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| Author: | Joshie225 [ Thu Oct 16, 2008 8:24 pm ] |
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It's not a claim, it's a fact. Adiabatic efficiency for an old Roots type blower like Terry runs is about 50%. Simply stated the air going through the Roots blower is heated twice as much as with perfect compression and they suck down a lot of horsepower in the process. Newer blowers made my Eaton are more efficient than the old Jimmy, but still don't match the efficiency of a turbo compressor. One big problem with the Roots blower is that it's a blower and not a compressor. Internal compression superchargers like the Lysholm are more efficient. Turbochargers are usually operated in ranger where they are over 65% efficient and often better with peak efficiency of many turbos now around 80%. Real World it is a big deal. Here is a great link to a bunch of compressor maps that show turbo efficiency and other good reading. http://www.rbracing-rsr.com/turbotech.html Most aftermarket Roots type supercharger systems are not intercooled as most put the carburetor or injection system before the blower which necessitates a short intake tract. There are intercooler systems that go under Roots type blowers, but they are water-to-air and quite expensive. The same intercooling problem exists with draw through turbocharging which makes blow-through turbocharging more popular as they are easier to intercool which allows more boost, denser mixtures and more power. The reduced low speed boost response from a turbocharger as compared to a mechanically driven positive displacement supercharger is a big negative for a lot of people, but turbocharged engines are more efficient and make more power than supercharged engines at a given boost level. Blowers are good at making low to moderate boost (5-10 psi) which makes for a nice power increase and is adequate in many cases, but turbocharged and intercooled engines can run more boost with lower intake temperatures and make more power. |
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| Author: | terrylittlejohn [ Fri Oct 17, 2008 3:56 pm ] |
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is that why most blower stay under 10psi, they are not so efficient and spinning them faster only heats the charge which produces more detonation? |
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| Author: | Joshie225 [ Fri Oct 17, 2008 4:43 pm ] |
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For the most part, yes. Cranking the boost up will see a decrease in adiabatic efficiency which means more heat and more power (in proportion to the boost delivered) required to drive the pump. I was looking at the efficiency plots of the Lysholm 2300R compressorand it's peak efficiency is 66% and that 66% is a very small island. Looking at the map it appears you could operate the 2300R at or above 60% efficiency under most speed and boost conditions. Old GMC superchargers are worse than the internal compression Lysholm. The high helix and teflon sealed blowers are better than the old style Jimmies, but the sealing strips don't last long. The Lysholm compressor has a maximum designed pressure ratio of 2.2:1 which is 17.6 psi boost assuming 14.7 psi atmospheric inlet pressure. Plenty of turbos will do pressure ratios of 3:1 (29.4 psi boost) and have adiabatic efficiencies above 75%. For example the Garrett GT40 56 trim will do 3:1 at 77% efficiency between 48 and 55 lbs/min of air flow. If you only want a pressure ratio of 2.4:1 which is just over 20 psi boost the efficiency is higher and the air flow range is wider. Nowhere on that GT40 turbo map is the efficiency below 65%. A Garrett GT42 will do a 4:1 pressure ratio (44 psi boost) at just under 74% efficiency. On a 77°F (25°C) day with a 50% efficient compressor (like your blower) making 10 psi boost the outlet temperature is 105°C or 221°F. That's a 80°C/144°F temperature rise. A 77% efficient compressor like a Garrett GT series turbo would have an outlet temp of 73°C/163°F for a temp rise of 48°C/86°F. Also, if that engine made 200hp with the blower and the blower took 30hp to drive the turbo engine would make about 240hp if we loose 10hp in exhaust back pressure on the turbo engine. 18hp of the difference alone is due to the cooler, denser charge delivered by the more efficient turbo compressor. |
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