Home made 4 stroke race outboard.

[NZ Sidewinder]

I'm building a diy 4 stroke race motor that I hope will compete with the Merc 300 and 400r in circuit racing.
Maybe longer river races as well.
The Powerplant I'm investigating is the 2400cc K series Honda.
Turbocharged.
It's going to be a big job, and will be documented in this thread.
So far I've built the midsection and the mid currently runs Merc 2.5 15.5inch boxes.

Edit update
I decided to build a new midsection specifically for this build and retain the Poor man's 15inch motor as a 2.5L platform.
So this thread documents the build of an entirely new engine.

The mid is set up like an f1 in that it doesn't run an exhaust plate system.
This helps keep the powerhead low on the boat.
The K24 motor is 24inchs from the base to the top of flywheel, so it's tall, something like a v8.

I can't afford a r motor, so this is my way of racing competitively in the future.

Currently I'm studying the oiling system on the Honda outboards which use the k series motors.
I'll use a closed water cooling system

Should be an entertaining thread

[Greg G]

Subscribed!

[aj06bolt12r]

Awesome.. Following! Don't forget to keep us posted!

[DangerNewb]

Tremendous endeavor. Heck, I use up all my mechanical skills just servicing and swapping components to keep things running.

[NZ Sidewinder]

So the mid is done already.
It uses a 20inch Merc clamp that supports a large horizontal flat Aluminium plate that the powerhead bolts down to.
That plate, which I call the platform, is supported by large steel wings that are welded to the origional steering arm boss.
The steering arm is cut short and the boss is machined so that the connection to the steel wings is very strong.

Here is a photo of the platform supporting a 2.5 2 stroke and also a photo of the assembled leg
Also, hete is the old broken Honda I picked up to study.
This is a k23 marine motor, but my intend engine will be the car k24.
The oiling systems are the most important part of the job

[laser_ED]

Following this thread,from your past posts I think we all will be watching this one...

[LakeFever]

Just wait til that Vtec kicks in yo


in all seriousness this is precisely the kind of four smoke boost concept I think the world needs. A smaller initial platform to keep the weight down was my thinking but I can’t argue with the proven power and reliability some of those Honda guys have achieved with those K motors. Going to enjoy watching this come together

[NZ Sidewinder]

Ok, well the process has started albeit in small steps.
First I purchased an early 130hp Honda which is an f series type motor, but the goat was to see how Honda dealt with the oil system.
There was also the possibility that some of the parts from that outboard might be useful in the k24 build.
These 130 hp engines are not the car engine just tipped up put on an outboard leg, they are purpose built outboard powerheads.
So what did I learn from the 140 motor?
Interesting design.
They don't have main bearing caps or a girdle, they have the caps as part of the sump pan.
Sort of combined caps, girdle, pan.
The oil pump is about where a cars clutch is in the bell housing.
It's unlike any pump I've seen.
Flat like a saucer and is driven by a pair of flats on a coupling that drives the main input shaft for the lower unit.
Below all this is the oil actual sump.
Surprisingly the oil pump is able to lift the oil out of this sump without being primed
I can't see any system for holding the pump in a primed state.
This is good news.
Next step is to buy a good k24 3 lobe runner and start working with that.
I paid 500 buck for the 130 motor and reckon that the drive shaft coupling alone is worth that to my job.
I may use the sump and oil pump as well, even if the 130 pump is used only as a lift pump for the k 24

These engines have a flat crank and balance shafts.

[NZ Sidewinder]

This photo and the general leg layout is well published already, but for the sake of this thread, here is a photo of the leg and powerhead platform to be used.
The Aluminium base plate in this example is cut to suit a 2.5 merc, but another one will be cnc cut to suit the 4 stroke block.
Those black wing shape things are welded to the turn tube boss after the origional steering arm is cut off and boss is machined etc.
The part that's welded to the boss is 1inch thick and then sets down to 1/2inch where the Aluminium plate fits to it.
Shift shaft doesn't go through the tube, it is 2inch further aft.
The water tube in this example suits the 2.5, but can protrude through the base plate at any appropriate location.
A simple pan is fabricated for the cowl to fit to.

[Speed Jr.]

Well that went from cool to ahh. The K24 would be something to see if you get it to launch at all. The boosted 600+ HP Mitsubishi that was put on a champ mid wouldn't even start in gear, not enough torque. There's a reason everything new has a large displacement

[NZ Sidewinder]

We don't drag race here.
We circuit race.
The boats hit the
start finish line at full speed and go through the turns at over 70mph with ideal speed in the straights of high 80s.
We already have the big v8s here and know what's required to beat them.

[89LASER]

With the oiling system, would there be a way to dry sump it, having a tank in the boat and running an electric pressure pump to the engine? Just trying to think out of the box a little and save some weight off the transom. You would be able to prime the oiling system before start-up.

[NZ Sidewinder]

I've been thinking about this issue.
The typical honda outboard system is probably the simplistic.
However the Honda outboard pump just acts as a lift pump to feed the stock car version pump.
The car one is typically a k20 pump fitted to the k24.
I'm also most likely going to build a 10 or 12 inch leg instead of using my existing 15inch.
This is so that I can get the powerhead weight even lower and also, this enables us to continue running the 2.5 on the 15inch leg.
The priming of the top pump can be managed in the reservoir system where the lift pump meets the main pump.

[NZ Sidewinder]

Today I picked up a k24 block and head from a wrecker to use as mockup pieces for this build.
When it comes time to get an working engine I'll buy the whole car so that the motor can be tested properly and I'll have all the parts I need.
Wrecking yards here don't want to sell complete engines, they just sell long blocks and people need to use their own bolt on parts or buy them.
So here we go.
As with all this type of builds I do, first is to do a rough concept sketch.
I did this one during dinner yesterday.
The idea is to get the powerhead down very low, but retain a 20inch transom.
So to do this, as was the case with my previous 260 build, I lowered the powerhead by deleting the exhaust plates.
With this build I'll take this idea one step further and set the powerhead back and down, on a 9 or 10inch midsection.
This will place the propshaft around 2inches below the pad when the Merc offshore clamp is hard down on the transom.
That's about as high as a circuit race boat should be.
These engine parts are only for mockup testing etc but themselves are good for parts
Note; the sketch shows an exhaust snout.
This build won't have exhaust in the mid, it will be dumped straight from the turbo.

[NZ Sidewinder]

I've decided to put this powerhead on its own leg.
The origional plan was to use my diy 260 2 stroke leg, however due to the extra weight of the 4 stroke I've decided to build a new, even lower midsection as described.
It will be along the same lines as the APX and run up to 500hp.
Vee bottom circuit racing in Mod Vp.