Exhaust chamber areas

[ian redshaw]

Hi folks, my current project in full size was powered by a converted Ford 10hp four cylinder sidevalve motorcar engine. My half scale model is powered by a Zenoah 62 but uses the crankcase of the Ford both as the Zenoah mount and the exhaust chamber. Now to the question. Has anyone got any reasoned suggestions as to where a baffle should be, and what type. Some say that you need to make the gasses travel further so they are much cooler on exit, others say baffle to death and make the gasses try harder. My preferance would be to put two plates in, both standing vertically running front to back, the first with a gap at the front, the second with a gap at the back, so the gasses have an unrestricted flow but have to travel three times the length of the chamber. The actual outlets will be four tubes on a manifold to match the full size. Here's some pics. I know its heavy, but it needs to be heavy and strong as it needs the noseweight, holds a Zenoah and also has a bracket and strut off its head to hold the top wing in register.

What say ye? Ian.

[John Greenfield]

Ian

I am not a fan of the "baffle to death" school of thought as too much back pressure can be created. Your idea of 2 plates and moving the gas around is sound but those long plates are going to need some cross support otherwise they will flex under the pressure of each power stroke. This will lead to a "tinny" sound and early fracture of the plates.
My personal preference for your model would be to divide the silencer into 3 chambers front to back and have a tube in each baffle that runs almost the full length of each pair of chambers. This will give a long gas flow path and the baffles will be better supported.

Easier to explain on the phone rather than in words so give me a call if you want.

John

[Andy Macqueen]

"I am not a fan of the "baffle to death" school of thought as too much back pressure can be created"

here here, It is a 2 stroke motor all said and done,as you know they do rely somewhat on back pressure but too much will create heat build up and the motor will run like a dog... your current idea will work,might be a bit loud and performance will not be the optimum but it will work never the less, as john said it would be better if you insert/weld interlocking cross sections between the plates to stop the internals falling to bits from vibration,i would simply drill holes in the cross sections rather than using a tube as again you can create too much back pressure depending on the size/s of tube you use especially through 3 chambers.
hope this helps
andy

[Cary Bailey]

Ian, speaking to my friend (my former race mechanic & exhaust tuning manufacturer) he recommends that for the Zenoah (this motor is a long stroke I presume) you will need to form a pipe from the exhaust port that enters your expansion chamber (which you showed me) that I presume sits on top of the engine, which this then allows the exhast gas to expand. The workings are that 2 strokes need a back pressure and a wavefront is formed from the exhaust pressure. The exhaust gas pushes its way into the expansion chamber which is already occupied by gas from previous cycles, pushing that gas ahead and causing a wave front. Once the gas flow itself stops, the wave continues on by passing the energy to the next gas down stream and so on to the end of the pipe. Too much back pressure and you will burn up the piston at the exhaust port and too little will cause the motor to run badly and have no power, therefore you will need to install 2 baffles across the chamber with a series of holes drilled in the plates. The size and number of the holes can either be matched to the throughput volume of the exhaust gases or slightly under less than this which acts as a "stinger" that helps the wavefront to be formed. There is a formula that can be used to calculate the volume etc which you can then transfer to the appropriate number of drill holes or size of drill hole. I can get you the formula for working this out if you want it so that you can get the best out of the motor.
Cary

[Phil Clark]

Would be very interested in that formula as well Cary......I've made several st/st muffler systems for Zenoah 45's and 62's........all have run well with a 3 chamber expansion chamber (so 2 baffles) but the number & size of holes has been purely guesswork, so it'd be nice to know if I've been anywhere close.

Phil

[Cary Bailey]

Phil, in response to your request here goes:-
My former race mechanic is Bob Clowes who is a professional racing exhaust system manufacturer with a big pedigree in the 2 stroke racing machines such as bikes and racing karts. This formula can be developed to the Nth degree but the starting point is a follows:-
Using the engine size (cc) of the engine you can multiply that figure by 15 to form the size of the expansion chamber. Therefore using 62cc then the expansion chamber needs to have a capacity of 930cc. At this point we do not need to worry about the shape of this chamber. Then we look to the Cross Sectional Area of the exhaust port to calculate the size of the tail pipe (or pipes). For example let's use 20sqmm as the size of the exhaust port, we then use the following calculation to find the tail pipe size:-
square root of (20 X 20 X 0.785) divided by 2
To explain the formula is that you use the square of the CSA then multiply it by 0.7854 (this is pi divided by 4) then that is all divided by 2. The resulting figure is 157.08. Now you need to find the square root of this figure which is roughly 12.55. Therefore the tail pipe size needs to be as close to 12.55mm diameter as you can. This size tail pipe diameter forms the mechanical device that enables the 2 stroke waveform or backpressure require for good running of the engine. A more tuned engine will require the cross sectional area worked out by using the bore and stroke figures of the engine, but seeing as though you already have an exhaust port suitably made on the engine itself you can use that CSA.
So now we have a 20sqmm exhaust port feeding the expansion chamber and a 12.55sqmm tail pipe requirement at the end of this. The engine will run fairly well using this calculation but will however be noisy as there are no baffles in the expansion chamber. 2 trains of thought develope from this, one being an add on silencer after the expansion chamber or fitting of baffles within the chamber. There are several methods applied here either by having a chamber larger than calculated but have the first baffle fitted at the desired expansion chamber size and then have baffles to suit after this point depending on how much noise reduction you require. The baffles then need to have holes drilled in them at this point. The size of hole can vary so long as the total hole sizes exceed the required tail pipe size, then the silencing works, however the larger the holes the less it reduces the noise and the smaller the hole you will need to drill a lot of holes. Again a compromise is reached at this point as to what noise reduction you need.
Yes I know this is a long winded explanation to what looks a simple task but years of design into exhaust systems has been carried out to even get a simple formula that seems to be a rule of thumb, the calculations can run into formulas that need a computer to work it all out. It took Bob over an hour to explain it to me again so that I could understand it myself & he used to make my exhaust systems during my racing days!
Cary

[ian redshaw]

Cheers for that bit of bumph Carey, much appreciated. I'm going to see if I can accomodate these figures into my Carden Ford.

Ian.

[Phil Clark]

Thanks Cary.......I will copy & paste that somewhere safe for future reference.

Just 1 thing to confirm.....you talk about the tail pipe exit in sq mm and diameter........which one is it (I'm assuming the sq mm as per the exhaust port)

Phil

[Peter Clare]

Hi Carey, I know this has nothing to do with Ian's enquiry but your calculations from Bob Clowes seem to present some problems.
The exhaust port area of 20 sq mm seems awfully small to need a tail pipe diameter of 12.55 mm. This 12.55 mm diameter then changes without explanation to 12.55 sq mm area.
My maths isn't very good but perhaps a problem has crept in with units (mm/cm) and diameter/area.
Peter Clare

[Cary Bailey]

Peter, yes I seem to have got the info a little cross wired. I will check with Bob and get the figures corrected.
Cary

[ian redshaw]

Many thanks folks for the ideas. I've finalised the plan now, two plate dividers fitted, to split the chamber into 3 layers. These plates have an 6mm rod up through them in the middle to support them a tad, plus the open end has a return with the gas flow to stop it flexing. Gasses in at the back of the lower chamber, to the front and up into the middle chamber, then to the back and up into the top chamber. The top chamber is accessible by removing the cast ali cylinder head. This will allow modification to the baffles if necessary and also perhaps include some exhaust wool which has been suggested by another race exhaust builder. The proposed four exhaust pipes can be tweaked to adjust back pressure by making them extend into the box more or by being closed up a bit. We'll see!

Cheers again, Ian.

[Alan Alldritt]

Ian,
It does seems an eminently sensible idea to make the exhaust system modifyable should it be required.
You don't need to be an Adolf Schnürle,Walter Kaaden or Joseph Erhlich to know that some aspects of 2 stroke port and exhaust system design still remain a black art.
Whilst worked on racing 2 strokes ( motorcycles,cars and outboards at de Havilland who paid for EMC's adventures) - and yes they did have a racing dept! - as an apprentice in the mid 60's my overriding impression was that it was all very much a game of chance and mystery.
Certainly herr Doctor Erhlich - a cross between the nutty professor and Captain Nemo - frequently displayed signs of terminal madness when just by changing the shape,size or volume by almost unbelievably small amounts it could reduce the power and torque to virtually nothing.
Perhaps Ernst Degner took the real secrets with him when he defected to the West - or rather far East - in the early '60's?
Whichever way you decide to design it it remains a stunning model and I look forward to see it aloft.
ATB
Alan

[ian redshaw]

Cheers Alan, it'll certainly fly once!! I've finished the bulk of the exhaust, just the final four pipes to sort, but I'm tempted to run it up soon to see how and if it performs, I'm pretty confident it'll be quiet enough though but we'll see. Just needs a good clean up with a die grinder and a flap wheel as I aint no welder!! The Carden Ford weighs 2.5 kg so I doubt it will rattle to much. The model might need some noseweight as well though, again we'll see. Thats what is so great about our hobby, the experimentation, highs and lows.

Ian.