Henry,
I have always maintained and was persuaded to believe that 2 factors play a
part in out manifold trounle that is other than the usual aging process
that eats antique car manifolds equally.
First is the fact that the bolt pattern is not uniform to the pressure
exerted on the manifold. There is a weak area at the center bottom, look
at the bolt pattern. As the manifold heats up, it will tend to bow out at
the center blowing out the gaskets and cracking the manifold in extreme cases.
The second is, as you pointed out, the manifolds heat up more than a car
because of the torque the motor has to create to drag the coach carcus.
How long can mere cast steel take such abuse, maybe if they were made of
Cryptonite they would last longer!
Actually, I did find another problem with manifolds that have been surfaced
several times. So much of the mate surface gets sanded away to flatten the
area that there is no longer enough room for the loi dip stick tude to go
between the left manifold & head. It hols the manifold away from the head
and causes gaskets to burn out. I have one in the shop now that has this
ailment right now that we just rebuilt!
Headers are the only viable fix but they do not come without trouble of
their own. You must constantly tighten the bolts on headers, ask Nate
about that! They do a combination of compressing the fiber gasket that
must be used on headers and back out the bolts from the heating & cooling
processes that they must endure.
You have an interesting question and one I would like to see the data on to
support & refute my understanding on this matter.
Survey on, young Skywalker!
Jim Bounds
- -------------------
>>Dear Netters,
>>
>>Does anyone know what is different about our GMCs that causes us so much
>>grief with the exhaust manifolds (i.e., warping and cracking)?
>
>I don't know that we're all that different from other gasoline big block
>engines used in heavier RVs or small blocks used in some lighter RVs.
>
>I DON'T KNOW IF IT'S TRUE, but here's what I was told by mechanics who
>regularly service the coaches when we first started looking at motorhomes:
>any engine run at wide open throttle for long periods of time at low speeds
>(ie climbing long grades) will significantly stress the exhaust manifold.
>Evidently the right side manifold on the 350s used in a number of Class C
>MHs is a chronic problem around here. And they saw similar problems with
>the 454 in bigger MHs.
>
>The comments were pretty universal from mechanics in the area - they see a
>lot of cracked exhaust manifolds in comparison to the same engine in a
>passenger car or lightly used truck.
>
>Note: this is not a scientific sampling. The mechanics might be mistaken,
>they may be over-reacting, or they could be nuts. But then, they might just
>be right.
>
>>Does an overly
>>lean running engine heat the manifolds?
>
>I don't know about lesn heating the manifolds, but I can tell you that
>leaking exhaust manifolds can cause a local leaning of the exhaust that
>raises temps on the exhause valves.
>
> Do these babies glow in the dark
>>during a hard pull?
>
>I haven't looked to see if they literally glow, but I can tell you that
>they get pretty hot.
>
> What about the mufflers mounted ahead of the Y. Is this
>>extra weight hanging from the manifolds significant enough to cause warpage
>>if indeed the manifolds were glowing red hot and bouncing down a rough
>>stretch of road? Do I sound crazy?
>
>No crazier than the rest of us. After all, you DO own a 27 year old MH.
>
>Good luck in your information quest!
>
>Henry
>
>Henry Davis Consulting, Inc / new product consulting
>PO Box 1270 / product readiness reviews
>Soquel, Ca 95073 / IP reviews
>ph: (831) 462-5199 / full service marketing
>fax: (831) 462-5198
>
http://www.henry-davis.com/ http://www.henry-davis.com
>
>
>
