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

tend to lean things up? ( 4 stroke )
Ok ..I understand that a more free flowing exhaust will allow more exhaust gasses to vacate the combustion chamber but why would that “lean” the mixture as the void will simply be replace by fresh fuel/air mix from the carb/injector on the next induction stroke? Is it because with a standard exhaust some fuel remains unburnt in the cylinder and so normally adds to what is being drawn in….. when this is removed the mix leans out a little?
If anyone is in the know and could explain in simple terms it would be much appreciated
thanks
Bill

The cylinder is cleared more efficiently of the unburnt gases after combustion stroke so more room for fresh air/fuel mix same amount of fuel more air? maybe off the top of my head

I may well be completely off the mark with this but..
any pressure in the exhaust system outside the cylinder will affect the amount of waste gas left in the cylinder. A more freee flowing exhaust system will encuourage those gasses to move along the pipe, lowering the pressure so next time the exhaust valve opens, there is less to block the exhaust gasses leaving the cylinder. ideally a vacuum at the exhaust valve would suck all the gasses out (Yamaha ExUp IIRC). Less free flowing exhaust-more restictive exit along the pipe=more gasses at exhaust valve hindering exit of new gas from cylinder.
maybe.

reduction in back pressure from the exhaust means more air in the combustion chamber and therefore leaner running

I was under the impression that only a tuned length exhaust, irrespective of it’s ‘freeflowingness’ would change the scavenging on a four stroke sufficient to need the jetting changed, though my interest stopped in the mid-eighties so I’m probably wrong.

Used to be that manufacturers would always jet an engine on the rich side to compensate for fuel variation, especially across markets, so most of the time there were performance gains to be made simply by rejetting according to the characteristics you wanted. Whether this applies to injected engines, I have no idea.

Whether this applies to injected engines, I have no idea

Almost certainly not, the ECU monitors how much oxygen in the exhaust and fuels accordingly. Of course the OP’s bike may not be fuel injected.

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

Changes of the exhaust configurations should be accompanied by suitable changes to the inlet.

But of course, what people really want is a “California tune-up” – remove baffles from the exhaust for more noise and lighter throttle spring for the impression of more power. Like those unshaven fat scowling weekend warriors wobbling around on Harleys.

Because they use less stringent/different noise/emmission limits and weight as well.

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

Because the aftermarket exhaust is far better quality and more expensive (Akrapovic make some truly exotic ti/carbon systems), and/or because it no longer conforms to the same noise/emissions standards, because it’s “for track use only” isn’t it?

I took 5kg off my bike’s weight with just that one change, and added around 6rwhp

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

I don’t think people really do, do they? As nick alludes, it’s all for the image, and I’d include the manufacturers in that. Look at how the aesthetics of exhausts have largely gone round in cycles for decades.

To be fair, my ‘Blade did go extremely well with the new system on, but it’s not as if it was lacking in the grunt department as standard……..

It does still apply to fuel injected bikes. Most modern bikes only run closed loop control of the fuelling in a very small band to meet emissions criteria. BMW are the only bikes I’ve seen that run closed loop more extensively.

The rest of the time they are running open loop and will require modifications to the fuelling to account for any changes to the exhaust.

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

My bike went from 109BHP to 124BHP on the same dyno after fitting an exhaust made by a man in a shed in Essex. It needed significant changes to the fuelling too.

To be fair, it’s getting harder and harder to get more power by changing an exhaust these days. A few years ago, the origional 1200 Bandit was heavily restricted (for the restricted Euro market) in the end can, and just switching the can would give ~15bhp. But that was a carbed engine. These days it’s easier and cheaper to bosh a new FI map in for different markets, and the manufacturers have heaved so much money into development that there’s very little the ‘home tuner’ can do becasue the engines are so highly specced in the first place. I doubt a modern 600/1000/V-twin etc would gain more then 1-2bhp from an exhaust change, and loosing 5kg is absolutely pointless on the road. Just run with 1/2 tank of fuel instead.

THE main reason, like it or not, that people change exhausts is to show off. That is all.
If you admit that, fine, crack on, just don’t kid yourself that there’s any other valid reason 🙂

I’ve had a couple of bikes with loud cans. My Speed Triple annoyed me so much I replaced the expensive Triumph carbon can with a road legal Blue Flame and left the baffles in. It was fine at full chat but it pissed me off BBBRRRRRRRRRRRRRRRRRRRRRing all the time when at normal speeds. I couldn’t tell the slightest difference in the power, and Triumph supply different engine maps for their cans for free.
My last 2 have remained bog stock standard. People have so much to complain about with motorbikes already, then bikers whinge like little kids when they get done for cans and small plates etc. It’s just not worth is any more. Don’t give ’em an excuse to pull you, stay quiet and out of the way!
Plus, if you get the right bike, the airbox makes a far superior noise, right under your chin! 🙂

Carburettors are tuned for mixing air/fuel at a given rate.

Higher volume of air flowing through can exceed the carb’s capacity to add fuel to it.

Carb needs rejetting.

You might even need to change ignition advance and also change to a hotter or colder spark plug (I can’t remember which – would have to google).

Oh – and you’ll have to change girlfriends because the current one will get sick of you spending hours dicking about with your motorbike.

Plus, if you get the right bike, the airbox makes a far superior noise, right under your chin!

Removing/replacing the filter and/or air box depending on where and what you were riding was the easiest and cheapest performance increase going in my youth.

My bike went from 109BHP to 124BHP on the same dyno after fitting an exhaust made by a man in a shed in Essex. It needed significant changes to the fuelling too.

I’d be interested to know whether changing just the fuelling on the stock exhaust produced similar figures, although quoting peak power changes doesn’t mean much if you can’t see the curve.

I doubt a modern 600/1000/V-twin etc would gain more then 1-2bhp from an exhaust change

My 2007 GSX-R1000 gained 8BHP by removing the cat and changing the end can.
My 2010 R6 gained 15BHP by changing the entire exhaust system and air filter.

There are still big gains to be had on modern bikes with exhaust changes, although the restriction is not normally in the end can.

I could see the point of mucking about with exhausts 30-40 years ago but the bikes these days have more than adequate power.

I think it was 34bhp to do 100mph, and 45bhp to do 120mph on an unfaired bike. Where are people using the 100+bhp? Do they all have private test tracks?

I very much doubt average speeds point to point are much better.

Vinnyeh, the standard fuelling on the standard exhaust was very good, there were no big gains to be had anywhere.

The new exhaust made more power than the standard exhaust from 6000RPM right through to 16500RPM

I came late to this post, shame.

In essence, as said above, an exhaust which has lower back pressure will allow more exhaust to exit, and therefore more air into the cyl, and usually the ECU will keep control of this on cruise but under any load (where it’s important) you’ll run lean if the ECU is speed-density or throttle position based, any system running an air flow meter (don’t think they exist on bikes, could be wrong) will compensate at all stages.

Easier breathing generally only affects the upper end of the RPM range and assuming you’ve still got your desired tuned length the same you can only benefit.

I put a laser duotech exhaust on my DL650 because the standard one looked like it belonged on a skootah, ruining the look of the bike IMO and made it sound like a sewing machine.
With the duotech it actually sounds like a v twin now, but could be mistaken for stock sound at normal speeds and looks like it belongs on the bike. 😀
Also seemed to help a lot with economy, I get an extra 20 miles per tank for some reason!

THE main reason, like it or not, that people change exhausts is to show off

Have owned a couple of Harleys and it’s important that the birds fall out of the trees when starting up.

Almost certainly not, the ECU monitors how much oxygen in the exhaust and fuels accordingly. Of course the OP’s bike may not be fuel injected.

If the OP’s bike’s using the original ECU this certainly won’t be the case. Even if it is equipped with closed loop fueling a narrow band lambda won’t be able to provide anything meaningful for the ECU to do with the fuel. The sensor voltage will simply fall away to approx 0.1 V if it detects a lean condition & probably ellicit a warning light or “limp home” mode.

A short run on a dyno equipped with a wideband O2 sensor would confirm either way.

D.

Coffeeking, I’ve never seen a bike with an air flow meter. Everything I’ve seen has always been speed density/throttle position based.

I very much doubt average speeds point to point are much better.

Even as a non-motorbiker I can see that you’re missing the point.

It’s because it’s MORE, isn’t it.

I think it was 34bhp to do 100mph, and 45bhp to do 120mph on an unfaired bike. Where are people using the 100+bhp? Do they all have private test tracks?

On the first point: Acceleration? It’s all very well being able to do 100mph, but pointless if it takes the lngth of the M1 to get there.

On the second: Track days? I know a few contractors that I work with that think nothing of paying £150-£200 for a day, then the same again in tyres, the same again in fuel, pads, etc and they usualy come back with tales of woe about clutches or other mechanical breakdowns. And when they’re not on track they’re raceing MX/Enduro’s. And I thought MTB was an expensive hobby!

My 95 ish bmw was fuel injected – a freeflowing exhaust gave 6 bhp extra at the top end adn 10 bhp in the midrange but leaned it out – a chip that richened it up gave another couple of BHp

I very much doubt average speeds point to point are much better.

Made a big difference to my lap times at Snetterton. Not quite point to point but does that count?

Coffeeking, I’ve never seen a bike with an air flow meter. Everything I’ve seen has always been speed density/throttle position based.

Me neither, but my bike specific knowledge is quite limited so I thought I’d be open to the possibility. 🙂

Loud exhausts are fun, I’ve got one on my car – not to show off, but because it makes me smile. I don’t give a damn what anyone else thinks about it quite frankly! 🙂

molgrips – Member
“I very much doubt average speeds point to point are much better.”
Even as a non-motorbiker I can see that you’re missing the point.

I’ve been on motorbikes all my life and I thought the point was to get there faster. 🙂

Handling and braking are higher priorities to me.

Harmitans – Member
I very much doubt average speeds point to point are much better.
Made a big difference to my lap times at Snetterton. Not quite point to point but does that count?

Racing is different, so no.

My 95 ish bmw was fuel injected – a freeflowing exhaust gave 6 bhp extra at the top end adn 10 bhp in the midrange but leaned it out – a chip that richened it up gave another couple of BHp

I have new found respect for you TJ – not only have we agreed several times recently but now I find you’re a vehicle modifying enthusiast. Excellent.

An old mans motorcycle as well coffeeeking . 🙂
Epicyclo – I tend to agree – that BMW was 100 bhp and 225 kgs – and Even with decades of road riding experience it was a rare day I could actually thrash it on the roads. Its thrashing a bike that is fun. Modern sports bikes simply cannot be thrashed on the roads

coffeeking:

Loud exhausts are fun, I’ve got one on my car – not to show off, but because it makes me smile. I don’t give a damn what anyone else thinks about it quite frankly

Yep couldn’t agree more, on all my bikes I have changed the stock can for one with a more fruity noise, that is except my current one…. which came with a loud high level can which I’ve replaced with a secondhand stock unit 😕

EIT: just going to go on it right now 😀

It’s not just top end power that can benefit from a more open pipe. Mid range torque can go up a fair bit too as bikes are often tuned to get through noise tests at those revs.
Chart is for GSXR1000 from the Akrapovic site

TandemJeremy – Member
…Modern sports bikes simply cannot be thrashed on the roads

That’s why I was querying the need to change the exhaust for more power.*

I got one of the first BMW K1s in Oz, and although it was much faster than my boxer twin it never seemed to get to the other end any quicker. I then had a Ducati Monster 900 and it was actually slower point to point because it didn’t like any irregularity on the roads. (I’m talking about riding between towns 275 miles apart and normal Oz road conditions).

*The same money spent on suspension and tyres is more likely to make the bike faster in the real world.

I’ve been on motorbikes all my life and I thought the point was to get there faster.

I meant that you probably weren’t trying to make your average day more efficient by arriving a few minutes earlier.. 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.

It’s not just top end power that can benefit from a more open pipe.

To be fair, again I’m coming from a car world 6-8K is still top end to me. Anything 10K+ is astronomicalbatshitmental 🙂

epicyclo – the bmw it did improve throttle response as well

I’m not actually currently intending to dick about with intake and exhaust ( it’s a honda dominator btw ) obviously carburated not Fi .I just like to understand things fully and I’m still not 100% clear about this.
coffeeking summed it up as “In essence, as said above, an exhaust which has lower back pressure will allow more exhaust to exit, and therefore more air into the cyl”
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 ?

Why is it that SOME bikers think it is acceptable to make as much noise as they like?? I have heard the safety argument (“people can hear me coming a mile away etc..”), but it’s a bit lame (most of us manage to get by on silent 2 wheelers). I mean, if every vehicle on the road had the same dB level as some motorbikes (many of which I assume are using aftermarket exhausts?), we’d all go freakin’ nuts! Or are they all within the regulations and I’m just being a grumpy old git? 🙄

Kaiser, you’re assuming a linear relationship between air flow through the carb and the amount of fuel delivered. I have limited experience of tuning carbs but I’m fairly certain this isn’t the case.

When we fitted a full exhaust system to a carbed CBR600 it took many hours of dyno work and plug chops to tweak the fuelling and it was still a compromise. It makes fuel injection tuning seem a doddle!

You’d be better off buying a decent book on exhaust/carburettor tuning if you really want to understand it. The relationship between exhaust back pressures and flow velocities etc are fairly complicated stuff. I would love to have the time to understand it better too.