454 Riser Max Height

[JohnV8r]

Hey Guys,

The mechanic handling the rebuilt head install is suggesting riser extensions to prevent any future water intrusion into the exhaust ports on the heads. What is the max height I can extend the risers to? Also, at one point Shambala had exhausts manifolds with risers at the end of the manifold. The last go round, the mechanic who replaced them recommended risers in the center of the manifold. Does the riser location on the exhaust manifold make make any difference in engine performance? I think having the riser more aft would allow a slightly higher height extension on the riser, but it would also mean less distance for water to travel.

Any better options? I won't know about the possibility of surge tubes until I get the rest of the deck off and start re-routing the exhaust.

Thanks in advance.

[Rawleigh]

I have had the log type manifolds on my Chryslers since 1966 with no problems. Any leakage would occur behind and below the ports with those. If your engines are high enough above the waterline those are what I would use since they are failsafe.

[Bruce]

The center riser manifolds vs the log aft riser are tuned just a bit better but nothing that you would notice.
Riser height above water line should be at minimum 13" with a downward slope of the exhaust to the outlet. All my 31 repowers no matter who's engines were used I shot for 14" lightly loaded and 12" full heavy loaded. Never had any issues on any boat repower I did with these standards.

In those installs where that can't be achieved because of deck height, lift mufflers are used.

Btw, the rust which created the port issues were almost exclusively from exterior water and moisture being that this is the hottest point on the engine/manifold.
Hatch leaks, side vent leaks etc.

If they were leaking internally from too low riser, water would have gotten into the cylinders and locked up the engine.

[Pete Fallon]

JohnV8er,
When I repowered my 31 express back in 1997 I put in 454 Throttle bodied fuel injection with 3" extension risers on the middle of the manifolds. I had 5" exhaust outlets at the transom with 18" Vernay FRP mufflers under the cockpit sole( straight back from the Y pipe that was on the centerline ) no fish boxes under the sole. If your using fish boxes look at Bob Lico's install of the muffler system.

I used blue silicon 5" hose with 5" FRP pipe between the mufflers and the exhaust ports and the Y pipe. The outlets at the rear of the exhaust manifolds were 3-1/2" that were adapted to 5" into the Y. The 3" risers where used because I did a lot of backing down on sailfish and giant blue fin and I didn't want water getting into my new engines even with the rubber flaps over the exhaust outlets and mufflers that had baffles inside them. I didn't use serge tubes because I used the original Bertram installed oak encapsulated with plywood engine beds. If your using new aluminum beds you will have room for serge tubes.

The manifolds with the 3" extension risers just fit under the new style on piece lift back engine boxes. I originally had the old style 2 piece boxes that I had to cut a notch so the risers would fit. The notch was between where the two hatch covers met. I never has a problem with this type of installation and there was hardly any back pressure in the exhaust system. Back when I did the exhaust system the blue silicon hose was $22.00 a foot wholesale but it lasts almost forever. Use T style clamps on all the fittings and secure the system to the inner or outter stringer with either copper or plastic strapping. Also make a couple of cradles for the front and back of the mufflers to keep the system from rattling under the cockpit sole. I used some scrap FRP for the cradles epoxied to the hull for strength., you can use 3/8" plywood or Azetk or Penske board for the cradles.
Good luck with your repower, take you time and think things out thoroughly before you start slapping things together.
Pete Fallon

[JohnV8r]

Guys,

Thank you so much. That is exactly the information I needed!

JohnV8r