Swapping 4.3 to 5.7 Engine....How much speed increase?

[jserr68594]

I have a 1994 20Ft Regal with a 180 HP 4.3LX. I top out at about 48 MPH now. If I were to swap to a '87 5.7 260 engine with a 1.47 gear, how much of an increased top end would I see (Obviously properly propped)? Would I gain a lot in my holeshot? Would fuel consumption be similar?

Thanks in advance!

 

[Silvertip]

Re: Swapping 4.3 to 5.7 Engine....How much speed increase?

If you keep the same gearing and the same prop -- no speed increase at the same RPM. A more powerful engine allows a lower gear ratio (higher gearing) which means prop speed increases at the same rpm as before and hence boat speed increases.

 

[ewenm]

Re: Swapping 4.3 to 5.7 Engine....How much speed increase?

i think thats what the OP was saying moving to a 1.47 gear set, i think his 4.3 would wave probably had a 1.97.

personally i think you should go to an on line prop calculator and dial in your numbers, factoring in a slip percent of 10 - 15%

and see what you get

 

[Tail_Gunner]

Re: Swapping 4.3 to 5.7 Engine....How much speed increase?

Maybe a 5-7 mph gain and a whole lot of work

 

[TilliamWe]

Re: Swapping 4.3 to 5.7 Engine....How much speed increase?

1994...180 HP 4.3LX... '87 5.7 260 engine... Would fuel consumption be similar?

You realize that the 94 4.3 is 180hp at the propshaft, and the 87 260 is rated at the flywheel, right? So the HP gain is not 80, it's more like 50.
The 5.7 will use LOTS more fuel.

 

[HT32BSX115]

Re: Swapping 4.3 to 5.7 Engine....How much speed increase?

The 5.7 will use LOTS more fuel.

Only if he pushes the boat harder... Going approx the same speed, the boat will burn approx the same fuel....

 

[TilliamWe]

Re: Swapping 4.3 to 5.7 Engine....How much speed increase?

Only if he pushes the boat harder... Going approx the same speed, the boat will burn approx the same fuel....

HT, he's going to push it harder. That's the whole reason for the swap.

And he won't cruise at the same speed, he'll cruise at the same rpms as with the 4.3, only going faster. So he'll burn a lot more fuel.

 

[HT32BSX115]

Re: Swapping 4.3 to 5.7 Engine....How much speed increase?

HT, he's going to push it harder. That's the whole reason for the swap.

And he won't cruise at the same speed, he'll cruise at the same rpms as with the 4.3, only going faster. So he'll burn a lot more fuel.

One must maintain control of ones urges!!