| But that is a grossely broad statement; I'll clarify: The 4.08 NA rear does not put as much load on the engine as the 3.96 TT rear does in the same gear. A comparison of spoolup response, say, on my car, with the NA rear end and then swapping out to the TT rear on my car will show slightly better spoolup response with the TT rear in same-gear comparisons. This is analogous to the effect of trying to build boost with the car in neutral: without the load on the motor to keep it from shooting straight to the rev limiter, the turbos do not have enough time to get spooled up to produce any significant positive manifold pressure. In a 1st gear comparison, from a dead stop to redline, traction not a factor, my Z will build more boost with the TT rear over the course of that short burst. As the gear is increased for the comparison, the difference becomes less and less, because there is more load on the motor and hence, more time for the turbos to get wound up. The turbos are almost always playing the game of "approaching infinity", analogous to that of two masses with differing temperatures contacting each other and watching the changing rate of temperature transfer between the two bodies: the greater the difference in temperature, the greater the rate of change in temperature towards equilibrium. Consider the difference in temperature analogous to the difference of load; the more load, the quicker the turbo response. In a fully loaded state, where the engine RPM is prohibited from increasing while at WOT, you will see the peak boost attainable by the setup at the given RPM hold. Consider this your "maximum" available boost pressure for that RPM. The lower the gear, the further away from this potential you will be for any given RPM. I should make a few 3rd gear pulls to show you the difference in spoolup response on my own car - I can plot three different boost plots and can show you how the spoolup response is affected by the amount of load put on the engine (in this case, by being in different gears rather than changing out the differential). The plot I posted was a loaded 4th gear pull - the 3rd gear plot shows a shift of manifold pressure to the right, by about 400RPM. However, the effect of having the 4.08 differential in the car, although it does not provide as much spoolup response as the 3.69 does, the additional ~10% increase in torque at the wheel with the 4.08 ratio more than makes up for the decreased spoolup response. This example would make for quite an interesting diff. eq: we dont have to do the math to figure this out though as there is empirical data that clearly suggests that the 4.08 (or the 4.10 leader gears) ratio is more beneficial for actually getting the car moving from a standstill, even while it does negatively impact spoolup response.
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