Horsepower=Torque*(RPM/5250)

If you take a look at this equation, you can see that by making more torque at the same RPM, you make more horsepower. By making the same amount of torque at a higher RPM, you also get more horsepower (think Honda S2000). So these are the only two ways to make more power - either develop more torque, or, shift the torque peak (and the powerband) upward - or both, of course!

I am by no means an expert, but I'd say that a fairly accurate response to your question is that most simple cam swaps do not really add much 'overall power' to a Volvo motor - you need to do more stuff to alter airflow, like headwork, headers, etc... to really broaden the total output of the motor. With just a cam swap, you are going to sacrifice a little here or there to get the power curve you want. The M-cam is excellent for low-end grunt - below 2500RPM, it can't be beat. The B-cam is a good step up from that, which helps to broaden the powerband and produce good response from the low-end to the high-end. The A-cam is slightly more radical, shifting the torque peak higher for better HP, but less low-end torque. Next is probably the VX3, then the Phase1/V15, K, Phase/V16 and H cams, GT6, etc... each giving up progressively more low-end torque for a higher peak HP number, at a higher RPM.

And then of course you can alter your cam phasing relative to the crankshaft to further tune any given cam - advancing a cam brings the torque peak down, retarding the cam takes it up.

Let's use my car as an example, 'cuz I have some experience with it! My '83 240 with M-cam fell flat on it's face after about 4000RPM. A sport exhaust (2.25") helped a bit, but I wanted more power. I popped in a K-cam. Now, the car has little grunt under 2500RPM, but as the tach needle sweeps past 3500 the car moves like an athelete, and keeps pulling to the redline. With the M-cam, the HP rating was something like 113HP/135lb-ft. With the K-cam, it's probably around 130HP/136lb-ft. Peak torque changes only slightly, but because it occurs at a much higher RPM, the resulting horsepower figure is greater. As long as I keep my engine above 3000RPM, the car has more power. To get it, though, I sacrificed my low-end torque. Because the K has such a high power peak, though, I can use an adjustable cam gear to advance the cam and get some of the low-end back, while still keeping most of the high-end power. I'm trying this over the weekend, so I can give details then.

So, to wrap up my now long-winded post, it all depends on what kind of driving you want to do - don't just run to the cam quoting the highest HP figures. The K is probably as radical as you'd want to go on in a street machine. Check out the Turbobricks forum, as there have been a lot of cam discussions and comparisons lately, on turbo and NA cars.
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Speed Racer, '83 240 R, '74 164 E, '93 940 OL1 (Manchester, CT)