Carputer + ECU + G forces = ?

Hey guys I've got a r31 skyline at the moment, but I'll be wanting to transfer the carputer over to a hr31 when I get the funds and I get off my P's. Anyway, the car is a 1988 model so I'm not sure what OBD it has, or if it even has one :S.

Now what I was wanting to do is have an aftermarket ECU hooked up to the carputer for instant access to things such as rpm, hp etc.... but the main thing that I'm after (here comes the hard part) is also G forces. So I can see what kind of G's I'm pulling around corners. I've looked at a few aftermarket ECU's and all I can find is the APEXI G-sensor, or I've heard that Motech has G force acceleration graphs like I'm wanting. Now I'm gonna be on a budget (as you can see with a r31 skyline :P) so I want to see if there is a cheap solution to get a carputer with the G forces integrated rather than having a external unit such as the APEXI G-sensor as the motech costs 8k apparently!

I was talking to my mate about it and he said motech most likely figures out G forces by means of mathematical equations, is this true? If so, would it be hard to implement a mathematical equation into a different aftermarket ECU which is cheaper? :P

Cheers

~Danz