The Merkur Encyclopedia

Component: Turbocharger

OE: The Turbocharger consists of 4 basic parts: compressor housing, turbine housing, center rotating assembly, and wastegate (including actuator). From a design and performance standpoint there are 3 parts: compressor, turbine, and wastegate. The difference is that the compressor and turbine wheels are mechanically part of the center rotating assembly, yet they are integral to the performance of their respective housings. Housings are sized by the A/R which is the area of the scroll divided by the radius from the center of the scroll to the shaft centerline. The only difference among the AirResearch turbos is the turbine (hotside) housing; early Merkurs with Automatic transmissions and early SVO s had A/R of 0.63, all others have A/R of 0.48. The early SVO s also were not water cooled. Late TC s had IHI turbos. These are smaller than the AirResearch T3; the IHI will self destruct over 15psi boost. The sizing of a Turbo to a vehicle is a compromise. Some people want max power while others want less lag. Increasing the flow of the engine increases wheel speed which affects turbo performance. The compressor on the FORD 2.3 s is marginal in size but the turbine side is pretty good. The AirResearch turbo is designated by the size family T3 . AirResearch makes other families of turbos such as the larger T4 and the smaller T025 . Compressor.

Aftermarket: The compressor size on the 2.3 is marginally too small. This is because there is too much air flow at too high a boost ratio for the compressor to operate at maximum efficiency. By operating at the top of the flow and pressure regions, the shaft speed is approaching its limit. By operating at less than peak efficiency, the compressed charge air is heated. An efficiency of 65% means that 35% of the energy is heating the charge air above that dictated by the thermodynamic equations. There are several upgrades available to increase flow, pressure ratio, and efficiency. Either the wheel can be changed or both the housing and wheel can be changed. The wheel and housing from a T4 can be adapted to fit onto the T3 center rotating assembly. This is called a T3 / T4 or a Grand National compressor (named after the Buick the compressor came from). Keep in mind that increasing flow capacity will increase turbo lag since a larger wheel has more inertia that needs to be spun up.

Modifications: Using a sanding roll to smooth the housing will improve efficiency slightly. Removing any bumps and evening out the cross sectional area of the inlet and outlet will also improve efficiency. Both will result in lower charge temperature and higher flow capacity.


Aftermarket: The larger the A/R, the more the turbine will flow, or for the same flow a larger A/R will have a lower shaft speed. Both A/R.63 and larger turbines are available, although changes to engine flow have a small effect on turbine sizing.
When using a larger A/R, switching to a T4 may be more cost effective and have better performance.

All the air that flows through the engine flows through the turbine, so using a sanding roll to smooth the surface of the inside will help. DO NOT open up the clearance between the wheel and the housing.

Other: None.


Modifications: The EEC-IV uses the BCS (boost control solenoid) to limit boost to 10 psi under 4000 RPM. Adding an adjustment to the wastegate controller is fairly easy by disconnecting the vacuum lines from the BCS and connecting them will allow full boost at any RPM. Some vehicles need a small orifice to replace the BCS to limit maximum boost. A simple orifice is a metal tube with a crimp in it easily done with diagonal cutters.
The full boost limit is set by the orifice size ratio in the brass fitting in the compressor housing. This fitting has orifices that function as pressure dividers and control absolute boost levels. Drilling out the bypass orifice will increase maximum boost.

BE CAREFUL ON THIS AND TEST WITH AIR COMPRESSOR BEFORE DRIVING VEHICLE. You may need more FUEL to compensate for the added air from more boost or you will run LEAN.

Other: The wastegate works by bypassing exhaust past the turbine wheel. Some turbocharged vehicles have too small a wastegate and under sustained WOT the boost actually increases. This can lead to lean mixtures and burned pistons or exhaust valves. On our T3 the wastegate is sized large enough to effectively limit boost so we don t have a problem with boost creep .

Turbocharger Map: T03 .60 A/R compressor


This map shows the performance of the AirResearch turbocharger compressor installed on XRs, TCs and SVOs. The axis are Boost and Air Flow. Normally these are shown as pressure ratio and Lbs/min, but for this book we are using Boost and CFM since they are more common for the instrumentation available. The efficiency for the compressor is shown as it relates to pressure and flow, and the shaft RPM is also shown.
As shown by the map above, 300 CFM at 18 psi is the maximum (assuming 72% efficiency).


Part Numbers:


Part No.



 XR E5ZE-9G438-AR 466586-Z OD1137  .60  M11
 84-85.5 SVO        
 85.5-86 SVO        
 84-86 TC        
 87-88 TC        

Turbocharger Sizing

go to

If you're driving even a mildly prepped SVO, plug in the following assumptions, which are generous according to what I hear are the flow limits of the iron head: under "RPM": 5,000 rpm for all the rpm boxes, 'cause that's peak hp unless you've retarded your cam.

under "boost": 9, 12, 15, 18, 21

in the five boxes under "VE" (volumetric efficiency of the engine): I use 80%, which is generous, assuming port matching of all the manifolding and good valves.

under "compressor efficiency": that's known, at least for the T3-60 SVO turbo -- peak efficiency is 74%. Other compressors can have higher efficiency, but you'll find that a turbo map is not impressed by a change from 74% to, say, 78%.

under "intercooler": leave the default box checked if you're running the stock IC

under "engine": hopefully you already have this info: 3.781 bore x 3.126 stroke, etc.

under "air temp": the applet wants Centigrade, so I use 26.7C, which is 80F (my San Diego bias) :)

under "IC efficiency": mathematically the stock IC likely is about 70%, the TC perhaps 85% (again, likely generous).

Now -- plot that data against every map you're interested in, such as the stock T3-60 that's in your car, the T3-Super 60, the T04B-H3, the T04E 40 and 46.

Then you'll quickly see which ones plot into the highest efficiency island (good), which ones fall to the right into lower efficiency and hotter air (bad), and which ones plot to the left near and past the surge line (very bad). Note also the rpm at these boosts levels, indicated by the "rib" lines that arc from left to right (or is it right to left -- I'm a Republican, it's always left to right).

Cool, huh? Now you have your answer in living color, and all thanks to some enterprising Aussies!

Compressor Surging
What to do about surging. Experts at Turbonetics and elsewhere can certainly add depth to what I have to say here, but you can't expect too much out of them for free. You're right, the obvious thing is to limit boost to stay out of the surge zone, by adjusting the wastegate. Preferred solution is to select the most suitable compressor for your application that allows you to run desired boost without surge. Since surge tends to be an issue mainly at lower rpms due to the nature of turbocharged engines (study a compressor map if you're not already familiar with this phenomenon), a possible solution can be to select a larger turbine housing to delay the increase of boost with rpm so that the WOT boost curve on the compressor map misses the surge zone. A sophisticated electronic control *could* be devised to guide the turbo past the surge zone, but I've never heard of this actually being done ever. It would certainly need to be carefully calibrated for the application, taking into account varying environmental and operating conditions. I work in a group that develops engine management systems for natural gas truck and bus engines. Our strategy is to size the turbo such that surge is impossible under any normal operating conditions. In addition, our electronics will be able to detect, from rapidly changing airflow and manifold pressure, if surge is inadvertently encountered. A severe intake restriction or operation at very low atmospheric pressure such as at very high altitude are two possible causes of surge in a engine that normally operates nowhere near the surge zone. In this unlikely event, our system will take action to quickly take the engine out of surge. This corrective action will effectively derate the engine, which is proper and necessary under such conditions in any event.

Manifestations of surge. There are four surge phenomena I have personally observed. One, which I am not completely sure was surge, occurred when I was test driving the used Merkur that I subsequently bought. Unbeknownst to me at the time, the wastegate hose was broken. It was the first Merkur (or turbocharged car of any kind) I had driven any distance, so I was completely unfamiliar with expected turbo behavior and boost levels. At full throttle on this test drive, boost rose quickly through the red zone, setting off the buzzer. As this happened very quickly and as explained above, I did not realize it was abnormal, I did not let off the gas immediately. At somewhere over 20 psi, the engine bucked noisily a couple of times before I finally let off the gas. All this took only a second or two from the moment the buzzer sounded. After I bought the car, I soon found and fixed the wastegate hose. Another incidence of surge that is quite familiar to me is the fluttering sound (several times per second) synchronized with the rapidly up and down changing volume of the turbo whistle made by a intercooled throttled engine with no compressor bypass valve, when the throttle is closed suddenly while the boost is high, e.g. during initiation of a manual shift. The third manifestation I have observed occurred during an engine development project. The engine was running at steady state WOT, when suddenly its torque just seemed to collapse and the resistance of the dyno stalled the engine. This occurred near peak torque rpm but not at higher rpms. We repeated this test many times while troubleshooting our fuel and ignition systems. These were eventually eliminated as potential causes. Our data from these tests showed the onset of rapidly fluctuating air flow, with the fluctuation increasing in magnitude prior to the engine stalling. I theorized that the turbo could be surging. Unfortunately, we did not have ready access to a compressor map to check this theory, so I just reduced the wastegate setting from 18 psi to 15 psi. The problem disappeared. A fourth I encounter occasionally with my modified Merk. I have the 0.36 A/R turbine housing, which increases low rpm boost and an intercooler which increases mass airflow rate, other things being equal. Both of these effects move the compressor operating point toward the surge line. It's no big surprise that I sometimes get a mild "rocking" sensation when at a fairly steady state condition climbing a hill at say 2000 rpm with 15 psi boost. I suppose this could be the wastegate or knock control, but I never get it at higher rpms, and if I lug the engine down lower than this it gets worse. As I've said, it occurs rarely and is pretty mild, so I am content to leave things as they are and drive around it.

What is happening. The compressor blades are analogous to a wing. As they move through the air, they direct the movement of the air according to their aerodynamic design. If an airfoil is operated at an angle of attack beyond its design limit, the flow on the low pressure side of the foil separates instead of following its contour. This condition is known as "stall". The surge line indicates where the pressure differential, air velocity, and compressor rpm conditions combine to define the aerodynamic limits of a particular compressor. If operated at or beyond this line, the compressor stalls and surges in and out of stall until external changes are imposed to keep the compressor away from this zone.