Turbochargers



V8 Turbochargers Description

A Garrett T25 turbocharger is used for each cylinder bank, mounted on a short cast iron manifold at each rear side of the engine bay. The turbochargers feature water cooled bearing housings and integral wastegates, operated by a pressure capsule. The pressure line between the turbocharger compressor outlet and the wastegate operating capsule is routed via a frequency valve, in order to permit boost control by the engine management ECU.

The turbochargers are not handed, and as such require different designs of exhaust manifold. Each cast iron manifold maintains separate gas streams for cylinders 1 & 4, and 2 & 3 (RH cylinder bank), and cylinders 5 & 8, and 6 & 7 (LH bank) up to the turbocharger turbine intake, in order to optimize pressure wave effects. Nine stainless steel studs and self locking nuts are used to secure each manifold to the cylinder head, with a 3laminate steel gasket sealing the joint.

Each turbocharger is provided with an oil feed for its fully floating turbine shaft bearings, from an oilway in the cylinder head via a tapping at the outboard centre of each head. Oil from the turbocharger returns to the sump via a drain pipe directly into the side of the sump. The turbocharger bearing housing is cooled by the engine coolant in order to help control oil temperature and prevent carbonizing during heat soak conditions after engine switch off. The coolant supply is tapped off the front end of the cylinder block outboard water jacket, and is returned to the rear end of the inboard jacket. Steel braided hoses are used for coolant and oil feed plumbing, with additional heat protective sleeving on the LH oil feed and LH coolant return hoses.

The wastegate capsule and actuating rod are calibrated by the turbocharger manufacturer to limit the mechanically controlled boost pressure to 0.35 bar (5.0 lb/in2), representing the base setting which can be increased by the ECU controlled frequency valve bleeding pressure from the capsule pressure line. The base setting can be checked using a using a dial gauge and hand operated pressure pump with 0 - 1 bar (15 Ib/in2) pressure gauge:

  • Mount the dial gauge against the end of the actuating rod.
  • Connect the pressure pump directly to the capsule, and record the pressure required to produce an actuator rod movement of 0.4 mm (0.015 in).
    Specification = 0.34 - 0.36 bar (4.9 - 5.2 Ib/in2).
  • If necessary, adjust the actuator rod length after removing the 'C' clip and releasing the rod extension from the wastegate lever arm: shorten the rod to increase pressure; lengthen to lower pressure. CAUTION: Do not turn, twist or force the threaded actuator rod emerging from the capsule and affixed to the diaphragm. Hold this rod stationary whilst slackening the locknut and screwing the extension piece on or off the actuator rod as required. Refit to the wastegate lever arm and recheck pressure setting.

The wastegate capsule may be removed by disconnecting the pressure hose, uncoupling the actuator extension from the wastegate lever arm, and releasing the nuts securing the capsule to the mounting bracket.

Back to top of document



V8 Turbocharger & Exhaust Manifold Assembly Removal

In view of the restricted access and harsh fixings environment, it is generally expedient to remove the power unit from the car before attempting to remove the turbochargers or exhaust manifolds.

1. Remove the power unit from the vehicle (See Sub-Section ED. 17).

2. Disconnect the inlet ducting to the turbocharger compressor.

3. Release the three fixings securing the exhaust system to the turbocharger.

4. Remove heat shield wrap, and release the clips securing the compressor discharge hose.

5. Release the turbocharger oil and water hoses:

- Disconnect the coolant feed from the outside of the cylinder block, and the coolant return pipe from the turbocharger.

- Disconnect the oil feed hose from the rear end of the cylinder head, and the oil drain hose to the sump. - Disconnect the wastegate capsule pressure hose.

6. Release the four fixings securing the turbocharger to the exhaust manifold, and remove the turbo, and/or; 7. Release the manifold heat shield (two fixings) and remove the nine exhaust manifold fixing nuts, and withdraw the manifold or turbocharger/manifold assembly.

Back to top of document



V8 Inspection of Components

 
  • Check the condition of the exhaust turbine wheel, and compressor impeller. If any damage to the vanes is apparent, the turbocharger should be replaced.
  • Note that a significant amount of free play in the bearings is a feature of the turbochargers, and is of no concern unless sufficient to allow the wheels to contact the housing.
  • The turbocharger shaft spindle should spin smoothly and freely, without any noise or roughness.
  • Use new hose clips to secure the compressor hose to its duct.
  • Any sign of oil in the compressor housing may be an indication of turbocharger oil seal failure.
  • Check the flatness of the manifold joint face, and machine if necessary, removing the minimum material.
  • Thoroughly check the manifold for cracks, evidenced by discoloration.

Back to top of document



V8 Turbocharger & Exhaust Manifold Assembly Refitting

Reverse the removal procedure with the following notes:
  • Ensure that the dipstick tube is inserted into the block before the RH exhaust manifold is fitted.  Use new gaskets throughout.
  • Apply anti-seize paste to all exhaust fixings.

Torque Settings:

  • Turbocharger to manifold (M10 Kaylock): 40 Nm (30 Ibf.ft)
  • Manifold to head (M8 Aerotight): 20 Nm (15 Ibf.ft)

Back to top of document



I4 Turbocharger Description

The cast iron exhaust manifold converges the gas streams from four branches into two, joining cylinders 1 with 4, and 2 with 3. These two gas streams are kept separated up to the turbine housing of the Garrett AiResearch type TB03 turbocharger which incorporates a water cooled bearing housing and integral wastegate. The exhaust gases spin the turbine wheel at speeds up to 100,000 rpm, before exiting the turbocharger and flowing through the catalytic converter and exhaust silencer. The turbine wheel is fixed to a short shaft supported by fully floating bearings in a water cooled housing, with the other end of the shaft carrying the compressor wheel. The compressor draws filtered air from the airbox and centrifuges the air out of the compressor housing into the air/water chargecooler, where the hot, compressed air is cooled by a circulated water supply before entering the intake plenum chamber.

The extent to which the intake air is compressed is dependent primarily on throttle opening and engine speed, but is mechanically limited by the action of an exhaust wastegate, which is designed to open at a specified boost level and bleed exhaust gas away from the turbine through a by-pass channel. The wastegate consists of a coil spring/pneumatically operated flap valve fitted between the turbine housing inlet and outlet which, when opened, diverts a proportion of the exhaust gas away from the turbine to limit the boost pressure built up in the inlet. The flap valve is linked to an pneumatic capsule which contains a spring to hold the valve shut, and a diaphragm pressure chamber connected by a hose to the boost pressure at the compressor outlet. As boost pressure builds up, the force in the pressure chamber, opposing the spring pressure, rises until the flap valve is opened.

In order to allow for controlled boost pressure in excess of this mechanical setting, a solenoid valve is fitted into the boost pressure sensing hose and under the control of the engine management ECM, is able to bleed off a proportion of the hose pressure in order to delay the opening of the wastegate and allow a higher boost pressure to be developed. As an engine safeguard, in case of a boost control system failure, the ECM will shut off the fuel pump and ignition if boost pressure in excess of 1.03 bar is detected for more than three seconds. For full details see section EMH.2 - T of the Lotus Service Notes.

The turbocharger bearings are supplied with an oil feed from the oil gallery cover at the right hand rear of the block, and an oil drain hose is provided to return oil to the left hand side of the sump. In order to help protect the turbocharger bearings from the effects of heat soak after the engine has been stopped, a water feed and return system is provided, and connected between the heater take-off at the rear of the block and the header tank. Water circulation around the bearings continues after engine switch off, by thermo-siphon action, and reduces the possibility of carbonization of the oil in the turbocharger.

The turbocharger unit is, with the exception of the wastegate capsule, a non-serviceable item which must be renewed if faulty. A certain amount of free play in the shaft bearings is a design feature, and should cause no concern unless the amount of play allows the turbine or compressor wheels to contact any part of their housings. The shaft should turn freely and smoothly and the turbine and compressor blades should be free form signs of mechanical damage. Note that great care must be taken when working on the engine to prevent any foreign bodies from entering the turbocharger or the wheels will be severely damaged.

Back to top of document



I4  Wastegate Capsule Replacement

To remove the capsule, pull off the pressure sensing hose, remove the circlip from the flap valve operating arm to release the actuator rod, and release the two capsule fixing nuts.

After fitting the new capsule, it is necessary to set the actuator rod length to achieve the specified maximum boost pressure of 0.65 bar (9.5 Ib/sq.in). For this purpose a hand pressure pump (e.g. seat lumbar support bulb) and 0 - 0.7 bar (0 - 10 Ib/sq.in) pressure gauge are required:

Apply pressure to the capsule, and observe the pressure required to produce an actuator rod movement of 0.40 mm (0.015 in).

Specification = 0.58 0.02 bar (8.6 0.25 Ib/sq.in)

If the pressure is below specification, the actuator rod should be shortened, and if above specification, lengthened.

CAUTION: Do not turn, twist or force the threaded actuator rod emerging from the capsule and affixed to the diaphragm. Hold this rod stationary whilst slackening the locknut and screwing the extension piece on or off the actuator rod as required.

When the correct specification has been achieved, fit the rod onto the flap valve arm with the circlip, and connect the pressure sensing hose.

Note:

  1. The practice of 'revving' the engine before switching off should be discouraged since the turbo will continue to spin, due to its inertia, after the engine has stopped and the pressurized oil supply has ceased.
  2. After a fast run, the engine should be allowed to idle for a few minutes before switching off in order to maintain oil circulation whist the turbo cools down and prevent oil carbonization from heat soak.

Back to top of document