New high flow ports for Norton’s

jseng1 · Jun 19, 2015

Here's the "Narley" intake port cross sections superimposed at 15 30 and 45mm from the port and also at the head manifold mating surface.

cjandme · Jun 20, 2015

jseng1 said:
Here's the "Narley" intake port cross sections superimposed at 15 30 and 45mm from the port and also at the head manifold mating surface.

This is amazing info, now all we need is a fullauto head already cast with this shape/design :mrgreen:

ILLF8ED · Jun 21, 2015

I'm concerned with these performance modification strings that some of the un-initiated road going Commando owners may be tempted to follow ending up with an un-reliable machine. In perspective this string is for the racers working an edge to pull ahead competing with similar period base designs. Definitely a place for this, yet it might be good to separate the forum. For street use keep in mind the original manufacturer stretched the 1948 500cc twin with about 25hp to the 750 combat with 65hp (claimed) with low reliability result. Back peddling, the 850, detuned yet even larger displacement brough back some reliability. Stay in perspective. If you expect to ride your Commando for years of street use with the least amount of headache, don't try these mods at home.

Hobot · Jun 21, 2015

The kidney bean ports may be a drag for city traffic and slow trail bike use but the Combat Bomb was only d/t slip shod manufacturing errors in cases and cranks and ignition advance (not lack of SuperDuperBlends) so Jimmy and his developers helper will not let us down like Norton did. Save your pennies and may be able to buy a ticket to watch them win races.

jseng1 · Jun 21, 2015

illf8ed said:
I'm concerned with these performance modification strings that some of the un-initiated road going Commando owners may be tempted to follow ending up with an un-reliable machine. In perspective this string is for the racers working an edge to pull ahead competing with similar period base designs. Definitely a place for this, yet it might be good to separate the forum. For street use keep in mind the original manufacturer stretched the 1948 500cc twin with about 25hp to the 750 combat with 65hp (claimed) with low reliability result. Back peddling, the 850, detuned yet even larger displacement brough back some reliability. Stay in perspective. If you expect to ride your Commando for years of street use with the least amount of headache, don't try these mods at home.

Of course you are right. But It will be unlikely that anyone but two or three extreme racers will try these mods. This is cutting edge and only for the most ambitious and very few if anyone will have the courage to do this. Your notes on reliability are well taken. But the lightweight pistons change everything and raise the bar when it comes to blowing up Norton motors. Sometime soon I will post a letter and photo in a new thread from a customer who put his 850 Commando racer on the Dyno for one minute at 8400 RPM - I don't think it would have survived without lighter pistons, longer rods, valve train with BSA lifters and Beehive springs. I would not have done such a High RPM test myself on an 850 and I am surprised that he was able to get an 850 up that high but I have revved my own hot street 750 in between 8000 and 8500 RPM and now I have over 20,000 miles on it. Crank or case breakage is the usual result when using stock parts (heavier pistons and short rods).

lcrken · Jun 21, 2015

With modern aftermarket bits, it seems entirely possible to take the 89 mm stroke Commando engine to much higher rpms than even the factory race bikes of the period (and serious privateers like Ron Wood, etc,). The real question is are they making any more horsepower at higher speeds (like 8500 rpm). Other than the occasional need to over-rev going into a corner, to avoid changing gear, or going a bit past the power peak to stay in the power band after shifting, what is the point of revving farther past the horsepower peak? I think it is going to be pretty difficult to come up with a build for an 89 mm stroke Commando that has a horsepower peak much above the 7000 - 7500 rpm range, and still have a rideable bike. Maybe for a drag racer or landspeed bike, and with a close ratio gearbox, but not for a street bike or road racer, where you need a broader power band. If you have a head that flows enough to make peak horsepower at 8000+ rpm, it's probably not going to have enough flow velocity at lower speeds to run well. There's also the limiting effect of ring friction losses at higher rpm, and we've covered that pretty well in other threads. Again, this is with 89 mm stroke. With the shorter and shorter stroke Commando engines we are building now, it's a whole different story.

It looks to me like the principal advantage of the XR design is that it allows a larger short side radius going into the valve seat. It's not really a matter of getting enough cross-sectional area in the port. You can already get that by opening up the existing ports, but it's difficult to do so and not get flow separation and turbulence over the short side radius.

In any case, like Jim, I'd love to see someone do all the work it would take to try to apply the XR750 port design to a Commando head. It would take some serious flow bench and dyno time to sort it out, but it might work really well on a race bike. Then again, it might not, but that's why they call it development.

Ken

jseng1 · Jun 21, 2015

lcrken said:
With modern aftermarket bits, it seems entirely possible to take the 89 mm stroke Commando engine to much higher rpms than even the factory race bikes of the period (and serious privateers like Ron Wood, etc,). The real question is are they making any more horsepower at higher speeds (like 8500 rpm). Other than the occasional need to over-rev going into a corner, to avoid changing gear, or going a bit past the power peak to stay in the power band after shifting, what is the point of revving farther past the horsepower peak? I think it is going to be pretty difficult to come up with a build for an 89 mm stroke Commando that has a horsepower peak much above the 7000 - 7500 rpm range, and still have a rideable bike. Maybe for a drag racer or landspeed bike, and with a close ratio gearbox, but not for a street bike or road racer, where you need a broader power band. If you have a head that flows enough to make peak horsepower at 8000+ rpm, it's probably not going to have enough flow velocity at lower speeds to run well. There's also the limiting effect of ring friction losses at higher rpm, and we've covered that pretty well in other threads. Again, this is with 89 mm stroke. With the shorter and shorter stroke Commando engines we are building now, it's a whole different story.

It looks to me like the principal advantage of the XR design is that it allows a larger short side radius going into the valve seat. It's not really a matter of getting enough cross-sectional area in the port. You can already get that by opening up the existing ports, but it's difficult to do so and not get flow separation and turbulence over the short side radius.

In any case, like Jim, I'd love to see someone do all the work it would take to try to apply the XR750 port design to a Commando head. It would take some serious flow bench and dyno time to sort it out, but it might work really well on a race bike. Then again, it might not, but that's why they call it development.

Ken

Yes I know and when I made what I thought was a screaming top end 850 for racing - there wasn't much point in pushing it past 7500 but there were times I had to because I had a (close ratio) 4 speed (I didn't think a 5 speed Quaife could handle it). I literally revved it as high as it would go because that's racing. The Narley ports have the advantage of a raised floor and what I'm trying to get through is that the XR 750 specs (all of them) are there for Norton racers to look at and use. Nortons haven't reached that power output or development level. The road map is there waiting to be traveled. Todays Norton motor with modern parts may be able to take the stress and put out more power than ever.

New high flow ports for Norton’s

jseng1

cjandme

ILLF8ED

Hobot

jseng1

lcrken

jseng1

Similar threads