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Righto gentlemen, I have built some swisho alloy exhaust manifolds to lower the engine a wee bit.

I am planning to go directly out the stern with each side seperate.

The manifolds are 45mm diametre and the pipes will steadily increase to 60mm at the stern.
Water will be injected about mid way to quieten them down and cool them.

Is there a gain to be had if I put in a balance tube between the pipes??

Dont know about jet boats, but on cars a balance pipe adds torque down low.

Agreed, it should have ample torque though as it only pushing about 300kg.
BTW it is a webberised EA81 stripped of all its non essential crap.

Its revs that I want, with appropriate torque levels of course, not silly road bike revs.

A decent exhaust note would be good too.

A balance pipe added to a system that did not have one before also added a deeper note and took away the cackling on the overun.
How hard would it be to add afterwards, so you can compare results for your self?
Nearly all factory systems that I can think off-road bikes included- that have seperate engine pipes include a balance pipe somewhere if the pipes dont join together fairly soon after they leave the engine, exept Subies, and they make that lovely throbbing noise because of it I think. Maybe its worth finding out the results of both ways for yourself?

Well that all sounds like good advice, Cheers Busdriver.

I can very easily fit the tube and most likely will now.
Gracias Amigo.

what happened to the plans of an EJ22 in the thing?

Weight, simplicity, reliability (both good but one is legendary), the hassle of a custom header set up for an EJ...

The fact that with my single stage impellor jet unit I am limited to 110 horsepower and not the 130 I thought previously.

I had the '81 built, running and I have NO MONEY to play jet boats with at the moment.

I like EA81s.

And the Kicker...Will suggested it, so it must be a good idea.

I don't understand why you'd want to run two pipes from the heads to atmosphere . I wouldn't think with std cam etc you would gain anything from not going into one and at least get something from a little extraction action .
Scavanging is all about creating a temporary area of low pressure beyond the exhaust valves just before they open .
Considering that most flat fours suffer a slight penalty because they fire the cylinders in each bank before crossing to the other side they get two pressure rises in each exhaust tract followed by a pause as the other side does its thing . Unless the engine pipes merge and you have a bit of exhaust pipe length you will lose bottom end torque because half the number of cylinders can't scavange properly - even for a flat four .

That boxer burble that people get off on is the sound of a four stroke piston pump that isn't gettting all the advantages that most inline fours do .
It's also a part of why Roos rarely get up near class winning fuel economy .
Don't ever think that these engines have horizontally opposing power strokes because they don't .