when you fit a more free flowing exhaust on a motorbike why does it ..

It’s as much about the SMIDSY’s as individuality.
The biggest bug-bear as a motorcyclist is inequality.
Exhaust – points/fine.
Small’ish number plate – points/fine.
Breathing on a thursday – points/fine…..

Yet – Chavtastic boyracer in his Punto/Scooby/GTISRXTWHATEVER…. can black his windows, stick a length of drainpipe under his bumper, pump up his ridiculous amp with 4 of his chav “BOYZ IN DA MOTA!!” and happilly float past the plod whilst they are undertaking a “random” spot check on every Biker that happens to be out at that time for “irregularities….”.
And before anyone says “it doesn’t happen like that” – my best friend and riding partner is a senior traffic officer with disciplinary’s against his file for refusing to undertake said “random” checks.

what confuses me though is that ok ..more air will come into the cylinder but that air is coming through the carb and is/should be mixed with the appropriate amount of fuel on the way in .So why is it leaner than it should ideally be ?

Carbs are not linear and can use multiple needles to inject at various throttle positions but in general they’re safer than EFI in such conditions, they’ll at least try to match.

As for sound – my car is loud, quite loud. At full chat it’s really quite loud. Bumbling around the estate I keep it low and slow and it’s fairly quiet – only really gets opened up in teh countryside or on motorways. So I suppose you’re being grumpy, it’s the occasional car/bike making a noise, not constant din. If it were constant din I too would complain. I’m not keen on the local kids with 1 litre saxos and 4″ exhausts who cut out all silencers and make a massive noise at idle.

i feel i am fairly well qualified to comment. i have raced motorbikes for about 20 years, and recently i have spent probably 50 hours on a dyno testing and developing exhausts, inlet trumpets, manifolds, throttle bodies, carbs, different inlet valve sizes etc on the same motorcycle engine. (i have run it on carbs, and on a custom made injection system) and many many days testing and racing at tracks.

anything that changes the volumetric efficiency (how much air gets sucked in during one cycle) of the engine will need the fuelling altering to compensate. ie different jetting or fuel mapping.

i also happen to work at a very large automotive OEM, and i can tell you that it takes thousands and thousands of man hours to calibrate the fuelling of a powertrain so it is correct (production vehicle).

my particular engine makes most power at an AFR (air fuel ratio) of about 13:1 (13 parts of air to 1 part of fuel, in weight)

if i increase the volumetric effeciency by a small amount by an exhaust tweak or changing the valve size of something, then there will be slightly more air in the combustion chamber, therefore more fuel will be needed to keep the 13:1 AFR

you cannot comment on the effectiveness of a modification until you have checked and if necessary altered the fueling. Fact.

molgrips – Member
…What I meant was fun is the point. Average speed surely is not the mark of fun? Speed at certain points or through certain sections probably is, so I suppose there’s a correlation at least.

Fun is cranked over, both tyres sliding and the bike bucking on the ripples passing the white-knuckled wobbling guy who’s wasted all his money on power increases. 🙂

When you already have more power than you can sanely use, it’s pointless increasing it. Spend the money on the chassis, brakes and tyres first.

Porter_Jamie, what bike was all the development for? Sounds like an interesting piece of kit.

I put a bigger barrel, bigger carb and bigger exhaust on my monkey bike, and went from 6.5 bhp to about 8 bhp.

Cool; it sounded like a Ducati!

this. it is a supermono – the class rules are:
1. one cylinder
2. four stroke
3. meet standing acu regs.
thats it!

my brother and i (him mostly) made most of it including the frame, swinging arm, tank, exhausts and so on, and i did a most of the machining on the engine. he won the 2010 british supermono championship on it.

That’s fantastic. Thought it was going to be a supermono based on all your mods!

I race with my brother too except we’re in endurance so closer to supersport regs.

porter_jamie – Member
this. it is a supermono…

Great looking bike.

That is a good example of the point I’m trying to make. I’ll bet on any ordinary winding road that would be faster than a big four bike with a 100bhp more.

Be perfect for a blatt round Loch Ness 🙂

Why do people think they can ad-lib their exhaust flow better than the manufacturer with millions of pounds spent on research?

Riders of some fuel injected Guzzis may ask how a manufacturer can produce such a poorly developed fuel injection system?

Thanks!

It is not road legal, but it weighs 110kg wet, and has 72 rwbhp on a dynojet. It’s all about the corners!

I’ve not changed a main jet or dropped a little needle shim on the floor for years now. Feels wrong 🙁

porter_jamie – Member

anything that changes the volumetric efficiency (how much air gets sucked in during one cycle) of the engine will need the fuelling altering to compensate. ie different jetting or fuel mapping.

Weirdly, my bike came rich from the factory and a simple end can swap and rubbish panel filter left it closer to optimal than Suzuki had managed. The assumption that the bike’s close to perfect in the first place is a bit of an unwarranted one IMO, especially with an older bike.