Supercharging has by far the biggest visual impact of any high-performance mod. There are some basic concepts that must be understood to achieve the best results for a given application. One of the fundamentals is that you have a definite end result in mind before you begin. Do you want peak performance that requires special fuels and constant tuning, or do you want a car that runs on gasoline and can be driven every day? Either application is really a configuration issue. It is possible to have any degree of the two extremes as long as the relationships between the components are correct. Let’s see more details about how a blower works.
Conceptually and the internal combustion engine is nothing more than a pump. Most high-performance modifications are aimed at increasing the flow of this inherently inefficient pump. Under normal atmospheric pressure, the displacement of an engine dictates the amount of fuel / air mixture it can ingest. Special camshaft select taps or pocket taps can make carb flow smoother, but the engine is still limited in its actual physical ability to suck in a measured amount of the air / fuel mixture. Supercharging will certainly affect airflow, but more importantly, it affects the density of the air or the amount of air that is actually trapped in each cylinder on the compression stroke. Basically, the blower “tricks” the engine into thinking that its effective displacement is greater than it actually is by forcing more air into the same small cylinder space. Forcing air through a pair of crank-driven rotors will push pressurized air into 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 top of the inlet and be driven by a gilmer timing belt to eliminate any potential slippage. Surprisingly, even if the blower is beltless, the motor vacuum can slowly turn the rotors allowing the motor to run. The most common automotive blower design is the “roots” blower, which is actually a pair of rotors that trap air together as they rotate and push this pressurized air through the intake. This is why the fan gear ratio or the relative size of the upper and lower pulley is so important. The more you accelerate or turn the fan rotors faster than the crankshaft, the more boost you will get. The large pulley at the top is underdrive and the large pulley at the bottom is overdrive. If your base compression ratio is low enough, say at 7: 1 or 7.5: 1, you can actually overload the blower with 93 octane gas. If, on the other hand, you have a 9: 1 or 9.5: 1 compression ratio, 12% to 15% sub-frame can still work. Note that it is not a single modifying variable, but the combination that is important.
The enemies of supercharging are heat, lean fuel mixtures, detonation, and kickback. A supercharged engine will produce more tax and heat cooling systems. A supercharger installed in an already marginal cooling system can quickly bypass the cooling loop. Also, a poorly performing carburettor set can cause a tremendous rise in cylinder temperature. The result can cause exhaust gases 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 explosions of fuel that hit the tops of the pistons, sometimes even to destruction. Finally, the counterproductive effect is the condition in which the force of the fuel explosion is propelled up through the blower. Backfire can be caused by driver error in throttle operation or improper timing. The result can be bent blower rotors or cracked housings. 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 provides a way for pressure to be relieved in the event of a blowback condition.
Although the enemies of supercharging may seem imposing, as long as you keep your initial plan in mind before you begin, these problems can be avoided. Now let’s just go over the short list of concerns to consider when changing your plan.
Blower Size A 6-71 blower can be quite tall and require a modification to the hood, but also the blower drive belt and pulley assembly can add another 8 inches to the front of the engine’s harmonic balancer. Be aware of clearance issues both at the top and at the front. Also note that your HEI ignition will not clear the fan housing and a standard distributor will be needed.
Fan Drive Ratio The pulley combination you select will be based on the overall compression ratio and engine configuration. Whether the blower is overloaded or not, keep in mind that the less turbulence you create, the denser the air will be when it is pumped into the cylinder. There is a law of diminishing returns the faster the blower is turned, since all this beaten air heats up and is less dense. The idea behind intercoolers is to prevent this form of expanded air from nullifying the more positive effects of the blower.
Carburetor setup Typically, blower carburetors will need to be injected according to the application, and I recommend that you find someone familiar with blowers to help you with this fine-tuning. These blower carburettors will often be mechanical secondary and injected 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 few adjustments. Another problem that is often overlooked is the carburetor actuator connections and fuel lines. These “accessories” are often not included in the blower “kit” and can be a significant additional expense.
Accessory Drive Belts – Racing applications generally forego any accessory so you often run into a rather aggravating set of circumstances attempting to spin the alternator or power steering pump. Make a plan from the beginning and stick with it.
Fuel Delivery System – Mechanical fuel pumps can cover many blower applications, unless performance demands start to increase. Everything increases in proportion, including the cost, so take care of your wallet! Electric fuel pumps can not only be expensive, but also a hassle for living with a daily driver.
Rear Axle Ratio – The best blower job is done at lower RPM, so take advantage of this point. Have a plan and stick to it!
Internal Engine Components Cam selection should be made with breathing in mind. Blowers will tend to mute the more radical camshafts, but there are “blower grind” cams that are better suited to the job. Base compression ratios should be calculated according to the type of fuel you intend to use. I always try to be conservative.
Everyday life with a blower Yes, it sure looks great, but what about every day? Blowers can generate an excessive amount of heat and noise. The whine of the blower was fine for a while, but on a long drive it can be overwhelming. Blowers can also be quite messy. The goo is often found seeping down the side of the blower, requiring constant cleaning. Automatic transmissions will shift prematurely without allowing the engine to stall. This is because the transmission is seeing a pressure signal instead of a vacuum signal and is a bit confused on what to do. Now you have to go for an automatic or four-speed manual valve body and more $$$$, but nothing says “forward” more than a 6-71 coming out of the hood. I say LET’S DO !!
Cost – Don’t be fooled! Blowers can be quite expensive. The typical blower “kit” will cost between $ 1,500 and $ 2,500. And there is still plenty to buy. Calculate that you can spend an average of $ 2,500 for a Weiand 144 blower driving down the highway, and $ 4,500 for the large 6-71. Exchange blowers are often incomplete or broken, so the buyer should be careful. There are some bargains, but know what to look for or bring someone who has them.
The bottom line is without a doubt this: is it one blower for everyone? Probably not. But for those looking for great performance and looks, it’s a great tried-and-true method. There are many posts written on blowers and their application, so I recommend that you do some research for yourself. There is nothing better than the smell of burning tire smoke in the morning!
