What is considered to big of a cam for 5.3L?
09-15-2018, 09:20 PM
#22
Mine is 218/224@.050 but on a 110LSA, it really doesn't hit the powerband hard till about 3600
. Widening the LSA out to 112 would help the down low, just not make quite as much peak power.
The more compression you can get away with, the less loss of low end you will have. .
. Widening the LSA out to 112 would help the down low, just not make quite as much peak power.
The more compression you can get away with, the less loss of low end you will have. .
Wider LSA = better low/mid-range power + smoother idle.
Tighter LSA = narrow powerband, more low end torque, more chop at idle .
Wider LSA = broad powerband, more high rpm power, smoother idle.
So which is it?
@whitecrew and @beau seem to be contradicting each other here. [/b]
09-24-2018, 07:47 AM
#24
Best thing to do to understand what "wide" vs "tight" LSA does, is to look at the valve events themselves.
Everything is measured off of the intake centerline. EVERYTHING. That means, "wide" LSA puts the exhaust centerline farther from the intake centerline, and the reverse for "tight" LSA.
In the engine cycle, let's say starting at the firing instance of TDC, the engine turns another 180° more or less, and as the piston reaches the bottom, the exh valve opens. As the piston returns toward the top, the exh valve continues to open, and reaches max opening somewhere around halfway up; then as the piston continues to approach the non-firing instance of TDC, the exh valve closes. As the piston passes through TDC again, the int valve begins to open. As the piston descends again, the int continues to open and reaches max opening about halfway down. It closes as the piston reaches BDC again, then the piston begins ascending the 2nd time, and firing occurs as it reaches the top the 2nd time.
As you can see, the exh and int peak opening points are right about ¼ of the engine cycle apart; or, about 90 cam degrees. (the cam turns ½ turn for every full turn of the crank) However, the valve events aren't really ever at the exact TDC, BDC, halfway, etc. points; that's part of the art/science of cam design. Since the engine is a pump, the exact timing of the valve events is intended to optimize the engine's performance as a pump for a specific RPM range. And, just like a pump, that means that if it's optimized for one particular RPM range, it will suffer in some other range. In reality, practically all modern car engines are heavily optimized toward higher RPM operation, with the int opening later to allow for the valve to stay open longer near BDC and take advantage of the inertia of the air in the intake tract to continue filling the cyl; and the exh opening earlier to get that process in progress as soon as possible as well. Therefore the ACTUAL LSA is pretty much always above the "theoretical" 90°; typical real-world values are anywhere from 108° to 116°, with most between 110° and 114°.
Note that the exh opened first, then the int opened afterwards. That means that as the LSA "widens", either the exh opens earlier, or the int later, or both. BUTT... since all of the angles in the "specs" are based off of the int operation, the net effect is ALWAYS that the exh opens earlier. The "int centerline" spec, which is controlled by the cam installer by way of adjustable/selectable timing gears and the like, determines the int centerline (duh); but the LSA, which is the # of cam degrees that the exh is ahead of the int, is controlled by the cam grinder. LSA specifies the relative location of these 2 events, not their absolute values.
So... for any given int centerline, "wider" LSA ALWAYS means, the exh opens earlier. Well, if it does that, then it means that it begins to open significantly BEFORE the piston reaches BDC in the power stroke; which means, it gives up some period of time that the pressure in the cyl is retained and remains able to push on the piston (make torque); which means, torque (engine output) is lowered. OTOH it also means that the cyl has longer to "blow down" out the exh valve, which means that the pressure in the cyl is lower when the int opens, which means that it's easier for the cyl to begin filling; as well as, there's less "reversion" into the intake tract, which is what "lope" comes from. It also optimizes the exh for higher RPMs. A "tighter" LSA produces the reverse of this: higher torque generally (and peak torque at a slightly higher RPM), more lope, peak HP at a lower RPM, and a faster power drop-off once the RPMs pass the peak.
The net-net effects of widening the LSA then, are: better idle quality, lowered torque, wider powerband in the higher-RPM direction, and a less "peaky" output curve with the curve sort of lowered a bit overall and "smeared out" toward the high-RPM end, which makes the car faster if the engine RPM must wander over a wide and uncontrolled range, like normal street driving. A tighter LSA produces the opposite effects: rougher idle, higher torque, peak HP at a lower RPM (narrower RPM range between the peaks) with a faster drop-off above the peak HP RPM, and thus a more "peaky" curve, which will make the car go faster IFF the RPMs are kept within this band as is common in racing situations with gears, converters, etc.
Everything is measured off of the intake centerline. EVERYTHING. That means, "wide" LSA puts the exhaust centerline farther from the intake centerline, and the reverse for "tight" LSA.
In the engine cycle, let's say starting at the firing instance of TDC, the engine turns another 180° more or less, and as the piston reaches the bottom, the exh valve opens. As the piston returns toward the top, the exh valve continues to open, and reaches max opening somewhere around halfway up; then as the piston continues to approach the non-firing instance of TDC, the exh valve closes. As the piston passes through TDC again, the int valve begins to open. As the piston descends again, the int continues to open and reaches max opening about halfway down. It closes as the piston reaches BDC again, then the piston begins ascending the 2nd time, and firing occurs as it reaches the top the 2nd time.
As you can see, the exh and int peak opening points are right about ¼ of the engine cycle apart; or, about 90 cam degrees. (the cam turns ½ turn for every full turn of the crank) However, the valve events aren't really ever at the exact TDC, BDC, halfway, etc. points; that's part of the art/science of cam design. Since the engine is a pump, the exact timing of the valve events is intended to optimize the engine's performance as a pump for a specific RPM range. And, just like a pump, that means that if it's optimized for one particular RPM range, it will suffer in some other range. In reality, practically all modern car engines are heavily optimized toward higher RPM operation, with the int opening later to allow for the valve to stay open longer near BDC and take advantage of the inertia of the air in the intake tract to continue filling the cyl; and the exh opening earlier to get that process in progress as soon as possible as well. Therefore the ACTUAL LSA is pretty much always above the "theoretical" 90°; typical real-world values are anywhere from 108° to 116°, with most between 110° and 114°.
Note that the exh opened first, then the int opened afterwards. That means that as the LSA "widens", either the exh opens earlier, or the int later, or both. BUTT... since all of the angles in the "specs" are based off of the int operation, the net effect is ALWAYS that the exh opens earlier. The "int centerline" spec, which is controlled by the cam installer by way of adjustable/selectable timing gears and the like, determines the int centerline (duh); but the LSA, which is the # of cam degrees that the exh is ahead of the int, is controlled by the cam grinder. LSA specifies the relative location of these 2 events, not their absolute values.
So... for any given int centerline, "wider" LSA ALWAYS means, the exh opens earlier. Well, if it does that, then it means that it begins to open significantly BEFORE the piston reaches BDC in the power stroke; which means, it gives up some period of time that the pressure in the cyl is retained and remains able to push on the piston (make torque); which means, torque (engine output) is lowered. OTOH it also means that the cyl has longer to "blow down" out the exh valve, which means that the pressure in the cyl is lower when the int opens, which means that it's easier for the cyl to begin filling; as well as, there's less "reversion" into the intake tract, which is what "lope" comes from. It also optimizes the exh for higher RPMs. A "tighter" LSA produces the reverse of this: higher torque generally (and peak torque at a slightly higher RPM), more lope, peak HP at a lower RPM, and a faster power drop-off once the RPMs pass the peak.
The net-net effects of widening the LSA then, are: better idle quality, lowered torque, wider powerband in the higher-RPM direction, and a less "peaky" output curve with the curve sort of lowered a bit overall and "smeared out" toward the high-RPM end, which makes the car faster if the engine RPM must wander over a wide and uncontrolled range, like normal street driving. A tighter LSA produces the opposite effects: rougher idle, higher torque, peak HP at a lower RPM (narrower RPM range between the peaks) with a faster drop-off above the peak HP RPM, and thus a more "peaky" curve, which will make the car go faster IFF the RPMs are kept within this band as is common in racing situations with gears, converters, etc.
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