View Full Version : need more HP
clone trooper
18-06-2007, 07:53 AM
hey guys, I have a 385 explorer and need more horse power from my 20hp yamaha. my brother has purchased a 25hp on the back 395 tinny and I can't keep up with him even when he loads his boat up with 3 big blokes.:-[
I understand that the 20hp is the same moter as the 25 but with bigger carbys( please correct me if I'm wrong)
so do any of you know who I can can talk to about moding my motor to get 25hp or more out of it. things like carbys, after market reeds, anything to make it go. I have put on a bigger prop which gave me more top end speed but in turn my brother did the same::). so if some one can point me in the right direction.
cheers brad.
FNQCairns
18-06-2007, 08:14 AM
Dont tell him, raise your engine 10/20mm and water test, the higher you go the better the boat becomes until it all goes to crap, finding the balance is where it is at, gotta take a little of the bad to get the mucho benefit of good.
The engine probably had a different tuner which is not a small mod. If the engine has the same displacement as your brothers, he should in theory only have the advantage in top end, you can also in theory even that score with engine height but keep it a secret.
cheers fnq
Kleyny
18-06-2007, 08:21 AM
i Don't know about your 20 but i have heard some models only require the carbies swapped over.
I would ring yamaha to confirm if this is true with yours. (straight from the horses mouth) I would hate to spend $$ to find out that it doesn't even run.
Depends on how old your motor is you could try outboard wreckers to get the carbs you require.
If all else fails steel your brothers carbies when he isn't home.;D
seriously you could borrow them to see if it makes a difference.
Noelm
18-06-2007, 08:36 AM
OK, here we go again, on MOST (but not all) the difference between two similar block but different HP motors is usually not just the jets or a carby change, seen it done heaps of times, and you end up with a poor performing engine that uses more fuel than it used to, remember your mates motor is 5 HP more, but that relates to 25% increase over yours! and setup as mentiones can male a huge difference in top speed, though it has to be really bad to give bad "out of the hole" performance.
bushbeachboy
18-06-2007, 09:17 AM
I have spoken with a Yamaha dealer about this very thing. I was informed that the main difference between the 20 and 25 two-stroke Yammies is an exhaust restrictor that can be easily removed. Well worth trying before mucking around with the carbies. I suspect it may be similar to fitting a better exhaust system to your car - the gases escape more quickly and allow it to perform better. If you were to combine this with raising the engine and keeping it quiet, you might win that race!
Noelm
18-06-2007, 09:44 AM
OK then, so you are saying that a 25 HP engine is exactly the same as a 20HP but has 1 less part, but costs more?? then I think Yamaha should get ready for a world wide law suit for some sort of fraudulent activity!
FNQCairns
18-06-2007, 10:20 AM
Go here to view the schematics easily, part numbers between HP sizes will tell all, for the savy spender - good bunch to buy from as well.
www.boats.net (http://www.boats.net)
cheers fnq
clone trooper
18-06-2007, 01:59 PM
I managed to get hold of workshop manual from my 20hp. thanks everyone for your help.
Noelm
18-06-2007, 02:51 PM
and the differences are????
Sailing Breeze
24-06-2007, 07:18 PM
[To my knowledge it depends on the year and model. Yamaha has changed the 20/25 models over the years with the main difference being carburation in some years and the other being cylinder capacity. ( 430 & 496 cc)
If your brothers 25hp has significantly more power it may have the larger capacity block.
Big G
25-06-2007, 11:28 PM
What year model is your 20HP.
I looked up the specifications on both the 20 and 25 hp 2007 models and they appear the same bore /stroke(67/ 56mm), same compression ratio(7.2:1)same ignition system, same gear ratio, same maxium rev range, .....
The difference would appear to be in the tuning, carburettors, exhaust ports or reed valves.
also for some reason the 25hp was 1/2kg heavier at 47.5kg?(20hp 47kg)
cheers
Big G
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