Supercharging Your MGB

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Written by Carl Heideman

From the Nov. 2003 issue

Posted in Drivetrain

Supercharging has been a part of the British car scene for a long time. From factory-built Blower Bentleys and supercharged MGs of the 1920s and 1930s to aftermarket offerings from Judson, Marshall, Shorrock, Wade and others in the 1950s and ’60s, many British cars have relied on force-fed power. So it only makes sense that Moss Motors is reviving the tradition of blown British cars.

A few years back, Moss came out with an aftermarket supercharger kit for the MG-TD. Similar in appearance to the Marshall units available in the past, the Moss kit features a modern Eaton supercharger for higher efficiency and better drivability.

Now Moss has come out with a more mainstream product, releasing a supercharger for the MGB. Following the formula and experience derived from the T-series kit, Moss has adapted an Eaton M-45 supercharger unit for use with the MGB’s 1798cc four-cylinder B-series engine. Currently there are two versions of the supercharger available: one for the 1968-’74 chrome-bumper cars, and another for the later rubber-bumper cars. Early MGB and MGA kits are looming on the horizon.

We obtained one of the first Moss supercharger kits and got to see if there was power behind all the mythology. We bolted the kit onto our 1978 MGB test mule and put it through its paces. First, we ran the kit straight out of the box as delivered from Moss. Then we pushed it further with more boost, more camshaft and an improved cylinder head. We ran it on the dynamometer every step of the way, and now we can report on it.

Supercharging Basics

Before we get into the details, let’s discuss the basics of supercharging. Using an old analogy, an engine is basically a pump. It takes in a mixture of air and fuel and pushes out exhaust gasses, making power in the middle. If the engine can inhale more air/fuel or exhale the exhaust gases more efficiently, then it’s going to make more power.

Traditional ways of increasing the pumping capacity of an engine include cylinder head porting, camshaft changes and exhaust improvements. Each of these methods gives the air/fuel mixture and exhaust an easier path to follow—sort of like leading a horse to water.

With supercharging, that horse is forced to drink. The supercharger is a high-volume, low-pressure air compressor that simply forces a larger air/fuel charge into the engine. Once that charge is burned, it’s got no choice but to force itself out through the exhaust.

This all sounds too good to be true. It is. Supercharging has some inherent side effects that need to be addressed. Number one is heat. Compressing the air/fuel mixture and forcing it into the engine causes much more induction heat than that seen in a normally aspirated engine.

This heat can cause all sorts of problems, but the worst is higher incidence of detonation (pre-ignition, also known as knock). Another problem is the parasitic power loss to drive the compressor. It takes a fair amount of power to spin the belt-driven compressor, so supercharger efficiency is paramount to its performance.

Historically, supercharging has enjoyed periods of dignity and disgrace. The years of high interest in the 1920s and ’30s were followed by some lulls. Supercharging saw some resurgence in the 1950s and ’60s, followed again by another long, empty period until the mid-1990s. Armed with the tools of modern computer-controlled fuel-injection systems and precision manufacturing techniques, superchargers have again been embraced by automakers and the aftermarket alike.

On to the Details

Obviously, MGBs don’t employ modern fuel-injection systems, but these cars can take advantage of the well-built, long-lasting superchargers installed on other types of engines. Eaton superchargers are used in many original-equipment applications including those from Jaguar, Mercedes-Benz, Ford and GM. They’ve also become a staple in aftermarket kits for Miatas, Hondas, Toyotas and others.

Moss has taken Eaton’s smallest supercharger, the 45-cubic-inch M-45 unit, and integrated it into a comprehensive kit that includes every nut, bolt and component necessary for installation in the MGB. The kit includes the supercharger itself as well as a specially cast intake manifold, brand-new SU HIF 44 carburetor, K&N air filter, new water pump, multi-ribbed serpentine drive belt system, new accelerator and choke cables, and all the miscellaneous pieces needed to finish the installation. The kit for the chrome-bumper cars also includes a new alternator. A 22-page installation manual finishes the kit.

The $2895 price may initially seem steep, but when compared to other power-adding options (see the sidebar on page 84), the return on investment seems strong.

We installed the supercharger kit on our 1978 MGB that’s no stranger to dyno testing. (See the Feb. 2002 issue of Grassroots Motorsports for more testing done with this car.) The testing originally began when the car had a fairly stock, unrebuilt 75,000-mile engine, albeit upgraded with a stock, earlier-spec camshaft, dual-SU conversion and Peco header and exhaust system.

We have never dyno tested this car in its pure stock configuration, but we’ve tested other stock, single-carburetor, rubber-bumper MGBs on the dyno, and they usually peak with a little more than 50 horsepower at the rear wheels. Our car never made more than 64 horsepower at the wheels, even with its upgrades—considerably better than stock, but still no screamer. The car has always had excellent drivability, and we hoped the supercharger wouldn’t affect this.

With the Moss kit bolted on as delivered, we could immediately tell good things had happened. After the installation was complete, the car had no problems starting or running, although with a slightly rich mixture. On the street, the engine sounded better, less like a sewing machine. The MGB felt a little more torquey below 3000 rpm; past that mark, it woke up and let everyone within earshot know that this whining engine was no stocker, pulling very smoothly to its 6000 rpm redline. Quite a few MGB owners have since driven the car, and it puts a smile on every face.

Performance Improvements

After about 100 miles of use, the time had come to record some zero-to-60 times. Previously, the car required 14 seconds to reach 60 mph; with the supercharger, the car was consistently coming in at 10 seconds—a significant improvement.

We installed an oxygen sensor and Edelbrock air/fuel meter to help monitor the combustion process, and we could see that the car was running slightly rich. For a better analysis—including the opportunity to measure horsepower and torque—we took the car to Baker Engineering for a few pulls on their Dynojet chassis dynamometer.

Our initial runs showed that things had improved, as peak power at the wheels had gone from 64 horsepower to 88. While this was a big increase, it was somewhat of a disappointment since we were expecting to break 90 horsepower. Looking at the air/fuel curve from the dyno data, we saw that the mixture had gone extremely rich above 4000 rpm, robbing us of a little power.

On the dyno, the car would pull well until we hit about 5300 rpm. At that point, it let out a large cloud of black smoke and rapidly lost power—probably about 5 horsepower, judging by how the graph looked. It never did this on the street, but the increased load from the dyno must have been exacerbating the mixture problem.

While SU carburetors don’t have the programmability of a modern fuel-injection setup, they are extremely flexible in mixture adjustment due to their multi-stepped mixture needle and available damper springs. A call to Joe Curto Inc. netted us some new needles and springs to try, and we made a little progress in eliminating the top-end losses.

Not wanting to leave good enough alone, we decided to push the Moss kit to its limits. The next simple test would have been to add more boost via the optional higher-boost pulley, but we first had some other plans, as we wanted to see how traditional breathing improvements would help the supercharger’s efficiency.

We had Mike and Sean Brown port a stock cast-iron MGB cylinder head for us. Their Oregon-based shop has been porting heads for street and race MGBs for years, and they’ve developed an excellent reputation for delivering power through their work.

We first installed their head ($995 plus shipping) along with a 270-degree-duration Crane camshaft (part no. 342-0010, available from Moss as part no. 222-270 for $325, including lifters). We also used an adjustable timing gear set (part no. BTGK-LW, $109) and an Automotive Racing Products head stud kit (part no. HSB-142, $136) from Advanced Performance Technology. The set allows adjustment of camshaft timing via different placement of the gears. We initially installed the camshaft straight up, but will be trying some camshaft timing changes in the future.

Performance improved immediately after we made these modifications. Zero-to-60 times dropped to 8.5 seconds. On the dyno, we made a maximum of 96 horsepower. Next, we installed the optional smaller pulley on the supercharger, raising boost from 6 psi to 8. Peak power went to 101 horsepower.

Installation Details

In addition to the supercharger’s performance, we’re very impressed with the kit’s packaging and instructions. The actual installation went very quickly, taking only about five hours at our professional shop, Eclectic Motorworks. The experienced home mechanic can expect to install the kit as a weekend project. We did encounter a few small problems, however, which we’ll get to in a minute.

The installation process involves removing the stock carburetor (and exhaust manifold if the car still has the single Zenith Stromberg setup) plus the radiator, water pump, alternator and lower crank pulley/harmonic balancer.

Then the new parts are installed: A replacement lower pulley with grooves for the multi-ribbed belt replaces the balancer, the new water pump and pulley goes on, either a new alternator and pulley (chrome-bumper cars) or a new alternator pulley only (rubber-bumper cars) is fitted, and then the supercharger is installed with a new exhaust manifold or header if necessary.

This brings up the first tricky part of the installation, as there are different flange thicknesses for various exhaust manifolds and headers out there. These flanges meet up with the intake manifold flanges at the mounting points, so alignment is critical to avoid leaks. This was further complicated since the mounting flanges on the supercharger manifold varied in thickness by up to .070 inches.

To match the thicknesses of these parts, Moss includes shims and super glue. Instead of using the glue, we chose to weld together our shims, but the Moss solution, though not as elegant, will work.

Since our car had a Peco exhaust header instead of the stock cast exhaust manifold that Moss recommends, we found an incompatibility. The oversized mounting flange of our exhaust header interfered with the oversized mounting flange of the Moss supercharger intake manifold, meaning one of them had to be cut back. We cut back the header flange, not wanting to weaken the supercharger’s mount. This step took one hour of our five-hour installation time.

Once the supercharger was mounted, we could install the belt and its tensioner. Here we encountered our next problem, as the alternator tensioning system didn’t have the right lower adjustment bolt for our car. To fix the problem, we needed to make up a new piece with a long bolt and a spacer. Moss has since corrected the problem on production units.

After this, we reinstalled the radiator and followed Moss’s instructions to cut and modify the lower hose, rerouting it with their included pieces. We’re not completely satisfied with the lower radiator hose configuration, but we are living with it for now.

After installing new accelerator and choke cables, we hooked up the lines for the fuel, vacuum, PCV and anti-run-on systems before bolting on the air filter assembly. Once everything had been installed, we were ready to fire the engine. The engine started and ran very well. With a timing and idle-mixture adjustment, we were ready to go for our first drive.

We noticed a problem on deceleration, however, as the air filter assembly was hitting the brake booster. Turns out the situation was due to a worn-out left-hand engine mount. In the instructions, Moss strongly suggests replacing the mounts, so we were warned of the potential problem.

Despite these minor detail problems, we feel Moss has done an excellent job in providing this comprehensive kit. Not only did the system install quite easily and included all the pieces needed for the installation, but it retains important systems like the PCV and anti-run-on system, which most performance parts ignore or bypass. Anyone installing a supercharger should expect to deal with some minor issues along the way, especially given that many cars have been modified from stock at some time in their past.

Tuning and Ignition

While we were impressed at how well the car ran with the supercharger installed right out of the box, we need to emphasize that anyone embarking on such an installation must have a clear understanding of how to tune an MGB. Since we installed the supercharger on an already well-tuned car, we had no difficulties. If the kit was installed on a poor-running car, it would probably run worse. The supercharger isn’t going to cure worn valves.

Most inexperienced tuners start by blaming the induction side of the engine equation, but remember an old saying: Most carburetor problems are ignition-related. Installing the supercharger will only make people more likely to blame the induction side of the engine, and we caution them to look carefully at their ignition system before even beginning to install the kit.

Moss wanted to include a new distributor with the kit, but elected not to because of the resulting increased cost. Nonetheless, we feel the correct advance curve is paramount to a successful installation. Our car runs the Lucas 45D “Eurospec” distributor (Moss part no. 143-110, $239.95) with an electronic conversion using a Pertronix Ignitor ($91.45). This is probably the best distributor setup to start with, followed by the earlier 25D units.

Later units have too much mechanical advance built into them, resulting in not enough advance at low engine speeds and thus causing sluggish acceleration. Likewise, they have too much advance at higher engine speeds, which can lead to harmful detonation. On this low-compression engine, we found 32 degrees total mechanical advance (vacuum advance disconnected) at 3500 rpm to be the best configuration for power without detonation. Cars with higher compression ratios will probably need a little less advance. On the dyno, we played with timing and advance curves and found that 10 horsepower could be lost with poor timing choices and/or advance curves. This assumes, of course, that the engine isn’t damaged in the process.

We also found that with the stock head and camshaft, we’d couldn’t give the car more than 32 degrees total ignition timing advance (vacuum advance disconnected) before detonation would set in. The ported head and improved camshaft would take more advance without detonation, but still made peak power at 32 degrees.

Moss includes colder spark plugs with the kit to help avoid detonation issues (NGK BPR7ES, which are equivalent to Champion RN7YC), but we found that they caused a hesitation with our car. We went back to the stock-spec plugs (NGK BPR6ES, equivalent to Champion RN9YC) and eliminated all problems.

Once we were happy with our ignition setup, we turned our attention to the carburetor. We actually did very little, except to find a better needle and spring setup. For the ported head and performance camshaft, we were pretty pleased with the slightly rich BCA needle Moss included, but we did change from a red 4.5-ounce spring to a green 12-ounce spring, which can be supplied by Moss or Joe Curto.

More Power?

Power is addictive, and you need to know when to stop. To be honest, this setup is great right out of the box. Our camshaft and head swap made a significant difference, but at a pretty big price—about $1500 not including any labor. And we think we can keep doing better by adding more boost. We’ll be trying to get 10-plus psi of boost soon—there may be five-plus horsepower in this simple upgrade.

Adding too much power starts affecting other areas that also impact the budget. For example, with the improved setup, we started to experience fuel starvation above 5400 rpm, so a higher-volume fuel pump will be in order as we get more serious. We’re also having to push the engine to higher rpm levels to make our power, which is not going to help longevity at all.

If you’re contemplating a project like this, carefully make your plan and consider what you’re willing to trade for more power. It may be money, it may be drivability or it may be longevity. Pick the right balance before you get carried away.

Silver Bullet?

For years, there have been many power recipes for MGB engines, ranging from simple ignition upgrades to a more involved engine swap. Now there’s one more recipe.

With performance, people are always looking for the silver bullet and often come up empty-handed. The Moss supercharger kit is not quite a silver bullet, but it comes pretty close. It’s an especially well-suited upgrade for the lower-compression engines of the later cars.

While its price requires serious thought, it’s on par with other proven power-adding methods. If you budget enough for the supercharger, the proper ignition system, an exhaust change if necessary, and—very important—get the engine properly tuned, you’ll end up with a significant power increase and good drivability for around $4000 or less. If you want to rebuild or improve your engine before you install the supercharger, you could spend another $1000 to $5000 pretty fast. Worth it? You decide. Meanwhile, we’ll keep smiling.

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