Technical question on a burnt valve

[chrisg]

Dear All

what causes an exhaust valve to burn out on a turbo ? Something simple or a bit more complex ?

Attached photo of the said valve, note its the exhaust valve on cylinder #4 and there's no soot in the combustion chamber compared to the other three cylinders.

Technical or non advice gratefully received.

Chris

 

[sawood12]

Cylinder #4 keeps on cropping up in all sorts of forums I read. It seems cylinder #4 is a bit of a weak spot and prone to having problems. Not 100% sure why but I think it has something to do with heat buildup.

Anyway the main (or most common) reason for burned vavles is the AFR is too lean therefore hotter. It could be that the injector on that cylinder has not been working properly or that your AFR's are out full stop and cyl #4 has suffered the effects first.

 

[Diver944]

Scotts right. Cyl#4 does seem to run slightly hotter or leaner than the others which is why the headgasket always seems to go in that area and if you have any problems with plugs or valves it's that one that shows up first. I've seen a few theories, the intake lets slightly more air down that runner for example. Lindsey also believe the water jacket has an air pocket near cyl#4 so it can't cool sufficiently. They have developed a 'steam vent kit' to supply coolant directly to that air pocket.

http://www.lindseyracing.com/mm5/merchant.mvc?Screen=CTGY&Store_Code=LR&Category_Code=STEAMVENT

When was your AFR last measured and do you have anything to monitor it, just in case something has happened to make you run leaner?

 

[chrisg]

Thanks for the advice chaps. In answer to your question Paul, I don't have an AFR fitted but had the car on the rollers last December at Weltmeister - all looked fine and dandy with the AFR OK. Since then its only done ~ 3.5k albeit mostly on the track and on the way to and from the Ring.

I suspect it may be a combination of things, including old age and a blocked/ reduced flow injector on #4

C

 

[sawood12]

I've often thought about maybe trying to cool the underbonnet temps a bit. I've looked at Rick Cannells car and seen the insulating tape he has used on the exhaust manifold, which should also help with turbo spool up as well, and his NACA duct in the bonnet and wondered what effect that might have. You want to keep the mainfold gas temp up for better turbo performance, but want to keep that heat in the gas and not leaking out into the bonnet cavity.

 

[peanut]

. If you look at the valves in cyl #4 there is no deposits on either valve whereas #3 has plenty.
cyl 4 has been getting much too hot and has literally burnt the deposits off the valves.The hot exhast gases have burnt the hole in the valve which may have been sticking and not seating properly so detonation has attacked the edge of the valve The plug will prob be grey or white compared to the others