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Who's running a single 40 DCOE?

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GeorgeOhr Nonya Business
Yes, confused, USA   USA
I have a 40 DCOE on my 1500 and it's "ok" but never been truly right. I think I'm gonna throw some parts at it to try to get it right but don't wish to order a ton of maybes. I'm trying to get an idea of what range I need to look at. (I'm right at sea level too.)

Who here is running a single, on what, where (street) and how is it jetted and choked? Thanks.

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spitlist Avatar
spitlist Joe Curry
Sahuarita, Sahuarita, AZ, USA   USA
Been there, done that. It never will be "right" because the nature of the runners required to connect the two output ports to the input ports on a 1500 head require that they are unequal length.

The inside two cylinders run ruch compared to the outside two. So you can never truly balance the carb.

The ideal situation is to have twin DCOEs so that you have a single barrel feeding each cylinder. Then you will have the tuneability that you need.

Joe

P.S. a single DCOE can be made to work great on an earlier 6 port head because each barrel is feeding one siamesed port on the head.

GeorgeOhr Nonya Business
Yes, confused, USA   USA
The single is actually from a dual set that I have but I don't want to run duals.

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2manyspits Avatar
2manyspits Thomas Schlundt
south bend, IN, USA   USA
RUN THE DUELS THEY WORK GREAT

GeorgeOhr Nonya Business
Yes, confused, USA   USA
This is a mostly low rpm street motor and I prefer low end power to high rpm.

bobbylrowland bobby l
cochran, ga., USA   USA
they do make a crossover intake, got mine from rimmer. don't know if it works any better than the regular intake, haven't tried it yet.

Doug in Vegas Avatar
Doug in Vegas Douglas D
Las Vegas, NV, USA   USA
I suppose you could come up with a custom intake so all paths are equal.

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Doug in Vegas Avatar
Doug in Vegas Douglas D
Las Vegas, NV, USA   USA
In reply to # 1506468 by GeorgeOhr This is a mostly low rpm street motor and I prefer low end power to high rpm.

If it's a street motor you NEED the power to keep up with traffic.

Also, if it's a street car the main thing you need is the ability to deal with long idle times. You don't want to hit the gas when the light finally turns green and have the car die in the middle of the intersection. It's embarrassing and all the talk of tweaking this and that amounts to an occasional tow in the rain.

Ideally the car will be so smooth at idle you have to look down at your ign and oil warning lights to tell if it's running.

spitlist Avatar
spitlist Joe Curry
Sahuarita, Sahuarita, AZ, USA   USA
In reply to # 1506474 by Doug in Vegas I suppose you could come up with a custom intake so all paths are equal.

It is difficult to do on a Spitfire because of the limited lateral space.

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Doug in Vegas Avatar
Doug in Vegas Douglas D
Las Vegas, NV, USA   USA
In reply to # 1506476 by spitlist
In reply to # 1506474 by Doug in Vegas I suppose you could come up with a custom intake so all paths are equal.

It is difficult to do on a Spitfire because of the limited lateral space.

I know, most attempts end up looking like a tuba.

GeorgeOhr Nonya Business
Yes, confused, USA   USA
In reply to # 1506475 by Doug in Vegas
In reply to # 1506468 by GeorgeOhr This is a mostly low rpm street motor and I prefer low end power to high rpm.

If it's a street motor you NEED the power to keep up with traffic.

Also, if it's a street car the main thing you need is the ability to deal with long idle times. You don't want to hit the gas when the light finally turns green and have the car die in the middle of the intersection.


Correction, you need low end torque which doesn't come via 800CFM on a 1500.

Spitfirejoe Avatar
Spitfirejoe Joe Guinan
Fremont, NE, USA   USA
1980 Triumph Spitfire 1500 "The Phoenix"
I run duals - Dellorto DHLA in my case, but very similar to Weber DCOE. I found I had to go down to a 28mm choke to get them to provide that low RPM grunt that I wanted. Well, "grunt" is maybe overstating it... This also limited the upper end - runs out of guts just before I get to 6 grand, where the bigger chokes would flow well higher than that. I didn't want to push the 1500 higher than 6000 rpm, so the trade-off was a good one.
The 32mm chokes it came with were way too much, 30's were a little better, and 28's worked great. Of course, not much help to your single DCOE issues, just wanted to throw in that idea in case you decided to try a dual set-up because of the inherent difficulties in getting a single to work right on an 8 port head.



Joe Guinan
Fremont, Nebraska

grumpicus Steve Jackson
Leicester, Leicestershire, UK   GBR
If you've got a pair of 40 DCOEs, but only want to use one, why don't you give up on the idea, sell the pair of them & buy a pair of HS4s + Euro manifold with the proceeds? That setup will perform as well as a single 40 DCOE, and avoids the strange manifold needed for the single 40 DCOE.

Of course, you could always use the 'split Weber' approach where two 40 DCOE carbs are used, using only one choke in each carb. You would probably need a modified inlet manifold based on the Euro twin HS4 unit, but at least the induction tracts would all be the same length! It may even work as well as the twin SU setup......

clshore Carter Shore
Beverly Hills, FL, USA   USA
In my opinion the 'unequal tract length' theory is a Red Herring.

If you examine the timing of the intake pulses, 1-3-4-2, you'll find that the intake pulses on
each choke are paired, 2-1 followed by two 'dead time' intervals, and 3-4 followed by two 'dead time' intervals.
These Weber carbs were developed to support one cylinder per choke, which they do well.
But asking them to support two consecutive intake pulses likely yields unequal mixtures to the two cylinders.

It's worthwhile to note that this situation is identical to the issue faced with dual SU, on which despite having
equal length runners
, the factory still had to still employ a balance tube to achieve equal mixture distribution.

So my suggestion would be to add a balance tube to your manifold.
Use the stock dual SU manifold as a guide for sizing and location ...
about 1/2" ID, just ahead of where the runner splits to each cylinder.
Drill and tap a hole in the top (avoids any puddled fuel), screw in a pair of 90 degree hose fittings,
and cut a piece of 1/2" ID hose to fit between them.

'Oh no, you will ruin that manifold!'

To which I reply, 'It's ALREADY not working properly, so what have you got to lose?'

GeorgeOhr Nonya Business
Yes, confused, USA   USA
In reply to # 1506514 by grumpicus If you've got a pair of 40 DCOEs, but only want to use one, why don't you give up on the idea, sell the pair of them & buy a pair of HS4s + Euro manifold with the proceeds?

Cause I already have those too and don't like them. winking smiley

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