Alright carb math junkies -- I have a puzzle for you. According to the Weber chart provided in Pat Braden's book my Chevy 292 wearing three Weber DCOEs (with an individual cylinder displacement of about 800cc) should be able to provide enough vacuum to activate the main jets with a 40mm Venturi in a 50mm throttle body. BUT... That chart assumes that you have each throttle body feeding a single cylinder (in other words, one cylinder is only drawing through one throttle body exclusively). With the Chevy 292 Clifford manifold and the siamesed lump-port each intake event draws air through BOTH barrels of one carburetor. I assume that the Weber calibration chart calculates vacuum based on air velocity generated through a single barrel. But this velocity would be halved if the cylinder had to draw the air through both barrels of the Weber DCOE. So here is the big question... Is the air velocity of a venturi with a cross-section area of X equal to the the air velocity of two venturis with areas of half of X? (I'm sure there would be additional surface area friction on the two smaller venturis that would make it slightly lower, yet similar.) 1 x 40mm venturi = 1256mm (area) 2 x 28mm venturi = 1230mm 2 x 30mm venturi = 1413mm Therefore, if I was feeding a single cylinder with an isolated single throttle body, then I would run a 50 DCOE with 40mm venturi. But since the single cylinder will draw on both barrels should I be running a 40 DCOE with 30mm vents?
... I'm guessing all of the carb junkies are already in bed. This engine was built by Sissel (Mike Kirby) and run on a dyno several times back in 2007ish. The guy who ordered the engine sold it off because it always ran rough for him. I've got all of the build sheets and dyno sheets, and looking at the dyno sheets you can see that the curves are somewhat erratic until -- BANG -- everything jumps into line at 3,000 RPMs and holds steady to 5,100. Looks to me like there isn't enough vacuum acting on the main jets under 3,000 RPMs, and it just so happens that the engine was set up with 45 DCOEs with 36mm venturis. During the dyno runs they swapped in jets, tubes, and air correctors, but they left it set up with the "ideal" 80% vent/throttle body ratio. Even with this set up it showed torque at 335 and hp at 300. Anyway... I'm just thinking this engine might really come to life if the carbs were better matched for the siamese intake ports.
Looking forward to the correct info on this. My experience with Webers, (IDTP's) has not been rewarding.
If they work like the IDA's, there are three circuts. Idle, intermediate, and main. The mains start around the 3k you mentioned. You might have some emulsion tube tuning to do. Before the WOW, at 3k to 5k, was it on the lean side or rich?
I'll have to go over the pull sheets to see if they offer any rich/lean information. But isn't that all secondary (the jetting) to determining the right size venturi? Venturis are activated by air speed and that is affected by cylinder volume and RPM. If the venturis aren't right, then jetting can't fully fix that. Experimenting seems to be a costly proposition, but I'm really tempted to throw on a new set of DCOE 40s. They come configured: "Prim.vent 30mm Aux.vent 4.5mm Main jet 115mm Air corrector jet 200mm Emul.tube F11 Idle jet 045mm Pump jet 40mm." From there it seems you would have to have a tackle box full of parts to dial in a set of three. $1,000 for the three carbs and nearly that much (probably) in a selection of small parts to tinker with.
Here is a bump for an interesting subject. BTW, have you discussed this with any repair shop guys that deal with old British cars with oem Webers? .
I am fairly well versed at this stuff, but I have always worked with isolated runner stuff so I'm going to watch from the sidelines on this one. You might try contacting Pierce Manifold or Redline inc. I think if you have a chanceat finding someone who has worked with this situation it would be through one of those companies.
Yeah, same here... The one time I tried a DCOE on a siamesed head it was a disaster, and I went back to the progressive two barrel downdraft to get it to run right again. ( probably because the intake I bought for it was crap ) What I have found in general with Webers, is that sizes are chosen way too large most of the time. Specially on street driven engines. So my gut feeling is that the 40's with 30mm venturies would be the best. I wouldnt buy new ones though. 40's are the easiest to get of all, used. ( because they were OE on Alfa Romeo's and others )
Those 50mm carbs are way too big for a 292, unless you want to twist it up to 9 grand or so You can choke them down with smaller venturi's, but the throttle response will still be lousy and they will not carburate very clean at lower RPM. I would get a set of 40's for sure
I've already got the 45's (came with the engine), so I've decided (been told) to set it up with 32mm vent chokes instead of the 36mm that are in it and see how well it tunes in. The engine is for a race car, so it will probably spend most of it's life between 3k-5k revs. If it still stinks with the 45/32 set up, then maybe I can talk the boss into letting me swap them out for a 40/30 set up.
Without knowing the differences of flow efficiencies probably a good starting point is to assume you have slightly over half the peak flow when comparing siamesed ports to a port per cylinder arrangement. This is backed by Wareneford in his "Guide To Tuning Weber Carburettors". In the guide he compares 4 cylinder engines; port per cylinder vs. siamesed inlet ports and makes the following recommendations. With a 250 cc capacity per cylinder: PpC - 27mm choke std, 28mm choke hi perf, 30mm competition, using 40DCOE X 2 Siam - 28mm choke std, 32mm choke hi perf, 35mm competition, using 40DCOE X 1 With a 500 cc capacity per cylinder: PpC - 33mm choke std, 35mm choke hi perf, 37mm competition, using 45DCOE X 2 Siam - 36mm choke std, 38mm choke hi perf, 40 mm competition, using 45 DCOE X 1 My experience with using DCOEs on a turbocharged Corvair taught me that too much carburetor kills any low end response for the very reason you state, no air velocity at low rpms resulting in no pressure drop within the secondary venturi to suck any fuel. So your 45 DCOEs with 36mm chokes do sound like way too much carb. If you have the patience and parts to tune swapping to 32mm chokes might work fine but also if you have 3.5 mm secondary venturis you probably should swap to 4.0mm or 4.5mm at the same time.
Does the X 1 or X 2 mean one or two carburetors for the entire 4-cylinder engine? Otherwise it looks like he is recommending a larger throat on a siamese port, and that doesn't make sense from a velocity standpoint. Why do you recommend a larger secondary venturi if the primary is smaller?
Sorry about not making it clearer; the siamesed port engine uses one carb and the port per cylinder engine uses two. Regarding going to the larger secondary venturi: The secondary venturi is essentially a stack with a small venturi in the center that exits just upstream of the main venturi (choke) where the air velocity is the highest. The purpose of this venturi is to provide the "suck" to draw the fuel in the main fuel circuit. This venturi is not only a player on the richness of the main circuit, it also determines the point the main circuit kickes in. Or another way to look at it is what engine rpm the main circuit kicks in. For a given flow the smaller the choke (throat of the main venturi) the higher the air velocity, therefore "rule of thumb" indicates going to a larger secondary venturi when going to a smaller choke, assuming similar main fuel characteristics are desired.
Based purely on a bit of basic math (292 x 16.387064 = 4785cc; 4785/3 = 1595cc) I'd be inclined to recommend the stock spec for a DCOE-equipped PpC four like a Fiat 1600 twin-cam as a base to work from. Bigger venturis on siamesed ports makes sense to me, as doubling the number of intake events per cycle will double the effective velocity in charge mass per cycle.
[QUOTE/] Bigger venturis on siamesed ports makes sense to me, as doubling the number of intake events per cycle will double the effective velocity in charge mass per cycle.[/QUOTE] No, not that simple. Remember that in a four stroke engine air/fuel is being drawn into the cylinder only one quarter of the time, the rest of the time there is no intake flow to that cylinder. Because the pulses of a siamesed port engine are staggered; in theory, while the total mass of flow is doubled through the siamesed port, the actual velocity is the same since the time mixture is being drawn is also doubled. In practice, however, fluids do not like to start/stop instantaneously and no intake port is 100% efficient which result in a carburetor requirement of slightly greater than half on a siamesed port engine when compared to a port per cylinder engine everything else being equal. Remember, in this case each cylinder is drawing through two venturis, not just one on the siamesed port engine. If both cylinders fired simultaniously then you would be correct. It will be interesting what Redline Weber has to say . . .
We run triple 45 DCOE on a Jag of about 235 cu in. Power is a bit lower than your 300 (212 at rear wheels). Our manifold is a log, so not isolated pulse to pulse, but the carbs are so close to the ports that I suspect there is little mixing between them.
The way I understand it, a Weber senses the pulse in the intake more than the flow ( because they are meant to run on individual runners ), so any connection between the ports will affect them.
I had some spare parts that came with the carbs (vents, jets, tubes). I've got them set up with 32mm vents, 140 mains, 180 air correctors, and 60 F8 idle. The auxilliary venturis have the number 45 on the outside of the ring, but I can't tell if that is supposed to be the carb they go in (45 DCOE) or if the . is missing from 4.5. They appear to have been sanded around the outside. I don't see any other numbers, so would this mean they are 4.5? I'm waiting on a list of parts to finish the assembly so I can fire it up. It will probably be 2-3 weeks, but I'll keep you posted. In the meantime I'm reading a stack of books on Webers to make sure I've got a plan for dialing them in as much as possible before dyno day.
Related Input: While reading The Dellorto Book, in an attempt to tune a confused carb. The Book Said: These carbs are designed to read pulses from one cylinder per barrel. To use them in any other manner PROBABLY will not work, but if you insist on trying... I did and it never really worked like the one barrel per cylinder successes I was accustomed to. These carbs are like fuel injection with a float bowl. Separate the intake runners, sooner than later.
I got a lengthy reply from Bud Pauge at Redline Weber, and he said he wants to work with me to get the carbs just right for our build. He needs more information about the car and racing situation before making a recommendation, but he is thinking that 45/36 is as large as he would recommend.
Sounds like you may already have an answer to your secondary venturi question, but just in case; the only number I've seen stamped on the venturi is the size, no decimal point. Sounds like yours is a 4.5. Regarding being sanded, not seeing it it is hard to tell but as long as it doesn't impact the carb body to venturi port seal it may not be a problem. As stated earlier, it will be very interesting to hear what Bud at Redline has to say. With the siamesed ports 36mm just sounds too big to get any flow at lower rpms for proper fuel mix unless the idle and intermediate circuits are richened so much they impact your main. There is no reason to run more carburetor than the engine can handle, hopefully you got air/fuel on your runs since in your case its critical information. And the arguement that a Weber will only work on a port per cylinder engine just does not wash. Absolutely, a Weber set up to work on a port per cylinder engine could turn out to be a disaster, but properly dialed in they should work fine. It does get tricky when the intake pulses are not evenly spaced, but that is true for any carburetor and is really a manifold problem. Let us know what you hear.
Bud said that 45s with 36mm chokes should be able to work fine, and he said he'd like to help me get it up to 38mm chokes. He said the problem with my configuration is likely the idle jet. He is going to get back to me with a recommended starting configuration. So much for my theory. But I'm just glad to get an expert opinion.
I agree with that. A single Weber or two split Webers ( using one throat of each carb ) was the hot setup for racing Mini Coopers, and cars like that. And those had siamesed ports too. I acctually seen a buddy of mine in a spaceframed Maguire Mini out accelerate and outrun a Etype Jag on the track. And he had one single IDA on his... I think the reason I couldnt get mine to work was because of the manifold. ( too many restrictions and bends )
Interesting, interesting. Like your theory, what I've read backed by some tinkering say otherwise, but I'd love to hear if proved wrong. Please keep posting updates.
The Aussies managed to get 2 IDA's adapted to a single 4 barrel manifold to work better than a 4 barrel carb. This was the hot setup in the late 70's with Group C touring car regulations. Peter Brock / Jim Richards had their most dominant victory ever in a Holden Torana A9X Coupe at Bathurst using this set up.
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