2-Valve Cam Options and Turbocharging
11-15-2005, 06:35 PM
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2-Valve Cam Options and Turbocharging
I'm biting the bullet while I have my manifolds out of the car for my K-26 turbo conversion to change the camshafts. I have a set of Euro S cams I was really set on using, but after some close evaluation I believe these would have been way too much cam for the twin turbo. My goal was to increase lift and duration without tapping into valve overlap. If my car had a header manifold system it would be different, but since I'm running collector log stlyes the overlap is a big no-no. It would appear as though when the intake valve is opening on the Euro S cam, that the exhaust valve is still virtually 3mm off the seat. I fully expect exhaust pressure before the turbine to be higher than intake manifold pressure.
I have some numbers on the various grinds:
The US cams 80-82 have 213/202 duration and -20 overlap at 0.050” lift. Total lift is 10mm/9mm intake/exhaust
The US 78/79 cams have 220/210 duration and -13 overlap at 0.050” lift. Total lift is 11mm/10mm
The Euro S cams have 232/220 duration and +6 overlap at 0.050” lift. Total lift is 12mm/11mm
For comparison, a 944 turbo cam has 223/215 duration and -9 overlap at 0.050” lift. Total lift is 12mm/11mm. The 944 NA cam has the same lift but a tighter LCA for 1 degree of overlap at 0.050". So it looks like even Porsche pulled the LCA out to minimize valve overlap.
As a result of this analysis I have decided to go with the 78/79 cams...the lobes are in just about the same spot as the 80-82 cams, but there is 1mm more lift that results in a tad bit more duration, I guess just short of 8 or so degrees. The 80-82 cams are pretty darn small when it comes right down to it. This is one big reason the US power drops off quickly up in the higher RPM. I have dyno proof that the Euro S cams and headers on a NA 82 can make a full 30 HP with minimal loss of mid range.
The 78/79 should give me that extra top end torque holding power. I’d guess at least another 500 rpm on the torque curve. I can afford to give up some torque down low, the car already makes 390 or so ft-lbs to the wheels. I'll be picking up a lot of spool with the K-26 in the lower gears, so I hope this will all balance out nicely when complete! My IHI turbos are sent off to Florida to be run on my Brother's 84 Callaway 928.
Nothing quite like combing the spare parts bin, now is there? I considered welded cams for a second...but the cost and complexity with these cams can cause headaches. $850 for a weld and regrind is crazy in my mind. It would be best to start with new blanks and cut what you need, but for my car I believe the 78/79 cams to be the answer.
I have some numbers on the various grinds:
The US cams 80-82 have 213/202 duration and -20 overlap at 0.050” lift. Total lift is 10mm/9mm intake/exhaust
The US 78/79 cams have 220/210 duration and -13 overlap at 0.050” lift. Total lift is 11mm/10mm
The Euro S cams have 232/220 duration and +6 overlap at 0.050” lift. Total lift is 12mm/11mm
For comparison, a 944 turbo cam has 223/215 duration and -9 overlap at 0.050” lift. Total lift is 12mm/11mm. The 944 NA cam has the same lift but a tighter LCA for 1 degree of overlap at 0.050". So it looks like even Porsche pulled the LCA out to minimize valve overlap.
As a result of this analysis I have decided to go with the 78/79 cams...the lobes are in just about the same spot as the 80-82 cams, but there is 1mm more lift that results in a tad bit more duration, I guess just short of 8 or so degrees. The 80-82 cams are pretty darn small when it comes right down to it. This is one big reason the US power drops off quickly up in the higher RPM. I have dyno proof that the Euro S cams and headers on a NA 82 can make a full 30 HP with minimal loss of mid range.
The 78/79 should give me that extra top end torque holding power. I’d guess at least another 500 rpm on the torque curve. I can afford to give up some torque down low, the car already makes 390 or so ft-lbs to the wheels. I'll be picking up a lot of spool with the K-26 in the lower gears, so I hope this will all balance out nicely when complete! My IHI turbos are sent off to Florida to be run on my Brother's 84 Callaway 928.
Nothing quite like combing the spare parts bin, now is there? I considered welded cams for a second...but the cost and complexity with these cams can cause headaches. $850 for a weld and regrind is crazy in my mind. It would be best to start with new blanks and cut what you need, but for my car I believe the 78/79 cams to be the answer.
