Long velocity stacks

I went one step further and tried them on 33mm Keihin CRS smoothbores. Yes performance is improved but not as much as you get when using a bigger cam that can take advantage of the ram/air column momentum effect.

jseng1 said:

I went one step further and tried them on 33mm Keihin CRS smoothbores. Yes performance is improved but not as much as you get when using a bigger cam that can take advantage of the ram/air column momentum effect.

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When you say 'performance was improved' did you ride the bike on both long and short circuits to find out where you gained the power advantage ? Under different circumstances, you use the motor differently. I usually work up to speed and find out what the bike is doing and where, before I get silly and race.

When you change the cam, you usually change the shape of the power band and where the top and bottom of the usable rev range are positioned. You can fit a different cam and get a boost somewhere in the rev range, but the inlet and exhaust need to work with it to get the best result. Effectively you are doing the same thing as fitting a different cam, when you lengthen the inlet tract. But it all needs to end up in balance.
I suggest the power characteristics of the motor must suit the circuit. If it is tight and twisty, more torque is needed. If it is open with long straights, you need more top end. However a gearbox is a torque converter. With a Commando motor you usually need more reach as you get towards the ends of the straights.
I probably will not race again, but in a similar race to my last one, the opposition would need 140 BHP. I had them cold, even with the 4 speed box.
It was very frustrating, my brain has self-cancelling effect. If I remind myself to do something, I usually forget to do it. I have never had a fuel line come off at any other time. But it had to be when I was well out in front.

To state the obvious, tuning the induction system is all about getting as much air into the combustion chamber as possible and keeping it there until the inlet valve closes. There are 2 additional methods for increasing this which I haven't noticed being discussed in these pages. I wonder if anyone has tried them?

Reducing the air temperature by using water/methanol injection into the air intake. This assumes you are running on petrol, not alcohol and it is not banned for your use.

Adding a plenum at the intake fed by pressurized air ducted from the front of the bike. This was used successfully on the Norton Rotary racer, but maximum benefit only comes at high speeds.

I dusted off my copy of 'The high-speed internal-combustion engine' by Ricardo. There is an interesting section on port sizes, valves & cams (Just because it is 100 years old doesn't mean it is no longer valid).

hybridracer said:

...There are 2 additional methods for increasing this which I haven't noticed being discussed in these pages. I wonder if anyone has tried them?...

Reducing the air temperature by using water/methanol injection into the air intake. This assumes you are running on petrol, not alcohol and it is not banned for your use....

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Yes I did exactly that. Look at the photo below and you will see a small clear plastic bottle and vent hose in front of the top fork yoke. This was WOT water/alkohol injection I experimented with. I got the idea from a hobbyshop friend who flew WW2 fighter planes with water/methanol injection. It was a boost they used called "Emengancy war power". He said you could tell when it was being used because black smoke would pour out of the plane's exhaust as a result of 10% HP increase. My results were mixed and I didn't continue it because other racers were suspicious. That bike took me to 2nd place at 750 BOTT Laguna Seca in 1984. Note the long intake runners & velo stacks (hand made dual gang guillotine carbs).

Changing one thing at a time can be deceptive. When data is multivariate, cause and effect are iften a result of a pattern. With a motorcycle it is the combination of all the factors which is more important, not so much each individual factor. If you look at one of Escher's pictures, you will see the monks climbing the stairs forever - perpetual motion. It is a trick of the mind. When we look at a picture, we can only really see one small part of it at any one time.
If you look at one part of one of Eschers pictures, you become deceived - as a whole they do not make sense.
On one TV coverage of MotoGP the host said 'they only changed one thing and had a win' - Daryl Beattie was there and quietly said 'usually it involves changing several things'.
If you only ever race on one circuit, getting the best out of your bike is easier - you probably cannot generalise about the effect of one factor such as the length of the inlet tract.
If you are tuning for a land speed record, you cannot expect your bike to be good on a tight twisty circuit. The length of the inlet tract works in combination with the valve timing and the configuration of the exhaust system.
The last time, I raced I was a long way ahead of all the other duds when the motor stopped - does that prove anything ?
Outright speed is less important than relative speed.

My mate built the 500cc short stroke Triton which was the first bike I raced. It was built with all the theories. His best theory was the tuned length was 28,000 divided by the revs gives you the tuned length in inches. You only had to twitch and that bike would try to kill you. He did not race for 20 years after he broke his arm and leg at Bathurst in about 1957. When I raced it, getting balked in a corner was always a nightmare. The bike had obviously dropped out of the power band while he was in the corner, then threw him into the armco fence when it came back on song. When it had separate pipes with megaphones, it was simply dangerous. With a two into one exhaust, it was almost ridable. When I rode it, I was always extremely careful to be smooth and very positive. Even then, I once had tp spear-off at high speed to miss one guy. Once I was in a corner, I was absolutely commtted. I could not back-off.
I really liked that bike 'some things are so bad that they are good'. I sold it back to my mate. When it was on petrol, it was not so mean.
I rode it one day when he had it. It still went sideways in one corner, even on petrol.
In all I think I crashed it at high speed about 8 times, in as many years. Mostly due to brakes. It was an education.

hybridracer said:

To state the obvious, tuning the induction system is all about getting as much air into the combustion chamber as possible and keeping it there until the inlet valve closes. There are 2 additional methods for increasing this which I haven't noticed being discussed in these pages. I wonder if anyone has tried them?

Reducing the air temperature by using water/methanol injection into the air intake. This assumes you are running on petrol, not alcohol and it is not banned for your use.

Adding a plenum at the intake fed by pressurized air ducted from the front of the bike. This was used successfully on the Norton Rotary racer, but maximum benefit only comes at high speeds.

I dusted off my copy of 'The high-speed internal-combustion engine' by Ricardo. There is an interesting section on port sizes, valves & cams (Just because it is 100 years old doesn't mean it is no longer valid).

Click to expand...

I suggest it does not matter much what size ports are except when they are too big and destroy the power curve when you use the bike on a short circuit. With any race bike you need to balance and optimise to suit it's intended purpose. My 850 motor has had almost nothing done to it, and the bike is still competitive against larger capacity four cylinder bikes.
I did not expect the Commando motor to be that good. It is extremely responsive to tuning. The standard crank balance factor kills it - but with a close ratio box, it is excellent

With some two strokes, very short inlet tracts give better performance. Ram effect might not be the major factor when making decisions about inlet tract length ? With four-stroke kadency effect is also probably present.

Some people use formulas to build expansion chamber exhaust systems. It might be interesting to use a formula which works for a two-stroke, and use it to design an expansion chamber exhaust pipe for a racing single cylinder four-stroke engine - then use a dyno to see what it does. I think the chamber would be huge, but it might prove something. I am very careful to avoid having steps in my exhaust system where it is attached to the cylinder head.

I made these for my Commando :



Only 30 mm long, but I like the idea of keeping the bowl vent away from the incoming air stream.

ludwig said:

I made these for my Commando :



Only 30 mm long, but I like the idea of keeping the bowl vent away from the incoming air stream.

Click to expand...

WHY did you choose to isolate the pressure from the inlet tract?

If the bowl vent is not isolated from the inlet tract, the lenghts of the inlet tract would affect the jetting. A longer inlet tract might increase the pressure in the bowls. With jetting, it is easy to deceive yourself that you have made a gain. It probably depends on how you use the bike. Changes to inlet tracts and exhaust systems give advantages under varying circustances. I have never tuned a bike for Bonneville, so I do not know what would work best there. With short circuits, the circumstances usually change depending on where you are on the circuit. With MotoGP bikes, individual gears get swapped to suit various parts of the circuit.
With Triumph twins, the cams are independent., and it is difficult to get more torque by moving only the exhaust cam, but with the Commando engine , you can get more torque by advancing the cam - both valve opening points are advanced. So there is probably a balance which is dictated by the lobe centres. The length of the inlet tract and the type of exhaust both contribute.

acotrel said:

Some people use formulas to build expansion chamber exhaust systems. It might be interesting to use a formula which works for a two-stroke, and use it to design an expansion chamber exhaust pipe for a racing single cylinder four-stroke engine - then use a dyno to see what it does. I think the chamber would be huge, but it might prove something. I am very careful to avoid having steps in my exhaust system where it is attached to the cylinder head.

Click to expand...

I used physics to design /build/test a 2-stroke exhaust system for a college project before discovering the joy (and pain) of Nortons.
2-strokes require a strong positive return pressure pulse at the exhaust port when it is closing in order to force back into the cylinder the fresh mixture, which had just been sucked out of it by a preceding negative pulse. By this stage the transfer port has already closed, so without the positive pulse returning mixture to the cylinder it would be lost. It is the reverse cone with restricted exit pipe at the end of the expansion chamber that produces the positive pressure pulse.
4-strokes require a negative pressure at the exhaust valve which reduces the pressure in the cylinder and thus helps the fresh charge into the cylinder trough the inlet port. It is the tuned length exhaust pipe / megaphone / interference pipe (2 into 1 pipes on a twin) that provides that negative pressure.
Ergo 2-strokes and 4-strokes have incompatible exhaust system requirements. Good suggestion though.

Regarding steps in the exhaust, you might want to reconsider. Yes, in a 2-stroke steps are undesirable, but in a 4-stroke a step from a smaller port to a larger pipe does little to impede the high pressure flow out of the port and may reduce any reverse flow back into the cylinder when "off" the pipes.

concours said:

WHY did you choose to isolate the pressure from the inlet tract?

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Because of the proximity, the incoming air creates a small pressure drop in the inlet to the air metering screw ( not the float bowl)
Most likely insignificant. Just another variable to get rid off..

Not norton related but engines is engines...

Back when I was drag racing we would install exhaust collectors of different lengths based on weather/track conditions at the time. We had three different length pairs. Admittedly, some days we guessed wrong and lost at the start with too much torque/tire spin.

What startled me was when a mate fitted a megaphone in place of the silencer on a well tuned Velocette Thruxton - rode across an airstrip and immediately burned a valve. I would not have thought there was much so difference in the jetting requirements. With my short-stroke 500cc Triton, I was always trying to get more torque. It is amazing what DOES NOT work. With the 2 into 1 exhaust, I lost 2000 RPM off the top of the usable rev range. I kept cutting the collector back and fitting larger tail pipes. When the tail pipe offered zero restriction, I had torque and got reasonable lap times. And I had only lost 1000 RPM. Then the bike was too loud. The problem was also I used to get lazy and often did not adjust the jetting to suit the changes in the exhaust system.
The general rule is to change only one thing at a time - sounds sensible, but it is wrong. Change one thing, then optimise the others to suit the change.

hybridracer said:

I used physics to design /build/test a 2-stroke exhaust system for a college project before discovering the joy (and pain) of Nortons.
2-strokes require a strong positive return pressure pulse at the exhaust port when it is closing in order to force back into the cylinder the fresh mixture, which had just been sucked out of it by a preceding negative pulse. By this stage the transfer port has already closed, so without the positive pulse returning mixture to the cylinder it would be lost. It is the reverse cone with restricted exit pipe at the end of the expansion chamber that produces the positive pressure pulse.
4-strokes require a negative pressure at the exhaust valve which reduces the pressure in the cylinder and thus helps the fresh charge into the cylinder trough the inlet port. It is the tuned length exhaust pipe / megaphone / interference pipe (2 into 1 pipes on a twin) that provides that negative pressure.
Ergo 2-strokes and 4-strokes have incompatible exhaust system requirements. Good suggestion though.

Regarding steps in the exhaust, you might want to reconsider. Yes, in a 2-stroke steps are undesirable, but in a 4-stroke a step from a smaller port to a larger pipe does little to impede the high pressure flow out of the port and may reduce any reverse flow back into the cylinder when "off" the pipes.

Click to expand...

I suggest we should think about what works with two-stroke expansion chambers - A fatter stinger gives more torque, but less top end. A fatter midsection gives more torque. A fat header pipe is not used on road racing motors. A level face at the top of the exhaust port causes an unacceptable noise problem. What you are saying is the expansion chamber is about vacuum and not about stuffing mixture back into the cylinder by using resonance ? A bigger stinger should reduce the vacuum and cause less torque ? - But it doesn't - Sidecars with two stroke motors have fatter stingers. The volume must have an effect

When I built my Suzuki T250 racer, I did not design the chambers. I simply took two Yamaha TZ250 chambers and made header pipes for them. They worked perfectly, even though the two motors are of different concept internally.

acotrel said:

When I built my Suzuki T250 racer, I did not design the chambers. I simply took two Yamaha TZ250 chambers and made header pipes for them. They worked perfectly, even though the two motors are of different concept internally.

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You were lucky, I tried that, picking a successful race engine of fairly similar cylinder dimensions and it was useless on my engine.