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Tuning the C20LET - Breathing/Cyl Head
These 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:
If 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.
If 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.
More on porting can be found at the bottom of this page
A good guide on head porting is here (local copy here)
Run 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.
That's how it should look after the operation:
The 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.
Now 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 labelled.
Here Tappets (Hydraulic lifters) are covered in more depth
If 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!Because of all the work involved in changing them, I would only use original Vaux seals. I've learned this the hard way...
The 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.
The 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.
This is a brand new LET valve spring.
Height: 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.
I'd 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.
On high boost turbo applications certain unique conditions can occur that n/a engines never experience.
If 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.
While 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.
Cams and their optimisation are HERE. Also the famous "XE inlet" mod.
They 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.
These 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.
Most 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)
If 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...
This is the assortment of parts for the LET Coscast head (pic by Caveman)
And this is the head put together
Below is a cut-out of the LET cyl head. Gives an idea of the thickness of the aluminium walls
There are two kinds of cylinder heads used in LETs:
- Vaux-made ones using el-Cheapo production techniques, heads tend to go porous if overheated.
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 curry look.- Cosworth-made 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.
Here's how to tell the difference: (Photo posted on MIG)
More differences between early/late models are here
More info on Coscast heads here
A well known successful modification for tuned LETs is to use the head gasket from the Z20LET, which is cheaper as well.
One 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).
Another 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.
Thickness 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.
...And 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
It's 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?
If 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
Additional reading:
Here is how to measure the volume of a combustion chamber
Here 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 and here)
Some conventional (!) porting fundamentals from Tomorrow's Technician
Make your own Flowbench with these parts
From the same source here is a DIYFlowbench forum (ah the wonders of the internet!)
On to the Combustion Chambers...