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View Full Version : 302 to 351 upgrade for Donzi 16



donzidan007
07-07-2011, 03:24 AM
I'm thinking of rebuilding my 302 why the leg and bell housing are off is it worth up grading to 351 will it make much difference at the moment I'm getting 54 mph with gps with a not so good prop and bell housing shaft that's getting fixed

Donzi_Dude
07-07-2011, 06:05 AM
your exhaust location will not be the same(not to say it wont work). if i was going to the trouble i would use a 408 stroker.


i think a 400HP 347 would be a gas in a 16.

mattyboy
07-07-2011, 07:38 AM
the 351 is taller and as dd said exhaust is one issue hatch clearance is another. the 351 fits but you might need change things like flame arrestor, and the distributor cap. the front motors mounts may need to be address mainly how they attach to the motor. i think they used a different setup on the 351, I know they used a few different setups on the 289/302. my 302 has a plate/stand that bolt to the timing chain cover i didn't want to take a chance and have to mess with the mounts in the boat so I stayed with the 302 and had it worked on.

wrussellw
07-07-2011, 02:09 PM
I changed my '67 16 from a 302 to a 351. The height difference required a hatch modification because of the exhaust. I put tear drops in to accommodate it. I see low sixties on GPS.

mattyboy
07-07-2011, 04:42 PM
my 351w fit without any hatch mods but it was rigged by Holman Moody

ClassicFanatic
05-19-2014, 08:43 AM
I consider doing the same thing to my C16 93'.

It has a OMC Cobra 5.0 (302) and as far as i know all the "marine-parts" are interchangeable with a 351w. I am tempted to stroke it up to 408, but i think the little Donzi will work good enough with a 351w with 650 Edelbrock and a Edelbrock Performer.

Hope I don't have to space out the engine hatch...

mattyboy
05-19-2014, 09:40 AM
yes most parts can be reused , new intake and dist. maybe some new exhaust hose the 351 is wider and taller so you will need to see how much clearance you have now with the hatch . the 351 is 2 1/4 inches wider and 1 1/2 inches or so taller same length so motor mounts should not be an issue. will probably need a bigger prop not sure if they made a different gear ratio for the 5.0 and the 5.8 the ratio in the omc was determined by a combo of the upper and lower gears so when all is said and done you might need to jump an inch of pitch or two .

good luck keep us posted

ClassicFanatic
05-22-2014, 07:31 AM
yes most parts can be reused , new intake and dist. maybe some new exhaust hose the 351 is wider and taller so you will need to see how much clearance you have now with the hatch . the 351 is 2 1/4 inches wider and 1 1/2 inches or so taller same length so motor mounts should not be an issue. will probably need a bigger prop not sure if they made a different gear ratio for the 5.0 and the 5.8 the ratio in the omc was determined by a combo of the upper and lower gears so when all is said and done you might need to jump an inch of pitch or two .

good luck keep us posted

Thanks, Mattyboy.

Yesterday I bought a complete OMC 5.8 1989 engine. The carb is out of tune and I will order a Edelbrock 1409 600 and Edelbrock Performer intake. I also consider buing gt40 heads, but I'm not sure if I'm better of with the P's (will there be a sparkplugg-clearence-issue with GT40 og GT40P?). What about using the 5.0 head? The valves have the same diameter and it looks like it would ad some needed CR to the 351w?

I also consider new Pertronix ignition, but I will wait to see if it will clear the hatch.

mattyboy
05-22-2014, 09:47 AM
Thanks, Mattyboy.

Yesterday I bought a complete OMC 5.8 1989 engine. The carb is out of tune and I will order a Edelbrock 1409 600 and Edelbrock Performer intake. I also consider buing gt40 heads, but I'm not sure if I'm better of with the P's (will there be a sparkplugg-clearence-issue with GT40 og GT40P?). What about using the 5.0 head? The valves have the same diameter and it looks like it would ad some needed CR to the 351w?

I also consider new Pertronix ignition, but I will wait to see if it will clear the hatch.

I used a breakerless electronic mallory dist. on mine. any ford powered donzi I have seen the spark plugs are a bitch. The gt40 heads will have a thermactor holes that need to be plugged

mattyboy
05-22-2014, 06:19 PM
the P have straight plugs not angled which may cause clearance issues with the risers

ClassicFanatic
05-23-2014, 02:54 AM
the P have straight plugs not angled which may cause clearance issues with the risers

In my head it sounds great that the plugs sit straight. With angled plugs (like it is today) it's hard to change the plugs.

ClassicFanatic
05-23-2014, 02:56 AM
http://www.correctcraftfan.com/forum/forum_posts.asp?TID=9522&title=gt-40p-valve-train

Check out the second picture from TRBenj. Looks very good with the GT40P's.

mattyboy
05-23-2014, 06:05 AM
Yes that looks better on a boat where you can get get in and take a pic like that Just remembering my 16 with my short stubby arms any thing lower was an issue the runners on the PCM are part of the body of the riser wonder if that makes a difference too

Do you have all that other omc stuff on your risers the harness plug and electronic stuff?

mattyboy
05-24-2014, 09:42 AM
been looking at the different designs for Ford SB manifolds each are a bit different as far as water channel and runner config

osco are different from PCM from Barrs the one that may be an issue with the P heads would be the GLMs they have a water bar under the runners so I guess you have to take a look at your risers and see what kind of clearance you have under the runners

ClassicFanatic
05-30-2014, 03:58 AM
Update on my 302 to 351w upgrade.

Next week I will drop the engine. Based on a 1989 OMC Cobra 5.8 (351w). Edelbrok Perfoermer intake. Edelbrok 1409 Marine carb. BARR exhaustmanifold and risers.

I will make this setup work properly before I install GT40P's.

Estimated HP?

Will upload pictures after engine install.

mattyboy
05-30-2014, 02:03 PM
the 89 5.8 omc was rated at 215 hp intake carb maybe 240 250 ish PCM made an HO 5.8 pro boss rated at 310 hp I'll see if I still have the specs

mattyboy
05-30-2014, 05:59 PM
all the PCM 5.8HO pro boss motors used gt 40 heads the HP ratings from carbed to MPEFI were 285HP to 310 HP.

They used the PCM center rise style exhausts but were mainly for ski boat apps so the water dump was pointed sharply downward more so than the flatter angle of thru hulls on center risers. That may let them run a more aggressive cam without fear of reversion.

ClassicFanatic
06-02-2014, 03:00 AM
Is it possible that my 302 makes 200hp and the 351 makes 215? I've heard 250hp on the 4 brl holley, and to me that sounds about right...?

ClassicFanatic
06-02-2014, 03:35 AM
It seems like there is several versions of the OMC 351 windsor. In 1989 this engine size ranged from 210 to 260 hp.

The Cobra version is rated 240hp.
King Cobra 260hp.

I don't know if I got this right, but they also made engines only marked OMC 5.8 delivered with diferent setups - 210, 215 and 235. They also made a "OMC 250" - 250 hp.

My engine is a 5.8 Cobra, so my guess is 240hp, but thats really not relevant as it will be customized.

mattyboy
06-02-2014, 06:09 AM
The SBF was rated all over the place

Merc used the SBF as the 188 302 188 hp and the 233 351 233 hp in their line up in the 70's

the older light blue OMC had the 302 at 170 -190 hp and the 351 at 200-220 hp

holman moody had 200 210 225 and 235 out of the 302

they had a HIPO 289 at 271 hp

290 in 69 then 260 after that for the 351

usually this is all based on what the car world was doing at the same time with the same motors.

my 5.0 HO started as a car motor at 225 hp with a little upgrade to the cam and intake the guys who built it figured it is around 250 hp. I missed the boat the builder wanted to make it close to 300 hp but i didn't want to change out the exhaust or cut any holes. In hindsight I should have gone much bigger but I wasn't sure I was going to leave the boat or tear it apart.

so go as big as you can

ClassicFanatic
06-09-2014, 04:15 PM
News from Norway:

The engine hatch clears without a problem. I don't know if it works with stock intake and carb, but it sure wasn't a problem with the Edelbrock Performer intake and Edelbrock 1409 carb. The center Barr exhaust also clears fine. The only problem I'm having is fuelpump clearing the engine mount. I'm curently using the old fuel pump from the 302, but the old **** desintegrated and filled my new carb with all sorts of dirt. Other than that it all worked great and it's a killer compared to the 302.