I MELTED MY HEAD!!!???

[robinairtechsupport]

i recently had an interesting experience that i would like to share, as well as maybe get some insight as to what exactly went wrong, though i believe i already know... 93 mazda protege, 1.8, single cam FI, I4 cyl, 16 v, gasoline engine, stock ignition, 67000 miles on the motor... just because i was bored one day, i decided to put a dual exhaust on the car... so i had some 1 7/8 laying around, and some 2 1/4,... and some 3 inch... so i took the 2 1/4, and flattened one side of a 4 inch peice, cut the angles on the 1 7'8, and made a perfect y pipe... from one pipe to two, perfectly transitioned, deburred with a die grinder for flow... 12 inches total, attached the 2 1/4 to the downpipe, stock exhaust manifold, out the back, about four feet of two 1 7/8 pipes welded together coming into the side of a six inch piece of three inch, cut and formed to transition into 1 7/8 to exit in front of the rear tires... i thought that hitting that "T" would create enough back pressure, but i believe things went horribly wrong because of inadequate heat transfer...end result...HOLES BURNED AROUND MY EXHAUST VALVE SEATS, INTO THE PORTS, ZERO COMPRESSION, ALUMINUM DEPOSITS ON THE PISTONS. i wasnt driving it at the time, but the ignition was not excessively advanced, and it was not running excessively lean, wich leads me to believe it was the exhaust, not detonation, though detonation may have occored once the metal started melting. any thoughts???

 

[sreid]

I think it has something to do with the oxygen sensors in the exhaust causing a mixture lean out leading to the damage you describe.

 

[patprimmer]

It is a lean out and or detonation. One normally introduces the other. Lean out at high loads does burn holes. Lean out at high load with detonation burns them a lot faster.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Cipher982]

I am fairly confident that the problem you ran into was caused by a back-pressure cooled head. I was in the same situation as you were a few months ago, I had an old subaru which I was going to strip the car, take doors off, remove the exhaust and go off-roading (it was AWD) till the car died.

My friend noticed I was removing the exhaust and told me it would not be wise because the cylinder head was cooled from the exhausts back-pressure, and removing a majority of the exhaust would cause my ports to burn up. This sounds eerily familiar to what you described.

 

[patprimmer]

Cipher982

I would not take to much notice of that friends advice. They obviously know nothing of the laws of physics as they apply to automotive exhaust systems.

The more effective exhaust system will draw more gas out of the chamber at TDV overlap.

This can lean the engine out and induce detonation which will then burn holes in pistons.

Increased back pressure increases exhaust heat.

Exhaust heat is normally a lot hotter than the head as the exhaust gas is just burnt and often still burning air petrol mix. Petrol air mix burns very hot.

The head is always colder as it has water cooling it.





Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[mishar]

It can be pure coincidence too. If the valve seat was loose before and you tested your new exhaust …