r1_pete
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| posted on 6/1/09 at 08:31 PM |
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Weber 40 / 45 DCOE question
OK this might be a dumb ass question, but what is the difference between 40's and 45's? I understood it was the carb bore.
But looking for a zetec manifold for 40's most appear to be for 40's and 45's, so if the bore is different by 5mm the manifolds
can't really be suitable for both types.
Can anyone enlighten me?
Cheers.
Pete.
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Xtreme Kermit
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| posted on 6/1/09 at 09:07 PM |
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No expert here, but both 40 and 45 dcoe have removable choke tubes - my 45s have 35 or 36mm chokes.
Smaller chokes = more low down torque but less top end power.
Bigger chokes = more top end power but less low end torque.
40dcoe carbs will have a power limit due to the amount of air/fuel mixture flow rate.
45dcoe carbs will have a much higher max flow rate therefore will be able to fuel a higher power engine.
I am prepared to be shot down here but I believe the function of the two carbs is much the same.
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flak monkey
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| posted on 6/1/09 at 09:29 PM |
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You are quite right. The manifolds are usually around 40mm bore, they need to be matched to 45's if you are being picky. The diameter refers to
that of the butterfly.
45's will drink more fuel and be able to give you a bit more top end power as you can fit bigger chokes. But you can equally over carb an engine
and make the car a pain to drive at low revs. A 2litre engine is on the brink of the 40/45 changeover.
David
Sera
http://www.motosera.com
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Canada EH!
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| posted on 6/1/09 at 09:30 PM |
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Size of the bore at the manifold end, both carbs have interchangable chokes.
The 45 will take bigger chokes as will a 48 or 50.
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big_wasa
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| posted on 6/1/09 at 09:47 PM |
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Yep my manifold had been "matched" to my 45's
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Danozeman
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| posted on 7/1/09 at 07:45 AM |
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The cheaper manifolds are about 43mm so will cover both 40's and 45's.
I have seen some that suposidly cover 40 45 and 48's.
You dont ideally want a 43mm manifold for 45's but tbh unless your going for every bit of power you can you probably wont really notice the
difference.
Dan
Built the purple peril!! Let the modifications begin!!
http://www.eastangliankitcars.co.uk
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02GF74
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| posted on 7/1/09 at 10:54 AM |
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wot FM says but I am not sure it follows that a larger carb will drink more petrol.
I would have thought this was a feature of the jets and choke diameter selection?
The number 40/45 is the bore through the carb and a choke, whose outer diamter is this dimension, fits into the bore to reduce the area through which
the air flows.
From memory, there is a bit of overlap in the choke size between successive carb models i.e. a 45 could be fitted with a choke that is smaller than
that fitted to a 40.
The choke diameter affects how air flows through the carb - smaller choke creates a larger vacuum which helps to draw and atomise the fuel but it
restricts the amount of air at higher rpm so power will drop off. A larger choke has the oppostite effect resulting in poor performance at low
rpm.
Juggling the choke size, jetting etc, is a compromise.
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MikeRJ
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| posted on 7/1/09 at 12:42 PM |
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quote:
Originally posted by 02GF74
wot FM says but I am not sure it follows that a larger carb will drink more petrol.
If the larger carbs are TOO large for the engine, accurate calibration is not possible and fuel economy will suffer.
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