bo-at ??'s

[Innerpiece]

bo-at ??\'s

hey all --
I have a 94 ski boat with a chevy 350 I/O weber quadra jet. As far as I can find, it has automotive "stock" heads on it with a lopey cam in it. The exhaust system, iggy, alternator, and external cooling system are all screwy marine stuff though... It's a 1994 mercruiser 5.7 I/O alpha I gen 2 (250hp) btw. a few questions.

(1) how much performance gain could I get out of slapping new heads on her?

(2) are "automotive" heads marine compatable?

(3) do k&n filters count as spark-arresting?

any thoughts or prognostications appreciated.

 

[fr8train]

Re: bo-at ??\'s

If you are lucky you might be able to find a boat shop that can port and polish your heads. Also check with Mercrusier and see if you can get your COMPLETE stats on your motor. You can contact Crane cams and see what they can tailor for you as far as more power and effieceney for your setup, as far as, cams are concerned. MSD makes a marine box also so you can upgrade your igintion. As far as a regular automotive K&N being a flame arrestor, sorry, no dice. Hope some of this helps. I had to help my uncle with his Fountain and the biggest PITA was working in and around the water inake system.

 

[cpqtim]

Re: bo-at ??\'s

Peace,
I have been playing with marine motors for some time. Torque is what makes a boat go. I have seen guys build killer motors that would make a truck flat arse fly an in the boat they lost speed. It is a delicate balance between torque, hp, hull drag, prop slip and weight. You don't want to put too much hp behind that Alpha or it WILL scatter, think of an ALPHA as a GM 200 tranny. What boat are you reffering to in particular, would need to know drive gear ratio (usually stamped on the alpha tag), prop pitch, current max RPM and speed. You may want to poke around www.offshoreonly.com in the message board. You may also try www.boatered.com. Tom Z has had several post's on building killer small blocks for marine use. On the offshore only site there is an add for a book by Dennis Moore (spelling?) and I think he has a pub on building marine sb's. Good luck and be carefull. DO NOT, DO NOT let a machinist near your block or heads unless he has done marine work before, it will scatter. Clearances and temps are all different. Special attention needs to be paid to the valves and seat angles etc. Remember a truck engine only gets thrashed for a short duration, you may think you are hard on it but running a boat at high rpm would be like dragging a Winnebago up a hill for an hour at WFO with all the brakes on.
Tim T.

 

[Thunder]

Re: bo-at ??\'s

The previous post sums everything up very nicely. Here are some of My thoughts.
Leave the heads alone. You will gain nothing by "slapping new heads on her"
marine heads are quite different than auto heads. they have smaller valves and are are made to generate power and torque at low RPM, If you put a set of automotive high performance heads on your ski boat you may not even be able to pull a skier out of the water.
YES K&amp;N <a target="_blank" href=http://www.knfilters.com/marine/default.htm>http://www.knfilters.com/marine/default.htm</a> makes spark arresting filters for marine use.They are less restrictive than the stock spark arrestor too. Dont use Automotive K&amp;Ns. They are not US coast guard approved and could cause an engine fire. That's A real bummer when far offshore.
Also dont put too much power behind that Alpha 1 It will blow. If you are going to start generating some serious HP look into a Bravo drive.
The MSD marine ign is a good add on It gave my boat a better hole shot.
Your hull playes an important factor in how fast you will go. You can sink tons of money in your engine. but you will only be able to go as fast as your hull will let you.
One word of warning Dont start replacing" Screwy marine stuff" with automotive parts. Marine parts are built and designed to perform in a far harsher enviroment than auto parts. Also most marine parts are designed to prevent fire and explosions from happening. If a fire breaks out in Your truck you can just jump out and run away.BUT<font color=red> How fast and far can you swim?</font color=red>
I have found this with boats. If you want a faster boat, Buy a faster boat. It will cost you less in the long run.

<P ID="edit"><FONT class="small">Edited by Thunder on 08/13/01 05:56 PM.</FONT></P>

 

[Innerpiece]

Re: bo-at ??\'s

I've seen people make the "autozone part in a boat" mistake ... ugly ..

I have a 94 chapparral 205se ltd. (20.5' cuddy) with 130 hours. I'm not sure of the prop size (it's been a while since I played with it), but I top out at about 56 mph (non stock speedo) at 4600 rpm (book is 4400-4800 WOT). I'm running cheap hydrofoils, a stainless prop, and the hull is smooth as a baby's ass deep V no steps. I'm happy with the top end, I'd just like to be able to pull fat people out of the water quicker without sacrificing top speed (ie. I don't want to swap out a higher pitched prop). I think that if I let her breath better (those stock spark arresters are serious flow limiting) with a marine K&amp;N I may develop a bit more power, and go to a higher pitched prop for a quicker pull with the same top end. whatdya think?

 

[Thunder]

Re: bo-at ??\'s

Well The K&amp;N may help. But the most performance changes come in the prop department. Changing props is trial and error there is no real cut and dried formula that works for every boat. Here is some info that may help you.<a target="_blank" href=http://www.gkpropeller.com/Help.html>http://www.gkpropeller.com/Help.html</a>
You could try a 4 bladed prop the same size as yours. They pull a lot better than a 3 blade without loosing much on the top end.
If you want pulling power dont go to a higher pitch prop. Go lower like from a 21" to a 19" That will also increase your WOT rpm.
I have to have 2 props for my boat one for fishing and top end, and 1 for pulling skiers with a full load.
Getting a ventelated prop also can help. These props have 1/2" holes drilled in the hub in front of the blades to allow exhaust gases to flow around the blades of the prop causing it to cavitate.When you nail the gas this allows the engine to get up in its torque curve almost immeadiatly. After you get moving the water pressure seals off the holes and the prop behaves normally. Merceury makes ventelated props that come with a set of rubber gromets that will increase or decrease the size of the holes so you can tune them to your needs.