How to improve MPG on 94 R100RT

[vanr100rt]

This is probably a long shot, but I am currently averaging 40 mpg while traveling on the interstate averaging 75mph. (I get 45-50 putting around home on the Blue Ridge Parkway and 45mph) I know, the interstate is not as much fun as the other roads but I always need to go from point A to point B as quickly and economically as possible. Besides that, I actually enjoy the interstate, especially at night. As I said I am averaging 40mpg and the R100RT has a 5 gallon tank which includes a 1 gallon reserve. I do not like to ride on the reserve, so at 160 miles I stop and get the 4 gallons of gas to top off the tank, and then go do it again in another 160 miles. When going 1000 miles from point A to B this gets really monotonous. Any Ideas on increasing the gas mileage would be greatly appreciated. ECM Change? Re jetting the Carb? Timing?

I have also entertained the idea of adding an auxiliary fuel tank. I never carry a passenger and I must recover the original seat before spring anyway. I am going to convert the original seat to a solo seat lowered at the butt and narrowed between my thighs to eliminate a very annoying pressure point on each side that kills me at 5-600 miles and remains for the rest of the day. Anyway back to the "Drop Tank".
Has anyone ever added a fuel cell between the rider and the top case?
I think a 5 gallon fuel cell would fit in there nicely being secured to the luggage rack on each side and to the top case rack on the back.
I could then run the auxiliary fuel line into a shutoff valve and then into a tee in the fuel lines on each side before the stock fuel shut off valves.
This would give me 9 gallons before the reserve at 40mpg for 360 miles before a fuel stop, which would be fantastic!

 

[kgadley01]

Sounds like your bike is running fine. your mileage is average to good. I would add the Aux tank to give you more range. believe it or not the best place to look for a Aux tank is at a lawnmower repair shop. most lawnmower tanks are plastic now days, and come in a bunch of different sizes. a friend of mine bought one that was round ( like a bedroll ) it held 2 gal. and worked great. he even used a lawnmower fuel cut off valve....

 

[vanr100rt]

Thanks for the tank idea. I was thinking about a boat portable tank for an outboard engine as well. It has filter, anti-spill vented cap if turned over, and is pretty tough because of it being banged around so much. I can easily fabricate a frame to mount the tank on the bike.

 

[pglaves]

Take a look at the safety standards for auxlliary fuel tanks used by the Iron Butt Association. They would not allow the boat tank for sure, and probably would not allow the lawn mower tanks either, but they might. They are concerned with the integrity of the tank in the event of an accident.

There are several makers of tanks (fuel cells) used in dragsters and other applications. These are available in sizes from 2 gallons to 5 gallons or more and could be counted on to be far more durable than other options. Jaz, and Summit Racing are a couple of places to start looking.

 

[Ken F]

www.racingjunk.com would be another good place. Kind of like craigslist for racers.
Used parts usually go for about half of retail

460

 

[97077]

mpg

I have an 85 R80RT that the fuel mileage was lower than I would like(During a Saddle Soar 1000). I was only getting in the high 30's low 40's) . I found that my right side carb. choke needed adjustment. The bike was running with the choke on just a small amount and after the adjustment of the cable my mpg did go up, Now into the middle 40's.The gear ratios are different from a R80 to R100 but the adjustment did help. Good luck. Snowing here again. Is it spring yet?Kevin

 

[fracture]

You will not get much more mpg out of that '94 R100RT. Final drive ratio is 3:1 as opposed to something like 2.8:1 or 2.9:1 for the twin shock models. The engine is not very efficient with that 8.2:1 compression ratio, either.

Some gains can be obtained by breathing improvements (less restrictive exhaust, ported heads) and raising compression but you may not think it is worth the cost.

 

[boxermaf]

As others have said, this is not bad fuel mileage for a late model R100RT.

You could change your "ECM" on the bike, which is your brain telling it to go 75 MPH ;-)

What is your tire pressure? Depending on tires, if they still handle well with 40 PSI in them instead of 35-37, if that is what you're running, could get you an extra 1-2 MPG.

You don't want to lean those late airhead engines out much, they're running close enough to "ping" territory as it is, and bumping compression up will just make matters worse.

There are (or were) some CLASSY Luftmeister auxiliary fuel tanks on fleabay a little while ago - but these fit on the twin shock bikes and not the monolever airheads, AFAIK.

 

[Ken F]

8.2:1 compression ratio?? I dunno...could be wrong....wouldn't be the first time but.....

Four-stroke, two cylinder, horizontally opposed "Boxer" engine, air cooled
Bore x Stroke 99 x 70.5 mm (3.89 x 2.77 in)
Displacement 1085 cc
Max Power 90 hp (66 KW) at 7,250 rpm
Max Torque 95 Nm / 70 ft lb at 5,500 rpm
Compression Ratio 10.7 : 1
Valves Per Cylinder 4

 

[Ken F]

>How to improve MPG on 94 R100RT<

Slow Down. I know that sounds like a really smart A#* remark, but I did not intend it that way...

Seriously, the maximum fuel milage:
Fuel Consumption (at constant 90 Kph) 4.7 litres 100Km / 60 mpg / 50 mpg US

90 Kph is basicly 56 Mph. My '95 RSL which is lighter, and pushing less wind only gets about 42 at a steady 75 Mph, so I'd say you're right in the ballpark. If you are capable of keeping it at 56, you should improve dramaticly. If you are like me, learn to buy more gas & smile. Do your part and contribute to global warming...havn't you noticed how cold it's been lately?

460

 

[dduelin]

A 94 R100RT has 8.45 to 1 CP and is getting good mileage for 75 mph. Did the late model RT's really lose a gallon of fuel over the twin shock models?

 

[bikerfish1100]

8.2:1 compression ratio?? I dunno...could be wrong....wouldn't be the first time but.....

Four-stroke, two cylinder, horizontally opposed "Boxer" engine, air cooled
Bore x Stroke 99 x 70.5 mm (3.89 x 2.77 in)
Displacement 1085 cc
Max Power 90 hp (66 KW) at 7,250 rpm
Max Torque 95 Nm / 70 ft lb at 5,500 rpm
Compression Ratio 10.7 : 1
Valves Per Cylinder 4

those are oilhead specs.
no stock airhead ever came anything close to 90 bhp; 70 hp in the big valve '78 motors was the pinnacle of ooomph for airheads (the 4 valves per cylinder was your other clue of non-airheadedness).

 

[fracture]

A 94 R100RT has 8.45 to 1 CP and is getting good mileage for 75 mph. Did the late model RT's really lose a gallon of fuel over the twin shock models?

I do not think the tank was any smaller on the late model airheads. Instead, the flapper and the filler neck design made it impossible to completely fill the tank. I think this was to provide an airspace for the fuel vapors to collect as part of the vapor trapping system.

 

[Ken F]

You are right Bikerfish...my bad! I read R1100 at the top of his post.
I just love getting old

460

 

[vanr100rt]

You guys are all great and it's truly a pleasure to be a member of the BMWMOA.
Thanks for all the responses!

The 94 R100RT does have that fill neck someone mentioned and it takes forever to get the 4 gallons back in the tank. They look at me like I am crazy when I am trying to get the last .5 gal in. Hum, bump, click. Hum, bump, click. Hum, bump, click. Hum, bump, click. Hum, bump, click. Hum, bump, click. Hum, bump, click. Hum, bump, click.Hum, bump, click. Like a thousand times. burping the spring loaded door after each Hum, bump, click.

I had new Mich. on the bike at the 2 up pressures due to the load I was caring and the bike handled great. I wouldn't want to increase pressure beyond that because of the likely hood of running in rain.
I had rebuilt the front forks after blowing the seal on the left fork the year before last and had ridden it with the seal broken for the rest of that year. So on the 1st trip out (2300 miles) the bike handled far better than when it did after losing a lot of the oil out of the left fork. As well as the second round trip. 2000 miles. I average 40mpg on both, so I guess I am good on tuning. I will tune up and adjust valves before this summers trip.

As to the auxiliary tank, I am looking and the racing cells now and they make a 5 gallon 13X13X9 cell, with all the safety, that holds 5 gallons and you build your on box to drop it into. That's what I am looking at right now and will use 3/8 Aluminum to build the box to hold the cell.

I have the stock windshield which needs to be cropped across the bottom where the center holes have broken out (that way when I got it) and re-drilled. Would a different windshield or modification make a significant difference? I currently ride with it in the lowest position. The stock one is wide and tall. I like the height but the width could perhaps be less.

 

[wecm31]

>I have the stock windshield which needs to be cropped across the bottom where the center holes have broken out (that way when I got it) and re-drilled. <



Before you take the saw to the windshield... My centre holes broke out as well. I made a sheet metal gusset plate bent around the bottom of the windshield and then riveted together. I then drilled the original holes through the sandwich and remounted with the stock centre bracket.

 

[bikerfish1100]

i got a bit curious about your tank capacity, as i recall my '78 having a 6.2 gaqllong tank stock, and could not quite imagine that the monos lost over a gallon of fuel ni that change.
here's what i found-
Dimensions/Weights


Length x Width x Height
2175 x 960 x 1478 mm (85.62 x 37.79 x 58 in)

Wheel Base
1447 mm / 56.96 in

Fuel Tank Capacity
22 litres / 4.83 gal / 5.81 gal US

Unladen Weight with Full Tank
229 Kg / 504 lb

Max. Permissible Weight
440 Kg / 969 lb

Ground Clearance
140 mm / 5.51 in

Seat Height (Unladen)
807 mm (31.8 inches)

Fuel Consumption (at constant 90 Kph)
4.3 litres 100 Km / 65 mpg / 54 mpg US

Top Speed
185 Kph / 114 mph

Acceleration 0 - 100 Kph / 62 mph



also, in reading, I found that the primary reason for lost capacity is not the filler flap, but rather the need for placement of the electronics that sit under the tank and the resulting reshaping of the bottom of the tank.
so- your tank has just been increased from 5 to 5.8 gallons (a gain of about 30 miles, based on 40 mpg), at no cost whatsoever!!
additionally- you might want to consider actually using the reserve capacity of your tank, as that is probably good for about 1 gallon/40 miles. for example, if you were to refill tank at 5.5 gallons (rather than your current 4 gal mark), you would have an approximate range of 220, vs the 160 you're sitting on now. given the 75 mph rating you gave yourself, that is about 3 hours of saddle time. At that point, your brain (and likely your bladder), and probably your butt as well, is in need of a break. lower speeds (more common for Eastern US) would translate into even longer saddle time.
just some thoughts to consider.

 

[donbmw]

I have a 82 R65 and a 1975 R90/6 both tanks hold 6 gallons. The longest I can set on either is around 150 miles. The 1979 Honda CX500 I had before the BMW only had a 2.3 gallon tank so you where stopping every 100 miles for gas. The year I rode it to the west coast I put a 2 gallon Wheel Horse gas take on the rack so I could go alitte farther between fuel stops. The only time that I realy needed the extra fuel was crossing Nevada on highway 50. My Butt and Blader won't let me more that 2 to 3 hours of ridding.

Don

 

[bikerfish1100]

I have a 82 R65 and a 1975 R90/6 both tanks hold 6 gallons. The longest I can set on either is around 150 miles. The 1979 Honda CX500 I had before the BMW only had a 2.3 gallon tank so you where stopping every 100 miles for gas. The year I rode it to the west coast I put a 2 gallon Wheel Horse gas take on the rack so I could go alitte farther between fuel stops. The only time that I realy needed the extra fuel was crossing Nevada on highway 50. My Butt and Blader won't let me more that 2 to 3 hours of ridding.

Don

150 miles on 6 gallons capacity? that is notably under 30 mpg. that is notably not good! a correct running R90 should be getting closer to 50+ mpg... i had seen as high as 62 on my '74 90/6, but that was back when premium leaded was available. even so, something is wrong there don.

 

[lkchris]

Install high compresssion, i.e. 9.5-1 pistons. BMW 9.5 pistons will drop right in, assuming you get your A, B, or C correct.

High compression returns pretty much by definition better efficiency (see diesel).

You'll be burning less but it will be premium unless you dual plug (rumor has it).

A higher rear gear would be a nice compliment, but that's pretty tough with the single-side bikes.

Of course the higher compression will also give you more power, so it will pretty easy for you to ruin your mileage improvement by having extra "fun."