hornbm

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.

BadAssRX-7

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 ***** 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

Hades12

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.

DriveFast7

iTrader: (1)

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.

81Rex6port13b

1MPG freeway/0MPG street

lol sorry to much simpsons

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

prefix

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.

Hades12

Is there any other way to drive.


STOP = Spin tires on Pavement.

DriveFast7

iTrader: (1)

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.

seanrot

iTrader: (2)

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

hornbm

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?

jimmdog

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 ****.
Whew glad I got that out!

hornbm

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

Anyway, back to those jet sizes...

Aviator 902S

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?

Siraniko

iTrader: (13)

can i get a summary? j/k

fitzwarryne

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..

Aviator 902S

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.

hornbm

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

When I still had my fuel injection, I got **** 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.

alien_rx7

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

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.

Aviator 902S

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.

hornbm

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?

Aviator 902S

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.

JacktheDrifter

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.

Aviator 902S

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.

Sanspistons

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!)

Aviator 902S