Re: [omc-boats] Through Transom Exhaust

I'd have to agree with Lee -- if the powertrain isn't designed to take
advantage of it, you probaly won't even notice the difference other than the
boat being a lot more noisy and obnoxious to your neighbors. (I'm a
powertrain engineer, btw.)

Also worth noting that the SAE standard (yes, the "SAE" on the oil can --
Society of Automotive Engineers covering land, sea, air, and space travel)
for measuring horsepower changed around that time. I don't remember the
exact year or if the change also applied to marine engines, but I know many
cars had to drop their advertised Hp.

Andy Perakes
1967 Johnson Reveler

On Tue, 12 Sep 2006 08:34:42 -0600, lib1 wrote
> Another good question that actually has some historical background.
>
> Just after OMC quit building our boats and sold the molds and rights
> to Chris Craft in 1970-71 they increased the rated power of their
> small block 307 Chevy V8.
>
> In the 19-footer E/J's that OMC built the 307 was rated at 210 hp.
> This was in models years 1969-1970.
>
> In 1971 they bumped the 210 to 215 and as far as I can tell from
> researching parts manuals no parts were changed in the process.
> Perhaps someone in engineering got a better dynomometer, who knows?
>
> But in 1971, they also introduced a 235 hp 307 Chevy. (Not to be
> confused with the later Ford-based 235 hp electric stringer). These
> 235 hp Chevies were painted red (as opposed to gray) and had
> Corvette/Camaro Z-28 cast aluminum valve covers. Theses engines also
> had the more agressive Corvette camshafts and THRU-THE-TRANSOM
> exhausts. They didn't idle down as well as the 210-215's but they
> sure do sound cool (or annoying depending on your point of view)!
> Not a lot were sold but you see them in the early Pachanga's
> (a Donzi-like v-hull boat).
>
> The in 1972 -74 OMC upped ante again and moved the 215 to 225. The
> "Corvette-Based" 235 jumped to 245. The engines were built to OMC's
> spec's by Chevy as they never offered a 307 in their car or truck
> lines with the QuadraJet and the hot Corvette camshaft. The OMC
> shifted focus to 302 and 351 Fords and the short-shaft stringer and
> Chevy 307 was phased out. There are more 225/245's out there than
> 215/235's becuase they were made for a couple years longer. As far
> as I can tell 210-215-225's are all the carb, cams, and heads. An
> the same applies to the 235/245's which are indentical twins as well.
>
> My advice (and this is based on talking to experienced OMC mechanics)
> is to leave well enough alone. About all you will accomplish in
> changing the exhaust is a lot more sound, er noise. You may think
> you're going faster but the power gains will practically un-
> detectable. That's because they engineered the back-pressure and
> matched it verywell to the 210-215-225's exhaust manifolds and
> camshaft. A hotter cammed 290-hp 350 might need less back pressure,
> but not our engines. Even the thru-hub exhaust of the 400-800
> stringer probably didn't add more than fractional horsepower gains.
>
> Probably more than you really cared to know......
>
> Lee Shuster
> Salt Lake City
>
> ----- Original Message -----
> From: "Scott E. Veazie" <scottveazie@...>
> To: <omc-boats@...>
> Sent: Tuesday, September 12, 2006 12:08 AM
> Subject: [omc-boats] Through Transom Exhaust
>
> > Would there be a horsepower increase by changing the exhaust from the
> > stern drive to a through transom setup? I've always wondered this,
> > besides the "muscle sound" gained going this route.
> >
> > ~Scott
> > -----
> > To get off this list send mail to omc-boats-unsubscribe@...
> >
>
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