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In this series, I will not repeat the information that is already available on how superchargers work, but instead focus on their selection, installation, use and tuning on older motorcycle engines. Here I will make various observations and comments on function, and in some cases how to make improvements. New material will be added regularly, and eventually better organized. Click below to jump directly to an individual topic. |
Supercharger Topics | |
Function |
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Purpose |
Supercharging simply means forcing more air into an engine than is possible by only atmospheric pressure. Atmospheric pressure, not vacuum, is what fills an engine with combustible mixture to be burned. This is about 14.7 lbs. per square inch (29.92 inches of mercury) at sea level. When an intake valve opens on an empty cylinder, atmospheric pressure fills it - not the engine.
What can you expect from a supercharger? More power in every RPM range, power immediately without the need to rev the engine, far more power than can be produced without some external source (nitrous &c.). How much power? The limit is generally related to the head gasket seal and compression ratio, but 1.5 HP per cubic inch is easy, 2 HP per cubic inch is fairly common, and 3 HP per cubic inch is possible. That’s a 750cc making 135 HP! Yes, it can be done by the traditional methods (carburetion, cam, compression), but a supercharged engine lasts longer at the same power level, requires less expensive speed equipment, and is generally easier and less critical to design and build.
Superchargers have been successfully used on motorcycles for over 75 years, it’s not a new idea. The type of mechanism used to compress the gas varies with the design. The complexity of the subject makes it lengthy, so for ease of reading I will cover the topics in separate pages.
There are few superchargers, and even fewer commercial kits suitable for motorcycle use currently available. For older motorcycles, the real problem is not the source of the supercharger itself (called the “head unit”, meaning the supercharger itself with no drive mechanism, adaptors or mounts), many used superchargers are still available and some are still in production. The difficult engineering task is to select a type, size and model that best serves your purpose, drive it from the engine, and re-locate (or substitute) some original components to permit mounting it.
I will not explore the kits on the market, as they not only rather expensive but would require major modification to both the kit parts and the motorcycle to adapt to an older model.
There are several different design concepts used by superchargers with advantages and disadvantages to each. There is no “best”.
Power developed by boost is much kinder to the engine than power developed naturally aspirated (“NA”). Maximum combustion pressure occurs later in the cycle and continues longer making a long “push” rather than a quick “bang”. NA motors also require much higher RPM to make maximum power, which breaks parts faster than pure power.
Any boost pressure has the effect of raising the static or nominal compression ratio, and therefore raises the octane requirement. This places a limit on the combination of compression ratio and boost versus the octane value of the fuel chosen; if either boost or compression is raised the fuel’s knock resistance must be examined and improved as indicated. Engines with low compression ratio and mild boost will still be within range of pump gasoline, but all must be analyzed on an individual basis.
The combined effect is difficult to quantify, but is explored here:  .
How much power?
What can you expect from a supercharger? More power in every RPM range, power immediately without the need to rev the engine, far more power than can be produced without some external source (nitrous &c.). How much power? The limit is generally related to the quality of the head gasket seal, and the compression ratio, but 2 horsepower per in3 (235” = 470 hp) is quite possible. Yes, it can be done by the traditional methods (hot cam, increased compression, more carburetion), but a supercharged engine lasts longer at the same power level, requires less expensive speed equipment, and is generally easier and less critical to design and build.
Power by increased compression
Since the cylinder volume and the combustion chamber volume are both pressurized to the same extent, the original compression ratio remains unchanged by boost pressure, and the compression ratio is not increased. However, cylinder pressure and knock sensitivity are increased due to greater charge density.
Power by increased charge density
Instead, the same two volumes hold more molecules of both fuel and air at higher density so the mass is increased, providing a large part of the power increase.
Despite what has been printed on the subject, the power added by boost is not the mathematical product of (atmospheric pressure + boost) ÷ atmospheric pressure. E.g., ATM = 14.7 psi, boost = 14.7 psi, therefore power is doubled. This is complete rubbish, although it appears in much popular “technical” literature. In all cases, the power increase will be much less due to thermal expansion and pumping loss, and the power increase decays with higher levels of boost (the first few pounds of boost are the most effective in terms of added horsepower per psi).
The claims of large power increases with mild boost must be considered as to the basis of comparison. The increase in delivery varies as the square root of the change in pressure. Unless there are other changes to the engine, 4 psi of boost pressure is only about 90% efficient in adding volume to the engine; the engine receives about 3.6 psi in terms of density, and in this case adds about 15% to the power, but that's compared to a very low restriction (large) carburetor system showing perhaps .75 psi (1.5” Hg) vacuum at WOT. The more restrictive the original normally aspirated carburetor was at WOT, the greater the increase by comparison: adding 4 psi of boost pressure to a high-restriction (small) carburetor running at 2 psi (4” Hg) vacuum at WOT will perhaps gives 20% more power. By comparison, the popular prediction is merely the above error repeated in miniature by adding 4 (boost) to 14.7 (for atmospheric pressure), then dividing by 14.7 for 127%: (14.7 + 4) ÷ 14.7 = 1.27.
The popular prediction is that 7 psi of boost pressure adds 48%: (14.7 + 7) ÷ 14.7 = 1.48. Actually, 7 psi is perhaps 80% efficient; the engine receives about 5.6 psi in terms of density. Compared to a large carburetor pulling .75 psi of vacuum at WOT, it's worth about 21% more power. Compared to a small carburetor with 2 psi vacuum at WOT perhaps 26% more power.
For a better but still approximate estimate of boost vs. power, the first few pounds of boost is about 85-95% efficient, which tapers off to 50% by 15 psi. Multiply the boost pressure by the efficiency to get the relative density index. Now estimate the WOT vacuum of the carbureted system in normally aspirated mode. Find your actual local atmospheric pressure (I use 14.7 psi for convenience - which is only true at sea level). Where:
HP+: horsepower increase
ATM: local atmospheric pressure (RAD is preferred, but difficult to calculate)
D: relative density index (boost pressure × efficiency)
V: full throttle vacuum, normally aspirated (typically between .75 psi and 3 psi), then
HP+ = ((ATM × D) ÷ (ATM - V)).5
For another example, 10 psi of boost (estimated at 65% efficiency) is applied to an engine that pulls 2 psi of vacuum at WOT. With elevation at 1,000 feet, local atmospheric pressure is 14.16 psi. For “D”: 10 psi × .65 = 6.5. For “ATM + D”: 14.16 + 6.5 = 20.66. For “ATM - V”: 14.16 - 2 = 12.16. 20.66 ÷ 12.16 = 1.699. For “HP+”: the square root of 1.699 is 1.303; adding 10 psi in this case adds about 30.3% power.
This calculation does not include the effect of boost pressure on volumetric efficiency.
Power by increased VE
Volumetric efficiency is always increased by boost, and in some cases will exceed 100%. This is due to the fact that the cylinder includes the volume of the combustion chamber, so the total volume can be higher than the nominal displacement (especially when the compress ratio is low). E.g., a 39.25” cylinder with 8:1 compression ratio has a chamber volume of 5.61”, totaling 44.86”, or 14.3% larger than its displacement.
In normally aspirated mode the chamber is still (almost) full of exhaust gas from the previous cycle, and the 39.25” cylinder volume is the maximum available at 100% VE. However, positive boost pressure (and carefully chosen camshaft events) “flush” the residual exhaust gas from the chamber, and permits extra volume of fresh mixture to be introduced increasing VE by as much as that 14.3%, raising VE to 114.3%, &c.
Under ideal conditions, the factors of increased VE and increased density operate in sequence, and the power is the product of their multiplication. The 30.3% example (mentioned earlier) produced by 10 psi boost, multiplied by the 114.3% VE, can result in a power increase of 48.9%.
Why not a turbocharger?
Very few turbos are small enough to work on 500-900cc engines (exceptions include IHI for Subaru, Geo Sprint, T15, etc.)
A turbo is not “free power” from the exhaust - the exhaust load is higher than NA (normally aspirated) under all conditions, which means more heat and pumping loss [superchargers do consume more power, but also have a slight cooling effect under boost].
Without a bypass system and a small (restrictive) turbine housing there is no boost at low speed.
The installation is much more difficult:
» The entire exhaust system must be gas-tight (exhaust pressure = boost pressure × 2) from the
port down.
» The exhaust system must be re-routed away from the engine, fuel, rider etc. [the stock exhaust
works fairly well on a supercharged application].
» With few exceptions they need pressure oil, and oil return which means either a separate pump
and sump, or a position higher than the existing oil tank [most superchargers have an internal
lubrication system, or sealed bearings].
» Sizing of the entire turbo, compressor trim, turbine housing size and A/R ratio are far more critical
than any supercharger. A mistake (no boost, low boost, overheating, too much boost) cannot be
cured by adjustment but by replacing expensive parts - perhaps several times [many supercharger
adjustments only require a single pulley or sprocket change]. |
See these Victory Library booklets |
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