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Click here for Turbo theory by Mike Kojima.
(writer for Sport Compact Car and brains behind this car)
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Turbo Theory, By Mike Kojima

Project Ultimate SE-R, Searl’s 200SX SE-R

Here is a preview of Project Ultimate SE-R. I would like to thank Peter Medina of F-MAX Fabrication and Clark Steppler of JWT for making this project possible. I would also like to thank Rick Head at Turbonetics for assembling this very custom turbo in such a timely manor.

Conventional wisdom has always been that in order to reduce turbo lag one would have to run a small turbo. Big, powerful turbos were laggy and unstreetable. When we started this project , we set out to show that a big turbo can be better for reliability, power and still have a reasonable, streetable amount of lag.

It has long been my contention that the tiny Garret T-25 turbo which is OEM on the SR20DET and used by most listmembers when doing a turbo conversion or when installing an SR20DET is much too small for extreme levels of performance.

These turbos were spec’ed from Nissan to have very little lag. Quick response at low OEM like boost levels is what the original design intent set by Nissan was. If one is just interested in a stock (DET) level of performance these small turbo are fine. 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-25 is challenged. The Pulsar GTI-R’s T-28 is only marginally better being good for about 10 to 15 psi. Higher boost levels are possible with the T25, 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

At 10 to 15 psi, the tiny T-25,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.

On Searl’s car we are doing some radical steps to reduce both the backpressure induced reversion and intake charge heating. The turbo we are using is a pretty extreme application that in theory should work well. It is basically a Garret TO4, T3 hybrid with a few twists. The compressor is a TO4E rather than the common TO4B that most people run. The TO4E is remarkably efficient, being able to maintain close to 78% efficiency from 10 to 20 psi of boost pressure. I think the Miata supercharger boys are exalted to declare around 60% efficiency at only 6-7psi of boost which I believe is a good comparison in the potential difference in power that this turbo is capable of producing over the supercharger. This means that the intake charge will be about 100 degrees cooler than the T25 even before it even hits the intercooler. The high efficiency also means it will take less shaft horsepower to turn the compressor wheel. This allows a relatively free-flowing turbine to be used because it will not have to recover as much energy from the exhaust stream to spin the shaft and compressor.

On a side note, the TO4E originally came from a big diesel truck motor and is a mid eighties design. The common TO4B was designed in the late 60’s. Thus the T04E has the benefit of being designed after at least ten years of fluid dynamic research which shows in its better performance.

On the exhaust side is a T-3 turbine, but it is the biggest T-3 turbine available, known as the stage III by Turbonetics. It is used on their all out Buick Grand National turbo It was originally OEM for a Navistar Diesel engine! This is a pretty free flowing turbine selected in our case mostly to eliminate backpressure.

These parts were also chosen in part to reduce lag! The T04E wheel makes a good amount of boost at 80,000 rpm, down quite a bit from the 250,000 the T-25 spins at. So even though the T04E is quite a bit bigger and heavier, it does not need to be spun up to such a high rpm to make big boost numbers. The big wheel also starts to move air at a much lower shaft speed so the boost onset rpm is kept low. Thus the lag will be kept reasonable even with big honkin reciprocating parts.

The lower shaft speed of the TO4E results in a more gentile handling of the air and thus less charge heating. A more technical explanation is that the super fast spinning T-25 has the compressor wheel tips traveling at near sonic speed. When the tips reach the speed of sound the air forms shock waves inside the compressor and the turbo stops pumping. Operating at conditions near this cavitation point are not so hot for efficiency which creates more charge heating.

An exotic ball bearing center section was selected to eliminate the traditional sleeve bearing’s oil-induced viscous friction in the center housing. The Nissan GTP car used this technology to reduce turbo lag and we will use it here. The ball bearings should help the turbo spool 5-600 rpm sooner.

I feel that the big gun turbo will start to produce boost by 2500 rpm, be impressive at 3500 and get scary at 4500 rpm! This is about 1000 to 1500 rpm later than the spool up point that is enjoyed by the ultra responsive turbos run by the T25,T28 crew but is still quite streetable. The power of the big gun will climb strongly until redline instead of surging hard to 5000 to 6000 rpm then falling off. The smaller turbos start to peter out in the 5000 to 6000 rpm zone because the backpressure starts to rise, leading to power robbing reversion. That is one of the reasons why the Stillen car turbo car was not as fast as what everyone had hoped it would be. This is not to speak badly of Stillen as their design intent was to produce an ultimately responsive, factory like, super-streetable turbo while maintaining a reasonable price, a mass market turbo kit so to speak. I believe that they effectively accomplished that goal. Our goal however is to push the envelope of SR20 performance! Price was no object here as this turbocharger alone will retail at about 1400 big ones.

By reducing reversion and intake air temperature we should be reducing the chances of detonation which should allow us to run more boost on pump gas. When Searl completes the low compression bottom end, I am figuring that at least 17 psi should be possible on good old 92 octane pump pee. On stock compression we are hoping for 15 psi with the aid of a sophisticated 3-D mappable water injection system and 10-12 psi without it turned on.

Although Searl is not planning to drag race his car, nor is his car set up for drag racing, if he did, with a drag racing suspension and slick set-up, this combination could push his car into the 11’s with about 420-450 hp to the wheels@20-23 psi on race gas. Searl’s main intent is to give those turbo Porshe’s hell at Willow springs, once we figure on how to get that hp to the ground!