2001 25H Johnson outboard question.

[PIPER354]

I noticed that the 25 and 30 horse engines have the same displacement and same basic weight as each other. Where does the 25 lose the 5 HP? Is it just a detuned 30 HP? Does anyone know how to get back those 5 horses?

Thanks,
Mike

 

[JB]

Re: 2001 25H Johnson outboard question.

There are probably many differences that don't show up on spec. charts, Piper.

This question surfaces about once a week about engines with the same displacement and different ratings.

In some rare cases the lower HP engine is detuned with a restrictor plate, but I do not know of OMC or BRP ever doing that.

In preparing 2 stroke carbed engines for racing, the following things are often changed:

* Carb venturi size.
* Size, number and flex of reeds.
* Crankcase volume.
* Size, shape and timing of intake and exhaust ports.
* Compression ratio.
* Piston shape, material and number of rings.
* Exhaust geometry.
* Ignition advance curve and max advance.

All affect longevity, torque and horsepower. There are others that modifiers don't talk about. OMC and BRP don't talk about any of them.

None of the above factors change engine displacement.

 

[PIPER354]

Re: 2001 25H Johnson outboard question.

Makes sense. Thanks.

 

[bigtwin]

Re: 2001 25H Johnson outboard question.

I own a 98 25 hp rude, I wanted a bit more power, so I did some research and found some differences, with the help of ebay and T/C Outboard I was able to make some changes, prior to these changes I was spinning a 13" prop, engine rpm 5450 @ 27-28 mph, now i'm spinning a 15" prop, engine rpm 5570 @ 32 mph8) First I had the small all metal carb. on my o/b, for $20.00 I bought a bigger 30 hp carb. off ebay, next thing I needed was a larger intake manifold $20.00 at T/C outboard, I also bought for $20.00 @ T/C Outboard a new bypass cover with a nipple for the oil recirculation line in the #1 port, I then needed a different throttle cam $6.00 from my jonny/rude dealer, and since I had the intake off, I installed a set of Boyesen reed valves for good measure $65.00 at Bob's machine,I also had to get gskts. and some small rubber hose to reroute the primer and oil recirculation systs. The mods took about 2 hrs. with a link & sink (less if I did'nt have elect. start) I raised the motor about 3/4" on the transom so the a/v plate is now even with the bottom of my boat, I also had to switch to a cupped 10x15" prop ($67.00 @ iboats) because the engine was now turning 6000 rpm with the stock 10x13" prop. Total cost about $180.00 for everything, it's alot faster and it now runs so smoooth now
Some 98 25 hp already have the larger carb. and the different bypass cover installed so all that is needed is to replace the intake, rejet the carb and change the throttle cam. in other words every motor is different so it is best to go to the BRP web site parts diagrams and verify what you have and compare it to the 30 hp.
Good luck!!

 

[PIPER354]

Re: 2001 25H Johnson outboard question.

Wow. Thanks for all that info. I do not have a the hot rod I have always wanted so my boat has become my hot rod. Always looking for a way to modify it.

Mike

 

[steelespike]

Re: 2001 25H Johnson outboard question.

What big twin said will no doubt give you some speed some more guts.
But if you do a very careful setup of your motor you could gain 2 to 3 mph, depending on where your at now.
Raising your motor an inch could gain 2 to 3 mph alone and careful trimming and prop selection might tweak out a little more.
With the anti vent plate about even with the bottom of the boat just raise the motor 1/4 inch at a time until just before ventilation becomes a problem.Prop selection could help that even more.And of course trim is important as well.If one notch doesn't seem enough but 2 seems too much you can make or by wedges to give you half a notch.
If your allready set up then the mods are the only way to go.good luck.

 

[bigtwin]

Re: 2001 25H Johnson outboard question.

Be careful raising your motor. I found my motor would loose water pressure before prop ventilation occures, keep an eye on your indicator (I think the water pump pickup is mounted high on these engines)
Please fill us in on what happens, I would like to know what you did and how well it works.

 

[steelespike]

Re: 2001 25H Johnson outboard question.

It appears like the intake may be on both sides of the lower unit.The screen looks like it could be modified to improve the flow allowing the upper portion to be blocked off.