[OMC-Boats] OMC Exhaust Manifolds (long post)

Hi Jason,

Always good to see pics of OMC Boats particularly in their element. Thanks for sharing.

Your 307 original was fitted with the log-style manifold. I see from your photo you are running straight, thru-transom exhaust outlets? Are they in use?
Is your goal to return to stock, original OEM style, as factory fitted? If so, with all the talk about obsolete parts, it's nice to know these are still in production and available from Barr Marine. In fact, Barr has manifolds for all OMC sterndrives: http://barrmarine.net/wordpress/category/omc/

Just a word or two about marine manifold design, might be in order. The original log-style OMC design does not incorporate a riser-trap. The trap is that upward bend (simialr to the one under your kitchen sink). The purpose of the trap is to keep sea water from coming back into the manifold and potentially causing a hydraulic locked engine.

With OMC's design tradition and focus on reducing NVH (noise vibration harshness) they designed the exhaust to flow through each side of the intermediate housing (on V-engines) and then exhaust gases are combined in a generous 3-inch port down the (long) leg to exit behind the prop through the anti-torque rudder.

This design not only resulted in acceptable exhaust back pressure, but combined with the long-leg (height differential) between manifold and underwater exhaust in effectively eliminated the need for a trap and it's quiet and it safely gets fumes away from the boat (under water).

While I would imagine your open-exhaust boat sounds very cool, in some area it might violate ordinances. More importantly, as a warning, I would hate to see you get water in your Chevy.

Which gets me back to Barr's manifold offerings. You could install later OMC/Generic style, with center or rear outlet with risers, which would give you some height differential and the protection of a trap for routing your non-stock thru-transom edxhausts. Installing external "flappers" also help. The later style manifolds are heavier and to take up more space.

On a side note, while OMC never offered anything other than the 200 hp, 300 cu in Buick V-8 and the 210 hp, 307 cu in Chevy V8 in our 19-ft hulls, but beginning in 1971 they offered a hotter version of the 307 Chevy. It produced 235 or 245 hp depending on the year. These featured hotter cams, higher CR, oil pan shields, straight-thru-transom exhausts, and the expensive cast aluminum Z-28/LT-1 valve covers, also the blocks were painted red-orange instead of OMC gray. Some of the 245's were offered with the OMC Jet Drives, while others twisted long-leg electric shift stringers and were installed in a few fast Panchanga boats (a Donzi competitor).

Few small block Chevy people realize that Tonawanda built these fairly respectable "hot" 307's to OMC specs in an era where the 327 and 350's got all the performance attention.
Most hot-rodders loath the 307, but frankly it's one the smoothest, most economical and reliable marine engines OMC ever sold.

OMC found they could get 302 and 351-W Fords a lot cheaper and began supplying more of those as replacements for the Buick V6 and Chevy V8's. It wasn't until Chevy introduced their V6 that GM got back into bed with OMC.

Lee Shuster
Received on Friday, 8 May 2009