Ask Away! with Jeff Smith: Carburetor Recommendations for a 355 Chevy with a Weiand Supercharger

I picked up a Weiand 144 small supercharger for my 355ci small block Chevy and now I’m wondering what carburetor to use? Obviously fuel economy isn’t one of my main concerns but do I really need a monster carburetor to help this thing make power? Speaking of that, what kind of power can I expect to make with this blower on my engine?

T.R.

These two questions are considerably open-ended, mainly because you didn’t offer more details about the engine.

We’ll start with your first question of how big of a carburetor to use. The simple answer is that any carburetor will work but since it sounds like your second question is more about getting the most power out of this small supercharger, then the next level answer is the larger the carburetor, the better. 

The reason for this is that the carburetor represents a significant restriction in terms of air inlet efficiency. Superchargers are like any kind of pump. In this case, they are air pumps. Pumps, as a rule, do a great job of pushing air out but are not nearly as good at pulling air in. This means that any restriction on the inlet side will reduce the supercharger’s efficiency on the output side.

A smaller carburetor will heavily restrict the airflow into the blower and, just like computers, garbage in is garbage out. Even a slight restriction on the inlet side will mean losses in terms of blower output efficiency. We once asked noted engine builder Kenny Duttweiler why an 8-71 blower engine on his dyno had two monster carburetors and he told us that the least restriction on the inlet side will produce the best power. An ideal situation would be no restriction at all which might be accomplished with a giant pair of throttle bodies that would be tied into an EFI system.

A simple recommendation that would minimize the restrictions yet still be streetable would be a 750 cfm carburetor. To confirm this, you will need to boost-reference the power valve to ensure that it opens as soon as boost is created. The typical power valve adds roughly the equivalent of six to eight jet sizes of additional fuel on the primary side. The reason for a power valve is to allow running leaner primary jets during light throttle operation to improve fuel mileage.

Often the blower will “mask” inlet manifold vacuum to the bottom side of the carburetor which may prevent the power valve from opening and the air-fuel ratio may be too lean. To prevent this, you can convert a Holley to reference the boost pressure under the blower to open the power valve. As an aside, there should not be a power valve on the secondary side. If there is, block it off and increase the jet size accordingly. This is generally between six and eight jet sizes. Many carburetor restoration companies offer this service.

Before you spend the money to boost-reference-modify your carburetor, you might try hooking a vacuum gauge to read manifold vacuum at the bottom of the carburetor. With the blower and carburetor installed, try going to wide-open throttle for a very short time (a second or two) in a safe location while running down the highway and have a friend watch the vacuum gauge. If the gauge reads five inches of manifold vacuum or less, then you don’t need to modify the carburetor. But if boost pressure leaks into the top half of the blower (which can occur), then the carb may see pressure which will not allow the power valve to open.

We’ll assume you have a relatively mild 355ci small block with 9:1 compression, a mild cam, stock heads, and now a 750 cfm carburetor. Let’s say your engine makes around 350 horsepower to the flywheel. These small blowers are generally worth about 25 to 30 percent increase in power with single digit boost numbers of around six to eight psi.

Theoretically, if you had a supercharger that makes 14.7 psi (atmospheric pressure at sea level without costing any horsepower to run, which is impossible) then your engine could double its power from 350 to 700 hp. This happens because you’ve doubled the normally aspirated air pressure feeding the engine. But since it requires horsepower to drive the blower, we must subtract that power from the net power at the flywheel. Plus, with only 7.4 psi (for example) this would be half of an additional atmosphere. So now our engine is making somewhere around 25 to perhaps 30 percent more power.

With a 350 hp normally aspirated engine, this little blower could push your engine up to around 425 to 450 horsepower (350 hp x 1.3 = 455). That may not sound like much, but a 30 percent increase in power is something you will most definitely feel in the seat of your pants. Plus, Roots style superchargers are known for making boost right away, which means the torque gain at low speed is very strong. So if the engine was making 350 lb.-ft. of torque normally aspirated, it now could potentially make 450 lb.-ft. at 7.4 psi.

Another way to look at this is that the blower is roughly equal to adding 30 percent to the engine’s displacement. So that would push your engine’s 355ci displacement up to around 460 cubic inches. Not bad.

Don’t be surprised if your initial combination makes less boost than anticipated under maximum rpm. Often, a stock blower drive will generate around three to five psi, which will also means it will add less horsepower than the above estimates. To increase the boost will require either a larger blower drive pulley on the crankshaft or a smaller pulley on the blower to spin the supercharger faster. If the stock pulley combination does produce seven to eight psi we would recommend staying with that. Often spinning the blower faster to achieve 10 to 12 psi produces excessive heat that could hurt overall power. More boost does not necessarily mean more horsepower.

Keep in mind that the additional power will also demand specific engine tuning with jetting and timing in order to make the most power. If that’s something you want to achieve, the easiest way might be to take the car to a chassis dyno shop and let them tune it to achieve best power. Keep in mind that the numbers at the rear wheels will likely be 100 to as much as 150 hp lower than the flywheel numbers we have been discussing.

We don’t have any details on your drivetrain, but it’s not unusual to see a 100 to 125 horsepower loss through a typical automatic transmission and rear axle assembly on its way to the rear tires. That will mean that if your engine is actually making 450 flywheel horsepower, the rear wheel horsepower on a dyno will indicate somewhere between 325 and 350 hp. Again, this doesn’t sound like much compared to the claims people make, but this is the reality of the situation.