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.
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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.
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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.
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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)
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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.
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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:
- 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.
- 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.
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