Removing baffles

[Mike54]

I'm no expert, but think I read somewhere, even 4strokes need a bit of back pressure to ensure good performance, so taking out baffles negates that, unless you change mixture, etc.

back pressure myth

https://forums.mightycarmods.com/forum/ ... it-s-wrong

[bowber]

I'm no expert, but think I read somewhere, even 4strokes need a bit of back pressure to ensure good performance, so taking out baffles negates that, unless you change mixture, etc.

The term back pressure quite often gets confused with an increase in pressure, in fact it's referring to the reverse pulse created when the positive wave of energy leaves the cylinder and then reaches some form of edge and then reverses and goes back to the cylinder to help evacuate the rest of the exhaust gasses, a tuned system will work better as a totally open exhaust with no restrictions, just look at MotoGP and F1.
It's a very complicated subject and I suspect that computer simulation has brought it all on a long way so that we can have high power outputs from quiet exhaust systems, however it still messes with the motor.

Steve

Beat me to it Mike

[catcitrus]

I'm no expert, but think I read somewhere, even 4strokes need a bit of back pressure to ensure good performance, so taking out baffles negates that, unless you change mixture, etc.

back pressure myth

https://forums.mightycarmods.com/forum/ ... it-s-wrong

I've read the article--and its really a bit mixed up--the original question was about removing baffles from a STRAIGHT THROUGH absorption type "silencer"--sort of a double negative/hit really over the carefully designed OE system. The point is that it messes up the SYSTEM which is designed to provide the best cylinder filling THROUGHOUT the operating range. Having an open exhaust and appropriate large valve overlap may well suit an engine at the top end (moto GP--the large valve overlap is driven by gas velocities operating without positive displacement and these are largely engine speed INDEPENDENT--so more valve opening in crank degrees is a by product of the flow being TIME limited), but this large valve overlap designed for top end coupled with reduced back pressure will certainly affect cylinder filling at lower RPMs with potential loss of fresh charge into the exhaust system (there is more TIME at lower RPMs for basic pressure to have an effect on the gas flow FROM the cylinder). A two stroke manages the positive and negative pulses from the changes in CSA of the spanny to both extract and then push fresh charge back in-but as I said pressure wave velocities back down the system are not engine speed dependant to any extent, so this "tuning" can only operate in a narrow rev range---aided these days by speed sensitive variable port height systems. You will note the Helmholtz chamber fitted to the latest 4 stroke KTMs--these produce a resonance and are positioned such that a positive pressure wave is designed to arrive at the exhaust port at medium/low revs to aid cylinder filling otherwise compromised by large valve overlap (for top end). Hondas, and all other modern 4 strokes need the back pressure as designed by the manufacturer to operate at their best--as a SYSTEM. If you think that more noise out of the back end means more power then go for it. It may well have been half true 40 years ago when computational fluid dynamics was starting to get going--but not now.

[HedgeHopper]

in fact it's referring to the reverse pulse created when the positive wave of energy leaves the cylinder and then reaches some form of edge and then reverses and goes back to the cylinder to help evacuate the rest of the exhaust gasses

Nope .......a reflected pulse can push unburnt fuel back into the cylinder of a 2 stroke instead of it going straight out the exhaust*, its not a feature in 4 stroke exhausts and if it was it would hardly help draw exhaust out as the pulse is in the other direction

* this happens because the exhaust port and intake port are partially open at the same time

[bowber]

Nope .......a reflected pulse can push unburnt fuel back into the cylinder of a 2 stroke instead of it going straight out the exhaust*, its not a feature in 4 stroke exhausts and if it was it would hardly help draw exhaust out as the pulse is in the other direction

* this happens because the exhaust port and intake port are partially open at the same time

When the positive wave is reversed it changes polarity and becomes an negative wave, these go back and forth reversing all the time and it's the timing of these that 2 strokes take advantage of to good effect but 4 strokes do too, and is why multi cylinder systems have specific lengths and joining points.
You can use a megaphone to help increase the strength of this wave or pulse but at the expense of a very narrow usable rev range (Manx Norton era road racers), adding a reverse cone to this megaphone increases the usable range but reduces the strength.
I have a shelf of books on this subject and used to spend days calculating it all with different valve timings etc for our racing motors, however all of this has been overtaken by computers being used to tune exhausts and motors and along with computer controlled ignition and fueling they can now use valve timings and hugely over square motors that would have taken huge amounts of time to get running properly.

The result of this is that I'd rather run the quiet standard exhaust than a noisy open one as it's works very well, the only benefit would be weight, and my original post stands in that to get the best out of an open system (no baffle) the OP would need to get the motor tuned on a dyno.

Steve

[catcitrus]

Nope .......a reflected pulse can push unburnt fuel back into the cylinder of a 2 stroke instead of it going straight out the exhaust*, its not a feature in 4 stroke exhausts and if it was it would hardly help draw exhaust out as the pulse is in the other direction

* this happens because the exhaust port and intake port are partially open at the same time

When the positive wave is reversed it changes polarity and becomes an negative wave, these go back and forth reversing all the time and it's the timing of these that 2 strokes take advantage of to good effect but 4 strokes do too, and is why multi cylinder systems have specific lengths and joining points.
You can use a megaphone to help increase the strength of this wave or pulse but at the expense of a very narrow usable rev range (Manx Norton era road racers), adding a reverse cone to this megaphone increases the usable range but reduces the strength.
I have a shelf of books on this subject and used to spend days calculating it all with different valve timings etc for our racing motors, however all of this has been overtaken by computers being used to tune exhausts and motors and along with computer controlled ignition and fueling they can now use valve timings and hugely over square motors that would have taken huge amounts of time to get running properly.

The result of this is that I'd rather run the quiet standard exhaust than a noisy open one as it's works very well, the only benefit would be weight, and my original post stands in that to get the best out of an open system (no baffle) the OP would need to get the motor tuned on a dyno.

Steve

Agree--unfortunately people like to mess--and some aftermarket firms make a fortune out of this with very little science behind them (IMHO), A piece of perforated tube with open cell "wool" behind it (interconnected SMALL chambers ) will only attenuate high frequencies--and frankly I've had enough of Hardly Ablesons rattling my windows with that sort of thing fitted (and serving no purpose except to generate annoying low frequency noise--sad barstewards.)

[Richard Simpson Mark II]

H-D: Turning petrol into noise without the inconvenience of horsepower since 1903

[chunky butt]

I'm no expert, but think I read somewhere, even 4strokes need a bit of back pressure to ensure good performance, so taking out baffles negates that, unless you change mixture, etc.

The term back pressure quite often gets confused with an increase in pressure, in fact it's referring to the reverse pulse created when the positive wave of energy leaves the cylinder and then reaches some form of edge and then reverses and goes back to the cylinder to help evacuate the rest of the exhaust gasses, a tuned system will work better as a totally open exhaust with no restrictions, just look at MotoGP and F1.
It's a very complicated subject and I suspect that computer simulation has brought it all on a long way so that we can have high power outputs from quiet exhaust systems, however it still messes with the motor.

Steve

Beat me to it Mike

Back pressure......nah it's pulses and velocity.....just testing you guys, I'm an expert now.... btw,good find that mike54

[Scott_rider]

I've just put the standard exhausts BACK onto my CBF1000A.... . It had Delkevic stubbies on it when I bought it but after nearly 5,000 miles in 6 months I've had enough of the noise, it seems odd but it was knackering over a few hours even wearing ear plugs.

The benefits are...much quieter, much smoother, and just much nicer to ride despite a very, very slight loss of 'shove' but it's barely noticeable.

[Richard Simpson Mark II]

I remember an interesting exercise done by, I think, Performance Bikes many years ago.

They had two Yamaha R6 bikes. One had been 'de-tuned' with cam profiles from a Yamaha Thundercat sports-tourer.

They invited a number of readers to ride the two back-to-back around a handling circuit they had constructed at Bruntingthorpe. They said one of the bikes had been modified, but didn't let on which one or how.

The bike with the mild Thundercat cams recorded the better time under the majority of riders...and was also the bike that the majority of riders picked out as the more powerful of the two.

It was actually less powerful than the stock bike in terms of peak power, but had more area 'under the line on the graph' when it came to torque.