Looking for advice on new motor buildup

[SuperNova]

I posted a while ago that I had finally collected a pair of roller-cam vortech 350's by various means. Both engines are in decent shape, but need to be freshened, you know, rings and bearings, valve seals, etc. I was on one the the high-performance mail order websites and started looking at some high-performance marine stuff....I am after all a hot-rodder at heart. So anyway, I started salivating, of course, but then I started thinking about things I can't change easily on the boat, like the exhaust system....right now it is thru the prop and the way the boat is designed, there is no clean way to do thru-hull. So I guess my question is this: How much horsepower do you think thru-prop exhaust can support before it becomes too restrictive?

--
Stan

 

[Bondo]

Re: Looking for advice on new motor buildup

How much horsepower do you think thru-prop exhaust can support before it becomes too restrictive?

Ayuh,... I dare say 300hp is stretchin' it... though I've never heard of any actual Testing of flows...

It's actually the Manifolds that choke up a SBC,... It's the common plenum thing...

 

[Don S]

Re: Looking for advice on new motor buildup

What drives do you have?

 

[SuperNova]

Re: Looking for advice on new motor buildup

Ayuh,... I dare say 300hp is stretchin' it... though I've never heard of any actual Testing of flows...

It's actually the Manifolds that choke up a SBC,... It's the common plenum thing...

I kinda thought about that too, but if I do an upgrade, I would probably put some high-flow performance type manifolds and risers on as well.

What drives do you have?

They are first gen Alphas. The boat is an '86.

 

[SuperNova]

Re: Looking for advice on new motor buildup

You don't want to put much more than 300 ponies in front of that Alpha drive.

I appreciate that, but I'm not worried about the torque capacity if the drives.....I've already put over 500 hp through Alpha's and 7000 rpm. I have witnesses......that same alpha still lives and is doing duty behind a 375 hp motor to this day.....I know because I'm friends with the guy who currently owns it.....and yes those were dyno-proven numbers....it's all about prep, prop, weight and bite.......I will tell you that same motor (the 500 horse 7000 rpm motor) toasted my BIL's alpha outdrive within a couple of hours......I didn't prep that one.... it took all the teeth off the upper gearset.....locked up solid. So....enough of that....back on topic.......

 

[SuperNova]

Re: Looking for advice on new motor buildup

Ok, through the prop exhaust is not really an issue and often has less back pressure than through the hull due to the vortex effect of the water over the pop hub. (lots of factors come into play with both.) Another thought to consider would be side exhausts if you don't have room to run them through the transom. The only draw back to that is the god awful racket and more and more places are starting to put the clamps on the noise. (A good thing in my opinion.)

I guess the boat had an option for side exhaust when it was new, but mine is not so equipped. The noise is an issue for me as well. My BIL's boat has twins with thru-hull and boy does that get old quickly. I don't much like riding with in it and I really don't like running next to him. Gotta admit though, it does sound cool as hell idling through a no-wake zone....just running at speed down the bay it get's to be a little much. Also when your cruising into an anchorage late at night or starting out early in the morning, it makes you cringe cause you just know people are hating you.....so anyhoo thru-hulls are pretty much out for me unless I can figure a way to fit them under the molded in swim platform.

 

[wca_tim]

Re: Looking for advice on new motor buildup

I'm not sure where I've read it, but I'm pretty sure you can get away with as much as 350 or 400 horsepower (depending on displacement and rpmsto a degree of course) through prop. it'll be a little harder to make... kind of like overcoming the factory cats on a modern muscle car...

I despise the sound of side exhaust...

no way to go through hull? surely if you get creative you can manage it ok...

 

[SuperNova]

Re: Looking for advice on new motor buildup

no way to go through hull? surely if you get creative you can manage it ok...

Yeah, I could, but there are a lot of downsides to doing it...besides, I'm not looking at making crazy numbers here....just want to maybe get 350 or 400 out of each at absolute most......really only wanted to do a little more than a stock roller vortech. So uprade the cam, maybe aftermaket heads and a good intake and carb, aftermarket ignition including distributor so I can set the curve...you all know the usual drill....maybe do a pair of strokers if I can sell a few things to fill out the budget a little more. I also want to convert the port outdrive to counter-rotating (both outdrive currently rotate the same direction) I did find new flywheels for the one piece rear-main engines for 75 bucks instead of $200 so that helped free up some money......