I have not taken the head off the car yet, however, I believe I have a burnt valve. What would cause this? A/F is 11.7 on full boost and 14 during normal driving. Thanks in advance.

 

 

Possibly a bad injector.

 

Excessively retarded ignition timing will cause the EGT's to go too high resulting on a burnt exhaust valve.

Who tuned the car?

TonyG

 

The typical cause is just carbon getting caught on the valve seat and not allowing the valve to seat all the way. Other possibilities are the one cylinder could be running leaner than the others. This can be caused by a dirty injector or weak valve springs. Also just because the AFR is good, does not mean your EGT is good. Timing and exhaust gas back pressure can play a big role.

 

I can run a turbine so tight it will spool at 1800rpms, and choke the engine off by 5000 rpms.. and the EGT's will be fine. But if you crank out too much timing, the EGT's go straight up every time. No amount of fuel will change it. And a burnt exhaust valve happens every time.

This is always the result of some tuner trying to keep the engine out of detonation by pulling timing.

TonyG

 

I was told carbon build up on the valve, super heats over and over again, and can burn a hole. Just swapped my head for the same thing. My car was abused by the PO.

 

I've heard that. But I've never seen it.

What I have seen, with an EGT gauge, is what happens when the timing is too retarded in order to keep the engine out of detonation for a given fuel (typically... pump gas).

When an engine is tuned this way, it's always a street driven car. And this type of setup can live for some time because there is no sustained WOT operation. EGT's on the rise don't spike... they just sort of keep going up slowly. So for a car that sees an occasional burst of WOT for short periods, it's ok. But... depending on the exact tuning scenario, a little too long at WOT, too many times, will result on a 1/4" hole in the exhaust valve... and a dinged up piston top.

TonyG

 

Had the excess carbon on the valve in #3 burn a beautiful wedge shaped slot in one of my valves a couple of years ago. Looked like someone used a cutting torch. Swapped head, rebuilt and flow matched injectors, lots of hours labour and it's running fine. Will it happen again...I hope not!!

 

So why would 1 cylinder have excess carbon and not the other 3 cylinders?

Rebuilding the injectors won't decrease their flow rates.


TonyG

 

Spite?

I'm running a 2.5bar on a tune requiring a 3.0bar and AFRs look good now. I wonder if my injectors are flowing too much.

My car was abused by the PO (21-24psi, super rich Autothority Stage 2).

 

I burned the #1 valve a year and a half ago at Watkins Glen. It was a cool morning and my first run on the fifth lap. Car was fine the month before in much warmer weather running Vitesse MAF with very good AFRs.

Machine shop said the valve springs were down about 20% from acceptable so I replaced them with new genuine Porsche springs and a used valve and sent the virtually new 75lb injectors out for service to see'if #1 was low. All were within 3% stock and 1% when they were done.

Only explanation was that valves weren't sealing and/or something got caught under the #1 which kept it open. Ran last year With no issues.