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Originally posted by RWD fords rule
Yes if you want to get more technical I agree with that, after testing the vacuum signal to the main jet fuel system with a different size chokes in 45mm carbs at multiple different throttle openings I can say for sure that choke size is critical for a road engine aswel as rally and race, you want to use the smallest choke size that will give the power you are after but that does not tell the whole story as throttle plate diameter and throttle plate diameter vs choke diameter are also very important
With a 45mm weber I flow tested the following choke sizes, 36, 37, 38, 39 and 40, at a variety of different throttle plate angles with no inlet manifold attached just full flow through the carb with a trumpet in place, what I found is that the larger the choke gets the weaker the vacuum signal at wot, But, the biggest point is that the larger the choke gets the weaker the vacuum signal at part throttle which creates a very lazy engine when accelerating from idle up to 3000rpm or so when the main fuel system has fully taken over and is now working well
For example you could have a 180bhp pinto with 45mm webers and 38mm chokes, then fit 40mm chokes and the engine makes very similar peak hp but acceleration out of corners will drop markedly
Using a choke diameter 7mm less than the throttle plate diameter is a good general rule to stick to with any sidedraft weber as the maximum choke size that should be used in a rally or race engine unless it is going to keep above 4000rpm at all times, then you could go larger but in most cases using larger than 7mm smaller will result in a slower engine over all
For a fast road engine I would use 32mm chokes in 40mm webers or 36mm chokes in 45mm webers, with 40's for a mild fast road engine and the 45's for a fast road limited mileage engine
The vacuum signal strength to the main fuel jets as the throttle opens and the main jet starts to work is very important, if you use chokes that are too large vs the throttle plate diameter then the vacuum signal to the main jets as the throttle is slightly opened will be very poor vs a smaller choke size, which can be proven on any flowbench
The next thing is the throttle plate size, if you fitted say 48mm webers to a mild 2.0 pinto engine with even "small" 38mm chokes, then even though the choke size looks good vs the throttle plate diameter the choke is way too large for the engine, basically the engine would not be flowing enough air through the carbs to create a good vacuum signal to the main jets until moderate rpm and would drive like a pig up until 4000rpm or so with no torque at all exiting corners and pulling away from a standstill
This is why I am saying 40mm carbs with 30 or 32mm chokes are a better option for a mild fast road engine that will be driven often, around town and on long runs etc, vs 45mm webers and 36mm chokes, the 40mm webers have smaller chokes that require a lot less airflow to create a decent vacuum signal to the main jets at part throttle which is very critical for a fast road engine that will be driven in traffic etc
All of this can be proven on a flowbench and also in the car where it counts
A long answer but there is a lot more to this than most people think
With a 45mm weber I flow tested the following choke sizes, 36, 37, 38, 39 and 40, at a variety of different throttle plate angles with no inlet manifold attached just full flow through the carb with a trumpet in place, what I found is that the larger the choke gets the weaker the vacuum signal at wot, But, the biggest point is that the larger the choke gets the weaker the vacuum signal at part throttle which creates a very lazy engine when accelerating from idle up to 3000rpm or so when the main fuel system has fully taken over and is now working well
For example you could have a 180bhp pinto with 45mm webers and 38mm chokes, then fit 40mm chokes and the engine makes very similar peak hp but acceleration out of corners will drop markedly
Using a choke diameter 7mm less than the throttle plate diameter is a good general rule to stick to with any sidedraft weber as the maximum choke size that should be used in a rally or race engine unless it is going to keep above 4000rpm at all times, then you could go larger but in most cases using larger than 7mm smaller will result in a slower engine over all
For a fast road engine I would use 32mm chokes in 40mm webers or 36mm chokes in 45mm webers, with 40's for a mild fast road engine and the 45's for a fast road limited mileage engine
The vacuum signal strength to the main fuel jets as the throttle opens and the main jet starts to work is very important, if you use chokes that are too large vs the throttle plate diameter then the vacuum signal to the main jets as the throttle is slightly opened will be very poor vs a smaller choke size, which can be proven on any flowbench
The next thing is the throttle plate size, if you fitted say 48mm webers to a mild 2.0 pinto engine with even "small" 38mm chokes, then even though the choke size looks good vs the throttle plate diameter the choke is way too large for the engine, basically the engine would not be flowing enough air through the carbs to create a good vacuum signal to the main jets until moderate rpm and would drive like a pig up until 4000rpm or so with no torque at all exiting corners and pulling away from a standstill
This is why I am saying 40mm carbs with 30 or 32mm chokes are a better option for a mild fast road engine that will be driven often, around town and on long runs etc, vs 45mm webers and 36mm chokes, the 40mm webers have smaller chokes that require a lot less airflow to create a decent vacuum signal to the main jets at part throttle which is very critical for a fast road engine that will be driven in traffic etc
All of this can be proven on a flowbench and also in the car where it counts
A long answer but there is a lot more to this than most people think