More HP, more pitch needed?

[wncrjb]

Hi All,
My latest project is a 1994 chaparral 180 SS.
mercruiser 4.3, alpha 1 GEN 2 drive


The original motor was missing when I bought the boat, but was rated at 205 HP with a 1.84 ratio drive and a 23P prop.

I'm building a motor to stock specs except:
bore is 30/1000 over and I am adding vortec heads with a 4BBL carb.

I know the increase may only yield maybe 25-30 HP, but is this enough to where I might have to increase pitch a little bit or maybe lower my ratio to a 1.62 ?
I' asking this because my HP will be in the neighborhood of the 305 V8 engines, but I'm not sure of the torque differences between the V6 and V8 yielding the same (or close ) HP values.

I need a new prop anyhow, so I better ask before buying one.

Thanks,
wncrjb

 

[John_S]

Re: More HP, more pitch needed?

If you were proped on the high end of rec wot rpm range, with the boat before the mods, then you will have to increase pitch or change gear ratio to use that additional hp for increased wot speed. Being at 23" pitch I would recommend changing to 1.62 ratio which is what comes on the equivelent 4.3L today. If your current prop is runable, you may want to wait until you have some test data before changing.

 

[Bondo]

Re: More HP, more pitch needed?

Ayuh,.......

I think I'd Wait til you've done some Sea-trials,+ See where you need to go......

Over the last year or so,.... Merc. has been offering standard props up to 25",....
Instead of the 23" that was Max before....

You may have to step up in Ratio,.....
But,....
Stepping up in Prop Pitch is Much Cheaper to pull off......

 

[wncrjb]

Re: More HP, more pitch needed?

The prop that came with the boat is ground down to to the hub. I will need a prop to start with and that is why the post.... It did have a 23P on it but I have no idea if the motor could hit max RPM with it.
I am thinking of getting another 23P and if it's not 100% right, at least I have a spare.

wncrjb

 

[Lakester]

Re: More HP, more pitch needed?

I'm building a motor to stock specs except:
bore is 30/1000 over and I am adding vortec heads with a 4BBL carb.

I know the increase may only yield maybe 25-30 HP, but is this enough to where I might have to increase pitch a little bit or maybe lower my ratio to a 1.62 ?
I' asking this because my HP will be in the neighborhood of the 305 V8 engines,

hello,

how are the vortech head dif from the stock? is cam same? will u be upgrading your ignition system?

regards
lakester

 

[wncrjb]

Re: More HP, more pitch needed?

hello,

how are the vortech head dif from the stock? is cam same? will u be upgrading your ignition system?

regards
lakester

The vortec heads came out around 1996 and have a different combustion chamber for better flow. These heads also require a different intake manifold as the intake only uses 8 bolts instead of 12 on the older models. The figures show these heads can produce an added 25-30 HP over older heads.
I'm not sure about the cam difference, since the engine I got is a 1997 model, I am going to get a replacement cam for a 1996 marine 4.3.
My boat uses the TD IV ignition and I will be using that, since from what I hear, that is pretty good ignition system.

wncrjb