This outline assumes the following:
- No A/C. If You want A/C, it requires plenty of extra work, but the same techniques
apply.
- Wiring harness will contain no extraneous wires or circuits.
- The completed harness will look as good or better than the original
harness.
- All functions including the gauges and check engine diagnostics will
work correctly.
- All mechanical functions and connections will be as good or better than
the factory produced.
- The 20v motor comes complete with ecu, engine wiring harness, and some
of the body harness. If there is no ECU, you will have to integrate an
aftermarket EFI system to the motor.
Mechanical notes:
- Remove the old motor (with engine harness still attached), transmission
and wiring from the engine compartment and put it side by side with the 20v.
This makes the swap over of functions much easier.
- Many of the vacuum tubes on the back of the 20v motor can be blocked
off. There is one available that works nicely for the brake booster, which
now will route on the back side of the motor with the other lines. The motor
lifting hooks and all the associated vac. plumbing on the rear hook can be
removed once the engine is in place.
- Use the front motor mount from the MR2 (2 bolts, 1 stud) as opposed to
the 20v mount (2 bolts). This seems to be a little sturdier, but you can make
the 20v setup work too.
- Use the crank pulley from the MR2 and timing belt tensioner bolt from
the MR2. The timing mark location on the 20v is not easily visible unless
using the original MR2 pieces. While you are at it, replace the timing belt.
- Use the alternator bracket from the MR2. This is simpler and lighter
than the 20v version, which has an additional idler pulley.
- For the exhaust, you can use the header that came with the 20v
if you also have the 20v bell housing which mounts the starter on
the intake side. Note that the down pipe may be very hard to get,
and fabricating one is complicated by the lack of an available flange
to bolt to the header. Alternatively, the crappy TRD header will work,
but the flange mounts needs to be lengthened for the #1 and #4 cylinders
so it will bolt up. This will require good welding and fabricating
skills, or a good shop. The following photos show what it takes to
make it work: cutting off the outside 'ears' and rewelding them out
further. I just used about 12" of pipe and a stainless steel
Supertrapp welded to it for my 'Exhaust System'. This proved to be
way too noisy, so I used sections of 2" pipe and a free flow
muffler to weld up a custom system.
The outer mounting tabs in the
correct location - Note the large gaps
After extending the outer
mounting tabs
- The 20v motor did not use an oil cooler, and it may not need it, but its
still a good idea, and routing the coolant lines away from the exhaust is
simplified.
- Use the Oil pressure sensor from the old engine. The 20v sensor is a
different voltage range and will not drive the dash gauge properly. Later
MR2's may use the later sensor, so this may not be required. It is easy to
tell the difference between the sensors: the old one is about twice the
diameter of the new one. If your old and new sensors look the same, you will
most likely not need to swap them.
- The coolant overflow tank will no longer fit in the stock location, so
move it or replace with a smaller unit which will fit the space (a 81-84
Starlet tank works great in the stock spot).
- Reroute the cooling system connections. Actually, the 20v configuration
is better than the original. All the connections come off the back of the
motor. Mount the filler neck towards the back of the motor near the fire
wall. It took me quite a bit of experimentation before I was happy with the
job. I ended up using 1 1/4" od .065 6061 aluminum to connect sections
of hose and for straight runs.
- The fuel line from the fuel filter to the engine needs to be 2-4"
longer, or move the filter so it will reach. I was unable to get the correct
fittings to make up a new fuel line (such as Aeroquip or Earls), so I spliced
together half of the original line and half of the 20v line to get what I
needed.
- The throttle cable is at least 4" too short to work well. A cable
should be made up which is 1-2 feet longer so it won't hang across the top of
the motor.
- If installing the motor in a formerly supercharged car, make sure the
intake plenum is bolted on the engine before you install it. The right side
allen head screws are difficult to access due to the placement of the engine
cover latch. The engine cover latch also needs the bottom 1/4" or so
ground off so it won't contact the intake plenum.
- If you want to keep the 20v starter on the intake side of the motor,
and/or want to use the factory header, you will need to mate the bell housing
section from the 20v transaxle to the original MR2 transaxle. You will also
need to replace the speed sensor with the speedo drive from the MR2. This is
a good time to replace the syncros or in general service the transaxle. Most
likely, you will want a shop to do this part.
- The motor and transaxle bolt up differently on the 20v than how the MR2
is configured. The 20v uses a fairly large aluminum stiffener to attach the
engine to the bell housing. If you are using the TRD header, the stiffener
will not fit. This requires that the right rear engine (to transaxle) mount
be drilled out so that the large bolt can pass through it and thread into the
transaxle (this hole was not used for the 20V).
- Use the full back plate (behind the flywheel). If using the TRD header
as it will help secure the engine and transaxle together better than if using
the two piece plate from the original motor.
- The 20v flywheel is like the late model MR2s. Any years flywheel and
clutch will work, but they need to be used as a matched set due to variances
in the clutch diameter. I used the HKS/TRD flywheel (and metal/ceramic clutch),
which is of the same diameter as the 1985 flywheel, and is the lightest steel
flywheel available.
- Replace the Timing belt and belt idler if needed. The 20v motor uses a
111 tooth timing belt as opposed to the 112 tooth belt of the MR2.
The distributor cap and plug wires also may be a problem to get.
- You can use the original O2 sensor, or get a generic 3 wire sensor to
mate to the 20v harness. There will be a 4th wire left over.
- Here's what mine looked like when I was ready to put it in (after a
little custom paint, K&N breather and bolting on the HKS/TRD lightweight
flywheel):
Electrical notes - The heart of the conversion:
- To do this correctly, it will take lots of time - be patient and make
sure you know what each wire is for, or you may ruin the ECU, which is
probably irreplaceable. If you start to tag everything you know on both the
20v motor and the original, the remaining wires are a lot easier to figure
out.
- Decide which accessories you will use - this will also determine how
much of the 'old' engine harness you will need for the new motor. I used the
main battery and power supply wiring (to the starter and to the body) from
the old harness, as well as various bits and pieces where convenient.
- It is not necessary to remove the 20v harness from the motor, or to
strip down the wiring which goes to the injectors.
- Some relays/functions are in different places, e.g. the starter relay is
mounted on the transmission for the 20v and in the fuse box for the MR2. This
will take some figuring and tracing of wires to hook these up correctly.
- Remove the wiring harness from the body and lay it out as it would
appear in the car - this requires the removal of part of the interior,
especially to get the fuel pump wires out.
- Tag all plugs and remove the complete engine harness from the 'old'
motor
- Tag all plugs and remove the complete rear body harness, including the
fuse box and rear lighting wiring. The harness starts on the DR. side kick
panel, goes behind the DR. side seat at the fire wall, to the fuse box and
several plugs in the engine compartment, and finally to all the rear lighting
and a plug near the ECU.
- Strip all electrical tape and wire protectors off both harnesses, but
leave everything in its original shape.
- Trace and label every wire on both harnesses and match by function (not
just wire color as they vary some, and some of mine were different than shown
on other published 20v ecu pinouts).
- Trace and remove (complete preferably) the engine bay cooling fan
circuit from the MR2 Engine harness and transfer to the 20v (This
is a simple circuit and easily transferred, but like the cooling fans,
this fan is not needed in my car dure to the extra cooling vents in
the engine cover and the reduced load from the significant weight
reduction. I have run with no cooling fans for years and never had
the temp go over 1/3). The ambient air sensor is on a bracket at the
top of the motor close to the firewall on the left side. The Relay
is bolted to the ecu. This entire circuit will need to be grafted
into the 20v engine harness.
- Eliminate unused circuits. There are a number of circuits you may not be
using (there was an unused emissions box on mine along with a/c and others),
which you can remove from the harness. Eliminating all the unnecessary wiring
is a great first step to see what you really need to do and saves about
10lbs.
- The 20v ignition coil, ignitor pack and capacitor (?) are standard late
model Toyota parts. You can also just clip off the connectors and attach the
old type connectors in order to use the original coil, etc.
- There are only 3 plugs that connect the engine harness to the body (2
gray plugs under the fuse box and a square white plug in the trunk). Now that
all the wires are labeled, it is fairly easy to match the 20v harness wires
to the MR2 body harness. For now, just twist the matching wires together and
put some tape on them to keep them from falling apart. Note: Most of the
wires will match in color/stripe, but not all.
- On my motor, I had a few plugs that were not immediately obvious what to
do with like the speed sensor connector and the MR2s injector resistor pack.
The 20v car used an electronic sensor instead of a speedometer cable. This
connector goes to another mystery connector, which is a Black, 2 row 22 pin
connector with only 9 or so pins used. Most of these pins map to the square
white plug in the trunk, and some are just eliminated. The MR2 uses a big
aluminum resistor (for the injectors) that mounts on the battery support
bracket. The 20v motor does not require this resistor, so it can be removed.
The 'INJ' relay in the fuse box is also no longer required.
- The original 20v wire routing will work pretty much as-is on the MR2.
This will end up much neater than the original MR2 rear harness and
eliminates the wiring going through the passenger side trunk area. The ECU
connections will be mostly all set to go. The tedious part of the job is
getting all the ancillary functions swapped over.
- The ECU will now mount on the driver’s side of the trunk instead of the
passenger side. Make up brackets to hold it securely - vibration can easily
damage the ECU.
- After all the wiring is done, and temporarily laid in the car, adjust
wire lengths and solder everything together and put all the wires in looms.
- For ultimate neatness, shorten the trunk harness to adapt to the new ECU
location.
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