Ahh Whiplash, you have hit on 2 of my favorite subjects, but I will try and keep it brief.
Hp/tq is relatively difficult to predict, but engine builders that repeat a particular build can get very close to what you will actually get. Conversely, you cannot predict hp/tq by knowing a few of someone’s engine specs. I have seen rwhp for many cobras in our club. 351's are typically around 350-375 rwhp, 393's are typically 390-430 rwhp (I built my 393 and got lucky with just over 470 rwhp) there are only a couple 408-427 small blocks in our club and I'm not sure what their numbers are. I do know of a fuel injected 427 with a 9lb blower that made 650 at the crank, but the fuel delivery is questionable so that number is somewhat unreliable. I have been told by an owner of a dyno that has seen the crank numbers and then the rwhp numbers. He states on average you will lose 60-70 with a manual trans and 90-110 with an automatic.
Diff ratios are absolutely dependent on how you will drive the car and what you expect from it. BDR almost always ship their cars with a 3.10 open diff and 25% lockup. That is actually a wise choice on their part and is debatable better for road racing which is the spirit of their cars. An open diff is much more forgiving to mistakes and that is what most guys will be doing when they first get behind the wheel of these cars. My car is much more harder to handle now with a 3.64 ls after breaking the output shaft on the original open 3.46 (like I said "almost always"). It is important to note that it happened on a quick wheel hop rather than a total torque strain and at the time I had 3200 miles of very spirited driving and several trips to the drag strip. It is amazing what these little bimmer diff will take, but they do have limits. It happened in front of the Hampton after the London show, but thankfully after the main street event....I would have hated to miss that. Anyway, BDR was very good about sending me a replacement diff that was limited slip and the gear ratio I wanted. The biggest reason I am glad to have a limited slip diff is the more positive spread of power to both wheels, which make it squirrelly. I still have the 15" BFG's which further bears out their weakness for traction. I must switch to 17" to get a decent tire selection or drive around with drag radials, which is the only alternative for 15" tires that are 295s. With the added traction will come the added strain on the e36 irs. If you can get the m3, do it! You will never regret it.
Here are what I consider to be the hot setups:
400+ rwhp, t56 6speed, 3.73 ls diff with 50% lock on m3 irs, 17" rims
400+ rwhp, tko with .82, 3.64 or 3.73 ls diff with 50% lock on m3 irs, 17" rims
I think I said I would keep it brief and I really did try. Sorry if it all seems a little cryptic.
