Newb question - cracked manifold.

It doesn't really damage the engine so much, but it makes noise and, because it's ahead of the cat, it releases CO (poison) into the engine compartment that can get into the passenger compartment

 

An exhaust header has rather powerful sonic activities. As the exhaust valve opens a powerful sonic blast runs down the pipe, then a bit later a powerful sonic near-vacuum runs back up the pipe.

When an exhaust header has a crack in it, the powerful sonic vacuum sucks in air. This air contains oxygen (standard 21%). The oxygen sensor reads the mixture as too lean and throws more fuel at the problem. This washes down the cylinder linings (of their protective oil coating) and accentuates top ring wear and top of the bore wear. The washed down oil burns and helps clog up the cats, but does most of its damage to the exhaust valves. The excess carbon layers deposits on the exhaust valves. Normally this would erode off, but now sticks to the face and stem because of the lower exhaust temperatures of the rich mixture.

Presto, one engine in need of expensive maintanence.

 

Don't use factory headers.

 

I think the problem for the cylinder concerned (that has the manifold leak) is probably more due to running too lean rather than too rich. It is due to the fact that the cylinder is running LEAN of stoichiometric that valve damage may occur. The other cylinders on the same bank will more than likely be running too rich and hence bore wash is possible.

 

I recently bought a 1995 F355 that was found to have a small crack in the left header. I was told not to drive it until it is fixed. The crack appeared as a thin white line on the header heat shield, but was a concern that under this was an actual crack. The dealer said that there is a new rev of the OEM headers that reduces the cracking issue and I am having it installed.

Has anyone hit a pot hole or shallow run-off grating across the road that resulted in the suspension/axle popping up and denting the headers? Apparently this happened to this car. Not sure when or where or if it happened in my possession, but there was talk of this causing the crack.

 

That happened to mine, due to the factory suspension being dropped all the way by the PO, though that didn't cause the failed header pipe.
The heat shield had heavy rubbing from the axle. Raised it back up, installed Fabspeeds, and enginefxr is almost done with the head rebuild.

 

I'm not an owner but I've never heard of a new revision factory header. Consensus seems to be that the all fail, it's just a matter of time.

 

I've also seen exhaust manifolds/headers that crack right next to the head cause cracked exhaust valves.

The exhaust valves are the hottest component in the engine, that's why many are made of better material than intake valves, sodium filled, titanium ect... there's way more engineering going into exhaust valves than intake valves because the intake valves have a cold wash of air/fuel flow over them all the time so they stay cool.

So when that very slight pulse of air from the cracked manifold/header is sucked back into the engine and hits the very hot valve it can crack. Of course this depends on the design of the engine and other factors. I have seen multiple valves crack on 2000+ hp big block Chevy from a leaking header gasket, and cracked headers.

Think the high school experiment when the teacher heated the test tube up and put it into water and it pretty much exploded.

 

Would removal of the cats help reduce the cracking on original headers or has someone still experienced eventual failure?

Lastly, would anyone of you guys tune your 355? I believe the handbook (UK specs) says 95 octane is ok for the car. A tuner said he can advance the timing to accept 97 RON and extract my horses from the car. Have any of you done this?

Thanks in advance.

 

I know 14.7 AFR is stoichiometric and is ideal for idle/cruise but what AFR is best for WOT on the F355 running 91 octane? I'm think somewhere between 12 and 12.7 but I'd like to hear what Mitch, Dave, Bruce, Goth, etc have to say.

Rob

 

There is one other point to make:

It may not be such a big issue on a mid engine car (but it still can be) but a cracked manifold allows Carbon Monoxide to enter the car's cabin because its not dealt with by the catalytic convertors or existed through the tail pipes at the very rear of the car.

 

Your theoretical numbers are quite close...... for last century.

Change what we are given to burn and you must change the numbers. A great deal of testing has gone into what the new numbers should read and I am fairly close to pinning that down.... And then the Feds turned it into a moving target which required me to make the new system adjustable, they have NO clue as the unforeseen consequences of their actions. This is one of the reasons I have put so much effort into the Gold Connector kit, I need the engine management at its peak performance to hold a very fine line. Once that is achived I must then modify how the management controls the system and to what numbers it holds.

The manufacturers are still, to this day, using the theory that we are burning gasoline... think again and read the pumps most anywhere worldwide. I have three components I designed to deal with this and testing is about ready to start on them. They will require the engine management to be in top working condition but early testing has showed we can actually make some more power from this fuel when we properly design for it.

I have wasted far too much time designing new components just to contain this new fuel. With that behind me the finishing touches are being put on the components to control the combustion and the final stage are components to optimize its use. Any one facet without the others is merely a waste of time. Now... if we could get the politicians to keep their noses out of this and pin down the moving target we call fuel...... I could finish this work and start on the next project.

Short answer to your questions is no, those are no longer the numbers that work or are correct unless you are filling the fuel tank from a steel drum.

 

Ok, so are the new numbers richer or leaner?

I appreciate all of your efforts Dave and am looking forward to your ideas/designs.


Rob

 

An interesting side note to AFR's is that the emcs for both the 2.7 and 5.2 in closed loop mode are regulated at/near 14.7...however when the ecms is in open loop mode their appears to be a significant(atleast to me) difference between the 2.7 and 5.2 AFR's all the way up to rev limiter.

Maybe someone else can confirm this also.