D.O.H.C. ? I want one of these

acotrel · Oct 28, 2014

Fullauto said:
If you REALLY want that, then buy a modern Ducati. All you asked for and more and at a fraction of the cost.

Depends on the intended use - what race classes are available. If it looks right, such a bike might provide really competitive rides in an historic racing class. A tricked up RD350 Yamaha would probably be faster - so you'd need to get it into a pre-72 class. If you want to do a mind bender, start thinking about how you can get a decent competitive race ride at reasonable cost. Certainly not on a Ducati of any sort. It takes very big bickies to up with the front guys on those. As far as I'm concerned I don't care what I ride as long as it is not a two stroke and the opposition are all on similar bikes. I really like VFR400 Hondas, we used to have a beaut race class for those. Our guys didn't even bother to rebuild motors - just bought a whole new grey import bike.

gortnipper · Oct 28, 2014

Fullauto said:
If you REALLY want that, then buy a modern Ducati. All you asked for and more and at a fraction of the cost.

Agreed. My ST4s has a 996 Desmoquattro, and will do the speed limit in first. But the C'do is just as much fun, and way more expensive now.

acotrel · Oct 28, 2014

If you can think of any bike which is easy to build and for which parts are cheap, and which can be raced - you are on a winner as far as sales are concerned. I really like the Chinese CT110 Honda clones. You can buy almost anything for them from 6 speed CR boxes, big bore kits, to 4 valve DOHC heads, and it is all reasonably priced. You don't need 200 BHP to race, all you need is a level playing field with minimum rules to have max. fun.

This is worth building without the blower, however there is currently no race class for it. You'd need to get it into bucket racing, then start selling replicas until you got enough fellas riding them to start your own four stroke 125cc or underbone race class :

You could build it this way - a $2000 racing motorcycle, I bought a good motor for $50 :

acotrel · Oct 28, 2014

https://www.facebook.com/underbone.tuning

PreservationCycle · Oct 28, 2014

The motor set up in the Dennis Manning double engine Norton streamliner used a similar drive off the end of the camshaft of the number one engine to run the fuel pump via a small belt and pulley. I always wondered how much load those cam bushes would take before you had problems. I guess if they can take the strain of that DOHC set up they must be doing ok...

acotrel · Oct 29, 2014

I think the comment about buying the Ducati might be sensible. I don't know any historic competition class which might suit a bike with a home-made DOHC head which did not exist before the required date. And I don't know any other reason for developing a DOHC commando engine. Did the Pantahs exist before 1982 ? I only know of one air-cooled non-bevel Ducati motor which was fitted with 4 valve heads, and it is impossible to buy replica heads. It is the bike Kevin Magee road for Bob Brown in the early 80s in Victoria before he moved to MotoGP. (I don't know of any 4-valve bevel motors except for some of the early factory racing singles).

Nater_Potater · Oct 29, 2014

PreservationCycle said:
The motor set up in the Dennis Manning double engine Norton streamliner used a similar drive off the end of the camshaft of the number one engine to run the fuel pump via a small belt and pulley. I always wondered how much load those cam bushes would take before you had problems. I guess if they can take the strain of that DOHC set up they must be doing ok...

In the case of running the fuel pump on the streamliner, even if the bushings don't last very long, they only have to last long enough to get a record. As for the DOHC, the "cam" shaft no longer has to carry the load of the valve train, acting only as a jack shaft, which should actually be much less strain.

Nathan

Rohan · Oct 29, 2014

Rohan said:
But we can stll dream of a wonderful torquey Commando motor,
that will also turn 12,000 rpms. !

How short stroke would a Commando have to go to be able to turn 12,000 rpm,
we wonder. ??
Without getting out the back of an evelope...

Some here have rather missed the point of where this is going, with talk of buying a Ducati instead. ??

With a viable DOHC 8 valve head to bring it into the 21st century,
a 961 Norton updated with a suitable bottom end is perhaps a little more attractive against a Duc.

Considering that at the moment a new 961 costs MORE than many a Ducati,
and you only get pushrods....

??

acotrel · Oct 29, 2014

Rohan said:
Rohan said:

But we can stll dream of a wonderful torquey Commando motor,
that will also turn 12,000 rpms. !

How short stroke would a Commando have to go to be able to turn 12,000 rpm,
we wonder. ??
Without getting out the back of an evelope...

Click to expand...

Some here have rather missed the point of where this is going, with talk of buying a Ducati instead. ??

With a viable DOHC 8 valve head to bring it into the 21st century,
a 961 Norton updated with a suitable bottom end is perhaps a little more attractive against a Duc.

Considering that at the moment a new 961 costs MORE than many a Ducati,
and you only get pushrods....

??

The 961 is probably neither fish nor fowl. I think the only time I would buy one would be if I'd owned an early commando and was in love with them. Ducatis are a pain in the backside - too much like Ferraris. Until recently a close friend ran a business in Melbourne fixing them. He told me it is very unusual for his bill to be less than $5000. He also said that most of his work came from people who tried to fix their own bike. He would always pull the bike down in front of the customer, so they could see the damage before he did his work on their bike. I'd really like to own a modern Ducati, I think they are the last remaining true motorcycle. However I shudder to think of the cost if they go wrong.

gortnipper · Nov 3, 2014

Not sure what bike this is, but it is interesting never the less.

[video]https://www.youtube.com/watch?v=X3i3mdlB4kM[/video]

madass140 · Nov 3, 2014

very interesting, goes to show how important those hardened lower valve spring retaining cups are.

acotrel · Nov 3, 2014

Imagine what a commando crank would do at those revs.

lcrken · Nov 3, 2014

Rohan said:
P.S. Why do these magazine articles never show the working part of an 8 valve - the combustion chamber shape/layout.

For lazy reasons, commercial reasons, or do they sign something that says they won't ?

I posted some scans of magazine articles on pages 3 and 4 of this thread,

valve-head-conversion-t9137-45.html?hilit=piper

showing the combustion chamber on the pushrod 8-valve head.

Unfortunately, they are not real great quality pics. I've never seen any that show the chamber on the Merlin DOHC head, but it is probably pretty similar.

Ken

acotrel · Nov 3, 2014

There is only one thing which makes development of a good 4-valve head impractical - the controlling body and the guys in historic road racing. The race classes are not intended to be 'development classes' regardless of all the cheating which occurrs. There has never been a better single cylinder 500cc bike than a manx because we've never had the opportunity to go there. I don't believe that even in the UK 4-valve engines would be permitted to invade something like the Lansdowne series. Perhaps we need a change of mindset to promote further development of the older designs ? 'Racing improves the breed'.

Rohan · Nov 3, 2014

lcrken said:
I posted some scans of magazine articles on pages 3 and 4 of this thread,
showing the combustion chamber on the pushrod 8-valve head.

Thanks Ken.
That seems to be the exception rather than the rule though,
magazines ALMOST never show the combustion chamber,
when it should be the 1st thing they show !!

Combustion chambers of race motors seem to be about in the same category....

Cheers. !

Rohan · Nov 3, 2014

acotrel said:
There has never been a better single cylinder 500cc bike than a manx

Are we completely ignoring or discarding the Matchy G50 then ??
For about 10 or 15 years (from the 90s ?) it was the bike to beat - finally got some breathing development late in its life.
All the replicas were faster than the manx, by a good deal, for quite a while there.
Recall how Barry Sheene RIP switched from a manx to a G50 in classic racing,
figuring that if you couldn't beat em then join em....
(Yes, we all know the G50 was sohc, goes to prove dohc is not entirely superior at lower revs...)

If we go back a few decades, into the late 40s early1950s, then the Velo KTT and the AJS 7R joined the fray.
Prewar, the Excelsior Manxman and the Rudge 4 valve Ulsters could all whup a manx predecessor on a good day.
And there were OK Supreme lighthouses, Dunelts, Hudsons, AJS cammies, some french oddities etc etc etc.

The manx came out on top only because of an accident of history - Nortons survived, and the rest succumbed... ?
How soon we forget, eh ?

Rohan · Nov 4, 2014

acotrel said:
There has never been a better single cylinder 500cc bike than a manx

There is also the minor matter that Moto3 (250cc singles) bikes these days are almost up to the performance levels of a good manx.

Aren't they around the 50 bhp mark ?
And that is with some rpm restrictions, if they were allowed MotoGP technology they'd blitz a manx...

frankdamp · Nov 4, 2014

You really don't want one of those. While I was at N-V there was a program to develop a DOHC version of the 750 engine. The diagrams in the articles posted look a lot like the one we were trying to develop. A couple of development engines were built and dyno-tested.

The big problem was the camshaft chain length. Because the design tried to use the existing push-rod tubes to route the chain to the head, the chain was close to 36" long. There wasn't enough room to put tensioners in to keep the gradually wearing chain in tension and within a few hours of dynamometer running, chain wear reulted in the valve timing going to hell.

If someone wants to do a DOHC versiion of the engine, they need to get the camshaft chain a lot shorter.

Rohan · Nov 4, 2014

frankdamp said:
If someone wants to do a DOHC versiion of the engine, they need to get the camshaft chain a lot shorter.

While there is much merit in this comment, HyVo etc chains were developed to combat wear in automotive chain applications.
And it does run in oil, so running dry and wearing badly should not be a problem.

With a full engine redesign, which was what was really needed for a new engine project (!),
the chain presumeably could be shorter.
Laverda did a variety of twins and triples, as brand new designs, and camchains didn't cause them problems ?
It is noticeable that some japanese designs did go to gear driven cams though....

Fullauto · Nov 5, 2014

Rohan said:
frankdamp said:

If someone wants to do a DOHC versiion of the engine, they need to get the camshaft chain a lot shorter.

Click to expand...

While there is much merit in this comment, HyVo etc chains were developed to combat wear in automotive chain applications.
And it does run in oil, so running dry and wearing badly should not be a problem.

With a full engine redesign, which was what was really needed for a new engine project (!),
the chain presumeably could be shorter.
Laverda did a variety of twins and triples, as brand new designs, and camchains didn't cause them problems ?
It is noticeable that some japanese designs did go to gear driven cams though....

A complex and costly exercise only needed because that particular manufacturer couldn't design a decent cam chain tensioner for, say, about three decades.

D.O.H.C. ? I want one of these

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