Roller Rocker Suggestion for a 408 w/ big cam
10-12-2012, 08:55 PM
#42
Launching!
Yes. I am aware of that.
Current lift on my cam is .605 intake and .612 exhaust with a 1.7 rocker arm.
With a 1.8 the intake lift changes to .640 and .647 exhaust. Obviously, PTV clearance will need to be checked but the duration of my cam is not huge so in that regard will probably be fine.
I've got Comp 926 springs that I'd like to keep but I'd also like to not spend 1500 dollars on a set of shaft mount rocker arms if I can help it.
OP - Sorry, I don't mean to thread jack. Hope the YTs work for you. They didn't for me.
Current lift on my cam is .605 intake and .612 exhaust with a 1.7 rocker arm.
With a 1.8 the intake lift changes to .640 and .647 exhaust. Obviously, PTV clearance will need to be checked but the duration of my cam is not huge so in that regard will probably be fine.
I've got Comp 926 springs that I'd like to keep but I'd also like to not spend 1500 dollars on a set of shaft mount rocker arms if I can help it.
OP - Sorry, I don't mean to thread jack. Hope the YTs work for you. They didn't for me.
Last edited by FLYZNTN; 10-12-2012 at 09:21 PM.
10-13-2012, 07:34 PM
#43
Lift has nothing to do with PTV.
Changing rocker ratio to 1.8 from 1.7 does much more than adding lift.
It increases the acceleration of the lobe in turn making it much more aggressive. I guarantee if you go to a 1.8 RR from a 1.7 you will have valve float issues from increasing the acceleration and intensity of the lobe. They make lobes specifically for high ratio rockers because of this.
Changing rocker ratio to 1.8 from 1.7 does much more than adding lift.
It increases the acceleration of the lobe in turn making it much more aggressive. I guarantee if you go to a 1.8 RR from a 1.7 you will have valve float issues from increasing the acceleration and intensity of the lobe. They make lobes specifically for high ratio rockers because of this.
10-14-2012, 09:03 AM
#44
Launching!
Lift has nothing to do with PTV.
Changing rocker ratio to 1.8 from 1.7 does much more than adding lift.
It increases the acceleration of the lobe in turn making it much more aggressive. I guarantee if you go to a 1.8 RR from a 1.7 you will have valve float issues from increasing the acceleration and intensity of the lobe. They make lobes specifically for high ratio rockers because of this.
Changing rocker ratio to 1.8 from 1.7 does much more than adding lift.
It increases the acceleration of the lobe in turn making it much more aggressive. I guarantee if you go to a 1.8 RR from a 1.7 you will have valve float issues from increasing the acceleration and intensity of the lobe. They make lobes specifically for high ratio rockers because of this.
I'll be sticking with the stock rockers for now but I am concerned about my valve tips because of the heavy 926 spring I'm using (Brian Tooley has me thinking about this). I have hope that Comp will release a 1.7 version of the rocker I posted a link to. However, in the interim, I was looking for some feedback from anyone who may have used the 1.8 version.... wasn't really considering any other dynamics other than durability (I broke a YT before going back to stock).
Still, I assumed that with a spring with these specs:
Extended .675” max lift, 159 lbs. at 1.835” installed height for aggressive cam designs
Cutting edge dual valve spring design with added processes for durability & performance
Engineered specifically for the new COMP Cams® LSR ™ Cams for GM LS engines
SEAT LOAD 159 lbs. @ 1.835”
OPEN LOAD 448 lbs. @ 1.160”
MAX LIFT .675”
COIL BIND 1.066”
.... and a cam with LXL lobes and, the aforementioned lift and duration, that my valve train was stout enough to handle the jump to a 1.8 rocker.
Clearly, I have made an incorrect assumption somewhere (mea culpa).
I am neither an engineer nor a mechanic and my experience in valve train dynamics is limited. However, I am a reasonably intelligent individual willing to learn so, if you don't mind, can you explain why lift has nothing to do with piston to valve clearance?
Last edited by FLYZNTN; 10-14-2012 at 09:30 AM.
10-14-2012, 09:04 AM
#45
I have a few comments.
1. Valve tip damage. The damaged described was caused by the stock rocker arm exceeding its designed range. The stock rocker arms are designed to contact the valve tip on the curved surface but once you exceed their limit, then the sharp tip of the rocker becomes the contact point.
2. Oil with Zinc. All conventional automotive oil contains some amount of the anti-wear compound Zinc DialkylDithiophosphates or ZDDP. However, some oil has more than others in the range of ~800 ~1500 ppm. The EPA has regulated that level down on production cars due to the claim high amounts reduce catalytic converter life. I doubt the level of ZDDP was the root cause of the valve tips damaged described.
3. Yella Terra Ultra-Lites. The first design of the YT ultra-lites had a higher than normal failure rate around the push rod cups so YT increased the material around the cups and other areas by 10% after June 2010. If the "1.7" is stamped in the center directly under the push rod cup, then thats the first design. If its stamped off center, then thats the second design. I exchanged emails with them and there are 2 other factors that can cause them to break. The first is excessive spring pressure. They recommended that you use spring below 420 lbs open spring pressure. The second is valve train stability. If you have any valve train instability or valve float, the rocker arms can experience much higher than 420 lbs momentarily.
4. Fuel Cut off. For a stock LS, the fuel cut off is designed to cut fuel at a predetermined RPM. Then after it falls 225 rpm, fuel is resumed. So if you hold the throttle down and keep it , the RPM will bounce up and down rapidly at the fuel cut off point by 225 rpm. This can cause unusual valve train harmonics which results in higher stress on the rocker. The solution is to avoid bouncing off the rev limiter and to reduce the RPM decrease in the tune before fuel resumes.
5. Springs. When installing new springs on a car with a new extreme high lift cam, you MUST measure the installed height and check for coil bind. For example, if the springs have a spec of 420 lbs at an 1.100 installed height. If you check the actual installed height because its .050 inch lower, it may be be 435 lbs., so you should measure it. There is a spec you can look up for coil bind. It may be something like .030" but you have look it up. Measure it and if its not in spec, that needs to be fixed.
1. Valve tip damage. The damaged described was caused by the stock rocker arm exceeding its designed range. The stock rocker arms are designed to contact the valve tip on the curved surface but once you exceed their limit, then the sharp tip of the rocker becomes the contact point.
2. Oil with Zinc. All conventional automotive oil contains some amount of the anti-wear compound Zinc DialkylDithiophosphates or ZDDP. However, some oil has more than others in the range of ~800 ~1500 ppm. The EPA has regulated that level down on production cars due to the claim high amounts reduce catalytic converter life. I doubt the level of ZDDP was the root cause of the valve tips damaged described.
3. Yella Terra Ultra-Lites. The first design of the YT ultra-lites had a higher than normal failure rate around the push rod cups so YT increased the material around the cups and other areas by 10% after June 2010. If the "1.7" is stamped in the center directly under the push rod cup, then thats the first design. If its stamped off center, then thats the second design. I exchanged emails with them and there are 2 other factors that can cause them to break. The first is excessive spring pressure. They recommended that you use spring below 420 lbs open spring pressure. The second is valve train stability. If you have any valve train instability or valve float, the rocker arms can experience much higher than 420 lbs momentarily.
4. Fuel Cut off. For a stock LS, the fuel cut off is designed to cut fuel at a predetermined RPM. Then after it falls 225 rpm, fuel is resumed. So if you hold the throttle down and keep it , the RPM will bounce up and down rapidly at the fuel cut off point by 225 rpm. This can cause unusual valve train harmonics which results in higher stress on the rocker. The solution is to avoid bouncing off the rev limiter and to reduce the RPM decrease in the tune before fuel resumes.
5. Springs. When installing new springs on a car with a new extreme high lift cam, you MUST measure the installed height and check for coil bind. For example, if the springs have a spec of 420 lbs at an 1.100 installed height. If you check the actual installed height because its .050 inch lower, it may be be 435 lbs., so you should measure it. There is a spec you can look up for coil bind. It may be something like .030" but you have look it up. Measure it and if its not in spec, that needs to be fixed.
Last edited by Mez; 10-14-2012 at 01:39 PM.
10-14-2012, 01:44 PM
#46
Thanks for the reply Martin.
I'll be sticking with the stock rockers for now but I am concerned about my valve tips because of the heavy 926 spring I'm using (Brian Tooley has me thinking about this). I have hope that Comp will release a 1.7 version of the rocker I posted a link to. However, in the interim, I was looking for some feedback from anyone who may have used the 1.8 version.... wasn't really considering any other dynamics other than durability (I broke a YT before going back to stock).
Still, I assumed that with a spring with these specs:
Extended .675” max lift, 159 lbs. at 1.835” installed height for aggressive cam designs
Cutting edge dual valve spring design with added processes for durability & performance
Engineered specifically for the new COMP Cams® LSR ™ Cams for GM LS engines
SEAT LOAD 159 lbs. @ 1.835”
OPEN LOAD 448 lbs. @ 1.160”
MAX LIFT .675”
COIL BIND 1.066”
.... and a cam with LXL lobes and, the aforementioned lift and duration, that my valve train was stout enough to handle the jump to a 1.8 rocker.
Clearly, I have made an incorrect assumption somewhere (mea culpa).
I am neither an engineer nor a mechanic and my experience in valve train dynamics is limited. However, I am a reasonably intelligent individual willing to learn so, if you don't mind, can you explain why lift has nothing to do with piston to valve clearance?
I'll be sticking with the stock rockers for now but I am concerned about my valve tips because of the heavy 926 spring I'm using (Brian Tooley has me thinking about this). I have hope that Comp will release a 1.7 version of the rocker I posted a link to. However, in the interim, I was looking for some feedback from anyone who may have used the 1.8 version.... wasn't really considering any other dynamics other than durability (I broke a YT before going back to stock).
Still, I assumed that with a spring with these specs:
Extended .675” max lift, 159 lbs. at 1.835” installed height for aggressive cam designs
Cutting edge dual valve spring design with added processes for durability & performance
Engineered specifically for the new COMP Cams® LSR ™ Cams for GM LS engines
SEAT LOAD 159 lbs. @ 1.835”
OPEN LOAD 448 lbs. @ 1.160”
MAX LIFT .675”
COIL BIND 1.066”
.... and a cam with LXL lobes and, the aforementioned lift and duration, that my valve train was stout enough to handle the jump to a 1.8 rocker.
Clearly, I have made an incorrect assumption somewhere (mea culpa).
I am neither an engineer nor a mechanic and my experience in valve train dynamics is limited. However, I am a reasonably intelligent individual willing to learn so, if you don't mind, can you explain why lift has nothing to do with piston to valve clearance?
If the lobe is correct for the rocker ratio the spring doesn't have to be near as high in seat and open pressure. On the other hand, even with a stout spring and a lobe family that has a very fast/aggressive lift curve it sometimes isn't enough to control the valve in some situations.
Lift has nothing to do with PTV because duration/lsa/icl is what determines the valve's open and close points in relation to the pistons travel in its bore. Lift only determines how far the valve is opened during those valve events.
You can have a cam that is very short in intake and exhaust duration with a lot of lobe lift that will never come close to having PTV problems where as you can have a cam that is long on duration that will keep the valve open longer to the point where the valve is still open/closing when the piston decides to occupy the location the valve is currently occupying because the duration/lsa/icl of the cam has kept it open long enough to interfere with one another. Even if the cam had a very minute amount of lift the valve would still be open during the time frame that the piston needs to occupy that space.
The exhaust valve is the most troublesome in PTV instances because the piston chases the exhaust valve. As the exhaust valve opens the piston is coming back to TDC and the exhaust valve is opening on the exhaust stroke at the same time the piston is traveling upwards to TDC towards that opening exhaust valve. Where as the intake is less likely to become troublesome. When the intake valve opens the piston is running away from the intake valve as it travels to BDC on the intake stroke, completely opposite from the other scenario.
10-14-2012, 04:37 PM
#47
Launching!
Actually if you were to use the LXL lobes you would be fine with a 1.8 ratio rocker. They are designed to work with higher rocker ratio's due to their slower lobe acceleration rate. Those springs would actually be slightly over kill for those lobes also even with a 1.8 RR.
If the lobe is correct for the rocker ratio the spring doesn't have to be near as high in seat and open pressure. On the other hand, even with a stout spring and a lobe family that has a very fast/aggressive lift curve it sometimes isn't enough to control the valve in some situations.
Lift has nothing to do with PTV because duration/lsa/icl is what determines the valve's open and close points in relation to the pistons travel in its bore. Lift only determines how far the valve is opened during those valve events.
You can have a cam that is very short in intake and exhaust duration with a lot of lobe lift that will never come close to having PTV problems where as you can have a cam that is long on duration that will keep the valve open longer to the point where the valve is still open/closing when the piston decides to occupy the location the valve is currently occupying because the duration/lsa/icl of the cam has kept it open long enough to interfere with one another. Even if the cam had a very minute amount of lift the valve would still be open during the time frame that the piston needs to occupy that space.
The exhaust valve is the most troublesome in PTV instances because the piston chases the exhaust valve. As the exhaust valve opens the piston is coming back to TDC and the exhaust valve is opening on the exhaust stroke at the same time the piston is traveling upwards to TDC towards that opening exhaust valve. Where as the intake is less likely to become troublesome. When the intake valve opens the piston is running away from the intake valve as it travels to BDC on the intake stroke, completely opposite from the other scenario.
If the lobe is correct for the rocker ratio the spring doesn't have to be near as high in seat and open pressure. On the other hand, even with a stout spring and a lobe family that has a very fast/aggressive lift curve it sometimes isn't enough to control the valve in some situations.
Lift has nothing to do with PTV because duration/lsa/icl is what determines the valve's open and close points in relation to the pistons travel in its bore. Lift only determines how far the valve is opened during those valve events.
You can have a cam that is very short in intake and exhaust duration with a lot of lobe lift that will never come close to having PTV problems where as you can have a cam that is long on duration that will keep the valve open longer to the point where the valve is still open/closing when the piston decides to occupy the location the valve is currently occupying because the duration/lsa/icl of the cam has kept it open long enough to interfere with one another. Even if the cam had a very minute amount of lift the valve would still be open during the time frame that the piston needs to occupy that space.
The exhaust valve is the most troublesome in PTV instances because the piston chases the exhaust valve. As the exhaust valve opens the piston is coming back to TDC and the exhaust valve is opening on the exhaust stroke at the same time the piston is traveling upwards to TDC towards that opening exhaust valve. Where as the intake is less likely to become troublesome. When the intake valve opens the piston is running away from the intake valve as it travels to BDC on the intake stroke, completely opposite from the other scenario.
I knew that duration and lsa and icl were far more relevant to PTV than lift but I guess I never really took the time to consider that lift was actually irrelevant.
Now that I've conversed with you I may reconsider the 1.8 afterall.
The LXL lobe was recommended to me by Pat G then my installer's recommendation was virtually identical.... so it was a no brainer.
The springs I knew were more than what I needed but I was thinking "more was better" when I bought them. Of course, when I broke a rocker arm I began doing some in depth research and discovered some surprising things about Yella Terra rockers and heavy springs.
Now I am reading about valve tip wear and heavy springs, with stock rockers, so I find myself looking at roller rockers again.
Live and learn.
10-14-2012, 07:41 PM
#48
I'd much rather have stock rockers than roller rockers and a heavy spring as that just creates more problems in of itself.
I'd run a stock rocker until the application no longer called for it due to a change to a mechanical cam.
If I'm running a hydraulic in a LS motor it's going to be with a stock rocker.
I'd run a stock rocker until the application no longer called for it due to a change to a mechanical cam.
If I'm running a hydraulic in a LS motor it's going to be with a stock rocker.
10-15-2012, 07:28 AM
#49
Launching!
So, there is concern about what the added weight of the roller rocker will do to the hydraulic lifters.
This was a good exercise for me. I'm glad I asked - Thanks.
This was a good exercise for me. I'm glad I asked - Thanks.
