Engine swap gearing and prop

nbertolan

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Dec 20, 2013
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I have a 16 ft 1974 monte carlo with the mercruiser 165 4.1L straight 6 with 1.65:1 gear ratio and we are putting a sbc 350 in it. It will be for tubing and speed. What prop and gear ratio do i need?

Thanks,
Nick
 

jimmbo

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A 350 in a 16 footer is a bit much, but... for gearing something in the 1.51:1 or 1.4:1 range, and props in the 21 - 23 range will be close.
My vortec 350 VP on an 18 1/2 ft has 1.51:1 gears and swings a 20in Raker or a 20 in Enertia at around 4900 rpm.
I did assume your 350 is a marine engine. If it is a car/truck engine, it will be next to impossible to get propped for proper WOT and still have good holeshot. If it is a car/truck engine, you will need USCG certified Carb, Flame Arrestor, Distributor, Alternator, and Starter. You might be able to reuse the Alt from the 250
 

proshadetree

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My 1994 truck engine has done great in my Bayliner for years. It twists 5500 WOT. Dont remember leg gearing but its off of a 4.3 boat. 19 pitch prop. I did use the original marine intake,oil pan, windage tray, carb, distribitor, exhaust. The internal cast numbers of the crank,rods,pistons, were all the same. Might have a diffrent cam but I love the way it runs so I wouldnt change a thing.
 

nbertolan

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It is a marine engine and thanks for the advise. Are there marine hei distributors? The one thats on this engine is hei with the vacuum advance. I thought vacuum ports were only on car/truck engines
 

Bondo

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The one thats on this engine is hei with the vacuum advance. I thought vacuum ports were only on car/truck engines

Ayuh,.... Welcome Aboard,.... That is in fact, Correct,....

Google up a Delco Voyager distributor,....

'n a V8 motor wants the 1.50:1 gear ratio outdrive,....

The input shaft of yer ole drive is also gonna be 'bout an inch short,....
 

jimmbo

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If it has a vacuum advance it is not a marine distributor. Marine distributors are either mechanical advance, or electronic advance. Since the 80s most I/O ignition systems have been of the electronic/transistor/CD variety. 'HEI' was just GMs name for it.
 

QBhoy

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I'd think you would need a 1.47.1 ratio, as mentioned. Most v8's have this ratio or similar.
If it helps you any, I have an 18ft bow rider with a 5.0 MPI running at around 300hp with the 1.47:1 ratio Alpha 1.
On this set up, the best all rounder I have found is a 19"p laser 2 prop at a smidgen over 5000 rpm (when trimmed and running light). This gives good hole shot and good top end.
It will also run with a 21"p which is great for cruising and top end, but not so much for water sports. This will run at about 4400 rpm flat out and running light.
On the assumption that your boat is a little lighter than mine and the rev range on the older 350 is between 4400-4800 (and not 4600-5000 like mine)....I'd say you would be good with a 21"P for good all round performance.
All that said and as many will tell you, all boats are different and trying out props is the best way forward...but you wouldn't go too far wrong with a 21"p decent stainless as a starting point.
 

jimmbo

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5.0 MPI running at around 300hp with the 1.47:1 ratio Alpha 1..

Which engine is that? I don't recall seeing a 5L rated at 300hp. I do remember 5.7 at 300hp and 5.0 at 260hp. I might be wrong but I don't recall it
 

QBhoy

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You are right enough...the 5.0 MPI is rated at 260hp standard.
It has a cam and performance exhausts fitted. I'm guessing the exhaust only gives a little, but along with the cam, it makes a difference. Estimated 30-40hp. Add in the higher octane fuel we class as "regular" here in the UK, I'm giving her an optimistic 300hp. Haha.
 

QBhoy

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Down side is that she runs a little lumpy at idle. Sounds lovely though.
 

QBhoy

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Jimmbo how do you find that hustler 20"P ? Our boats probably have similar characteristics.
The laser 2 I have seems pretty spot on, but doesn't stop you wondering what if....haha.
 

proshadetree

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Davis Unified makes a marine HEI distribitor, dont think any ever came factory though.
 

jimmbo

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I had the Raker on my 18 ft with a 150 Johnsom. When I got my I/O I had it rehubbed and really liked it. The entire boat rode higher out of the water, not just the bow. Used it for 4 or 5 yrs until I tried the Enertia. The Enertia seems to accellerate a bit quicker than the Raker and it holds a bit better in turns and high trim angles, though the Raker is no slouch in that area. Both props are very good
 

QBhoy

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Jimbo
In your profile pic...the boat looks brilliantly trimmed and running high. Very similar to the stance my own boat takes at high speed. You reckon a cleaver prop would be of use ? Only good for one thing really, but would be fun to try just to see.
 

jimmbo

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A cleaver prop is generally a stern lifter. My boat was running the Enertia when the picture was taken
 

QBhoy

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Is that like an offshore series prop ? 4 blade ? What pitch was that ?
 

jimmbo

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It's a 3 blade Mercury prop that has a lot of rake, mine is 20"
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Pics get bigger if you click on them
 
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