Swapped from a 5.7 down to a 305. Now boat won't plane out.

[yukonrider]

Hey guys, I'm shooting in the dark, but I'll try an give you as much information as I can

My buddy had a cracked block on his 5.7, so he swapped in a 305 (he claims im not sure how to verify)

Got it running, timed right (except for an occasional random back fire at low RPM)

But now the boat takes forever plane out. It had a 21P prop on it, and I swapped down to a 19P, and that helped, it will eventually struggle out of the hole if everyone is sitting in the bow.

The difference from the 21 to the 19 makes me feel as though a 17 is the next logical step, but am I missing something here? I feel like this boat should have more power. If it makes a difference it sounds like the engine is lugging a lot when struggling up to speed but then seems to rev out to what sounds like about the right RPM when on plane.

The boat is an ~89 21ft Stingray SVB205 bow rider. Max speed with the 19P is about 29MPH The tach doesn't work so I can't verify RPM. The boat bottom is fine.

 

[Scott Danforth]

welcome aboard.

the 5.0 usually comes with a 1.62:1 drive and the 5.7 comes with a 1.47:1 drive if you did not switch out the drive, you may need to drop down to a 17p or even a 15p

however a boat heavy with wet foam could also be an issue. have you and/or your buddy weighed the boat recently?

 

[yukonrider]

welcome aboard.

the 5.0 usually comes with a 1.62:1 drive and the 5.7 comes with a 1.47:1 drive if you did not switch out the drive, you may need to drop down to a 17p or even a 15p

however a boat heavy with wet foam could also be an issue. have you and/or your buddy weighed the boat recently?

Interestig, I could probably get my hands on a 5.0 Out drive. Would this be the better route besides the prop? IE maintaining a reasonable top speed, and hole shot?

No, I haven't weighed the boat, but it was stored properly, never filled with water and is in good shape. How could I check for wet foam? I do know the boat tows heavy, but I don't know the original weight of the boat off hand to compare with.

 

[Scott Danforth]

I would swap props vs outdrives

http://boatspecs.iboats.com/Stingray...89/bp/66b89106

your boat weight empty is 2801# add weight of fuel and trailer (about 1100#) and you have a wet weight on the trailer of about 4000#. hit your local scale and do a comparison. if it weights lots more, then the foam is probably wet.

if it was my boat, I would have built a stroker motor vs downsizing to a 305.

 

[dan02gt]

You sure that motor is healthy? Timed right and jetted properly? Right cam?

I can see going down a 1-2" of pitch to get the hole shot and WOT RPMs where they need to be but you shouldn't have to go much further than that. What's the difference in hp between the 2 engines 35hp or so? Sounds like that 305 is not running like it should.

 

[QC]

A true 20+ footer may not have any foam. Where did the 305 come from? If it was a car, then I would be concerned about a lot of things, cam is definitely one of them. What all did you use from the 5.7? Just stuff to ponder.

 

[yukonrider]

I didn't do the swap, but supposedly everything g is straight from the 5.7 except the block. Would this be bad?

 

[Scott Danforth]

the combustion chambers on the 5.7 head are a bit big for the 5.0, so are the valves.

FWIW, my 24' has lots of foam in it.

 

[yukonrider]

the combustion chambers on the 5.7 head are a bit big for the 5.0, so are the valves.

FWIW, my 24' has lots of foam in it.

First, thank you so much for the help you've given so far.

Second, would the heads lead to loss of power? I think I actually could get my hands on some 5.0 heads if this was my issue, but I would need new gaskets eh? Hm..

 

[MikDee]

the combustion chambers on the 5.7 head are a bit big for the 5.0, so are the valves.

FWIW, my 24' has lots of foam in it.

Question I always had Scott, wouldn't the bigger valves of a 350 hit the smaller bore 305 block, when fully open?

 

[QC]

Mik, please read your PMs...

 

[Outsider]

The tach doesn't work so I can't verify RPM.

The first thing you need to do is fix the tach! Then run it and see what your RPM is at WOT, vice maximum range for a comparable 5.0. You're just changing expensive parts if you don't do that first. Oh, and healthy tuned engines don't backfire at low RPM