YES, the A15 4-speed is fine to use with an A15. It can do 150kph in a Datsun B310 with stock gearing (rear final drive).

Ok i'm a little confused! :/ so if i use the standars 4 speed of the a15 would be fine with a 100 hp motor?

* Acceleration (quickness) is dependent on torque
* Top speed is dependent on HP

It may take a looong distance to achieve top speed, since the torque decreases as high-RPM horsepower increases.

In circuit racing the goals are different from top speed contests. I would guess that in your type of racing, you want to select the steepest gearing (quickest acceleration) that allows the highest top speed for the given length of track, while also keeping in mind gear ratios used for slower corners.

That is rarely the goal of a street driven car, as even a stocker A12 will exceed the highway speed limits. So instead most owners will focus on either acceleration (steep gears) or fuel economy (tall gears) or some compromise of the two (middling gears). Perhaps the best performance combination for highway use (120 or 140 kph maximum) is steep final drive gearing (say 4.111 or more, which allows 120+ kph top speed) along with overdrive for cruising. The overdrive does not improve performance/power/acceleration but does reduce noise and increase fuel economy, and is used when not accelerating.

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I dont quite follow your logic there David.

5000 RPM in 4th will be slower than 5000rpm in 5th. When I mean slower I'm referring to road speed. Acceleration is dependent upon torque the engine generates.

The A15 makes more power at 5000 RPM than it does at lower RPM. If you shift it up to another gear, the RPMs go down and so does the power.

A 5-speed is good for fuel-economy, but is slightly worse for acceleration. A 5-speed does not increase the top speed -- more HP does. Highest top speed with a 5-speed is achieved in 4th gear.

I think i dind't ask correctly. What i want to know is: if i modify an a15 engine and use the 4 speed gearbox of the a15 wouldn't it stop making power on 4th gear pretty quickly since the engine has more power and therefore it goes through gears faster? I mean with my stock a12 it feels that at 140km/h (the engine now is revving hard on 4th gear) stops making power, it feels like it needs a 5th gear. Would that not be a problem?

The A15 can run at 5000 RPM all day long. Don't worry about it.

Mmm ok :/ the thing is i'm in malta so i definitely won't find that gearbox :( do you know any alternatives? Can i install a different gearbox? Or if i stick with a15 gearbox would it be ok?

If you use the A14 4 speed, it'll have the same gearing as the A12. Interestingly the extra torque means you can use less throttle to get the same speed, so it seems easier. The only way is to find an overdrive 5 speed to suit the A15, which are quite rare over here in Australia. They are easier to come by in South Africa, being used in the Baky ute until they stopped making it. It does make a difference fitting the 5 speed, & you never know where they'll be. You can only look, I suppose.

Thank a lot for you advice dude :) it is very useful! :D i have another problem though, its about the gearbox. With the a 12 at 130-140km/h i'm in 4th gear and its revving the balls out of it! Lol would i have the same problem if i modify an a15 with the 4speed gearbox of the a15?