Does anyone know why there is a 26 horsepower difference between the STS-V and the XLR-V? They are the same engines are they not?

The STS engine bay is larger, thus allowing more room for intake and exhaust manifolds.

And with 600lb less weight, I doubt you'll see a difference in performance in the XLR-V.

With 600lbs less weight, the XLR-V should still be faster. You gotta figure 1-10th in the 1/4 for every 100lbs of weight at those levels and 26HP should make up a tenth, maybe 2. The XLR should be a half second faster if the weight is correct.

The difference is due to the weight.
The engines are pretty much identical, likely the only difference is tuning and software. The main reason for the difference as mentioned is weight.
The XLR is much lighter since it's a sport coupe made of plastic and fiberglass. The STS is made with heavier components and more of them.

That's what GM told us at the DTS ride and drive event I went to.
They had an XLR-V there for us to look at. It's pretty impressive, but with the production numbers I have heard, you'll probably be lucky to see one on the road.

The oil pan on the engine in the XLR-V is also smaller and shorter (due to the proximity of the engine to the ground...) which causes additional windage losses on the XLR-V version of the engine....that and the packaging of the exhaust manifolds on the XLR-V account for the difference in the HP...