50HP merc powerhead weight vs's a 70hp merc powerhead weight

Does any body know what a 50hp 4cyl powerhead weighes and a 70hp 3cylinder powerhead weigh both in the mid 1980's 83-84 models. I'm think about using my short midsection from a 50 and putting a 70hp head on it when i run my boat with a outboard jet lower. I just don't want to put to much weight on the transom because i'm going to bw using a tiller and i need to even out the weight from front to back with the jet lower . HEHEHE might be fun to run the prop lower on it to sometimes!!!! 350 lb boat and 70hp

The difference in just the powerheads will be in the 50-60 pound range (obviously the 70 is heavier). I think you will find without doing some major re-fitting and jury rigging that you will never get a 70 powerhead to mate up with housing from a 50 - they weren't from the same production family (40, 45, 50, 60's shared parts) and (70, 75, 80 and 90's shared parts), but they didn't cross back and forth.

The 70 would need "it's" exhaust system to produce the 70 horses - even if you did get the 50's housing retrofitted so that it would bolt up, it would restrict the exhaust enough that you would never make 70 horses (so you would also have to refit the exhaust plate) - and then using that to run a jet unit would drop your horsepower a further 1/3.

A 70 horse jet, running properly, roughly equates to about 50 horses at the "prop". So anytime that you don't NEED to run a jet (due to very shallow water or for safety reasons), you should stick with a regular "prop" lower unit.

A "JET" sounds fast, and there is a "cool factor", but horse for horse they produce about 1/3 less power (at the prop end) than a standard outboard...

Well, the 82--- 50 HP longshaft weighs in at 176 LBS ------------the 82 -70 HP longshaft weighs in at 190 LBS--------Straight from the 82 factory brochure.------------Mid section / lower units on the two are essentially the same.

Most of the parts are the same for a 70hp exhaust tube etc. the 70 has a different exhaust adopter tho but they all use the same gasket to midsection from mid 1970's to the 1990's. It should bolt up . i do know 75hp mercs and up are different tho. I know I'll lose 1/3 of hp when i use the jet lower that's why i want a bigger powerhead to make up for the lose. 50hp at the jet should push a 350lb jon really well up and down the river. The 25hp merc prop motor pushes the boat 25mph with 2 people and all my gear up the river but I'm tired of repairing the lower unit and buying many props. In one spring i spent around $800 in repairs . That's just for the parts . I performed the repairs myself. The motor should be lighter since its going to be a short shaft with no tnt.

Just wished my midsection was from a 35hp short shaft motor then if i get stop by the warden i wouldn't have to have insurance on it.

You won't believe this, but I've done this swap twice (sort of). In my case, I use a Mark 55 tower with the 70 hp triple power head. The difference in POWER HEAD weight is about 20 lbs, that's all. My complete motor (see photo) weighs about 145 lbs. and makes lots of power.

Jeff

The video shows this combo in action, on a 9 foot dink boat.

http://www.youtube.com/watch?v=1oCSXMSWUEU

was this an easy install? how did the exhaust line up? i have a mark 55 midsection and i was thinking about doing this conversion myself because it was so light. also what lowerunit did you use?

Actually, I've now done this swap three times! My latest has a homemade external reeds setup (and makes excellent power).

The exhaust tuner WILL fit into the Mark 55 type DSH, but you need to remove that sound reducing device (2 screws) and do some carving where it was attached. (Be careful here, for it's easy to grind a hole right through the housing--I've done that already.) The triple's adapter piece (plate under the motor) will bolt up except for the rear holes. I grind a slot in the DSH so they can be used as well, with a strap that allows clamping the works in place. (Can dig up aphoto if you need one.)

The rest is pure busy work: The shifter I use is the triple setup, along with the slide for throttle and shifting. The driveshaft is a go--no change there--but you'll have to adapt the water tube of the power head to the Mark 55 LU.

The weight of my LONG shaft triple/ Mark 55 DSH set up is 150 lbs.--just weighed it. That's about 25 lbs. more than a stock Mark 55, but it has a LOT more power!

Jeff

Pretty good work ..will all that epoxy live with fuel and vibration? If you really want to hop up a 3 banger find a 650 XS front half and install it. When we raced years ago we had 2 of the 650XS motors on tunnel boats. That motor had the 14 pedal V-6 reeds, 3 -38MM Tillistons, stock porting and high dome pistons,ran on 100 octane av-gas at 7000+rpm and made 90hp. Below is picture of intake...

...."...will all that epoxy live with fuel and vibration?"

We'll see! Been looking for a 650/ 700XS power head for years that I can afford. Hard to come by in the SA but easier in Europe, I'm told.

Jeff