Centaur exhaust

[AJWParticipant @ajw]

My Centaur engine had got put on the back burner!

Guilt kicked in and I am now in the process of mounting it on a proper base (instead of a lump of MDF!) and getting it to drive a generator and running off propane.

One thing that is bugging me is the exhaust, I have made up a couple of trial silencers and been disappointed at the ‘light’ sound produced. Best sound I have achieved so far is using a long piece of 15mm pipe whereby it sounds nice and meaty!

What have you boys used in similar sized engines?

 

Alan

[JasonBModerator @jasonb]

A cast “pot” type silencer would look correct for the style of engine. I don’t think Graham Corry has any left, Adrian at Enginer’s Emporium also does some but not sure on sizes though easy enough to make from solid.

Most of my open crank and hit and miss engines that do have silencers tend to be the pancake type, two discs with a small space between

[AJWParticipant @ajw]

<p style=”text-align: left;”>Jason, thanks for your reply! The pot type could be the way to go, I’m guessing the larger the internal volume would be required for the beefy exhaust note I’m after?</p>
Might try and experiment with some different volume containers (tins?) to get an idea.

 

Alan

[JasonBModerator @jasonb]

I’m not sure how much effect the more “weighty” casting makes on the sound compared with a lighter tube or tin.

[Andy_GParticipant @andy_g]

I was going to suggest a pancake too. Gives a nice ‘whuff’ rather than a pop. You can play with the spacing between the two halves to change the sound if you’re so inclined.

 

I’ve shown this video before but it might give you an idea of what an 8.5cc engine sounds like.

 

 

[JasonBModerator @jasonb]

Then again there is something to be said in favour of a straight through pipe.

Adjusting the advance/retard will also alter the note as well as running speed, could even start playing with the cams as they are fairly easy to move about on the side rod and an engine like the Centaur being throttle governed will sound different to the Hit of a H&M engine. I’m working on an engine that is based on an early National Gas engine at the moment and have gone with astraight through pipe with that, couple of weeks should tell what it sounds like, in the mean time……..

 

[AJWParticipant @ajw]

https://youtube.com/shorts/qT4ZB5j1gvs?si=O8eH4S2JXX4Msyah

 

All food for thought, this is where I’m at presently. By adding the length of 15mm pipe the note changes to one with far more low frequency output.

I shall have a play.

 

Alan

[AJWParticipant @ajw]

Had some experimentation today and decided that any short length of pipe makes for a fuller sound. So having a rummage through the ‘could be useful’ bits and pieces I found the cast iron outer shell from a centrifugal clutch and an aluminium handwheel which could fill the open end.

This seems as though it is worth experimenting with, the clutch part is about 3 1/2in diameter and a couple of inches long so it would look pot like! Will try and get some photos up.

 

Alan

[MichaelRParticipant @michaelr]

The exhaust on my Centaur is made from a piece of 2″dia brass pipe with a 3/8″ bore outlet. my engine has a new owner now

https://youtu.be/jxTixF_dQp8?si=wi_sauynTm2m_o-s

[AJWParticipant @ajw]

 
<p style=”text-align: left;”>Nice engine and exhaustnote!</p>
<p style=”text-align: left;”>Produced this trial exhaust pot today so the exhaust enters via the flange and possibly fill the insides with a stainless (scourer?) and the exit pipe could be adjusted in length for hopefully a nice beat!</p>
 

Alan
<p style=”text-align: left;”></p>

[AJWParticipant @ajw]

[Howard LewisParticipant @howardlewis46836]

Many small staionary engines used nthe type of silencer already shown. These relied on back pressure to slow the exhaust stream, and decrease the gradient of the pressure wave

The larger ones which were fixed in place within a building often used a pot type “silencer”, fitted some way down the exhaustb pipe, somtines half way or more from the engine. The back pressure wasaproduced from the inertia of the gas column within the pipe., and in view of the volume, the pressure wave was reduced in amplitude.

The length of pipe can affect the exhaust note. What id not wanted, with a view to silencing is for the pipe to resonate within the engine’s running speed range, (Preferably below or above the running range).

On resonance the noise is likely to increase markedly, and power may improve, but possibly at the expense of fuel consumption if charge is lost down the exhaust pipe.  That can also produce some intersting explosions on the oiverrun!

[Howard LewisParticipant @howardlewis46836]

The big National at Herford does not “bark”, since it has  a fairly long pipe, the exhaust note is more prolonged and tends to be a bit “hissy”.

You wouldn’t want a loud exhaust in or near, an urban environment.

(At C A V one night, someone on nightshift, ran an engine withouit checking if it was still on open exhaust, after a day time test.  It was! The locals were literally climbing over the gates to complain!)

Howard,

[DiogenesParticipant @diogenes]

FWIW, and as a point for further discussion, it’s actually noise that the OP wants to produce; I’d have thought (from several decades of motorcycle experience) that with the exhaust cam timing being so soft, compression being so low, & exhaust gas velocity so sluggish, that you’d be better off with a straight pipe deliberately tuned to give the strongest coincidence of pressure waves at the outlet, and to try and use resonance in the pipe wall itself to achieve the note you want.

I’d use a straight pipe (not too big dia. to keep gas velocity up), make it as long as looks ‘in scale’ / tasteful, and then see if you can ‘telescope’ a piece at the end and ‘tune’ it – somewhat in the manner of a trombone) – to achieve the note you want.

If you want to refine ‘tone’, I might try and use thin wall steel or stainless pipe – copper tends to be a bit ‘dead’ in it’s soft state..

..as a last resort see what can be done to get the Ex valve to open and close with a faster action.

I must go to work..

..the classic way to deepen the tone would be a megaphone – probably not the right look for a S/E tho’?

[AJWParticipant @ajw]

Spot on Diogenes! You’re right I want the exhaust to sound like an exhaust and have reverted to the straight through pipe design with the length to be determined by experimenting.

This is in copper at the moment but I believe I have some stainless tube somewhere! Don’t want to start mucking about with cam shapes.

 

Alan

[Howard LewisParticipant @howardlewis46836]

As jason says, exhaust cam and ignition tiings will affectbthe note, since each will afect the pressure of the gas when the exhaust valve opens.

With a small engine, a large exhaust pope will tend to produce a sound reminiscent of an expansion box, so probably best avoided.

It used to be said that tuned Austin 7 (750cc) engines benefiref ron having a smaller rather than a larger exhaust pipe, presumably to maintain a bit of back pressure, to prevent charge loss, and power loss on suceeding strokes.

Ao 15 mm copper pipe might prove to be too large, and smaller bore stainless may be beficial in both sound and performance / fuel consumption. Again pipe length can have an effect, although for a small engine, running at low speed the length of pipe needed for a resonance is likely to be MUCH too long.

So a rel;atively short small bore is probably the way to go.

If it is any help, I made a “normal” motor cycle sounding exhaust for a 250 cc 4 stroke twin, by making oa silencer for each cyliknder,  with holes drilled around the pipe, (15 mm doameter) equal in area to that of the the pipe, each side of a central plug, passing through a 28 mm daimeter copper pipe, closed at each end.

Obvously, this would need to be scaled down for a smaller engine.

Howard

[Grindstone CowboyParticipant @grindstonecowboy]

I believe from reading a lot of tuning books (David Vizard, etc.) that back pressure should be avoided in all cases. Any apparent increases in power from small diameter pipes is due to a scavenging effect, pulling exhaust gases from the combustion chamber – it depends on the speed of the pulses of gas in the pipe apparently.

Rob

[Howard LewisParticipant @howardlewis46836]

In general, back pressure is to be avoided (Especially if a turbocharged engine) to minimise pumping losses.

In general, engines are developed working against a set back pressure.

There is a general “rule of thumb” that a turbocharged four stroke tends to find itself working against a back pressure similar to the boost delivered into the induction system. As in mechaically supercharged engines, the charge mass is increased by charge cooling. Marine enmgines benefit from this since there is an almost infinite supply of raw water for this.

Decreasing backpressure (and pumping losses) may increase power, but at the expense of fuel consumption, because of charge loss.  Similarly, tuning an exhaust system for ram effect at one speed, might have just the reverse effect at a lower speed. If you are interested in running “on the meg” low speed economy, tirque, or a steady idle will be of lesser interest than output at the chosen speed.

For a constant speed engine, tuning the exhaust and induction syatems can bring benefits. Possibly why the old low speed, single cylinder engines used long exhaust popes with just a relatively small “pot” as an expansion chanber (Minmal use for silencing since its volume was probably less than that of the cylinder), part way along a long exhaust system.

The smaller engines just used simple baffles to slow the exhaust, and reduce the gradient of the pressure wave, for silencing. (I once used a two pint can, with holes pumched in the bottom as an effective silencer for a small single cylinder engine generating electical power for a remote house!  Part expansion chamber, psrt baffle.)

BUT, in carbretted four strokes, depending on the valve overlap, too litlle backpressure can result in loss of charge (and therefore fuel and power)

Port controlled two strokes, again, are sensitive to back pressure for the same reason.

Napier Deltics on B R exhausted into a large receiver.  Unfortunately, sometimes unburned fuel carried over and ignited in the receiver!

At least with uniflow two strokes exhaust valve timing gives some form of control.

The big, scavenge blown, and more recently turbocharged, EMD engines, in US, operate against minimal backpressure, since there is almost no exhaust pipework.

Some railroads claimed improvements by changing from two outlets to four, on the earlier naturally aspirated versions. (Presumably having decreased inter cylinder reactions)

To reduce noise, the UK Class 59s and 66s (Using the same engines) exhaust into a silencer so may be subject to back pressure.

Howard

[Grindstone CowboyParticipant @grindstonecowboy]

Very interesting stuff, Howard, thanks. There’s obviously a lot more to it than meets the eye 🙂

Rob

[AJWParticipant @ajw]

Thanks guys! I took the non scientific approach and have been silver soldering up all sorts of combinations! Final design uses 15mm pipe entering a piece of 1 1/8 in the side and exiting at the other end also in 15mm and must say it sounds pretty good!

Stripped down for final assy at the moment but will put some photos up once assembled.

 

Alan

[AJW]

On AJW Said:

Thanks guys! I took the non scientific approach and have been silver soldering up all sorts of combinations! Final design uses 15mm pipe entering a piece of 1 1/8 in the side and exiting at the other end also in 15mm and must say it sounds pretty good!

Stripped down for final assy at the moment but will put some photos up once assembled.

 

Alans

Good stuff. Sounds like you made yourself what the sports car guys used to call a resonator.

[Howard LewisParticipant @howardlewis46836]

Resonators, or expansion boxes, give an exhaust a distinctive note, whilst imposing minimal back pressure.

Sometimes the transition from one pipe size to another is made conical. This is to avoid unwanted reflections of the pressure wave from the end of a sudden ntransition.  Having said that, some exhaust designs, notably on two strokes, deliberately used this effect as a means of extending the useable torque curve at lower speeds.

Pre WW2, a chap called Morrison, made up a rotating valve which connected to a large number of manometers, so that the pressure wave, at various speeds (With different pipe and silencer arrangements) could be observed and measured .

Freddie Dixon, again pre war, used to use resonance and ram effects, on both intake and exhaust systems to improve performance of his Riley racers. He paif great attention to every detail, and was very successful.

The problem with resonances is that nature will give with one hand, and take away with another, so that what is a major benefit at one engine speed might be a disadvantage at another.

Many years ago, I had a 4 cylinder side valve, with an independence exhaust manifold and a short stragiht through silencer. With a 18 mm choke tube in the carburettor, it coulkd just about match a standard vehicle with a 21mm choke, and exhaust system, on acceleration, until the exhaust hit resonance. Then the car was left in the dust!  Pity the brakes werm’t as good

Howard

 

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