Roots, Centrifugal, And Screw Superchargers- Boosted Power

Superchargers And Built Motors
So far, we have mainly looked at supercharging as a bolt-on for an otherwise near-stock powerplant. In such a situation, the kit manufacturer pretty much takes care of issues that may arise by limiting the power increase, limiting boost, and including an intercooler. Now let's move on and consider what issues you may need to address if you are looking for those monster numbers to annihilate the opposition at the track. Basically, we have to address four primary issues: detonation avoidance, control of excess heat, air mass through the system, and finally, the effective and reliable conversion of the air mass throughput into torque and horsepower.

Let's start with detonation avoidance. The bottom line is that if the engine is strong enough to withstand all normal forces and stresses, then detonation ultimately limits the power that can be extracted from the engine. First, the higher the boost goes, the lower the compression ratio needs to be to avoid detonation. There is a balance to be struck. If part-throttle fuel economy is an issue, then the balance needs to be struck in favor of compression. If ultimate power is the goal, the balance is strongly toward boost. Just how much compression can be used is determined by the engine's combustion chamber/exhaust-valve temperature, and the fuel's octane rating. Charge cooling by means of an intercooler and higher fuel octane are the two routes to success.

At this juncture, it is worth pointing out that water injection can effectively solve two critical problems simultaneously. Firstly, the octane rating of water is virtually infinite. (Think about it, just how high would the compression have to be to get water to detonate?) Don't imagine for one moment that a fine spray of water is going to put out the fire. Spraying even a huge amount of water as a fine mist into a gasoline fire serves only to make a lot of steam. It does not come close to putting out the fire. In addition to pushing the apparent octane of the intake charge up, the water also cools the peak temperatures to the extent that melting anything down is totally countered. To give you an idea just how effective water injection is, I can say while I was a student we successfully ran a 17:1 compression tractor engine on kerosene (less than 50 octane) and water injection. More in line with what we are doing here: how about a 1,100hp 350 small-block Chevy on 35-psi boost and 87-octane fuel? Water injection flat works, and if you are in the market for a system, check out Snow Performance.

Controlling excess heat can become a major part of any supercharged installation. The water injection just mentioned is a good start, but when very high output is in the cards, this needs to be backed up with a big radiator and piston oil squirters. The big radiator is self-explanatory, but to those who may be new to supercharging's little nuances, an explanation of squirters is in order. Here, engine oil is squirted through jets onto the underside of the pistons, thereby cooling them. This is a procedure I have used on all my serious nitrous and supercharged engines since the late 1980s. Many of the newer, small four-valve factory turbo engines are now adopting this method of increasing piston life. If you are supercharging a typical V-8 and live in the Los Angeles area, then it may be of some help to know that Performance Techniques in San Bernardino can modify your block to a squirter configuration at a relatively modest cost.

Now for point number three: maximizing air mass through the system. There are many misconceptions here, the most notable of which concerns the cylinder heads. Do not fall victim to the misconception that heads don't need to be good, because the supercharger forces the mixture into the cylinder. Ask yourself where the power to generate the force came from. The reality is, the better the heads, the more the power is for a given boost, because more air is passing through the system. Also, you should note that for higher boost figures, it is better to trade off some intake valve diameter for a larger exhaust diameter. We are currently putting 15 psi (and planning on 20) into a 331 small-block Ford, and are using a 1.94/1.70-inch valve combination, rather than the 2.02/1.60 combo. For what it's worth, be aware that the lower the compression, the larger the exhaust valve should be in relation to the intake.

Getting The Cam Right
The next factor on the agenda is the camshaft spec. Forget turbo installations for the moment. As for all others where the exhaust flow is uninhibited, we can set some ground rules that apply across the board. The most important factor is the lobe centerline angle (LCA). This is very dependant on the boost/cubes combination in relation to the circumference of the intake valve, and the intake-to-exhaust flow ratio. The bottom line is this: Assuming your engine had the optimal LCA for a normally aspirated application, the LCA would need to get progressively wider the higher the boost. Also, if the exhaust valve is too small for a supercharged application (as is most often the case), it will need to be opened earlier, thus widening the LCA in its own right. Although not so good for the bottom-end output, the early exhaust opening can appreciably help the top end. What we are trying to avoid is the boosted intake charge passing right through the combustion chamber, and out through the exhaust. Also, another factor to take into account is that for a street setup, you will find that the cam for the job can be shorter, thus boosting the low-speed output from a centrifugal type supercharger. If the heads are good, then the supercharger will take care of the top end.

Now on to the subject of turbo cams. One thing you should appreciate here is that a cam company cannot sell you an appropriate cam for a turbo motor until the pressure differential across the motor is known. Let me explain. When a turbo goes into boost on the intake side, it will have been driven there by pressure built up between the engine and the turbo. Other than in a pulse-driven turbo, it is this pressure that drives the exhaust turbine. A typical turbo kit has an intake-to-exhaust pressure ratio of about 2:1, but that can vary from 1:1 for a really well-designed installation, and right through to a mediocre 3:1. Let's assume that your turbo setup is going to be about 2:1. What this means is that if there is 15 psi of boost in the intake, there will be 30 psi of exhaust pressure in the exhaust manifold. If you install a cam with a typical street overlap, when the intake opens, rather than having the mixture enter the cylinder, the exhaust goes out through the intake and into the intake manifold. This heats the charge considerably, and more often than not, increases the likelihood of detonation. Working with a turbo fanatic friend of mine, we looked at cams with negative overlap in such a situation. We tested a cam that had about minus 25 degrees of overlap. Guess what? It worked great. Big torque right off idle, big power, and a glass-smooth 600-rpm idle.

When I build my own turbo motors, I really work hard to target a 1:1 pressure ratio across the engine. If this is achieved (our road race championship-winning turbo Ford Cosworth Sierras in England were this way), then whatever cam events work best for a normally aspirated engine also work best for the turbo setup.

One more valvetrain factor to take care of concerns the valvesprings. Because boost pressure tries to open the valve, a stronger spring will be required on the intake of a regular supercharged engine, and on both the intake and exhaust of a turbo motor.

Our last subject is the effective and reliable conversion of the air mass throughput into torque and horsepower. The golden rule above all else is to have the tune-up right. A lean mixture, or too much ignition advance, will kill a supercharged engine far quicker than a normally aspirated engine. I have seen a poorly tuned motor (too much throttle too soon in the tune-up session) eat itself in less than five seconds. I'm just glad it was not my motor. Also, the ignition and plug requirements are more stringent. The safest plan is to use plugs that run really cool, and go into overkill mode on the ignition system as a primary step. But even then, the situation can go down the drain. Once detonation starts, it leads to more detonation. The best safety measure I can recommend here is the J&S Safeguard, which monitors the onset of detonation and backs out timing as necessary.

As for the mechanical parts of the engine, there is some good news for those who are looking at bolt-on blower power, while maintaining stock bottom-end parts. Rods are often a problem, but it is rpm that kills rods much sooner than cylinder pressure. As a result, rods that may be good for around 450 hp in a normally aspirated engine may well be good to 550 in a supercharged situation. Of course, once we start talking of a built-from-the-ground-up deal for as much power as the chassis will take, then we need to address block and rotating assembly strength. Some production blocks, such as Ford's 5.0 blocks, are not as stout as we might wish for. To date, I have already seen two 5.0 blocks (used for about 550-600hp builds) crack right up the middle of the lifter valley. If about 900 or so horsepower is all you are looking for, you might want to consider a Bessel Engineering block conversion as a cost-effective alternative. As for four-bolt mains, they are a good idea on almost any block where big numbers are contemplated.

Also, if piston squirters are on the menu, it is a good idea to upgrade the oil pump. If we are trying to keep from ventilating the pistons, a thermal barrier coating is a great idea. I have not lost a coated piston engine to boost or nitrous in the last 20 years.