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Author: Subject: Rules for chokes, trumpet length and air filters/air box for DCOE carbs
Blairm

posted on 16/1/05 at 08:07 AM Reply With Quote
Rules for chokes, trumpet length and air filters/air box for DCOE carbs

Hello from down under.

Rightly or wrongly I have ended up with twin 45 DCOE on my 1680cc Cross flow that has been well worked. Many people have told me I should be running 40’s, but I got them from a gentlemen running a similar spec motor who removed his 40’s and replaced them with 45’s as he felt the 40’s where restricting the top end and I took his advice.

At present they have 36 chokes (his was 34 though he has other clients running 36 apparently very well) and 60mm trumpets. I suspect that 34 mm chokes may be better given the engine does not have huge displacement and air flow will be quiet low at low rpms. Thoughts?

I have about 80 mm from the carb to the bonnet on the front carb and with 60mm trumpets it is a bit tight. The 45s are on a 77 mm manifold I sourced from Burton Power and people have suggested I keep the trumpets as long as I can. At this stage I am not keen to cut the bonnet so reducing the size of the trumpets seems the only answer if I am going to have any chance of fitting some form of air cleaner.

What is the effect of reducing the trumpet lengths? and how much clear space should you have from the trumpet to any obstructions such as a air cleaner top?

It has been suggested I have an air box made up with a single large pod filter feeding air to both carbs. Do people think this is a good option and is there guide line to the volume the box should be to maximise performance?

All help and suggestions appreciated.

Thanks Blair

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britishtrident

posted on 16/1/05 at 10:33 AM Reply With Quote
I think you have a pretty good understanding of the the issues involved. The following is mainly based on a David Vizard article in CCC many years ago.

The main reason for fiting an airbox is that because the flow of air through the carbs is pulsing -- this causes a back flow which causes a cloud of fuel air mist to form at the entrance to the carb. This can sometimes be seen as wetness on an part of the body or chassis very close the the carb mouth. If the area around the carb mouth is exposed the airflow this cloud will get blown away fitting an airbox or filter retains this fuel cloud in the system in the system and obvious has an effect on mixture, power and fuel efficiency.
Lotus and Chrysler UK used air boxes on cars fitted with twin 40s --- Chrysler Arrow range workshop manuals had dire warnings that the seal around the two parts of the airbox had to be 100% or the mixture would be upset.

My understanding is the ideal airbox should be deep enough that the fuel cloud dosen't wet the airbox cover, Lotus used a shallow airbox on the baby Elan for space reasons but the one on the Holbay Hunters and Avenger Tigers was very deep -- from memory 5 or 6 inches.

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Blairm

posted on 16/1/05 at 05:45 PM Reply With Quote
Thanks British Trident,

The car is not yet a runner so maybe I should wait to see what happens with regard tot he need for a air box.

One of my reasons I considered an air box with a pod filter is the filters are far cheaper than thoose for th eend of DCOE's. Here in NZ K and N seem to be about he only option and the is half my morgage.

Either way I need to probably reduce the trumpet length. Have you a comment on the effect of doing this. Browsing the web, I see lots of short trumpets of various types, so I assume it is not a big issue?

Cheers Blair

[Edited on 16/1/05 by Blairm]

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britishtrident

posted on 16/1/05 at 06:32 PM Reply With Quote
On the Imps we use the shortest trumpets availalable , Elan used the next size up while the the Holbay Hunter GLS were built without trumpets but they had an odd balll version of the 40 DCOE and had extended auxilary venturis.
Obvious it is to do with the rpm band the engine runs in but I suspect it only would drop 2 or 3 bhp.

[Edited on 16/1/05 by britishtrident]

[Edited on 16/1/05 by britishtrident]

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