It has long been my contention that the tiny Garret T-28 turbo which is OEM on on many Japanese twin turbo engines and used by most speed shops when doing a turbo conversion or when installing an small displacement high efficency engines is much too small for extreme levels of performance.
These turbos were spec’ed from Garrett to have very little lag. Quick response at low OEM like boost levels is what the original design intent set by automakers. If one is just interested in a stock level of performance these small turbo are fine in pairs. If one wishes to compete with the big boys, something more is needed!
If boost levels above 10 to 12 psi are intended to be used, the T-28 is challenged. Higher boost levels are possible with the T28 but damage to their thrust bearings and even engines can result if these turbos are pushed higher than this. Running boost levels of over 15 psi are in the range of diminishing returns on these turbos. Even if higher boost levels are reached, the corresponding gains in hp will not correlate for the reasons which we will list below.
Above these boost levels the exhaust side of the turbo is physically too small to flow enough exhaust gases and the backpressure behind the turbine starts to increase. When the boost is turned up past the point previously mentioned, the back pressure in the exhaust manifold soars to over 50 psi. When 50 or so psi is present in the exhaust manifold and only 10-15 psi in the intake manifold, it is possible to get backflow of hot exhaust gasses through the engine during the overlap period where both the intake and exhaust valves are both open. Normally an engine depends on the inrush of relatively cool intake air and fuel during the overlap period to internally cool the engine’s valves, piston tops and combustion chamber. Because of this backflow or reversion, the engines internals start to get heat saturated under high boost. When things get real hot they can cause the fuel air mixture to auto ignite causing detonation and damage to internal parts like piston rings and so fourth.
At 10 to 15 psi, the tiny T-28 compressor is zinging to the tune of 250,000 rpm plus. At this speed the air is being sonically whipped to a froth so to speak and being beaten to death. This physical abuse of the air raises the temperature to over 350 degrees F, further increasing the heat load on the engine. Imagine your engine ingesting 350 degree air! Think how much the CAI helps you guys with NA engines when it drops the inlet temperature a mere 50 degrees!
All this heat accumulation results in three things, one the engine becomes prone to damaging detonation, two, the engine cannot make much power because of charge contamination by the reversion and three, a reduction in power caused by the hot thin air being pumped in by the turbo.