More power with more intake length

Someone was having a little mid range stutter with his hot cam so I suggested he experiment by adding velo stacks to increase the intake track length. So he attached a pair of velos and "95%" of the stutter was gone to be replaced by a significant increase in power. Approx 10" from the head to the end of the carb or velo stack is what's considered to be ideal by myself, Ron Wood and Leo Goff (Peter Williams used 12"). You can't always achieve that with an aircleaner and stock oil tank but any increase in length is an improvement because the longer air column gains momentum and creates a ram effect. All the best racers who have tried it know its value. The owner of the bike in the photos is working on an aircleaner now and is incorperating the velo stacks into the aircleaner box. Longer stacks are also being ordered for more experimentation.





Special order velo stacks are available from Japan as shown below. But I don't know how to order them as the website is written in Japanese. Extending the manifolds or boots should have the same effect but its best to keep most of the weight closer to the head.

Msny years ago, my mate built the first Triton I raced and he used all the theories. One of those theories was - the optimum intake length in inches , is 28,000 divided by the revs. In my ecperience the exhaust system has much more influence than the intake length. When he built the bike, he had separate exhausts with 4 inch diameter megaphones. He rode it at Bathurst. When he reached the end of Conrod Straight, he was going that fast that it would not stop. The choice between the escape road or the circuit presented the same angle, so he chose to go around, and got around. When he got to the top of the mountain he crashed into the armco fence and broke an arm and a leg. I rode the bike with those pipes. If I was balked in a corner and the revs dropped, it would drop out of the bottom of the usable rev range, so the clutch needed to be slipped to get power again. The bike would immediately go sideways. With that bike there was always a danger of hitting another rider. You could never back-off in a corner.
At TDC when both valves are open, the inlet tract is connected to the exhaust. A two into one pipe fixes that problem. However if there is any restrtiction in the pipe, you can lose 2000 revs off the top of the usable rev range..

On the Dyno recently I found that my engine produced slightly more power when fitted with pod filter adaptors than it did with either of the supplied length of genuine FCR stacks (on 39mm FCRs).

So there’s no real rule to this stuff, experimentation is the only way to fine tune IMO.

I stuck some 60mm Vstacks with unifilters on the 35mm FCR's in the early start of tuning and did notice a real difference ...leaning them out further with jetting ...made it all come together...

So now he took off the short stacks and temporarily taped on some long make shift stacks for a total overall length of approx 10" overall to the head and the mid range stutter dissappeared completely plus he has more power than ever with the ft wheel coming off the ground in 2nd gear.

PJ Motorsports in Oregon USA make custom stacks and know Keihin and Mikuni.

PJ Motorsports

As the automotive hot rodders in the USA say, "Every motor is different."

I have not played with velocity stacks on the 35mm FCRs, but did with the 34mm Mikuni VMs. Definitely ran better, but I don't like grit in my motor that much, so only used them at car shows when the bike was sitting in the bed of my '65 El Camino. Got a trophy even.

Anywho, yeah stacks can work. With a Seeley frame I would think the options for intake tract length are nearly limitless. I can't get more than 8.5 inches of length in my version of Norton frame. Well actually it isn't a Norton frame. It's a Matchless frame with a 750 Norton engine stuffed in it.

60mm Vstacks with oiled unifilters...(compromise)
New Zealand roads can be dusty

My two 34 mm Mk2 Amals have velocity stacks which are about 3 inches long. I don't think it matters. What is more important is getting the exhaust system and the jetting right. When you use methanol fuel as I do, getting the jetting right is easser because the jets flow twice as much fuel, so an error in needle jet dimension has half the effect, when compared with using petrol. With my carbs, half a thou in needle jet internal diameter, is the difference between fast and slow. If I was using petrol, that would be a quarter of a thou.
When you ride a Commando, you usually have a heavy crank. When you change up through a wide ratio gearbox, the revs usually fall. But you hold the throttle open. The inertia of the crank causes vacuum in the inlet tract, so you usually have a quicker taper needle in the carb. The mixture quickens faster, so the bike is slower. When you use a close ratio gearbox, you do not create as much vacuun, so you can use a slower taper needle, and the bike is faster. With methanol fuel it make a big difference. With petrol as fuel, it would be much more difficult to get the same level of control over the mixture.
When you fit longer velocity stacks - what are you changing - it must affect the amount of vacuum in the inlet tract ?
I tried to race with a normal Norton gear box - the bike was miles too slow, You can open the throttle fully and wait for the revs to rise. With lean needles and close ratios, that does not happen. I have as much throttle response with my 850 as I ever had with my 500cc short stroke Triton which had a very light crank and close gears.
All methanol fuel ever does which gives more power, is it makes the tuning easier to get right. The high lastent heat of vaporisation causes the jets to be larger. The energy content of methanol is about 0,.8 times that of petrol, but you can tune it better. Modern bikes use fuel injection and engine management systems to get the same effect.

I actually love the heavy Commando crank. it really makes me laugh. Lowering the overall gearing does not give quicker acceleration. When that crank is spinning high, nothing stops it. If I race, it never drops below 5,500 RPM. I have ridden a few bikes in my life, but never one better. My old mate rode it before he died, it put a really big smile on his face.

KiwiShane said:

60mm Vstacks with oiled unifilters...(compromise)
New Zealand roads can be dusty

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I parked my Norton for 27 years and it had UNI filters on the 34mm Mikuni carburetors. When I went to bring it back from the dead the UNI filters crumbled in my hands. I decided to use the gauze K&N filters that came with the Norton 35mm FCR kit.

I don't notice any stumble, stutter, or whatever in the midrange with the FCRs, but I do have a 2 into 1 exhaust that might be disguising it.

By the by, USA folk can also check to see if Dime City Cycles has a set of velocity stacks that can be adapted to work on the Keihin CRS bodies. The custom appearing stacks they sell are a little bulky, but it is another source.

KiwiShane said:

60mm Vstacks with oiled unifilters...(compromise)
New Zealand roads can be dusty

Click to expand...

A few years ago, I was racing at winton, and I did not realise there was a half inch gap under the seat on my bike. When I tried to start the motor, it woukl not start. There was a small stone under the slide in one of the carbs. The gap under the seat is now closed with rubber.

I am a bit surprised at a Norton race bike lifting the front wheel in second gear. My Seeley usually loses traction at the rear wheel first. The weight distribution is probably different. I like the centre of gravity to be well forward. When it is back, you need to take more care in corners.. There is only an inch in it.

acotrel said:

I am a bit surprised at a Norton race bike lifting the front wheel in second gear. My Seeley usually loses traction at the rear wheel first. The weight distribution is probably different. I like the centre of gravity to be well forward. When it is back, you need to take more care in corners.. There is only an inch in it.

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Do you normally race on a skid pad?

How in the heck could you ever break away from the back markers if your rear tire is spinning under race time acceleration.

Schwany said:

I parked my Norton for 27 years and it had UNI filters on the 34mm Mikuni carburetors. When I went to bring it back from the dead the UNI filters crumbled in my hands. I decided to use the gauze K&N filters that came with the Norton 35mm FCR kit.

I don't notice any stumble, stutter, or whatever in the midrange with the FCRs, but I do have a 2 into 1 exhaust that might be disguising it.

By the by, USA folk can also check to see if Dime City Cycles has a set of velocity stacks that can be adapted to work on the Keihin CRS bodies. The custom appearing stacks they sell are a little bulky, but it is another source.

Click to expand...

Even though the uni filters are washable and can be re- oiled...I generally toss them these days as I have a sack of them...I removed the K&N's and replaced them with the unifilters due to adding the Vstacks which goes almost right back to the battery.

A port and polish of the intake ports and runners can help smooth out engine breathing and make things go faster.
I have done so on V8 intake manifolds with sound improvement and performance.
I applied the same procedure on my Norton engine as well.
Its a monster of a job on a big intake manifold but the ports and runners of the Norton engine were a whole lot less labor intensive.
The intake ports on the Commando cylinder head were ported and polished when I had the head was off while restoring the bike.

Lineslinger said:

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Does polishing the outsides help much?

Triton Thrasher said:

Does polishing the outsides help much?

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It was brutally hot in my shop when I was working on this part of my restoration.
I powder coat certain components for protection and esthetics.
In answer to your observation and IMHO oxidized aluminum can develop a neglected appearance. Some don't care, I do.
If I've cleaned it once I have no interest in performing the same job twice. Some don't care, I do.
The little portable AC unit I keep in my shop couldn't keep up, my camera overheated, wouldn't go into Macro setting.
I continued on without any photo records of the porting and polish work showing the inside of the runners final polish, the connection seam at the runner to cylinder head and the carb outlet port to runner inlet port connection/seam.
Hope this satisfies your "curiosity".

With velo stacks on the street you want filters. Foam filters may work but here's a solution with a paper filter attached to the velo stacks via a carbon fiber plate.

Is there any benefit of increasing the length on a vm 34 on a single manifold?