ported prop over weekend

[walleyehed]

Re: ported prop over weekend

If you have 5/32" holes and have noticed nothing, I think either your placement is wrong or something.<br />Lower the pitch, smaller the hole....<br />What pitch prop do you have and how many blades?

 

[imported_JD__]

Re: ported prop over weekend

I have a Johnson/Suzuki 4-stroke on a 18' bass type boat. I'm running a 14 X 24 Stiletto SS prop which was made specifically for the engine. It came vented. I enlarged the holes one drill size and it helped the hole shot. I went one size larger and it "sat & spun". I took the prop to a machinist friend who prepped it for the Mercury type PVS plugs. It was either do this, or weld the holes shut and start over. I bought a few each of the smaller plastic plugs and proceeded to drill those out and re-test. I now have it fine tuned for hole shot. It ventilates enough to let the engine rev up some getting into the power band then bites and takes off.<br />It amazed me how 3/8" was about right and then only enlarging the holes to 7/16" caused the prop to blow out completely.<br /><br />When you're ready to try the next drill size, only drill one hole at a time, unless you have a welder and a machinist handy.

 

[tee-boy]

Re: ported prop over weekend

JD,<br />What HP is your 4 stroke? My prop is 16 pitch (unsure of diameter). Pretty certain I am drilling in right place (5/8 from leading edge of blade on aft or left side on a clockwise rotating prop; directly through the hub while ensuring not to drill through the webs). My father has older model 150 black max with huge holes (you can certainly hear and feel the difference with his prop vented-the boat literally jumps onto plane). From what I am getting here, the lighter load on the prop from the exhaust will not cause as high an RPM jump on the 4stroker due to the higher torque at lower RPM. The key here is how responisve the 4stroke is to load compared to the 2 stroke. However, I will continue to try just to satisfy myself.

 

[imported_JD__]

Re: ported prop over weekend

Mine is a 140 HP. A 2-stroke will rev up faster than a 4, plain and simple. I went from a 115 Merc 2- stroke that would push this boat around 45-46 MPH. I can now go about 50-51 MPH with the 140 which is around 100 pounds heavier. Performance wasn't my primary goal in swapping engines. I like the fast easy starting, hot or cold, quietness and not having to concern myself with adding oil and smelling the smoke. I can talk with my passenger at full speed without screaming. The wind rushing by is louder than my engine.

 

[Silvertip]

Re: ported prop over weekend

A little theory is order here gents. Four stroke engines do not fire each cylinder once per rev. Each cylinder fires once every other revolution. Therefore a four cylinder four-stroke has two exhaust pulses per rev. A two stroke fires each cylinder every revolution so you have one exhaust pulse for every cylinder each revolution. Hence twice the exhaust flow in a two stroke vs four with same number of cylinders.

 

[jim dozier]

Re: ported prop over weekend

4-stroke has 2 exhaust pulses per rev??? Maybe with multiple cylinders. Upinsmoke you need to rethink your explanation. The exhaust valve on a 4-stroke cylinder only opens every other rev. I don't know which engine type is pushing more volume of exhaust per rev or how it affects this prop issue because there are lots of variables here. But a 2-stroke farts exhaust once each rev and a 4-stroke every other rev.

 

[walleyehed]

Re: ported prop over weekend

Upinsmoke is perfectly correct...I see no errors in his post.<br />jimd, re-read his post, I think you'll see the part you missed about # of cylinders...

 

[Todd Humphreys]

Re: ported prop over weekend

You guys inspired me to drill vent holes in my prop. I have a 1982 75hp Johnny with OMC 13x19 prop and by the proceedure from the OMC service bulletin I used 3/16" holes. I haven't had a chance to test since the local river has been at flood stage, but I will let you know the results; very anxious and nervous.