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2002 300 hp reasonable from 5.0L GL

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nsxxtreme

New member
I have a 2002 reinell with a 5.0L GL motor and a penta sx outdrive.

I have heard that outdrive is ok to use with 300hp. I'd like to get a little more power. So I have 2 options. Build this 305 with some more performance parts or swap in a 350. Doubtful I will gain anything but maybe 20 more hp realisticly with the 305.

I can probably build a 350 for roughly $1000 that will get me 300 hp.
Couple questions I have, can I throw any old 350 4 bolt main motor in there?
Will the stock exhaust mount up? And any unforseen issues?

I've built plenty of engines before but never a marine engine. I know the 600cfm 2 barrel carb would probably have to be ditched for a 4 barrel.
If I was searching the junk yard for a donor motor what would be a good donor to start with?

I'm looking for suggestions any of you might have.
 
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Re: 300 hp reasonable from 5.0L GL

There are some things that you should avoid, and some things that, IMO, you should include.

Will this be using a closed cooling system? Or open (aka raw water cooled)?
Will you be using the thru prop exhaust?

Gotta run, but will try to get back to this later.
 
Re: 300 hp reasonable from 5.0L GL

Boats and cars and their engines are very different. For one thing, you neglected to mention what size model boat you have. The answer to your question will differ depending on if your boat is a 19ft bow rider or a 28 ft cruiser.

One base line bit of info for you... The duty cycle of a boat engine is different from a car. Boat engines spend their life in top gear going up a 20% grade at 75 to 80% of WOT RPMs.
 
Re: 300 hp reasonable from 5.0L GL

You can't get 300HP from 305 cubic inches in a boat, nor from a 350 for $1000.

You can in a car.

so, why not in a boat, you ask?

Boat engines can only turn 5000 rpm max due to the drive design (u joints/gimbal etc)

A 350 car engine will be turning 6000+ rpm when it is making 300HP

If you want 300HP in your boat, you have 3 options really:
1. 383 stroker small block
2. big block
3. 350 with multi port electronic fuel injection.

And as mentioned above, you will probably need through hull exhaust. and a different gear ratio in the outdrive
 
Re: 300 hp reasonable from 5.0L GL

You actually have a third option, and that would be the Duo Prop lower unit. I can tell you first hand that this makes quite an improvement.

Yes, your exhaust will work for most any SBC engines.

I agree with Hystat. You would need to include higher compression, and most likely increase valve lift and duration, in order to pull 300 HP from a 5.0L... and this would be at a higher RPM.
Marine engines in cruisers do not fair well when built this way.

We have to consider the RPM at which this HP is going to be rated at.
Often you will see that the RPM where these newer engines are rated, is an RPM that we just simply cannot and do not operate at.
The 315 HP 5.7L Vortec, for example, is rated at/near 5,300/5,400 rpm..... Yikes!
What good does that do us who own cruisers?

The 383 would be a good option also.... but this build requires very special attention, IMO.

Some of this is a cut/paste from a post I left for a member some time back:

If you were to custom build one, now is your opportunity to go with a piston/head combination that offers a Q/E (quench effect, or aka squish area).
I would distance myself from the use of a full dished piston (this is not as desireable as a Q/E piston would be).
The Q/E can be achieved by using an array of piston that provide this..... and in an array of brands, qualities, material selections, and even shapes, depending on which cylinder heads you would choose to use.

None of these pistons will have any portion of the dish that comes up under the wedge area of the cylinder head.
This is the most undesireable aspect of a full dished piston.
(I could load you up with some good articles if interested)

Instead, the Q/E piston will offer a flat area that comes up under the wedge area of the cylinder head that creates this effect (when the correct head gasket is also used).
It may still have a dished portion (to control C/R), but this will be offset from the wedge area.
A Low Compression Q/E can be a D-dished for a wedge type head......, or the "D" may have a tear drop shape as to accommodate the Vortec chamber wedge, for example.

Here is the important part, IMO:
A good Q/E combustion chamber configuration will reduce the chance of Detonation. (detonation is not pre-ignition)
Now you can throw some ignition TA at it, and make some real marine horse power, and with a reduced risk of Detonation!
All of this works wonderfully for the SBC marine engine.

I would, however, make my camshaft selection after I have made my piston/head selections.

If you care to, try to get your hands on a copy of Dennis Moore's "Small Block Chevy Marine Performance" and read the great information that he offers on building a more true SBC Marine Engine.
Beg, borrow or steal if you have to!
It is a great read!
717SXM8WJ3L._BO2,204,203,200_PIsitb-sticker-arrow-click,TopRight,35,-76_AA300_SH20_OU01_.gif
 
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Re: 300 hp reasonable from 5.0L GL

I beg to differ on the subject of thru leg vs thru transom (for 200/280 series drives at least). I don't know what the minimum diameter of the exhaust system "Y pipe is in the SX drive, but unless its at least 4" it's not enough (per the Merc manual for 350CID engines and there is no reason that Volvo's version of this SBC would be ANY different)... its a matter of the gas flow for a particular engine displacement at a particular HP and RPMs.

See below for my adventures in this area.

http://miniurl.com/51013
 
Re: 300 hp reasonable from 5.0L GL

Thanks for the posts guys. THe boat is 21ft for the person who asked earlier.
It currently has a 5.0L GL wich isn' a closed cooling system and has a penta sx outdrive I think that answers most of the questions.

As far as price wise go yes I believe I could build something for $1000. I rebuilt my blazer 350 motor for $500 I think I could pull off a 350 for the boat for under $1000. I'd obviously do all the work myself.

I like the 383 stroker idea as that really isn't any harder to do then a 350 and you can buy premade kits all ready to go. Although I haven't really looked at where in the rpm range they make their power. What would be a good set of heads to look for? I'm thinking a 10:1 compression ratio the boat already requires premium anyway. My simple plan was to use flat top pistons, vortec heads, 4 barrel carb and a set of roller rockers. That book looks pretty good I might just see if I can buy it.
 
Re: 300 hp reasonable from 5.0L GL

Re: Tha amount of HP you are putting on the hull..

Almost boats on calm waters can go 30
When you start to get to 35 to 40, then things start to get interesting
When you get above 40 well....

On a car, at most speeds :) all 4 wheels stay on the ground all the time.
A boat however (planning hulls) presents less and less of its hull to the water as speed increases.
Go fast enough in a hull not carefully designed for high speeds and you will soon have control and stability issues, especially if the water is other than mirror smooth.
For some data points: my approx 4000 lb displacement 23 ft (old school measurement) Deep V hull will run all day at 30 and pretty much just hit 40 wide open. On other than the smoothest water, 40 is... not pleasant... and in a good chop 30 is... jarring. I'm running 260 HP from a 2bbl carbed FWC 350 merc engine. On the waters that I boat on (Barnegat Bay), only the "cigarette" boats can run any faster than I do and they are ALL 7 ft+ longer overall than I am. When you are running in a chop, the length of your boat starts to become critical as speed increases. That's why all the "racers" are well over 30 ft LOA.

In short... I'd be very careful not to overpower your hull. While the porpoising that you see on TV that the big boys do when they are running wide open looks cool, running that way is on the point of loss of control on most "civilian" hulls.
 
Re: 300 hp reasonable from 5.0L GL

I dont care about speed really. My boat will do 35 and if I'm lucky and all things line up I have come close to 40. But I dont care about all that. I wakeboard and the extra power is really useful when you have a boat full of people and trying to get someone out of the water.
 
Re: 300 hp reasonable from 5.0L GL

nsxxtreme said:
I dont care about speed really. My boat will do 35 and if I'm lucky and all things line up I have come close to 40. But I dont care about all that. I wakeboard and the extra power is really useful when you have a boat full of people and trying to get someone out of the water.
Click to expand...

wow... we can save you about $900 of that $1K budget then.

By changing your prop to a lower pitch, you can easily reduce your holeshot to plane time by 30% or more
A 5.0 can never make low end torque. It doesn't have the stroke or displacement.

But a prop swap is HUGE for pulling power... it is like starting in a lower gear. Lets your engine rev to a range where it makes serious power. Vent holes in the prop (e.g. Raker) allow some slip too for added revving and power.

Only hitch is, you have to swap props a lot for water sports, but a floating propwrench and a few minutes hanging over the swim platform and voila.
 
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Re: 300 hp reasonable from 5.0L GL

Ya I know about changing the prop. Let me restate what I meant I don't want to reduce the speed the boat is currently able to do nor do I want to speed it up.
 
Re: 300 hp reasonable from 5.0L GL

i think rick answered your question on this whole matter. you dont want a lower speed but you dont want it to go faster. Then what you need is a duo prop set-up my friend. cut and dry answer. way better and cheaper than swapping in a differant engine. plus it comes out of the water like an inboard.good luck
 
Re: 300 hp reasonable from 5.0L GL

nsxxtreme said:
.........................
I like the 383 stroker idea as that really isn't any harder to do then a 350 and you can buy premade kits all ready to go. Although I haven't really looked at where in the rpm range they make their power. What would be a good set of heads to look for? I'm thinking a 10:1 compression ratio the boat already requires premium anyway. My simple plan was to use flat top pistons, vortec heads, 4 barrel carb and a set of roller rockers. That book looks pretty good I might just see if I can buy it.
Click to expand...
You'd best do some homework first, IMO. The F/Ts with the 64cc chambers of the Vortec cylinder heads, used in a 383 will put your C/R way too high for marine use.
Read post # 6 again if custom building an engine.

ruckus3313 said:
i think rick answered your question on this whole matter. you dont want a lower speed but you dont want it to go faster. Then what you need is a duo prop set-up my friend. cut and dry answer. way better and cheaper than swapping in a differant engine. plus it comes out of the water like an inboard.good luck
Click to expand...
Yep, that was my first suggestion in post # 6. However, this is NOT an inexpensive addition!
When I installed these on my twin engine boat, it was as though I suddenly had 160 more horse power, or so. Amazing!
 
Re: 300 hp reasonable from 5.0L GL

RicardoMarine said:
You'd best do some homework first, IMO. The F/Ts with the 64cc chambers of the Vortec cylinder heads, used in a 383 will put your C/R way too high for marine use.
Read post # 6 again if custom building an engine.
Click to expand...
Flat tops with 64cc heads will put the compression slightly higher then 10:1 which is essentially the same CR I have now in the stock 305 which requires premium only.
 
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