355 Anyone's stock manifolds fail twice?

Just read this ( new 94 348 here ) , I researched the reliability between the 348/355 and had a conversation with a U.K. Ferrari tech who was around @ Marenello , the U.K. concessionaire back in the 90 s when they started to import them .

Stood in his show room with just about every example inc 348 vs 355 spiders mulling over the differences , we opened the engine lids and he gave me a technical tour back to back so to speak .
No rush plenty of time ( later in the work shop under lifts )
Obviously we delved into the Bosch 2.7 vs 5.2 from what he has seen coming through the shop over the past 40 yrs .

Amongst other topics the cracked manifold/ (headers ) came up .

In a nutshell it’s a fuel pressure regulator issue the 5.2 mapping can’t cope in certain circumstances ( related to driving style) and results in excess fuel entering the manifolds over heating them resulting in cracking .

obviously it’s rare in the 2.7 found on 348 s , theoretically possible there could be a huge fuelling issue , not saying it can’t happen but extremely common to almost say odds on dead cert in the motronic 5.2 that Ferrari adopted , which is found in 355. Not withstanding the early 2.7 s .

Sure the weakest link the air injection rail on USA cars cracked first , but then on Ferrari just just chased there tail remanufacturing / modifying / beefing up .The crack moving to the next weakest place , for ever chasing as each mod as they came through.

Said in the uk ( no air rail ) he’s never seen cracked manifolds on stock 348 s but every 355 had new headers or will do at some point .Indeed he was involved in the U.K. factory recall campaign program, on 355 which initially ran a while until eventually they came back cracked .

Told me if you go aftermarket with a 355 or any for that matter and retain the CAT s but deleting the sensors it’s dangerous.
He’s seen many a overheated burned out CAT , red hot smouldering away cars flat bedded in and fair few 355s on fire insurance write offs .= All because of this excess unburnt fuel getting through to the CAT s un noticed , or guys with the sensors ignoring the “ slow down “ light .

He’s stressed don’t delete the sensors and or dash lights with 5.2 s .

Basically I have read this thread on a myriad of other constant running threads on “ cracked manifolds “ on this 348/355 page and there’s not one conversation on the aetiology, what causes it ?

All I read is fit aftermarket being allegedly better built , which just kicks the over fuelling further down the line to the CAT s which then are deleted or the sensors disabled if I understand the 355 culture on this site ?

 

I haven't seen anyone on Fchat recommend disabling the cat temp sensor system on a 355, and I have seen numerous posts warning against doing so.

I also doubt the veracity of your mysterious mechanic's claim about the cause of header cracking. A rich mixture makes an engine (and its exhaust) run cooler, not hotter. Otherwise a good theory....

 

The places that were melted on the stock manifolds I cut open were spots 2 separate primary tubes were touching. No space between them at all so they melted.

IMO it's both a material (probably 400 series stainless) and design issue. Tight radius bends don't help either as that thins the metal quite a bit at the outside of a bend.

Also, with the mismatch of port angle of the head to the header flange angle, it seems like this was possibly intentional to force more exhaust into the air injection tubes.

In the case of the 2.7 especially that would also force -some- exhaust into the other 3 exhaust ports from the one that just fired. Not something to think about usually but the mismatched angles and angle of the holes in the air ports would suggest they were trying to help exhaust flow into the air injection manifold part of the header.

 

Excess fuel unburnt ignites in the exhaust manifold , = over heats them = stress cracks from excessive thermal expansion.

 

There is insufficient O2 in the exhaust when running rich for the unburt fuel to ignite in the manifolds.

 

When raw gasoline ignites in the exhaust, you get a backfire. It's loud and unmistakeable. Backfiring typically occurs from ignition problems and unburned or partially burned air/fuel mixture is pumped into the exhaust where it ignites. When you have a rich condition, the engine just runs rich. The theory above sounds good but it's wrong on multiple levels and is just not how engines work. Sorry.

 

Yeah on the stuff I work on catalytic overtemp protection dumps extra fuel to save the cats.

 

Lots of theories abound - one is from Capristo that the combined primary and secondary paths create a condition where the bypass exhaust flows back through the primary cats super heating them and also messing with the o2 sensor readings, and separating the primary and bypass completely will help prevent the issue. They could just be trying to sell more exhausts. I'm still on original headers but have had test pipes or exhaust with separate primary and bypass for the 13 years and 17k miles I've put on the car and it had test pipes on it when I got it.

As catalytic converters are there to burn up any unburnt fuel, an excess rich condition generally will cause them to get glowing red hot as the fuel burns in the cat instead of the engine. I think that's usually caused by ignition issues or injector issues and not the ECU going over rich, but it's possible that the 5.2 condition described above could cause it if it really dumps a lot of fuel at times. The 2.7's have larger injectors so would seem to be more susceptible to this issues instead of less, I'm not sure what they changed in terms of FPR from 2.7 to 5.2 other than the change to a single vs. dual pump setup on the early 2.7's. The FPR's are the same.

 

Not to mention enclosing the poor quality stainless steel headers with insulation and then enclosing them with a stainless steel cover.