Basics of Supercharging An Internal Combustion Engine

Supercharging by far has the most visual impact of any high performance modification. There are some basic concepts that should be understood in order to achieve the best results for a given application. One of the fundamentals is that you have a defined end result in mind before you start. Do you want a maximum performance that demands special fuels and constant adjustment, or do you want a car that runs on pump gas and can potentially be driven on a daily basis? Either application really is a question of set up. It is possible to have any degree of the two extremes provided the relationships between components are correct. Lets look at more of the details on how a blower works.

Conceptually and internal combustion engine is nothing more than a pump. Most high performance modifications are aimed at increasing flow of this inherently inefficient pump. Under normal atmospheric pressure the displacement of an engine dictates how much fuel/air mixture it can ingest. Camshaft selection special intakes, or pocket porting can make the flow from the carburetor smoother, but the engine still has limitations on its actual physical capacity to suck in a measured amount of the air/fuel mixture. Supercharging certainly will effect airflow, but more importantly it effects air density or how much air actually is trapped in each cylinder on the compression stroke. The blower basically “fools” the engine into thinking that its effective displacement is larger than it actually is by forcing more air in the same small cylinder space. Forcing air by means of a crank driven pair of rotors will push pressurized air in each cylinder creating more fuel mixture resulting in a more forceful explosion. The end result of this is more horsepower, but what you can really feel is the tremendous increase in torque.

Typically a blower will sit on the top of the intake, and be driven by a gilmer toothed belt to eliminate any possible slippage. Surprisingly, even if the blower had no belt the engine vacuum can slowly rotate the rotors allowing the engine to operate. The most common automotive blower design is the “roots” blower, which actually is a pair of rotors that trap air between each other as they rotate, and push this pressurized air through the intake. This is why blower drive ratio, or the relative size of the upper and lower pulley is so important. The more you overdrive, or turning the blower rotors faster than the crankshaft, the more boost you get. Big pulley on top is underdrive and big pulley on bottom is overdrive. If your base compression ratio is low enough, say in the 7:1 or 7.5:1, you can actually overdrive the blower on 93-octane gas. If on the other hand you have a 9:1 or 9.5:1 compression ratio 12% to 15% underdrive can still work. Keep in mind that its not any one modification variable, but rather the combination that’s important.

The enemies of supercharging are heat, lean fuel mixtures, detonation, and backfire. A supercharged engine will produce more heat and tax cooling systems. A supercharger put on an already marginal cooling system can quickly overtake the cooling loop. Additionally, a lean running set of carburetors can cause tremendous rise in cylinder temperatures. The result can cause exhaust to glow red and even melt pistons or other internal components. Too much ignition advance, or too much compression for a given octane rating can cause detonation. The result is premature fuel explosions pounding the tops of the pistons, sometimes even to destruction. Finally, backfire is the condition where the force of the fuel explosion is driven back up through the blower. Backfire can be caused by driver error in throttle operation or incorrect timing. The result can be bent blower rotors or cracked cases. This is why you often see a spring loaded valve on the back of the blower. Often referred to as a “pop off” valve, it gives a pathway for the pressure to be relieved for a backfire condition.

Although the enemies of supercharging may seem imposing, as long as you have your initial plan in mind before you start, these problems can be avoided. Now lets go over just the short list of concerns that need to be kept in mind as you devise your plan.

Blower size A 6-71 blower can be quite tall requiring hood modification, but also the pulley assembly and blower drive belt can add another 8 inches on the front of the engine’s harmonic balancer. Keep in mind clearance problems both on top and in front. Also keep in mind that your HEI ignition will not clear the blower case and a standard distributor will be needed.

Blower drive ratio The pulley combination that you select will be based on the overall compression ratio and engine setup. Whether the blower is overdriven or not, keep in mind that the less turbulence you create the denser the air will be when it gets pumped into the cylinder. There is a law of diminishing returns the faster the blower is turned as all this whipped air is heated and less dense. The idea behind intercoolers is to prevent this expanded air form negating the blower’s more positive effects.

Carburetor setup Blower carburetors will typically have to be jetted according to application, and I highly recommend you find someone that is familiar with blowers to help you with this fine tuning. These blower carburetors will often be mechanical secondaries and jetted quite rich for supercharging with the exception of the smaller B&M style blowers. The B&M 144 blowers are quite nice as they are often just “plug and play” with little adjustment. Another often overlooked problem is the carburetor actuator linkages and fuel lines. These “accessories” are often not included in the blower “kit”, and can lead to considerable additional expense.

Accessory drive belts – Racing applications typically forgo any accessories, so often you are left with quite a aggravating set of circumstances trying to make you alternator or power steering pump turn. Make you plan early on, and stick to it.

Fuel delivery system – Mechanical fuel pumps can cover many blower applications, unless the performance demands start to rise. Everything rises in proportion including the cost so watch your wallet! Electric fuel pumps can not only be expensive, but also a hassle to live with on a daily driver.

Rear axle ratio – The blower’s best work is done at lower RPM, so take advantage of this point. Have a plan and stick to it!

Engine internal components Cam selection should be made with lots of breathing in mind. Blowers will tend to quiet the most radical cam shafts, but there are “blower grind” cams that are better suited to the job. Base compression ratios should be figured according to the type of fuel you intend to run. I always try to be conservative.

Day to day life with a blower Yeah, sure it looks cool, but what about every day? Blowers can make an excessive amount of heat and noise. The blower whine was neat for awhile, but on a long trip it can be mind numbing. Blowers can also be quite messy. Goo will often be found oozing down the side of the blower, requiring constant cleanup. Automatic transmissions will shift prematurely never allowing the motor to wind out. This is because the transmission is seeing a pressure signal instead of a vacuum signal, and gets a little confused as to what to do. Now you have to go to a manual valve body automatic or a four speed and more $$$$, but nothing says “go” more so than a 6-71 climbing out of the hood. I say LETS DO IT!!

Cost – Don’t be fooled! Blowers can be quite expensive. The typical blower “kit” will cost anywhere from $1500.- $2500. and there is still a whole lot of stuff to buy. Figure that you can spend an average of $2500. for a Weiand 144 blower driving down the road, and $4500. for the big 6-71. Swap meet blowers are often incomplete of broken, so buyer beware. There are some deals out there, but know what to look for or bring someone that does.

The conclusion is most certainly this: Is a blower for everyone? Probably not. But for those wanting great performance and looks it’s a great tried and true method. There are many publications written about blowers and their application, so I recommend doing some research for yourself. There is nothing better than the smell of burning tire smoke in the morning!