Chrysler 3 cyl porting

[bloodygazza]

Hi all,

Searching posts trying to understand where the horsepower differences are derived for the 3 cyl motors, assuming bore and stroke is the same from 70hp to 90hp.

I am trying to learn if it is worthwhile porting whilst rebuilding to improve the motor(s). I will be dismantling to rebuild my 70hp 707HE motor soon and am looking to improve top-end performance. I believe I will also benefit in midrange economy on the plane as the motor while not be working as hard for a given performance at partial throttle.

To begin improving my understanding, is the different horsepower ratings of the 70, 75, 85 & 90hp motors achieved through porting alone or are there other internal differences (compression ratio) and external differences (carburation, ignition timing) the make the extra rated HP.

Thanks in advance for your kind assistance.

Gary.

(PS: apart from the 707HE fitted to my boat now that I will rebuilding, I recently acquired an 856HA for parts which had a failed big-end/good gearcase. If I can find a reasonably priced s/h crank, I will rebuild it too as apart from that it is in good condition.
The weekend just gone saw me cheaply add an good running/good gearcase 707HF and a good running/very average gearcase 757HC to the 'fleet'. From all this I expect to have a good motor for summer - southern hemisphere - and some toys to play with. I will start another thread to identify the best performing gearcase to use.)

 

[steelespike]

Re: Chrysler 3 cyl porting

Go to second page "how to improve a 2 stroke".

 

[Frank Acampora]

Re: Chrysler 3 cyl porting

Read the post quoted by Steelspike, and YES, it is almost always worth doing port work on 3 cylinder Chrysler engines if you already have the engine apart.

There are SIGNIFICANT differences in the exhaust port size between the 70-75 and the 85-90 engines with the 85-90 being much larger. Doing the port work on your 70 then changing from the 1 3/16 venturi to 1 5/16 venturi carbs (like a set of WB7, WB13, or WB24 --Double check my numbers to be certain they are 1 5/16 diameter venturi before you buy) will greatly improve the performance. Carb change alone will do very little to increase power.

If you can find a 2 to 1 lower unit or gears for your 70, that will also improve speed by a couple of MPH by itself.

That 15 foot boat in the avatar is doing 36 MPH with three people in it. It is pushed by an 85 block that was ported and has a 2 to 1 lower unit. With just the driver, it tops out at somewhere around 42-45 MPH. Not too shabby for an old school small bore engine some 30 years old.

If you intend to disassemble the rods from the pistons and intend to re-use the wrist pin bearings, you must use a special tool or a spacer to prevent damage to the needles or wrist pin keepers. It is imperative to use a .301 inch spacer when pressing in the wrist pin to prevent the needles from being squeezed between the keepers. That would prevent them from rolling. For this reason I recommend Wiseco replacement pistons as they come complete with a circlip retained wrist pin and piston rings. They are much easier to install properly.