l-o-n-g inlet manifolds

What advantage will fitting these l-o-n-g inlet manifolds do to a 750 Norton

e-bay no; 230837521223

and why would a part for a race bike be chromed?

gory said:

and why would a part for a race bike be chromed?

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To blind the other competitors who try to get a sneaky look :!:

I have not done any flow bench testing but believe that the velocity of the charge and subsequent increase in atomization is a factor. I made a "heat isolator" out of aluminum that is pretty thick and set my single Mikuni back a bit. That particular engine has always had lots of low end torque. Maybe just a coincidence.
I hope that Jim Comstock can join this discussion. I always picture the dual carb set up on the Harley dirt racers as a good example of long intakes.
Mike

Long time ago, I had a pair of manifolds made from calculations I found in a book, the carbs were a good 6 inches away from the head. My seat of the pants dyno told me it was stronger than stock, but time marches on and for best performance, I would get another bike rather than try to get a couple of extra HP from a 60+ year old design.

Jean

Using long manifolds with well designed bell mouths can in combination with tuned exhaust pipes and appropriate camshafts help fill in low points in the power curve. There are many examples in the literature.

However fitting with no other integrated tuning plan would likly result in no substantial improvement.

Plus based on my experience long fixed inlet tracts like the ones pictured can often cause fuel frothing and starvation at high rpm unless they include proper rubber isolation.

Paul Dunstal does not recomend them in his Norton tuning book but I have had good results when tuning using inlet tracts on a dyno (supported by subsequent lap times on the track)

John.

Correct me if I am wrong, but I read sometime ago (a long time ago, so my memory might not be what it used to be) I understood that the shorter the inlet tract the higher the revs the engine is capable of.
The longer inlet tract delivers better lower end (border) spread of power.
Hence, the long inlet tract on the Commando is a compromise between delivering even lower bottom end power and max revs.

The optimal inlet tract length for manx nortons has long been quoted as ~14 inches.
I understood this promoted good laminar gas flow = many more cfms than shorter tracts.
Same rev range as the Commando too (although larger cylinder).

F1 and MotoGP race engines have had variable length inlet manifolds for quite some years back, electrically operated in concert with throttle/rpms. Optimal length tuning for revs...

Short manifolds only work for low rpm sidevalves ?
Dunno what these would do to a Commando = good for full throttle work ??

Tried 14 in. valve to jet on the 750 . Wouldnt idle on twqo below 2000 , wouldnt run below 1100 .No balance toob . .

Ram effect / harmonics and optimiseing output in a specific rpm band is the general objective .With close ratio gears a
narrow powerband on a race bike MIGHT get more horsepower . Or a higher horsepower peak .

Matt Spencer said:

Tried 14 in. valve to jet on the 750 .
.

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Total inlet tract length and "valve to jet" length are 2 very different things ?
Where did you put the carbs - behind the back wheel ??!!

Nay . tracts are only yay long . carbs go up behind the cross brace .

looking at Deans C'do you can see .

http://www.flickr.com/photos/jorvikiwi/5222237725/

The only time I have seen a reference for valve to carb jet distance was in an article on the AJS 7R by Vic Willerbie (sp?) Classic Motorcycle engines. (Based on work by Jack Williams - Peter Williams's father) From memory the distance was about 12 3/8 inches ????. Dont quote me because I am about 12,000 km from my books at present.

I have looked very hard over the years for theoretical or actual test data on the optimum location of the carburetor in the inlet tract system and this is the one and only time I have seen it specified. Standard texts like Smith and Morrison, Scientific Design of Exhaust and Inlet system, G Blair Four Stroke Design dont really discuss it. I have not read Vizard enough so maybe thats another good place to look. A G Bell Four stroke performance Tuning doesnt cover it either.

All these books give formulae to calculate the optimal lenght for first, second, third nodes etc tuning lenghts. (First is always way too long to fit). If you google inlet tract lenght design you will find lots of sites with the calculations. You can also find it in Jeffrey Diamonds Victory website and in his Amal carb books.

But like I say above you must also consider practical issues and isolate the carbs from vibration. Which becomes more difficult when they are on the end of a long tract. I realised this when I tested my bike on a dyno and saw the extreme movement of the carbs. This had lead to misfiring and fuel stavation at the end of long straights and was cured by using expensive Mikuni rubber mounts and holding the carbs in foam rubber insulated cradles.

John

Matt Spencer said:

Tried 14 in. valve to jet on the 750 . Wouldnt idle on twqo below 2000 , wouldnt run below 1100 .No balance toob . .

Ram effect / harmonics and optimiseing output in a specific rpm band is the general objective .With close ratio gears a
narrow powerband on a race bike MIGHT get more horsepower . Or a higher horsepower peak .

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14 inches is w-a-y too much.

There has to be an optimum size as regards to engine, camshaft and type of use, e.t.c.

14 inches is w-a-y too much.

There has to be an optimum size as regards to engine, camshaft and type of use, e.t.c.[/quote]


Actually no. All the published analysis would show 14 inches is actually too short to tune second node in the rpm range of a Commando. You would have to be spinning at 8500 rpm to tune for a for 14 inch inlet tract second node.

14 inch would tune third node at 6250 rpm and fourth at 4900 rpm.

My Dommie is tuned with 13 inch inlet tracts at about 6800 third node and 5250 fourth node.

At these lenghts the formula in Blair and Vizard agree exactly.

John

Here i a picture of what I made back then. I have velocity stacks under those foam air filters and my calculations were made from the valve head to the end of the velocity stacks. I don't remember what the lenght was, but see for yourself. I remember just running away from a 750 Kawasaki triple, but it could have been a slow rider on the Kwacker too. I had a lightened crank too which really helped with quick revs, but it vibrated more than a Harley so I put a standard centerweight soon after.



Jean

Homemade glass tank, homemade glass seat/oil tank, homemade electronic ignition, reversed cam plate to keep 1 up 3 down pattern, the rear disk is from a Datsun (Nissan now) and rear hub is a Kawasaki 900 front hub.

"Running away from a Kaw 750 triple"? No need to be frightened , just play nicely in the future- L.O.L.

As a matter of possible interest P.E.Irving`s 1965 classic, 'Tuning for speed ' [which includes inlet tract tuning] is available as a download from www.scribd.com/

Really neat looking and interesting sounding build there Jeandr.

You still have the bike?

Dances with Shrapnel said:

Really neat looking and interesting sounding build there Jeandr.

You still have the bike?

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I still have the frame, everything else is different, and looking at it, these new inlet tracts are longer than they are on a Commando




Jean

Ahhh, things get better with age. Nice.