Please explain advancing and retarding cam geometry...
06-27-2004, 02:24 PM
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Please explain advancing and retarding cam geometry... I forget which way it goes. It's been a long time since I played with cam timing. Does advancing the cam move the powerband up or down the rpm scale? Seems it was down, but I forget.
06-27-2004, 02:50 PM
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You are correct. Advancing the cam moves powerband down. Retarding the cam moves it up.
The hard part with adjusting cam timing with with multi-keyway crank gears is figuring out where the new dot-to-dot marks belong. I wanted to avoid that hassle so I got a hex-adjust cam gear. Almost idiot proof, LOL.
Another thing I have noticed is that large cams like advance and small cams like retard. Optimally spec'd cams would probably be best "straight up". I think part of the reason why brand new LS1s feel stronger after a few miles are racked up is because of chain stretch retarding the small stock cam. Just my opinion!
The hard part with adjusting cam timing with with multi-keyway crank gears is figuring out where the new dot-to-dot marks belong. I wanted to avoid that hassle so I got a hex-adjust cam gear. Almost idiot proof, LOL.
Another thing I have noticed is that large cams like advance and small cams like retard. Optimally spec'd cams would probably be best "straight up". I think part of the reason why brand new LS1s feel stronger after a few miles are racked up is because of chain stretch retarding the small stock cam. Just my opinion!
06-27-2004, 06:58 PM
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In essence a smaller LSA is a smaller cam but with more "overlap". Back when I got into motors, things were measured in overlap, opening, closing, and lift. The LSA number is related to overlap, but not really. A 112 LSA means something different on a 240/240 cam than it does on a 200/200 cam. It's the angle of the lobe peaks relative to each other.
06-27-2004, 10:25 PM
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Originally Posted by JZ'sTA
I have also heard that the LSA of the cam affects the powerband as well.
EX a 224/224 on a 110 with no advance built in would make peak power a couple hundred RPM's before a 224/224 on a 114 with no advance built in.
Is this true?
EX a 224/224 on a 110 with no advance built in would make peak power a couple hundred RPM's before a 224/224 on a 114 with no advance built in.
Is this true?
06-27-2004, 10:39 PM
#6
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If you retard the cam you are delaying BOTH the intake and exhaust valve events in relation to the crankshaft.
If you use a wider LSA all you are doing is advancing the exhaust valve open/close while retarding the intake valve open/close.
A narrower LSA delays the exhaust open/close and advances the intake open/close.
So, let's say we had a cam with a 112 LSA and no advance ground in. Now let's install the same cam except it has a 116 LSA and 2 degrees of advance. What has happened? Our exhaust valve now opens and closes 4 degrees sooner while our intake valve opens and closes exactly as it was before.
The result?
1. We have a MUCH smoother idling engine. That much is FOR SURE. Alot less overlap.
2. The engine gets better gas mileage due to less overlap.
3. The engine has cleaner emissions due to less overlap.
4. The HP peak is probably at about the same RPM. The intake valve events are the same.
5. The HP falls off slower after the HP peak due to less overlap. Higher shift points will be needed to best take advantage of the powerband.
6. The engine makes less HP throughout the midrange due to having less overlap (less scavenging effect.)
7. The engine runs smoother and makes better power at extremely low RPMs due to less overlap (higher cylinder pressure due to less bleedoff.)
If you use a wider LSA all you are doing is advancing the exhaust valve open/close while retarding the intake valve open/close.
A narrower LSA delays the exhaust open/close and advances the intake open/close.
So, let's say we had a cam with a 112 LSA and no advance ground in. Now let's install the same cam except it has a 116 LSA and 2 degrees of advance. What has happened? Our exhaust valve now opens and closes 4 degrees sooner while our intake valve opens and closes exactly as it was before.
The result?
1. We have a MUCH smoother idling engine. That much is FOR SURE. Alot less overlap.
2. The engine gets better gas mileage due to less overlap.
3. The engine has cleaner emissions due to less overlap.
4. The HP peak is probably at about the same RPM. The intake valve events are the same.
5. The HP falls off slower after the HP peak due to less overlap. Higher shift points will be needed to best take advantage of the powerband.
6. The engine makes less HP throughout the midrange due to having less overlap (less scavenging effect.)
7. The engine runs smoother and makes better power at extremely low RPMs due to less overlap (higher cylinder pressure due to less bleedoff.)
06-27-2004, 10:40 PM
#7
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"If changing the LSA from 114 to 110, the intake valve closing event occurs one or two degrees earlier, then that would drop the peak power a couple hundred rpms."
Right, two degrees earlier if degreed the same way. The other two degrees is on the exhaust side.
Right, two degrees earlier if degreed the same way. The other two degrees is on the exhaust side.
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06-28-2004, 10:52 PM
#8
Originally Posted by Colonel
If you retard the cam you are delaying BOTH the intake and exhaust valve events in relation to the crankshaft.
If you use a wider LSA all you are doing is advancing the exhaust valve open/close while retarding the intake valve open/close.
A narrower LSA delays the exhaust open/close and advances the intake open/close.
So, let's say we had a cam with a 112 LSA and no advance ground in. Now let's install the same cam except it has a 116 LSA and 2 degrees of advance. What has happened? Our exhaust valve now opens and closes 4 degrees sooner while our intake valve opens and closes exactly as it was before.
The result?
1. We have a MUCH smoother idling engine. That much is FOR SURE. Alot less overlap.
2. The engine gets better gas mileage due to less overlap.
3. The engine has cleaner emissions due to less overlap.
4. The HP peak is probably at about the same RPM. The intake valve events are the same.
5. The HP falls off slower after the HP peak due to less overlap. Higher shift points will be needed to best take advantage of the powerband.
6. The engine makes less HP throughout the midrange due to having less overlap (less scavenging effect.)
7. The engine runs smoother and makes better power at extremely low RPMs due to less overlap (higher cylinder pressure due to less bleedoff.)
If you use a wider LSA all you are doing is advancing the exhaust valve open/close while retarding the intake valve open/close.
A narrower LSA delays the exhaust open/close and advances the intake open/close.
So, let's say we had a cam with a 112 LSA and no advance ground in. Now let's install the same cam except it has a 116 LSA and 2 degrees of advance. What has happened? Our exhaust valve now opens and closes 4 degrees sooner while our intake valve opens and closes exactly as it was before.
The result?
1. We have a MUCH smoother idling engine. That much is FOR SURE. Alot less overlap.
2. The engine gets better gas mileage due to less overlap.
3. The engine has cleaner emissions due to less overlap.
4. The HP peak is probably at about the same RPM. The intake valve events are the same.
5. The HP falls off slower after the HP peak due to less overlap. Higher shift points will be needed to best take advantage of the powerband.
6. The engine makes less HP throughout the midrange due to having less overlap (less scavenging effect.)
7. The engine runs smoother and makes better power at extremely low RPMs due to less overlap (higher cylinder pressure due to less bleedoff.)
Why thank you sir.
Damn now I feel camtarted.
Acturally what you wrote was explained nicely and I understood for the most part.
Thanks again.
