Re: [OMC-Boats] Outdrive Gear Ratios v6 versus v8

Hi Doug,

So let's be clear about your goals? Your asking if it's possible to take the outdrive assembly from a V8 and replace it's UPPER GEAR SETS (21:16; 21:17, 21:18, 21:19) with UPPER gear sets from the Buick V6 (25:25) ratio's ? I'm unclear because your footer lists 68 and 69 Sportsman's which probably already have V6 (25:25) stock gear sets?

First make sure you have what you think you have: Is your V8 unit a four or five bolt cap unit? You want the later five-bolt cap units introduced after 1967. Secondly is it a long-shaft or short shaft?
Short shaft's usually came off later Ford V8's (170-190-235hp) which had different ratios (see my chart). You notice they all share common lower gear sets of 15:23. It is the upper gear sets that vary.

I've never personally done this job, I understand it can be complicated by year-to-year changes and differences in shims, bearings, etc. Mixing and matching usually leads to problems.

What I did was to exchange the entire power unit without cracking open the units. At my altitude, I considered the fact that my 21:16 V8 gearing is a little tall, but I have enough room to still compensate by running a relatively short 13-inch SST prop. The 21:19 set used in the 165-hp, straight six Chevy is ideal but hard to find. The 302 Fords came in 175-hp @... 21:18, and 190-hp @... 21:17 but were usually as I said before short-legs. Remember I have a 24 per cent power loss when I run at 7000 feet, so I seriously considered running lower V6 gearing in my V8. The benefit of running the taller V8 gearing is I get fantastic cruising fuel economy, (that is provided I'm not into the secondaries of the QuadraJet.)

So to summarize: Find out exactly what you have, by counting teeth, or precisely counting input to output rotational relationships. And either exchange entire down-leg units or find someone mechanically familiar with rebuilding the outdrive with the desired gear set ratios.

Happy Easter

Lee Shuster

  ----- Original Message -----
  From: DOUGLAS JULIEN
  To: Evinrude & Johnson Boats of the 1960's and 70's
  Sent: Saturday, April 11, 2009 10:51 PM
  Subject: [OMC-Boats] Outdrive Gear Ratios v6 versus v8

        Hi Lee and list folks:

        Lee, when you upgraded from the V-6 to the V-8, did you change the gears in your outdrive? Per your prop page, these engines used different ratios in the upper gears:

        http://hhscott.com/evinrude/props.htm

        I'm wondering how to do the reverse - to change the V8 gears in an outdrive to gears designed for the V6. Is it a simple matter to switch these things out?

        Thanks for any info.

        -Doug Julien, '68 and '69 Sportsman 155's

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Received on Sunday, 12 April 2009