Amal carbs CFM

acotrel · Nov 15, 2024

With carburation there are about three things which need to be in an optimal balance to get maximum torque - mixture, compression ratio and ignition advance. The first mistake most of us make when tuning a motor is increase inlet port size. Less vacuum means the needle jet needs to be larger to deliver enough fuel. Raising the bore size, means the compression ratio becomes higher which has a similar effect to leaning-off the mixture or increasing the ignition advance. The point is, if you fluke the optimum, you get more torque.
I am at an advantage using methanol instead of petrol. It does not matter what configuration the inlet port , compression ratio or ignition advance might be, I can jet to get the optimum - simply because when using methanol it has 0.8 times the calorific value of petrol, and 2 times as much is used due to it's latent heat of vaporisation. My needle jets get adujsted in half thou increments. Petrol needle jets come in increments of one thou. There are 5 notches on the needles with both fuels. The difference between fast and slow is almost nothing in needle jet internal diameter,
Back in the 1950s, the British could get their Nortons going as fast on petrol, as our guys could in Australia using methanol. But the British invented cricket and sneaky ways to defeat Germans.

mdt-son · Nov 15, 2024

baz said:
No idea about CFM it's all beyond me!
But I run a single tm40 flat slide mikuni on a home made manifold on my 750
And it'll easily pull beyond the redline in the lower gears

baz, this is of no significance, as the torque needed to reach 7 000 rpm (or 10 000 rpm for that matter) in lower gears is way below the torque needed to rev out with a 1:1 gear ratio.
The degree of cylinder filling of combustible gas termed VOLUMETRIC EFFICIENCY is what determines the amount of torque produced, all other factors kept constant.

A 2-valve engine with constant valve valve timing will struggle to exceed a volumetric efficiency of 60%. A 4-valve engine with variable valve timing and variable inlet duct length may reach 75% .

- Knut

baz · Nov 15, 2024

mdt-son said:
baz, this is of no significance, as the torque needed to reach 7 000 rpm (or 10 000 rpm for that matter) in lower gears is way below the torque needed to rev out with a 1:1 gear ratio.
The degree of cylinder filling of combustible gas termed VOLUMETRIC EFFICIENCY is what determines the amount of torque produced, all other factors kept constant.

A 2-valve engine with constant valve valve timing will struggle to exceed a volumetric efficiency of 60%. A 4-valve engine with variable valve timing and variable inlet duct length may reach 75% .

- Knut

As I said
It's beyond me but compared to twin amal 32s it's better in the mid range and the same at top end

MexicoMike · Nov 15, 2024

I can't speak to this re a Norton engine but I have experience with other conventional engines that routinely exceed 110-120% volumetric efficiency at optimum RPM with appropriate intake/exhaust length, etc.

kommando · Nov 15, 2024

MexicoMike said:
I can't speak to this re a Norton engine but I have experience with other conventional engines that routinely exceed 110-120% volumetric efficiency at optimum RPM with appropriate intake/exhaust length, etc.

Correct VE's of over 100% are not uncommon, this is where you have a swept volume of 414 cc per cylinder but at certain revs with a wide open throttle more than 414 cc's of mixture enter the combustion chamber. All comes down to the cam, cam timings, valve sizes etc. ECU software is written to allow values over 100% for that very reason.

acotrel · Nov 15, 2024

Some of you guys might not realise that when we talk about combustion , we are talking about thermodynamics. There is a difference between the American system and the British system. In American texts there is a minus sign which appears because of the way they think of entropy. To me, order is when everything is in it's ground state, and it takes energy to construct. In the American system, a construction is order., and destruction releases energy.
It makes a difference in the way we think about things - even the creation and evolution.
Entropy change to me, is the amount of energy absorbed divided by the temperature of the change. In the American system energy is released to produce change. To Americans entropy is the amount of order in the universe - not the amount of disorder.
When I was studying at night school, one of my mates stole a book on thermodynamics. The minus sign appeared everywhere - he threw the book out of a first floor window in disgust. The American book cannot be safely used in Australia. Australia is British.
'In God we trust' has a few implications.
Whichever system we use, we always need to be consistent - they both do the same job and return the same result, but confusion is dangerous.

mdt-son · Nov 16, 2024

kommando said:
Correct VE's of over 100% are not uncommon, this is where you have a swept volume of 414 cc per cylinder but at certain revs with a wide open throttle more than 414 cc's of mixture enter the combustion chamber.

Please present all engine data this table refers to. The values look like the result of a simulation??

- Knut

Thomasa · Nov 16, 2024

I am a bit hazy in the meaning of laminar flow. Is that the average over a period of time or the instantaneous flow during the period when the piston is actually sucking? It seem to me that the instantaneous-flow maximum during the inlet stroke might be well in excess of the theoretical flow over a minute's time. As has been discussed, the resonant effect of a tuned inlet tract in concert with the resonant exhaust effect apparently can produce greater than 100% V.E. over some time period. How much greater might that be if one considers only the time during which the inlet event happens?

For example, with audio speakers we must consider the instantaneous current required to move the speaker cone when a big bass wave happens. That's why those big Monster cables seem so large if you only consider the average current on a 50W speaker. So would carburetors need to be oversized for the same reason? Neglecting, low and mid-range driveability.

In my youth, hotrodders would run 6 2bbl carbs on a log manifold to feed two banks of a V8. You could buy the manifold kits from J.C. Whitney and weld them up yourself. And Stromberg 97 carbs were dirt cheap in bone yards. The size of the logs made a difference it was said, because they provided a reservoir of air for the cylinders to draw from when it was their turn. The number of carbs was immaterial as long as they could flow more than the average requirement. Any ram-tuning happened between the log and the valve port. There were long cross rams available as I recall.

acotrel · Nov 16, 2024

One of my friends was a top A - grade rider in Australia, and an excellent motor mechanic. I once asked him 'how many variables do you think there are when you road-race a motorcycle ? ' - he said 'a lot'. The superbike guys ride motorcycles with sensors all over them and collect heaps of data. I don't know what software, they use to analyse it. It would have to involve pattern recognition - some form of AI.
Cause and effect often does not depend on just one variable - it is usually about the combination of many variables.
That combination can be rationalised to a certain extent by the human brain. My boss did it when we were making steel - he adjusted the chemical composition by rationalising data from many melts. he actually got a result.

acotrel · Nov 16, 2024

Thomasa said:
I am a bit hazy in the meaning of laminar flow. Is that the average over a period of time or the instantaneous flow during the period when the piston is actually sucking? It seem to me that the instantaneous-flow maximum during the inlet stroke might be well in excess of the theoretical flow over a minute's time. As has been discussed, the resonant effect of a tuned inlet tract in concert with the resonant exhaust effect apparently can produce greater than 100% V.E. over some time period. How much greater might that be if one considers only the time during which the inlet event happens?

For example, with audio speakers we must consider the instantaneous current required to move the speaker cone when a big bass wave happens. That's why those big Monster cables seem so large if you only consider the average current on a 50W speaker. So would carburetors need to be oversized for the same reason? Neglecting, low and mid-range driveability.

In my youth, hotrodders would run 6 2bbl carbs on a log manifold to feed two banks of a V8. You could buy the manifold kits from J.C. Whitney and weld them up yourself. And Stromberg 97 carbs were dirt cheap in bone yards. The size of the logs made a difference it was said, because they provided a reservoir of air for the cylinders to draw from when it was their turn. The number of carbs was immaterial as long as they could flow more than the average requirement. Any ram-tuning happened between the log and the valve port. There were long cross rams available as I recall.

My carbs are two 34mm Mk2 Amals on 30mm ports. I don't know why I did that. I am surprised that using slower taper needles made a significant difference,

acotrel · Nov 16, 2024

baz said:
As I said
It's beyond me but compared to twin amal 32s it's better in the mid range and the same at top end

What does volumetric efficiency really mean when the other factors keep changing when you ride the machine ? If you use lean needles in the carbs, it means you reduce the luxury of whacking the throttle open, but when you wind the throttle on slower, the bike might accelerate faster.
The taper on the needles compensates for loss of vacuum in the inlet port.
When you change configuration, you need to optimise the associated variables to suit the way you intend to use the motorcycle. We usually only ever change one thing at a time and often do not understand the interactions. If a race circuit is tight and twisty, a torquey motor is often better than a peaky motor - it depends on the lengths of the straights and whom you are racing against.

acotrel · Nov 16, 2024

I think the piston only sucks when you are starting the motor. After that. everything else is resonating sound waves. The explosion causes the exhaust to work, and that sets the rest going. At TDC on one stroke, both valves are open. Kadence effect is mixture stuffed back into the cylinder from the exhaust.
Triumph 650 engines have independent inlet and exhaust cams. When you adjust timings independently the effects can be detected.

maaseyracer · Nov 17, 2024

Madnorton said:
How is it efficiency? surely what is being measured is % of operating capacity. Surely using single carb is over capacity when trying to supply 2 cylinders and thus operating at over 100% of it's capacity at certain rpm. If the above was the case then the racers would use a single carb and use the other spare 64% capacity - 'just thinking logically'

With a single carb you generally have a plenum volume which is a storage of sorts, and the size of that plenum is what allows the support of air flow to support higher RPM operations. So with a small single carb manifold the plenum is small and does not have the capacity to support the air needed for higher RPM. At the same time the smaller plenum often aides in lowering the point where torque comes in on the RPM band.

maaseyracer · Nov 17, 2024

kommando said:
Correct VE's of over 100% are not uncommon, this is where you have a swept volume of 414 cc per cylinder but at certain revs with a wide open throttle more than 414 cc's of mixture enter the combustion chamber. All comes down to the cam, cam timings, valve sizes etc. ECU software is written to allow values over 100% for that very reason.

Its even harder on an air cooled engine. A MAP sensor and O2 sensor would really go a long way on these things when it comes to tuning these things.

acotrel · Nov 18, 2024

I suggest the most critical thing which affects performance is carburation. Amal carbs on petrol are probably too difficult to get really right and then compensate for the weather. A computerised engine management system with fuel injection would probably be streets ahead . I like the idea of different ignition advance curves to cope with different riding conditions. Some people seem to believe you cannot have too much horsepower, however controlling how it is delivered must be good.

Amal carbs CFM

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