SU or Weber Carburetors?

It’s an age-old question—and sometimes fuel for an argument: Which carburetors are better, SU or Weber? The answer is elusive. Everyone has an opinion, but no one seems to be able to offer definitive proof. 

We set out to find some concrete answers by measuring a couple of mule engines equipped with various popular Weber and SU configurations on a dynamometer. We’ll even spoil the surprise by skipping to our conclusion. Which one is better? We have three answers: either, neither, and it depends.

How can this be? Well, it turns out that both choices make about the same power, neither choice is a silver bullet, and the answer really depends on the application. The way your car is built, the way you drive, and even your own cosmetic preferences can help you make the best decision.

Clockwise, from left: We tested four setups on our MGB engine: 11/2-inch SU HS4 carbs on a factory MGB intake manifold, a Weber 45 DCOE carb on a Pierce intake manifold, a Weber 32/36 DGV on a Pierce intake manifold, and 13/4-inch SU HS6 carbs on a Maniflow intake manifold.

The Numbers

We’ve made thousands of dyno pulls in our time. While we’ve rarely tested SU and Weber carbs back to back, we can tell you that it’s been clear through these pulls that either carb combination can make similar power numbers. We figured you’d want more proof than that, however, so we finally performed some thorough tests to show that both setups work well.

For our first set of tests, we used a 1978 MGB fitted with a bone-stock 1.8-liter B-series BMC engine. We compared dual 11/2-inch SUs, dual 13/4-inch SUs, a single side-draft Weber DCOE, and a single down-draft Weber 32/36 DGV. The venturis in each test were sized to logically suit the volumetric efficiency and displacement of the engine/carb setup being tested. We dynoed all of the combinations on a chassis dyno and came up with the following horsepower numbers at the rear wheels:

For our second set of tests, we turned to our Modern Midget’s more radical 1380cc A-series BMC engine. This engine features a well-ported cylinder head, higher compression, more cam, and many other performance tricks. It makes good power throughout a big rpm range.

We tested three setups on this engine: the dual 11/2-inch SUs, a single 13/4-inch SU, and a Weber DCOE. Each one was tested on an engine dyno, so our numbers represent the horsepower at the engine’s crankshaft. (The chassis dyno figures always include driveline loss, hence the lower figures for the MGB.)

As the numbers show, each carb setup shines slightly at a specific rpm point, but on average they’re all nearly identical. Nitpickers may say that the Weber DCOE is the best, but in our book it’s only by a hair—a one- or two-horsepower advantage doesn’t make a clear winner in our book. Context is king when choosing a carb setup.

It Depends: Like We Said, Context Is King

The MGB testing was done on Baker Engineering's Dynojet 248 chassis dyno.

So, the numbers show the Weber DCOE winning by a hair, but each setup produced nearly the same numbers. Which do you choose: One or two SUs, a side-draft Weber DCOE, or a downdraft Weber DGV? Here’s how we decide.

First, we’ll consider our budget and skill set. Ifa we already have SUs on our car as well as the tools and skills needed to rebuild them—or the carburetors are in good shape—we’ll usually keep them. We’ll just make sure that they’re properly dialed in. 

If we think that a professional rebuild is in order—that alone can often cost $400 to $800 per set—then we’ll start thinking about a switch. We’ll also consider a swap if we just don’t have the right carburetors on our car. (Picture a rubber-bumper MGB fitted with its original single Zenith-Stromberg.)

Next, we’ll think about drivability features we like. If a manual choke is viewed as a pain for those who drive the car, then we’ll look toward the downdraft Weber DGV as it’s available with an automatic choke. The DGV can be purchased with a choke that’s either heated by water or operated by an electric circuit. If manual chokes are okay, then the DCOE or SUs are on the list.

Accelerator pumps make for good throttle response and can also influence our decision: Webers have them and SUs don’t. As a result, many people feel that a properly dialed-in Weber provides slightly crisper throttle response. We’ve been unable to prove this on a proper dyno, but our seat-of-the-pants impressions agree.

Another drivability consideration is our ignition system. SUs and Weber DGVs work well with vacuum advance distributors. Meanwhile, DCOEs are less suited for them; most kits don’t even have ports for vacuum advance, so custom work is involved and the results can be mixed. 

We’ve long been fans of vacuum advance on street cars, as it adds a bit of economy. Most importantly, a vacuum advance increases cruise and idle drivability as well as engine smoothness. For this reason, we’d say that the DCOE is a little less than desirable for street use. 

Next we’ll move on to our tuning needs. If we plan on dialing in things in once and then basically forgetting about it, any carb is fine. However, if we like to tweak our tuning, the DCOE is probably the easiest to service. The SUs come next, with the DGV bringing up the rear. 

The DCOE and DGV have easy-to-reach idle mixture screws. Some SUs (HIF and HD models) have screws, too, but they’re not always as conveniently located. Other SUs (H and HS models) have their idle mixture controls hidden underneath the carbs, so reaching and adjusting them can be an exercise in contortion.

Further mixture work with the DCOE comes through the removal of a wing nut followed by a few turns of a screwdriver—the process is very simple and fast. SUs are nearly as easy: three screws to remove the carb top followed by another screw to change the needle. 

DGVs are probably the least convenient to service. To change the jets, the air cleaner as well as the whole top of the carburetor must first come off. 

Emissions are another factor. SUs can easily be equipped with factory PCV systems and work with the evaporative loss systems that absorb fumes. Many Weber setups lack provisions for these things. If keeping smells and emissions at bay is important to you, you’ll have more work to do if you select a Weber.

Another thing to consider: Who do you know? If your friends in the sports car community are Weber people, maybe you should follow their lead. That way you can freely exchange knowledge and parts with each other. The same is true if you hang out with SU people. 

The last consideration, whether you want to admit it or not, deals with aesthetics. Maybe you like the look of polished dashpots. Or maybe you need two—or three!—Webers with velocity stacks. Don’t discount this concern as silly. After all, you pick your paint colors and accessories for aesthetic reasons, so maybe your engine compartment choices should follow suit to some extent.

Looking for some concrete answers? Both SUs and Webers have their strengths and weaknesses. Either is good and neither is perfect. The things that matter the most to you in the context of your car, your tuning expectations, and your drivability priorities will make the choice for you.

Size Matters

Okay, so if you’ve been paying attention, you’ve figured out that any choice can be a good one if made in the right context. However, making a good choice isn’t just about SU vs. Weber. When shopping induction setups, you also have to pick the right size and number of carburetors.

Many people oversimplify the carb’s job. They see it as a piece of equipment that simply gets fuel into the engine. That’s true, but it’s only a small part of the equation. The carb also gets air into the engine and, of course, properly mixes it with the fuel.

Carbs are “sized” by the amount of air they can flow, and this sizing should closely match the engine’s needs. The engine’s capacity is one factor of this sizing, but its maximum rpm and volumetric efficiency are also important. For example, a large engine running at low engine speeds may need less carburetor than a smaller engine running at higher speeds.

The bottom line is that an engine’s CFM needs are determined by this formula: 

If we assume 80 percent volumetric efficiency for a stock engine, we’ll see that a stock MGB (roughly 1800cc or 110 cubic inches) needs 153 CFM at 6000 rpm. A stock Triumph TR6 (approximately 2500cc or 152 cubic inches) needs 211 CFM at 6000 rpm. 

If we add performance modifications to either engine, we’ll see the VE increase—along with the CFM needs. Likewise, if we raise or lower our maximum rpm, the CFM needs will also be affected. This chart shows several of these scenarios:

Note that a normally aspirated engine will rarely achieve 100 percent volumetric efficiency. However, with forced induction, 100 percent or greater is more easily obtained.

Now that we’ve figured out the math regarding our engine needs, we turn to our carburetor choices. This chart shows the flow potential for our common carburetor choices:

Note and disclaimer: This data was taken from multiple sources, some averaged. Flow numbers may vary slightly due to testing conditions and methods or tools used. Nonetheless, this information should be useful in carb sizing decisions.

Now it’s time to put this all together. Once we know the CFM needs for our engine, we can pick a carburetor size and setup to match it. It’s better to go a bit oversized than undersized, but don’t go too far. Installing too much carburetor will reduce power at lower engine speeds and allow the fuel to fall out of the air/fuel mixture. 

Of course, there’s still one more factor to consider: Just because a carburetor flows a certain amount of air doesn’t mean it will flow that much when it’s on the engine. Why? The manifold between the carb and the cylinder head has to flow well, too. Some designs flow better than others. This is another reason why you’ll want to pick a carb that flows a bit better than you need—you can probably expect losses from the manifold, especially if it’s a stock piece. (On a related note, we’ll get into some head-to-head manifold comparisons in a future issue.)

Tune It Right

Here’s a harsh reality when it comes to carburetors: Time and time again we’ve seen people worry and fuss about which setup to install, only to spend little or no time on the jetting required by their particular engine’s needs. 

The right carburetor setup teamed with the wrong jetting will lose power and potentially damage your engine. Make sure your jetting is correct or call in an expert. We’d go so far as to say that an incorrectly sized carburetor that’s properly jetted is better than the right carburetor that’s not properly jetted. 

How do you correctly jet a carburetor? First of all, understand that there is a difference between setting the idle mixture and the mixture under load. That second one involves the all-important main jets. 

In the case of most carbs, the idle mixture can be tweaked with a screw (or jet nuts on some SUs), but the main mixture is adjusted by changing jets or needles. While the idle mixture can be adjusted while the car is standing still, adjusting the main mixture requires the car to be running—and usually moving—under a load. The details of main mixture adjustment can easily fill many pages, but we do have three tried-and-true methods. 

First, if your engine features a fairly straightforward combination of parts, there’s probably a chart or expert you can consult that will get the jetting very close to your needs. We’ve found factory manuals as well as Internet sources—try Teglerizer Engineering and Consulting (teglerizer.com) or Pierce Manifolds (piercemanifolds.com)—that can offer great charts and recommendations. Any reputable carburetor vendor or installer will be a good resource for a personal consultation, and they should be rewarded with your business. 

Second, you can use the proven reading-the-plug method to determine carburetor jetting. If following this route, don’t do it after the car has been idling—you’ll just be reading the idle mixture off the plugs. Here’s the best way to go about it: Make an acceleration run in a remote, safe area before immediately shutting off the engine and coasting to a stop. Now read the plugs.

The third option is probably the most accurate: Use a chassis dyno fitted with a wide-band oxygen sensor. We usually use our first method to get a baseline tune before doing the fine work on the dyno. 

You’ve probably noticed that we’ve suggested spending a few hundred dollars on tuning, ideally via a dyno session. We’re consistently surprised when people don’t heed this very important advice—they’ll spend extra on a carburetor setup they may not need but will skimp on the tuning. Money spent on a good tune is always economical. 

Now What?

If we’ve done our job here, then we’ve settled the SU vs. Weber debate: They’re both good, but neither is an overdog. 

Here’s where we want to leave you: First, don’t worry so much about which carburetor is better, but rather which setup best fits your application and tastes. Next, make sure to size the setup to your engine needs. Finally, spend the time and money to tune it right.

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Comments
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wspohn
wspohn Dork
8/13/20 1:19 p.m.

Good article. It is particularly true that on siamesed port engines the benefits of Webers re low and given that new Webers come set up for some other application and the tuning to your engine is a combination of experience and science that most owners lack, it really isn't that attractive a mod except to those who desire the look of the Webers for braggig rights.

 

Go to one intake port per cylinder and things shift a bit more toward the Webers and it becomes worth the exercise to fit and tune them for a street performance car or a race car (I have always used Webers only on my race cars powered by MG engines).

MG engine with after market 4 port crossflow head:

MG engine with factory DOHC head:

dougie
dougie Reader
8/13/20 11:10 p.m.

Would have been nice to see some test results on the SU 2" HD8's. Tuning on the dyno is the only way to accurately tune any performance motor.

https://youtu.be/8CmGFL54VlA

Sam12693
Sam12693
8/14/20 6:13 p.m.

I've worked with SU's and Webers.  Lucky enought to live close to Pierce Manifolds for my Datsun 240Z setup.  Webers let me tweak settings as I add modifications or change my driving style.  Jets x 6 are not cheap, so getting good advice from folks running similar modifications saves $$ and time.

wspohn
wspohn Dork
8/15/20 2:37 p.m.

Dougie - the BMC factory was in the habit of speccing too large SUs for racing. The Healey didn't need 2", nor did the race MGB, nor the street Rover 2000 TC. They even tried them on the factory MGA Twin Cam race cars.  Flow is too slow at part throttle and they don't run ell except on wide open throttle, and even then only when tuned exactly right.   I have tried them on my race cars (uop to 1950 cc MGB motors) and prefer the 1 3/4" . It gets you little to have a horsepower or two more at top rpm if you lost time getting there.

Sam - we are lucky to have local shops that specialize in Italian cars and keep full sets of jets that they will charge a fee for plus whatever it costs to replace stock once you know what you need. I am lucky that I never had to do anything but fine tuning on my V12 Lamborghini, but that would have been the way to go.

IIRC the Datsun wasn't homologated with Webers but with Solex PHH 44s, but that's pretty much the same thing except for jet availability. We had local racers that got them properly set up and they were one sweet set up once they got them optimized!

dougie
dougie Reader
8/17/20 9:03 p.m.
wspohn said:

Dougie - the BMC factory was in the habit of speccing too large SUs for racing. The Healey didn't need 2", nor did the race MGB, nor the street Rover 2000 TC. They even tried them on the factory MGA Twin Cam race cars.  Flow is too slow at part throttle and they don't run ell except on wide open throttle, and even then only when tuned exactly right.   I have tried them on my race cars (uop to 1950 cc MGB motors) and prefer the 1 3/4" . It gets you little to have a horsepower or two more at top rpm if you lost time getting there.

Sam - we are lucky to have local shops that specialize in Italian cars and keep full sets of jets that they will charge a fee for plus whatever it costs to replace stock once you know what you need. I am lucky that I never had to do anything but fine tuning on my V12 Lamborghini, but that would have been the way to go.

IIRC the Datsun wasn't homologated with Webers but with Solex PHH 44s, but that's pretty much the same thing except for jet availability. We had local racers that got them properly set up and they were one sweet set up once they got them optimized!

That hasn't been my experience, I'm very familiar with all the Healey Works and BMC prepared competition cars. With modern cam design, true technical head porting, and a few additional American tuner tricks the period factory teams couldn't do, didn't have knowledge of or have time to install and test, I've achieve performance far beyond any Healey they prepared for road racing. It was easier for them to switch to Webers, then to do additional testing & tuning with the 2" SU HD8's.

wspohn
wspohn Dork
8/18/20 12:43 p.m.

Well done if you have managed a triple 2" set up that has good transition through the rpm range. I don't think that could ever be done with the twin 2" on an MGB engine.

My favourite set up on the C series was a triple 1 3/4" intake that resolved all of the problems with the end cylinders that the stock twin carb set up caused (I think I've posted a pic before). The flow rate on those totals 630 cfm.

I beleive that Vizard said that a 2" SU can flow 330 cfm on the bench and I would think that a 3 liter pushrod C series would be hard pushed to need 990 cfm at red line

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