Cams & Lifters...what's hot
11-09-2008, 02:19 PM
#16
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What many of you have failed to realize that as we modify our engines and increase boost. The valve springs need to be upgraded. There is a relationship between boost pushing the backside of the valve "open" and countering that effect so that you don't have the valve spring bouncing off the rocker and camshaft. The Catch 22 is >>As you increase the spring tension, you begin to weaken the ability for the "stock" hydraulic lifter to pump up and maintain the valve clearance.
I have looked at all the popular cam choices from the stock SC cams, 964, 993NA, RS, EVO street, and Race sticks, this includes comparing what RUF and FVD have done. When compiling all this data, one can go back to the 930 engine and see the effect of what a SC vs 964 cam does. It becomes clear when you look at the RUF and FVD cams as to what the are and the performance gains from them.
On grave mistake is to take a hydraulic camshaft that has a very different ramp and install it with non hydraulic rockers. This induces harmonics or "jerk" to the valvetrain. Basically once one figures out the lift, base-circle, the overlap, and cam timing, you can start to CNC precision grind masters. From masters you can grind a set of camshafts for testing. This is where you set the cams up in camhousings and measure harmonics and distortion. The cams coming out of Porsche are good, however, there seems to be better grinds for "factory race" applications.
Yes, I have looked at all the "known" camshafts. And have come up with a grind that I feel is excellent choice for the twin turbo application. I can assure you that this new cam is not just a copy or regrind, but a complete redesign for a turbo application.
I have looked at all the popular cam choices from the stock SC cams, 964, 993NA, RS, EVO street, and Race sticks, this includes comparing what RUF and FVD have done. When compiling all this data, one can go back to the 930 engine and see the effect of what a SC vs 964 cam does. It becomes clear when you look at the RUF and FVD cams as to what the are and the performance gains from them.
On grave mistake is to take a hydraulic camshaft that has a very different ramp and install it with non hydraulic rockers. This induces harmonics or "jerk" to the valvetrain. Basically once one figures out the lift, base-circle, the overlap, and cam timing, you can start to CNC precision grind masters. From masters you can grind a set of camshafts for testing. This is where you set the cams up in camhousings and measure harmonics and distortion. The cams coming out of Porsche are good, however, there seems to be better grinds for "factory race" applications.
Yes, I have looked at all the "known" camshafts. And have come up with a grind that I feel is excellent choice for the twin turbo application. I can assure you that this new cam is not just a copy or regrind, but a complete redesign for a turbo application.
11-09-2008, 03:26 PM
#17
Three Wheelin'
Sorry but I certainly am not qualified to answer this, the little I know is that the GT2 race cars revved to 7600-7800 RPMs in many instances and were primarily built to sustain 24 hour races on the other hand.
But I am not sure if the problem with the hydraulic lifters was directly related to RPMs (whereas springs and retainers were) but rather with minimizing moving parts, increasing reliability, and lightening the assembly. The GT2 parts are not the same as the 964 ones, they were adjusted via shims, and did not have an adjustment nut.
As to cams, the type of car usage is what dictates the cam profile choice, the ones that are most discussed in this forum are the proven solutions (964, RS, EVO, etc..), but there are so many others.
But I am not sure if the problem with the hydraulic lifters was directly related to RPMs (whereas springs and retainers were) but rather with minimizing moving parts, increasing reliability, and lightening the assembly. The GT2 parts are not the same as the 964 ones, they were adjusted via shims, and did not have an adjustment nut.
As to cams, the type of car usage is what dictates the cam profile choice, the ones that are most discussed in this forum are the proven solutions (964, RS, EVO, etc..), but there are so many others.
11-09-2008, 10:09 PM
#18
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The GT2 EVO rocker arm parts are slightly different, for the GT2 ones, you have a multitude of different cap thicknesses to adjust the clearance and the shaft is also different.
Part number: 993 105 308 81
I believe this is what TB has on his factory GT2 twin plug heads.
The only other change between the GT2 EVO and the stock 993TT concerning the valve/spring/rocker assembly were the valve sizes and spring set on the 1997 EVO model only (the '96 still had the stock springs). There are two different factory provided GT2 EVO cams as well.
Spartan, all (or most) the GT2 EVO race parts are still available from Weissach.
11-10-2008, 12:29 PM
#20
Kevin
1. It would be interesting to see some dyno figures that show how your cams perform (HP/TQ) compared to stock. Can you please provide that information?
2. Also what "kit" were these designed to work with???? Stage 1, 2, 3?
1. It would be interesting to see some dyno figures that show how your cams perform (HP/TQ) compared to stock. Can you please provide that information?
2. Also what "kit" were these designed to work with???? Stage 1, 2, 3?
11-12-2008, 04:08 AM
#22
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Chris, I am hoping that we won't have any issues. Tuning with proper fueling should get us to pass.
LA, They are for Stage 3 engine packages..
LA, They are for Stage 3 engine packages..
12-09-2008, 09:05 AM
#25
Nordschleife Master
The issue with the camshaft profile has to do with the acceleration and deceleration ramps of the camhshaft profile. This is the primary difference between a 964 camshaft and a 993 camshaft. The hydraulic lifter type rocker arm cannot be accelerated as quickly as the mechanical one and as Colin mention, it bleeds down at higher RPM. So, technically, you could run a high lift camshaft, however, to maintain the maximum acceleration rate for the hydraulic rocker arm, the valve would be opened so slowly, it would not allow proper cylinder filling.
As you move to a more agressive camshaft profile, the pressure required to keep the valve on the seat when closing and on the nose of the camshaft at peak lift needs to be greater to keep the valve under control. Additionally, the heavier the valvetrain mass (valves, retainers, etc.), the greater the pressure required to keep the valve under control. F=MA. Porsche uses very light spring pressures on their stock cars and on the 993tt, the valve will not be controlled properly with stock springs above about 6900rpm with the stock camshaft profile. Once you add an agressive profile, the the valve will be uncontrolled. The small amount of boost pressure - call it 15psi for discussion - adds very little extra work a valve spring must do.
All of the aircooled Porsche factory camshaft profiles are old technology and an aftermarket camshaft grinder can improve upon the design which will improve the valve motion control. All of the camshafts I install are designed specifically for the application.
As I found out recently, the rocker arm profile also effects the camshafts ability to control the valve. I just completed some testing on Colin's new billet rocker arm and my camshaft designer commented that the design of the rocker allowed him to lift the valve at a higher acceleration rate and have more peak valve velocity. This was done using a spintron machine and some camshaft software he has. The new camshaft for my engine therefore will have more "area under the curve". I suspect there is a similar difference between the stock mechanical and hydraulic rocker arms.
As you move to a more agressive camshaft profile, the pressure required to keep the valve on the seat when closing and on the nose of the camshaft at peak lift needs to be greater to keep the valve under control. Additionally, the heavier the valvetrain mass (valves, retainers, etc.), the greater the pressure required to keep the valve under control. F=MA. Porsche uses very light spring pressures on their stock cars and on the 993tt, the valve will not be controlled properly with stock springs above about 6900rpm with the stock camshaft profile. Once you add an agressive profile, the the valve will be uncontrolled. The small amount of boost pressure - call it 15psi for discussion - adds very little extra work a valve spring must do.
All of the aircooled Porsche factory camshaft profiles are old technology and an aftermarket camshaft grinder can improve upon the design which will improve the valve motion control. All of the camshafts I install are designed specifically for the application.
As I found out recently, the rocker arm profile also effects the camshafts ability to control the valve. I just completed some testing on Colin's new billet rocker arm and my camshaft designer commented that the design of the rocker allowed him to lift the valve at a higher acceleration rate and have more peak valve velocity. This was done using a spintron machine and some camshaft software he has. The new camshaft for my engine therefore will have more "area under the curve". I suspect there is a similar difference between the stock mechanical and hydraulic rocker arms.
12-09-2008, 12:31 PM
#26
Thanks Geoffrey....I'd be going w/a mechanical lifter...but I get what you are saying about valve train stress and OE parts. I understand that to do cams one should change many components to address this issue. I'm guessing w/Kevin's program as it was designed for OE cams it probably won't respond too well to any mods in that area....I guess only Kevin knows....but that seems the logical conclusion.
