Some thing for a bit of debate for the turbophiles. Pick the holes in these comments.: -

The speed the turbine spins at for a given engine speed is determined by -

1. Volume of exhaust gas flowing into the turbine.
(More gas from bigger engine = more turbine speed). 1500cc engine will produce approximately half the gas from a 3000 cc engine at the same engine speed.

2. A/R ratio of the turbine housing.

The higher the A/R ratio number, the slower the exhaust gas speed but the less back pressure the engine has to work against. (Off boost response improves.) Smaller engines use lower A/R ratios to increase gas speeds at lower Rpm.

3. Turbine blade trim

(coarser trims accellerate the turbine quicker at lower Rpm, but create back pressure at higher gas volumes) Smaller engines usually use coarser trim blades to get speeds up at lower rpm.

4. Pressure of the exhaust gas.

(More pressure = more turbine speed up to a point). Increased gas pressure can be obtained by changes to exhaust cam profile. Opening exhaust valve earlier allows higher gas pressure, but reduces energy input to crank shaft. Trade offs abound.

5. Compressor wheel diameter ratio's trims.

(Larger wheel trim ratio's give greater flow at lower engine speeds). They also require greater amounts of energy to make the same amount of boost pressure that a smaller diameter wheel will, even though they have greater flow. On smaller engines, smaller compessor wheel diameter ratios allow the engine to make higher boost pressures with reduced flow, without creating excess exhaust back pressure.

6 Compressor housing A/R

The A/R of the compressor housing helps determine the amount of air that the compressor can produce without losing efficiency. The larger the A/R ratio, the more flow for a set turbine speed and pressure. Smaller engines use smaller housing ratios. This allows higher boost pressures with reduced flow rates.


With a VG30 turbo on an A15 it will take twice the normal amount of engine speed to produce the gas flow to get the compressor on boost. The A15 also uses half the air of the 3000 cc as delivered by the compressor. This means, the ratio of boost pressure to exhaust back pressure remains the same on both engines. (It just means the whole compressor map is moved up to twice the engine speed of the VG30T engine)

If the turbo came on boost at 2000 on the VG30,I'm tipping it will come on boost at approximately 4000 rpm on the A15.

It will probably go like hell at higher engine speeds but I think it will be a bit soft at speeds below 4000 rpm though.