beginner question - boost & compression
Thread Starter
Safety Car
Joined: Dec 2004
Posts: 3,565
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From: stafford country, va. Avatar: Me on turn 3 @ Bristol (The World's Fastest Half-Mile)
beginner question - boost & compression
please forgive me if this is covered in boost 101 but i was wondering....
it's my understanding that when one supercharges an engine they nromally drop the natural compression of the engine then add boost to basically bring it where they want it. why couldn't one keep a medium compression (11:1) and still add just a bit of boost. wouldn't this also add power ? would one be able to achieve the same peak numbers that another would achieve with low compression high boost ? does a turbo help relieve backpressure and therefore also help a supercharger achieve even better results.
it's my understanding that when one supercharges an engine they nromally drop the natural compression of the engine then add boost to basically bring it where they want it. why couldn't one keep a medium compression (11:1) and still add just a bit of boost. wouldn't this also add power ? would one be able to achieve the same peak numbers that another would achieve with low compression high boost ? does a turbo help relieve backpressure and therefore also help a supercharger achieve even better results.
Safety Car
Joined: Oct 2001
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From: Dallas/Valley Ranch TEXAS!!!
Originally Posted by Zig
please forgive me if this is covered in boost 101 but i was wondering....
it's my understanding that when one supercharges an engine they nromally drop the natural compression of the engine then add boost to basically bring it where they want it. why couldn't one keep a medium compression (11:1) and still add just a bit of boost. wouldn't this also add power ? would one be able to achieve the same peak numbers that another would achieve with low compression high boost ?
it's my understanding that when one supercharges an engine they nromally drop the natural compression of the engine then add boost to basically bring it where they want it. why couldn't one keep a medium compression (11:1) and still add just a bit of boost. wouldn't this also add power ? would one be able to achieve the same peak numbers that another would achieve with low compression high boost ?
Jeremy
Burning Brakes
Joined: Sep 2005
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From: St. Lucie, FLORIDA 757 RWHP 694 RWTQ
Originally Posted by Zig
please forgive me if this is covered in boost 101 but i was wondering....
it's my understanding that when one supercharges an engine they nromally drop the natural compression of the engine then add boost to basically bring it where they want it. why couldn't one keep a medium compression (11:1) and still add just a bit of boost. wouldn't this also add power ? would one be able to achieve the same peak numbers that another would achieve with low compression high boost ? does a turbo help relieve backpressure and therefore also help a supercharger achieve even better results.
it's my understanding that when one supercharges an engine they nromally drop the natural compression of the engine then add boost to basically bring it where they want it. why couldn't one keep a medium compression (11:1) and still add just a bit of boost. wouldn't this also add power ? would one be able to achieve the same peak numbers that another would achieve with low compression high boost ? does a turbo help relieve backpressure and therefore also help a supercharger achieve even better results.
Melting Slicks
Joined: Feb 2006
Posts: 3,271
Likes: 182
From: Manassas VA
Originally Posted by Zig
please forgive me if this is covered in boost 101 but i was wondering....
it's my understanding that when one supercharges an engine they normally drop the natural compression of the engine then add boost to basically bring it where they want it. why couldn't one keep a medium compression (11:1) and still add just a bit of boost. wouldn't this also add power ? would one be able to achieve the same peak numbers that another would achieve with low compression high boost ? does a turbo help relieve back pressure and therefore also help a supercharger achieve even better results.
it's my understanding that when one supercharges an engine they normally drop the natural compression of the engine then add boost to basically bring it where they want it. why couldn't one keep a medium compression (11:1) and still add just a bit of boost. wouldn't this also add power ? would one be able to achieve the same peak numbers that another would achieve with low compression high boost ? does a turbo help relieve back pressure and therefore also help a supercharger achieve even better results.
What you have to decide is if the extra expense of new pistons, gaskets etc. is worth the increase. For all out performance the answer is yes. For street driven cars probably not.
Thread Starter
Safety Car
Joined: Dec 2004
Posts: 3,565
Likes: 5
From: stafford country, va. Avatar: Me on turn 3 @ Bristol (The World's Fastest Half-Mile)
please forgive me for my ignorance.
it's been my understanding that detonation can be caused by cylinder temps. being too high. basically the temp get's too high and the fuel self ignites or ignites early, due to residual heat on the plug tip and or other areas in the cylinder.
why would a lower compression ratio allow you to have more volume or air/fuel ?
isn't "the compression ratio a single number that can be used to predict the performance of any internal-combustion engine. It is a ratio between the volume of a combustion chamber and cylinder when the piston is at the bottom of its stroke, and the volume when the piston is at the top of its stroke. The higher the compression ratio, the more mechanical energy an engine can squeeze from its air-fuel mixture. Higher compression ratios, however, also make detonation more likely."
http://en.wikipedia.org/wiki/Compression_ratio
a "supercharger (also known as a blower) is an air compressor used to compress air into the cylinders of an internal combustion engine. The additional mass of oxygen-containing air that is forced into the cylinders improves the volumetric efficiency of the engine which allows the engine to burn more fuel and makes it more powerful."
http://en.wikipedia.org/wiki/Supercharger
doesn't lowering the compression ratio actually reduce the volume (area) ava. in the cylinder
by adding boost aren't you in essence forcing more into the cylinder than what would normall be 'sucked' in. you are increasing the density of the air, forcing more into the cylinder.
wouldn't high compression and too much boost be an easy way to 'explode' the engine. basically, blow it apart from too much internal pressure and the 'other' parts begin to fail.
while low compression and boost allows more 'room' before the pressures become too great.
imo, i would think that the most power could be obtained from a high/med. compression engine with just a few lbs. of added boost. i would think that one would want to keep the lbs of boost just below the failure point.
imo, it sounds like low compression engines use more boost to bring them back up into the same range as their high compression cousins, while attempting to maintain structural integrity.
maybe i'm completely wrong, i'm just trying to figure why it works the way it does before i run out and put one on.
it's been my understanding that detonation can be caused by cylinder temps. being too high. basically the temp get's too high and the fuel self ignites or ignites early, due to residual heat on the plug tip and or other areas in the cylinder.
why would a lower compression ratio allow you to have more volume or air/fuel ?
isn't "the compression ratio a single number that can be used to predict the performance of any internal-combustion engine. It is a ratio between the volume of a combustion chamber and cylinder when the piston is at the bottom of its stroke, and the volume when the piston is at the top of its stroke. The higher the compression ratio, the more mechanical energy an engine can squeeze from its air-fuel mixture. Higher compression ratios, however, also make detonation more likely."
http://en.wikipedia.org/wiki/Compression_ratio
a "supercharger (also known as a blower) is an air compressor used to compress air into the cylinders of an internal combustion engine. The additional mass of oxygen-containing air that is forced into the cylinders improves the volumetric efficiency of the engine which allows the engine to burn more fuel and makes it more powerful."
http://en.wikipedia.org/wiki/Supercharger
doesn't lowering the compression ratio actually reduce the volume (area) ava. in the cylinder
by adding boost aren't you in essence forcing more into the cylinder than what would normall be 'sucked' in. you are increasing the density of the air, forcing more into the cylinder.
wouldn't high compression and too much boost be an easy way to 'explode' the engine. basically, blow it apart from too much internal pressure and the 'other' parts begin to fail.
while low compression and boost allows more 'room' before the pressures become too great.
imo, i would think that the most power could be obtained from a high/med. compression engine with just a few lbs. of added boost. i would think that one would want to keep the lbs of boost just below the failure point.
imo, it sounds like low compression engines use more boost to bring them back up into the same range as their high compression cousins, while attempting to maintain structural integrity.
maybe i'm completely wrong, i'm just trying to figure why it works the way it does before i run out and put one on.
Originally Posted by ronsc1985
You can achieve higher power by lowering the compression ratio and increasing the boost. The reason is simple. The limit on pre spark plug fire cylinder pressure is when you start to detonate. With a lower compression ratio you have a larger volume of air/fuel mixture at top dead center. If the pre fire cylinder pressure is the same between the higher and lower compression cylinder you obviously have more air/fuel mixture to explode and therefore you can make more power. There are some second order effects like increased temperature from higher compression ratios or increased temperature from the compression required to achieve more boost but these are not significant enough to preclude making more power with lower compression and more boost.
What you have to decide is if the extra expense of new pistons, gaskets etc. is worth the increase. For all out performance the answer is yes. For street driven cars probably not.
What you have to decide is if the extra expense of new pistons, gaskets etc. is worth the increase. For all out performance the answer is yes. For street driven cars probably not.
Melting Slicks
Joined: Feb 2006
Posts: 3,271
Likes: 182
From: Manassas VA
Originally Posted by Zig
please forgive me for my ignorance.
it's been my understanding that detonation can be caused by cylinder temps. being too high. basically the temp get's too high and the fuel self ignites or ignites early, due to residual heat on the plug tip and or other areas in the cylinder.
why would a lower compression ratio allow you to have more volume or air/fuel ?
isn't "the compression ratio a single number that can be used to predict the performance of any internal-combustion engine. It is a ratio between the volume of a combustion chamber and cylinder when the piston is at the bottom of its stroke, and the volume when the piston is at the top of its stroke. The higher the compression ratio, the more mechanical energy an engine can squeeze from its air-fuel mixture. Higher compression ratios, however, also make detonation more likely."
http://en.wikipedia.org/wiki/Compression_ratio
a "supercharger (also known as a blower) is an air compressor used to compress air into the cylinders of an internal combustion engine. The additional mass of oxygen-containing air that is forced into the cylinders improves the volumetric efficiency of the engine which allows the engine to burn more fuel and makes it more powerful."
http://en.wikipedia.org/wiki/Supercharger
doesn't lowering the compression ratio actually reduce the volume (area) ava. in the cylinder
by adding boost aren't you in essence forcing more into the cylinder than what would normall be 'sucked' in. you are increasing the density of the air, forcing more into the cylinder.
wouldn't high compression and too much boost be an easy way to 'explode' the engine. basically, blow it apart from too much internal pressure and the 'other' parts begin to fail.
while low compression and boost allows more 'room' before the pressures become too great.
imo, i would think that the most power could be obtained from a high/med. compression engine with just a few lbs. of added boost. i would think that one would want to keep the lbs of boost just below the failure point.
imo, it sounds like low compression engines use more boost to bring them back up into the same range as their high compression cousins, while attempting to maintain structural integrity.
maybe i'm completely wrong, i'm just trying to figure why it works the way it does before i run out and put one on.
it's been my understanding that detonation can be caused by cylinder temps. being too high. basically the temp get's too high and the fuel self ignites or ignites early, due to residual heat on the plug tip and or other areas in the cylinder.
why would a lower compression ratio allow you to have more volume or air/fuel ?
isn't "the compression ratio a single number that can be used to predict the performance of any internal-combustion engine. It is a ratio between the volume of a combustion chamber and cylinder when the piston is at the bottom of its stroke, and the volume when the piston is at the top of its stroke. The higher the compression ratio, the more mechanical energy an engine can squeeze from its air-fuel mixture. Higher compression ratios, however, also make detonation more likely."
http://en.wikipedia.org/wiki/Compression_ratio
a "supercharger (also known as a blower) is an air compressor used to compress air into the cylinders of an internal combustion engine. The additional mass of oxygen-containing air that is forced into the cylinders improves the volumetric efficiency of the engine which allows the engine to burn more fuel and makes it more powerful."
http://en.wikipedia.org/wiki/Supercharger
doesn't lowering the compression ratio actually reduce the volume (area) ava. in the cylinder
by adding boost aren't you in essence forcing more into the cylinder than what would normall be 'sucked' in. you are increasing the density of the air, forcing more into the cylinder.
wouldn't high compression and too much boost be an easy way to 'explode' the engine. basically, blow it apart from too much internal pressure and the 'other' parts begin to fail.
while low compression and boost allows more 'room' before the pressures become too great.
imo, i would think that the most power could be obtained from a high/med. compression engine with just a few lbs. of added boost. i would think that one would want to keep the lbs of boost just below the failure point.
imo, it sounds like low compression engines use more boost to bring them back up into the same range as their high compression cousins, while attempting to maintain structural integrity.
maybe i'm completely wrong, i'm just trying to figure why it works the way it does before i run out and put one on.
What you are missing is that with the higher boost in a lower compression engine you are also adding more fuel to preserve the air to fuel ratio. Don't believe me. Check out the CR and boost pressures used in Top Fuel, Top Alcohol or Sports Compact drag racing. In all those cases they use CR's in the 6-8 region and boost pressures somewhere north of 40-50 psi. While these boost levels are way out of the range for a street car they do illustrate the principle.
