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Today's quick and dirty physics lesson: Polar Moment of Inertia.
What Polar Moment of Inertia means to you: if something has its weight concentrated nearer the ends, it has HIGH polar moment. If something has its weight concentrated nearer the center, it has LOW polar moment. The higher the polar moment of inertia of an object, the more difficult it is to rotate the object.
Getting a car to turn-in effectively with the least drama is all about getting it to rotate. Autocross, racing, drifting... you want your car to rotate freely.
Look at any modern front engine, RWD performance car, like an S2000 or a 350Z. Notice that the engines in these cars sit almost entirely behind the front wheels. The bulk of the weight of the car is as close to the center as possible, so that the car will rotate more easily.
Now let's look at our old friend, the S13. With any of the stock four cylinders, the engine hangs a little bit forward of the front axle centerline, but not much. It's not optimum, but it was a good design for a cheap car in 1988. Now let's replace that engine with an RB. All that space formerly occupied by lightweight nothings like the fan and fan shroud is now filled by two more cylinders mounted in an iron block. You've added more weight further out towards the ends of the car - you've added more polar moment, making it more difficult to rotate.
Now, in the real world, you're not adding MUCH polar moment, and it can be recovered by relocating the battery to someplace central like the rear seat area (ever notice that Honda and Mazda put their batteries up against the firewall? polar moment! this is why those cars handle so damn good for FF).
If you've really got your heart set on an RB, and you're not a competitive racer, don't worry about it. It won't make the car suddenly handle like a pig. You might even like the change, who knows.
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