Gas mileage guestion?

[beby99]

With gas prices being what they are I thought I would start riding a little slower
to conserve fuel. But this has caused the reverse effect?@ 70-75 mph I average
about 50 mpg and @ 60-65 mph I get around 45 mpg. I have run this test 3 times
using the same gas station and get the same results?.anybody know why this is?
Suzuki 650 twin with BS36SS carbs. Idle set at 1200 rpms with tan plugs. Compression
is 140 on both with new oil and air filter?..thanks

[gspaul]

Bret, there's something not right somewhere, have you balanced the carbs recently(dead easy on a twin ,no gauges required )
Idle mix adjustment, there can be quite a bit of overlap with the needle mid range.
Valve clearances?
I believe the ignition timing is electronic on this model but worth checking.

[beby99]

Sync the carbs at start of this season and did a valve adjustment about
a month ago...all within specs.

Guess I'll resync and check timing (not checked) and see what happens.
Sure does idle nice through all ranges...no hesitation, hicups or sputters....??

[Hoomgar]

It makes complete sense. It is totally dependent on your gear ratio and the power band of the engine. I would guess that when your running at the lower speed your in a range where the engine does not produce as much torque or maybe HP and is therefore working a bit harder to push the bike? Now when running at the higher speed you are most likely running in the engines peak performance range and it can therefore move the bike with greater ease and is using less gas as a result. It simply means that this is your bikes cruising speed if economy is your concern. Many car engines are the same way. For years the EPA was telling everyone that 55 was the most economical speed to run your car at. They were full of shltt! Some cars yes, others, not even close. My car will give me way better fuel economy on the highway when I run it between 75 and 80 than it will running between 55 and 60. Your engine being a twin cylinder engine is even more susceptible to this than a 4 cylinder would be but it applies to all wet fuel burning engines.

Hope that made sense?

[gspaul]

I have to dissagre on that point Mark, if a motor is well tuned through out it's rev range, fuel consumption must be relative to speed.More so in the speed range that we are talking about as that is where the "drag factor" really start's to kick in.

[Hoomgar]

I have to dissagre on that point Mark, if a motor is well tuned through out it's rev range, fuel consumption must be relative to speed.More so in the speed range that we are talking about as that is where the "drag factor" really start's to kick in.

Duly noted. But this is how it is. This one is not my opinion. I don't think he has a problem at all.

[beby99]

For years the EPA was telling everyone that 55 was the most economical speed to run your car at

Yep...this is why I thought I could ride slower to conserv fuel :?
And now that you mention gear ratio....I had forgot that I changed
the front sprocket up one tooth for better cruising speeds (last year).

All makes sense now....so I should ride at 80-90 mph and if I get
busted I'll just say I'm doing my part to conserv fuel

Thanks guys.

[gspaul]

more speed, more power, more energy(fuel) ............''

[beby99]

more speed, more power, more energy(fuel) ............''

I know...I know....this is why I'm having trouble with this....both
you guys make sense too me....will try a few adjustments and
see's what happens.

[Hoomgar]

more speed, more power, more energy(fuel) ............''

I know...I know....this is why I'm having trouble with this....both
you guys make sense too me....will try a few adjustments and
see's what happens.

Yup. The part I believe your missing though Paul is still the same mistake the EPA was making. More speed does not always equal more power. That's the whole point here just so you understand. In Beby's case he is actually using LESS power at the faster speed. Bump it up to 100 MPH and everything changes again. All engines produce different amounts of power at their own RPM ranges. It is often the case that a slightly faster cruising speed will use less power and give better fuel economy than a slower one that runs said engine in a range that it does not produce as much power. Doing that make is work hard. Working hard equals giving it more fuel. It's what economics is all about buddy

[mark m]

The part I believe your missing though Paul is still the same mistake the EPA was making. More speed does not always equal more power.

Mark,

1) Unless you have found a way to repeal the laws of physics, more speed ALWAYS takes more power. It is impossible to avoid this. Power required goes up as the cube of the speed (that is, twice as fast = 8 times the power required) and this is a fixed law of nature.

2) Engines are ALWAYS more efficient at large throttle openings and lower revs because frictional and pumping losses are lower at this condition.

What I expect is happening is that the change in speed is causing a slight shift in the carb circuit the bike is running on and the lower speed range one is set slightly richer. The simple solution is to cruise where the mileage is the best, regardless of why this occurs. If the bike is running well and you are happy with it, I would not tinker just because of this quirk. Besides, it gives a good excuse to cruise at higher speeds...


Mark

[gspaul]

I see your point Mark(Hoomgar)(Hey! are we as bad as each other or what)But the power and torque curves for these motors have a fairly linier ascention, so I do not understand how at a lower revs they could use more fuel.
At 60mph I use 50-55, at 50mph 55-60, but when I get carried away 40-45.

[redliner1973]

Also keep in mind that an engine produces it's best efficiency at it's peak torque output. the closer you are to this at whatever speed ( reasonable, of course) the more efficient the engine will run,and better mileage it will produce.

[gspaul]

I don't think I would get better fuel consumption at 8000rpm rather than 4000rpm......'8O'
But hey I could be wrong!.........''

[mark m]

Also keep in mind that an engine produces it's best efficiency at it's peak torque output.

This is only for the full throttle condition, where intake and exhaust pulse tuning and cams dominate. At lightly loaded, part throttle conditions (such as driving down the highway) the pumping losses and friction are the biggest factors, so lowering revs and using bigger throttle openings will produce the highest efficiencies for cruising along.


Mark