Outboard porting????

Hi All!

I have read a little of the "2 stroke tuners handbook" posted on the forum. Some of it is a little heavy going...for me a least (not being a mechanic or anything). I understand some of the basics but appreciate that this stuff is delving into the 'nitty gritty' of getting more from a 2 stroke.

Now the tuners handbook seems to be referring more to single cylinder motor bikes (???) from what I can gather. It was also written qute a while ago.

Can any one tell me what differences (if any) may apply to a 2 or even 3 cylider 2 stroke outboard in the modern age?
I'm guessing it goes without saying that any porting done should be identical in all three cylinders?
Also, the tuners handbook refers quite alot to the 'exhaust' ports and shapes of / dimensions of etc. but I didn't read too much on the transfer ports. Perhaps I missed it as I only skimmed over it.
Are the exhaust ports the only ports that get touched or must there be some modification to transfer ports as well?

Sorry in advance if these are dumb questions! Like I said I'm no mechanic but am trying to expand my knowledge or understanding at least.

No need for expansion chamber stuff as not allowed to run them.

Thanks Haych:cool:

It is one of the most intimidating books I ever read. The first few times I read it I think I re-read each page 3 times before going on to the next page.


As described in the book, what you do to any of the ports depends on what you want the motor to do differently from what it does now.

What to do to the transfer ports is well described in the book.

There is nothing really different with multi cylinder motors compared to singles as far as porting goes.

Little or nothing that applies to outboards has changed since Jennings wrote his book. Most of the basic research on 2 strokes has always been done by the motorcycle companies, so that is almost always the best place to look.

If you tell us what motor you are thinking of modifying and to what end, it will be easier to help:)

MARK75H: "It is one of the most intimidating books I ever read. The first few times I read it I think I re-read each page 3 times before going on to the next page."

Only 3 times!!! Lol!
I know what you mean.

Sorry should have mentioned what engine.....3 cylinder Suzuki DT 25/30 C.

These engines are known for having good low down torque but getting them to rev is the challenge. That and the fact that the rev-limiter is built in to the CDI box and not easily by-passed.

I guess what I would be looking for is good top end without sacrificing too much lower end torque.
I suspect that you can't have the best of both???

Standard rev limiter is around 5500-5600 RPM depending on your CDI box.
Without the limiter, I don't know what sort of upper RPM limit you should look at for at least some amount of reliability. I don't want a "hand grenade"!

At the end of the day, in any racing application, speed is what is sort the most so I guess thats what we are looking for.

Q = Is porting the place to start?

Thanks for replying by the way.

Haych :cool:

quoting SAM...As described in the book, what you do to any of the ports depends on what you want the motor to do differently from what it does now.

i DONT MEAN TO OFFEND .. BUT i do not advise messin with porting unless you KNOW exactly what yer doing here as many a motor have been ruined attempting this especially on an outboard ... the power / performance needed here is a LOT different than what a dirtbike etc uses (no gearbox ) to offset the loss of low end power which a typical bike motor lacks unlike a outboard ... which is typically a low rpm engine /hightorque ...as compared to the others mentioned here .simple explaination
as far as yer question about single verse multiple cylinder engine same applies only x times how many cylinder ...yes they would all be ported the same // radical porting in todays typical bike / jetski / sled motors NEED an expansion chamber to WORK with the porting to achieve the performance
also generally speaking exhaust mods can bring about ....sometime`s... major hp improvments ... as its raised hp goes up along with rpm`s where
it is achieved at the same time your dropping power at the low end which can BE REAL BAD for an outboard ... you are dropping compression as you raise the port also there are a lot of factors involved in this !!!!
as sam said ... what is the engine your interested in and what is the boat ??also what is it `s use ????? weight also ??? often times a properly set up motor with the rite prop can make a major difference in performance without messin with the engine , or perhaps a bigger engine would be a better bet ... NOT to say that it CANT be done , as it has been done many times by a lot of guys ... including me !!:)

Wouldn't it make more sense to get beyond the CDI rev limitation first before opening up the engine? If you can't resolve the RPM limit issue ther is no point in cutting up the engine.

Have you done all you can with trim, setup and props?

Hi All!

Thanks for posting.

First, no offense taken.
I do not have the intention of hacking into ports in the hope of making it better. I don't have the experience or knowledge.
What I would like is to have a better understanding of what works and what doesn't with an outboard, especially, as you point out, an outboard does not have a gearbox full of gears to help compensate for any modifications made.

Bigger engine not available as already running biggest allowed eg: 30 HP.

Boat set up is pretty good but always tinkering but rules do have limitations to that too.
Always looking at props to see what might work better.

Hey Mate, looks like your in the Northern Territory. :eek:

racing tinnies must be fun!! firstly.. your mate up the front does he have to be there?.. the reason i say this your boats sitting very flat.. how much trim are you running? how highs the cav plate ( this is the piece directly above the prop, part of the gearbox) in regards to the bottom of the boat? :eek:

what size prop are you running?

i see you have the boysen stickers on the boat.. your running those?

850.

I think the greatest possible power increase is going to come from increasing your compression ratio.

How much will depend on your class rules and locally available fuel.

It looks to me like your type of course and boat depend on both acceleration and top speed and a compromise in either would not be helpful.

Hi All!

Thanks again for posting. I'll clear a few things up.

850cc racer - No not Northern Territory.....Riverland, South Australia. We have enough trouble dodging logs let alone 'crocs'! :D
The boat is sitting flat in the picture as we are in one of the tight twisting creeks. My 'navie' is at the front to help get the boat to turn through the twisty bit. As soon as we hit the open water he moves back to the back seat with me to get the nose up. Not much boat in the water then!!
In the photo, you can make out the mouth of the creek with the River in the back ground, so we have just come steaming into the creek as the photo was taken to get around a couple of logs. :D
Cav Plate is and must be level with the planning surface of the hull. Can not run surface piercing props.
This year we ran standard class with an Alloy prop (10 1/4 X 15). Now planning to jump up to run 'Sports' and will use 10 X 17 OMC stainless. Must run though hub exhaust prop NO CLEAVERS.
As for trim, standard class must run fixed pin. We ran it on the 4th pin setting which was pretty good for both open water and in the creeks. For Sports Class, we are setting up a manual hydrolic pump type trim system so will run anywhere from 1st pin to 5th pin and beyond.

David L_6 - Rev Limiter is not an issue so we can discount that. It will rev to whatever you like depending on the prop on the bottom.
Can not run external exhaust but porting as far as I am aware is not restricted in our 'Sports' Class.
So any porting I guess comes down to creativity and alot of engineering/mechanical know how. Finger ports though....I have no idea what that means.
You are spot on though about not wanting to sacrifice bottom end torque for outright top end speed. There needs to be a compromise for sure.

Mark 75 - Yeah compression sure helps which can be helped with some new rings. That and a little off the head maybe.
Can run oversize pistons as long as they are "factory specification" and "Mass produced" (ie: Can not have three pistons made - must be available to the public).
Fuel, 98 oct unleaded with Octane Boost (which doesn't help much from what I read). Avgas is available and allowed in 'Sports Class'.

Hope some of that lot helps. Cheers Haych :cool:

Quote:

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Originally Posted by Haych

Hi All!

Thanks again for posting. I'll clear a few things up.

850cc racer - No not Northern Territory.....Riverland, South Australia. We have enough trouble dodging logs let alone 'crocs'! :D
The boat is sitting flat in the picture as we are in one of the tight twisting creeks. My 'navie' is at the front to help get the boat to turn through the twisty bit. As soon as we hit the open water he moves back to the back seat with me to get the nose up. Not much boat in the water then!!
In the photo, you can make out the mouth of the creek with the River in the back ground, so we have just come steaming into the creek as the photo was taken to get around a couple of logs. :D
Cav Plate is and must be level with the planning surface of the hull. Can not run surface piercing props.
This year we ran standard class with an Alloy prop (10 1/4 X 15). Now planning to jump up to run 'Sports' and will use 10 X 17 OMC stainless. Must run though hub exhaust prop NO CLEAVERS.
As for trim, standard class must run fixed pin. We ran it on the 4th pin setting which was pretty good for both open water and in the creeks. For Sports Class, we are setting up a manual hydrolic pump type trim system so will run anywhere from 1st pin to 5th pin and beyond.

David L_6 - Rev Limiter is not an issue so we can discount that. It will rev to whatever you like depending on the prop on the bottom.
Can not run external exhaust but porting as far as I am aware is not restricted in our 'Sports' Class.
So any porting I guess comes down to creativity and alot of engineering/mechanical know how. Finger ports though....I have no idea what that means.
You are spot on though about not wanting to sacrifice bottom end torque for outright top end speed. There needs to be a compromise for sure.

Mark 75 - Yeah compression sure helps which can be helped with some new rings. That and a little off the head maybe.
Can run oversize pistons as long as they are "factory specification" and "Mass produced" (ie: Can not have three pistons made - must be available to the public).
Fuel, 98 oct unleaded with Octane Boost (which doesn't help much from what I read). Avgas is available and allowed in 'Sports Class'.

Hope some of that lot helps. Cheers Haych :cool:

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Haych, My exp. is in messin with gettin every boat I ever owned to go faster sooo.. Ill 1st agree with getting the rev thing done & out of the way, you might be surprised. You seem to have your ducks in a row with props, set up, weight & rules... Yeah the tuners book will take me a long time to go thru as well but eventually. I mess with crossflow motors & it seems to be agreed on that to square off the tops of the ports w/o changing the timing will help a little w/o the need of ext. exhaust. Not sure how that would do in a looper tho. Id give it last resort status for now. Raising the compression might be good for the acceleration needs.