idou o logos tou HGF

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idou o logos tou HGF

Δημοσίευση από kostantinos » Δευ Οκτ 09, 2006 5:40 pm

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 main cause is the distortion the aluminium engine suffers under the thermal gradients that occur particularly during warm up and warm down cycles. The original stat design was designed for a low weight/ max economy/min emissions vehicle, not a mid-engined sports car such as the MGF or Lotus Elise. The solution to this issue was the PRT stat , fitted as standard to many MG Rovers from 2003 onwards (particularly the TF and the Rover 75 - Ed).

The temperature difference between the inlet and outlet of the K-series using the standard thermostat during warm-up. Not what you'd call a stable rationship. Read more on Dave Monk's measurements and developments on the MGF here or clicking the above figure.
With the original stat, water temps were so badly controlled that at warm up and warm down particularly, a 60 degree Celsius temp gradient can be measured over the engine between cylinders 1 and 2. (This can to some extent be seen in Dave Monk's telemetry experiments - see figure opposite. He found that inlet and outlet temperatures did not have a particularly stable relationship during warm up with some alarming temperature spikes. You can read more about this work here. - Ed) 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 (this is not exactly true - see article written by someone on the ECV3 team here - but we'll let Simon off the hook on this one - Ed). So it had the immensely stiff crank carrier design when, for instance, the Japanese 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 alloy, 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 and goes soft - and over a period of time the fire ring frets it's way into the head surface. This results in a loss of clamping force, allowing cylinder gasses to escape and pressurise the coolant system, forcing water into the cylinders and oil, and oil into the water - and that's when you get the classic symptoms of "HGF". The reality is that failure is a slippery slope and occurs long before symptoms become visible.

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 (Pressure Relief Thermostat - read more here - Ed).

The new MLS gasket (Multi Layered Steel Gasket - read more here - Ed) 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, and the PRT works very well.

Landrover, or 'Euro4' oil rail top, compared to original item below. Picture thanks to Dr Dave @ forums
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 core plates 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.

This is what my cylinder head looked like after a spectacular HGF. Pale valves are the hot exhaust valves, and the indentation in the cylinder head (arrowed) around the combustion chamber is clear to see - yes, that surface should be completely flat! Ed.
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. On failed cylinder heads following HGF, clear indentation rings are often to be seen on the exhaust side (and therefore hottest side - Ed) of the cylinder head where the fire ring had pushed into the head and therefore has consequently leaked cylinder pressure (see picture opposite - this is from my own engine following an alarming HGF blow-out last year... ouch! - Ed).

Why is this an Achilles heel? Well 226 alloy, as used in all other 4 cylinder petrol engines, is not a heat treated alloy - it doesn't matter (as much) if it gets hot. There is no heat treatment to destroy. Ergo - by virtue of material choice, the K-series is rather more vulnerable to temperature damage than the majority of other engines - and therefore more likely to write off the cylinder head in the event of a HGF.

A classic mistake made in the UK was to weld up heads around the fire ring. As we've already established that excess heat is the last thing that a quenched LM25 alloy cylinder head needs in terms of maintaining its surface hardness, it is clear that welding is possibly the worst thing anyone could do to a K-series head; a head so treated is sadly practically guaranteed to fail in short order! So beware dodgy eBay adverts!

Hope this helps explain some of the intricacies of the K series, and how and why head gaskets can fail on these engines. - Simon
rover 214 si
throtle body 52mm
KN air filter !!

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Εγγραφή: Τρί Νοέμ 15, 2005 11:02 pm
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Δημοσίευση από zed » Δευ Οκτ 09, 2006 9:24 pm

Το'ξερα, Το'ξερα!!...οτι κατα βάθος έχω μια F1 που καίει φλάτζες! Τι να κάνουμε ούτε ο Σούμι δέν τη γλύτωσε...Τα κακά του επαγγέλματός μας...

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Δημοσίευση από nikolas » Δευ Οκτ 09, 2006 10:25 pm

πολύ ενδιαφέρον κείμενο..αν μπορείς όμως πες και από που το βρήκες γιατί απ'ότι κατάλαβα υπάρχουν και κάποιες φωτογραφίες οι οποίες δεν φαίνονται στο παρόν κείμενο..
εξηγεί αρκετά καλά πάντως τους λόγους για την αστοχία της φλάντζας..και τελικά φαίνεται πως η νέα φλάντζα αυτή και μόνο δεν πρέπει να θεωρείται πως λύνει το πρόβλημα γιατί σύμφωνα και με το κείμενο..η φλάντζα είναι απλά το αποτέλεσμα μετά το πρόβλημα που δημιουργείται στην κεφαλή και περιγράφεται στο κείμενο..
πρέπει να συνδιάζεται οπωσδήποτε με αλλαγή του "οδηγού" κάτω από το στρόφαλο το λεγόμενο "oil rail" και τη χρήση νέων κολάρων με τον νέο θερμοστάτη..
ενδεικτικά να αναφέρω ότι oil rail είναι το κομμάτι στο οποίο βιδώνουν οι "βίδες" και σφίγγουν την κεφαλή με το μπλοκ του κινητήρα..


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