Calling all Weber DCOE gurus

[rsporsche]

if the m30 is using smaller numbers (all with stock components), it would seem to me that the BMW head is flowing better. or am i all wet here.

 

[jamesw]

rs: If you're flowing w/ the right mixture for the jets you are running then that is just what your head can do.

If you can go w/ bigger venturis and bigger jets and still have a ~14 A/F ratio then that means you have a better flowing head - because it is flowing more air and fuel, but still burning it all.

Cheers
James

 

[rsporsche]

rs: If you're flowing w/ the right mixture for the jets you are running then that is just what your head can do.

If you can go w/ bigger venturis and bigger jets and still have a ~14 A/F ratio then that means you have a better flowing head - because it is flowing more air and fuel, but still burning it all.

Cheers
James

James,

i understand what you are saying, but there is a difference between efficiency and how much it can take. here's my point - if all things are equal (for this discussion - lets say displacement and output power), an engine that runs with smaller venturis and produces the same power is running more efficiently - producing similar power with less - that to me is a well flowing head / combustion chamber. if you throw more gas / air at it and still produce the same power, that is inefficient and doesn't breathe as well.

if you balance the engine design to the same carb + venturis, historically you end up with a more powerful engine - this has been my experience in the case of bmw engines. my last 2002 had a korman built engine w/ 11:1 compression, schrick 316 cam - and it took twin 45s for it to breathe - it produced 180+ hp at the crank. it was a very radical engine for the street but was extremely quick ... not much low / mid range torque - but its power / torque at the upper end was phenomenal. in terms of free flowing heads - that was an amazing engine for a single cam ... but it was no where near stock. it would get 30+mpg on the highway, but drank gas in town if you put your foot in it. my point is, a stock build bmw doesn't need 45s to produce good power.

i have been told that too large and you kill mid-range for some gain at the top (assuming it isn't too big). Smaller tends to increase the bottom of the powerband with a drop in the top end and often increases the throttle response.

 

[shanon]

Suggestion: get it 'close' as you can and then book some pulls on a dyno to truly know what is happening.

Know what cam you have, Korman or Pierce Manifolds can get you close to a base with a few questions, then the rolling road will tell the rest.

Look at the entire system, exhaust too! This is where a dyno and a good weber guy at the session will get ya dialed. Worth every penny!

Once dialed in, Webers can be 'set and forget'.

HTH,
-shanon

 

[jmackro]

If you can go w/ bigger venturis and bigger jets and still have a ~14 A/F ratio then that means you have a better flowing head - because it is flowing more air and fuel, but still burning it all.

A 14 A/F ratio where? The challenge with Webers (or any carburetors) is that you will get quite different A/F ratios under different situations: one when the accelerating pumps kick in, another at idle, at high rpm, when going uphill, coasting downhill, ..... Sure, the goal is to chose jets so it behaves like EFI where the A/F ratio doesn't vary much. But with carbs, you can only get so close.

 

[jamesw]

Right, I tune mine for high 13's at cruise, 11 at WOT and somewhere in between for on/off throttle.

Cheers
James

 

[jmackro]

Right, I tune mine for high 13's at cruise, 11 at WOT and somewhere in between for on/off throttle.

OK, thanks. Sounds like a good "rule of thumb".

 

[CookeD]

Here's a question-- It's been said on the board more than once that DCOEs don't like stock (U.S.-spec) compression, and that you also need more cam duration.

Is this not the case, and/or is the DCOE flexible enough with venturi and jet tuning that the compression / cam can be worked around?

 

[Stevehose]

I've seen posts from members having success with stock engines and 40DCOE's. 45DCOE's typically need the upgrades. I will run 32mm chokes for torque and hopefully should have them on next weekend and will report whether or not this is the case . After a drive Saturday morning the Zeniths come off and I will detail the engine and install the electric fuel pump and cutoff switch. The phenolic gaskets and redesigned linkage are on the way (unexpectedly expensive components-I am way over budget and my wife is wondering why i am so nice these days :mrgreen.

Here's a question-- It's been said on the board more than once that DCOEs don't like stock (U.S.-spec) compression, and that you also need more cam duration.

Is this not the case, and/or is the DCOE flexible enough with venturi and jet tuning that the compression / cam can be worked around?

 

[coupe2800cs]

SK Carbs

I have a rare set of 45mm SK carbs with manifolds, and linkages I picked up for cheap a while back on e-bay for my e9 M30, I will have a look a the jet settings and chokes then re-post...