horsepower

[sillyhilley]

I have Chevy 350 with 270 hp. I'm look at new engine with 325 hp. How much fast if any will she do. I run from 50-55 mph now..just wondering hp=mph?

 

[JB]

Re: horsepower

Be a good idea to put it in some kind of boat, SH, then tell us about the boat.

 

[45Auto]

Re: horsepower

A generic rule of thumb in the 50-70 MPH range is that it takes 10HP to go 1 MPH faster. Since you're adding about 50HP, your boat will be about 5 MPH faster.

 

[Tahorover]

Re: horsepower

A generic rule of thumb in the 50-70 MPH range is that it takes 10HP to go 1 MPH faster. Since you're adding about 50HP, your boat will be about 5 MPH faster.

Slip % starts to climb after 60. I added 220 hp and only got a 17mph increase due to slip.

 

[Hogger]

Re: horsepower

What kind of boat?
That much increase in HP, you will probably have to re-gear your outdrive, and re-prop.

 

[45Auto]

Re: horsepower

Slip % starts to climb after 60. I added 220 hp and only got a 17mph increase due to slip.

That's why I said it was a "generic" rule and he would go "about" 5 MPH faster.

If he gets the same gains you did (17 MPH for 220HP), then he will gain 1 MPH per 13HP. A 55HP increase would gain him 4.2 MPH. To me, 4.2 MPH is the same as "about" 5 MPH.

That much increase in HP, you will probably have to re-gear your outdrive, and re-prop.

With a 270HP 350 as his current motor, he should already have a 1.5:1 outdrive. Nowhere to go from that with gearing unless he tries to track down one of the old 1.32:1 gearsets.

Only difference he should have to make is an inch or two on prop pitch. He should be able to go about 10% faster (5 MPH gain at 50 MPH is 10%) with his new horsepower. To do that while running at the same RPM as his old engine means he's going to have to go up 10% on prop pitch. If he's currrently running a 21" pitch, that means he would have to go to a 23' pitch.

 

[sillyhilley]

Re: horsepower

Be a good idea to put it in some kind of boat, SH, then tell us about the boat.

Its a 201 liberator. It had a 350 king cobra, however the engine piston broke on Sunday. Correct me if I'm wrong but i think the king cobra out drive can handle and bolts to a 350, 383 ,400 or a 454. all I need to do is change the prop.

 

[Mischief Managed]

Re: horsepower

Drag is a function of the square of speed. If you go 55 on 270 HP, you'll go 60.3 MPH with 325 HP, assuming your new prop is just as efficient as your old prop.

 

[Bondo]

Re: horsepower

Its a 201 liberator. It had a 350 king cobra, however the engine piston broke on Sunday. Correct me if I'm wrong but i think the king cobra out drive can handle and bolts to a 350, 383 ,400 or a 454. all I need to do is change the prop.

Ayuh,... There were several King cobras

 

[Bamaman1]

Re: horsepower

And, you've not been clear on what kind of new motor you're going to be running.

If it's a marine engine running the same max. rpm's, you'll have to change the pitch to get more speed. Otherwise, you'll just accelerate very fast.

Like it was said above, a change in propeller pitch is warranted on that much additional horsepower. I agree that you have 60 mph in your future.

Be sure to let everyone know what the outcome is after the boat's in the water.

(Sometimes people use auto engines when they re-engine I/O's--and they run substantially higher rpms--and top end speed. Car engines will accelerate slower from a start, and they may not last as long due to the extreme torque stress a marine engine dishes out a steady medium rpms.)

 

[sillyhilley]

Re: horsepower

This gives me a good idea what to except thanks.