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Considering I will not be needing any torque to speak of, just out of the hole revs (and quickly).
Can I do away with the y pipe arrangment and go with twins all the way?

They will be short, will this affect performance?

I am wanting this engine to sit as low as possible it the hull, I was thinking of an EJ20T manifold and bringing it up around the top.

LegacyTT had it set up on Fatz's L sedan that way, nice clearance indeed.

Any thoughts would be appreciated.

The Y pipe does help with cylinder scavenging, but I reckon you'd be ok with twins for more grunt out of the hole.

LS1s are now using what they call an "X-pipe", which is a piece of exhaust that's used instead of a balance pipe between the left and right banks. That way you still get exhaust-pulse scavenging with the free-flowing qualities of a twin pipe.

This link shows you what I'm talking about

I wouldn't go any bigger than 50 mm though.

Thanks Mudrat, options galore when you build from scratch!

A hot exhaust is a problem in a small boat, I may have to water inject the pipes to cool them, then shield them with a cover.

I had thermal wrapping on my headers (until the first time I took them up the bush and it all got torn off!) and I could actually touch and hold my header-pipe after a long drive. It wasn't cool by any means but it wouldn't burn you if you brushed against it.

The wrap stuff is expensive though.

PS, it's also a good idea to keep your exhaust gasses as hot as possible too, for reasons that don't have to be gone into here.

Mate in Perth Dave has a jet boat with Toyota V8 & blower

seagull wrote:Mate in Perth Dave has a jet boat with Toyota V8 & blower

And Matt in Tasmania is building one with an EJ22. What's your point?

The EJ20T headers would be a considerable option, I've heard the uneven length manifolds provide better bottom end torque than even length type. This isn't to say that they'd be better than the EJ22 style headers for torque though, just better than those aftermarket even length EJ20T jobbies. The noise would be better, nothing like the uneven pipes to get that true WRX note (can't get it any other way).

It sounds silly, but your jet boat reminds me of the Motori Moderni F1 car. It ran a boxer motor (12 cylinder Subaru no less) to lower the centre of gravity. The motor had to be raised however to accomodate the exhaust!

Muddy's on the ball there with the pipe diameter too, you'll want to keep some form of back pressure or you can wave goodbye to your exhaust valves!

Cheers Tim, I am leaning towards the EJ20T headers and then wrapping them as the Muddy one suggested.

The reasoning behind lowering the engine is-
a) it helps with the driveshaft angle and
b) I am building a hinged centre steering console which will cover the engine.

Yet again I am trying to fit an elephant into a bread box!

Matt, you will need an EJ20T water pump aswell if you use the turbo headers, otherwise they don't clear the NA pump.

If you are injecting water to cool the rear of the pipes, make sure you drop the pipes first, or you may get water back into your turbos. also, don't feed it in at right angles into the pipes, but at around 30 deg ( in the direction of flow). You may also need a spray nozzel on each side.

To avoid drag from an additionl water pump on the engine, use an electric one, like an electric deck wash pump, which turns on with ignition.

I used to race drag boats ( shooting flames 2mts + out the pipes ) and there are many tecniques to deal with hot pipes. I would also recommend the thermal lapping, but would also go for some heat shields as well.

Don't forget, that with the extra torque, that depending on the boat, you will need large trim tabs on the back and when the throttle goes down, I have seen many a boat flip over with torque.

also, be carefull if you are thinking about lightening the flywheel. I don't knnow enough about what it will do, but I know that there is a huge difference between land and water. I think they are supposed to be heavier for some reason.

been thinking about this...brings to mind the options of extractors that were available for my Ford six..the average were interference design to give best torque and the other was tuned length for all out top end power. I've had both and prefer the interference for torque. For your needs wouldn't you just wanna get the gases out quick ?, so no interference ? and would larger diameter pipe run cooler ?

A larger diameter pipe , may run slightly cooler ( I used 4" megaphones) but are harder to cool. They also get in the way of anyone moving around in the boat. I was running a big block chev and needed some back pressure, but running "TT's" no back pressure is required, behind the turbos.

Intercooling would give you more options as far as boost though. Might be an interesting look with a front mount...