have to be smooth - again, not spit shine, but certainly finger smooth.
Casting flaws and ridges have to be smoothed away. The entry of the
ports can be made slightly larger than the manifold runners (use the
amended gasket as a guide to draw the new edges).
It helps prevent reversion (a bit!).
A Dremel extension would be quite handy:
you're going to enlarge the ports, remember that most of the gains are
at the TOP of the port, that's where you should focus.
you feel that all this is totally over your head, then just cleanup
and polish things, and put it back together. It's very easy to botch
it up and make the head flow worse in the end! As this
is non-reversible, it's best to be safe than sorry.
on porting can be found at the bottom
of this page
good guide on head porting is here
(local copy here)
your finger down the valve stem. If you can feel any anomalies that
are not due to carbon deposits, the valve (and probably the valve guide)
has to go. The valves can be soaked in degreaser overnight, and have
the rest of the carbon taken off by fitting it on a drill and using
sandpaper to do the work. If in doubt, use the smoothest paper you can
find. Do protect your eyes and lungs from all the carbon dust that will
float about. Also be VERY careful not to touch the valve seat. It's
sacred. If sandpaper hits it, all the grinding paste in the world won't
bring it back.
how it should look after the operation:
valve heads have three-angle cuts, not bad. They can be further improved
by rounding up slightly the two outer angle edges (the ones further
away from the valve seat). This will increase VE, by improving flow
at the lower valve openings.
it's time to lap the valves. Look at the seats carefully, both around
the valve heads and on the cylinder head. If they look fine, then only
use fine grinding paste. 10 seconds lapping should do it, wipe them
clean, apply a small drop of fresh paste and lap once again. If the
smudge from the paste is a full circle with consistent thickness, you're
done. Wipe everything clean and proceed to the next valve. Every valve
will have to go back to it's old place, so keep them separate and clearly
Tappets (Hydraulic lifters) are covered in more depth
the head happens to be lying around and the valves are already out,
it's a good idea to change the stem seals. They're cheap, and the engine
will burn less oil. After 60-70K miles they might start leaking. The
symptoms are similar to the turbo bearings going south, so with turbo
cars you're never really sure if the stem seals leak or not. Change
them while you're at it.
If you buy pattern ones, better order a few spares, as once they are
pulled out they can't be used again. Then some may disintegrate as you
push them in place (the tiny coils brake loose - don't force them, or
it may happen with the engine running!).
Use a deep 11mm socket to push these seals in place, much safer than
pushing them by hand!
of all the work involved in changing them, I would only use original
Vaux seals. I've learned this the hard way...
stock valve springs are not too bad, despite what resellers of aftermarket
'uprated' items might tell you. They are the best compromise for lobe
long-life, power-sapping and smooth valve operation at all revs.
first thing to check when the head is dismantled is the 'free height'
of the springs. As they age they tend to shrink, losing
their elastic properties and allowing the possibility of valve float
at high revs.
is a brand new LET valve spring.
42.25mm. Note that the accuracy of the instrument is far higher
than the margin of error in this case, as the spring top and bottom
surfaces are not totally level.
treat this as an indication that if the springs are less than
42mm tall I'd keep an eye on them, and if they are 41mm or less
I'd consider replacing them all.
high boost turbo applications certain unique conditions can occur that
n/a engines never experience.
boost pressure is high enough, it can literally force the inlet valves
open. Similarly, if exhaust backpressure is high enough, it can (potentially)
interfere with the exhaust valve timing. It's not always cut'n'shut
because the intake/exhaust pressure ratio is also important. But to
keep things simple, lets just say that normal 'race' engines need stiffer
valve springs for reasons of overcoming inertia forces (revving at much
higher rpm) while tuned turbos don't usually have to rev any higher
than stock. So their valvetrain issues are completely different, and
non-turbo experts are the wrong people to consult when it comes
to such 'uprated' parts that can do more harm than good.
Spark Plug Protrusion
the head is out, tighten the spark plugs temporarily and check that
they all protrude equally in the combustion chambers. If any one is
different, it has to be fixed before the head goes back on. Use this
opportunity to thoroughly clean the threads. Crossthreaded plugs are
not funny while the engine is in one piece.
and their optimisation are HERE. Also the famous "XE inlet"
have to be cleaned in a similar way to the Intake valves. Most probably
they will have more carbon deposits and their seats will be in worse
condition. Coarse grinding paste might be needed for all of them, but
once the seats look decent, wipe it off and switch to fine paste. You
don't want to overdo it and damage the seats with too much lapping,
you'd need new seats then. These will have to be deeper in the
head, and you only have a couple of millimetres leeway for that, or
else the tappets can't follow the valves - i.e. the head is scrap.
valves are hollow and sodium-filled (that's why you shouldn't throw
them in the bin with ordinary waste). Filling the valves with sodium
allows them to cool better because the sodium liquifies at operating
temps and allows convective cooling to occur, as opposed to the conductive
cooling of solid valves.
probably they'll be full of carbon. Thoroughly clean and polish them,
but do NOT enlarge them. Focus mainly on the upper part of the ports,
as they are responsible for most of the airflow (it's the outer side
of the turn, as the gases rush out of the chamber)
there are sheared studs (as above) now is the time to get them out.
Do not put the manifold back if there are missing studs, as they hold
the weight of the whole turbocharger assembly. A leaking exhaust gasket
can lead to localised overheating of the head or a burnt valve. Drilling
slowly is one way to get them out (eventually). Giving them to a specialist
machine shop is not as macho, but far safer. This one above didn't even
need helicoil afterwards, despite the 2 (two) broken drill bits that
I skilfully inserted before finally taking it to specialists...
is the assortment of parts for the LET Coscast head (pic by Caveman)
this is the head put together
is a cut-out of the LET cyl head. Gives an idea of the thickness of
the aluminium walls
Vs Vaux heads
are two kinds of cylinder heads used in LETs:
ones using el-Cheapo production techniques, heads tend to go porous
It's either sloppy casting, substandard alloy or just bad
design. The fact is that after a number of expansion/contraction
cycles a crack forms allowing pressurised oil to seep into
the waterjacket. (focus on the red circle in the middle-left
part of the pic. it shows where <probably> the water/oil
holy union takes place in porous heads).
Nasty feeling when the coolant expansion tank has that semi-digested
taken by Gary
and posted on the MIG
ones, made by people who know what they're doing. These have 'CosCast'
printed on them, but it's quite hard to find with the head in place.
how to tell the difference: (Photo posted on MIG)
differences between early/late models are here
info on Coscast heads here
Gasket - Z20LET mod
well known successful modification
for tuned LETs is to use the head gasket from the Z20LET, which is cheaper
difference is that it has 4 rivets on the edges (highlighted in yellow
circles). Apart from the top-right one, they have to be drilled out
first (easy, with an HSS drill bit).
difference is the diameter of the cooling holes, which is a bit smaller.
This (when done in moderation) has the effect of raising the coolant
pressure inside the head, something desirable on a high-performance
application that has to shed more heat overall and risks more hotspots
than a std-boost engine.
is essentially the same, but it has the advantage of splitting into
separate layers, so a thicker one can be easily and cheaply made up.
It's probably stronger than the stock item too.
vent and Camcover mod
if you think crank ventilation is girl stuff (real men
only focus on power-making mods) then take a look at
the contents of this intercooler
the stock item from a Toyota SoarerTT, piston blowby and ineffective
crank ventilation made sure it was half-full with engine oil. We don't
want an oil drain plug on our intercooler, do we?
you're feeling adventurous and want to further refresh the engine, then
a new set of rings might not go amiss, especially if the engine has
done 70K+ miles. Here is what to do with the block
is how to measure
the volume of a combustion chamber
is an alternative
way of porting, by making
the ports smaller. This guy swears that it gains lots of power,
but I'm not convinced. It may work on ports that are too big from the
factory perhaps but I don't see this happening on the LET. The article
is interesting though. (Local copies here
conventional (!) porting fundamentals from Tomorrow's
your own Flowbench
the same source here is a DIYFlowbench
forum (ah the wonders of the internet!)