Fuel economy better with an exhaust?

[racefactory]

I can't quite get my head around if carburettor bikes are better on fuel with standard exhausts or after-market ones. Who knows for sure?

Of course there is a weight saving from removing the stock item but in terms of flow they are very restrictive and would hamper efficiency right? Less efficiency means more fuel consumption? If a free flowing can is put on it should make the engine more efficient but then again you have to jet slightly richer making the bike use more.... so I'm confused.

[ducatilover]

My BROS650 had a full Moriwaki exhaust and it ate gas like a motherfucker, but, we suspect it had a big bore kit and cams too (71rwhp), which really wouldn't help haha It would do 130km before reserve...
My VT250 Spada always averaged 4L/100km over the three and a half years and 130km I did on it, that was with five different exhaust set ups and the most power I could squeeze out of it
My CB400 is missing some baffles and I think it's using more gas, but, that'd be the re-jet, it averages 6.2L/100Km

[TIBLE_90]

I found that fuel consumption does not change much with an aftermarket exhaust compared to stock. The biggest factor in fuel consumption is how you ride of course, and I found that with riding hard I use pretty much the same amount of fuel. Have not tried riding economically yet with the new exhaust though.

When I got an after market air filter on my old 250 I found that that actually did increase fuel economy, don't know if that helps though haha

[imdying]

but in terms of flow they are very restrictive

That's your first mistake right there.

[racefactory]

That's your first mistake right there.

How do you mean?

[superman]

The back pressures different exhausts provide cause can cause all sorts of engine improvements and be just as detrimental.

Fact is if something makes you faster it must either allow the engine to use more energy or be more efficient with wasted energy. Something like a turbo for example increases efficiency with no detriment to fuel economy.

Reducing the back pressure of an exhaust similarly increases performance. Bad ones can make you lose low-RPM torque and fuel efficiency but usually well designed exhaust systems improve fuel economy as well as give higher performance. Really it all depends on the quality of the exhaust system and it's design relative to your bike.

[steve_t]

Something like a turbo for example increases efficiency with no detriment to fuel economy.

[ducatilover]

Fact is if something makes you faster it must either allow the engine to use more energy or be more efficient with wasted energy. Something like a turbo for example increases efficiency with no detriment to fuel economy.

Only in the case of let's say, newer VAG made cars....
A 4G93T CM5A Mitsubishi Lancer GSR for example VS a CD5A 4G93 Lancer, the Turbo GSR strolls along at 110km/h at similar revs, but uses 10L/100km IF you're lucky and are not on positive boost.
Whereas the CD5A Lancer uses just over 7....why? Because it has no turbo, the injector timing is much "smaller" and the turbo motor is also less efficient off boost because it has a much lower compression ratio, needing more retarded timing and generally being a retard.
BUT, newer Turbo engines are massively efficient due to taking advantage of partial throttle torque and also labelled as efficient because they make less emissions than a car with the same engine size and HP that's naturally aspirated.

[willytheekid]

I would have to say its more in the carb set up than the pipe.

I run a full termignoni race system with 41mm keihin carbs, when I first bought the bike I was lucky to get 160k per 15ltr tank full !!!
Now after a carb rejet (bigger jets!?!) and retarding the floats (leaning out the carb), the result is now 230k per 15ltr tank full
...and it only took me 4 different sets of jets and hours...and hours! of playing around with temperamental bloody carbs to get that saving and still obtain some "poke" from the carbs

oh the fun!

[bogan]

The biggest factor in fuel consumption after an exhaust change is whether the jetting/mapping is done correctly, next would be whether you ride differently. All things being done properly and equal, I don't think it would make bugger all difference (i'm yet to notice any on mine).

However, the efficiency of engine is effected to the temperature in the cylinder, hotter is more efficient, so back pressure may help there, but dunno whether it offsets the pumping losses associated with the back pressure.

[nosebleed]

That's your first mistake right there.

Almost, but not quite...

...carburettor bikes ....

Theres the problem.

[ducatilover]

The biggest factor in fuel consumption after an exhaust change is whether the jetting/mapping is done correctly, next would be whether you ride differently. All things being done properly and equal, I don't think it would make bugger all difference (i'm yet to notice any on mine).

However, the efficiency of engine is effected to the temperature in the cylinder, hotter is more efficient, so back pressure may help there, but dunno whether it offsets the pumping losses associated with the back pressure.

With the right exhaust set up you increase gas flow and scavenging in the engine (This is for cams with big overlaps) so theoretically will improve economy, just a matter of how much.
Fuel wants to go in cold, but, ignition needs to be hot, so there's a play off there, this is why the Toyota 7AFE lean burn model wasn't so great, too hot, the intake charge was less effective. It's all about compromise.
Ideally we would have a pneumatic valve train, multipoint injection (Direct and non-direct), variable length and volume intakes...variable valve angles and all sorts of shit, but, cost dictates that these aren't happening too soon on bikes (Pneumatic valve train that is...)


I see a project? Or making an emulated VarioCam system....

[bogan]

Fact is if something makes you faster it must either allow the engine to use more energy or be more efficient with wasted energy. Something like a turbo for example increases efficiency with no detriment to fuel economy.

Turbos only increase efficiency when the manifold is under boost, otherwise they just increase backpressure and pumping losses, superchargers would be much the same but with more awesome. Faster also isn't a global term, exhaust systems can pick up power or efficiency in som areas and drop it in others. Wave scavenging and inertial scavenging are what effects the performance of the engine, and each has tuned length requirements, the tuning of these lengths governs where the scavenging is beneficial or detrimental to the flow of the engine. Though inertial scavenging is likely much more effective at WFO, as you obtain the gas velocity the exhaust was designed for. Guess what I'm saying is it's pretty complex eh!

[bogan]

Ideally we would have a pneumatic valve train, multipoint injection (Direct and non-direct), variable length and volume intakes...variable valve angles and all sorts of shit, but, cost dictates that these aren't happening too soon on bikes (Pneumatic valve train that is...)


I see a project? Or making an emulated VarioCam system....

but I reckon solenoid controlled intake valves with exhaust ports at the bottom of the stroke, and a supercharger
They are doing good things with multipoint injection and stratified charge ultra lean burn I hear.

[imdying]

How do you mean?

I mean for one it is wrong, and for another it's looking at the problem through a gross over simplification that ignores many of the mitigating factors. Or put another way, it's just not that simple.