Been trying to find out something

Rubecula

I have tried google and not yet got a definative answer to this, maybe one of you might know. The current crop of T.Rex posts reminded me to ask in here.

Allosaurus and the later T.Rex both top predators of their time (allegedly)

How would you compare them.

Size, effectiveness, killing power etc etc.

T.Rex supposedly had feathers, even though I think birds had already evolved by then, but back when Allosaurus was around had feathers evolved at all? If so is it likely that 'Big Al' also had feathers.

Finally, were the two related in any way?

Galvasean

The recent discovery of a feathered heterodontosaur that lived in the early creataceous shows that feathers were not unique to the theropod (group that includes meat eaters like Velociraptor, Allosaurus etc.) dinosaurs. Feathers therefore must have first appeared in dinosaurs during theTriassic, before the split between the two major dinosaur groups. So essentially, feathers in dinosaurs were much more common than once taught and not just limited to theropods.
Source.

Did Allosaurus have feathers? To put it simply, we do not know. No feathered fossils of Allosaurus or any of it's close relatives have been found. However, it seems highly likely that their chicks would have had downy feathers - as is most likely the case with all theropods.

Size etc.: While the largest of Allosaurus specimens approached 40 feet in lenght (comparable to T. rex), they were fairly lightweight for such large animals. A tyrannosaur of the same lenght would be much more massive and 'well bult' than an allosaur of the same lenght. For example, a 40 foot long Tyrannosaurus would have weighed about 6 tonnes, while a 40 foot long Allosaurus would have weighed about 2.
Allosaurus would have been faster than a T. rex in terms of flat out speed in any case. However, it's brain was somewhat primitive in comparison to that of T. rex. Tyrannosaurrus most likely had better reflexes than Allosaurus. Allosaurus' eyes were also on the side of it's head, meaning it lacked stereoscopic vision (like T. rex). This would have been useful for precise strikes. Allosaurus' favoured game appears to be large long necked sauropods and possibly Stegosaurus. It probably attacked the flanks, avoiding the bones so that it's thin blade like teeth cut out large amounts of flesh. It also had a massive jaw gape, comparatively larger than that of T. rex to maxmize this effect. Tyrannosaurus also had sharp teeth, but they were thick and robust, capable of crunching through bone as well as tearing through flesh. It is believed that T. rex's teeth/jaws were used to inflict a crippling bite, allowing it to finish off the wounded prey at it's leisure. Allosaurus, on the other hand, would try to inflict massive damage on fleshy parts. Blood loss would most likely be the killer after Allosaurus wore down it's prey with repeated assaults.

Two similar looking animals alright, but hunted in very different manners.

Rubecula

Brilliant summation Galvasean, enjoyed the read.

Is there any evidence that Al was ancestor of T.Rex? I have a feeling that they were totally unrelated in any way, but I am not sure enough to say so outright.

Galvasean

Forgot to mention that in my last post. Sorry.
If you look at this cladogram:


Allosaurus is in the carnosauria (as is Giganotosaurus and it's kin), while T. rex is surprisingly a memeber of the coelurosauria (a group which also includes the 'raptors', therizinosaurs and even birds).
So basically, Allosaurus would be like a distant second cousin of T. rex. While it was once thought that Allosaurus was ancestral to Tyrannosaurus, discoveries in relation to the tyrannosaur/coelurosaur family in the last 20 years have shown that this is not the case.