1993 20hp 2stroke mercury prop help

[boater1234]

I have a 1993 20hp 2stroke merc that is currently running a aluminum 13 pitch prop and it's on a 1648 alumacraft flat bottom jon boat.The motor will get it,has awesome power so i believe i have the right prop on it.The question is i was thinking of going to a ss prop but i don't know for sure but i believe isn't the rule of thumb to drop a pitch going from aluminum to ss? How much increase in speed could i really get by switching to a ss prop also?Is it worth the extra $100.I know a ss prop will outlast an aluminum one by many yrs from what i have read.I could use i mean a hair bit more hole shot if i had to complain about anything i don't like but it's not that bad.I love the wot performance, i get near 30mph with close to 1000lbs in the boat,that is just amazing for a 24yr old outboard and a 20hp to boot.I am thinking of either staying with a new 13 ss prop or going to a 12.Any help would be awesome,thanks.I just want to know what to expect if i go either way up or down.Most of the time it will be just me or me and a friend so that is all the weight.

 

[flyingscott]

Why do you think ss props outlast aluminum either one will last a long time. If you want to go faster find the carb from the 25 hp motor more bang for the buck

 

[boater1234]

Is that the only difference in the 20/25hp merc from that yr,the carb. What about the exhaust or reed valve?

 

[flyingscott]

Just the carb, the 20 hp carb is much more restrictive than the 25 hp carb.

 

[Sea Rider]

Swapping carbs issue will only work if both engines have same CC powerheads. Instead playing blindly with alum or SS props, you need to install an induction tach, go for a wot spin you and your friend and check max achieved wot rpm, compare them to that 20 HP factory wot range, will know if actual current prop is performing right, if not, can go for a alum less prop pitch maximization to pull revs higher towards max wot range with you 2. Could be lugging or over revving badly that engine which is no good and not knowing anything about it.

Happy Boating

 

[boater1234]

The reed valve is the same also?I did a 9.9hp ayamaha once and the reed valve was involved also.Man if i can gain an extra 5hp for like 5 minutes work ,just unbolting the carb and putting the new one on that would be worth it.There is also different number carbs,like wmc 31/32A and then wmc 32,whats the difference and how do i know which to get if i do this????

 

[boater1234]

The engine runs perfect with the standard 13 pitch aluminum prop, I just wanted to get a ss for longevity and a bit better hole shot without losing any wot speed as i have been told the ss prop will do both.In my 1648 we get 28.3 mph with near 1000 lbs of total weight so it's not lugging a bit which is awesome for a 20hp but if i can get a few mph faster for a few hundred,why not. Mercs 20/25hp has been the same power head for near 25+yrs as far as i know and as long as it's not the rebaged tohatsu so that is no issue,the max rpm is 5500 and i know we haven't over revved because i'm sure it would act up.I ran wot for 30 straight minutes a few days ago and the ss prop crossed my mind as i have seen so many people post the benefits of them.The power head is 400ccs as as far as i remember unless were going way,way back then i'm unsure,the only difference in rpm is 500 going from a 20/25.I also just looked up a diagram on a site and every part was exactly the same except for the carb so Flyingscott is correct.

 

[flyingscott]

The 18/20/25 hp blocks are all the 25 CID blocks internally they are the same. Here is how it works the 18/20 hp motors share the carb but the 18 has it's own exhaust tuner. The 20/25 hp share the exhaust tuner but the 20 ,as stated before shares it's carb with the 18. Those motors showed up on a lot of 16' boats as a package deal and worked really well the prop is the same 13" pitch 500 more rpm. Stay with the carb for your motor has, the ignition you have will determine that look at the linkages to determine which one you need. Stay with the carb that has the plastic spacer in the fuel pump.

 

[boater1234]

I will just talk to the place i'm ordering it from if i do so,how much would i really gain in speed is the question,how much extra gas will it eat to.I get over 30mph with just me in the boat, i have never seen any 20hp do that since i have been alive,at least stock anyways.I have owned allot of merc 25s and i have to tell you this thing flat out gets it for just a 20hp even know i get it's a detuned 25.If i got like 3 to 5mph extra and a bit more power then $300 is worth it but i am not doing this like tom,so i have time to think on this.Your correct as a guy on you tube explained the very thing you just said with the different carb and exhaust as he had the 18hp and made it into a 25 with exactly what you just said,hit it on the head,thanks for the info.