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Does anyone know why the side valve models have a larger diameter exhaust pipe than the OHV models?..An M20 for example has a pipe of 1 3/4" diameter whereas a B33 has a pipe of 1 5/8"...Ian
email (option): ian@wright52.plus.com
The hotter gasses????? Ron
email (option): ronpier@talk21.com
The length and volume of the exhaust system seems to has great effect on silencing and it also on the cylinder filling.
Besides the pipe the silencer of the sidevalve is also much bigger than the OHV. Looks like lower revs needs more volume ??
And as the silencer of both is more or less in the same place the increase had to be found in the pipe ?
Will see if I can find something in the books..
M22 has 1.85" (47mm) pipe, so I doubt in that OHV versus SV theory. Pipe with larger diameter just looks cooler:muscle:
['M22 has 1.85" (47mm) pipe, so I doubt in that OHV versus SV theory....']
The Gold Star has an 1.750" pipe...and Daytona models have a 1.875" pipe...Exhaust pipe diameters and the reasons for the variations are not well covered in the literature from what I can find, or not find!...
Combustion temperatures are the same in both SV and OHV engines....The former run at higher temperatures only because the design is not as efficient at turning heat energy into power and disposing of the resulting 'waste' heat...I always find it a fascinating idea that a 100% efficient engine would run completely cold...
Length is covered more than diameter in the literature but even that is a subject I haven't really got my head round to the extent that I don't yet know enough to work out the correct length for a given engine....'Tuned' lengths are a complicated subject!!...Ian
email (option): ian@wright52.plus.com
I fitted the larger Daytona pipe on my Goldie café racer purely because I like the 'fat' pipe look...However, I'm looking for the information now as I have to make a decision on the best pipe size for my sprinter project and that is a more critical application!...
email (option): ian@wright52.plus.com
This is the next page but you will probably have this book yourself:
Seems the shorter the pipe the bigger the diameter, and use of megaphones require an even shorter pipe with even bigger diameter.
Maybe take the M20 pipe and make it easily adjustable in length for testing??
The Husqvarna V twin racers had the end sticking out a metre behind the rearwheel. :slightly_smiling_face:
BR Michiel
Don't know if this helps.
I have a technical book somewhere but it might take some finding. Also I have a calculator for determining optimum dimensions. I can send the working version by email (can't seem to figure out how to attach it here). Below is a print-out - the only figures you would need to enter are along the top row.
email (option): cas.vanderwoude@gmail.com
I've found some information after a bit of searching...Exhaust pipe diameter (along with other factors) determines the Mean (gas) Flow Velocity in the exhaust...As a general rule a larger pipe slows gas speed and a smaller one increases it..
As usual no two engines are the same and many other factors must be taken into account when calculating the optimum diameter...
A 'typical' Mean Flow Velocity is in the range 280ft/sec. to 320ft/sec.as far as I can deduce...
A quick formula to calculate Mean Flow Velocity is:
Piston speed (in feet per second) divided by 60 x the cylinder bore (in inches) x the cylinder bore (in inches) x the pipe I.D(in inches) x the pipe I.D (in inches)...
The piston speed part of the formula wold be a 'mid range' figure for a road bike in terms of the RPM figure used to calculate that....For a sprinter like mine which is going to do a 'flying quarter mile' peak power RPM would be the figure to use...
I won't go into it any deeper here as I'd imagine most forum members aren't that interested in the subject...So far I've spent this morning researching and doing calculations...After lunch I'm going to make some parts to rubber mount my carb, far more relaxing!!...Thanks to all for the input which got my brain cranking over....:laughing: Ian
email (option): ian@wright52.plus.com
Ian it would still be interesting to hear what pipe size you calculate for the sprinter.
BTW as far as rubber mounting the carb, a guy round here had a "tuned" M20 and the carb was hanging out the side of the barrel and wobbling around on the end of several inches of car radiator hose.....I use the word "tuned" loosely?? Ron
email (option): ronpier@talk21.com
Here's the carb...The parts I've just finished making are the alloy body top, top locking screw and mounting plate and the upper rubber mount...
I'm not using the flange mounting shown in the photo, the carb will be directly rubber mounted to the inlet stub...The main rubber mounting is an Amal part but the mounting that is at the top/rear of the body is what I've made...That gives additional support to the carb and prevents it 'flapping about' on the mounting rubber...The Mk2 Amal body is drilled and tapped to take a mounting but that has to be produced individually to suit the application...The stud is overly long at the moment as I haven't made the bracket that connects to it yet....
I'll post my conclusions on the pipe sizes when I've finished working it out....Ian:stuck_out_tongue:
email (option): ian@wright52.plus.com
Many years ago I had talked with Roland Pike about this question, he told me that the large pipe diameter was a carryover because sometimes factorys do not like to make changes (hard headed). He told me the story that Ariel was complaining that there twins were not making the power that BSA were making, when they were tested Ariel (twins) were not making the same power and it came down to the BSAs had smaller diameter pipes, demonstrated reason to Ariel. Ariel said thanks did not change diameter but no longer complaned."Side valves had big pipes that the way it is". My son and I are building a B50 roadracer and we are testing a steped diameter exhaust small to large and length due to valve overlap picking up more midrange power. Increase so far is impressive. worked for months to get length and diameter correct. This race season we will see what happens.
email (option): millcrocfarm@gmail.com
On a website that I cant retrace anymore I read about Roland Pike and a tale about tuning he and others did on a BSA Goldstar.
By accident they found out that a step in the inlet made for a increase in power.
So new Goldstars where delivered with a smaller inlet in the head compared to the carb size.
More then one owner, when finding out that the carb was bigger then the inlet, enlarged the inlet in the head and lost the gain power again.
The article also gave information about this 'step' in the inlet, in fact a venturi. see drawing I made below.
Apparently the idea was the step improved 'swirl' for better cylinder filling and air/fuel mixing...I'm struggling to absorb the information on this subject at the 'deeper' level beyond 'ball park figures'....The flow through the inlet, the engine itself and the exhaust system are all interlinked, affected by and dependent on each other...
I know I'm going to have to read more and think more if I'm to get past the basics to a reasonable level of understanding...
It's one of the things I like about tuning...To rebuild a standard engine you follow the laid down specifications and procedures and you can successfully get a bike running without really understanding how it works, only how it goes together....Tuning requires knowledge of the theories governing how the engine works if improvements are to be found beyond the obvious alterations like compression ratios and the normal cam changes....
Although I've had my moments with power outputs achieved, I have nothing but admiration for the designers and tuners who reached a point far beyond anything I have managed so far or am ever likely to!!....Ian
email (option): ian@wright52.plus.com
I can certainly testify that my race pipe is too short. It’s about 800mm long, the standard pipe is another 150mm roughly before the muffler. I realised this when the engine went back in the road bike. The tale below explains!
At this year’s, 2021, Speed Week I ran my road engine again as the short stroke is still it ready, it won’t make 2022 either, but that’s another story. Anyway the engine has my first version of a special head, which allowed 65-2442 inlet to be used, about 11.5 mm lift. For various reasons I didn’t finish the setup until the day of packing for the trip and it appeared to be very rich and hesitant mid range but ok(ish) at the top end, just reving offload. At the meeting the best I could manage was 73mph, down on previous setup by 20mph but that was the top in 3rd or 4th and it felt like the issue was carby setup. I’ve been hampered in doing much over the year with lockdowns, as I only have my parking space and cage in the apartment so have to use a communal workshop. Anyway I got as far as finding lots of reversion (pulses back through the carby to the extent that the fuel overflowed the float chamber, it’s a Mikuni VM36) and then preparing some valve caps to go with some beehive springs to give about 130lbs seat pressure and trying a pulse-jet plug insert. In Dec I put the engine back in the road bike as I wanted to go on a bike run!! I’ve only one working engine! After some teething troubles with fouled plugs (the pulse-jet came with an 8 grade NGK)I’ve found the setup is pretty good. Only one bad spot just of the stop only annoying when cruising on a level road. The run out to Malmsbury about 130km for a post Christmas BSA meeting was the first real test and I’d got water in the maggy the day before so was later than even I’d planned so I took the Calder Freeway once I was out of Melbourne, that’s a 110km/h road which I’d normally avoid at all costs. However felt I had no choice and much to my amazement I was easily able (hills excepted) to keep up with traffic and overtake a few, ie easily keep at 65-70 indicated. At one point I thought I’d wind it open and the needle as around 80mph but I had a scary moment of weave / slow tank slap requiring to be brave at keep throttle open and gradually quell the wobble and gradually back off. But I was pleasantly surprised by the top end speed, and that’s down to the pipe length at least in the main.
email (option): dickie.bobbie@hotmail.co.uk
Yes, having done some more reading and some simulations, it is now blindingly obvious that scavenging is the aim, reflected waves during the overlap will be just bad news all round.
We do tend to forget production needs and designs fitting those needs, BSA models parts are very well standardised, making for more choices of interchangeable parts for us now, and more spares available in general :)
email (option): dickie.bobbie@hotmail.co.uk