unequal trumpet lengths
FASTdan - 21/4/10 at 06:51 AM
Because our ZX9's didnt come with trumpets and we have since bought some that dont fit 
I am making up some adaptor/extensions to make them
fit.
Which got me thinking. Why do some bike carbs have the two middle trumpets longer?
I have read somewhere something about trying to keep the total induction length the same on all four cylinders from trumpet mouth to valve. That being
the case if the two outer runners on the manifold are a longer length (which if the carbs are not respaced then they are) then perhaps this is the
reason to have longer middle trumpets? To make up that extra distance?
Whilst im making these extensions i might aswell tune the individual lengths if it is seen to be beneficial - however i cant seem to find much info on
it.
FASTdan - 21/4/10 at 07:03 AM
ha, just had a measure up on CAD and my manifold lengths only vary by 2mm!
Hardly worth bothering about. but still interested to know the reasons for having differing lengths...
atspeed racing - 21/4/10 at 07:13 AM
We asked the same question to a top engine builder that came and used our rolling road some time back.
He was involved with the development of some race bike engines a few years ago. Without the standard air box on and removing the odd length trumpets
killed the power by a conciderable amount. We proved this some time ago when a customer came in with a bike engined kit car and he was using a foam
filter, when we replaced the foam filter with the standard airbox and trumpets, it transformed the engine and gave more power and increased the torque
all the way up.
Induction can be a strange thing.
FASTdan - 21/4/10 at 07:53 AM
quote:
Originally posted by atspeed racing
Without the standard air box on and removing the odd length trumpets killed the power by a conciderable amount.
So thats running without trumpets at all? Or do you mean removing odd lengths and making them all the same?
quote:
Originally posted by atspeed racing
he was using a foam filter, when we replaced the foam filter with the standard airbox and trumpets, it transformed the engine and gave more power and
increased the torque all the way up.
Induction can be a strange thing.
Did you try trumpets under the foam filter? just wondering how much the airbox contributed to that?
Interesting stuff.....
gingerprince - 21/4/10 at 08:44 AM
When I was getting my 'blade carbs jetted on the rolling roaded a couple of years ago, the fella was talking about how the inner 2 cylinders tend
to run slightly hotter than the outer 2 due to their position. In an ideal situation therefore they'd put slightly different sized main jets on
the 2 outers to the 2 inners to compensate.
Perhaps this is another way of compensating for the same effect but in a different way?
atspeed racing - 21/4/10 at 08:45 AM
The car that came to us still had the odd trumpets on. we just removed the foam filter and used the original air box.
We didnt try it with the same length trumpets.
Dave Ashurst - 21/4/10 at 09:20 AM
quote:
Which got me thinking. Why do some bike carbs have the two middle trumpets longer?
I thought it was an effort to spread the torque across a broader rev range (compared to a more localised torque peak that might result from 4 inlet
tracts of equal length).
Exhaust header lengths are more complex (because unlike your bike carb intakes which are individual pipes, exhaust headers connect together) but can
have similar effects.
As I understand it.
brianthemagical - 21/4/10 at 09:26 AM
It's pretty much to spread the torque.
Having two tuned speed ranges, making them both less pronounced.
Mini's with the LCB systems had a similam effect.
The air box also has some tuning effect. To try and out-do a mojor bike manufature is non-sensicle.
The induction does need to be tuned to the cams though, but it'll be bearly noticable.
There's a fair bit of info around, most it is a bit sparce and generic but there are some simple calculaters on the net.