Weber venturi polishing

p400 · 10-16-2006, 06:41 AM

Is there any documentation or history on polishing the primary and secondary venturis in a Weber 40 IDA 3C?
would polishing improve anything? [IMG]venturi[/IMG]

turboteener · 10-16-2006, 07:42 AM

If they are the cast venturies yes there could be some improvement. If there are any casting marks. It should be done on a lathe to maintain the proper shape. You won't gain much by polishing the secondary. You can gain some by making the supports on the secondary a little more aero friendly.

If this is a race motor with a carb restriction then there maybe some gain, but for a street motor I wouldn't waste the time. You would be better off going to a bigger carb.

davidppp · 10-16-2006, 08:07 AM

Hello, p400.

I have no flow figures.... I'd predcit minimal or no effect..

It is not possible to do this on a lathe I'm afriad..unless you use a special set-up.

The choke bore is eccentric to the carb bore on the ID3's...

The theory would support polishing ..this is the highest velocity in the whole induction system and also no issues re deposition..

Kind regards
David

turboteener · 10-16-2006, 02:16 PM

Your suggesting that unlike all other weber venturies that it has some different profile than round? If it is round like every other weber carb then you can chuck it up in a lathe and take a piece of sand paper and polish until your hearts content

p400 · 10-16-2006, 04:27 PM

There seems to be a rough cut band around the entire venturi throat at the point of max velocity. Smoothing and polishing would seem to be a desirable thing to do.

I dont find any previous talk of this. Or maybe i have not been paying attention. These could be chucked in a lathe and smoothed or even just by hand. Having all the venturis smoothed might lead to easier tuning.

p400 · 10-16-2006, 04:45 PM

The secondaries would require some custom hand work, but not too difficult. It would seem important to smooth the casting line on the cylindrical portion, and smooth the leading edge of the airfoil support wings.

turboteener · 10-16-2006, 05:08 PM

As far as secondaries go I wouldn't bother. You might gain a little but you might not. There is a boundary layer which in effect insulates the airflow from the effects of those small disturbances along the flow path.

p400 · 10-17-2006, 05:54 AM

As i look at the 40 IDA 3C 4.5 aux venturi i have for review in the midst of a rebuild, i am not impressed with the final state of the precision castings, primary and aux venturi. It is not a matter of getting higher air flow, but a consistant, repeatable air flow thru each throat with some concern about turbulence and eddys. It looks like i doesnt take too many mechanics pulling aux venturi until the trailing edge of every aux is deformed. Is the air velocity so low it doesnt matter at these points? i dont think so. Are these concerns addressed in some period publication i am not familiar with?

911pcars · 10-17-2006, 12:47 PM

I suppose one could trim excess material away from the sec. venturi; make it a sharp edge rather than blunt as it now is. The advantage of slide valve throttle bodies is the absence of restrictive items in the throttle bore (e.g. throttle plate, throttle shaft, sec. venturis, discharge tubes, etc.). How close can you get with judicious trimming and flow improvements? Maybe a little.

Sherwood

davidppp · 10-18-2006, 04:03 AM

Hello again.

The primary venturis on these carbs are indeed unique in my experince.

The bores are round, but eccentric to the bore of the carb body.

It is not possible to use a three-jaw set up to machine or polish them for this reason.

Kind regards
david

p400 · 10-18-2006, 05:33 AM

Thanks David, I only have my 30mm venturi to reference. They are unmodified. they are concentric. 45.85 OD with a 30mm cast bore, approx 8mm wall all around.
i have read about boring these 30's out to larger dimensions (32-34) and certainly that could be done off center.

So polishing is not a difficult task. I am just surprised to find no references (1966 thru 2006) to address this easily accomplished task. Race car types wouldn't be doing this type thing?

Best regards
Craig

davidppp · 10-18-2006, 05:37 AM

Hello, Craig.

there are lots of ventiris put there..not all are Webers, not all were for these carbs.

FWIW I have used concentric ones in the past..they work oK..

I do not know for sure that all orginal ones were offset..but I do know many were..I have plenty here!

Kind regards
david

jluetjen · jluetjen's Garage

Quote:

Originally posted by p400
There seems to be a rough cut band around the entire venturi throat at the point of max velocity. Smoothing and polishing would seem to be a desirable thing to do.
I dont find any previous talk of this. Or maybe i have not been paying attention. These could be chucked in a lathe and smoothed or even just by hand. Having all the venturis smoothed might lead to easier tuning.

I'd be careful messing around with the venturi. If you want more flow, use a larger venturi (within reason). If they're to be used in a race car to rules which mandate using a certain size venturi, polishing or "porting" them will most likely be illegal and get you in trouble during a tear-down. Not to mention, my understanding is that venturis have very specific shapes in order to promote fuel atomization. I suspect that it would be a lot easier to mess things up then it is to improve on them.

p400 · 07:03 PM

The material removed in cleaning and matching up venturi components would be so minimal that it is of no adverse technical concern. I would have thought the cleaning of flashing, ensuring no rough surfaces, etc, to be a more commonly undertaken tuning aid than it appears to be.
I am not saying a home mechanic with 40 grit sandpaper, a hi speed Dremel tool, and a bead blaster couldn't screw this up.
But i think within the 40 years of 3C this would have been old stuff for Weber experts, i thought the oldtimers would have had a comment or two
thanks
P400