Erik,

GT6 offers a good overview of the issues. It is a complex topic, as many factors are involved. If you focus too much on one spec, you could end up with a cam that is great in that spec, but not great in other specs and that actually ends up providing worse or no better performance. Lift is the most popular spec. That is how far the valves open. More is usually better for performance. Duration is how long they open. More may not necessarily be better. Overlap is how long both of the valves are open at the same time. More may not necessarily be better, but it is obviously inevitable if you have a lot of duration. Lobe angle separation is how far apart the lobes are on the base circle and it affects overlap. More may not necessarily be better. And there are dual pattern or asymetrical cams. Those may or may not be better than a symetrical cam (lift and duration is the same for both intake and exhaust).]

To answer all those maybe questions, you have to start examining the engine and its modifications and its intended purpose. Generally, as you go for higher performance, there is some tradeoff in low end performance for mid range and upper rpm performance. It is really hard to avoid that unless maybe you have variable valve timing.

Generally speaking for a turbo motor that is street driven, you want more lift, but not much more duration, and you want a wider lobe separation angle. People will argue about degrees on the durantion and lobe separation angle, but generally agree that as much lift as you can get is good. You also want a cam that is symetrical or that favors the exhaust, but not one that gives the exhaust less lift or duration than the intake. These factors are an issue due to the presence of a high ratio of turbo boost pressure to exhaust back pressure. This ratio is often 2:1 or more. All that back pressure can cause reversion back into the head during the overlap period.

For a race turbo car, the ratio of the turbo boost pressure to the exhaust back pressure before the turbo (in the exhaust manifold) is often around 1:1, and the cam choice generally is not turbo specific. A cam that works well on a non turbo may be the best choice. But a race turbo motor will probably make no boost below 4000 rpm and will have a powerband to 8000 rpm, and would not be fun as a daily driver. So a race turbo cam or a non turbo cam might be high lift, more duration and thus more overlap, and less lobe angle separation.

You will read about people getting "performance" cams for motors that are not really performance motors or getting a non turbo performance cam and putting it in a turbo and then not being happy with the weak bottom end. Then they buy a cam gear and alter the cam timing, thereby shifting the power band back down. All they are doing is taking 5 hp and 5 lbs of torque from up top, where they may not notice it and moving it down to lower rpm where not much power is made and they will notice it. Still, though, the cam timed this way may offer better performance than the stock cam. But it is far from ideal. You really don't want a cam that you have to time half a tooth off to enjoy. You are leaving a lot of performance potential behind. The IPD/MVP VX3 is such a cam, as the 3 signifies that it is advanced 3 degrees from the factory. The VX signifies that it uses the V grind on the intake and the X grind on the exhaust. So it favors the intake (V is higher lift than X in the Volvo cam specs). This cam was used in some OEM cars in areas with higher speed limits, where off the line acceleration was not as important as acceleration once moving.

You have to study some, be prepared to compromise a bit, and make a choice. I used an A grind in my 83 242T for several years and liked it a lot more than the T grind. I used a boost retard ignition to give more initial ignition timing to help off the line response. Also that car was a manual trans. In my 89 745T, I am using an Enem V15 turbo spec. It has the best specs I could find for a high performance daily driver. I also have a boost retard ignition and a higher stall speed converter, to help with the softer bottom end. I have not yet put this engine combination on the road so I can offer no driving experiences. Also, I did a lot of mods, so those would affect things in a way that probably would not let me distinguish the effects of the cam.

Philip
pbonsalb@juno.com