70hp Evinrude performance mods?

[Swoll50]

I'm pretty new to boating and was wondering if there were any tricks/mods to get some more power out of my 78 Evinrude? I'm a Mustang guy and know there are always little things you can adjust/swap/etc for cheap power gains on 5.0Ls, but not sure if the same applies with outboards...(ie. swapping carbs from another similar outboard/cold air).

 

[LubeDude]

Re: 70hp Evinrude performance mods?

You know the old saying, "pull the radiator cap and drive a new car under it", Well, thats the way you get more power on your boat. There really isnt any way to gain much more power from outboards. Just make sure it is running at its potential, and its proped tight for your application.<br /><br />You would get a slight improvment switching to synthetic oil both for the engine and the lower unit, but marginal at best.

 

[phatmanmike]

Re: 70hp Evinrude performance mods?

to add to what lube dude said... this is because on a car, you can modify the exhasut with headers, you can swapp in a hotter ignition and distributor, you can change camshafts and swapp lifters. none of these items exist in a typical 2 stroke outboard. like the dude said, synthetic oils help a smidgen, and thats just about it, you can get a jack plate for the boat and play with your tilt/trim angles, and lighten the load or even do some go fast bottom paint. "souping up" an outboard would only make it perform new again.<br /><br />m ikey

 

[Chinewalker]

Re: 70hp Evinrude performance mods?

Hi Swoll50,<br /> In some cases there are some relatively easy mods you can do to pick up power, .ie the 9.9hp to 15hp conversion on may of the Johnrudes of the 1970s and 1980s. However, there are some subtle, not-east-to-duplicate differences between the 70hp and similar 75hp. The 75hp tends to breath easier at top end, while sacrificing a little off the bottom end vs. the 70hp.<br /> As was pointed out, the easiest way to get more power is to get a bigger motor or a smaller boat. Set-up is everything. I see more people than I can count try to "soup up" a rig that has sea grass growing on the hull, a hook in the keel, a prop modified by the bottom of the lake, and a motor that hasn't had a good decarb and checkup since the Carter administration. <br /> Compression on that motor should be on the far side of 130psi in each hole and more or less even top to bottom. (My 1980 OMC triple punches the gauge to 145 psi in each hole.) You should be able to spin about 5800-6000 revs at WOT with a light load. Make sure you do a link & synch on the carbs to be sure the carbs are opening all the way at wide open and the timing is reaching full advance. 25-year old linkages have a way of loosening up a bit and allowing a bit of play to creep into the system.<br /> Check out your hull, too. Is it straight? Nothing growing on the bottom? No waterlogged foam flotation hiding under the floorboards?<br /> If you really want to go hog wild with an OMC triple, you'd need a true performance hull - light and fast. Because of gearing, etc. you can't get a stock OMC triple to spin high enough and still make enough power to plane off a family runabout. You may add more power to the motor, but you'd need a taller prop to bring the revs down, which inversely reduces the low end pulling power necessary to plane off a boat. No easy way to explain it...<br /> Just a teaser - I've seen a Johnson Stinger 75-powered hull blister the water at well over 100mph. The powerhead itself was basically stock with some mild porting. The real artwork comes in the tuning - a regular welder's nightmare of pipes and chambers to pull the ehaust out of the motor. That's where you make your power.... That, and the boat was a sliver of plywood, lexan and epoxy that weighed maybe 200 lbs....<br />- Scott

 

[Mercathode]

Re: 70hp Evinrude performance mods?

Swoll <br />I am not sure about the avail now that OMC bellied up but they use to offer some mods for the 3's years ago most common was a high performance head. But again you may be cheaper to upgrade to a larger motor. The way you gain performance from an outboard is in porting. The more fuel you dump in and the exhuast you can expell out the more power you make with in reason. Lets go to 1979 OMC made a 149 cube V6. The horsepower ran from 150, 175,200,235 all from the same block. The 235 had large ports,higher compression, more carb to give it the 235 rating. Each lessor horsepower block was detuned to give it its respective horsepower. Like a old I-6 Merc 90 has ports the size of your finger whereas the I-6 150 has ports you could put a fist through. So Outboards can be modified. This is how the manufactourers achieve different HP with the same blocks. The problem with your 70 is that to achieve lets say 100 HP would tax it to the limit for recreational use. But outboards can be pumped up if you have the $$$$$$$

 

[Bass Runner]

Re: 70hp Evinrude performance mods?

A friend of mine changed carbs, took a set off a 75 stinger and it ran a lot stronger i looked at the two sets of carbs and they are identical i think the difference was in the jetting, it did run a little rich at idle and fuel econemy went down but it ran a lot better, might check into rejetting your carbs with 75 stinger jet sizes, or mayby find a salvage that has a dead stinger for the carbs. an old omc tech told me that is the only difference between the 70 and 75 horse moters, but i don't know.

 

[Bass Runner]

Re: 70hp Evinrude performance mods?

by the way how heavy is your boat, i am running a 70hp 74 model on my 16' texas maid deep V and it will run like hell with a 13x19 raker prop, but is a little slow out of the hole i can drop to a 13 1/4x13 and it will almost turn over backwards comming out of the hole but i loose some top end. My boat weighs in at 540lbs. with out gear and has a nice clean hull and it has a 6'9" beam by the way welcome to the forum.

 

[rickdb1boat]

Re: 70hp Evinrude performance mods?

Some boysen reeds may get you a little extra...

 

[phillnjack2]

Bass runner there is no such prop as a Raker 13x19, the omc Raker only come in 13.5 diameter for the 13 spline and range from 18 to 24 pitch all even numbers.
the 13x19 omc stainless is a SST propeller, totally different prop in every way.
I run 13x19sst and would love to get hold of a 13.5 x 20 Raker to get my bow lifting up and moving a lot faster.

A 70hp evinrude can be taken to 90hp !!!
Now before you get all excited and start thinking you can get a 50 % increase , that would purely be for a racing application on a very light boat, and
it wouldn't last too long on an every day run about boat.
there is just so much that can be done to the omc triples, its just how far do you want to go ? and how much to spend as well.

For performance you can first of all see about getting the engine set up right on the transom, i.e height for top performance all round.
once this is done if you want a bit more power there is CCM reeds for better idle and smoother running, then there is finding out what compression you have.
If its in the 125 psi range then get the head milled down by 1mm to get you up nearer the 150 to 160 psi, this will give you a fair bit of low down power.
for top end its a case of porting the engine, by this I mean raise the exhaust ports and clean the inlets to get things realy moving.
then there is exhaust modifications that can be done to get more power.
with just these youl be looking at a raise in rpm along with a power increase to run more prop.
its not too hard to get a 70hp to 6500 to 7,000 rpm at around 80hp and still be a reliable long lasting engine.
if you want to go real mad you can get these motors to run over 8,000 rpm but then its realy screaming and wont be too good for longevity.

the evinrude hustler75 and Johnson stinger75 were both 75hp at the crankshaft and these had just a few minor differences to a 70hp in 1978.
carbs can sometimes be swapped out , especially if you have small bore carbs !!
if you go onto Boat Racing FACTS .com (all small case and just one word because iboats dont like other forums being mentioned) you will
see there is a lot of knowledge about the omc triples for those who want to go fast, or for those who just want a little bit more .
there is plenty of 75mph boats out there running 3 cylinder omc engines, and not all of them started as a race motor.

first thing is to do a compression test.
then tell us what rpms you get at full throttle needs to be minimum of 5500 rpm at wide open throttle with normal load
what boat is it on
what propeller are you running and is it stainless or alloy 9stainles is a upgrade)
what oil do you use
do you have a jack plate
how heavy is the boat
what is your maximum speed
do you have power trim
etc.
.

 

[jimmbo]

Lubedude had it correct. The easiest and CHEAPEST way to more power with a outboard boat, is a bigger engine. Trying to wring more power out of a current engine is. 99% of the time, like banging you head against a brick wall, feels better when you don't. I admit there is 'more power' to be found in these engines. BUT, you won't like how the engine and the boat will respond to it. Even some production engines, like the 3 cylinder OMC 75hp, the 3 cylinder merc 70, and the merc inline 150hp were less than optimum. All were trying to attain what was 1.5 hp per cubic inch. For a non racing outboard that pretty much the edge of what makes a usable recreation outboard. These engines all had soggy holes hot, erratic running in the 2000 - 4000 rpm range, but from 4000 - 6000, they could haul @$$. Further hopping up just magnifies these characteristics.

As mentioned you should look at making your setup more efficient. Trim, engine height, hull condition, proper propping, will help. Remove all unnecessary weight. Wives, kids, and especially mother-in-laws, are to be left ashore. Girlfriends and Mistresses on the other hand, are not a hindrance to performance and can come aboard.

 

[flyingscott]

By the time you spend the money to modify your motor you could buy a different one. The 86 and newer 70 hp motors are a large jump in power over your motor and can be found for reasonably cheap. An example is on my 78 SS160 I had a 79 70 hp evinrude that did 31 MPH with a 17" stainless sst prop. Same boat same prop 1986 70 hp 38 MPH and overreved by 200 rpm so with a 19" stainless would have been over 40. That is bone stock they don't have the best idle but they are more powerful. Your motor is rated at 5500 rpm the 86 and newer are at 6000 rpm. Just a thought.

 

[jbcurt00]

Closed

Phil/Jack, it's been 12yrs since this topic stopped being active. Back in to the depths of the archives it goes.

Please remember not to post into inactive topics. After 12yrs, this most definitely fits the inactive status................