obviously v8 car will have more torque, so it will kill the 4.
You beat me to this point... Sad but true.
You can crunch the numbers all you want, but when it comes down to it, the V8 will own you 9 times out of 10, due to the fact that the V8 will have double the displacement, and gobs of torque throughout the powerband. Also keep in mind that V8 will start making power low in the revs, and reach peak power a lot sooner, due to a shorter rev range.
Sure, you can talk about forced induction, but it's only a way to help reduce the shortcomings of a 4-cyl platform, A 4-cyl will need to work twice as hard to make the same power a V8 does.
You can offset the torque differnce through weight reduction, better tires and a susension that will help your car put that power to the ground, but the sad truth is that any 4-cyl platform will be playing catch-up in terms of the amount of modification to make it on par with most v8 powered platforms.
Now, lets say we compare two cars of sililar performance levels; a Mustang GT to an STi. Both are pretty close to one another in terms of performance: both cars put down about 300 HP. The mustang lays down 320 Ft Lbs of torque while the STi puts down 300 Ft Lbs. of torque The STi weighs in at around 3298 Lbs. and the new mustang GT weighs around 3487 Lbs. the torque difference will offset the weight difference, so, they're evenly matched in terms of performance.
Out of the box, they will be evenly matched from the factory (we will not consider the cobra mustangs here) The AWD setup on the STi will pay off on the initial launch if both drivers are equally matched, however, turbo lag and lower torque band will see that gap shorten quiclky after that. Still the STi can win against a stock car, but the race would be interesting to see whoever wins.
With some exhaust work, headers and an Intake
, numbers on that mustang will vastly improve, However, an STI owner will be spending more for similar gains, since the STi is pretty hot from the factory. you will need a turbo upgrade, better intercooler, etc; the mustang, by contrast, has a lot of room for improvement on it's 4.6L modular engine; the STI powerplant has most of it's optimization done at the factory. lastly, that base-trim mustang GT will cost you less money to buy and build up than an STI.
It's not impossible, but it's also not as cost effective. Most import fans claim that almost any 4-cyl can be made to be more ferocious than it's 8-cyl forefathers, but I have seen little proof of this in reality, most of the time, a V8 will hand you your ass if you drive a 4-cyl, boosted or not (My mom's bone-stock, automatic, rusted and out of tune Cougar XR7 walking all over a DSM eclipse with $10K in turbo and other upgrades comes to mind)
Don't forget, V8 owners can go FI too, superchargers add a LOT to the performance of a V8 musclecar, since torque and HP is in ample supply from the get-go. In the end, it could be considered a losing battle most of the time.
If you want to take on a V8, displacement is your friend. Go with the biggest motor you can get. Nissan Sentra Sepc V has a big 4-cyl, and produces lots of torque. with the right mods, the weaker Spec V will beat a GTS celica, mostly because of that extra displacement and torque.
Part of the mystique of import dominance comes from cars like the Z cars and the supra, don't forget these cars come with 6-cyl engines, more displacement, and more torque.
THe last way to offset this difference is to play to the strong suit of the 4-cylinder engine: Rev range. A 4-cyl engine has the potential to rev much higher, and sustain those revs longer. Since a inline-4 achieves peak torque and HP in the high revs, making the motor rev faster, and get to the sweet-spot faster will help you out, knife-edging the crank, lightening the car up, and lightening and strenghtening the valvetrain will go a long way to do this. very few stock V8 engines can rev past 6K safely (except certain ferarris, corvettes or the motor in the mustang cobra R) this is your advantage. The one thing everyone can agree on; you need to beat on a 4-cyl to win in a race with a v8, keep those revs high, and keep the engine screaming to make the same power as the typical V8, which can lazily turn and still shred tires into clouds of smoke.
Of course, this all applies to the drag race, road racing would be another story, since the STi will corner and brake better than the mustang GT.
You need to look at car specs when thinking about building a car to take on a V8, don't look at peak HP numbers, because that's just the PEAK power, you need to check a dyno graph; does the car make 400 Peak HP? Where is it making that power? probably close to the redline. V8's will make peak power closer to the midrange, and will keep making that power up to redline. Also, don't neglect that torque curve, usually, torque decides who wins a race, not the HP numbers.