2b_pablo
|
| posted on 3/1/08 at 11:47 AM |
|
|
carb choke size
two quick questions re webers/dellorto carbs:
1. if a set of 40s have 36mm chokes will this perform the same as a set of 45s on 36mm chokes?
2. will 36mm chokes on a set of dellorto 45s be enough to run a 2.1 pinto with a lairy cam to 7k? (320deg rally cam).
Im for upgrading my weber 40s but the guy selling me the 45s has 36mm chokes in them so Im wondering is there much point as I can put 36mm chokes in
my 40s?
thanks all, dont want to be wasting cash.
|
|
|
|
|
coozer
|
| posted on 3/1/08 at 12:08 PM |
|
|
Why not use bike carbs, they don't have seperate chokes so the size is as it says 40mm = 40mm.
Much cheaper to purchase and easier to setup.
1972 V8 Jago
1980 Z750
|
|
|
2b_pablo
|
| posted on 3/1/08 at 12:12 PM |
|
|
dont want bike carbs, already spent £100 on a manifold for the dellortos and there are 100 people who can tune them over here. noone to do bike carbs
unfortunately 
|
|
|
flak monkey
|
| posted on 3/1/08 at 12:38 PM |
|
|
The venturi effect you get on 36mm chokes in 40mm carbs is different to that in 45s (its not as optimised if you like). You get better performance
from 36mm chokes in 45mm carbs, but you will allso take a hit in fuel economy over the 40s.
David
Sera
http://www.motosera.com
|
|
|
Memphis Twin
|
| posted on 3/1/08 at 12:40 PM |
|
|
36mm chokes in 40s will flow less air than in 45s as you lose the venturi effect.
36 chokes will work just fine for your modified Pinto, only losing out to 38mm chokes at extemely high revs. But how long are you going to be at
7000rpm? Keeping the choke sizes sensible will give a much fatter midrange, a more driveable car, and ultimately, faster acceleration.
Fact.
|
|
|
2b_pablo
|
| posted on 3/1/08 at 12:40 PM |
|
|
dont care about fuel economy, just a track toy really.
so 45s are still a better bet, thanks.
Id still be a bit concerned that 36mm (even on 45s) wouldnt be big enough for my engine.
|
|
|
2b_pablo
|
| posted on 3/1/08 at 12:43 PM |
|
|
quote:
Originally posted by Memphis Twin
36mm chokes in 40s will flow less air than in 45s as you lose the venturi effect.
36 chokes will work just fine for your modified Pinto, only losing out to 38mm chokes at extemely high revs. But how long are you going to be at
7000rpm? Keeping the choke sizes sensible will give a much fatter midrange, a more driveable car, and ultimately, faster acceleration.
Fact.
car is primarily for track use so balls out power would be good, but will be used on the road too so I guess a good split would be ideal but erring on
the side of performance.
|
|
|
2b_pablo
|
| posted on 3/1/08 at 12:46 PM |
|
|
thanks for all the replies by the way.
reason for changing is that the carbs (imo) are running out of puff at 6k where the cam isnt really getting going (324deg burton rally cam) meaning a
lot of wasted power
|
|
|
flak monkey
|
| posted on 3/1/08 at 12:50 PM |
|
|
The ideal choke size for the 2 litre pinto is 36 to 38mm. There is no point having larger chokes than the inlet ports on the engine. Even on heavily
modded heads these are never more than 38mm (pretty much as cast by ford).
David
Sera
http://www.motosera.com
|
|
|
britishtrident
|
| posted on 3/1/08 at 02:25 PM |
|
|
I am with Memphis Twin
The Pinto was never designed to be a high revving engine even by 1970 standards play to its strengths not its weaknesses. The gain in going to 45s is
small.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
|
|
|
02GF74
|
| posted on 3/1/08 at 03:06 PM |
|
|
quote:
Originally posted by Memphis Twin
36mm chokes in 40s will flow less air than in 45s as you lose the venturi effect.
Can someone explain why? As I understand it, the carb body is a tube (presumably the diameter is the carb model i.e. 40, 45, 48) with the throttle
disc and various inserts - i.e. the choke - is fitted which restricts the internal diameter further, the idea being the diameter is chosen so the
volume of gas (as determined by engine breathing) is within a certain range for fuel pick up to work best.
So how will a 38 mm hole in a 40 body be more restrictive than when fitted in a 45 body?
|
|
|
Dingz
|
| posted on 3/1/08 at 06:31 PM |
|
|
I think the idea is the venturi effect is better in the larger body, ie reduction from 45 to 38 then back to 45 gives better atomisation that 40 to
38. But there are so many obsticals in the way as well in a side draught , auxiliary venturies etc. just to throw a spanner in the works.
|
|
|
britishtrident
|
| posted on 3/1/08 at 06:37 PM |
|
|
quote:
Originally posted by 02GF74
quote:
Originally posted by Memphis Twin
36mm chokes in 40s will flow less air than in 45s as you lose the venturi effect.
Can someone explain why? As I understand it, the carb body is a tube (presumably the diameter is the carb model i.e. 40, 45, 48) with the throttle
disc and various inserts - i.e. the choke - is fitted which restricts the internal diameter further, the idea being the diameter is chosen so the
volume of gas (as determined by engine breathing) is within a certain range for fuel pick up to work best.
So how will a 38 mm hole in a 40 body be more restrictive than when fitted in a 45 body?
The 40mm body may have a 40mm hole but it has a lot of obsructions -- ie aux venturi and its struts, throttle plate accelerator pump jet. All of
these obstuctions have thier own boundary layer of dead air surrounding them contibuting to the total "blockage".
As a result a 36mm venturi in a 45mm throttlebody will pass more auirflow and hence have a higher flow velocity than a 36mm venturi in a 40mm
throttlebody.
[Edited on 3/1/08 by britishtrident]
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
|
|
|
