Mercruiser engine swap, HP questions

[Lukelona]

Hey folks, after just buying my boat about a week ago, and finding it to have a spun rod bearing, I found a super nice motor and went and bought it on the spot. Came from a externally damaged 97 bayliner with less than 100 hrs on it.
Now my original motor had roller rockers and EFI setup (TBI) and the "new" motor has flat tappet and a 2 barrel carb.
What kind of HP should the "new" motor have had with it's stock setup?
How much will I gain from EFI and electronic ignition swap VS losing my roller rockers?

Or to simplify it, roughly how many HP should I expect with a flat tappet 5.7 with TBI fuel injection?

 

[Scott Danforth]

first. what year was your original motor. what is the casting number of the block and heads. how many bolts holding the intake manifold down.
post the casting numbers of your new motor along with the number of intake manifold bolts

the '97 bayliner should have had roller cam. however if its a left over '95 motor its a non-vortec

the cams will be different between the two with one being EFI and one being carb. the lob centerlines are different.

so if you have a vortec motor now and the new to you motor is a non-vortec, your intake manifold wont fit and you will be loosing about 40hp just from the heads.

so before you get excited and start swapping things, what are you starting with?

 

[Lukelona]

first. what year was your original motor. what is the casting number of the block and heads. how many bolts holding the intake manifold down.
post the casting numbers of your new motor along with the number of intake manifold bolts

the '97 bayliner should have had roller cam. however if its a left over '95 motor its a non-vortec

the cams will be different between the two with one being EFI and one being carb. the lob centerlines are different.

so if you have a vortec motor now and the new to you motor is a non-vortec, your intake manifold wont fit and you will be loosing about 40hp just from the heads.

so before you get excited and start swapping things, what are you starting with?

Ok for starters, my original motor was a 95 TBI 5.7L.
Replacement motor is a 97 5.7 carb.
And it was actually backwards if what you'd think, the 95 is a roller motor and the "new" 97 is flat tappet
I've already swapped 98% of the parts over and intake fits etc. It is a 12 bolt intake

 

[Scott Danforth]

a 12 bolt intake indicates that its a pre 1996 motor period. in 1996 (actually septemter 1995) all GM motors switched to the vortec heads). your '97 is a left over '95

motors from 1988 to 1995 in the automotive world were roller motors, in the boat and truck world, they were hit or miss. the block will accept either cam. is your new to you motor machined for the lifter spider hardware. if so, swap cams.

your carbed motor has a different cam than the efi motor (lobe separation is different) carb would be 112 degree, EFI would be 114 degree.

 

[Lukelona]

Yes it is machined for spider, I was trying to avoid swapping cams bc of water intrusion to old motor, spun bearing and possible metal shavings.
How big of a difference are we talking between the 3 builds? Roughly of course

 

[Scott Danforth]

I would pull the cam and inspect it.

the EFI programming relies on the larger lobe center. you can run a carb on the EFI cam, however EFI doesnt like to run on a carb cam.

 

[Lukelona]

Dang. I already have the Motor installed. Wiring it up as we speak. Is this gna be a huge issue? Like is it gna run like crap? Or are we talking like a small HP loss?

 

[Lukelona]

Well, motor is in and I've brought it out 4 times now, it runs great, smooth even power throughout.
Running a 19p 4blade Aluminum prop I'm getting about 37mph @ 3900rpm.
Does this sound about right? I thought these boats were known to clear 50mph?
Does the carb cam on efi motor make this difference?

 

[cecho]

Well, motor is in and I've brought it out 4 times now, it runs great, smooth even power throughout.
Running a 19p 4blade Aluminum prop I'm getting about 37mph @ 3900rpm.
Does this sound about right? I thought these boats were known to clear 50mph?
Does the carb cam on efi motor make this difference?

What size and weight is your boat? Are you heavily loaded? My 3000 lbs 20 foot cuddy cabin cruiser with a tired, 39 year old 198 HP 2 bbl 5.0 easily does 42 MPH when trimmed right with three people and full fuel at 4500 RPM on a torque shift prop at WOT... sometimes I can even sneak 45 MPH out of it. 3900 rpm seems low to me. I think you're over over propped for your current HP. The speed may or may not be right depending on your boat, my guess is that it's low as well though.

What's your hole shot like?