Weber MAX CFMs (Weber Carbs and Max Air Flow)

[Russ Noble]

Re: Weber MAX CFMs

The point some people seem to be unable to understand is that with a common plenum air is flowing through the carb and as each individual cylinder draws on it, it has the full area and flow of the carb available to it. 90 degrees later the next cylinder is going through the same cycle and the airflow is comparitively consistent. There is overlap of the cylinders of course but in simple terms that is what is happening. At all times the full flow of the carb is available to the whole engine.

With the Weber or any other single choke per cylinder induction the cylinder only draws through the (relatively tiny compared to a 4bbl) carb for a small proportion of the 720 degree four stroke cycle. That is why Webers strangle big cube motors. The rest of the time, when the inlet valve is closed, the potential flow of the carb is not available to the rest of the engine. With injection larger throttle bodies are available and overcome the problem, but you can't use big enough venturis with Webers.

There are also significant revese pulses etc generated in a Weber system and these can act both to the benefit and detriment of the torque curve at different revs.

 

[Randy V]

Re: Weber MAX CFMs

The point some people seem to be unable to understand is that with a common plenum air is flowing through the carb and as each individual cylinder draws on it, it has the full area and flow of the carb available to it. 90 degrees later the next cylinder is going through the same cycle and the airflow is comparitively consistent. There is overlap of the cylinders of course but in simple terms that is what is happening. At all times the full flow of the carb is available to the whole engine.

Agreed - Another benefit is that the single plenum carburetor will maintain a relatively good and relatively steady vacuum signal to it during the entire range of operation (RPM). Given that a carburetor is a device that relys almost solely upon differential pressures - when there is no differential, the carb literally stops working and has to draw the fuel up through the system each time there is an intake stroke (in the case of an individual runner (IR) carb such as the Webers discussed here)..

With the Weber or any other single choke per cylinder induction the cylinder only draws through the (relatively tiny compared to a 4bbl) carb for a small proportion of the 720 degree four stroke cycle. That is why Webers strangle big cube motors. The rest of the time, when the inlet valve is closed, the potential flow of the carb is not available to the rest of the engine. With injection larger throttle bodies are available and overcome the problem, but you can't use big enough venturis with Webers.

Agreed again - The flow capacity of the IR Carb cannot be shared - therefore is unavailable. But I'm not so sure about your last sentence. I believe that the venturi would be sized according to use / engine design (cam profile, cylinger head flow capacity etc.. The larger the venturi, the longer it will take to build up the appropriate vacuum signal to make the carb draw the fuel and atomize it before dumping through the main metering system. Go too large of a venturi on a smaller motor and you'll likely find the limits of the accelerator pump circuit in short order.

There are also significant revese pulses etc generated in a Weber system and these can act both to the benefit and detriment of the torque curve at different revs.

Curious about the potential benefits of intake reversion on the torque curve.. Given that reversion will not only stop the carb from working (since there is now a positive pressure in the main metering system) - it may potentially purge the main metering system of fuel by forcing it back into the float bowl..

 

[Ross Nicol]

I'm rebuilding my race motor at the moment and when it goes back in, I'm fitting the 4bbl Motec manifold that I bought with this engine.Previously I ran my 8 stack throttle body set up with this motor.I should be able to give impressions from the race seat but the extra cubes may blur this a little. I get the pistons back tomorrow after minor alteration yippee!

Ross:thumbsup:

 

[Mike]

Re: Weber MAX CFMs

Crash 33, an 1100 cfm 4 barrel will flow 8800 cfm if you use the same logic that a weber engine will flow 2640. Each cylinder can pull about 330 cfm through a weber and the same engine with an 1100 cfm carb will give a max of 1100 cfm to EACH barrel. I have dyno numbers that show the stock 48 IDA could not pull my engine much over 500hp at the crank. My heads are Yates and they will flow over 360 at .7 inches of lift. The webers can't keep up. The dynatek system was at 500hp at the tires and still climbing at 6000 rpms when it hit a rev limiter that I didn't know about. The dynatek system will flow 90 cfm more per barrel than my webers. It is all about the air flow. You will not get all your engine has to offer if it is a big HP engine if you use the webers.

The error in this line of thinking is that vacum is the same for both the open plenum and the IR setups. Obviously they are not equal, so yes, the 1100 will possibly flow more than the rated 1100, but it is also quite possible that the 320 CFM Weber is flowing much less than the rated 320. Is the 1100 EVER gonna flow 8800 CFM? No, I don't think so.

May entire point all along was just that it has a lot more to do with the "quality" of flow than it does with a CFM rating. An open plenum engine will be greatly different than a IR engine. Especially in the cam department. That is, if one wants to maximize whatever system they choose to use. In other words, I was trying to point out that it seems you are focused on CFM #s and, whatever system you use, there is a lot more to maximizing power output than just purely carb choice based on CFM.

 

[Jack]

Amost every problem with drivability that I have encountered with Webers was directly related to attempting to use too large venturi's. Don't get trapped by the CFM numbers.

 

[Ian Anderson]

Just thinking out loud here (and I probably shouldn't)

With IDA / IDF you get t carb throat / inlet per piston
So 302cu in * 6000 rpm = 1812000 cu in (divide 12 divide 12 divide 12 = 1048.6 CFM / 8 = 131 cfm But only a charge stroke every 720 degrees = 65.5 cfm

So if the IDA / IDF is set up to run it only does 65.5 cfm

Now 720 degrees divide 8 = 90 so 2 cylinders must be getting filled at the same time.

So do a plenum and stick a single IDA / IDF on the top (2 barrels too cope with 2 cylinders filling at the same time)
Then you only need a carb that flows 65.5 / 2 = 131 cfm for a 302cu inch engine at up to 6000 rpm!

The maths all looks correct but I happen to know that most GTD's etc run a 600 cfm Holley or similar!

My brain now hurts so I'll stick to accounts unless someone can explain in monosylables

Ian

 

[Bob Putnam]

I did a spreadsheet adding up very crude estimates of the flow for 8 cylinders sucking fuel at 45 degree intervals. The peak collective flow is (very) approximately 250% of the flow of a single cylinder.

 

[Ian Clark]

These are great mind bender type discussions because so many conventions and beliefs are challenged.

Generalities of configuration such as 1 4bbl vs 4 2bbl or 8IR (no common plenum) on a V8 draws valid points on absolute horsepower.

I doubt there's any debate that an optimised 4bbl, cam, head and header combo will beat 4 48IDAs (optimised setup too) in top end power above 500hp.

Where the debate gets personal is in the intended use of the car and your priorities in terms of authenticity.

For me nothing compares to the Webers for looks and properly setup they work beautifully.

What I'd like to see is the difference in torque across the power band of a street driven 450>500hp small block motor, 4bbl vs IDAs. What the seat of my pants tells me is that the Webers produce a broader torque curve (less cammy), better low speed throttle response and of course sound incredible!

So it comes back to perspective and intended use. A track car operating from 3500 > 6500 rpm and able to put 500 plus horsepower on the pavement would work great with a 4bbl and cheaper to build than a mainly street use, occassional track day motor on Webers imho.

The EFI Weber look-a-likes produce flow more, make more power because of larger throttle bores and lack the chokes required in the Webers. I suspect in the the power band, below the limits of the 48IDA, would look the same for either system.

Back in the day, Le Mans 1969, the tired old GT40 covered 3,105 miles in 24 hours averaging over 129 mph (208 km/h) with something around 450hp. It got the job done - on Webers...

Cheers

 

[Mike]

Sorry. I think I kinda threw a wrench into the mix when I mentioned the 4 bbl. All the original thread was about was if the particular cfm IR carbs would be enough for the >500HP engine envisioned. My point was that the CFM #s don't really tell the story. It's how the whole enchilada is set up that will make or break it. With the knowledge that I may throw another wrench into things, let me just make this observation. The Weber is basically a 320 -350 CFM carb. I would bet dollars to donuts that an experienced engine builder will get more HP and TQ out of a single Weber or Holley 350 CFM carb than you or I will with a 500 CFM unit on the same displacement engine. How much more depends on how good they are, and how much the customer is willing to pay. The point I was trying to make was that CFM doesn't necessarily determine how the engine will perform and is not neccessarily THE determining factor.

 

[fostereast]

I've found two large bore Weber IDA type carbs. Berg 58mm IDA Weber and Jay Cee 62mm Weber IDA. Both are pretty pricey and both are not quite off the shelf parts. However if you consider the complete cost associated with aftermarket fuel injection on a TWM type IR unit, the final cost is probably similar.

I was hoping to also find a DCOE (sidedraft) large bore carb. Weber offers 55mm DCOE, but after checking I found out they have no venturis or chokes available, so I had to rule those out. The Corvette Grand Sports used 58mm Weber DCOEs but I can't find a source for these yet. The reason I like the DCOE is that it should be easier to tune the length of the ram tubes and manifold runners, and I like the look better.

As I lean towards the IDA type carb, there are several guys I know that have build very similar engines, and I should be able to get some good advice on cam selection, etc...

Regarding CFM, when you start reading about Weber's and V8's the most repeated info is that they do not flow enough for big inch, high horsepower motors, hence the need for something bigger than a 48mm IDA.

Thanks for all the discussions on this so far. I suspect someone makes the 58mm DCOE, but it does not seem like very public info so far.

 

[Ian Anderson]

Foster East

Have you tried looking on the VW tuner sites - they also use IDA / IDF carbs and "replacement" ones with big bores

Worth a look

Ian

 

[Mike]

Foster East

Have you tried looking on the VW tuner sites - they also use IDA / IDF carbs and "replacement" ones with big bores

Worth a look

Ian

I think that's exactly where he has gotten the Berg info. Gene Berg is a VW builder primarily IIRC.

 

[Jac Mac]

I did a spreadsheet adding up very crude estimates of the flow for 8 cylinders sucking fuel at 45 degree intervals. The peak collective flow is (very) approximately 250% of the flow of a single cylinder.

Was that a two stroke 8 cylinder motor Bob, otherwise it should have been at 90° intervals.

 

[Mike]

Just remember it this way. There are 8 cylinders. Because only half the cylinder are on an intake stroke on any given revolution, there are only 4 that are "sucking". Thus 360/4=90

Or you can look at it as Ian did. 720/8=90 since it takes a full 720 degrees or two complete revolutions to cycle ALL the cylinders through their "sucking" stroke.

 

[Jac Mac]

Re: Weber MAX CFMs

Curious about the potential benefits of intake reversion on the torque curve.. Given that reversion will not only stop the carb from working (since there is now a positive pressure in the main metering system) - it may potentially purge the main metering system of fuel by forcing it back into the float bowl..

The reversion pulse continues to pull fuel from the metering system ( The aux/booster venturi works with flow in the opposite direction too ), that fuel is then drawn back ito the engine when normal flow restarts. The only way fuel can be forced back into the bowl via the main system is when an inlet runner explosion or backfire takes place to create pressure at that point, fairly hard to do on a weber since there is no choke butterfly. Any flow thru a venturi creates a lower pressure which is what draws the fuel from the bowl.

If there was to be a perfect carb it would have a very complex venturi that worked like a 3d version of the iris of an eye or camera that would reference off airflow demand.

 

[Mike]

Re: Weber MAX CFMs

The reversion pulse continues to pull fuel from the metering system ( The aux/booster venturi works with flow in the opposite direction too ), that fuel is then drawn back ito the engine when normal flow restarts. The only way fuel can be forced back into the bowl via the main system is when an inlet runner explosion or backfire takes place to create pressure at that point, fairly hard to do on a weber since there is no choke butterfly. Any flow thru a venturi creates a lower pressure which is what draws the fuel from the bowl.

And one of the very reasons that one will see a "reversion cloud" over an engine with large overlap on the cam at high RPMs on the dyno. This effect is even more prevalent on IR setup due to the aforementioned physical pulses having no where else to go. Yet more proof that the common plenum is appropriate.

 

[Mike]

Re: Weber MAX CFMs

If there was to be a perfect carb it would have a very complex venturi that worked like a 3d version of the iris of an eye or camera that would reference off airflow demand.

Hence the slide and variable venturi models like the Predator(Also known affectionately as "the toilet bowl")

 

[fostereast]

Re: Weber MAX CFMs

And one of the very reasons that one will see a "reversion cloud" over an engine with large overlap on the cam at high RPMs on the dyno. This effect is even more prevalent on IR setup due to the aforementioned physical pulses having no where else to go. Yet more proof that the common plenum is appropriate.

Actually the reverse is true. In a plenum type manifold for FI or a 4 barrel carb the cylinder on the middle (strongest draw) of its intake stroke will pull exhaust from the cylinder with an open exhaust valve, not fully closed and its piston still pushing exhaust out. In this case the reversion is pretty strong.

In an IR, one cylinder can not pull exhaust from another cylinder; however for the same cylinder; when the intake valve is opening and the exhaust valve is closing, reversion can happen, however since the intake stroke is not at full drawing power, the reversion is much less.

An IR setup can also run more cam duration than a 4 barrel because of this very issue.

In general a 4 barrel can't beat an IR setup (If carb size is correct in both cases and initial setup is correct) on any level except cost of ownership. Initial cost and cost to tune, etc...

 

[Cliff Beer]

Very interesting debate regarding single port and common port/plenum flow rates. I still don't see how a common port/plenum set up is going to flow more "because each cylinder has access to the full capacity of the plenum" so to speak. Take that same IDA/IDF that's feeding a single cylinder and plonk it on top of a common plenum intake and it's not going to flow any more than before (on a single cylinder) - the choke is still the same diameter and will restrict air flow at the same rate. The only thing that's going to change that flow rate is changing the atmospheric pressure upstream (aka turbocharing/supercharging). As I recall, the Maserati biturbo utilized a turbocharger upstream of a smallish choked IDF and, wow, that made that car fly!

Just my opinion, and I fully appreciate the logic of others opinions (which are likely just as correct or more so!). It certainly does sound like there's some consensus that the IDA/IDF's run out of steam above 500hp flow rates....

 

[Russ Noble]

Re: Weber MAX CFMs

An IR setup can also run more cam duration than a 4 barrel because of this very issue.

In general a 4 barrel can't beat an IR setup (If carb size is correct in both cases and initial setup is correct) on any level except cost of ownership. Initial cost and cost to tune, etc...

That is right and you only have to look back at the Can Ams which were a virtually unrestricted formula to see them running IR in the form of massive injection systems. If they could have got more out of a common plenum system they sure as hell would have done it. If Webers were available big enough they would beat a 4bbl hands down. But they aren't and thats the problem.