Compression Ratios and Supercharging
When matching superchargers to engines, the most important factor to be resolved is the engine’s mechanical compression ratio. Since a supercharger can elevate the volume of air ingested by each cylinder beyond that magical 100 percent VE figure – the compression ratio is much higher than it was prior to supercharging – this new dynamic dictates the engine’s output and the life expectancy of the engine’s suddenly higher stressed internal components.
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There’s a significant difference in the calculated static compression ratio and the dynamic compression ratio. When you pencil out a static compression ratio, you compare the volume of air the piston’s movement will displace with the volume of air that will exist between the piston dome and the combustion chamber surfaces at Top Dead Center (TDC). In supercharged applications, however, the chamber volume will be included in the displaced volume, increasing the Bottom Dead Center (BDC) displacement figure without an associated increase in the TDC volume. This effectively increases the compression ratio, at least under boost.
To optimize a supercharged engine, reducing the static compression ratio will allow higher intake manifold pressure (more boost) without wandering into problems with detonation. Unfortunately, having too low a static ratio can affect drivability. If the vehicle will be expected to negotiate traffic most of the time, the static compression ratio must suit those conditions and be high enough to maintain adequate spark plug and combustion chamber temperatures for low-speed operation. This usually requires a compression ratio above 8.0:1.
To allow higher boost levels, the static compression ratio can be set at approximately 7.5:1 with pistons having shorter compression heights than stock. This ratio is useable on the street only if the engine is exercised on a regular basis. Otherwise, you’ll be changing fouled spark plugs a lot more often than you’ll care to.
The payoff, however, is awesome performance under boost, which can be raised to at least one atmosphere (15 psi) without worrying about melting your mill. Most high-performance engines with low static compression ratios will handle considerably more than a single atmosphere of boost, but it takes some careful planning to control the spark and fuel demands.
The fuel requirement of a supercharged engine is not precisely related to the increased volume of air under boost. Programming the fuel curve to reflect the increased airflow using the fuel requirements of an unsupercharged engine will result in a very lean mixture under heavy boost. The additional fuel required under heavy boost often calls for an auxiliary fuel pump to control the combustion temperature in the chambers.
While the excess fuel in the engine will absorb large amounts of heat in the process of achieving optimum fuel vaporization, if oxygen no longer remains in the chamber to support combustion, the vaporized (and usually some raw) fuel will leave through the exhaust valve. But without what sounds like a wasteful process, the engine would self-destruct under detonation.
Since the amount of air within the intake tract has been increased, the compression pressure will be higher than normal. The ignition timing must usually be retarded to compensate for the quicker burn time of the dense, rich air/fuel mixture. The entire concept of variable ignition timing is to begin the combustion so that maximum cylinder pressure occurs slightly after the piston passes TDC. Lean mixtures burn more slowly than rich mixtures do, and highly compressed, dense mixtures burn very quickly. The proximity of the fuel molecules with the oxygen in these mixtures ensures fast flame travel.
It’s a given that the more air/fuel (higher VE) you stuff into an engine’s combustion chambers, the more exhaust gases will have to come out. Under naturally aspirated situations, the exhaust lobe center on a conventional hydraulic, flat-tappet camshaft has enough duration for the engine to be able to cleanse itself of the exhaust in-between the intake, compression, ignition, and exhaust strokes. However, with a blower, dispensing this exhaust in an efficient manner using the existing valvetrain components may prove somewhat problematic. To more fully understand the dynamics of camshafts and street supercharging, we spoke with Chas Knight, Domestic Valvetrain Product Manager from Crane Cams.
“When it comes to supercharged street engines, we like to use dual-pattern camshafts wherever possible. Traditionally, the dual-pattern camshaft provides the broadest power and torque band with favorable idle characteristics and good performance. To put it in laymen’s terms, what you have with the dual-pattern camshaft is a different profile on the intake lobe master and a different profile on the exhaust lobe master. Traditionally, you have a slightly longer duration lobe (112 to 116 degrees) on the exhaust because of the excess heat and boost that’s generated by a supercharger to allow the exhaust valve to stay open longer, remembering that most popular American V-8 engines feature an exhaust port that doesn’t flow nearly as good as the intake, and is governed by a smaller diameter exhaust valve. By extending the exhaust lobe duration slightly, it is a very efficient way to increase exhaust port efficiency and relieve back pressure.”
What is the best design dual-pattern blower cam? Hydraulic or flat tappet? Roller or non-roller?
“A blower cam can be anything from a hydraulic flat tappet, hydraulic roller, a conventional flat-face solid lifter, or a solid roller. You must remember as a rule, the average street supercharger enthusiast isn’t big into high RPM. So, normally for these kinds of applications, you could use a hydraulic roller camshaft. Of course, with a hydraulic roller camshaft, you can set and forget the valve pre-load once instead of having to go back and readjust the lash intermittently like you would have to do with a solid grind. I guess you could say that it’s a more user-friendly camshaft.
“The hydraulic roller cam profiles that are available also provide much more efficient valve motion than you can obtain from a flat-face lifter. With the hydraulic roller cam, you get better idle, better torque, and better horsepower!”
How would you go about selecting the ideal profile blower camshaft?
“This is where it starts to get a little more involved,” says Knight. “Normally when you go to a supercharged application, chances are you’re going to be lowering your static compression ratio because you don’t want to run the risk of constant detonation, or have to run exotic (and oftentimes expensive, and illegal) racing fuel on the street. When lowering your compression ratio to prevent detonation, you’ll also need to go to a milder grind camshaft with less duration on the intake, because you’ll now be force-feeding the intake mixture into the engine mechanically. Instead of depending on the vacuum pulses of the intake system to draw fuel into the combustion chambers, you’re going to be doing it more positively allowing the supercharger itself to fill the cylinders or do the work.”
Prior to delving into turbo cams, we posed a hypothetical, yet commonly asked, question. What kind of camshaft would I use on an 8.5:1 compression, Vortech VS-1A-supercharged 5.0L at 6 psi and 5-speed manual transmission?
“With an application like that we would use a hydraulic roller cam with 226 degrees (duration) at 0.050 inch on the intake, and 232 degrees (duration) at 0.050 inch on the exhaust, with something like a 0.544 to 0.559 (inch) max valve lift. This type of cam would feature a lobe separation up to 114 degrees to reduce overlap so you won’t be blowing the intake charge right back out the exhaust.”
What type of camshaft would you typically use in a turbocharged application?
“That’s another deal altogether. For a basic street-driven small-block Ford application, we would probably recommend a single-pattern grind camshaft where the intake and exhaust would be the same duration, or quite possibly what we call a reverse-pattern cam, where the duration on the exhaust is actually shorter than the intake.”
Knight went on to explain that the turbocharger is a thermal pump. It lives on heat, and uses the engine’s exhaust to drive itself.
“The more efficient the exhaust system, the more efficient the turbocharger becomes. What you want to do is capture that heat (through a less aggressive, shorter exhaust lobe duration), and use it to drive the turbocharger. With a singlepattern grind, you’ll spool up the turbo quicker. You’ll get better throttle response. You’ll get better power, and less turbo lag.
“Again, your ‘average’ street turboequipped car will be turning 6,500 rpm or less, so a single-pattern hydraulic roller cam will work just fine. Anytime you start getting over 6,500 rpm, you’re probably going to be better off with a solid-lifter roller cam application.”
Carburetors Versus Fuel Injection
In the long-running dispute over which is the best induction system, the results have generally been favorable for EFI, and in particular sequential electronic fuel injection (SEFI). However, if you’re fascinated by things like that (and if you’re reading this book, we guess you are), you might be interested in why that is.
The first point for EFI is its amazingly quick response time. The second point isn’t as simple, because the tenure of carburetion has been a long and successful one.
Let’s cover the importance of response time first. Think about how many events, in terms of fuel delivery and ignition, occur every second while an engine runs. At 6,000 rpm, the crankshaft is turning 100 times per second, the ignition system is charging and discharging 400 times per second, and the fuelinjection system will be timing the pulse length for each of the injectors at the same rate as the ignition system, varying the amount of fuel delivery at intervals of milliseconds to optimize the mixture cylinder by cylinder. That’s a darned good trick, by any standard!
A well-tuned carburetor is a wonderfully precise fuel delivery system, but it cannot respond to the requirements of an engine on a cylinder-to-cylinder basis. Instead, the carburetor is dealing with the engine’s gross airflow for all eight cylinders. The carburetor responds to a generalized airflow and fuel requirement, where an SEFI system can be capable of responding to each cylinder as it comes up on the compression stroke.
The fuel injection system is able to control fuel delivery so precisely because it can estimate the amount of time between a cylinder firing and the resultant spent charge passing the oxygen sensor in the exhaust system, for a given engine RPM. No carburetor is capable of that kind of interactive calculation. Typically, fuel injectors are measured in pounds per hour, which is a measure of the actual static flow, or output of the injector. For example, the SEFI system on your average stock 5.0L Mustang V-8 uses a set of 19-lb/hr injectors. Ford engineers decided this was the optimum size to provide the best performance and overall drivability, while meeting stringent emissions requirements. Of course, once you start bolting on big-bore throttle bodies, blowers, etc., all those parameters begin to change. To make more power, you need more fuel, hence, larger fuel injectors.
However, carburetion shouldn’t be dismissed entirely. After all, carburetors were the mainstay of the internal combustion engine for close to a century prior to the introduction of EFI.
In one particular instance, however, the carburetor will continue to rule supreme. We’re talking about the “carbin-a-box” setup used on the Paxton supercharged 289 Shelby GT-350 Mustangs.
Installing a carburetor between the supercharger and the intake manifold eliminates a lot of unnecessary plumbing, gets around most of the problems with pooling fuel in the intake manifold, and presents a more tidy appearance than having the blower draw air through a carburetor. When a carburetor is placed in an air box following the supercharger, it lives (albeit with a few modifications) happily in an environment that changes dramatically as the boost builds. This works well because the carburetor’s fuel curve is adjusted relative to the ambient pressure inside the box. Whenever the boost climbs, the carburetor’s circuitry has no clue there has been a change in manifold pressure. All of its air bleeds and the air volume above the carburetor floats see no change in the relative pressure between the ambient and internal systems – therefore, it always thinks its airflow circuitry is at normal pressure.
One more important variable must be considered. The fuel pressure must rise proportionately to the airflow inside the box or the carburetor will lean out under full boost. To accomplish this, you’ll need to run a fuel pressure regulator that is variable, using a signal from inside the air box to control the fuel pressure in the same relative manner (relative to boost) as the airflow circuits in the carburetor.
There remains one completely fixed variable in this scenario, and that is the maximum airflow capability of your carburetor. If the carburetor is too small (too few cfm) the engine will run lean at higher boost. For that reason, selecting the right size carburetor is very important to maintain a proper fuel curve. Earlier in this book, you can find airflow curves under various boost levels for all Ford V-8s, and your choice of carburetor(s) should be based on the boost available for your engine in terms of displacement and application.
Keep in mind that airflow ratings assigned to most carburetors are listed at the point where the carburetor will actually start to flag, so your target for selecting a particular carburetor should include a margin of about 10 to 15 percent overage. In other words, if the engine airflow at maximum boost is about 700 cfm, you should select a carburetor with a cfm rating somewhere between 770 and 820 cfm. In most cases, this amount of overage is not sufficient