boost controller

[Martin F]

The turbo will spool quicker and faster because of the pressure differential across the turbine blades being greater than when the cats are fitted.

I'm not sure if this is the best (simplified) way to explain it but i'll give it a go. If the exhaust gases coming out of the exhaust port exert say 30psi of pressure on the turbine blades and there is nothing beyond the turbine blades then the pressure differential across the blades is 30psi, and hence they will spin at a rate relative to that differential.

If you now put a restriction on the other side of the turbine blades (cat, restrictor ring, small bore exhaust, etc) then a certain amount of back pressure will be present as the gasses can't exit as quickly as they did previously. Lets say this restriction creates 3psi of back pressure, now the differential across the blades will only be 27psi so therefore the blades will not spool up as quick or have as fast final speed.

The wastegate works in a fashion where it diverts gasses away from the input side of the turbine blade, thus reducing the pressure.

Fitting a restrictor ring to a Jap spec car is indeed a kludge, as you are compensating for the fact that Jap spec wastegates can not divert enough gas away at high rpm, so by putting a restriction in the exhaust you are reducing the pressure differential and limiting the max pressure differential possible.

But (aligned with what you are saying) this reduced pressure differential will be there at all points of spool up, so a car without a restrictor ring\restriction will always spool quicker than one with. However I suspect in most cases the difference is quite marginal.

HTH