Lotus Elan

Weber Trumpet Length

PostPost by: AlfaLofa » Fri Mar 08, 2013 4:17 pm

Why is the trumpet used on the 40 DCOE 18s 1.75 inches long?

Why can't it be 1" or 1.5" (as on the DCOE 31) or any other length which will fit, without causing obstruction, in the airbox?

What difference does the length of the trumpet make to the induction process?
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PostPost by: Tahoe » Fri Mar 08, 2013 6:05 pm

Excellent question. I was looking at mine last night and they are only 1.25 (approx) on 18's. I was also wondering if I need to get longer ones.
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PostPost by: john.p.clegg » Fri Mar 08, 2013 7:05 pm

You are now dabbling in the Black Art of airflow tuning,which unless understood fully is best left to the experts..

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PostPost by: CBUEB1771 » Fri Mar 08, 2013 9:52 pm

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.
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PostPost by: john.p.clegg » Fri Mar 08, 2013 10:23 pm

....told you so....

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PostPost by: Pistacchio sprint 72 » Sat Mar 09, 2013 8:03 am

Thanks ross :mrgreen:
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PostPost by: CBUEB1771 » Sat Mar 09, 2013 10:46 am

john.p.clegg wrote:....told you so....

John :wink:


As usual, I find I made a mistake after pontificating. :o When I said that Lotus had decided on the 1.5 inch inlet trumpets I was using the +2 work shop manual as my only reference. The +2 manual covers only 40 DCOE 31 type Webers. I checked the Elan manual and find that both 18 and 31 variants are included. I don't know what the change point was but it seems the 18s were changed to 31s during S3 production or maybe introduction of the S4. The 18s were fitted with 1.75 inch trumpets. The change in trumpet length might have been driven by introduction of the Sprint cams but that is based on my creative interpretation of the likely changeover date. If you have 40 DCOE 18s and change from 1.75 to 1.5 inch trumpets the difference on the road would probably be tough to notice, a .25 inch change in trumpet length does not change the total inlet tract length very much. That being said, many modern vehicles have variable inlet tract length controlled by the engine management system. Chapman was not the only guy who would not spend money on changing inlet tract length if it didn't make a measurable difference.
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PostPost by: AlfaLofa » Sat Mar 09, 2013 12:45 pm

Hi Russ - thanks for answering my initial question.

So there is logic (i.e. science) behind the different lengths used - and it is best to stick to the lengths recommended in the workshop manuals.
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PostPost by: CBUEB1771 » Sat Mar 09, 2013 4:49 pm

I finally got a chance to look through the Elan Service Parts List, it took longer than usual as got we over half a meter of snow yesterday and getting to the garage took real work. The Service Parts List shows the transition from type 18 to type 31 40 DCOEs as "from 1968 on". The S4 was introduced in March 1968 so I guess that was closer to the actual transition. The "D" cams also were introduced with the S4 and carried into the Sprint. So maybe I was not too far off in my guessing. It might also coincide with a change in what Weber was producing. In 1968 lots of changes were underway to deal with the new US emissions laws. Changing to 1.5 inch trumpets might not hurt. Many people think that cylinder #4 is starved because the end of the inlet trumpet is so close to the wall of the air box. Tony Thompson makes a deeper air box for exactly this reason.
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PostPost by: Bud English » Sat Mar 09, 2013 5:06 pm

I wouldn't get too wrapped up in how much that quarter to half inch is changing your tuning. A switch to tubing, 4-2-1, headers from the cast manifold changes the tuned power range as much or more than the change in intake length. Most everyone that makes that change doesn't think twice about doing it, especially on a street driven car.
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PostPost by: SJ Lambert » Mon Feb 23, 2015 8:58 am

_57.jpg and



_56.jpg and



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Managed to get a set of non concentric ones for 40/42s and a second set for 45s in one day!!
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PostPost by: el-saturn » Mon Feb 23, 2015 11:43 am

last spring, when the engine was back in the bay, my motor man bruno schaffner told me i should go to 75mm after he drove and listened to noise: I understood that as a compliment: all steel and forged bits; about 1:11 and 10,75 lift - it's quite noisy using a 3L mondeo "silencer"!! sandy (if anyone wants more info, 'll share it)
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PostPost by: Chancer » Mon Feb 23, 2015 12:13 pm

The trumpet lengths may well have been what were supplied as standard by Weber with the carb bodies they were producing at the time, Lotus were a small scale manufacturer and probably couldnt specify their own special carbs without a significant surcharge, they will have changed the jets but probably baulked at changing chokes, auxiliary venturi's and trumpets.

Then weber changed their carb models and with it the trumpet length.

Just a hypothesis.

I know when I have bought new carbs I had to spend an awfull lot or money to get what I really needed, the aux vents and trumpets were the highest cost for the least return and the items that I would sometimes compromise on.
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PostPost by: bill308 » Mon Feb 23, 2015 10:44 pm

I remember reading many years ago, that exhaust tuning favors the lower end of an engine's torque curve and intake tuning favors the upper end of an engine's torque curve.

Of course there are lots of ways to tune an engine depending on application. One that comes to mind is looking at the inlet trumpets on big block Chevy's used in the old CANAM series. Half the inlet trumpets were shorter then the other half, a strategy designed to widen the torque curve, albeit at lower levels than if all were the same length.

I selected 1-3/4 inch trumpets for the 40DCOE18's on my S2 SE and kept them after switching to Sprint cams, tubing headers, and doing the Vizard inspired port work. This car always had good low and high speed torque and would pull nicely from about 1500-7000 rpm.

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PostPost by: SJ Lambert » Fri Feb 27, 2015 11:40 am

Are the original trumpets nickel plated?
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