Je retourne sur seloc immediatment, because Simon est l ennemi jure de DVA...et que generalement chacun de ses poste technque fait 12 pages de fight avec DVA, la fin de l article situe l ambiance a venir...
Bonne lecture...
The cause of "HGF" in the K series
This is long verdue but here goes.
What is the cause of "so called HGF" in the K series.
Well it is a complicated issue, there are many contributing factors, but the cause, is the distortion the aluminium engine suffers under the thermal gradients that occur particularly in warm up and warm down cycles. The original stat design was designed for a low weight/ max economy/min emissions vehicle, not the track car that is the Elise. The solution to this issue was the PRT stat , fitted as standard to all MG/Rovers from 2001. Why Lotus chose not to ask for it God only knows.
With the original stat water temps were so badly controlled that at warm up and warm down particularly, a 60deg c temp gradient can be measured over the engine between 1 and 2 cylinders. This causes water to leak down the church window between 1 and 2 on the exhaust side , causing small air pockets in the head and consequently overheating. The water loss is insidious, but so small it burns off in the oil without being noticeable.
Why does this small phenomenon affect the gasket? - it doesn't. It affects the head. K is special, it was designed as a stand out engine, throwing all conventional practice aside, most of it's features came straight from F1 design - it was to be a ground breaking engine like the all ally bonded chassis it was designed for - ECV3. So it had the immensely stiff crank carrier design when for instance the japs have only just caught up, until the k20/FOC20/ vvti all their engines relied on the old fashioned bearing cap design.
Hand in hand with the new 4 cylinder design went the choice of material. LM25 is a very expensive ally, the only other mass produced engines to use it up till now are all of Aston Martin's recent engines and AMGs engines For Mercedes. All BMWs hondas, toyotas , Fords etc are made of cheap 226. LM25 is much stronger - so you use much less of it to get the required mechanical strength. About 40% less - hence the weight advantage of the K compared to the "Industry standard " engines, but in order to get that strength LM25 undergoes a heat treatment - a quench. That treatment gives it it's strength but is also it's Achilles heal. - If the operating temperatures and hence the temperatures the engine sees are not carefully controlled the metal loses it's quench, it's hardness, goes soft and over a period of time the firering frets it's way into the head surface , losing clamping force. The cylinder gasses then escape pressurising the coolant, forcing water into the cylinders and oil, and oil into the water - thats when you get the classic symptoms of "HGF". The reality is that failure is a slippery slope and occurs long before symptoms become visable.
The gasket may go on to get hot and elastomer come free from the core plate, but that's a secondary failure. The head is what fails, and once it is soft no amount of new gaskets or skimming will ever prove more than a short term prop. The head is dead. Only a new hard head can redeem failure, together with of course, fitting of a PRT.
The new MLS gasket was never a solution to "HGF" it was however part of a number of new measures that helped the engine survive the stresses it underwent in use. The new gasket is if you like a bigger fuse, but the problem is the uncontrolled temp gradients that occur with the original stat location. The remote stat is an improvement, the PRT works very well.
Far more significant was the introduction of the revised oil rail for Euro4 engines and now available through Landrover as a retrofit. This new casting is a stiffer design made in ANSI357 alloy - the same metal as F1 blocks and is as stiff a cast alloy as you can currently make. The purpose is to resist the bending forces the engine is subjected to in those warm up/down cycles. During all the elaborate stress analysis done on the engine in the last 5 years the original oil rail was found to collapse at the towers that hold the bolts and allow the coolant leakage that will eventually destroy the head.
Consequently fitting the new rail is almost as important as fitting PRT. However it is ESSENTIAL to fit it, if a MLS gasket is being used, because the new MLS gaskets achieve only 75% of the clamping of the old elastomer gaskets [the elastomer compresses more than the 5 steel coreplates of the MLS] and consequently the MLS is much more prone to leak than the elastomer gasket without the new oil rail, particularly without a PRT fitted.
Consequently no head should be fitted without a hardness test. New heads register 110-120 brinnell, I would never use one less than 100 brinnell,less than 95 would be foolish. Most of the heads I have collected here from failed engines have hardness ratings between 70 and 80. Mark Bowles's exchanged head that lasted just 750 miles before he suffered "HGF" on his very first track day measure just 82 brinnell, Clear rings could be seen on the exhaust side of the engine where the firering had pushed into the head and leaked cylinder pressure.
Why is this an Achilles heel, well 226 , used in all other 4s is not a heat treated alloy - it doesn't matter if it gets hot. There is no heat treatment to destroy, and the problem is that engine builders are used to handling this alloy, and totally unprepared for the intricacies of LM25.
A classic mistake made in the UK was to weld up heads around the firering, that is the worst thing anyone could do. A head so treated is guaranteed to fail in short order.
So, if welding the head is a mistake, welding the block up is just as foolish. I have a number of scholar closed top blocks here , all from failed engines. The welding has softened the metal back to 40 odd brinnel, so rather than stiffen the block the treatment has weakened it.This is a classic example of tuners not understanding the issues , finding wholly inappropriate solutions, which further undermine the integrity of the engine. I call it butchery.
Hope this helps explain why understanding the engine is essential, and why the "good old boys" building engines for years are for the most part totally unequipped to handle K.
simon
Balèze le Pob !
