triple Webers

Belgiumbarry

Belgiumbarry

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has anybody with triple webers ever changed the venturis ? I believe stock DCOE45 carbs have 36 mm venturis.
According the Weber calculation this is good for a 3.5l aiming max HP at 5500 rpm.
If we want this at 6000 rpm and above we should use 37 mm/38 mm but loose some response in lower rpm's

Just wondering if anybody has experience with that … as most of those calculations are for "race" engines.... perhaps 35 mm is even better for street use ??
 
Hi Barry,
I do not have experience with it yet. However I have a set of 34 mm chokes that I plan on trying in my DCOE45 carbs - as my engine is not a 3.5 liter nor a race engine. So I guess I am going the other direction...
 
I bought a set of triple webers that supposedly had been used on a running M30B30 engine, and installed them on my car. They had 32mm venturis, and when I got high in the rev range - I think it was around 4500 rpm -- the engine was not getting enough air. I changed to a set of 34 mm venturis and she now will continue to pull without dropping off as before, and I did not see any reduction in low end torque changing to the larger venturis.

I have not (yet) tried 35s in my car.

My engine is an M30B30, with piano top pistons and very good compression. Not sure about the cam; it is something of a mystery I have not run down.
 
interesting experience Ohmess…. thanks !
yes, with 32 mm you would max out at 5000 rpm according the Weber calculation.
Also proof 34 mm is OK for a 3 liter and perhaps we only see drop low end torque once we go to big as if you would test 35..36 mm.
I assume for street use the best is as little as possible so not to max out air flow before max rpm. ( the higher the velocity in the venturi, the better fuel evaporation..due created vacuum )

Perhaps that setup with 32 mm came from a 2800... could have worked until 5500 rpm.
 
Belgiumbarry said:
yes, with 32 mm you would max out at 5000 rpm according the Weber calculation.
Click to expand...

The "Weber calculation" that I was told is:
Choke Dia (mm) = Square root of (Indiv. Cyl. Disp. (cc) X max rpm / 2500)
So plugging 32 mm and 5,000 rpm into this and solving for Indiv. Cyl. Disp. gives 512 cc, which corresponds to a 3,072 cc, 6-cylinder engine.

If you think about this formula, it is saying that the choke's cross-sectional area should be sized to keep the air flow speed through the carburetor within a reasonable range between idle and maximum rpm. Cam profile, compression ratio, ignition timing... have no bearing on this; you're just modeling the engine as an air pump that will pull a fixed volume of air with each revolution. Since air is compressible, it obviously isn't quite that simple; nonetheless, it's a good first approximation.

( the higher the velocity in the venturi, the better fuel evaporation..due created vacuum )
Click to expand...

Yes, but high air velocity only helps you up to a point. You don't want the velocity so high that the airflow becomes turbulent. So I assume the formula I quote above provides sufficient air flow at idle to provide some fuel evaporation/mixing, but not so much that flow is impeded by turbulence at high rpm.

OK, let the fluid dynamicists begin...
 
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I've changed chokes before, from 32 to 34mm, because my main jets were coming on too early in the progression and had a very rich cruise. I did not notice any lack of low end torque as a result and the engine runs better overall rpm's. I don't plan on going any larger as I have a stock motor with a 284 cam. I know @m5bb has a monster engine with big chokes so perhaps he can chime in. I also found the Weber graphs to not be very accurate in the real world as I did not run out of air at the redline with the 32's. Changing chokes/venturis usually require new jetting as a result. No thanks!
 
My experience differs.
I found a notable difference in low end torque and top end power (>5000rpm) when changing the chokes/centuries by 2mm. The smallwe made cold weather start/warmup better and easier to lay rubber, which clearly indicated more torque below 2000rpm. But as you guys said, the engine would feel breathless at high rpm. Same experience at. Sea level as well as at +7000ft altitude.

I ended up using the smaller for everyday driving and the bigger ones for track days.
 
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my 76 korman 2002 had 45 dcoe's with big chokes - it developed all of its power between 4500 and 6500 rpm ... no low end torque ... but it did not run out of breath at high revs .... it just kept going and would rev to 8000. it was a beast ... nice loping idle too due to the schrick 316 cam. not a civilized engine ... not interested in repeating that for a street car
 
jmackro said:
The "Weber calculation" that I was told is:
Choke Dia (mm) = Square root of (Indiv. Cyl. Disp. (cc) X max rpm / 2500)
So plugging 32 mm and 5,000 rpm into this and solving for Indiv. Cyl. Disp. gives 512 cc, which corresponds to a 3,072 cc, 6-cylinder engine.

If you think about this formula, it is saying that the choke's cross-sectional area should be sized to keep the air flow speed through the carburetor within a reasonable range between idle and maximum rpm. Cam profile, compression ratio, ignition timing... have no bearing on this; you're just modeling the engine as an air pump that will pull a fixed volume of air with each revolution. Since air is compressible, it obviously isn't quite that simple; nonetheless, it's a good first approximation.



Yes, but high air velocity only helps you up to a point. You don't want the velocity so high that the airflow becomes turbulent. So I assume the formula I quote above provides sufficient air flow at idle to provide some fuel evaporation/mixing, but not so much that flow is impeded by turbulence at high rpm.

OK, let the fluid dynamicists begin...
Click to expand...
Jay's formula seems to align with my experience with the 4.7l 928 engine . Also 580 cc/cilinder and 6200 rpm , but 8 cilinders instead of 6 .
38 mm venturis perform better then the 36 mm ones , whithout any feel for low end loss.
Those are IDA48's ... don't know what difference it "can" make with the CS 45 DCOE's.

So finally i enlarged to 38 .... curious to see driving performance.

38 = Square root ( 580 x 6200 / 2500 ) :)

Only remark/wonder is that many formulas use "rpm at max HP" .... others just max rpm.

On the 928 with 36 venturis we had to go to small jets ( 135...145) .... which indicates to much fuel vaporazation i assume, to much venturi effect.
With 38 mm we got back to "normal" sizes as 155..160 mains.

I drove the CS with 155 mains, but was very rich . Now 38 will make that already leaner.
 
Just been going through this process w/ my tweaked 3.2L (9.3 piano top pistons, Alpina oversized valves, 300deg cam)

Initially we had tuned them choked down from stock 36mm to 34mm venturies with 145 mains. But then after our first tune session, my tuner had an epiphany told me to go back to the 36mm and step to 150mains because he felt it was making more power than he anticipated. We haven't been able to get on the dyno (life issues) yet so we've been doing some 'seat of the pants' tuning.

Have I noticed a difference from the 34mm/145s to 36mm/150s yet? Hard to tell exactly, crappy levee road conditions and illegal rates of speed issues have kept my foot out of it....sorta. It sure sounds bitchen and it's pulling like a train to 6K. We hope to get it on a dyno over the winter, we think its about 95% dialed currently.
After 2 track days (HPDEs) and possibly a third in Nov (to end the year properly), I have been very pleased with the results on my conversion's "journey".

TIP: before removing the choke, mark both the choke and barrel wall with a dash from a Sharpy, then transfer the dash from one choke its replacement. This will expedite reinstallation of the choke alignment with the set screw greatly....and don't forget to put the air correctors back in! ;)

Stay tuned! (pun intended)
 
seems interesting that you are at 36 (/150) with your 3.2 l. Again almost exactly according the above mentioned formula.

I demounted the carbs to inspect at the same time the gaskets etc..... so no problem align the set screws. Also those bend safety washers underneat would be a pita fiddling in the car , no ?

nono, sure air correctors mounted.... i always place them by 6 in front of me and do the change in one time. I do need a magnifying glass to read the numbers :rolleyes::D

PS the IDA's are more tuning friendly , no 4 set screws per carb , the auxiliary venturies and chokes are simply held in place by the trumpet screws. Chokes have no fix rotation and the auxiliary venturies are aligned by a "spring" sliding in the carb groove opposite of the fuel connection inside.
 
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Belgiumbarry said:
seems interesting that you are at 36 (/150) with your 3.2 l. Again almost exactly according the above mentioned formula.

I demounted the carbs to inspect at the same time the gaskets etc..... so no problem align the set screws. Also those bend safety washers underneat would be a pita fiddling in the car , no ?

nono, sure air correctors mounted.... i always place them by 6 in front of me and do the change in one time. I do need a magnifying glass to read the numbers :rolleyes::D
Click to expand...
"..those bend safety washers underneath would be a pita fiddling in the car , no ?" Yes! a pita but less time overall than removing the carbs. learned through the 'journey' to just remove velocity stacks & filters to increase arm reach and hand access.

"I do need a magnifying glass to read the numbers"....and a nice lighting!

rally on!
 
i do have seen now that those safety washers are very soft types..... i expected much stronger ones , not that easy to bend with carbs on engine.
 
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