Getting the best sound


The sound you get from your locomotive when fitted with the Summerlands Chuffer (or indeed any type of chuff pipe) is influenced by a number of factors:


The Chuffer Design

The sound is generated by the chuffer at the sounding slot – but the tone is determined by the primary resonator volume above the slot. We maximise this volume by machining it out before silver soldering the cap on. The size has been determined by extensive trials where the Chuffer has not only been tested for sound, but also for the free flow of hot gasses from the burner.


The Chuffer Position

This is crucial as it generates a secondary resonance to amplify the Chuff. You can demonstrate this for yourself with a piece of 1/2” or 15mm tubing. Blow sharply through your Chuffer – as if you were blowing a candle out – and you will here the basic sound. Now put the Chuffer as far as you can into the tube and chuff it again. You will hear a marked difference. Now slowly move it out of the tube , a little at a time, and you will hear the note change.


What you have heard from this test is the reason why we must have the sounding slot well inside the chimney. It is why we have modified the instructions for Russell – where the shorter chimney meant that the slot was not far enough in. We did consider making a shorter Chuffer body mounted higher in the chimney – but the gain from the secondary resonance was lost with the smaller primary resonator.


How far into the chimney you can mount the Chuffer is not only limited by the fact that it should not protrude from the top (obviously!) but also, on the Roundhouse Twin-exhaust version, by the fact that the adapter must be well clear of the bottom of the chimney (at least 4mm) to allow free passage for the hot gasses on internally fired locos.


So for short chimneys try to get the whole chuffer body inside the chimney, even if it means the top is level with the top of the chimney (paint it black). For long chimneys, such as on 7/8ths scale, try to get the slot about a third of the way up (this will probably mean lengthening the chuffer connecting pipe with 5/32" K&S brass tube sleeved on - or contact us for a longer, exchange chuffer)


The Chimney/Smoke-box Resonance

The length and diameter of the chimney, as well as the construction, all have a influence on this. Thin tubing is louder than cast – but there are ways that the quality of your chuff can be improved – see “Improving your Chuff”. Some chimneys can be extended into the smokebox using thin walled brass tubing. The K&S range often has one that is either a fraction too small - in which case it can be made oval to push fit - or too large, when a vertical cut in the tube can make the difference and allow it to fit. Please note that the extension must end above the top of the boiler flue level and that this is not an option when using the twin pipe adapter.


The Load

The chuff is generated by the exhaust steam – just like on a full sized steam loco. The harder it works, the wider the regulator is open and the more exhaust steam there is. Give your loco a good load for maximum chuff. Some folk have tried a "brake" van where the is some rolling resistance built in - there are many possibilities.......


The Wheel Diameter

This simply determines the distant travelled per revolution – and there are usually four chuffs per revolution. At a given speed – the larger the wheels, the clearer and more defined the chuff. This also presents a problem with geared locos and with multi-cylinder locos like Garratts and 3 cyl Shays - too many chuffs becomes a buzz!


The Speed

This will be obvious – but the slower the loco goes – the more distinct the chuff. And the faster it goes, the more it blurrs into a buzz. If ever there was a reason for driving slowly with a good load on  (just like the real narrow gauge railways) then this is it! Watch the Fowler video to see what I mean (here).


The Boiler Pressure

I have found that for a given load, the chuff is louder and more distinct when running at lower boiler pressure - this may be because the regulator will be wider open rather than just cracked open.


Exhaust pressure

For the chuffer to work correctly, it needs the full exhaust pressure. If any sort of exhaust throttling is added, or in the original design (as in the Finescale Quarry hunslets) then the chuff will be reduced.


Throat Clearing

With your Chuffer you will experience what I call "throat clearing". This is where condensed steam fills the resonator, which goes quiet for a moment before it drains and clears. This is perfectly normal. I have been asked about drilling a drain hole, but this will significantly raise the pitch of the chuff.


Driving technique - Accucraft reversing valve locos.

These locos, which include the UK generic locos such as Ragleth and Lawley and any of the US locos which use the piston reversing valve, can be driven in a way to enhance or reduce the amount of chuff. The reversing piston acts as a throttle on both the stem supply and the exhaust, so if the loco is driven slowly with the regulator open, but the reverser just cracked open, the exhaust will be strangled and the chuff reduced. If, however, it is driven with full forward or reverse gear, and the regulator cracked open, it will chuff sharply.


For manual locos, this is straightforward as they are normally run in full gear and controlled by the regulator. For single channel R/C with the control on the reverser, it is important to use small regulator openings so that a good amount of forward gear is used. If twin channel, then as for manual, give it full gear and control with the regulator.

Improving your chuff?

Earl Martin in the USA has come up with an idea to improve the Chuff  on his Accucraft Ruby by reducing the exhaust pressure. His idea, originally to help drain the condensate, was to drill a single 1mm hole through the side of the adapter - half way between the threaded part and the top where the copper tube emerges.

Before drilling this hole, mark the adapter when in place, to ensure the hole does not point back down the flue. I am informed by a Youtube correspondent that this can put the burner out!


The effect on the Chuff appears to be that although the volume is slightly affected, the sibilant (hissy) edge that is often a feature of Accucraft locos is greatly reduced.


I have now tested this in my garage and outside in a brief pause in the foul weather. The video of the results is shown below


If you are in any doubt about your ability to carry out this modification, please do not try it. If, however, you do try it and don't like the result, then slip 10mm of K&S 3/16" OD tube over the adapter and the hole will be covered. This what I did in the test.


My thanks to Earl and to where he first posted the idea.