Motorcycle itb's , duel tbs?

[tpv]

There was a post about motorcycle itb's and the consensus was too big.
I've seen 1108 fire engines run well on twin 40dcnf carbs, although the choke size reduced the effective bore.
Injection systems tend to have better low end manners than carbs plus although the 600/1000cc etc bike engines developed peak power at very high rpm they still run well at low rpm and often have more power than my engine there too!
I saw a mini 1000 with a twin cam 8 valve head running 40mm itb's and it pulled hard from low down to 8000rpm no problems.

With injectors spraying into the inlet ports fuel stay atomized even when air speed slows down unlike having an oversized carb.

I think someone should definitely experiment with some bike itb's.

My first car was 1000cc and I had a 40dcoe, even on a standard engine it ran excellently..... Admittedly it was just 2 Venturis for 4 cylinders.
I also had a 2cv with a 40dcnf adapted to standard manifold with an awfully home made adapter with a steep angle!......no extra power.

Another example is the fiat 126 manifold for the single inlet port head for the 40dcoe!
Huge carb into a tiny hole!
I can't see how that will be of much benifit unless the area of the head port is similar to the cross sectional area of the carb Venturis?

What is the port diameter of the fire 1108 engine?
I'm sure there must be some itb's to suit?
36 to even 40mm.
Obviously you won't make use of the itb's max potential but if set up right should run ok with a big gain in power.

In fact Newman cams suggest twin carbs or itb's and kenvey etc don't do smaller than 40mm....I think?

I saw some itb's go for £50.... with injectors.....!

Or alternatively make a manifold using 2 itb's?

 

[bugsymike]

Is it also affected by the speed of the inlet fuel charge, too big a bore speed slows and low end performance poor.
In the old days SU carbs gave good all round performance from low down with possibly a slight loss at top end compared with fixed choke ventauri carbs . There was also the Stromberg and Fords VV carbs with rubber diaphragms which did a similar job but were prone to backfires splitting the rubber diaphragms. CD carbs Constant Depression?
Re the 2CV, didn't the one they used in the James Bond film have the four cylinder GS 1220 flat four air cooled engine which came with the twin choke carb as standard. "For instance, in order to make the chase scenes more exciting, stunt driving maestro Rémy Julienne ditched the standard car’s feeble 602cc two-cylinder engine in favour of a more powerful four-cylinder unit from the Citroen GS."
Some may argue this point:-
“A carburetor can make more torque, more horsepower, and use less fuel than EFI,” said Dave Braswell of Braswell Carburetion, Tucson, Arizona. That remains true, he said, even as drag racing V8s continue to grow in size and output, and can spin at 10,000 to 11,000 rpm.1 Jan 2022.
In the old days I fitted an inch and a half SU carb to a swan neck adaptor on a 1500 Ford engine replacing the 997cc, it sounds daft but it ran much better using the old carb and manifold from the original 997cc engine. Not ideal but as a poor young apprentice in the very early 1970s it was a vast improvement, though not compared to a petrol customer at the garage who had fitted a Lotus Twin Cam engine along with the twin 40 DCOE carbs, but he used to tune and race cars at the Hot Rod Circuit.

 

[tpv]

Yes but due to the injector spraying at the valve it remains atomized?
Re carbs make more power....I don't understand why he would say this.
Every race engines even in the 80's used injection (well most works engines and dedicated race engines- dfv etc).
Mechanical injection was used on many.
You can have the correct tuned inlet size - length AND width,these resonating pulses can be used more effectively with a smooth inlet- a carb has the Venturi, this will always make injection superior for power.
I saw a fiat sohc 127 (under 1300cc) with mechanical injection the diameter must have been 45mm at least.
Hillman imp race engines use huge inlet cross sections.
Zacspeed race engines used injection.
The bhp increase from itb's over carbs on engines like vauxhall red tops is far more than carbs.

Also it depends where the injector is aimed as to gas velocity, but the finer the spray the less likely it is to de-atomise.

Another thing is that the bigger cross sectional area the inlet has the less resistance to flow.
It's like having 2 inlet valves instead of 1, that slows down the inlet velocity but it improves power.


Check out Dyno comparisons between even 50dcosp even 55's(yes they exist- used on lotus Talbots!): and itb's.

With respect the man who said carbs will always make more power is wrong or he said it in 1943!
Or top fuel dragster wouldn't use injection!

 

[porta]

There’s loads using their carbs and…
There are videos, and I think one of the wheeler dealer programmes, where motorbike injection throttle bodies have been used, particularly on smaller capacity car engines, to great effect…the difficulty is in the electronics, which is why many use a hybrid electro/mechanical system

 

[bugsymike]

Yes but due to the injector spraying at the valve it remains atomized?
Re carbs make more power....I don't understand why he would say this.
Every race engines even in the 80's used injection (well most works engines and dedicated race engines- dfv etc).
Mechanical injection was used on many.
You can have the correct tuned inlet size - length AND width,these resonating pulses can be used more effectively with a smooth inlet- a carb has the Venturi, this will always make injection superior for power.
I saw a fiat sohc 127 (under 1300cc) with mechanical injection the diameter must have been 45mm at least.
Hillman imp race engines use huge inlet cross sections.
Zacspeed race engines used injection.
The bhp increase from itb's over carbs on engines like vauxhall red tops is far more than carbs.

Also it depends where the injector is aimed as to gas velocity, but the finer the spray the less likely it is to de-atomise.

Another thing is that the bigger cross sectional area the inlet has the less resistance to flow.
It's like having 2 inlet valves instead of 1, that slows down the inlet velocity but it improves power.


Check out Dyno comparisons between even 50dcosp even 55's(yes they exist- used on lotus Talbots!): and itb's.

With respect the man who said carbs will always make more power is wrong or he said it in 1943!
Or top fuel dragster wouldn't use injection!

I do agree with you, but having just seen"Dave Braswell of Braswell Carburetion,comments re dragsters" I thought I would throw it into the pot.
The massive increases in engine output over the years with injection, computers, super/turbo charging etc. have made a great leap forward, with modern saloon cars making the horse power per litre what only Grand Prix cars could dream of in the past and more important with reliability to do 100k miles plus.

 

[porta]

I do agree with you, but having just seen"Dave Braswell of Braswell Carburetion,comments re dragsters" I thought I would throw it into the pot.
The massive increases in engine output over the years with injection, computers, super/turbo charging etc. have made a great leap forward, with modern saloon cars making the horse power per litre what only Grand Prix cars could dream of in the past and more important with reliability to do 100k miles plus.

And if you consider the ‘modern’ multiair, with its induction, that technology on the last iteration of the S4 delta, (the one that was carbon fibre and Kevlar, but never raced) it was reportedly pushing 850+ bhp from the venerable Fiat TC at 1759cc

 

[tpv]

Not wanting to appear arrogant but speaking as someone who is an expert due to to sitting on my arm chair and fantasizing I must be right! Lol
Joking aside I appreciate your feedback!
I'd so love to be rich enough to prove it either way! Lol