I got asked "why lower compression ratios work better with superchargers & TURBOS"
Id guess the best way to answer that is to point out what a supercharger or turbo does to build the extra potential power in the first place, and that's to pack MORE AIR AND FUEL,INTO THE COMBUSTION CHAMBER, AIR THAT CONTAINS OXYGEN packed into the cylinders on each individual compression stroke that can be mixed with fuel and burnt to produce cylinder pressure, MORE than a non- boosted engine can do, which is basically limited to your displacement.
the standard atmosphere at sea level has approximately 21% OXYGEN content and its the 21% OXYGEN content that can be mixed with gasoline as fuel to produce the burn and pressure that forces the piston rapidly away from the combustion chamber on the power stroke.
that pressure is what produces the potential torque, and its a direct result of the volume of the fuel/air mix being efficiently burn.
yes you can reduce, heat and the resulting cylinder pressure with an inter cooler that drops fuel/air temperatures, or use longer cam timing and a wide LSA to bleed off low rpm cylinder pressure, but you'll push a great deal of wasted f/a mix out into the exhaust if you do so, without understanding the concepts correctly
that pressure from burning the extra fuel/air mix , and the greater volume of oxygen that it contains that's trapped above the piston produces usable power up to the point of reaching the pressure and heat levels where detonation results if any more pressure or heat is generated, its NOT THE BOOST PRESSURE its the added OXYGEN and the FUEL that can be BURN with that added OXYGEN, due to the larger volume of air that produces the potential hp gains
lets as an example say we are dealing with two similar 383 engines, one at about 11:1 compression is naturally aspirated, where each cylinder holds roughly 48 cubic inches of compressed fuel/air mix, the second has 6:1 compression ratio, all the same specs but a piston designed to produce lower compression, the combustion chamber area above the piston on the supercharged engine will need to be approximately double that of the higher compression engines volume to achieve the lower compression ratio, but the supercharger or turbo will pack that 48 cubic inch volume with about 90 cubic inches of fuel air mix
yes your supercharged engine DOES tend to show a slightly richer plug color and fuel/air ratio meters do tend to indicate your effective mixture is a bit richer for the exact reason you stated.
thats one reason super charger cams tend to have a wide 112-116 LSA, so that theres a bit less overlap time where compressed boost could enter and partly exit into the exhaust port while both valves were open
in the first case we have 1/2 the volume of fuel/air mix at 11:1 cpr or about 180 psi before ignition takes place
in the second case we have almost twice the combustion chamber volume of boosted fuel/air mix to burn at 11:1 effective cpr or about 180 psi before ignition takes place, with about 12 psi of boost pressure filling the cylinder
we can at least in theory burn more than three times the volume of fuel/air mix that's crammed into the larger combustion chamber and produce greater power with the supercharger,or turbo, in the real world theres a great deal more to this and burning twice to three times the amount won,t work as a set ratio due to heat/pressure, and detonation limits but theres still a great deal more power produced with the supercharger, because the larger volume of the combustion chamber volume in the lower compression ratio engine that allows more fuel air mix to be burnt, per power stroke, before you get into detonation.
been there, ( helping tune and trouble shoot ,BOOSTED LT1 engines) several times, so let me point out a few things
there are fairly well established limits to the temperature and cylinder pressure that an engine will tolerate, before you get into detonation at a given altitude , engine temperature and fuel octane, and ignition advance.
the higher the static compression ratio the less boost can be applied before those limits are exceeded, if your starting with something like a 10:1 cpr your at a distinct disadvantage compared to something like an 8:1 cpr if your intending to make big gains thru supercharging simply because you run into problems with detonation far faster, now IM not about to say you can,t make significant gains, over a n/a engine but you still need to stay within the limits the fuels octane, engine temperature and detonation limits you too. ,And a purpose built engine of similar design with components selected strictly to work with a supercharger or turbo will produce far better results than slapping a turbo or supercharger on a high compression ratio engine combo designed to maximize the n/a performance. select larger port less restrictive cylinder heads, lower compression pistons and a fuel supply system designed to fill the engines demands, and a cam designed to work with a supercharger or turbo and your in a totally different ball park from the guys who try to mod the stock n/a engine and get it to work, build the combo with that turbo or supercharger as a matched component and the difference can easily be several hundred extra horsepower over and above whats possible with the stock engine
probably 90% of the useful infos in the links, and use of posted calculators , so take the time to read thru them carefully, if you don,t understand the basic concepts you'll fall for the B.S. that gets constantly repeated on different sites, you can do the MATH and DETERMINE the cylinder pressure curves and you'll quickly find out why boost, high compression and low octane pump gas won,t play well together.
it doesn,t mater if your running high static, compression or you boost the cylinder pressure with a turbo or supercharger, theres a limit to the cylinder pressure, heat and ignition advance curve with a given octane, before detonation results, and that will quickly result in busted pistons and rings
YOU CAN,T just randomly start slapping components together and then add a supercharger or turbo and expect to run trouble free without regard to the laws of physics and thermal dynamics.
you better start thinking about use of an inter cooler to lower air temps and alcohol or toluene, added to the fuel injection to raise the octane levels, if your adding significant boost levels
viewtopic.php?f=52&t=727
http://auto.howstuffworks.com/supercharger1.htm
http://en.wikipedia.org/wiki/Internal_combustion_engine
viewtopic.php?f=86&t=1644&hilit=+detonation
http://www2.zhome.com:81/ZCMnL/PICS/det ... ation.html
http://victorylibrary.com/mopar/otto-c.htm
http://shop.brutespeed.com/85-91-Corvet ... 98_c20.htm
viewtopic.php?f=52&t=1343&hilit=+detonation
http://victorylibrary.com/mopar/cam-tech-c.htm
http://kb-silvolite.com/calc.php?action=comp
http://www.crower.com/misc/valve_timing_chart.html#112
http://www.kb-silvolite.com/calc.php?action=comp2
http://www.wallaceracing.com/boost-comp ... o-calc.php
http://www.rbracing-rsr.com/compression.htm
http://www.dragsource.com/index.php?nav ... ctoview=3#
http://www.turbofast.com.au/TFcompB.html
http://www.rbracing-rsr.com/comprAdvHD.htm
Id guess the best way to answer that is to point out what a supercharger or turbo does to build the extra potential power in the first place, and that's to pack MORE AIR AND FUEL,INTO THE COMBUSTION CHAMBER, AIR THAT CONTAINS OXYGEN packed into the cylinders on each individual compression stroke that can be mixed with fuel and burnt to produce cylinder pressure, MORE than a non- boosted engine can do, which is basically limited to your displacement.
the standard atmosphere at sea level has approximately 21% OXYGEN content and its the 21% OXYGEN content that can be mixed with gasoline as fuel to produce the burn and pressure that forces the piston rapidly away from the combustion chamber on the power stroke.
that pressure is what produces the potential torque, and its a direct result of the volume of the fuel/air mix being efficiently burn.
yes you can reduce, heat and the resulting cylinder pressure with an inter cooler that drops fuel/air temperatures, or use longer cam timing and a wide LSA to bleed off low rpm cylinder pressure, but you'll push a great deal of wasted f/a mix out into the exhaust if you do so, without understanding the concepts correctly
that pressure from burning the extra fuel/air mix , and the greater volume of oxygen that it contains that's trapped above the piston produces usable power up to the point of reaching the pressure and heat levels where detonation results if any more pressure or heat is generated, its NOT THE BOOST PRESSURE its the added OXYGEN and the FUEL that can be BURN with that added OXYGEN, due to the larger volume of air that produces the potential hp gains
lets as an example say we are dealing with two similar 383 engines, one at about 11:1 compression is naturally aspirated, where each cylinder holds roughly 48 cubic inches of compressed fuel/air mix, the second has 6:1 compression ratio, all the same specs but a piston designed to produce lower compression, the combustion chamber area above the piston on the supercharged engine will need to be approximately double that of the higher compression engines volume to achieve the lower compression ratio, but the supercharger or turbo will pack that 48 cubic inch volume with about 90 cubic inches of fuel air mix
yes your supercharged engine DOES tend to show a slightly richer plug color and fuel/air ratio meters do tend to indicate your effective mixture is a bit richer for the exact reason you stated.
thats one reason super charger cams tend to have a wide 112-116 LSA, so that theres a bit less overlap time where compressed boost could enter and partly exit into the exhaust port while both valves were open
in the first case we have 1/2 the volume of fuel/air mix at 11:1 cpr or about 180 psi before ignition takes place
in the second case we have almost twice the combustion chamber volume of boosted fuel/air mix to burn at 11:1 effective cpr or about 180 psi before ignition takes place, with about 12 psi of boost pressure filling the cylinder
we can at least in theory burn more than three times the volume of fuel/air mix that's crammed into the larger combustion chamber and produce greater power with the supercharger,or turbo, in the real world theres a great deal more to this and burning twice to three times the amount won,t work as a set ratio due to heat/pressure, and detonation limits but theres still a great deal more power produced with the supercharger, because the larger volume of the combustion chamber volume in the lower compression ratio engine that allows more fuel air mix to be burnt, per power stroke, before you get into detonation.
been there, ( helping tune and trouble shoot ,BOOSTED LT1 engines) several times, so let me point out a few things
there are fairly well established limits to the temperature and cylinder pressure that an engine will tolerate, before you get into detonation at a given altitude , engine temperature and fuel octane, and ignition advance.
the higher the static compression ratio the less boost can be applied before those limits are exceeded, if your starting with something like a 10:1 cpr your at a distinct disadvantage compared to something like an 8:1 cpr if your intending to make big gains thru supercharging simply because you run into problems with detonation far faster, now IM not about to say you can,t make significant gains, over a n/a engine but you still need to stay within the limits the fuels octane, engine temperature and detonation limits you too. ,And a purpose built engine of similar design with components selected strictly to work with a supercharger or turbo will produce far better results than slapping a turbo or supercharger on a high compression ratio engine combo designed to maximize the n/a performance. select larger port less restrictive cylinder heads, lower compression pistons and a fuel supply system designed to fill the engines demands, and a cam designed to work with a supercharger or turbo and your in a totally different ball park from the guys who try to mod the stock n/a engine and get it to work, build the combo with that turbo or supercharger as a matched component and the difference can easily be several hundred extra horsepower over and above whats possible with the stock engine
probably 90% of the useful infos in the links, and use of posted calculators , so take the time to read thru them carefully, if you don,t understand the basic concepts you'll fall for the B.S. that gets constantly repeated on different sites, you can do the MATH and DETERMINE the cylinder pressure curves and you'll quickly find out why boost, high compression and low octane pump gas won,t play well together.
it doesn,t mater if your running high static, compression or you boost the cylinder pressure with a turbo or supercharger, theres a limit to the cylinder pressure, heat and ignition advance curve with a given octane, before detonation results, and that will quickly result in busted pistons and rings
YOU CAN,T just randomly start slapping components together and then add a supercharger or turbo and expect to run trouble free without regard to the laws of physics and thermal dynamics.
you better start thinking about use of an inter cooler to lower air temps and alcohol or toluene, added to the fuel injection to raise the octane levels, if your adding significant boost levels
viewtopic.php?f=52&t=727
http://auto.howstuffworks.com/supercharger1.htm
http://en.wikipedia.org/wiki/Internal_combustion_engine
viewtopic.php?f=86&t=1644&hilit=+detonation
http://www2.zhome.com:81/ZCMnL/PICS/det ... ation.html
http://victorylibrary.com/mopar/otto-c.htm
http://shop.brutespeed.com/85-91-Corvet ... 98_c20.htm
viewtopic.php?f=52&t=1343&hilit=+detonation
http://victorylibrary.com/mopar/cam-tech-c.htm
http://kb-silvolite.com/calc.php?action=comp
http://www.crower.com/misc/valve_timing_chart.html#112
http://www.kb-silvolite.com/calc.php?action=comp2
http://www.wallaceracing.com/boost-comp ... o-calc.php
http://www.rbracing-rsr.com/compression.htm
http://www.dragsource.com/index.php?nav ... ctoview=3#
http://www.turbofast.com.au/TFcompB.html
http://www.rbracing-rsr.com/comprAdvHD.htm
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