Thread: GP1200 Pump tunnel mods.
04-04-2010, 12:25 AM #1
GP1200 Pump tunnel mods.
I have a spare pump tunnel and I have been thinking about why GP1200's wont go 70+ without major power gains. Think about this, I measured the water volume of the pump tunnel only. 5.5 ltrs of water. The pump including nozzle takes 2.8 ltrs of water. The pump tunnel is twice the volume of the pump ! I have looked at the GPR tunnel and its much smaller being molded into the hull , I would like to measure one. At speed all this water must go somewhere or load the engine down with extra drag. Look at the new FZR 's the pump inlet is quite small and the pump volume is smaller ,yet the heavier hull albeit with more power really moves.
The GP1200 weighs 238kg or close too and the FZR weighs 366kgs yet it still has the thrust needed to propel it at high speeds. GP1200 with triple pipes made 185 hp and barely made it past 70mph yet the single pipe 155hp did 67-68mph , something is slowing the hull. If the trim tabs are good then what is it , Drag ! From the pump ! Thats all it could be , i have seen guys with ride plates shimmed up with the nose sky high with barely any hull in the water except for the pump. The one thing in common with Yamaha's post 99 is the pump tunnel is smaller and molded into the hull.
What if we could fill some of the volume of the Aluminium pump tunnel with epoxy resin to reduce the amount of water required to fill it ? Less Drag and quicker pump priming would result. Faster hull speeds.
04-04-2010, 06:21 AM #2
Adam,this had been talked about before, and I am sure it would work.
You can buy a R&D/riva pump stuffer for the 144mm superjet pump-basically a peice of plastic thet sits in there making the pump smaller, and that apparently is worth 2-3mph.
I bought some 3mm perspex sheeting on ebay a while ago, and this can be warmed with a heatgun/hairdryer and made very bendy-Just and idea - try lining the tunnel with something like that, and trimming to fit-you can then add more layers to progressivly make the tunnel smaller for testing-AND take it out again if it dosent work!
04-04-2010, 06:44 AM #3
Yes , Worx has a stuffer grate that does the same thing. I thought that an epoxy of some sort would be be best to mold into the right shape and volume. Im going to do it with my spare tunnel and take photos and post. I think I will need to fill about 2-3 litres if possible to try to balance the volumes of the pump and tunnel.
The GP800 had a 144mm pump and smaller tunnel, 120 hp and still went 59-60mph. There has to be a way to get more speed making a more efficient pump !
04-04-2010, 11:01 AM #4
Interesting theory Adam..
I wonder if there is/was a reason for Yamaha to design the pump tunnel this size..??
Now would some sort of pump relief be required if the tunnel pressure is going to be increased..??
Will it cause the ski to buck..??
I remember a few years back on my stock GP getting tossed over the side for NO apparent reason..??
Was riding flat out in some light chop, about 6" to a foot max..
Next thing i know I'm being fired off the side of the ski at about 60mph..
I do like the sound of what you are going to attempt..
Getting back to Yamaha's design of the tunnel....
Was it simply made this size because they more or less just lifted it straight out of the 1100 venture/raider..
So a case of stick with what works..??
At that time in the early nineties jetski's really were still in the infancy stages..
Will be following your progress..
04-04-2010, 11:19 AM #5
Rpms is the story with the triples. We are turning great speeds around 7300rpm. But that is a problem with the triples. the power band for the triples starts at about 7400 and really don't feel it til about 7500 and some change. Using the 14/20 j prop brings your rpms down too low to create power with the pipes. This why the speeds are so close with triples and singles. Triples are making no power at that rpm. If you start pitching the prop to get rpm you lose your prop angle to make speed. Triples were made to run CC racing not for top speed.
04-04-2010, 02:14 PM #6
I am just curious as to where this data comes from? The GPR's problem is too much water in the tunnel, which caused the buck and it became necessary to relieve water from the tunnel via the pressure relief mod. Also the speed from a GPR comes more from the stepped hull design then any thing else.
04-06-2010, 12:56 AM #7
04-06-2010, 02:15 AM #8
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