Another DCOM question.

[mattbibbey]

Hello chaps,

As some of you might already know, I have a nice set of triple Weber 40 DCOM carbs for my modified L24 engine. Ive got a large valve E88 head with a port and polish and a fairly exciting cam in it plus the DJ exhaust system. Just wondering if anyone thinks that I should go to bigger carbs than 40s? 45s maybe? Would there be any improvement? A friend of mine just bought a MGB Roadster race car and I think he said it had a 48 on it!

 

[jonbills]

I wouldn't think you need bigger than 40s - I think you can get 34 deffo and maybe 36mm chokes in there, and that's all you're going to need.
the MGB is a 2.0 4 cylinder so that's 500cc per cylinder which warrants something bigger than a 40DCO , whereas you're 400cc per cylinder. (which will be fine on 40s)

 

[mattbibbey]

Cool thanks, so the size of the carbs/venturi is really dictated by the cylinder volume.

 

[SKiddell]

Cool thanks, so the size of the carbs/venturi is really dictated by the cylinder volume.

Not quiet that simple I'm afraid
Its linked to CC and any other factor that influences air/fuel flow such as cam, seat angles, port venturi effect even the exhaust manifold has an effect on the induction system (scavenging) its about the volumetric effeciency of the system as a whole.........which changes with engine speed hence why some high output engines have dynamic induction lengths linked to speed/load.

 

[jonbills]

yes that's right - the critical point is the progression from idle jet to main jet; the main jet doesn't start delivering fuel until the air speed in the choke is high enough, and air speed is a function of air required (from cylinder size) and choke size, so if you have too big a choke you can be flowing more air than the idle jet can satisfy yet the air speed is too low to actuate the main jet. Net effect is the engine goes lean and bogs down until it picks up enough revs to actuate the main jet.

Conversely, if the choke is too small, the engine will pick up great, but might be limiting the air flow at the top end and so limit max power at high revs.

There's a formula in Des Hammil's book on it - as I recall, it comes out at 34mm chokes are what you want for a 400cc cylinder engine that you rev to around 7000 rpm.

 

[jonbills]

So, in order not to look like I'm contradicting Steve (I know that won't end well), when I say 'yes that's right', I mean 'yes that's a reasonable approximation'

 

[SKiddell]

So, in order not to look like I'm contradicting Steve (I know that won't end well), when I say 'yes that's right', I mean 'yes that's a reasonable approximation'

For a ballpark yes that works, but the real interesting part of the internal combustion engine is the 10% that sits outside of the generalism.......IE all those little nuances that help the educated tuner gain that little bit more....those are the bits that are real fun to find out about and research, those are the bits that the turbo brigade sadly bypass with a boost dial.

 

[mattbibbey]

those are the bits that the turbo brigade sadly bypass with a boost dial.

Haha. A turbo can substitute for a lack of actual driving talent too! Point and shoot.

My cam spec below:

Intake: 36-74
Exhaust: 74-36
Valve lift: .490°
Advertised Duration: 290°
.050 Duration: 242°

The carbs are Weber 40 DCOM not DCOE. The chokes are 34 as you suspected. According to Weber, they were an improvement on the DCOE but there aren't many differences. A diaphragm accelerator pump, a different design of emulsion tube that wasn't available on DCOEs, no progression access cover etc. All other jets are common to the DCOE.

The head as I said is a large valve E88. It's the one that has the machined recess for injection. Mild porting of the head which has also had a 36thou skim but not quite sure without checking it. Planning on using (if I can get hold of one) a Felpro 1mm gasket or similar.

The complete DJ exhausts system which I know is too big really and I'll lose some low down torque and power but I bought it because of how it'll look and sound more than performance gains.

What jetting are people using?

 

[jonbills]

I've got Dellorto DHLA 45s, so isn't going to compare directly...
I'm running 135 mains and 48 idle (I think) and 45 accelerator pump jets with 36mm chokes.
It was on my 2.4 last year, and I've kept it the same on my 2.9 this year.
It was effectively over jetted for the 2.4 to compensate for the chokes being too big. Now I've got the right jets with the right chokes.

 

[zman240uk]

'A friend of mine just bought a MGB Roadster race car and I think he said it had a 48 on it!'

I think you will find its running a single 48 carb unless he has an expensive 8 port cylinder head fitted. Only 2 inlet ports on the standard head

 

[mattbibbey]

Yeah I knew it was a single carb on that.

I hadn't thought of carbs matching cylinder displacement before as I don't know much about the weber type carbs.