1982 Chrysler 75HP port timing and venturi matching??

[$uperJoe]

looking for some photos and info on the stated title. looking to get my mill to the best power it has available, without drastic changes in hardware. porting has always been best for me in two strokes so i figured why not here as well. just looking for some pointers before i make an anchor with inexperience on outboards.

 

[Frank Acampora]

Re: 1982 Chrysler 75HP port timing and venturi matching??

On these three cylinder engines you can port both exhaust and bypass. If you have experience with motorcycle or motocross two cycle engines, you need not do port mapping because these are straight crossflow engines. Unless you want to really get into racing them, there is no reason to change port timing. That would simply reduce low end torque and only increase horsepower at higher than factory recommended RPM.

Your engine has circular milled exhaust ports with the bottom of the port above the piston crown at BDC. SO, in addition to squaring the ports and streamlining the inter-port webs, you can extend the ports downward and make them rectangular until they match the piston crown at BDC.

Bypass ports are also circular but the lower half is masked by the piston at BDC. You can simply square these and streamline the webs. You can also remove some aluminum from the lower cylinder casting to streamline entry into the ports.

All ports should be trumpet shaped outside the cylinder to also increase gas flow.

Your 75 has three WB carbs with a 1 3/16 venture. after porting the engine can utilize three WB carbs off a Chrysler or Force 85-90 with a 1 5/16 venturi.

Open up the reed valve stop plate clearance to allow a bit more gas flow through them OR find a set of later squared reeds with the short, drilled reed stop plates. OR Buy a set of aftermarket composite reeds. Boyeson is one brand but there are others.

Run the carbs slightly rich to prevent melting pistons. These engines are very intolerant of lean runs.

The lower unit should be changed to a 2 to 1 ratio off a Force 85 or 90 and prop pitch increased.

These modifications will allow the engine to deliver an honest 90 horsepower at the prop.

If you do not wish to go to all the work, Later Force engines were manufactured with the center exhaust web drilled to increase port area and you can emulate that.

Again, as I mentioned in my PM answer, this may be way too much weight for your 10 foot tunnel. A 1982 75,85 will weigh about 275 pounds with power tilt/trim.

Post a couple of photos of the hull

 

[Frank Acampora]

Re: 1982 Chrysler 75HP port timing and venturi matching??



Do not port too aggressively. This modification reduced horsepower output at normal speeds. I have not yet tried it at RPM over 6000.

Last photo is later model Force reeds that I mentioned above. First photo is a better shot of factory porting. It is a 5 cylinder 150.

 

[$uperJoe]

Re: 1982 Chrysler 75HP port timing and venturi matching??

Thanks Frank. i went out and counted the gear reduction. its already a 2:1. i have been told that there are limited props available for the chrysler. does the force props use a different style prop? could i use any prop that fits the house and change the hub to match the chrysler spline? kinda unclear about this

 

[Frank Acampora]

Re: 1982 Chrysler 75HP port timing and venturi matching??



Force props will fit as will many others. You can use small gearbox Mercury props too. (like from the small gearbox 90) In fact, almost any 15 spline prop with the small 4 inch diameter hub will fit if you are willing to modify them slightly. Some Vortex props have internal webs that interfere with the anode around the prop shaft. You either mill the webs to clear or remove the anode. You can also buy a later model anode which does not have the high "walls" and the prop will fit.

Yes, you can press out the hub from a prop for another brand engine and press in a Force hub. That is what I did with the Ballistic prop

Some solid hub Mercury racing props will fit.

Photos show some props that swap on: Ballistic, Bronze two blade, aluminum etc. First photo is a Chrysler stainless cleaver.

The blue prop was modified. It was a Mercury test prop and the internal flange was too big in diameter. I cut off the flange, reduced it and epoxy glued it inside the prop flange. It now fits the Chrysler/Force engines.

 

[Frank Acampora]

Re: 1982 Chrysler 75HP port timing and venturi matching??



You must use a little imagination when mounting the Ballistic prop: It is high rake and will hit the exhaust snout unless it is special mounted with custom hardware.

There is no performance difference using an open or closed hub prop or using the tailcone or not.

 

[Frank Acampora]

Re: 1982 Chrysler 75HP port timing and venturi matching??



The forward gear cup bearing happens to be the correct size and taper to press in a hub. use an old spalled one. Lubricate the rubber with soapy water and let sit a couple of days after pressing in, before using the prop.

Stainless props use a higher durometer rubber than aluminum. You can sometimes re-use a hub but if you put an aluminum hub into a stainless prop, the prop will twist (not necessarily slip) excessively on the hub.