Swapping 4.3L to V8

[brainsboy]

I have a Chris Craft 202 Stinger. It has a mercruiser 4.3L 205hp V6 and runs 55mph at 5000rpms. I want to switch to a V8. I dont want to do anything crazy with power but would like to get another 10mph out of it. How much does it take to switch to a V8. Do I need to change the outdrive gears? prop? I plan on buying a mercruiser V8 complete so everything is there as far as bolting it in manifolds, coupler, pulleys, ect.
Is this something reasonable to do, or should I just leave the thing alone? Even if I can get 70mph, It would be nice to have the V8 sound to match the killer looks of this boat.


Thanks, Ben

[Jango]

You probably would only need to change the Prop - increase pitch to limit rpm's. Changes should be minimal. Your V6 is basically a V8 with the two middle cylinders removed.

Although, I doubt a "Stock" sm block will do what you want, unless your boat is a lot lighter than what I think it is. Calculations show that In order to Increase the speed of a 3000# boat (total weight w. pass. etc ) from 55 to 65 mph, requires abt 90 HP Additional at the Prop. This translates to probably 125 HP more Motor. Don't forget, the V8 will also weigh more.

[gonzo]

The correct gear ration for a V8 is 1.50:1. Just changing propellers is probably not enough. The diameter of propellers is limited by the cavitation plate. It won't have enough blade area to absorb the HP.

[Jango]

In the case of a V6 vrs. a V8, increased HP is absorbed by prop RPM. That's why V6,s normally have a 1.84 gear and V8,s a 1.50.

Dia is not an issue. If you can find a Prop with 4" of additional pitch from what you now have, you should be OK, otherwise a gear change may be required.

[brainsboy]

Someone told me if I dont change the gears out they will strip out because of the additional power. Is this true or not? What happens if I dont change the gears? Just the possibility of over reving right? Why not just use a MSD rev limiter?

[Jango]

Unless your gears have a problem, I doubt very much addtional power from a sm blk V8 will cause a problem. I believe,Both the V6 and the sm blk V8 use the same Alpha gear box.

An MSD rev limiter will work but will also limit your performance,both top speed and acceleration. All you will be gaining is the V8 sound.

As I indicated earlier, ADDING pitch will absorb the power and maintain desireable max RPM, with the benefits of added speed and acceleration.

[gonzo]

Propeller pitch and blade area are two completely different things. Blade area is what is needed to absorb HP. If pitch was all that was needed, you could use a little outboard prop with a lot of pitch. Pitch has to be calculated for the expected boat speed.

[Jango]

Gonzo, If your reasoning where correct, ALL you would need to do to increase speed is add pitch, NO increase in HP - the sky's the limit.

Minimum prop Dia, which translates to Blade Area is determined by load or Boat displacement. Since going to a V8 will not increase weight appreciably (perhaps 200lb), blade area (Dia)need NOT change. Adding 4" pitch to the current dia prop, with 100HP+ additional will increase speed 8-10 MPH while maintaining current RPM levels.

My own 20' inbord (straight inline) displaces 2350 lb with driver only and has a 12" dia 3 bladed prop. A 12" dia would be used if the boat had 100 HP or 400HP which it happens to have. My 12x15 @ 5700 RPM produces 77 MPH. With less than 6% slip - the prop is hardly too small.

[gonzo]

I said the pitch has to be calculated for the expected speed, not that a change of pitch will keep on increasing speed. Prop diameter alone does not determine blade area. It is a function of the amount of blades and blade shape and diameter. With a small diameter prop, as in an outdrive, it is more efficient to decrease the gear ratio to increase propeller RPMs. The prop tips don't reach cavitation speed.