View Full Version : Getting more bang our of my old 20hp merc
Apollo
05-07-2010, 03:48 PM
Got a 1987 merc on the back of my old boat (cruisecraft cierra 1300) that goes well, but it would be good to get a bit more bang out of her. Can't go to a bigger motor and hard to justify anyway if I could. Was told that the simpliest method is to put a 25hp carby on it as the motors were the same except the carby (guessing only the jets). Any opinions?
Cheers
Steve
finga
05-07-2010, 04:14 PM
Kinda looks like it.
Compare all the parts numbers here between the buggers
http://www.crowleymarine.com/mercury-outboard/274.cfm
http://www.crowleymarine.com/parts.cfm
FNQCairns
05-07-2010, 04:55 PM
Pay attention to port heights too. Not just the bolt on bits.
Chimo
05-07-2010, 05:06 PM
Hi Steve
I got more bang out of my 2.3 HP just by lifting it until it lost grip and then lowering it a tad. :P Really pushes a 12 ft tinny well compared to what it was like with the motor way too deep in the drink.:'( Only down side is the need to shift weight to the stern on turns or turn gradually to avoid losing grip;D .
Got more bang out of a 9.9 by podding the tinny. That works a treat too, as the motor can be trimmed right out and the tinny now sits flat.:-*
Check out the simple stuff before rebuilding the motor IMHO::)
Cheers
Chimo
trymyluck
05-07-2010, 05:14 PM
Mate wack a couple of extra pods on the back and I'll do ya a deal on a 115hp Johno................:o:o;D Just like ranger only smaller..............;D
Apollo
05-07-2010, 05:28 PM
Thanks Mark, I appreciate your most useful assistance.
trymyluck
05-07-2010, 05:40 PM
No probs mate anytime......... ::)
Or you could try.... http://www.jennycraig.com.au/home/ ....:-X;D
finga
05-07-2010, 05:49 PM
Pay attention to port heights too. Not just the bolt on bits.
It looks like they use the same block FNQ.
I'd be checking the numbers for the reed valves etc.
Apollo
05-07-2010, 07:41 PM
No probs mate anytime......... ::)
Or you could try.... http://www.jennycraig.com.au/home/ ....:-X;D
Is that it? Is that all you are capable of coming up with?
Boat jumps and flies with just me aboard, so the problem must be YOU
trymyluck
06-07-2010, 09:10 PM
So a couple of pods and the Johno????;D;D
I'm pretty sure I'll be able to acquire the 25hp powerhead off the old man when I'm down Newy in a couple of months.
I was gunna say lose 20kg but I was beaten to it haha
seems though you have to lose the deckie to make it go :0
cheers Murf
trymyluck
07-07-2010, 05:40 AM
I was gunna say lose 20kg but I was beaten to it haha
seems though you have to lose the deckie to make it go :0
cheers Murf
It won't be long and there will be mutiny, Steve's kids will be making him walk the plank so that they can go faster...........They catch all the fish anyway......;D;D
Apollo
07-07-2010, 06:34 AM
Thanks to those that gave constructive, helpful assistance.
The other two - payback's a bitch. I can shed 20kgs, but there is no help for what is wrong with you pair.
Steve
Matt76
07-07-2010, 07:13 AM
Thanks to those that gave constructive, helpful assistance.
The other two - payback's a bitch. I can shed 20kgs, but there is no help for what is wrong with you pair.
Steve
Haha thats a classic.
Noelm
07-07-2010, 08:36 AM
as a general rule, most motors are not worth the trouble to make them the next HP up in a common motor run, most just result in a very poor performing motor that uses more fuel than it did before, in your model (from hazy memory) the exhaust is different, the carby size and manifold size, I think also the ports (in the block) are slightly different shape, and there could well be other differences, far better to get your current motor setup 100% and be done with it.
FNQCairns
07-07-2010, 09:19 AM
It looks like they use the same block FNQ.
I'd be checking the numbers for the reed valves etc.
Ahh! if that's the case the bigger engine might also have a RPM range higher than the smaller one??.
Bigger bore carby on an identical engine will end up producing doughy performance in the lower revs (torque will suffer) but it will come alive (sorta kinda) above 5200rpm .
The engine only has the ability to deal with the same stoichiometric ratio at same RPM....that can only change high in the rpm range with a bigger carb.
Wonder if the ignition timing via link and sync is a different specification, suspect they will not change ignition in any way.
Chimo
07-07-2010, 10:06 AM
Pretty obvious really! Good on you FNQ for bringing it up ::) ................http://www.engineeringtoolbox.com/stoichiometric-combustion-d_399.html
Cheers
Chimo
Noelm
07-07-2010, 10:22 AM
I once had the Stoichiometric with chips at a resturant once, or was that what I got the anti biotics for?
finga
07-07-2010, 11:03 AM
Ahh! if that's the case the bigger engine might also have a RPM range higher than the smaller one??.
Bigger bore carby on an identical engine will end up producing doughy performance in the lower revs (torque will suffer) but it will come alive (sorta kinda) above 5200rpm .
The engine only has the ability to deal with the same stoichiometric ratio at same RPM....that can only change high in the rpm range with a bigger carb.
Wonder if the ignition timing via link and sync is a different specification, suspect they will not change ignition in any way.
Ah, but what if the bigger carbi available is better suited to the engine dynamics and the carbi for the smaller hp rating is just not letting the engine perform to it's optimum ie it's restricting it's performance??
FNQCairns
07-07-2010, 11:45 AM
Chimo that's too complicated for me, my in depth knowledge on stoichiometric ratio's ends at about where air/fuel ratio starts :)
yeah can be true i think finga but with 2st outboard designs there is not a lot of room to move, they dislike running a little rich it will rob torque curve greatly and blow up quicksmart running lean.
still it's more about scavanging and pulse on these engines, larger bores even if jetted to supply the same amount of fuel V RPM can really only come into their own on the same block higher/highest up in the RPM range.
But yeah generally you are totally correct, and conversly with outboards always buy the engine with the hp you require but still the smallest HP output in that blocks series, it will be the better engine...changed a little now with electronics governing output.
As per Chimos advice the op could raise the engine and fit a prop that works ascertained via GPS and rev counter and he will blow 99% of all the other merc 25s out the water in pickup/longevity and top speed.
Spaniard_King
07-07-2010, 06:23 PM
Carbies and exhaust tunner need to be changed to make her go
wrenchman
07-07-2010, 06:37 PM
i have a (DICK) Johnson 20 hp on my 3.7 mtr tinny , the previous owner had the carby and inlet manifold from a 25hp Johnson fitted to it ... i dont know what it went like as a stock 20 hp , but it certainly has some mumbo now ... your best bet is to ask someone who works on them .. or even at the outboard wreckers , there usually up to speed with what fits what..
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