91 is the better candidate for an sr20det swap because of the dohc. With the sohc you have to change the power steering lines because the sohc power steering pump is on the passenger side of the motor and on the dohc the pump is on the drivers side. They don't line up and the lines aren't long enough. This can be fixed on an 89-90 model by switching the power steering lines over off of a dohc car. Usually found in the junkyard fairly cheaply, but it's an extra hassle. Also, as I stated earlier the tachometer off the sohc will not run the dohc motor. This can be fixed using the tachometer off of a 180sx. The gauge clusters are the same and the plugs fit perfectly so you can also just switch the gauge clusters. The down side to that is that you will now read in Kilometers (for this method you need to switch the speed sensor also). If you plan on the swap, the 15hp doesn't mean anything. If you don't want to swap and you just plan on building what you have, the 91 has the better motor. Either way, the car is going to be slow without doing a swap or adding a turbo. Sr20det swap (front clip and doing the swap yourself) about 3g's. Turbo for the Ka motor 3g's. The sr motor also responds extremely well to mods.
