Aftermarket Cam Questions
08-30-2006, 02:00 AM
#1
Three Wheelin'
Thread Starter
Aftermarket Cam Questions
Well, I have been doing lots of research and continue to do so, however I am very confused when it comes to cams on these cars. From my limited knowledge on camshafts, it is my understanding that with forced induction you want little to no overlap, or negative overlap if possible. My question to everyone is why are so many people using SC cams, 964 cams, and Gt2 cams on these turbocharged motors? From my calculations, this is what I'm seeing from these cams (all at .050*):
Stock 930 - Duration 208I/197E, Lift .368I/.339E, Centerline 110I/106E = -5.5* overlap
SC - Duration 228I/218E, Lift .450I/.395E, Centerline 113I/113E = -3* overlap
964 - Duration 238I/226E, Lift .470I/.430E, Centerline 113I/113E = 6* overlap
Gt2 - Duration 248I/232E, Lift .485I/.470E, Centerline 112I/112E = 16* overlap
That's the overlap numbers I'm coming up with. This just doesn't seem right to me. That much overlap on Gt2 cams makes them sound much less than ideal for a high hp forced induction application, and I would think they would be incredibly laggy with that much overlap at .050. This info is straight from Dougherty Cams website. Is my math just wrong? I'm going by some calculations from one of Corky Bell's books:
-Add the intake and exhaust durations
-Divide the results by 4
-Subtract the Lobe separation
-Multiply the results by 2
I currently have 964 cams, and I am starting to wonder if they are the reason my car is so sluggish and unresponsive. I've seen much smaller engines (non f6 motors, apples to oranges I know) spooling the same turbo that I have quite a bit faster and earlier. Though, it seems that lots of 911 Turbo people are using 964 cams and Gt2 cams, so I don't know if there is something I'm missing? I'm still an amature when it comes to aircooled f6's, so take it easy on me.
I guess I should state what I am looking for. I just got a Precision T71 with a Q trim turbine and .81 exhaust for my Carrera. I'm looking to hit somewhere around 500-550rwhp on pump gas at low boost. I also want to put out a few dyno queen runs at high boost and on C16, and I'm shooting for the ~700-750rwhp level. I'll be going with an AEM standalone from Chris Carroll at Turbokraft, including the AEM CDI so the fuel system and management is pretty much ironed out. Right now I'm trying to get a handle on the mechanical side of things. I'm going to stick with the 3.4l setup (Mahle 3.4 P&C, 3.2 Carrera heads), and was planning on going with gt2 evo cams, springs, and Ti retainers to sustain ~8k rpm. Though, I'm not sure if I should go with a custom grind instead to minimize overlap. I'm building the car for no real purpose other than to have fun, and high hp is what I'm looking for regardless of how ridiculous it may be.
Stock 930 - Duration 208I/197E, Lift .368I/.339E, Centerline 110I/106E = -5.5* overlap
SC - Duration 228I/218E, Lift .450I/.395E, Centerline 113I/113E = -3* overlap
964 - Duration 238I/226E, Lift .470I/.430E, Centerline 113I/113E = 6* overlap
Gt2 - Duration 248I/232E, Lift .485I/.470E, Centerline 112I/112E = 16* overlap
That's the overlap numbers I'm coming up with. This just doesn't seem right to me. That much overlap on Gt2 cams makes them sound much less than ideal for a high hp forced induction application, and I would think they would be incredibly laggy with that much overlap at .050. This info is straight from Dougherty Cams website. Is my math just wrong? I'm going by some calculations from one of Corky Bell's books:
-Add the intake and exhaust durations
-Divide the results by 4
-Subtract the Lobe separation
-Multiply the results by 2
I currently have 964 cams, and I am starting to wonder if they are the reason my car is so sluggish and unresponsive. I've seen much smaller engines (non f6 motors, apples to oranges I know) spooling the same turbo that I have quite a bit faster and earlier. Though, it seems that lots of 911 Turbo people are using 964 cams and Gt2 cams, so I don't know if there is something I'm missing? I'm still an amature when it comes to aircooled f6's, so take it easy on me.
I guess I should state what I am looking for. I just got a Precision T71 with a Q trim turbine and .81 exhaust for my Carrera. I'm looking to hit somewhere around 500-550rwhp on pump gas at low boost. I also want to put out a few dyno queen runs at high boost and on C16, and I'm shooting for the ~700-750rwhp level. I'll be going with an AEM standalone from Chris Carroll at Turbokraft, including the AEM CDI so the fuel system and management is pretty much ironed out. Right now I'm trying to get a handle on the mechanical side of things. I'm going to stick with the 3.4l setup (Mahle 3.4 P&C, 3.2 Carrera heads), and was planning on going with gt2 evo cams, springs, and Ti retainers to sustain ~8k rpm. Though, I'm not sure if I should go with a custom grind instead to minimize overlap. I'm building the car for no real purpose other than to have fun, and high hp is what I'm looking for regardless of how ridiculous it may be.
08-30-2006, 08:36 AM
#2
Nordschleife Master
Overlap is a byproduct of intake and exhaust duration. It is unavoidable and the more duration your run, the more overlap is found. You can only modifiy the acceleration ramps on a camshaft so much for a given valve mass and spring pressure before you punt the valve (rocker) off the camshaft lobe itself. The goal is to have the highest volumetric efficiency (cylinder filling) possible for a given engine combination and that means the intake valve needs to be open sufficiently to allow for incomming air. The exhaust valve also needs to be open sufficiently to scavage the cylinder of the spent gasses. Because of the air velocity differences, there is a large difference in requirements of an engine running at low speeds where the air velocity is slow and high engine speeds where the air velocity is high, and a turbocharged engine makes things worse because it does not have the compression at low speeds where the turbo is not working to help.
Cylinder filling is directly related to piston velocity which is a function of stroke, rod length and where the engine is operating at in the RPM range. You need the intake valve open sufficiently where the piston speed is it highest which on the 74.4mm crank and stock rod length is 73 degrees ATDC (If I remember correctly from my calculations). The earlier the valve is open (more overlap) the more cylinder filling that will occur. Cylinder filling also is a function of air velocity and cylinder head port size and velocity have a large effect on camshaft function as well. This is why a custom camshaft ground to you specific head flow (CFM) and velocity is important.
There is far more information on the subject, but I would offer this; Overlap is unavoidable and it is the intake closing event that will have the largest effect on the performance of your engine since it governs your dynamic compression ratio and will have a large effect on the operating RPM range of your engine. I believe in selecting a camshaft based on the engine operation and I DO NOT differentiate between a turbocharged and N/A engine. To give you a comparison, my turbo engine had camshafts with 54 degrees of overlap and still retained very good off boost response and drivability.
As for your mechanicals, the combination that you listed above is a common grouping of component and will work well. The only comment is that you won't really be revving the engine past 7000rpm, so you do not need valve springs for 8000rpm and the heavier springs will take away power...although we are nit picking here. If I read this correctly, you are using a Turbonetics T71 turbo? That is rather large for your application, a good T66 or one of the newer GT series turbos sized appropriately might be a better choice.
Hope this helps
Cylinder filling is directly related to piston velocity which is a function of stroke, rod length and where the engine is operating at in the RPM range. You need the intake valve open sufficiently where the piston speed is it highest which on the 74.4mm crank and stock rod length is 73 degrees ATDC (If I remember correctly from my calculations). The earlier the valve is open (more overlap) the more cylinder filling that will occur. Cylinder filling also is a function of air velocity and cylinder head port size and velocity have a large effect on camshaft function as well. This is why a custom camshaft ground to you specific head flow (CFM) and velocity is important.
There is far more information on the subject, but I would offer this; Overlap is unavoidable and it is the intake closing event that will have the largest effect on the performance of your engine since it governs your dynamic compression ratio and will have a large effect on the operating RPM range of your engine. I believe in selecting a camshaft based on the engine operation and I DO NOT differentiate between a turbocharged and N/A engine. To give you a comparison, my turbo engine had camshafts with 54 degrees of overlap and still retained very good off boost response and drivability.
As for your mechanicals, the combination that you listed above is a common grouping of component and will work well. The only comment is that you won't really be revving the engine past 7000rpm, so you do not need valve springs for 8000rpm and the heavier springs will take away power...although we are nit picking here. If I read this correctly, you are using a Turbonetics T71 turbo? That is rather large for your application, a good T66 or one of the newer GT series turbos sized appropriately might be a better choice.
Hope this helps
08-30-2006, 11:09 AM
#3
Three Wheelin'
Thread Starter
Thanks Geoffery! I thought you might respond. You always seem to be so full of information! 
I guess I need to do a little more research until I have a better handle on the subject of cams for a forced induction motor.
It's a Precision T71, which uses a custom CNC machinced compressor wheel that is very efficient (it's not the same as a Turbonetics T71). It's rated at 880fwhp. I've looked at several other turbos including a T61, T67, Gt4088r, and Gt4294r. The most similar in size and power output to a T71 is a Gt4294r although that 42r variant is slightly larger and looks to be a little laggier. The 4088r is probably perfect for 700rwhp, though after looking at the compressor map and seeing some real world numbers from them it seems like I'd really have to push it to hit 700rwhp. I honestly don't think this T71 will be that laggy. I've been seeing numbers from various smaller displacement motors and it seems responsive enough. From what I've seen, full boost should be at about 4500rpm, which is perfectly fine with me.
I guess I need to do a little more research until I have a better handle on the subject of cams for a forced induction motor.It's a Precision T71, which uses a custom CNC machinced compressor wheel that is very efficient (it's not the same as a Turbonetics T71). It's rated at 880fwhp. I've looked at several other turbos including a T61, T67, Gt4088r, and Gt4294r. The most similar in size and power output to a T71 is a Gt4294r although that 42r variant is slightly larger and looks to be a little laggier. The 4088r is probably perfect for 700rwhp, though after looking at the compressor map and seeing some real world numbers from them it seems like I'd really have to push it to hit 700rwhp. I honestly don't think this T71 will be that laggy. I've been seeing numbers from various smaller displacement motors and it seems responsive enough. From what I've seen, full boost should be at about 4500rpm, which is perfectly fine with me.
