Downdraft .

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Matt Spencer

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Back in the early 70s , every second ' Tuning ' article had the word ' DOWNDRAFT ' . Often severally .

The 72 F - 750 & Dunstall appear to have been ' in the fold ' in that respect . Unimpeded flow to the port , One Might Say .

Be intrested to see a souped up Commando tested with std . manifolds & square / straight through , with no other changes ,
as a comparison . Have to be something fairly hot & working hard , Semi race bike or decent Combat , to pick a differance ? .

Downdraft .


Close up of the manifolds ,
Downdraft .


Another Dunstall Straight In Manifold . perhaps even the same ,


The 155 mph daytona set up .
Downdraft .


Downdraft .


So , has GRAVITY been superceeded ?

Nuver Fred on this ere ; https://www.accessnorton.com/NortonCommando/straight-shot-manifolds-on-jp-norton.33855/
 
Hi all,
I read the article with interest.
In it’s day it must have been a very serious bike, I don’t think much would have stayed with it.
I guess it would have been slightly ahead of a Z900 when they came out.
A pity Norton couldn’t have provided that performance in a production bike whilst maintaining acceptable reliability
Al
 
Alan L said:
Hi all,
I read the article with interest.
In it’s day it must have been a very serious bike, I don’t think much would have stayed with it.
I guess it would have been slightly ahead of a Z900 when they came out.
A pity Norton couldn’t have provided that performance in a production bike whilst maintaining acceptable reliability
Al
Click to expand...

That Dunstall time is around 0.5 seconds quicker than a 1973 Z1 900 (12.4 @ 111 mph) and around 0.26 seconds slower than a 1978 6 cylinder CBX (11.64 @ 118 mph)
(The Z900 was mid 1970's)
 
I guess the question is, with the correct parts (without going super exotic) and the the best assembly what was the highest performance that would have been obtainable from a commando whilst still providing reliability that was factory warrantable ?
Would the Dunstall been able to be produced by the factory without a reliability disaster like the Combat?
just wondering
Alan
 
Time Warp said:
That Dunstall time is around 0.5 seconds quicker than a 1973 Z1 900 (12.4 @ 111 mph) and around 0.26 seconds slower than a 1978 6 cylinder CBX (11.64 @ 118 mph)
(The Z900 was mid 1970's)
Click to expand...
The Z900 and the 850 Commando were both 1973. Four cylinders have twice as many valves as two. But race times are a result of many different things.
 
Alan L said:
I guess the question is, with the correct parts (without going super exotic) and the the best assembly what was the highest performance that would have been obtainable from a commando whilst still providing reliability that was factory warrantable ?
Would the Dunstall been able to be produced by the factory without a reliability disaster like the Combat?
just wondering
Alan
Click to expand...
'Racing improves the breed'. Was the Combat raced ? I have got Dunstall's tuning notes - they are very conservative - nothing radical.
 
acotrel said:
The Z900 and the 850 Commando were both 1973. Four cylinders have twice as many valves as two. But race times are a result of many different things.
Click to expand...

The Z1 900 was 1973 and 1974, the Z900 1975 and 1976, the Z1000 in 1977.
#
Based on posts on this website, good luck with a Dunstall 810 cylinder lasting long term especially if it was a bike running 11.9 ET's
I don't think I would want to rev a 1971 20M3S 750 engine to 7000 rpm on a daily basis let alone do it sitting on a 2 inch thick seat foam.

The Norton with its long stroke gave loping power which would be very useful in the real world of production road bikes of that period and little different a half century later.

Could the factory build a hopped up version, yes and we know how that turned out especially when the buyers mentality was an unknown after the sale.
Bikes like the big Z were robust (unless you tried to spit a valve shim out) so even meatheads (Archie Bunker) were hard pressed to blow one up.
 
Matt Spencer said:
Back in the early 70s , every second ' Tuning ' article had the word ' DOWNDRAFT ' . Often severally .
Click to expand...
With respect to the title, Downdraft, the heavy Norton twin (including the Commando variants) have very little downdraft. That includes the JPN/Williams racers in the photos above.
Angling the carburettors upward does not create downdraft - it is a function of the angle of the inlet tract (inside the head) wrt the bore centreline. If you look at Works/Manx inlets you'll see the inlet tract (inside the head) has to be about 15 degrees downdraft. The Norton twins are almost zero. Same goes for road singles (ES2, Model 50, etc).
Good downdraft gives better cylinder filling efficiency, raising combustion pressure.
Not to say the Norton twins are not good engines - but - they have very little "Downdraft".
Cheers;)
 
Last edited:
'Racing improves the breed'. Was the Combat raced ? I have got Dunstall's tuning notes - they are very conservative - nothing radical.
Click to expand...
Naw .
As he bought the ' Domiracer ' race shop contents , he got the ' Norvil ' type re angled inlet Hemi'd head . Like the Norvil 750 . When they got around to it. Again .
Click to expand...

https://www.accessnorton.com/NortonCommando/the-wonders-of-modern-technology-1974.34342/#post-560283

Downdraft .


the reason there was no 850 Combat , was those peskey noise regulations, by then . Tho there mayve been a few weeknesses .
Pity there wasnt a rpm limited contactless ignition fitted . The got the Combat sorted . Villiers hadnt a clue at first .

Youd Think Hi tech Materials might sort out a Commando , with a outrigger / chaincase mainshaft bearing .
Not everybody thrased em to death back in the day . Theres still tidy functioning survivors .
Tho they wernt built on monday or friday . :rolleyes:
 
" With respect to the title, Downdraft, "

SO , You want DOWNDRAFT , Do You .

Downdraft .


Downdraft .


The Commando might not have a great deal of DOWNDRAFT , in the port ( Relitive to the gasket face .
But it certainly aint UPDRAUGHT . Not being a side valve . unless of course rubber mouted carbs have sagged . ! :D

Technically the Carb s , as far as flow goes , a part of the intake port . Id suspect that that 72 F-750 has the port roof raised ,
if those carbs are in line with the intake port about the rear face / flange , there .
 
Matt Spencer said:
" With respect to the title, Downdraft, "

SO , You want DOWNDRAFT , Do You .
Click to expand...
Sorry Matt.
With the thread title then immediately going to photos of Dunstals (okay - no apparent downdraft there!) and the JPNs with their raked carburettors might give some members the notion that they have substantial downdraft.
I was merely pointing out that is/was not the case.
Cheers
 
No Wollies . But regarding the 70's , Any downdraft was downdraft .

Now DOWNDRAFT may well be regarded as approaching vertuical , say , maybe , as low as 45 Deg. ( to Something )
In Comparison to say , a Triumph 650 head , The Commando head does incorporate a degree ( some ) downdraft .

There were people butchering Ford 4 Cyl. heads , to get ' full downdraft ' ? where the port entry was raised to the top head face , thereabouts ( below ) .

Perhaps the point being , the Norton Head i the port is more downwardly inclined ( than a Triumph ) , thus ' less kinked ' at the curve thru to the valve seat . ?

Perhaps this is where the Nortons Repution for free & deep breathing was originated . Where at lower speeds ther'd be less impediment to ' direct flow ' ,
before all the pressure cycle waves start coordinating & the Triumph takes off ! . ;)
 
Now dats downdraft .

Downdraft .


But tarted up volkswagons & Lotus Cortinas , its the CARBS . Whereas ' port inclination ' is really a differant kettle of fish .

Any port inclined above perpendicular to the bore , might be said to have a degree or portion of ' Downdraft ',
where one might refer to a port with say 5 deg slope as downwardly inclined . And from there . . . .
 
Line 1 Harry (Weslake) Dan (Gurney) is on the phone.

primotipo.com

Gurney-Weslake Ford V8…

Dan Gurney’s Lola T70 Ford during the Stardust Grand Prix, Las Vegas Can Am round, 13 November 1966… Gurney didn’t have a great weekend, the fuel injected Gurney Weslake aluminium…
primotipo.com primotipo.com

Downdraft .


Downdraft .


Weslake - Wikipedia

en.wikipedia.org en.wikipedia.org

"In the 1940s, Weslake set up his research company in Rye, and was contracted by Norton to work on improving gas flow on their Manx engine."
 
Time Warp said:
Line 1 Harry (Weslake) Dan (Gurney) is on the phone.

primotipo.com

Gurney-Weslake Ford V8…

Dan Gurney’s Lola T70 Ford during the Stardust Grand Prix, Las Vegas Can Am round, 13 November 1966… Gurney didn’t have a great weekend, the fuel injected Gurney Weslake aluminium…
primotipo.com primotipo.com

Downdraft .


Downdraft .


Weslake - Wikipedia

en.wikipedia.org en.wikipedia.org

"In the 1940s, Weslake set up his research company in Rye, and was contracted by Norton to work on improving gas flow on their Manx engine."
Click to expand...
If you look at the gasket line on that V8 (and assume a combustion chamber depth) the downdraft there looks to be about 20 to 30 degrees - pretty good!
 
I wonder why Weslake was not involved with Norton development in the later period. (over the 'car people Cosworth)
 
Time Warp said:
Line 1 Harry (Weslake) Dan (Gurney) is on the phone.

primotipo.com

Gurney-Weslake Ford V8…

Dan Gurney’s Lola T70 Ford during the Stardust Grand Prix, Las Vegas Can Am round, 13 November 1966… Gurney didn’t have a great weekend, the fuel injected Gurney Weslake aluminium…
primotipo.com primotipo.com

Downdraft .


Downdraft .


Weslake - Wikipedia

en.wikipedia.org en.wikipedia.org

"In the 1940s, Weslake set up his research company in Rye, and was contracted by Norton to work on improving gas flow on their Manx engine."
Click to expand...
What’s that engine next to him on the bench?
 
Fast Eddie said:
What’s that engine next to him on the bench?
Click to expand...
Chain driven double overhead cam twin. Sounds Italian but doesn't look like it.
Japanese style (70s/80s) header retention.
What are those "ports" at the front of the rocker covers?
Looks similar to what @EstuaryBoy has suggested though.
???
 
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