Are the 55's too much????

[Vagos]

Hi i have 2512cc engine 85mm stroke,big valve head by rallybobs spec,big cam e.t.c.Im currently running dual 48 webers.Can i go to bigger carbs like 50 or even 55!!weber DCO???

[GTJIM]

The only motor I have seen with 55mm DCO's on it is a DOHC 3 litre Formula III Ferarri 4-cylinder. They are huge - it produced 295 hp at 6,200 rpm

A 2.5L Opel motor is probably over-carb'd with 48mm DCOE's.

What improvement do you expect with bigger carbs?

[jordan]

Originally Posted by GTJIM The only motor I have seen with 50mm DCO's on it is a 3 litre Formula III Ferarri 4-cylinder. They are huge - a 2.5L Opel motor is probably over-carb'd with 45mm DCOE's. What improvement do you expect with bigger carbs?

I would say that a 2.5 depending on what cam it has would be right at home with a 48's. Though also I had no idea that 50mm+ existed.

[RallyBob]

I think that the size of the carbs depends on the size of the engine and the modifications to the head, but more importantly the type of use. For example the early Manta 400 competition engines used 48 DCO Webers, but later they also used 50 DCO's (EDIT: I found that the Phase 3 engines used 50 DCO's with 46 mm venturis!) since the horsepower climbed over the years (Phase 1 was 240 hp, Phase 2 was 255 hp, Phase 3 was 275 hp). Of course, the type of use was rallying, so maximum horsepower was not the most important thing, instead the engine needed torque and flexibility. In recent years racing versions of the Manta 400 engine use 55 DCO's, but they produce 320-330 hp, and low rpm power is not important.

For the 'true' CIH engines, of course these hp number are not attainable, but still you can use this as guidelines. I like to try to match the throttle plate diameter of the carburetors to the size of the inlet valves (or close to it). Then for a racing application, I use the size of the inlet port opening at the smallest point as the reference for the venturi diameters. For road use, it is better to err on the small side of this for venturi size - about 80% of the throttle plate diameter, but it is a good guideline anyway.

So for your 2.5 litre engine with 1.94" (49.27 mm) inlet valves, you could say that 48 mm carbs would be a good choice for mixed use (some road use, some motorsports), but for racing only then perhaps 50 mm would be better.
I would only consider 55 mm if perhaps you were running events for top speed, such as Bonneville or high speed events like the Silver State Classic where nearly full throttle is used for 90+ miles!

I will be using 52 mm throttle bodies for my Bonneville engine, but that is on only a 2.0 litre engine!

Bob

[bigjim5551212]

For racing, the 50 DCO are fine. There is no bottom end. But we did put some on a Datsun 510 with a 1600 and it did make about 200hp.

There are several issues that you have to be aware of.

48, 50 and 55 DCO series carbs mount on the same flanges as a DCOE but they are physically wider.

So that means they might hit each other on the manifold. I am not sure about the Opels. You have other more pressing problems like the heater core.

There are limited sizes of venturies. The 48 has a 42mm and the 50 has a 46. They are interchangeable. Still too large for street use. You can have custom made smaller ones.

The 55 is overkill for most applications. Cars where it works are Mazda Rotaries and V8's

The other thing is linkage. The 48 has linkage attachments on both sides of the carb. The 50's are only in the middle.

We can supply any of these carburetors for you.

jim@overseas-auto.com

[pcolina]

Hi, how much for a pair of Weber 48 DCO? Thanks

[bigjim5551212]

we could supply a pair of 48 DCO for about $1200 shipped