Radical difference in fuel consumption with exhausts(solved)

[Kev L]

Another layman having a stab in the dark here, but could it have something to do with the bike being jetted to cope with altitude. No, oh well just a thought.

[thunder]

OK... been thinking about this...

hows this theory regarding back pressure....

cylinder volume is (for example) 5cc's.
With stock exhausts NOT all the gases are vented (due to exhaust resistance/back pressure) leaving (for arguments sake) 1cc of unburnt/burnt gas still there for the next cycle.
Thus the induction is filling a partially filled cylinder, up to 4cc only.(less fuel)

With the aftermarket 'zorsts , there is no back pressure, thus all 5cc's of exhaust are vented, allowing a full 5cc to enter on induction cycle...? (More fuel?)

i know this may be out there but it's about the only possible reason i can think....

[agentpineapple]

try using some db killers/baffle, not sure if they will give you back pressure tho....

[Steve97]

OK... been thinking about this...

hows this theory regarding back pressure....

cylinder volume is (for example) 5cc's.
With stock exhausts NOT all the gases are vented (due to exhaust resistance/back pressure) leaving (for arguments sake) 1cc of unburnt/burnt gas still there for the next cycle.
Thus the induction is filling a partially filled cylinder, up to 4cc only.(less fuel)

With the aftermarket 'zorsts , there is no back pressure, thus all 5cc's of exhaust are vented, allowing a full 5cc to enter on induction cycle...? (More fuel?)

i know this may be out there but it's about the only possible reason i can think....

[tony.mon]

It's nowhere near that simple, unfortunately.
Back pressure isn't solely responsible for allowing the engine to vent it's spent charge, many other factors are also important.
Remember that if there's valve overlap, then if no other factors were taken into account the pressure in the cylinder head (assuming static piston movement at that point in the cycle) would be atmospheric.
Next, take into account the effect of the intake venturis (stack) etc- the point of those is to ensure that the intake air continues to flow in a series of waves, rather than stopping and starting each intake cycle. Same with the airbox, whether a ram-air type or not.
Exhaust design plays a major part, back pressure is one factor but again the system has a function in developing a flow of air form the ex port, but also uses waves of back pressure travelling quickly back up the pipe )generated at or near the end of the can, so shortening cans has an effect on this), which if timed correctly can at certain revs, help to scavenge the spent charge form the cylinder.

Design-wise, valve diameter and timings are very important, gas flow properties of the heads (terrible on Storms, by the way) valve seat angle, port angle, etc, etc, etc, etc.

So please don't think that there's anything like as simple an equation between the volume of spent charge "left" in the cylinder and the amount of baffling in the can, it simply ain't so.

Otherwise, the best power an engine could generate would be with no cans at all, and a simple dyno run tells you that isn't the case.

(if it was, all racing bikes wouldn't have an exhaust at all, other than getting the gasses away from the rider's boots).

I'm back to loss of power generated with standard exhausts makes you twist the twistgrip more to give you the power you're asking for, so more fuel used=poor engine efficiency in terms of power output= rider compensating by opening throttle more.

Get it on the dyno, then you'll know.

[thunder]

Hokay..... turns out i'm not crazy...

Original cans back on, same sorta riding , and i'm back up to 150-160km per tank. (depending if i fill the tank upright or side stand)

BTW Tony, thanks for the explanation

I have found a place with a dyno that can do the bike (apparently).
I will pack my loud exhausts (bit too loud), and take the bike to them to be tuned...

Question, those of you that have been to a dyno, did you experience a greater fuel mileage afterwards?

Because i'm just trying to figure out if a dyno (damn expensive) is worth it, because if the only benefit (the bike currently runs superbly , manual CCT) is the sound... i dunno

TX
Thor

[Richy]

Question, those of you that have been to a dyno, did you experience a greater fuel mileage afterwards?
Thor

depends if you find a dyno operator that knows what they are doing, and will set your bike up accordingly.

Just running your bike on the dyno won't fix it but will give you the power output.

The skilled operator can use this to adjust certain bits of the carb, and then re run and compare the graphs.

Eventually, you have a bike running right hopefully. Also an empty wallet.

[tony.mon]

Question, those of you that have been to a dyno, did you experience a greater fuel mileage afterwards?
Thor

Yes, for the reverse of the reasons I listed above.
The engine will run better and more efficiently, and develop more power.
So you need to twist the throttle less.

[thunder]

Hi all

Well have been running with he standard pipes for a while, but loving the deeper sound of the others (more free flow), i put em back on.
Again the crap fuel consumption... something wasn't adding up.

Airfilter!!!!!!!!!

New air filter and i am now on 155km (as opposed to 130km) and the red light has yet to come on!
Also, the bike is developing alot more power, everywhere from pullofff to top end. Loving it.
(i have been giving it stick ,more than usual to make sure my figures are not false)

Then old air filter was crap, i did 10 000km on it and god knows how much mileage it had when i bought it.

Will let you know the total range until red light.

So...lesson learned.
Thanks
Thor

[thunder]

"I cannot think of any reason the bike should be +-1/3rd heavier on juice with aftermarket cans on? Anybody?"

You haven't got something partially blocking your air intakes? or a really shitty air cleaner I presume?

Sigh... i gotta listen more

[darkember]

Just remember in SA you will have alot more dust therefore change the airfilter more frequently.

[Xenocide]

Glad you got it sorted mate!

[sirch345]

Good to know you got there in the end

Chris.

[VTRDark]

Is this with a standard air filter or a new aftermarket? My fuel consumption is pretty bad with a clean K&N, so I'm considering trying a standard filter as I've never tried one and a re-jetting to suit. I currently get an average of around 80miles and the light light comes on. I can keep riding with the light on to about 100miles but then I need to fill up quick before I'm running on fumes.

(:-})

[tasum]

Is this with a standard air filter or a new aftermarket? My fuel consumption is pretty bad with a clean K&N, so I'm considering trying a standard filter as I've never tried one and a re-jetting to suit. I currently get an average of around 80miles and the light light comes on. I can keep riding with the light on to about 100miles but then I need to fill up quick before I'm running on fumes.

(:-})

I was going to ask the same question. Is the standard filter more economical than a k&n?

I have a k&n, but not sure that there was any carb/jet mods done when it went on. If i went back to oem would I need to do any adjustments? i appreciate the obvious answer is to try it and see, but that's just too easy

Must be fun riding in SA, but do you not have any security issues when out and about or are you tooled up?