Nissan 350Z Performance
Intercooled Twin Turbo Installation Information
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APS Intercooled Twin Turbo System - Oil Drain Details

This notice is targeted at those who are experiencing light oil smoke emission from the exhaust under deceleration. Whilst APS has yet to encounter or reproduce this problem, it is most likely caused by insufficient oil drainage due to incorrect fitting of the molded rubber turbocharger oil drain hoses and/or overfilling the engine with oil.

APS had always intended that the APS Intercooled Twin Turbo system be installed by authorised APS dealers who are aware of the information contained in this article. The installation instructions are geared towards this fact however due to the increasing frequency of owner installations, the information presented here should be seen as an adjunct to the APS installation instructions.

The following technical article provides more detailed information to ensure the correct fitting of the turbocharger oil drain hoses and details the correct amount of oil to be used for oil changes (drain and fill - with and without oil filter replacement). If oil had entered the exhaust system previously, it may take some time for the oil to be completely expelled after the following procedure has been followed.

Oil is fed into each turbocharger under pressure and then gravity drains back to the oil pan via specially molded turbocharger drain hoses. It is critical that the oil drain from each turbocharger is arranged to ensure optimum drainage back to the oil pan.

Drain hose orientation

Ensure that there are no kinks in either turbocharger oil drain hose and that oil is free to drain without restriction along the entire length of hose. In addition, there must be a constant downward fall along each drain hose, otherwise oil pooling may occur inside the hose resulting in insufficient internal free hose volume and excessive oil pressure inside the turbocharger.

The images below show the correct orientation of the turbocharger oil drain hoses.

RHS turbocharger drain hose showing constant downward fall

LHS turbocharger oil drain forward of engine mount showing constant downward fall.

LHS turbocharger oil drain hose rear of engine mount showing constant downward fall.

Whilst partially restricted oil drainage has no adverse affect on the performance or durability of the turbocharger, oil may be forced through the turbocharger and into the exhaust - resulting in light engine smoke on deceleration.

Drain hose connection to turbocharger

In order to ensure the optimum oil flow from each turbocharger to the respective drain hose, it is important that the top part of each hose is trimmed appropriately (to achieve a constant downward fall as described above). In addition, it is critical that the drain hose (particularly on the LHS turbocharger) is not pushed too far into the turbocharger oil drain fitting. The turbocharger oil drain relies upon flowing oil into a straight section of hose, not into the bend of the hose.

If the hose is pushed too far into the turbocharger oil drain fitting - i.e. well into the bend of the hose, there is insufficient internal surface area immediately after the oil drain fitting to ensure adequate oil flow.

RHS Drain hose connection to APS oil pan

The oil drain fitting into the APS oil pan are of tapered thread construction to ensure the optimum seal. The fitting is inserted into the oil pan and depending on the force used to screw the fitting in, the resultant finished height may vary.

This additional height may result in a reduction in the amount of constant downward fall as described at the top of this article. In these cases, in order to achieve the required amount of downward fall, it may be necessary to trim the lower part of the molded oil drain hose.

Measure the height from the top of the machined face of the APS oil pan to the top of the nut section of the the tapered oil drain fitting. Then trim that same amount from the end of the molded oil drain hose and refit the hose end.

RHS Drain hose connection to APS oil pan

Measure the LHS molded oil drain hose, oil pan end by holding straight edges against the hose as shown and trim 60mm down. This will ensure the best possible downward fall and unhindered oil flow - by not allowing the fitting inlet to protrude into the hose bend radius.

Oil Restrictors

Each APS/Garrett GT Series ball bearing turbocharger is fitted with the optimum sized oil flow restrictor to control the volume of engine oil flow through the turbocharger. Under no circumstances should a smaller restrictor be fitted. Smaller restrictors can be easily blocked by foreign debris resulting in damage to the turbocharger's bearings. Fitment of any non-genuine restrictor will instantly void warranty on the Garrett GT Series turbocharger.

High Crankcase Pressure

Many built engines fitted with forged pistons utilising large piston to cylinder bore clearances (and indeed engines with worn or broken piston rings) can experience high crank case pressures.

Obviously the amount of crank case pressure within an engine is out of APS' control (especially if an engine is built to clearances other than Nissan factory specifications).

High crank case pressure may lead to insufficient or reduced oil drainage from the turbochargers due to the high pressure experienced inside the oil pan. In these situations, it is recommended that the condition of the engine is verified via a cylinder leak-down test and the stock Positive Crankcase Ventilation system's breathing capacity be improved utilizing a full oil catch can appropriately sized to alleviate the higher than stock crank case pressure.

When one is considering forged pistons with large piston to cylinder bore clearances, an efficient catch can system should be considered mandatory.

High Engine Oil Pressure

High engine oil pressure can result in increased oil flow through the turbochargers. Even though there is a good deal of extra flow capacity in each turbocharger's oil drain system, high oil pressure may in some cases contribute to oil smoke emissions from the exhaust.

The reason why this point is raised here is that whilst APS engineers have never encountered a VQ35 engine with excessive oil pressure, there have been discussions on internet forums where oil pressure as high as 120 psi has been mentioned (APS does not vouch for the validity of these claims).

The Nissan factory Service Data and Specifications lists a minimum oil pressure (shown below). Should engine oil pressure be significantly higher than these values, then the engine oil pressure regulation system (pressure relief valve) should be checked and rectified.

Engine speed (RPM)

Approximate discharge pressure
kPa (bar, kg/cm2 , psi)

Idle speed
More than 98 (0.98, 1.0, 14)
2,000
More than 294 (2.9, 3.0, 43)
6,000
More than 392 (3.92, 4.0, 57)

Engine Oil Capacity

The APS oil pan included with the APS Intercooled Twin Turbo system provides for a greater volume of engine oil. The following engine oil capacity figures are provided as a guide to achieve the correct oil volume. Due to the fact that the oil pickup is lowered by approx 1/2 inch (12 mm) into the larger APS oil pan floor, the correct engine oil level is achieved when the stock dip-stick reads at the low mark after being fully inserted.

NOTE: Litres, Imperial quarts and US quarts are different volumes - please ensure that you utilize the correct units and quantity when refilling the engine oil.

Litres
Imp quarts
US quarts
Without oil filter replacement (oil change only)
4.7
4.1
5.0
With oil filter replacement
5.0
4.4
5.3
Dry engine (brand new built engine)
5.7
5.0
6.0


When the turbocharger oil drains are installed to specification with the correct volume of engine oil there should be no oil smoke emissions from the exhaust system.