Logo

Exhaust manifold skin temperature how hot should it be

T

Tom Tarlton

Guest
"Engine gurus: I have a pair

"Engine gurus: I have a pair of Mercruiser 8.1 S Horizon engines (model number 3-S4871JN1)in a Bertram 33. A few months back, the port side exhaust manifold on the starboard engine began leaking where the hose fitting connects to bottom center of the manifold. The engines have closed cooling systems with heat exchanger. Original manifolds are cast aluminum; Mercruiser has since gone to a cast iron unit for replacement. I notice a tremendous difference in surface temperature between the aluminum and cast iron units on their lower sides near their connection to the block. For instance, at 700 rpm, the cast iron manifold averages about 250 degrees, the aluminum unit 185 degrees. At 900 rpm, 315 degrees vs 215, at 1800 rpm, 480 degrees vs 270 degrees, and at 2700 rpm, the cast iron manifold reaches 590 degrees while the aluminum unit averages about 330 degrees. During the initial runs after the cast iron manifold was installed, I noticed some smoke coming off it which led me to investigate. My mechanic though it was probably excess paint/grease, etc burning off, and the smoke disappeard after the fourth run. The upper area of the cast iron manifold is quite cool, about 10 degrees warmer than the aluminum unit, sufficiently cool to keep the engine guardian system happy, as overheat alarms are not being triggered by the thermostat in the top of the manifold. The engine room is a bit warmer, but I can't imagine how hot it would be if all four of the exhaust manifolds were cast iron. Any thoughts on this?

Thanks"
 
"Holy crap! My manifolds &#40

"Holy crap! My manifolds (Chrysler 360s) never exceed 140 'F. That sounds really hot! Are you reading the things with a laser-pointed temp gun? And, if so,, are you looking at the water cooled portion, or where the exhaust enters?

Jeff"
 
"Hello Jeff. I used a laser-p

"Hello Jeff. I used a laser-pointed temperature gun. The readings were taken on the underside of the manifold on the lobes which extend out from the exhaust ports. I put the temp gun about an inch away while taking readings off each of the 4 lobes, close to the block. The numbers are averages of the 4 lobes at each of the rpm settings. The upper side of the manifold is much cooler, about 110 on the aluminum and 120 on the cast iron unit. Once the water mixes with the gases, things cool off and the temperature sensor on the top of the manifold is not reaching threshold for triggering the warning system. It just seems way to hot down there."
 
"Tom,

I believe Merc cast i


"Tom,

I believe Merc cast iron manifolds have a small area (approx. silver dollar size) directly under exhaust runner that actually has exhaust gasses blowing directly on it without water jacket between it and exterior surface. cast iron isn't nearly as heat conductive as aluminum so I wouldn't be suprised at these temps in VERY SMALL areas. If it's very wide spread I'd be looking for a reason!

Bob"
 
"Hello Bob. The hottest part

"Hello Bob. The hottest part is on the bottom surface where the manifold connects to the engine block, as you mentioned in the direct flow of the exhaust gases. Temperature decreases as you move out and up the side, yet it is very hot to the touch, until you reach the upper area and top surface. Given the seemingly extreme difference in temperature of the two manifolds in that area, I was concerned there might be some difficulty. (Remember 451 fahrenheit is enough to burn paper.)"
 
"sounds completly normal that

"sounds completly normal that small area will get hot enough to burn off the paint ,but i'm curios why replace the original manifolds if they are fresh water cooled?"
 
"Peter: the aluminum manifold

"Peter: the aluminum manifold failed, corroding through just above the threads where the hose fitting connects on the bottom side. There is a small lobe on the side of the manifold in that area. Initial signs of the problem showed up about 9 months prior to the failure. First, the paint began blistering up, then chipped off in the area (about the size of a quarter). There was a chalk-like white dust under the paint chips and on the aluminum surface. Next thing I noticed were tiny round globules of what looked like crystalized metal on the paintless area. There was no evidence of water leaking or anything like that while running. Eventually I grew tired of wiping the stuff off and decided to cover it with one of those fabulous two part sealers, using the gray type for metal repairs. The patch I made was good sized, about twice the size of the original bare area. Eventually, the repair material unbonded and cracked through, revealing a steady water drip from the manifold. The folks at the boat yard hadn't seen a failure in that area before. Conclusion was the original casting was bad. I am just glad that I always go down into the engine room and have a last minute look around as we make our way out the canals. There could have been salt water spraying all over the place down there."
 
"Back again. While working on

"Back again. While working on same engine for other problems (posted another note), I noticed the paint beginning to flake off the aluminum manifold on the outboard side of the engine in the same place where the other mainfold failed. It wasn't evident two months ago. This aluminum unit is the original and is almost three years old now.

There have never been any system alarms for low water pump pressure or overheat registered on this engine. Must be heat and salt water cause the aluminum to deteriorate."
 
Back
Top