Why are they different lengths?


Andrew.

for playing different music?

on a more serious note, by using different length trumpets alters where peak power is in the rev range or something to that effect.

tom

They are different length for different inlet tract lengths. This alters the frequency at which they resonate, and boost torque. Often useful for filling in holes in the torque curve or boosting peak torque. Oh, they also smooth the airflow into the carbs/bodies and aid overall power because of this.

I have 2 longer yhan the other 2, it doesnt make sense to me, I could understand if they were changed as a set.

Longer gives more bottom end/mid range torque..

Shorter gives more top end.. in a nutshell..

Its a bit more complex than that, profile airbox design, position of filter and/or intake relative to trumpets..

I believe some blown engines use different length trumpets within a plenum to balance the airflow... but that is beyond my level of knowledge...

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Originally posted by dilley
I have 2 longer yhan the other 2, it doesnt make sense to me, I could understand if they were changed as a set.

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Its a blackbird, its all running fine, I am just curious.

The Duratec V6 uses two lengths of inlet tracts which are opened by a butterfly mechanism above 4000 rpm to boost the top end torque and closed to give more bottom end.

I suspect they use different length tracts on the inner ones so that they can spread out the torque characteristics by having two cylinders producing more torque at lower rpm, and the other two slightly higher, rather than one slightly larger peak but only at a single rpm.

Also the two inner cylinders generally run hotter than the outer ones due to having another hot cylinder on both sides, to the extent that they run different fuel mixture settings on the inner two (certainly on the injected R1) so maybe that also has a bearing on it.

chris is spot on, the R1 has to have richer jets on the middle 2 cylinders to balance the heat...

I think the longer trumpets MIGHT be for the 2 inner cylinders, maybe due to the shape of the airbox..

You will need to look at the owners manual to make sure... Haynes do one for the blackbird which should cover it

Yup, the longer two live in the middle, as Chris said, to spread the torque out a little. It helps level out spikes in the power curves.

So why is it better to fit an air box on bike carb's.

You can then tune the inlet even more to a particular rev range. It's an extension if the trumpet idea basically. It gives a bigger volume of air that is matched to a rev range to give a little positive pressure to the carbs. Again, used to fill holes in the torque curve.

Yep, if (and its a big IF) you can get the airbox the right size and shape, you can start getting the effect known as pulse tuning where the resonance (ie timing of the pulses) of the air coming into the airbox coincide with the inlet valves opening on the engine, so you get a mini supercharging effect. Its only small, maybe worth a few horsepower on a bike engine, but if you get it wrong it can have the opposite effect and almost cause a negative pressure so less air enters the cylinders than it would without the airbox.

Running without an airbox has its pitfalls too though, because you're sucking in hot air from the engine bay, and also sucking in quite turbulent air especially if the air filter sticks out the bonnet.

Pulse tuning of an airbox (and all airboxes do this) adds a small positive pressure at the tuned RPM and at half that speed and a quarter of that speed. In between these you get negative pressure which worsens torque. the effect is slight but can be useful to flatten a spike or fill in a dip in torque.

My R1 engines both have fairly low acceleration up to 5000 RPM when suddely they take off. By tuning may airbox to 4500 RPM, the power now rolls on gradually from 4200 to 5000 RPM. It also sounds happier at full throttle in the range that it did before.

If trumpet length does the same thing then perhaps you could use it to cancel the dips caused by the airbox tuning, just leaving you with the primary airbox resonance.

Have a look on the Jenvey website. They have a very handy theory page explaining this and giving advice on the correct trumpet length/ throttle body diameter for each capacity engine.
ATB
Phil

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Originally posted by smart51
Pulse tuning of an airbox (and all airboxes do this) adds a small positive pressure at the tuned RPM and at half that speed and a quarter of that speed.

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