Modifying Exhaust

**prattvan** · 05-19-2016, 10:43 PM

I did a google search on modifying an exhaust system on an 4 stroke outboard. I found alot of info stating not to do it, that you would burn your pistons.

Maybe they were referring to a 2 stroke engine, controlled by a carburetor. BUT, I'm referring to a 4 stroke outboard, fuel injected engine, controlled by a ECM.

So.........I'm kind of new to outboard engines, but have worked on car/truck engines, and go-karts for about 20 years (not professionally, just as a hobby).
For as long as 100 years, people have modified combustion engines in search of more power. If you buy a new car or truck, one of the first things some people do is change the exhaust system (because most stock exhaust systems are restrictive). And also change the intake system (such as adding a K&N cold air intake). Therefore, making the engine breathe better creating more power.
So I'm kind of confused on this. If someone puts headers on a 4 cylinder or V-6 engine in a car, you get more power. But, if you open up the exhaust on a 4 stroke outboard, it will burn the pistons ?? If you were to replace the exhaust manifold on a small 4 cylinder fuel injected engine, say 1.3L, with a header and connect that header to an piece of exhaust pipe(no muffler).......you would have a open exhaust. In theory, the ECM would sense the increased airflow and increase fuel flow to the injectors. Correct ??

Take a 1998 SUZUKI DF70 for example. It's practically the same engine that was in the 1998 Suzuki SWIFT car. Engineers made a modified intake manifold, a modified oil pan, put the fly wheel on the front of the engine instead of the back, a cast aluminum modified exhaust manifold with water jackets to keep it cool and from melting the hood/engine cover, and the engine is mounted vertical instead of horizontal, like in the car.

I'm open to ALL comments.

I have gotten alot of comments when I posted this on other forums.
They posted alot of comments about 2 stroke outboards. I don't have a 2 stroke. It's a totally different engine. Most 2 strokes are carbureted, and a 4 stroke is fuel injected. If you modify a carbureted engine to breathe better by creating more intake air and/or free flowing exhaust, you have to add more fuel. You either install bigger jets or a bigger carburetor all together. When doing the same to a fuel injected engine, it's a whole different animal. The ECM senses more air coming in and/or going out and increases fuel to the injectors........to a degree. If you modify the air intake/exhaust more than just a little, you change to bigger injectors.

By reading some of the comments I received, many assumed that I would remove the water cooled exhaust manifold and replace it with open headers or something to that effect. Not the case here. Simply put, I'm thinking of modifying the stock water cooled exhaust manifold.

Please feel free to reply. I would like all comments.

**Bodhi** · 05-20-2016, 12:20 AM

What outboard is it your trying to do this on?
Second, you answered your own question in the paragraph. ...by opening the exhaust you are leaning the motor out beyond the bandwidth of adjustment the ecu/emm/whichever is present in your motor. So after figuring out how to,program it richer, you'll then need to deal with the water cooled part. If the exhaust comes down the mid too hot, you'll start frying paint and water pump housings.
My opinion would be to leave it, we need to worry about ripping exhausts apart when the catalytic converters enter the scene....

**prattvan** · 05-21-2016, 06:07 PM

Originally Posted by Bodhi

What outboard is it your trying to do this on?
Second, you answered your own question in the paragraph. ...by opening the exhaust you are leaning the motor out beyond the bandwidth of adjustment the ecu/emm/whichever is present in your motor. So after figuring out how to,program it richer, you'll then need to deal with the water cooled part. If the exhaust comes down the mid too hot, you'll start frying paint and water pump housings.
My opinion would be to leave it, we need to worry about ripping exhausts apart when the catalytic converters enter the scene....

I was a general question / thought about 4 stroke engines in general. But with that being said, the engine I'm working on will be Suzuki DF70.

**prattvan** · 05-21-2016, 06:10 PM

I'm really surprised that on THIS forum, with a huge amount of modifications being done to outboards, hence a vast amount of mechanically inclined individuals.........that I only got one response.

**Bodhi** · 05-21-2016, 10:20 PM

Originally Posted by prattvan

I'm really surprised that on THIS forum, with a huge amount of modifications being done to outboards, hence a vast amount of mechanically inclined individuals.........that I only got one response.

The new stuff coming out is strictly computer related, none of this is like we know it......carbs, lean/rich, exhaust mod, intake mod.....it's all impossible nowadays without programs and other computer junk. If the was a way to mod your engine, bro, believe me this group on here is the best

**nitro_rat** · 05-23-2016, 04:58 PM

What is the largest outboard in the same family? With the newer computer motors the hp difference is all in the computer in a lot of them. Finding a dealer willing or even able to reflash your 70 to say a 90...

The charge for the program is probably = the cost difference of the different model anyways (otherwise everybody would buy the cheaper lower hp motor and reflash to the higher hp later). Good luck!

**prattvan** · 06-04-2016, 10:37 AM

Originally Posted by nitro_rat

What is the largest outboard in the same family? With the newer computer motors the hp difference is all in the computer in a lot of them. Finding a dealer willing or even able to reflash your 70 to say a 90...

The charge for the program is probably = the cost difference of the different model anyways (otherwise everybody would buy the cheaper lower hp motor and reflash to the higher hp later). Good luck!

Thanks for your reply. I understand what your saying. Makes alot of sense.