Re: [omc-boats] Through Transom Exhaust

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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Received on Tuesday, 12 September 2006