Intake Trumpet lengths - RV8
Discussion
Can anyone explain why the intake trumpets on the injected RV8 engines are all different lengths?
On all other engines (admittedly running carbs) the length of the intake trumpets and the shape of the rolled lip has always been quite a determining factor on how the engine performs.
Therefore I find it quite odd that the lengths vary so much.
What will the gains be if I shorten the trumpets to the same length?
By the smae token will there be much to be gained from cleaning and polishing the inside of the plenum area and the tops of the trumpets?
Cheers.
On all other engines (admittedly running carbs) the length of the intake trumpets and the shape of the rolled lip has always been quite a determining factor on how the engine performs.
Therefore I find it quite odd that the lengths vary so much.
What will the gains be if I shorten the trumpets to the same length?
By the smae token will there be much to be gained from cleaning and polishing the inside of the plenum area and the tops of the trumpets?
Cheers.
Isn't it something to do with equalising the distance the air has to travel after passing through the throttle, otherwise the 'outermost' cylinders (the two at front and two at rear) would run lean compared to the centre four?
ISTR when balancing the carbs on my Suzuki bike, the airflows had to be higher on 1&4 and lower on 2&3, much the same effect?
Also when I asked a similar question years back, I think it was Tim Lamont who suggested taking 15mm off the length of each trumpet, but the lengths would still vary...
ISTR when balancing the carbs on my Suzuki bike, the airflows had to be higher on 1&4 and lower on 2&3, much the same effect?
Also when I asked a similar question years back, I think it was Tim Lamont who suggested taking 15mm off the length of each trumpet, but the lengths would still vary...
The length that matters is the overall intake length from the valves back to the top of the trumpets. The runners within the manifold are different length, so the trumpets are set at different heights to keep the overall intake lengths the same.
Different engines have different trumpets fitted so there is no single answer. Depending which trumpets you have and what spec the rest of the engine is there may be gains from shortening them, or boring the manifold out and fitting shorter/wider trumpets, or just raising the top of the plenum chamber slightly (and there are various ways you can achieve this). However, there's no point going mad on the trumpets if the induction system has a bottle neck somewhere else.
Really ACT are the best people to speak to about this.
Different engines have different trumpets fitted so there is no single answer. Depending which trumpets you have and what spec the rest of the engine is there may be gains from shortening them, or boring the manifold out and fitting shorter/wider trumpets, or just raising the top of the plenum chamber slightly (and there are various ways you can achieve this). However, there's no point going mad on the trumpets if the induction system has a bottle neck somewhere else.
Really ACT are the best people to speak to about this.
I think you might find, that the shorter trumpets actually feed the opposite cylinder bank, and the longer ones, the same side.
The 4 trumpets on say the left hand side, do NOT feed the 4 cylinders on the left hand side, and vica versa.
the centre 2 feed same side, the outer 2 feed opposite sides
So overall inlet tract length may be very similar.
It could also be down to the shape of the plenum above, which is curved, perhaps to allow sufficient clearance above the trumpet.
The 4 trumpets on say the left hand side, do NOT feed the 4 cylinders on the left hand side, and vica versa.
the centre 2 feed same side, the outer 2 feed opposite sides
So overall inlet tract length may be very similar.
It could also be down to the shape of the plenum above, which is curved, perhaps to allow sufficient clearance above the trumpet.
As I said, trumpets are different lenghts, but inlet tracts are probably close to equal length due to the crossover in some of the pipes.
You can see the crossover on the outer pipes in this pic
http://gallery10880.fotopic.net/p714943.html
>> Edited by stevieturbo on Thursday 31st March 01:36
You can see the crossover on the outer pipes in this pic
http://gallery10880.fotopic.net/p714943.html
>> Edited by stevieturbo on Thursday 31st March 01:36
stevieturbo said:
As I said, trumpets are different lenghts, but inlet tracts are probably close to equal length due to the crossover in some of the pipes.
You can see the crossover on the outer pipes in this pic
<a href="http://gallery10880.fotopic.net/p714943.html">http://gallery10880.fotopic.net/p714943.html</a>
>> Edited by stevieturbo on Thursday 31st March 01:36
Wow. Shows how much I don't know but assumed.
The trumpets themselves are actually all identical lenghts, its the trumpet base that makes them appear different lenghts.
The different heights is to ensure that the overall induction lenght is the same across all 8 pots.
As steves photo shows, the inner four holes are closer to the valve (i.e. shorter) than the outer four, therefore the inner four trumpets are longer by the same amount.
I took 40mm of them, because with the westfield plenum they seemed too close to the top for decent air flow. Apparantley the clearance over the top of the trumpet should be at least 1 x the diameter.
Bigger pics at: www.mez.co.uk/westfield
>> Edited by eliot on Thursday 31st March 11:24
The different heights is to ensure that the overall induction lenght is the same across all 8 pots.
As steves photo shows, the inner four holes are closer to the valve (i.e. shorter) than the outer four, therefore the inner four trumpets are longer by the same amount.
I took 40mm of them, because with the westfield plenum they seemed too close to the top for decent air flow. Apparantley the clearance over the top of the trumpet should be at least 1 x the diameter.
Bigger pics at: www.mez.co.uk/westfield
>> Edited by eliot on Thursday 31st March 11:24
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