Johnson 30hp long transom rebuild

[nordy]

Hello,
I need to change short for long transom with a kit.
I worry about this, because the exhaust system's resonance length is increased about 11 cm.
This will cut maximum power a bit, because exhaust charging will happen at lower speed.
On the other hand maximum torque will increase due to better cylinder filling.
I prefer better power, so I consider measures to prevent this.
Is there a different exhaust tube (diffusor) for this ?
The series long transom versions of the 30 hp seem to have no different parts, which may shift back the resonance point to the short transom level.

Greeting from Germany
Peter

 

[Chinewalker]

Re: Johnson 30hp long transom rebuild

Hi Peter - don't worry about it. The resonance comes from the exhaust tuner under the powerhead - not the outer exhaust housing. If there is any performance difference it will be negligible and more than negated by having the correct shaft length for your transom...
- Scott

 

[G DANE]

Re: Johnson 30hp long transom rebuild

Hi Peter, good to hear you still have it running !! I am changing to long shart, too, got another boat. I bought a used shaft complete with lower unit. Since the motor is pretty slim down there, and the inner contains the drive shaft as well as the water tube, the volume increase is limited. I thought about it too, and I am not sure the crossflows will react as much to exhaust tuning as the loopers, with the small 2stroke scooters in mind !!. With the size of opening both at the end of the exhaust tuner and the outlet thru propeller, i dont think they put much weight on the exhaust tuning, back at the construction, which basicly was in 1976. Not that the rules wearnt known back then. I have seen racing outboards that had an over water muffler, on each of 2 cylinders, mounted at side of powerhead, with expansion chamber for each cylinder. Effecient, no doubt, but noisy, ugly and uncomfortable.