Headgasket

Much recent speculation has been made over headgasket failure on tuned L series engines.

Those who have been paying close attention will notice that the majority of people having problems are those with the later VP30 pump and have had the ECU remapped.

People have run around looking at various solutions to the problem without really grasping what the actual problem is.


Plainly and simply the maximum cylinder pressure is too high leading to the gasket leaking under high boost/load conditions. So why is it too high? Well a combination of factors but the obvious common link is the remap of the ECU which has changed the injection timing. I'm not saying anyone in particular is at fault after all we all know that you can't make an omlette without breaking eggs, and tuning engines is no different.


Increasing the injection advance means that fuel enters the cylinder sooner. As the fuel begins to burn the pressure increases in the cylinder. If fuel is injected too soon then the peak pressures get too high. High peak cylinder pressures cause excess noise and also gasket failure (or worse). So why increase the injection advance? Well the longer the time that the fuel has in the cylinder to burn, the more completely it burns, which means more power and less smoke, so you can see why it is an attractive proposition.


So what can we do to avoid this. Well the most obvious solution is to use a less radical injection map. However 5 alternatives I have proposed below each have pro's and cons.

The first is an "upgraded" headgasket. I suggested a solid copper gasket from the likes of ferriday engineering, however by their own admission the current gasket is good and there wouldn't be a huge performace benefit from this alone.


The second option is to reduce the compression ratio which will reduce the peak cylinder pressures. However doing this will affect the combustion under all conditions necessitate an extensive remap to retain good drivability, and performance. Inevitably as the compression ratio drops the fuel economy will also fall, but this is probably a very minor factor compared to the others. To alter the compression ratio you have a few options, you can either fit a thicker headgasket/shim, (see above), remove material from the head (as the head is virtually flat), remove material from the top of the piston/fit a shorter piston, or expand the bowel in the piston.


The third is to try to clamp the head to the block more effectively. This essentially means to increase the bolt torque for the head bolts. Some people have made some initial enquiries as to whether stronger head bolts are available, but as I suspected the current bolts are a high grade to start with. There are ARP stud kits which supposedly have a better clamping force but I've got no technical data on this at the moment.

The fourth option is to fit wire rings/wills rings to the cylinder block to help the gasket seal to the head more effectively. This is a good an effective option, however it requires that the block needs machining which means a complete engine stripdown and removal from the car which isn't for everyone.

Someone suggested putting the rings into the head, instead of the block, which I am a little sceptical about but I remain open minded and await the results. However they then found that they couldn't make it work. I may have a look into this further when I get time or if I encounter issues.

My concerns are that the wire rings will eat into the soft aluminium head and locating the rings in the head when you are fitting the head may be very problematic, but I remain open minded as it would be a lot easier than removing the block for modification!

The fifth which no-one seems to realise yet is that the turbo also has its part to play. Both in terms of the boost pressure that it is running which has an obvious effect on the cylinder pressure, however another which is less obvious is the exhaust housing of the turbo. This is pretty small on the standard and stage 1 and 2 turbos. So what effect does this have? Well as you have added a huge amount to the flow needed through this area then the exhaust turbine and housing becomes a big restriction. Why does this matter? Well if the exhaust manifold pressure is high then the starting pressure inside the cylinder is also high. So why is this important? Well if the cylinder is filled with pressurised exhaust gas to start with rather than a lower pressure then the cylinder pressure once compressed will be higher. Not only that but more importantly if there is a cylinder full of pressurised exhaust gas there, to get more fresh air in (to make power), you need to raise the boost pressure, which makes the cylinder pressures a lot higher, so it is a double whammy so to speak. So what can you do about it? Well there are a couple of things people could consider. The expensive option is to fit a bigger turbo exhaust turbine, however this is not easy, cheap, and would increase the revs at which the turbo comes on boost. So what else could we do. Well my idea would  be to fit an external wastegate to the location where the EGR used to be. Why would this help? Well it would bypass exhaust gas around the turbine reducing the exhaust manifold pressure. This means that the cylinder pressures are lower. It will also mean that you can reduce the boost pressure to get the same power level, again reducing the peak cylinder pressures and hence the gasket has more chance of doing its job! I'm not saying it will definatly work but the theory is sound. If people want proof then they can simply monitor exhaust manifold pressure and see how it climbs as the exhaust turbine housing becomes choked.

What would I do?

Well a less radical remap is first on my list if I had a VP30, followed by wire ringing the block, and possibly using a copper headgasket of the original thickness aswell and adding an external wastegate which will bypass the turbo, as a belt and braces approach.

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