Racing lower unit

[Mac_55]

Boat has been put away for the season, need something to work on.

i like the look of the racing lower units but will never get one, also like neutral and reverse haha so question is .....

has anyone ever taken a two piece 70's 3 or 4 cylinder lower unit, cut and welded the exhaust port to look and sound like a racing unit???? power gain, loss? , not enough back pressure? ,


thanks

 

[Frank Acampora]

Re: Racing lower unit



No, but I have ported a one piece lower unit--a lot more noise and very little power gain. Understand too, that porting the lower unit or venting it like the racing lower is considered a cutout and illegal (except for sanctioned racing) in most states (here in the U.S.).

The two piece lower unit can be ported the same way and, of course, no welding is required. It is however an exersize in futility. The two piece lower unit is about 3 MPH slower than the one piece AND the two piece will blow out at around 60--if you can get that high. By blow out, I mean that the shock wave from the nose cone becomes bigger in diameter than the prop so you lose all thrust.

 

[Frank Acampora]

Re: Racing lower unit

View attachment 174729View attachment 174730

No, but I have ported a one piece lower unit--a lot more noise and very little power gain. Understand too, that porting the lower unit or venting it like the racing lower is considered a cutout and illegal (except for sanctioned racing) in most states.

The two piece lower unit can be ported the same way and, of course, no welding is required. It is however an exersize in futility. The two piece lower unit is about 3 MPH slower than the one piece AND the two piece will blow out at around 60--if you can get that high. By blow out, I mean that the shock wave from the nose cone becomes bigger in diameter than the prop so you lose all thrust.

The way to get around this is to install a low water pick-up and jack up the engine so the gearcase is surface riding--but then you need a special surfacing prop and I don't think they are easy to find for the 18 splined shaft.

You would be much better off finding a later model junker and swap your engine block onto the one piece midleg and lower unit. (the one piece will not fit on your midleg) I did this in a post a long time ago with a 3 cylinder 90 HP and proved that the one piece lower unit is faster.

Lastly, with the one piece lower unit props are easier to find, and there is more selection. You also have your choice of single exhaust or dual exhaust.