Performance parts?

[jedimaster]

Well folks, just wondering if anyone knows what may be available for the 70hp johnson's as far as performance parts? Things like velocity stack, exhaust tuners etc... Trying to squeze out a few more rpm's to jump up a prop.
Its for a bass boat application.
I know I can get boyesen reeds, but can't find any velocity stacks, exhaust tuners etc... The last time I had the carbs out I notcied that the intake runners are reall bumpy, does anyone polish or port these things?
Can't go to a bigger motor, for weight issues, so I am maxed out on a 70, jst trying to squeze a bit more out of it. Already junning a 6 inch plate, and waiting to get it in the water on a new prop(hopefully tonight if the rain holds off)

 

[Chinewalker]

Re: Performance parts?

Your 70hp is already pretty well tuned for peak performance. About the only thing I can say is that if it is an older 49cube powerhead, you can try to find a 75hp block, which was ported a bit differently for better power at peak RPM. If it is the later 56 cube block, then there's not much you can do beyond the Boyesons. Concentrate on set-up and prop selection rather than trying to tweak the powerhead...
- Scott

 

[jedimaster]

Re: Performance parts?

Its an '81 so I would think its a 49. What years of 75hp block can bolt on?
Already running a 6 inch jack plate 3 inches below the pad, tried a few different props, just lookin for a bit more if possible. I don't mind spending a few hundred here or there.

 

[boatmonkey82]

Re: Performance parts?

shave the head , put some reeds in , order 2 10lb bottles of go fast juice and a few 100 hp jets and go have fun .

 

[jedimaster]

Re: Performance parts?

Not lookin to be that destructive.
With motorcycles, I know once you go with reeds, usually you jump up a jet size is that the same for outboards as well?
Also how much comes apart when changing the reeds? Can it be done vie the carb opening, or does the block come apart?

 

[Chinewalker]

Re: Performance parts?

Behind the carbs is the intake manifold. That manifold comes off, with the reed blocks bolted to the backside of it. I'm pretty sure the directions that come with the Boyesons also outline what to do with your jetting...
Your motor is indeed the 49 cuber. I'm pretty sure any of the other 49-cube blocks will work, from about 1973 to the mid 1980s. The "hot" one, as I understand it, was from the late 1970s. Not sure why, but those seem to be the ones the FE guys go for...
- Scott

 

[Dhadley]

Re: Performance parts?

The best of the 75 hp blocks was the 1975 but on a bass boat I doubt you'll see any difference. Most everything depends on where you are now. If you're at 6000 rpm you can go one direction. If you're in the low 5000 range you'll need to go another.

 

[jedimaster]

Re: Performance parts?

What if I am in the middle at 5700? Not getting enough bow lift up at wot and slow on hole shot.

 

[Dhadley]

Re: Performance parts?

At 5700 I wouldn't expect the boat to be a rocket out of the hole. You need rpm, holeshot and bow lift. Several things can improve that. A jackplate will help with changing the fulcrum point for easier bow lift, more X dimension and easier adjustability. A bow lifting prop like a Raker, Stiletto or a Turbo Lightning Jr (if you can find one). You can try venting the prop for more acceleration. bigger holes if it has some now. There are lots of options.

 

[jedimaster]

Re: Performance parts?

It had no holes before, but I put three in and the hole shot is slightly better, I also lowered the motor in the water a 1/4 inch.
Now it revs out to 6,200rpms.
Also has slightly better bow lift as I can trim it up mor without venting.
Still a bit slow on the bottom end, but its bareable now, how large can I go with the vent holes? Right now they are 7/16's of an inch.
Should I jump up a pitch?

 

[Dhadley]

Re: Performance parts?

Now things are getting confusing. First we were at 5700 rpm now we're at 6200. To figure this out correctly we'd have to lay things out as they are. If you were at 5700 and then changed X, Y & Z to get to 6200 then there's more to this story. In the performance chase you always start with a baseline and work thru one change at a time.

 

[jedimaster]

Re: Performance parts?

When I was revving 5700, I think the motor was too high as It felt like I couldn't fully trim out as it started to vent. Now that I can fully trim out I am hitting 6200 rpm's but not much more speed, than when I was sitting at 4700, I think the prop must be slipping, maybe its still venting a bit but it definatly has better bite even at 6200 rpms, then when I was sitting at 5700.
I can't think of anything else that has changed, other than lowering the motor height and drilling three holes inthe prop...Oh yah I also filed down the leading edge of the prop as I heard that can make a bit of difference, I sharpened them up maybe about the first 2 inches of the blade to razor sharp. Umm, oh yah, I also put a basketball in the jack plate. Supposedly its supposed to help keep the boat from riding down during hole shot.

 

[CATransplant]

Re: Performance parts?

What is your goal here, jedimaster? If you're trying to get more performance that you can get from that 70hp, you'll be fiddling until the end of time. It sounds to me like you've probably reached the max on this.

If the performance isn't up to your goals, maybe it's the wrong boat and the wrong outboard for you.

 

[jedimaster]

Re: Performance parts?

Well, part of i is I just have no idea what the performance should be. I here and read all kinds of people saying they are getting this or that, a guy says he has the same hull and can do 45mph with a 50hp(I think he is talking km/h but he says its mph)... Am I getting what I should be or is there more speed to be squezed out of this little guy? I am certainly not expecting 50 or 60 mph, but I would have thought I should be able to spin 40 mph, with a 70hp and a 750pound bass boat hull.

 

[TN-25]

Re: Performance parts?

I would say that speeds approaching 40 mph with a 70 triple on a bass boat are entirely normal. You would be doing amazingly well to reach 45, but if you are already maxxing out the revs then it is a function of the prop. A deeper pitch might gain a few mph, but with your equipment on board the thing likely would end up being a dog out of the hole. It might not even reach peak revs if over-propped so you would actually be going backwards in terms of performance.

How fresh is your engine? Does it have very good compression? Fresh rings can help if the engine has a lot of hours. Another thing that those triples can benefit from is making sure the carbs are all in sync.

What shape is the hull in? Is it true or does it have any cupping from sitting on a trailer? Make sure it is totally clean and fully waxed to reduce drag. Also make sure that the stringers or flotation are not waterlogged (that can really weigh a boat down)

Johnson & Evinrude offered a high-compression head for the 1975 75-horse triples. It probably would be borderline impractical to use due to the fuel requirements (assuming you could ever even find one). Super tuning can cost in terms of tolerance for octane or varying operating conditions. If everything is tuned just so, you might end up even burning a piston if everything is not working together (a lean-out on one carb at WOT for instance).

I'm just wondering out loud if it is possible to bump the timing just a bit more on your motor, all else remaining as is. If it is possible, be prepared to run higher octane. I'm not a mechanic so take it accordingly.


As for your friend claiming 45 mph on the same hull with a 50-horse, well it sounds like he's inflating things a bit. IF it is a very light and fast hull, maybe 40+ (but I don't think you are running a tunnel hull)

 

[jedimaster]

Re: Performance parts?

Well I finally found a guy that will let me borrow a few SS props for my rig. hopefully I can find out that will get me out of the hole and keep the rpm's in check.
The hull is sound and solid, it weighs 750 pounds, and I am told its a very fast hull, but tends to be slow out of the hole.