Thread: Recessing pump shoe
07-10-2010, 06:35 PM #1
Recessing pump shoe
I've heard mention of individuals milling the pump shoe mounting surfaces to close the pump tunnel slightly thus increasing top speed. I've also heard cavitation out of the hole can become an issue with this method. I've done a search on this specific topic and can't find anything solid except to make sure the shoe is all the way up and back, and should be recessed the thickness of a nickle. Blah, blah, blah... What if more were machined and the shoe could be tucked up further? I've listened to others and have read between the lines and have come to the conclusion that the people who are experimenting with this usually take between .050 and .070 off the surface. Does anyone have any input on this topic? What kind of speed increase are we talking about? Advantages? Disadvantages? I've seen a few videos (from one tuner in particular) who is noted for his pump shoe setup. In every video he is very gentle with the throttle when taking off but he has some of the fastest machines out there.
07-10-2010, 07:10 PM #2
- Join Date
- Apr 2006
- South MS
You probably won't any mph gain numbers since this mod you can't do back to back. In my opinion- it would be slightly faster since we know
that if the shoe is hanging any you will lose speed. Unless your a hardcore drag racer from a dead stop just I doubt you will notice a holeshot
difference with just .050" taken off.
You also have to remember what works or makes gains in speed can be totally different depending on what speed your ski
is doing now...what works on a 75mph is way different than at 85mph. So at 75mph it might not make .5mph but maybe more at 85mph....again just my opinion.
Far as the videos.....when any of us are making top speed vids we are using 800 grates so no use in doing a drag race start plus I know
on my ski I'm trimming it up and getting into position for top speed.
So this mod is a perform at your own risk. Once the shoe is in there its very hard to redo. I've personally just "cleaned" up the surface
of the shoe on my ski so there is no chance of it hanging down due to the thickness of the sealant don't really know how much is taken off but the grate also attaches to the shoe so everything moves up.
Seems to run great 78mph+ on 1200cc single so far....
07-11-2010, 12:13 PM #3
- Join Date
- Jan 2006
- Texas City, Texas
You should know that when the shoe is recessed that not only is the intake reduced, but other things also change.
For instance, the leading edge on the front wing also changes position because the WHOLE intake changes position. Normally, the front wing gives the incoming water about a 50/50 split between loading on the top and bottom.
Also, because it is so much more recessed, a vacume is created on the outside part of the shoe that is exposed to the water (where you bolt it to the hull). One other thing. The water that goes into the intake is deflected by the shoe up toward the impeller. The force that is exerted by this jet stream onto the shoe actually is driving the stern part of the hull down, helping to keep it planted.
Not saying that it will not work, because obviously it does. I've used a lazer to see these things, and the changes are subtle but measurable. It is rare when you make one small change and it is the only thing that happens.
The real trick to making real changes is not to redirect the incoming water flow (that's fine tuning) , but to REDUCE the water flow in a dramatic way..... to the tune of about 30%. This should help to resole "the buck" issue, but create a cavitation issue.
When you "reduce the intake by 70 thousandths", what you really mean is that a stream of water 70 thousandths by 4.75 inches (width of the intake) is reduced.
I've been told by a hydrodynamics expert (and by using a lazer have gotten close to the calculated values) that the incoming stream of water is about 2 inches by 4.75 inches. Reducing the flow by .070 inches by 4.75 inches is a small start.
07-11-2010, 09:41 PM #4
yUp, he said ^.......becareful to not suck up a shoe way up in the hull. you can lose speed. ya want the rear even with the hull but the front up. It affects ride plate angle too. Having the rear sucked up in the hull will also reduce the water inlet shape where the transom is. Its a fine line. This is how it was set up on my 76.7 mph unported 1200. The same setup is on my conversion (same hull)
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