more hp out of my 20 merc

[Mark75H]

I think you have fallen for urban myths on drilling the exhaust. 2 strokes perform best with a certain amount of back pressure on the exhaust; no manufacturer makes a motor with too little exhaust outlet.

A very few drag racing V-6's have benefitted from some exhaust relief when the normal exhaust outlet was far deeper in the water when the boat was launching than the manufacturer intended (small boat, BIG motor). Just because it helped one motor doesn't mean a modification should be done to all motors.

[Roy Hodges]

I think mark 75h opens up a "hole can of worms , with exhaust tuning . I notice his "coke machine" (6 cylinder merc) on his hydro has a set of stacks on it . They might have been made by Lon Stevens. He told me, about 30 Plus years ago, that a MK 75h (stock) was Extremely sensitive to exhaust tuning . So critical , that he removed the backing strip of sheet metal inside the megaphone (stack) & welded the seam up without the benefit of the strip , because on the dyno , the strip caused a power loss. He also told me that a short flare or bell on the end of the stacks helped tremenously on the stock MK75 . But , some of you guys knew him , so ? But, You are right about one thing, "What works on one motor might not work on another "

[atk81]

last year i cut my pistons, to explain myself, i lined the top and the bottom of the pistons up with the exhaust ports, and the intake. then i marked them to see were they were hitting. i then took 2mm of the top and the bottom of the pistons. i was hoping this would give the same effects as porting the head itself, but with less damage if i f..... it up. pistons are alot cheaper than powerheads. hahaha. ya think this will give the same effect as porting the head, or should i try the block???

[atk81]

oh and one more thing , my motor seems to be a little slow out of the hole,,, but just for the first few secs. as soon as my boat levels,somewhat., she picks her revs up very fast. i am currently running steel reeds, do you think boyesen will help, i've got a set on the way. i figured it nwouldn't hurt, if any thing. and at least if i pop a reed i wont ruin my block. i'm hoping they will help me witjh my problem. i've set my reed butters out 2mm from the original position, this helped lots, can you go too far with them??? i see some people with ss motors that don't have reed butters at all, i would think this would cause flooding???? correct??

[atk81]

yes dampers. ok i was just wondering some thing else . i was looking into boreing. and going oversized pistons. the cyls on my motor are "nicosol" plated i wa sscared to bore it out because of this. will it hurt the nicosol clys?
not quite sure how to spell "nicosol nicosil". hope you know what i'm talking about.

[John (Taylor) Gabrowski]

It seems that by description of your cylinders that your engine is a later model loop scavenged with flat top pistons model with the block not unlike the Mercury 25XS as opposed to the earlier models of crossflow/deflector engines that had a large deflector crown not unlike an earlier model of Mercury racing block termed as the Merc 25SS egnines. As far as I know and I stand to be corrected is that "nikasil" plated cylinders correspond to loop charged/scavenged engines unlike the earlier ferrous metal cylinders were diecast with the aluminum block and could be rebored to go oversize. I am not aware that "nikasil" cylinders can be bored over without replating? I am told they can't and it might even be cheaper to buy another block if the bore of the existing block is no longer useable.

In any event, its admirable that you tried different things with it but being that it is probably the newer Merc loop engine, your not going to get the results you hoped for anyway and you could spend the money if accumulated on a real Merc 25XS stock racing engine on the used market? That engine of yours is very limited in what can be done with it just from its engineering, many consider it weak and not capable of too much extra horsepower without putting its internals under more stress and possible failures.

Some possibilities to increase horsepower with that engine would be to put in *power reeds, if the *carburetter has a restrictor under it, remove it entirely, use a *colder sparkplug and rejet the carb to accomodate the colder plugs requirement for more air/fuel (richer), *different ignition timing that maybe tried with different carb jetting and sparkplugs.

Real horsepower increases to stock blocked engines in particular, 2 stroke Mercs and others is through the *exhaust system. Right now you have a closed off *stock exhaust chamber that collects exhaust and sends it down through the tower and the jet prop exhaust hub and out. That is basic exhaust known to most outboards these days for pleasure/fishing use. To get meaningful horsepower increases you need to uncap the existing exhaust system and install a *custom made exhaust system composed of exhaust guiding filler block and megaphone external pipes that could give from an estaimated *8% to 12 percent increase in horsepower. Those would be a very loud system fetching you some or even a lot of complaints in some parts.
Again those are serious time and money.

*Computer Desgined *Custom made and connected racing expansion chambers not unlike those found on snowmo sleds, racing motorcycles and karts can produce power increases of some 30% and in cases slightly less or more. They are quieter than megaphone systems but they are long, need supports and they also take a lot of serious time and money too.

Coupled to all this is that Merc 20-25 horsepower engines internals are not that heavy duty to begin with to handle major horsepower increases.

*It would be practical and safer in fact to buy a used Mercury 25XS and used stock racing boat to go with it, if your wanting to really deal with realistic performance aspects of the Merc 20-25 Loop engines. Otherwise you can spend a lot of money and time and come away pretty short and disappointed in the long run if you are unprepared for setbacks and setbacks cost you time and money better spent smarter with something that has application in hi performance already. Re-inventing the wheel in this case would be highly questionable a project in terms of cost and result. Hope I have helped you a bit.