Merc 20Hp -69 Increase output for racing

Hi

Im have several Merc 20Hp engine and i have planned to use one for racing( Merc 20 1969 ).

Does anybody know if connectong rod Crank shaft is bigger for Merc 35?

I was thinking maybe to drill up the block so 35 hp piston will fit.
The difference with diam is only 0,438" for the piston.

Re: Merc 20Hp -69 Increase output for racing

It will never fit as there is not enough sleeve material to bore that size, and if you could the stock reed block would not flow enough CFM's to make any real hp and it is limited to what you can make it flow.....

Re: Merc 20Hp -69 Increase output for racing

Not much you can do with the 20hp other than maybe square the ports. Are you running it on the stock full gearshift lower unit? Or, are you building up a backyard 25SS?

Re: Merc 20Hp -69 Increase output for racing

Faztbullet said:

It will never fit as there is not enough sleeve material to bore that size, and if you could the stock reed block would not flow enough CFM's to make any real hp and it is limited to what you can make it flow.....

Click to expand...

Hi
Im not doing anything to the gear.
Im working on a standard 20hp 1969 engine.
I will machine off cylinder cover.
Changing the nozzle in careburator.
May be increasing air channels and exhaust.

Re: Merc 20Hp -69 Increase output for racing

No way to machine the "cylinder cover" as it is part of the block. The cover you see is not a cylinder head - it is a water jacket cover.

Regardless, anything you do to a bone stock motor with the full gearshift leg will likely not be noticed in actual improved performance due to lack of propeller selection. Personally, I'd spend my time on set-up and making sure you're getting all 20 original ponies. There are a few props that are available - including a rare brass prop by Michigan.

If you really want to wake up the 20, put it on a direct drive lower unit from a Merc KE7 and build up a "Rat Racer".

Re: Merc 20Hp -69 Increase output for racing

Also, if you do square the ports, do NOT alter the port timing, just square them out, leaving enough of the webbing to support the rings. Make the webbing too thin and it'll break out...

Re: Merc 20Hp -69 Increase output for racing

Chinewalker said:

No way to machine the "cylinder cover" as it is part of the block. The cover you see is not a cylinder head - it is a water jacket cover.

Regardless, anything you do to a bone stock motor with the full gearshift leg will likely not be noticed in actual improved performance due to lack of propeller selection. Personally, I'd spend my time on set-up and making sure you're getting all 20 original ponies. There are a few props that are available - including a rare brass prop by Michigan.

If you really want to wake up the 20, put it on a direct drive lower unit from a Merc KE7 and build up a "Rat Racer".

Click to expand...

Hi
Thank you for cool pic.
How much rpm can you take out?

The boat i have is deep V-type weight 265 lbs
I will lift up engine to max.
Im planning to use a cleaver prop. Not sure about pitch. I have to try different types.
Or do you prefer Michigan prop ?

I have not yet managed to get cyl cover off since top part has not yet moved. 6 Pin bolts very rusty.
I will se if possible to take off material from the block.

What do you think if i chnage the nozzle to 0,081 for the carb ?
How much can i chnage the timing of ignition ?
Normaly little bit earlier have can give more power and lower exhaust gas temp.

Re: Merc 20Hp -69 Increase output for racing

Chinewalker said:

No way to machine the "cylinder cover" as it is part of the block. The cover you see is not a cylinder head - it is a water jacket cover.

Regardless, anything you do to a bone stock motor with the full gearshift leg will likely not be noticed in actual improved performance due to lack of propeller selection. Personally, I'd spend my time on set-up and making sure you're getting all 20 original ponies. There are a few props that are available - including a rare brass prop by Michigan.

If you really want to wake up the 20, put it on a direct drive lower unit from a Merc KE7 and build up a "Rat Racer".

Click to expand...

Problem with dismantling... soon time for sledge hammer

Re: Merc 20Hp -69 Increase output for racing

If you can find a cleaver prop that fits the propshaft and isn't too much pitch, go for it. Keep in mind, more pitch does NOT equal more speed. That motor needs to spin up into the 5500-6000 RPM range to make power. If you put a high pitch prop on it, the RPM will probably not reach that range. It's kind of like driving uphill in a car in high gear - the engine lugs and pings because it can't get into it's power range. Most of the stock props are 9x9 or 9x10, in both aluminum (common) and bronze (rare). It's not going to hurt to try others if you can find them, but use a tach to make sure the motor revs as it should.

Changing jets will not get you anything unless the motor is already running rich, in which case you would want to go leaner (smaller jet).

Changing timing will probably not get you much either. If you're operating off a throttle that moves both the timing and carb, then you'd need to set the max advance at about .235" BTDC. Any time you're messing with timing be sure to check your plugs. You want them to run a light chocolate color. If they run white or look blistered, you're running to much timing. Most racing applications fix magneto at the max timing and then operate the throttle off the carb only. Makes for hard starting but gives you snappy acceleration. My 25SS racing motor (stock 20hp powerhead on a racing lower unit) will rip the handle out of your hand when starting due to the advanced timing...

As for your need of a sledge - welcome to the world of 40+ year old Mercs.

Re: Merc 20Hp -69 Increase output for racing



Hi
As you can see there is not so much material to machine off.
Mainly the duct after careburator could be increased.
Also into combution space very limited.

Do you know a good web sait for spares?
Im seaching for starting motor and some other small things....

Re: Merc 20Hp -69 Increase output for racing

Electric starter are like hens teeth for these engines ..RARE!!!!

Re: Merc 20Hp -69 Increase output for racing

Faztbullet said:

Electric starter are like hens teeth for these engines ..RARE!!!!

Click to expand...

HI
I know its hard to find but im sure a standard starting motor could fit.
Its only important to a motor which fit the flange for the pin bolts.

Do you know any good web shop address

Re: Merc 20Hp -69 Increase output for racing

If you're trying to build up a high performance motor you do NOT want a starter motor on there. The reason is you would have to have the much heavier ring gear flywheel, which if you're trying to turn more RPM would likely bust the crankshaft. The lighter ring-gear-less flywheel is the one to have. Also, you need the original style starter motor. A different motor will not fit and will not align with the ring gear properly. It'll also have a different pinion gear...

For older Mercs, oldmercs.com is a good source.

Re: Merc 20Hp -69 Increase output for racing

Hi

i have leaht machine but how much can i machine off.

I will try to search from old merc.

Re: Merc 20Hp -69 Increase output for racing

On the last pic.
If i grind the hole to one inlet.
Do you think it will damage the piston rings ?

Re: Merc 20Hp -69 Increase output for racing

In the last pic, what you want to do is square the ports. DO NOT make them any longer or wider - just square them. DO NOT remove the webbing between the holes or you will break rings.

There isn't really anything you can machine off. The head is not removable and you can't remove anything from the crankcase side as it will make it impossible to reinstall the crankshaft.

Re: Merc 20Hp -69 Increase output for racing

Hi

Okey i will not make the inlet bigger.
What about the cover. If i machine off the "pluggs" then the air space will be bigger before entering combustion space.

I know. It only a cooling cover.

Re: Merc 20Hp -69 Increase output for racing

Not sure what you're referring to by "pluggs"... Are you referring to the intake port cover? If so, those "plugs" need to stay. Actually, some racers find the older style covers that have more contoured shape to direct flow into the ports. If anything, you want LESS volume in the crankcase, not more...

Re: Merc 20Hp -69 Increase output for racing

Also, if you want to open up the crankcase opening (where it meets the carburetor), open up enough so that it is the same size as the exit of the carb. You can also open up the reed block a bit - more towards smoothing out the casting than anything else. Leave the edges at the reeds squared up, though... Opening things up larger than the crankcase opening or the carb exit won't gain you anything. Also, make sure that the reed block intake is aligned with the crankcase opening. I've seen some that were off 1/8 inch or so with the pin aligning it incorrectly. Plus, make sure all the reed stops are set at the same height - I've seen them vary a bit, too...

Re: Merc 20Hp -69 Increase output for racing

Hi

Pic 195
Okey i will not touch these.

Pic 193
I checked the reeds. They all open in the same way. As you can may be see difficult to incease the ports.
Small wear on contact suface which can be grinded/polished off.

Pic 194
Here its may be possible to make the space bigger by machine off.

Pic 192
Exhaust gas port.
Here its possible to drill hole bigger so back pressure will be lower.

I knoticed that there is possible to out flywheel in machine and take away 5 mm.
Good ?