Singles Breathing

[MK III]

OK, Here's another take on the big "D" The port shape is modified to create a smoother exit environment around and after the exh valve. The same is done in the intake side - a "hump" is put in the port - but because it's on the cool side of the port we can use a very malleable and easily attached (to the Aluminum) substance - Epoxy.

On the "hot" side epoxy won't work so the "hump" was precut, filed, sanded, and after lots of reshaping to get the best flow numbers and fit, it was welded into the port - BUT- just at the outlet end. You really can't get the TIG back in there very far so there's a long leverage arm working on that aluminum insert. Jesse probably could have tapered the last inch down to a sharp edge and welded along the edges (?) but I don't remember - this was late 1970s. Flowbenches many times like changes that would seem "wrong headed". Jesse O'Brien is still "out there" and I have his e-mail address. I might just pm him and see if he remembers the "why".

Using the "D" as a "deflector" to prevent gas reversion would really seem counterproductive. We ran all of those race bikes with reverse cone Megaphones - which, in effect - act similar to the expansion chamber on a two stroke. With a radical cam - lots of overlap - the burned charge goes out the pipe, accelerates in the diverging cone, and pulls some of the fresh incoming charge into the pipe. Just about then, the sonic wave reflected from the reverse taper slams some of that fresh charge back thru the still (partially) opened exh valve. The flat face of that D would be a hindrance to this process. I think it was more of a "necessary evil" than something functional. But, I'll find out.

[graeme]

Hello MK III, was there a picture that didn't attach?

[LaceyDucati]

Mk III, That's pretty much what I said regarding the D shape port. Most of the advantage I see is the improved flow from the seat, reducing turbulence.

Regarding Tig welding up the port, it can be done. With all this welding, you will be putting in new seats so there is nothing to lose by welding from the other end. What I do is cut the seat pocket back towards head face, then weld in some decent material for the new seat to sit against. I usually also weld the seat pocket up to fit a narrower better proportioned seat compared to the original large seats. Once the important material around and under the seat has been formed, it is then just about possible to then back fill the lower part of the port as there is now something to weld to. The quality of this final welding is of little significance, it only needs to be secure and sufficient to be shaped.

I have really only semi used the D shape early in the exhaust port preferring to then return to round at the exit of the port. This has worked for me, but I do bow to the better knowledge of some engine designers I am involved with who have utilised D shaped exhaust ports in a new development. I will question them on the reasoning.

For all of those reading this without the facility to carry out this kind of work, rest assured you can still make large improvement and potentially win races without a Tig set! The 350 Sebring engine I used throughout my racing, used a head devoid of welding. It also finished 3rd in the Manx GP lapping at an average of around 96mph with Doug Snow on board. So interesting as some of this discussion is we should keep things in perspective of the diminishing returns of modifications. Yes if you want to take on G50's and Manx's racing with a 450 Ducati, then expect to get welding, but for the road or club racing I wouldn't get too carried away!

Nigel

[graeme]

Hello Nigel,

What rod length did you use with your 80mm stroke engine?

Graeme

[LaceyDucati]

Hi Graeme,
Standard 140mm. The rod ratio was 1.75 which worked fine. On the Desmo by the time I had used a twin piston with a lower crown and closed up the squish the bevels were only marginally further disengaged. On an 81mm stroke engine I did, I did have to cut and lengthen the bevels as it was a spring valve with more load involved. I used a Folan rod which utilises a small bigend which allowed clearance to the gearbox cluster. I have subsequently had Arrow rods manufactured to similar dimensions. The small 30/38/18 bearing has proved reliable over quite some time, in fact it has not been changed since the engine was built 10 or so years ago. Fuelling my thought that most of the factory problems with bigends were due to poor tolerancing, poor selection of clearance and possible component hardness consistency.

I used a 350 crank as it had more material around the pin holes than the 450. I offset bored the holes and made "top hat" sleeves which I pressed in. I then welded the sleeves in place and got the holes jig ground. The crank was then rebalanced.

Eldert went down the route of getting new cranks made for his stroked engines. A better solution, but I didn't have the resources at the time for this. That said the crank trued perfectly and has not given any problems in use. I have some pictures of the stoked crank I will post later.

Nigel

[graeme]

Thank you Nigel, that answers a number of questions re how the big end had transmission clearance at 80mm, and the piston pin position.
I've seen pictures of Elderts 78 mm crank, beautiful.

There are so many different ideas from different people to achieve a similar capacity increase.
All very interesting.
I have seen bore/stroke ranging from 87/75 that works nicely on a road bike to 90/78 to 92/75 for track bikes and your 87/80.
Thank you for sharing what you have done and had success with.

Regards
Graeme

[LaceyDucati]

Graeme,
Pictures of the 80mm stroked crank. I also did an 81mm crank running with an 88mm piston giving 493cc, this bike really is a joy to ride. I've always preferred the stroked route as you can't really take advantage of the theoretical extra revs you could run a shorter stroke motor to. Also the thicker liner and more material to the stud holes tends in my mind to result in a more stable cylinder shape. I did try a 92 x75 engine early on with a customer but wasn't keen on the cylinder stability. One of the advantages of enlarging the engine (whichever way) is the compression ratio rises and you can end up with a fairly high compression ratio with a nice shallow crown shape. On a single plug we still only need to run around 28 degrees for race use, signs of a good combustion chamber shape.

crank 2 (750x586).jpg

crank 1 (750x586).jpg

crank 3 (750x586).jpg

[Harvey]

Very interesting pictures of head jobs that are aimed at increasing the breathing at higher engine speeds. I can see that this approach is warranted for a 250/350 that can run at higher piston speeds, but to get the 450 to rev like the smaller engines can make them a bit brittle for everyday use.
My bike was a multi purpose bike, to do distance touring, a bit of Post Classic racing, and still dress up for Concourse. To that aim, I didn’t do much to the inlet port, for fear of the valve guide support cracking from the 12mm lift. Just a bit on each side of the support then, widen the port to get the area of a 36mm port. I think it would take a lot of surgery to get it to flow well, in the right direction.
I found that with a 88mm, 11.5:1 piston, the 12mm lift V Two cam, and the inlet tract opened up to run a 40 mm carb, there was a big increase in torque, from 2000 to about 6500/7000, it dropped off over that, HP about 8500. I guess you could say it had the torque characteristics, of a 900SS, so it was better to gear it up and use the torque, rather than trying to get it to run higher up the rev range.
As the bike was to do 1000km touring, I did most of the work to ensure reliability, engine vibration, and suspension compliance. So I agree with Nigel on using the torque of the engine, and leave the high rpms for racing.