This is the most interesting topic so far. This is an exciting topic because most first generation Lexus LS400/Celsior and Lexus SC400/Soarer owners are interest in forced induction like supercharger, turbocharger and NOS. The first generation Lexus 1UZFE engine is about 111-14 years old now and average about 120,000-160,000 miles (this is my estimation, none objective data available). Any first generation 1UZFE owners would think twice before investing $5,000 into a forced induction kit. The first thing is to find out about your engine condition. Running smooth and low mileage Lexus LS/SC is not a very good indication of your engine internal health. Another word we are not sensitive enough to detect the minor engine defects. Below are two tests that can be done by yourself (leakdown) or with your friend (compression test).
From my experience, I paid $80 for a compression test and $300 for a leakdown done by two different shops. I think the $300 for the leakdown test was little on the high side. I would expect the price for the leakdown test is around $150 or so. Anyway, you can save some money and learn how to do it yourself or with your friends.
Compression Test Kit:
Compression Test kit is pretty cheap and easy to perform. Compression Test
(CT) kit is about $25.00 from Ebay. Here is how CT looks like. A
basic kit usually comes with a pressure gauge along with spark plug adapters.
These kits are pretty much universal and work on both domestics and imports, but
contact the seller before purchasing
it. If you just do one test on this kit is worth the money already.
Keep on reading this article for instruction on how Compression Test is done.
Leakdown Test Kit:
Leakdown Test kit (LT) is slightly more complex, but still very simple. LT
usually comes with two gauges, pressure regulator and along with spark plug
adapters. 
This kit is
not 100% complete and no one sell a complete kit. The only that is missing
is a air compressor. You will need a air compressor or air tank to do this
test. Its wise to invest in a 2.5-3 hp, 6 gallon air compressor. The
price range is about $135. Luckily, I just got a air compressor and loving
it. The LT kit usually comes with two pressure gauges. However, most
air compressor also have a pressure gauge too and you can use that a your base
line and you can make your own leakdown test kit. You will need another
pressure gauge along with spark plug adapter. The DIY kit will run you
about $30, why mess with it. You can get a leakdown kit for about $55.
The picture on the left is an example of a leakdown test kit with out the air
compressor.
Below are instructions on leakdown
and compression test from Corky Bell’s Supercharger! book. He did such a
good job on it so I just scan the whole article and post it in this page.
Credit should be given to Corky Bell for his hard work.
Engine Condition:
The Combustion Chamber seal is me most important aspect or
engine evaluation If the chamber seals well, it follows that the valves, head
gasket, and compression (, rings are in good condition. The condition of the
crankshaft bearings and oil control ring are also of concern. Evaluating them is
nothing more than insuring the engine does not rattle or consume excessive oil
and maintains factory tolerance oil pressure. Excessive oil consumption is a
loose term but can generally considered a quart in 500 miles.
Checking
engine condition:
Two methods exist for checking the quality of the combustion chamber seal. The
cheapest, easiest, and quickest is also the
least reliable and the least information the chamber-pressure check on cranking
the engine. The other method, and clearly the superior one, is the leakdown
check. Proper procedure must be observed with both methods if the results are
to be the best possible.
Compression
check:
The quality of the cranking-pressure check depends on many factors, all of which
need to be adhered to for the results to offer good data. Here is the
Lexus OEM Compression test instruction.
Here’s
the procedure:
1. Warm the engine to standard operating temperature.
2. Remove all spark plugs.
3. Attach a battery charger
to the battery. A decreasing battery charge over course of a compression check
can erroneously indicate that the last cylinder checked has the lowest pressure.
4. Block the throttle open.
This is urgent when checking an engine with a throttle plate for each cylinder.
Imagine trying to squeeze the air to an acceptable pressure when the throttle
is closed and won’t let anything in to be squeezed. Not quite so important on a
single-throttle V8, but block it open anyway.
5. Insert the pressure gauge
into each cylinder in turn, crank the engine, and record each cylinder’s
pressure.
<![if !vml]>
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you find a bad cylinder, checking and correcting the valve lash for solid-lifter
engines will produce greater cranking-pressure accuracy. But time spent on
setting valve lash could be spent doing a leakdown check. A weak hydraulic
lifter will also cause errors where not enough air is let in due to less valve
action.
The results of a compression
check are only relative comparisons of the chamber pressures. If a cylinder
indicates less pressure, that remains the only fact known. No hint is available
to suggest where the pressure went.
Evaluation of the pressure variances is a judgment call. This writer would
sug≠gest that 5 psi from best to worst is an excellent chamber seal. A 10-15
psi variance is acceptable, but past 20 psi, it becomes time to refresh the
engine. Between 15 and 20, try the coin toss or do a leakdown check.
Leakdown check:
Good science and a little
more effort can give the investigator a much better insight into the quality of
the combustion chamber seal. This process is called a leakdown check. The idea
is to pump a given pressure into the chamber through the spark plug hole and,
with a separate pressure gauge, measure the pressure remaining in the chamber.
An air compressor or large
storage tank is necessary, to maintain a constant source of air pressure. It is
most convenient to use 100 psi for the pressure into the chamber, as the
remaining pressure after leakage will automatically indicate the percentage
seal. Fifty psi would prove equally convenient by doubling the numbers, but
slightly less accurate. The engine needs to be at standard operating
tem≠perature. A leakdown gauge can be bought at a racing supply, shop; it can
also be built.
Procedure:
1. Remove all spark plugs.
2. Place the cylinder to be
checked at top dead center with the valves closed. I may be necessary to hold the
crank in that position with the parking brake, while in gear, or by using a
front pulley wrench. The piston must be close to TDC so the air pressure won’t
cause it to walk down the bore. TDC can be found by inserting a narrow wood
dowel through the spark plug hole and feeling when the piston reaches TDC. With
a wrench on the crank pulley’s front nut, wobble the crank back and forth
slightly, right at TDC, until the piston exhibits no motion. That will be TDC.
Remember to put your transmission in Neutral
so you can freely turn the engine.
3. Install the leakdown gauge
in the cylinder.
4. Adjust the pressure gauge
to read 100 psi input.
5. On the output gauge, read
and record the pressure remaining in the chamber<![if !vml]>
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Judgment of the measured
numbers is somewhere in this area:
97-100 Very Good
92-96 Serviceable
89-91 Ok but impaired
<88 Fit it
Not only does the leakdown check indicate real percentage sealing capability, but much more data can be gathered with brief investigation. Obviously the leakage, regardless of the amount, must go somewhere. With the chamber pressurized remove the oil filler cap and check for airflow out the hole. This air must be passing the compression rings.
Likewise, listen carefully at the tailpipe for exhaust valve condition and at air filter (with the throttle open) for intake valve leakage. In most instances, airflow can be detected at each point, but by blocking the tailpipe and watching pressure gauge rise slightly, the percentage leak at the exhaust valve can be e mated. On a solid-lifter engine, if you determine that the valves are leaking, check the valve lash and repeat the leakdown check (certainly before taking the engine apart).
Remove the radiator cap, mindful of hot coolant under pressure, and check for bubbles. Bubbles will indicate head gasket condition or cracks in the head or block. A further value of the leakdown gauge is in checking leakage of valves and rings when rebuilding an engine. A cover plate bolted to the combustion chamber can “leak down” the quality of valve refacing. With a plate attached to the engine-block top surface, one can even leak down a cylinder. The cylinder can also be checked for leakdown at varying piston positions down the bore.
Which test I would recommend? Leakdown of course. This test give you so much information and you can do it by yourself. The compression test need two people. However, a air compressor is needed otherwise, a leakdown test is highly recommended before any type of forced induction. If your internal engine health is not satisfactory, you should fix it first and than do forced induction later. Trust me, its cheaper to fix your engine problem first and putting on forced induction and fix your engine later. A big mistake!