'm still trying to set up a race between my car and my engine builders ride; identical except for Webers and DIS ignition. I STILL get better mileage than him, though...!

Your statement that these vehicles are using 35% of power at 60 cruise seems a bit much, also. Car and Driver has stated that "most" vehicles need around 15 hp at a steady 50 mph; are your figures the result of a dyno test? Most dyno runs are at full throttle, NOT the partial throttle used on the street. I'm sure you know this; NOT trying to insult your (obvious) knowledge.

Sanspistons (and Sansdyno sheets; still running S.O.T.P's!)

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 piss 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.

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) ;)

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. :cool:

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.

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)

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.

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 ass!

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

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

DriveFast,

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

B

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.

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).

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