Lotus Elan

Weber Trumpet Length

PostPost by: el-saturn » Fri Feb 27, 2015 11:47 am

no, lots of em are corrosive standard sheet material. you can easily polish them (as you polish aluminum with the proper paste) making them looking perfect unless the corrosion is too "advanced". sandy from the snowy alps
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PostPost by: pauljones » Fri Feb 27, 2015 5:22 pm

If you increase the length of the induction system you have the effect of torque at the lower end. Decrease it and the torque arrives at a higher rpm. With this in mind and armed with a dyno print out along with the correct math equation then it is possible to select a length to "fill" a hole. This is achieved by changing the only part possible, the trumpets.

As we now have many more tuning products available to us then previously there are many options. You can now fit much deaper air filters then standard cars had.

What ever air filter you fit leave a gap of 25mm and calculate that as the length of trumpet. Other wise you may restrict air flow intake.
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PostPost by: SJ Lambert » Fri Feb 27, 2015 9:46 pm

P1100491.JPG and


After clean up session #1 (with a shiny ring in for comparison)
Ford Escort Mk1 Lotus Twin Cam
Elfin Monocoque (Twin Cam)
Elfin Type 300 (Holbay S65 - 120E) mechanic

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PostPost by: Galwaylotus » Mon Mar 02, 2015 3:31 pm

From where to where is the length being measured? Overall? Flange to inlet? :?
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PostPost by: pauljones » Tue Mar 03, 2015 9:27 pm

Its the length from the trumpet face to the valve seat.ie total length of track..however its only the trumpet length that can be changed as the rest is a fixed feature.
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PostPost by: Andy8421 » Wed Mar 04, 2015 9:00 am

CBUEB1771 wrote:The inlet trumpets are interchangeable tuning parts on Webers just are are the chokes and jets. Lotus specified 1.5 inch (38 mm) long trumpets for the Twin Cam as installed in the Elan and +2 as seen in the work shop manual's data section. Replacement Webers may very well come with the wrong length trumpets. Trumpets in all the various lengths are available from all the usual suspects. If had the wrong ones I would swap out for 1.5" trumpets, assuming normal road use. Generally speaking, the shorter the inlet trumpet the higher the engine speed at which optimum cylinder filing occurs, meaning the peak torque point occurs at higher revs. This is due to the length of standing pressure waves in the inlet tract. Shorter trumpets lead to shorter standing waves and therefore higher frequencies at which the standing waves optimize cylinder filling. For Lotus the 1.5 inch trumpets were probably the best compromise between the desired torque curve and engine bay dimensional constraints.


I spent some time looking at this a while ago. I had assumed that a engine inlet system was a one-way mechanism - air flowed through the inlet into the engine. It is much more complex than that - the flow is bidirectional. The inlet tract / carb / trumpet forms a tuned pipe, in a similar manner to an organ pipe. The formation of standing waves at certain engine revs leads to the fuel being blown out of the carb air inlet and forming a stationary 'cloud' of air / fuel vapour (standoff) hovering over the trumpet allowing the standing wave to be visualised. These dyno videos clearly shows the 'standoff' as the engine approaches full revs.

https://www.youtube.com/watch?v=Sx_V9t48YaM

https://www.youtube.com/watch?v=ssd9sLpYi0s

I was never able to figure out the maths to predict the appropriate length of 'pipe' but it seems reasonable that changing the length of the pipe changes the resonant frequency and the point at which the standing waves develop. The aim is to have a high pressure node develop just behind the inlet valve at the desired revs - effectively giving a 'poor man's supercharging' effect.
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PostPost by: Galwaylotus » Wed Mar 04, 2015 11:12 am

My question on trumpet length was referring to the 1.25" , 1.5" or 1.75" dimension, not the overall length of the inlet tract! :shock:
Can anyone tell me how the trumpet itself is measured? Thanks.
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PostPost by: john.p.clegg » Wed Mar 04, 2015 12:36 pm

"Mechanical Engineer, happily retired!" but still surrounded by buffoons....

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PostPost by: AHM » Wed Mar 04, 2015 1:02 pm

and we haven't even discussed the angle of the cone, the radius at the end, and the length of radius at the end ... yet!
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PostPost by: Galwaylotus » Wed Mar 04, 2015 1:51 pm

Still waiting. ... anyone? :?
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PostPost by: john.p.clegg » Wed Mar 04, 2015 3:41 pm

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PostPost by: Galwaylotus » Wed Mar 04, 2015 5:14 pm

Thanks for that. It looks like it's the length from the base of the flange (where it sits on the carb body) to the bell opening (air inlet). Now I can go and see what trumpets are on my engine! :D
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PostPost by: ort » Wed Mar 04, 2015 9:27 pm

well worth a read nothing like actual testing

http://www.emeraldm3d.com/articles/emr- ... th-intake/
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PostPost by: pauljones » Wed Mar 04, 2015 10:34 pm

The length is as stated. Stand them up on a fkat surface and measure with a steel ruler. There are many books that describe the ins and outs of ram air tuning. I like David Vizards but thre a few modern one's too with simple easy to follow calculations
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PostPost by: john.p.clegg » Thu Mar 05, 2015 6:56 am

pauljones wrote:The length is as stated. Stand them up on a flat surface and measure with a steel ruler.


I think you may be wrong there paul as part of the "trumpet" sits inside the carb body...

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