Re: [OMC-Boats] OMC-Boats Digest, Vol 38, Issue 5

Scott,

Starting in 1971 OMC introduced a 307 Chevy V8 that was uprated an addition 20 hp. In 1971 this was 235 hp and then from 1972 onward these engines were rated at 245 hp. (Oddly 10 hp higher than the 351 Fords). The engines were specially tuned using Corvette cams and other off-the shelf HP Chevy parts. Mostly they were installed in Panchanga performance boats. The point is you won't pick up significant gains in economy or power with just the switch in exhaust routing. (Alternatively a simple switch from an AL to SST prop will show noticeable improvement.)

So you have the following hp ratings for the 307 Chevy OMC, which all had the fantastic Rochester 4MV QuadraJet: 210 hp (1968-1970), 215 hp (1971) and 225 hp (1972-1976?). The straight exhaust versions of the OMC 307 were rated at 235 hp (1971) and 245 hp (1972-1976). (I still haven't figured out what OMC changed after 1971 to yield the extra 10 hp, unless it was the switch to the down-firing (late style) electric stringer underwater exhaust port, which placed the exhaust gases closer to the center of the prop-wash vortex.

When I was repowering or replacing my Buick V6 with my 307 Chevy in my Sportsman, I did extensive research on alternative exhaust systems and their effect on power, economy, and the environment (noise abatement). My conclusion: PLEASE DON"T DO IT.

The existing underwater exhaust has minimal back pressure, especially at the power levels our engines generate in stock form. One look at the diameter of the round exhaust collector inside the intermediate will tell you that, along with the scavenging effect of the underwater outlet the the prop wash.

Please don't break existing noise ordinances with an un-muffled thru-hull exhaust.

Besides you also run the risk hydro-lock damage of water ingestion when you drastically decrease the height between wet manifold and the exhaust inlet. That's never a problem with the long-leg stringer drive and it's stock underwater exhaust outlet. OMC engineers were obsessed over NVH, don't screw up their fine efforts. They went to great efforts to only have to poke a single hole in you transom. Why poke two more?

If you wanna make some serious noise, go get a different boat with a pair of big blocks. Then I'll curse when you don't observe the NO-WAKE zones and laugh at you when you get pulled over!

Sorry for the rant!

/ee

________________________________
From: omc-boats-bounces@... [mailto:omc-boats-bounces@...ultimate.com] On Behalf Of Scott Veazie
Sent: Monday, June 14, 2010 5:46 PM
To: omc-boats@...
Subject: Re: [OMC-Boats] OMC-Boats Digest, Vol 38, Issue 5

Had an interesting idea, I know this was done on v8 OMC applications, but I was thinking, could these be used on my Buick V-6? I don't see why they couldn't, maybe a performance boost? Also...the coolness of the sound could play a factor. Just an idea....

http://portland.craigslist.org/clk/boa/1792062090.html

~Scott
Received on Tuesday, 15 June 2010