Trade Intake Bias for Duration?
12-09-2006, 02:02 PM
#1
Trade Intake Bias for Duration? I'm looking to understand the benefits, if any, of an intake bias versus going bigger on duration and having to advance the cam to keep a targeted IVC. For discussion, lets look at 2 cams and ssume that they come from the same family of lobes. Assume a street legal car (LS6 Intake, Headers, cats, catback) with a target IVC = 38 and 0* overlap at .05.
cam 1 = 216/224 110 110
cam 2 = 220/224 111 108
The second cam gains 4* more intake duration but now requires 3* advance to get to IVC = 38. It also opens the exhaust valve 4* earlier but closes it earlier too. The first cam is 2* intake biased, the second cam is 2* exhaust biased.
1) What are the relative merits of selecting the smaller intake duration of cam 1 versus cam 2's extra intake duration but being forced to advance cam 2?
2) Why does 4* advance seem to be the limit used in most cams for the LS1?
cam 1 = 216/224 110 110
cam 2 = 220/224 111 108
The second cam gains 4* more intake duration but now requires 3* advance to get to IVC = 38. It also opens the exhaust valve 4* earlier but closes it earlier too. The first cam is 2* intake biased, the second cam is 2* exhaust biased.
1) What are the relative merits of selecting the smaller intake duration of cam 1 versus cam 2's extra intake duration but being forced to advance cam 2?
2) Why does 4* advance seem to be the limit used in most cams for the LS1?
12-09-2006, 05:30 PM
#2
This is a really confusing question and the selections seem arbitrary......
I don't get what you are talking about with "bias", maybe I just don't read enough BS on the LS1 section here to understand your vernacular?
1) If your goal is ONE certain IVC then the smaller duration lobe will work in the lower part of the RPM range better since it's IVO is latter as well. Overlap issues will not effect it as much, but it's also not going to have as much time(duration) to fill the cylinder. You also have more exhaust relative to the intake so that will also help in areas where the exhaust system is worse. Cam 1 will make more TQ below peak TQ than cam 2.
2) 4° of advance is the standard in the industry..... it's used mostly because thats what everone "feels" works the best? Honestly I have no clue why it's the standard, it's not always what I have seen works the best.
Bret
I don't get what you are talking about with "bias", maybe I just don't read enough BS on the LS1 section here to understand your vernacular?
1) If your goal is ONE certain IVC then the smaller duration lobe will work in the lower part of the RPM range better since it's IVO is latter as well. Overlap issues will not effect it as much, but it's also not going to have as much time(duration) to fill the cylinder. You also have more exhaust relative to the intake so that will also help in areas where the exhaust system is worse. Cam 1 will make more TQ below peak TQ than cam 2.
2) 4° of advance is the standard in the industry..... it's used mostly because thats what everone "feels" works the best? Honestly I have no clue why it's the standard, it's not always what I have seen works the best.
Bret
12-09-2006, 06:40 PM
#4
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Originally Posted by SStrokerAce
This is a really confusing question and the selections seem arbitrary......
I don't get what you are talking about with "bias", maybe I just don't read enough BS on the LS1 section here to understand your vernacular?
1) If your goal is ONE certain IVC then the smaller duration lobe will work in the lower part of the RPM range better since it's IVO is latter as well. Overlap issues will not effect it as much, but it's also not going to have as much time(duration) to fill the cylinder. You also have more exhaust relative to the intake so that will also help in areas where the exhaust system is worse. Cam 1 will make more TQ below peak TQ than cam 2.
2) 4° of advance is the standard in the industry..... it's used mostly because thats what everone "feels" works the best? Honestly I have no clue why it's the standard, it's not always what I have seen works the best.
Bret
I don't get what you are talking about with "bias", maybe I just don't read enough BS on the LS1 section here to understand your vernacular?
1) If your goal is ONE certain IVC then the smaller duration lobe will work in the lower part of the RPM range better since it's IVO is latter as well. Overlap issues will not effect it as much, but it's also not going to have as much time(duration) to fill the cylinder. You also have more exhaust relative to the intake so that will also help in areas where the exhaust system is worse. Cam 1 will make more TQ below peak TQ than cam 2.
2) 4° of advance is the standard in the industry..... it's used mostly because thats what everone "feels" works the best? Honestly I have no clue why it's the standard, it's not always what I have seen works the best.
Bret
12-10-2006, 10:25 AM
#5
I don't get what you are talking about with "bias"...
I am not going to drop an idea in publci but start thinking about 23X with an ICL of 114. Might work very well with an LS6 intake.
12-12-2006, 09:26 AM
#6
Joel, I honestly think you try too much stuff, Im not sure exactly WHAT your trying to do though! 
For one why would you want to have an IVC of 38? Unless its got no compression in it, I dont know why you would cut the legs off a LSx motor and make it stop at 6000 rpm. Not trying to give you a hard time, I just dont understand the guys that want something to shift at a stock rpm. These motors turn 6500 with ease and its almost free power.
For one why would you want to have an IVC of 38? Unless its got no compression in it, I dont know why you would cut the legs off a LSx motor and make it stop at 6000 rpm. Not trying to give you a hard time, I just dont understand the guys that want something to shift at a stock rpm. These motors turn 6500 with ease and its almost free power.
12-12-2006, 09:51 AM
#7
Originally Posted by Ragtop 99
The concept of bias is relative to the overlap period. If we have a single pattern cam and the LSA & ICL are the same, the overlap is centered over TDC (assuming symetrical lobes). Advancing a cam makes it more exhaust bias relative to TDC. Retarding a cam makes it more intake bias relative to TDC. From what I've read, an exhaust biased cam tends not to carry as well past peak HP as an intake biased cam.
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12-12-2006, 10:00 AM
#8
Originally Posted by GrannySShifting
...why would you want to have an IVC of 38? Unless its got no compression in it, I dont know why you would cut the legs off a LSx motor and make it stop at 6000 rpm.
If I were building a drag racer (or had 11:1 CR) I'd completely agree with you. I'm building this around the way I drive, which is a lot of intown driving where good throttle response at 2000 - 2500 is more important to me than free power at 6400 rpm. To get good throttle response takes a high DCR and on stock heads that means an earlier IVC then a cam optimized for 6600 shifts. A stock 02 cam IVC is at 34 and with headers it pulls to 6200.
I've had a number of cams and driven other cars with cams. Sure, on a dyno a 228/228 114 will show gains at 3000, but it will be sluggish feeling at part throttle at commuting rpms. I'd rather go with a tighter LSA and pack the power in a narrower and lower rpm band. My current cam does that, but it was set up for stock manifolds. With headers I can run a little more overlap without screwing up the low end, so I'm just upsizing a bit.
I do like to try things,
so who knows, maybe by this time next year I'll join the rank & file members with a 23X cam. 
12-12-2006, 12:15 PM
#12
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Originally Posted by Ragtop 99
Jeff:
If I were building a drag racer (or had 11:1 CR) I'd completely agree with you. I'm building this around the way I drive, which is a lot of intown driving where good throttle response at 2000 - 2500 is more important to me than free power at 6400 rpm. To get good throttle response takes a high DCR and on stock heads that means an earlier IVC then a cam optimized for 6600 shifts. A stock 02 cam IVC is at 34 and with headers it pulls to 6200.
I've had a number of cams and driven other cars with cams. Sure, on a dyno a 228/228 114 will show gains at 3000, but it will be sluggish feeling at part throttle at commuting rpms. I'd rather go with a tighter LSA and pack the power in a narrower and lower rpm band. My current cam does that, but it was set up for stock manifolds. With headers I can run a little more overlap without screwing up the low end, so I'm just upsizing a bit.
I do like to try things, so who knows, maybe by this time next year I'll join the rank & file members with a 23X cam.
If I were building a drag racer (or had 11:1 CR) I'd completely agree with you. I'm building this around the way I drive, which is a lot of intown driving where good throttle response at 2000 - 2500 is more important to me than free power at 6400 rpm. To get good throttle response takes a high DCR and on stock heads that means an earlier IVC then a cam optimized for 6600 shifts. A stock 02 cam IVC is at 34 and with headers it pulls to 6200.
I've had a number of cams and driven other cars with cams. Sure, on a dyno a 228/228 114 will show gains at 3000, but it will be sluggish feeling at part throttle at commuting rpms. I'd rather go with a tighter LSA and pack the power in a narrower and lower rpm band. My current cam does that, but it was set up for stock manifolds. With headers I can run a little more overlap without screwing up the low end, so I'm just upsizing a bit.
I do like to try things,
so who knows, maybe by this time next year I'll join the rank & file members with a 23X cam.