Overlap and LSA...have a simple question
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On The Tree
Joined: Apr 2002
Posts: 179
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From: Baltimore, MD
Overlap and LSA...have a simple question I know that a higher LSA means a higher-quality idle...but I'm confused as to the effects of overlap on the powerband.
My impression was that, all else being equal, when you decreased the LSA (narrowed the separation of the lobes), it would cause more overlap, which leads to more exhaust scavenging at high RPMs and better higher RPM power, at the expense of low-end torque and idle quality, and that when you INCREASE the LSA (wider separation), you decrease the overlap and, as a result, lose some of your scavenging effects and high-end power, but pick up more on the low-end and idle because compression/torque is restored.
I've seen sites that say this, and I've seen sites that say the OPPOSITE.
Bottom line: If you have two cams that have the same specs and one is on a 114 LSA and one is on a 112 LSA, which has more low-end power and which has more high-end power?
My impression was that, all else being equal, when you decreased the LSA (narrowed the separation of the lobes), it would cause more overlap, which leads to more exhaust scavenging at high RPMs and better higher RPM power, at the expense of low-end torque and idle quality, and that when you INCREASE the LSA (wider separation), you decrease the overlap and, as a result, lose some of your scavenging effects and high-end power, but pick up more on the low-end and idle because compression/torque is restored.
I've seen sites that say this, and I've seen sites that say the OPPOSITE.
Bottom line: If you have two cams that have the same specs and one is on a 114 LSA and one is on a 112 LSA, which has more low-end power and which has more high-end power?
On The Tree
Joined: May 2002
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From: Richmond, Virginia
Re: Overlap and LSA...have a simple question I went through this years ago with a small block. It seems that with less overlap, you increase the cylinder pressures at the low end. This is the reason that the cam mfr's stated minimum recommended compression ratios for their radical cams with a lot of overlap. I had a small block that I wanted to go more radical but still be able to start and idle since it had an automatic and power brakes. The cam mfr recommended a cam with a 114 LSA, which at that time was somewhat radical. I had a cam in with a lower LSA but it would not idle below 1400 RPM in neutral so you would get 900 RPM in drive, and it had a 3500 RPM Stall TC. Anyway checking the cylinder pressures with the low LSA, I found them to be below 100 psi. The mfr had me advance the cam which brought them up a little, but still was low, so I switched out the cam and life was good. The cylinder pressures came up to 150 PSI. Now you have to be careful, because if you bring the cylinder pressures up to high you will go nuts avoiding detonation and will lose so much power it is not worth the effort. Best to let the Mfr know what your engine has, compression and modification wise and go with their recommendation, assuming they are a reputable cam mfr.
TECH Fanatic
Joined: Nov 2001
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From: Corona CA
Re: Overlap and LSA...have a simple question Here's what I learned for all 4 stroke engines:
Given the same duration of intake and exhaust opening, a bigger overlap (less LSA) causes poor idle and higher torque/wider powerband.
The idle problem is caused by "8-stroking". The valves are both open, and the piston comes up, forcing some of the exhaust gases up the intake runner. The piston comes down, and sucks in a partial charge, which does not ignite (misfire). Now you have fuel/air mixture still in the cylinder under fairly low pressure when both valves are open again. This time, you get a better, denser charge in the cylinder, so it fires. All automotive/motorcycle cams will "8-stroke" at a given RPM. The greater the overlap, the higher the RPM this happens at.
About the powerband: With less overlap, the intake valve opens very early, and the exhaust closes very late. This aims the powerband up in the RPM range. But since the intake closes early, and the exhaust opens late, you don't get as good of a charge as you should. So you get a narrower, higher powerband, yet about the same peak power (if not less) than a large overlap cam.
Why use a small overlap cam? Emissions, idle quality are both improved. If these are not important, use the bigger overlap.
IMO...
Given the same duration of intake and exhaust opening, a bigger overlap (less LSA) causes poor idle and higher torque/wider powerband.
The idle problem is caused by "8-stroking". The valves are both open, and the piston comes up, forcing some of the exhaust gases up the intake runner. The piston comes down, and sucks in a partial charge, which does not ignite (misfire). Now you have fuel/air mixture still in the cylinder under fairly low pressure when both valves are open again. This time, you get a better, denser charge in the cylinder, so it fires. All automotive/motorcycle cams will "8-stroke" at a given RPM. The greater the overlap, the higher the RPM this happens at.
About the powerband: With less overlap, the intake valve opens very early, and the exhaust closes very late. This aims the powerband up in the RPM range. But since the intake closes early, and the exhaust opens late, you don't get as good of a charge as you should. So you get a narrower, higher powerband, yet about the same peak power (if not less) than a large overlap cam.
Why use a small overlap cam? Emissions, idle quality are both improved. If these are not important, use the bigger overlap.
IMO...
Re: Overlap and LSA...have a simple question </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by squirrels:
<strong>I know that a higher LSA means a higher-quality idle...but I'm confused as to the effects of overlap on the powerband.
My impression was that, all else being equal, when you decreased the LSA (narrowed the separation of the lobes), it would cause more overlap, which leads to more exhaust scavenging at high RPMs and better higher RPM power, at the expense of low-end torque and idle quality, and that when you INCREASE the LSA (wider separation), you decrease the overlap and, as a result, lose some of your scavenging effects and high-end power, but pick up more on the low-end and idle because compression/torque is restored.
I've seen sites that say this, and I've seen sites that say the OPPOSITE.
Bottom line: If you have two cams that have the same specs and one is on a 114 LSA and one is on a 112 LSA, which has more low-end power and which has more high-end power?</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">112 will have more low end and the 114 will have more high end and a better idle.
on top of that. If you advance the cam you will gt more low end, and if you retard the cam you will get more high end power
I hope that helps
As for the overlap discussion. Having to much overlap will hurt low end toque because you are just sucking air/fuel thru the chamber ans out the exahust at the low end. Overlap is directly related the idle quality, but a few other things as well. The reason some overlap helps is that the exahust creates a low pressure area in the cylinder on top of the low pressure generated by the cylinder going down in the bore. This starts the air moving into the cylinder from the intake. If you have too much overlap then you will not seal the exahust valve up at the right time and it will take the charge out of the cylinder and thru to the other side. If you ever look at a blower cam you will notice there is very little overlap because the air/fuel doesn't need much help getting into the cylinder. That's what the blower is for.
If I was looking to have a cam that did everything well (including make tons of HP) I would want as little overlap as I needed, some very vertical ramps on the lobes alot of lift and little duration. This would give me great TQ belwo the HP peak and enough flow to get the air/fuel in there at the top end. This is what you really want to make your car go faster.
Bret
<small>[ July 02, 2002, 01:56 PM: Message edited by: SStrokerAce ]</small>
<strong>I know that a higher LSA means a higher-quality idle...but I'm confused as to the effects of overlap on the powerband.
My impression was that, all else being equal, when you decreased the LSA (narrowed the separation of the lobes), it would cause more overlap, which leads to more exhaust scavenging at high RPMs and better higher RPM power, at the expense of low-end torque and idle quality, and that when you INCREASE the LSA (wider separation), you decrease the overlap and, as a result, lose some of your scavenging effects and high-end power, but pick up more on the low-end and idle because compression/torque is restored.
I've seen sites that say this, and I've seen sites that say the OPPOSITE.
Bottom line: If you have two cams that have the same specs and one is on a 114 LSA and one is on a 112 LSA, which has more low-end power and which has more high-end power?</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">112 will have more low end and the 114 will have more high end and a better idle.
on top of that. If you advance the cam you will gt more low end, and if you retard the cam you will get more high end power
I hope that helps
As for the overlap discussion. Having to much overlap will hurt low end toque because you are just sucking air/fuel thru the chamber ans out the exahust at the low end. Overlap is directly related the idle quality, but a few other things as well. The reason some overlap helps is that the exahust creates a low pressure area in the cylinder on top of the low pressure generated by the cylinder going down in the bore. This starts the air moving into the cylinder from the intake. If you have too much overlap then you will not seal the exahust valve up at the right time and it will take the charge out of the cylinder and thru to the other side. If you ever look at a blower cam you will notice there is very little overlap because the air/fuel doesn't need much help getting into the cylinder. That's what the blower is for.
If I was looking to have a cam that did everything well (including make tons of HP) I would want as little overlap as I needed, some very vertical ramps on the lobes alot of lift and little duration. This would give me great TQ belwo the HP peak and enough flow to get the air/fuel in there at the top end. This is what you really want to make your car go faster.
Bret
<small>[ July 02, 2002, 01:56 PM: Message edited by: SStrokerAce ]</small>
