Sidedraft vs Downdraft MANIFOLD flow

[vandor]

Hello Guy,

I just received your DVD and it's been a blast watching it! What a suberb workshop, but of course we expected nothing less of you :-)
Did I hear it correctly that the Fiat 'waffle' IDF manifold flows about aswell as a GC sidedraft manifold (without carbs)?
The reason I ask is that I want to convert to individual throttle bodies (and programmable EFI), so it does not matter how the actual carb flows.
If this is true I may stop pulling my hair out trying to fit a sidedraft manifold on a 124 Spider and just go with a downdraft waffle or PBS manifold.
Thanks,

Csaba
'71 124 Spider, much modified
'67 850 Coupe, waiting for TC transplant

[Guy Croft]

Csaba, hi

thanks for that kind review of the dvd especially since you were the victim of a dud disc 2, please accept my apologies for that and thanks for working with Jon Logan on that, we have a 100% check regime in place now.

Let's ignore the question of carbs vs injection for a moment. Yes, the IDF manifold from Fiat - 'waffle' type does flow very well mainly because it has a generous curvature on the upper section. But, interestingly, look closer at the graph of bare port flow with manifold and carb fitted, 44IDF 36mm choke vs 40DCOE 30mm choke, (same as the one in the dvd in fact) the data demonstrates clearly how the sidedraft DCOE proportionately flows much better. How do I arrive at that conclusion?

Well, it comes from a cursory examination of of choke and carb areas. To start with, mounted on a properly enlarged and matched waffle manifold, the 44IDF with 36mm choke is 'working' the air less than the DCOE. The true entry bore of the IDF44 is (actually only) 42.4mm and the ratio of choke to carb bore is 72% . The higher the ratio the better, although that said, not all engines can cope with big chokes. With the 40 DCOE mounted to a GC sidedraft manifold (which is near perfect in terms of size and match) the true entry bore is 37.3mm and the ratio of 30mm choke to that bore is 65% - not so good. So with that in mind we'd expect the IDF to outflow the DCOE per-se, and it does, but not by much! Examine the ratio of choke size: 30 vs 36mm. The IDF choke can flow 1.4 x as much air, but it isn't, nowhere near. Why not?

I surmise that the answer to why the sidedraft set works so disproportionately well is that the IDF manifold only flows good volume (on head) when it's fitted without a carb. The air is not forced in any particular direction at entry by the carb barrel. And in the free state the bulk of the airstream doesn't come in vertically, it prefers to flow at an angle. Carb chokes (primary and secondary) behaves a bit like a jet nozzle and when you fit a carb the high velocity air coming out of the chokes is forced towards the base of the IDF manifold and this is not a preferential route for good flow. I think I might have shown the flow vector with string in the dvd, not sure. In short, the carb upsets the flwo regime on downdraft - but not on sidedraft.

I have never got really impressive results from downdraft carbs on manifolds with 90 deg bends, and I am pretty sure this is the reason. Yes, going to a downdraft injection will be better than carbs because, since inj requires no choke there is no pumping loss in the body. You could typically see 10% more peak power and of course all the benefits of better torque curve, easier starting and better progression (well, let's face it - anything is better than IDF progression!). And, just maybe the absence of a choke might allow the air entering the manifold to take a better line. Maybe..

All in all the practical aspect of installation might well make the downdraft choice the best for your needs. I would though just emphasise that I have not tested a PBS manifold as such.

Hope this helps a bit,

GC

[vandor]

Guy,

Thanks for the reply, it now make sense. I think you are right that downdraft TB's would not be as restrictive as the IDFs, but most likely sidedrafts TB's would be best.
The PBS/Pierce IDF manifold has slightly larger runners than a stock IDFs, but no idea wether it flows better.

No problem on the fluke DVD, it was actually Disc #1, but Jon got the replacement out as soon as he received the defective one, so I was only delayed by a few days.
bye,

Csaba

[vcg]

Guy you just answered a question I had for years!:

I have seen one actual works 124BS1 (ex Lindberg) with the 44IDFs slightly tilted towards the outside away from inlet cam (manifold towards carbs face milled at an angle), and I have also seen pictures of other 124/131 Italian prepared with carbs slightly tilted. I have always wanderd why. My mind went on things like air horns to bonnet clearance/flow, etc.

I NEVER THOUGHT THAT SUCH A TILT WOULD MEAN A LARGER RADIUS AT INFLOW, AND THUS BETTER FLOW !!!

I guess the tilt must not be much otherwise it will affect floats.

How much in degrees do you think Guy such a tilt must be in order to improve flow but not hurt floats?

Thanks,

[Guy Croft]

Vassilis, hi

Yes, thanks, another good result from the flowrig I guess. In theory probably the same Weber data figure the DCOE will tolerate, 5deg, but I would not be surprised if you could get away with 7-10 deg to the vertical. Maybe more..

GC

[dkermode]

Guy,

Very interesting topic. What affect does this difference in flow have on top end horse power - say 44 IDF compared to 45 DCOE on the same engine. This is assuming one has maximised (ported etc) the flow on the manifold with the 90% radius.

Any idea when your new version of your book will be out.

Thanks!

doug kermode

[Guy Croft]

The short answer Doug is - 'enough to make it worth changing to sidedrafts if you can'.

On my LHD StIII 1800 124 CSA with remote braking system fitted it was 12bhp and 8lbf ft torque, since then i have never done another back-to-back test, that was proof enough for me.

GC