New high flow ports for Norton’s

These ports are taken from the most highly developed and best cylinder filling pushrod 750 ports – the Harley XR 750 dirt track motor. Its taken me a couple years in my spare time to collect the necessary info and put it together in a way to make it possible to adapt these ports into the Norton head. No stock or aftermarket head has enough material to allow for these port shapes so machining/welding (or boring and insertion of heavy wall tubes) is necessary before creating these shapes. Or a special FA head could be cast with some sand removed from the mold so more aluminum would be thickened up in the right places. The most important features of the XR 750 port shape are the gradual curve leading up to the valve seat and the extra wide “Cobra head” design up and around the guide area that allows for a raised port without weakening the guide area (the roof is not round). An ideal downdraught port with such a gradual curve to the valve seat would be so elevated that it would protrude into the rocker box area, so to avoid that the port must continue its curve throughout its length all the way from the valve to the manifold. Its counterintuitive but the manifold must be more level to avoid a kink in the port near the valve. Look at the side image below and you will see that the new port is not as steep as the original Norton port at the manifold end.

You see the same cobra head shape in some Nascar ports. It’s an advanced design and should bring some new HP levels to Nortons and heighten the competition. So far its been done to one (international) brit bike that I know of and it added an additional 10 to 15 HP to what was thought to be an already highly developed race motor. On the track it not only won its class but beat many of the modern bikes as well. Another motor is on its way. There’s more development to do and it will be interesting to see what happens if/when cutting edge racers/tuners take the plunge. Three variations of this port are available for short or long stroke motors of various sizes. The higher performance ports require other related modifications as well.


Side view (black lines = new Hi Flow port, green lines = original Norton port)


cross section of “cobra head” port shape at the intake guide


See re-angled manifolds below.

Very interesting info Jim, thanks for posting it!

The cross-section is interesting. I can understand the dip in the middle of the top that smooths flow past the guide and stem, but most modern ports are flattened and widened at the bottom to a nearly squared shape with radiused fillets, of course.

Yes - titillating!

What size valve are you using in the new port and what's the good news on the airflow of the new port?

From Comnoz' airflow postings here on the forum of his and others big valve (1.62" IN dia) heads, flow bench results suggest that airflow maximizes at ~ 155 cfm at 28" WC, which is about 73% of entitlement airflow (for the 1.62" valve). To produce 15 HP more would require an increase in airflow on the order of 50-60 cfm at 28" WC, which would be closing in on 100% entitlement for a 1.62" dia IN valve.

If somebody actually achieved those airflow numbers the potential exists to make power similar to an oval port XR, which is really exciting, and likewise, stress all the engine components to their limit and well beyond, as this range of airflow would support operation in the range of 10,000 rpm.

Reminds me of previous discussions here on Axtell's contributions to Norton performance where some members suggested his engines were unrealiable (blew up), while others suggested that the airflow he provided allowed access to an rpm and HP regime that far surpassed the strength of other engine components and the result was exactly what would be expected - a blown engine due to the weakest mechanical link. I'm in this latter camp. I've got the shields up so fire away.

As it was explained to me by a local dirt-tracker who built numerous scorching Triumph engines, the valve will flow best with the port running parallel to the stem, thus, the JB weld in the floor of the ports. I tried this on my 650 Ya-Mama-Ha, and it added a few more ticks to the top end.

WZ507 said:

Yes - titillating!

What size valve are you using in the new port and what's the good news on the airflow of the new port?

From Comnoz' airflow postings here on the forum of his and others big valve (1.62" IN dia) heads, flow bench results suggest that airflow maximizes at ~ 155 cfm at 28" WC, which is about 73% of entitlement airflow (for the 1.62" valve). To produce 15 HP more would require an increase in airflow on the order of 50-60 cfm at 28" WC, which would be closing in on 100% entitlement for a 1.62" dia IN valve.

If somebody actually achieved those airflow numbers the potential exists to make power similar to an oval port XR, which is really exciting, and likewise, stress all the engine components to their limit and well beyond, as this range of airflow would support operation in the range of 10,000 rpm.

Reminds me of previous discussions here on Axtell's contributions to Norton performance where some members suggested his engines were unrealiable (blew up), while others suggested that the airflow he provided allowed access to an rpm and HP regime that far surpassed the strength of other engine components and the result was exactly what would be expected - a blown engine due to the weakest mechanical link. I'm in this latter camp. I've got the shields up so fire away.

Click to expand...

Ron Wood said his cases would always crack at the end of a straightaway when the throttle was let off, so there's more to it than too much power at too much rpm.

I think it was my freaked out throttle snap shut on Peel over rev event that added the suckion loads that bent crank rather than just raw centrifugal sling and the witness marks, nay blown out or rubbed off areas imply something held pistons from descending as fast and crank so crank end pivoted at bearing to tip its ends out most at 7 and 4 oclock positions. Next time will just hold kill button if rev limiter failed.

Danno said:

Ron Wood said his cases would always crack at the end of a straightaway when the throttle was let off, so there's more to it than too much power at too much rpm.

Click to expand...

On a motorcycle with no brakes the throttle closing will be a pretty abrupt action to generate some braking, engine braking dependent on compression, so this effect would seem to be as much as anything a direct relationship to the compression ratio......

However, the phenomenon was not limited to Commando twin based machines in the '70s, but was observed on Manx Nortons (and other machines, but I think mainly singles!) in the late '50s early '60s......although I understand in these machines it would sometimes occur with just a little easing of the throttle intended by the rider to give the motor an easier time!

Am I correct in understanding that this design is based upon the later XR heads, the ones with the generally oval as apposed to the generally round ports?

xbacksideslider said:

Am I correct in understanding that this design is based upon the later XR heads, the ones with the generally oval as apposed to the generally round ports?

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Kidney shaped even....judging by the included illustration!

I'll try to answer the questions above in one post.

Danno & Backsideslider - The intake ports are more oval but the ex port is very D shaped as in the photo below. The dip in front of the guide is to add material to prevent cracking near the guide bore (see photo at top of page). The point is to raise the port around the guide so the fuel charge can turn downward more gradually to avoid turbulence. The port gets wider than the valve at the guide and is very shallow from the roof to the floor - this keeps the radius curve of the floor closer to the radius curve of the roof (trying to get the port parallel with the valve stem as Nater Potater mentioned)



WZ2507
The "Narley" port specs come in 3 stages starting with 41mm intake valves and going up to 42 and even 44m for larger bores. The XR uses up to 44mm intake with 37mm exhaust - this is for an all out 750 with 79m bore, 77mm stroke, 9000 RPM and a claimed 100 HP. I suggest 42 mm intake to start with for short strokes or larger bores unless you're going balls to the wall. I’m not providing any flow specs – that’s already been done ad infinitum by C.R. Axtell, Carl Patrick. Dick O Brien and many others who developed the oval/D port with endless hours racked up on the flow benches and dynos.

Steve A
Ron Wood was using stock cases and they could crack at the end of the straight when shutting off the throttle. The reduced cylinder pressure allowed the rods to try to throw themselves into the stratosphere. It’s the shaking forces that blew up the motors. A heavy piston motor would explode at elevated RPMs but today we have 1/3 lighter pistons and longer rods to reduce the jerk. This combined with solid cranks and heavy duty Maney cases to keep things together makes it a different game now as evidenced by 1000cc Norton equipped with the modern parts staying together through a full year of racing and 920 on Nitrous etc.

SteveA said:

Danno said:

Ron Wood said his cases would always crack at the end of a straightaway when the throttle was let off, so there's more to it than too much power at too much rpm.

Click to expand...

On a motorcycle with no brakes the throttle closing will be a pretty abrupt action to generate some braking, engine braking dependent on compression, so this effect would seem to be as much as anything a direct relationship to the compression ratio......

However, the phenomenon was not limited to Commando twin based machines in the '70s, but was observed on Manx Nortons (and other machines, but I think mainly singles!) in the late '50s early '60s......although I understand in these machines it would sometimes occur with just a little easing of the throttle intended by the rider to give the motor an easier time!

Click to expand...

FYI, flat track bikes have rear brakes only, which are used to break loose the rear wheel going in and initiate a powerslide.

In Ohio last year I saw various twins that just over powered rear as they tossed em over accelerating no rear brake needed. Flat track was the loudest cycle event I have been too as they essentially just hit and stay at WOT power band to ride the jet thrust on open pipes. Saw a fairly inexpensive mostly factory 850 blow away two non Norton 920s and a National level XR900 that may of had the oval ports, by like a dozen lenghts from last exit to finish line. Commando power plants variation confuse me no end.

hobot said:

In Ohio last year I saw various twins that just over powered rear as they tossed em over accelerating no rear brake needed. Flat track was the loudest cycle event I have been too as they essentially just hit and stay at WOT power band to ride the jet thrust on open pipes. Saw a fairly inexpensive mostly factory 850 blow away two non Norton 920s and a National level XR900 that may of had the oval ports, by like a dozen lenghts from last exit to finish line. Commando power plants variation confuse me no end.

Click to expand...

Hobot:
Jay Springsteen is the one who developed the dirt oval style of keeping the throttle wide open and using the rear brake to slow down the rear wheel to break traction entering the turns. He won races this way and others caught on.

Backside slider:
I'm somewhat aware of Axtell's contribution to the XR750 years ago. He was on the right track and I saw similarities in the raised eyebrow shape (to a small degree) in his Norton intake ports. Back then they actually used to view the port in a running motor through a glass.

Axtell's contributions to XR evolution were associated with the earlier round port design. The late Ken Augustine was the one responsible for creating the oval port design in the early 1980s.

Greg Sanders built a prototype head when he was working for Jerry Branch years ago. It mimicked the heads that were going to Harley @ the time. That places it in the early head category. One side of the head was still close to Norton production standards and the other side had extensive welding to accommodate the XR config. Greg lived in Long Beach when I last called him about 10 years ago. Still had the V4 Norton with no interest in selling it. Maybe worth checking out the head ?

jseng1 said:

I'll try to answer the questions above in one post.

The intake ports are more oval but the ex port is very D shaped as in the photo below. The dip in front of the guide is to add material to prevent cracking near the guide bore (see photo at top of page). The point is to raise the port around the guide so the fuel charge can turn downward more gradually to avoid turbulence. The port gets wider than the valve at the guide and is very shallow from the roof to the floor - this keeps the radius curve of the floor closer to the radius curve of the roof (trying to get the port parallel with the valve stem as Nater Potater mentioned)

The "Narley" port specs come in 3 stages starting with 41mm intake valves and going up to 42 and even 44m for larger bores. The XR uses up to 44mm intake with 37mm exhaust - this is for an all out 750 with 79m bore, 77mm stroke, 9000 RPM and a claimed 100 HP. I suggest 42 mm intake to start with for short strokes or larger bores unless you're going balls to the wall. I’m not providing any flow specs – that’s already been done ad infinitum by C.R. Axtell, Carl Patrick. Dick O Brien and many others who developed the oval/D port with endless hours racked up on the flow benches and dynos..

Click to expand...

That cross-section loks a bit different than the first. Is there any "necking down" in the port to accelerate flow, or is the cross section area more or less comstant?

Danno said:

That cross-section loks a bit different than the first. Is there any "necking down" in the port to accelerate flow, or is the cross section area more or less comstant?

Click to expand...

I refered to Axtell and a few other turners because that is where all this started. There has been continual development by others and I give credit to all of them even if I don't know all their names.

Note that I have shown intake cross sections and exhaust cross sections in earlier posts and the shapes are different near the carb or near the ex pipe – The ex port requiring a D shape at the pipe to reduce reversion.

The cross section changes continually. The oval shape starts in the manifold near the carb and morphs all the way to the valve. The guide is the obstruction and the port gets wider around it. See the XR 750 port mold photo below that was posted earlier in another thread on this discussion group.



Photo below is of a highly developed Nascar port – very similar in shape to the port shown above.


XR 750 port compared to an early 28.5mm Nort intake port. Small ports make for good velocity at moderate RPMs but note the radical departure from round and the out of round width factor required for efficient breathing at high RPM.



The XR 750 IN port gets down to about 26mm from floor to roof at one point whereas the small early Nort port is 28.5. I have specs in 3 stages so those who like small ports for higher velocity have the choice - but all stages have the similar efficient shape.

Seeing the whole port shape tells me it is almost like one port diverging into two at the valve guide and stem and then partially rejoining, at least in direction, just before the valve seat.

Speaking of the 8-valve ohc VR 1000 motor, H-D's Dick O'Brien once said, "The ports are too goddam big and the valves are too goddam small." Top speed was around 157 mph. Years earlier, Luicifer's Hammer, the resurrected Mark Brelsford XRTT 750 punched out to a full liter achieved 170 mph with god-fearing pushrods. Axtell, Jerry Branch, Don Tilley, Tom Sifton and others all had much to do with making these antiquated motors run with and past more modern machines including desmo Ducatis. They all learned to do more with less making side-valve KRs run way beyond their design capabilities.

While at Bonneville in 2006 Jerry Branch stopped by to talk with the builder of the bike my brother was riding. What a nice guy with a ton of stories to tell, he worked with some of the top teams back in the day. I'll never forget when he was walking up my brother asked "you know who that is?" I said no, well I guess I should have. I do now.
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