HD XR 750 & Full Auto port conversion

Status
Not open for further replies.
My brother runs two types of sidecars on speedway - methanol-fuelled three-cylinder two strokes, and methanol-fuelled twin-cylinder 880 JAP and Vincents. With the two-strokes you get extreme horsepower and very smooth power delivery. With the Vincents and the JAP you get less top end power but a more lumpy delivery. The two strokes are lighter and faster but much more scary - you need nerves of steel where the others are much easier.. The race times are probably similar.
 
acotrel said:
I have got the impression that the 95 BHP XR750 s you are talking about are flat trackers.
Click to expand...

Could that be, because I have said it so many times? Go back and Read my posts.

acotrel said:
Running a bike on the dirt is very different to running it on bitumen. On the dirt you are balancing slide and drive.
Click to expand...

It does not matter if it is dirt or pavement, the limits of grip are there for both. You are balancing sliding and drive exiting every corner, in all types of motorcycle racing.
The ability to control the bike while sliding is paramount to a fast lap in any kind of racing.
The rest of the world learned that when the American dirt trackers came to Europe.
Kenny Roberts, Freddie Spencer, Eddie Lawson, Wayne Rainey, Gary Nixon, Steve Baker, Doug Chandler, John Kosinski ,Nicky Hayden, all flat track racers before they were roadracers.
The rest of the world riders saw the controled sliding the Americans brought with them, and came to the USA to train and learn.
They all do it now.

Watch the Moto GP races, they slide under power exiting every turn.
MotoCross, constant sliding while under power.
Of course bikes are set up differently depending on the surface grip.
Lighter flywheels, heavier, firing order, number of cylinders.
In the last years of the HD flat trackers, the parallel twin Kawasaki's were beating them.
 
Hey Dan, give up. Al is a good sort, but pretty fixed in his opinions. You'll get tired of trying to change his mind pretty quickly. Besides, you've only been the AMA Tuner of the Year twice. What could you possibly know about all this racing stuff? :D

Ken
 
@Dan Kyle

Just had a chat with my buddy, according to him up to approx 120 hp and a redline of up to 13k rpm are doable with different cams, porting bigger valves and large throttle valves etc.

Kind regards

Christian
 
I can only assume that the ports in the XR750 motor must be extremely different to those in a Norton. Makes Joe Craig look like an idiot ? If you talk nicely to Fullauto, you might be able to buy a cylinder head in which the core box has not been used. Then you can carve out your own ports and combustion chamber from solid metal.
 
Last edited:
The only time I ever slide my bike on the bitumen is when it is nearly vertical. It sometimes wheel-spins when coming out of corners. On speedway solos, it is possible to be fast enough by riding around the pole line, but not the fastest.
 
Last edited:
It might be interesting to know if Andy Molnar has created a better port shape for a Manx.
 
Without diminishing the effort of Craig's or better kuzminskys (who was the real genius imho) work but imho Williams late head design is by some length the better design and I assume also hopwood did some really nice progressive work.
Anyways, almost 20years later and somewhat to my knowledge on a blank sheet of paper to design a full breed race engine, (after the supposedly initial disasters) continuously consulting and employing the best of the crop tuners (branch, Axtell, Augustine) that were used to look outside the box of course one gets better results than some insolvency endangered UK companies.
That's of course my personal opinion, but after I saw the high rise oval late xr750 port I was amazed that they had just approaches already decades and decades ago.
Of course Harry weslsake did some awesome development work on some of the Brit stuff but and that's a big buuuut, due to the fact that I used to live for years on the west coast I have a high opinion about the "if you go in the woods better take a sharpened axe with you" attitude!
Ok admittedly a somewhat pathetic statement but obviously there were less restrictions for a race engine under O'brien.

Kind regards

Christian
 
Last edited:
Manx Nortons were race-developed by the factory from about 1930 right up until 1962. Triumphs only figured in IOM TT results about three times since about 1948 to 1960. When I built my bike, I had severe doubts about the design of the motor. I don't have those doubts any more. The design actually works and extremely well. I believe that lightening the crank is a backwards step. It might help the motor perform better if you don't have a close ratio gearbox. But with close ratios, the heavy crank is superb. When you compare the Norton cylinder head with the Triumph 650 head, the Norton head is streets ahead. I don't doubt the port design. Of all the British motorcycle manufacturers, Norton should have known what they were doing when they designed the Commando engine. It is obvious that some things such as cam design would have been altered to get reliability and good fuel consumption in a road bike. But I think the port design of the Commando head, would always have been the real deal.
 
When you talk about Harry Weslake, you are probably talking about 4 valve cylinder heads. When you go from 2 valve to 4 valve, you usually pick up 10% in horsepower. But going 4 valve in an air-cooled motor is not easy. In Australia, there has only ever been one air-cooled Ducati which had 4 valve heads and it ran on methanol. It was the bike which Kevin Magee came up on. It eventually got sold to a Japanese collector. The 4-valve Weslake Triumphs did not do much that a 2-valve would not do.
 
acotrel said:
Manx Nortons were race-developed by the factory from about 1930 right up until 1962. Triumphs only figured in IOM TT results about three times since about 1948 to 1960. When I built my bike, I had severe doubts about the design of the motor. I don't have those doubts any more. The design actually works and extremely well. I believe that lightening the crank is a backwards step. It might help the motor perform better if you don't have a close ratio gearbox. But with close ratios, the heavy crank is superb. When you compare the Norton cylinder head with the Triumph 650 head, the Norton head is streets ahead. I don't doubt the port design. Of all the British motorcycle manufacturers, Norton should have known what they were doing when they designed the Commando engine. It is obvious that some things such as cam design would have been altered to get reliability and good fuel consumption in a road bike. But I think the port design of the Commando head, would always have been the real deal.
Click to expand...
You are a bit lacking in Norton history there, Norton didn't design the Commando engine, it dates right back to the iron head 500 Domi engine, bored and streched to 600, then 650, then 750 which was the Atlas - which was the first engine used in the first Commando that was sold to the general public.
 
You can't compare a 2 valve Triumph with as big as you like big bore with an 8 valve Weslake. The Weslake wins hands down! Its just not true. Never was & still isn't today!
 
  • Like
Reactions: baz
@Chris

True indeed, but nonetheless unfortunately the weslsake/nourish head needs considerable tender love and care to make it flow as it should/could.
I was working on two of those and it left me with a somewhat delusional feeling when I looked at the initial flow measurements.
Also afaik the cam timing events available could be a hint more suitable for a 4 valver.
@acotrel
So based on what Chris already wrote comparing a to my knowledge prewar based work mule to an almost thoroughbred is like comparing apples to pears.

Kind greetings from Italy

Christian
 
Seems a shame that the Piper/Kitts Green/Fair Spares 8-valve heads for the Commando never really worked out. I'm not sure what the status of them is now. Last I heard Andy Molnar owned all the rights, designs, tooling, etc. to them, but wasn't planning to do anything with them. That was several years ago, so I don't know if the status has changed any since then. I always liked the idea, but compared to a standard Commando head, they were kind of ugly.:( A friend of mine in Texas was working on a conversion to two intakes and one exhaust valve that would keep the stock exterior appearance, but wasn't able to keep the project going. Having both push rods at the front of the engine appears to make it a bit more difficult to do a Norton 8-valve head than for the Triumph. That long, heavy forked intake rocker arm has to be a liability.

Ken
 
At the risk of exposing my ignorance, Why didn't norton consider widening the crank cases, placing a 3rd bearing in the center along with a sprocket to drive an overhead cam/cams?...

I believe that's what the yamaha brand norton-clones did.... It would mean the cylinders would move further apart, which would allow for a bigger bore also, and then the crankcase wouldn't be an exercise in geometry with the pushrods...

Was that design even around to be considered by norton?...
 
I cant remember if one of the Norton experimental engines that have been rescued had a center crank bearing. I will have to look that up. Norton was bequeathed the P10 800cc project that they tried to rescue with a redesign ie the Z26. Petrol tank & superblends made it on to the Commando.
'
 
I'd say a couple of things:
1, - the law of diminishing returns applies to engine development. The more you spend the less you get for the extra money.
2. When you race a motor cycle, there are several variables which add-up to make a pattern of which there are a couple of common types. Often when you change only one variable it affects the others, and can preclude development in certain directions.\
I think a 750cc version of a Paton would be good.
 
If you watch MotoGP, have you noticed that a few bikes have very different angles of lean in corners and some corner faster than others ? Most simply form part of a procession, but some are different. I don't think it is necessarily the rider causing the difference.
 
Status
Not open for further replies.
Back
Top