[omc-boats] V6-to-V8 upgrade a success on 66 Sportsman

From: lib1@...
Date: Tue, 9 Aug 2005 20:28:21 -0600

Hi all,

I thought I'd give the OMC-List an update the progress of the V6-to-V8 conversion project in my 1966 Sportsman. I'd appreciate hearing your comments and questions.

Before you go off and think I'm plum loco for undertaking such an outlandish project, let me outline what the underlying reasons and goals were upon my undertaking this project:


Project Goals:

1) All our boating is done between 6500 ft to 8000 ft., in the mountains above Park City, Utah. The effects of altitude on air density and engine performance are well documented: see http://wahiduddin.net/calc/calc_hp_dp.htm In a nutshell, a normally aspirated engine produces about 15 to 20 percent less power at this altitude than at sea level. Top speed on my Buick V6 was down to about 34 - 35 or about what you'd expect the little 120-hp Chevy 4 to pull at sea level.

2) We also typically use our boat for family water sports, including towing one and three-man tubes, knee boarding and water-skiing. I can't remember the last time the boat carried fewer than three to four passengers, and typically she's carrying six sometimes seven adults onboard. It's pretty amazing how space efficient the Sportsman design is. We were constantly asking the little V6 to do more than it's fair share, and that was after we had reduced prop pitch a couple of times.

3) Given the first two points, I grew up in the sixties working after school at an Evinrude dealer where we sold the Sportsman, Playmate and Rogue. Back in the Midwest, the little Buick V6 was pretty respectable at 500 ft ASL, especially going against 75, 90, and 100-hp V4 outboards. The gull-wings were well constructed, but they are heavy for their size, weighing more than a lot of present-day 18-footers. It's a hull designed to handle moderate-to-heavy loads but it's not the most efficient hull design to push down the lake.

In short, I wanted to retain the boat's classic, original look and feel, but improve it's safety and reliability and restore her performance to at least the original sea level parameters. I also wanted a boat that came close to modern-day, turn-key operating standards, in short no muss of fuss.


Conversion considerations:

Last winter, I initially determined that approximately 200 to 245 shaft-hp would be required to meet the above objectives. I briefly considered rebuilding a Buick V6, even considering a larger 252 cu in version. While the Buick V6 can crank some serious power, it's not inexpensive to build-out one.

The popular Chevy 229/262 V6 are fairly common in boats, typically powering later OMC's, MercCruiser Alphas or Volvo SX outdrives. The Chevy V6 is a shoe-in, but getting 220 hp would require a virtually new engine running EFI; again not an inexpensive proposition. After pricing those out and realizing how much custom transom/sterndrive build-up would be required I focused on staying with the OMC stringer. Besides, I didn't want the "I coulda had a V-8!" syndrome to hit me later.

Taking careful measurements, I began to realize the fifty-year-old, Small Block Chevy (SBC) is only about 4-inches longer than the Buick V6. (The 300-inch Buick V8 was never considered, as it's even longer than the Small-block Chevy). The Chevy V8 is actually narrower than the Buick V6 and only weighs 90 pounds more. The SBC spawned a entire industry of affordable, available speed parts and know-how. OMC produced 283's, 305's, 307's and 350's, all with stringer outdrives. The OMC 307 was produced in 210, 215, 225, 235 and 245 hp versions and was fairly popular from 1968 to 1975. The 350 SBC was commonly available as a 260 hp model. I began searching for a suitable donor, late last winter.

In March, I located a clean, low-hour, 215-hp, 307 that came out of a well-maintained 1971 Reinell 22-ft cuddy cabin. The owners had decided to depart Utah for Alaska and needed a closed-cooling (antifreeze) sealed system and opted for a used 350 Chevy-Volvo SX combination. That meant I could grab the 307 and it's long-legged stringer drive with the taller 21:16 (upper) V8 gearing, for a fairly reasonable price. I found a good home for the Buick V6 and sterndrive: it's now repowering Brian Nelson's cool '64 OMC Deluxe 17; replacing a tired, 2-stroke, 88-hp sterndrive.


Construction details:

The early 65-67 model Sportsman all have an easily removable, one-piece, molded, engine bulkhead/seat. Once unbolted, this makes accessing the engine area a no-brainer. I'd hate to undertake this conversion on a later model Sportsman where the engine bulkhead is molded as part of the top deck. Having free access to the engine bay area made the job a lot easier.

Once the Buick V6 was completely removed (see: http://www.hhscott.com/evinrude/chevy_v8.htm ) ; it was interesting to see how OMC frame-mounted engines in the early (64-67) stringers. The transom was very thin fiberglass and was not going to be strong enough to support the Bennett Trim tabs. (SelecTrim was briefly considered, but ruled out due to complexity and space considerations). Also, it was very interesting to see the obvious "off-center" offset of the sterndrive aperture. I wanted to convert the early style round aperture seal to the later "rectangular" rubber seal. One the aperture hole was re-shaped, a piece of marine-grade plywood was glassed into place. Also, four engine mounting "pads" were glassed into the floor. OMC switched over to this engine mounting system sometime in late 1967. Engine wiring was upgraded and now uses the later style, (black and yellow), round connectors. The battery was slightly relocated using a box. The sterndrive assembly was completely refurbished by an experienced OMC stringer tech with 30 years experience.

At the same time, the steering system was changed over from the original rope-and-pulley style to OMC's mechanical TruCourse worm and gear system, which uses a sealed, push-pull cable system. Other improvements (that were easy to do while the engine was removed) are: an automatic, water-sensing bilge pump and separate bilge blower.

Once everything was properly aligned and positioned the engine was bolted to it's new, floor mounting pads. I must have been living right, as the hinged motor cover just clears the flame arrestor. But the engine water pump pulley ended up just brushing the front bulkhead. A small hole was cut in the vertical bulkhead surface and provided the additional clearance. A 3-inch, stainless, clamshell vent now covers the small hole and is the only visible evidence that the V8 is tucked away under the original motor hood. In retrospect, the slight clearance problem might have been solved by simply shoving the engine and drive further aft by about 3/4-inch.


How does she run out?

Well, better than I expected. The mildly tuned 307 idles so smoothly and quietly at 500 rpm, you have to look at the tachometer to make sure she's fired off and running. The boat's become, a "Sophisticated Lady" or maybe a "Miss Manners." It really showcases the design strengths of OMC's fully floating, vibration less, transom-isolated stringer mounting system. I thought the Buick V6 was smooth and quiet, but the small-block Chevy is just sooo. much more... incredibly smooth and quiet.

She even runs more economically, believe it or not! Starting is spot on, it's almost as well mannered as a modern fuel injected engine. I've yet to consume more than a full 16-gallon tank in a full day (6+ hrs) of skiing. But when you lean into her throttle, the QuadraJet-fed, mouse motor has a mean-sounding V8 snarl. Cool.

Spinning a 14 x 13-inch, pitch SST prop, she pulls 4350 RPM, lightly loaded at 6400 ft ASL. I haven't yet put the GPS on her yet, but she's showing 37 or 39 on the funky, original pitot-tube speedometer, when you average out, two-way WOT runs in smooth water. But for me, it's not so much just about top speed. Hole shots are improved and the trim tabs lift the back end and keep the nose down until she's up on plane. What's nice is the engine doesn't work nearly as hard. I look down and am amazed to see the tach loafing at 3000 RPM, pulling a tube, along with 5 or 6 onboard. It's unusual to push her past 3500 rpm. The Buick was struggling at 4000+ rpm to try and keep up under similar loads.

The other really wonderful, yet somewhat unexpected, surprise benefit is the TruCourse steering and rudder indicator. Few if any boats can out turn her or other OMC stringers with their 90-degree turning. The TruCourse really does add a measure of security and responsiveness. The rudder indicator is especially useful when skiing and docking. I'd actually rank the steering upgrade higher than the trim tabs in usefulness.


Any regrets?

The only downsides to doing this conversion are measured in pounds, dollars and time. The boat gained about 90 pounds in motor weight and an additional 50 pounds (est.) for the steering and trim tab gear. That effectively reduces her rated load capacity by 10 percent. The hit to my wallet was a little heavier, but I'm not regretting the financial aspects. In fact, I discovered an interesting fact about used boat (Blue Book) resale values: On older boats, the single variable factor that affects the boat's total resell value is the rated engine power, whether it's outboard, sterndrive, or inboard. No other improvement has such a dramatic affect on value. You'll never get much of your boating dollars back, but re-powering to a larger engine rating pays back a higher percentage than most boating expenditures.

I elected to have the conversion labor performed professionally, by an experienced OMC technician. It took a little longer than I would have liked, but I think the results were well worth it. Could you do this project? Probably, if you are the least bit handy with mechanical and electrical work and have a way of lifting the engine. It really isn't a technically challenging conversion if you stay with the old technology electric stringer. Converting over to a newer MercCruiser or Volvo SX would be considerably more difficult, requiring extensive fiberglass reinforcement to the transom and hull.

So what does the future hold? This winter I plan to upgrade the instruments and console wiring. But it's also fun to think what she'd do with a mildly warmed over 350 SBC? I ran some numbers using the data in Phil's Evinrude test station report. See the graph below to see how your early Sportsman (at sea level) compares:

Lee Shuster
Salt Lake City

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Received on Tuesday, 9 August 2005

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