Dumb question for the Doc!

[Guest_User]

hey Doc, I understand about the RAVE`s function. In the yamaha world some would pin the valves open for a specific performance gain, usually when going Big Bore. and the ski would react more like an on off switch but spend the majority of it`s run time in the upper RPM range. Quite a few have switched over to GAS valves, very similar to SD`s set up.
Yamaha power valves when working are good, when they fail, they take out that cylinder, unlike seadoo`s exhaust valves where it has a stop against the cylinder. although some have been known to snap and fall in realizing the same fate.

My curious question: I don`t believe I`ve ever come across any info on members anywhere pinning the seadoo exhaust valves open or simply deleting them and installing a BO plate.
What are the specific reason why members don`t remove or keep them pinned on the 787 engine`s? I realize the 717 engines do not have Rave`s.
What would be the pro`s and con`s doing so?
will you lose bottom end, will it fall on it`s face when trying to plane off?
will the engine still achieve max rpm`s?

please school me as I am curious, thanks...PR...

 

[Guest_User]

:lurk:
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[Dr Honda]

Sorry I missed this one.


Well.... the simple answer is... if you keep them open... the exhaust timing will be too aggressive, and your idle, and off idle performance will be bad.

Sure... a lot of engines run great without a "Power Valve"... but the port timing is set to a point where it's a compromise. But to just pin them open is silly. I don't care what kind of engine you are building... and what they ultimate use is... they do serve a use.


OK... here's the deal. the 720 engine makes 85 hp... and the 800 makes 110 hp. Do you really think Rotax could pull out an extra 25 hp with only a 75cc increase without the valves? Sure they could. BUT... the engine would be so rough, that it would be miserable to ride at low RPM's. (like in a no wake) So... you put that valve in... and when the RPM's are down below 4500.... then it runs smooth.

The other side of this is torque. In something like a dirtbike... you need that valve to be closed to get the "Ummph" to accelerate. In a watercraft... it's not as critical. Our pumps make thrust exponentially... so they require less torque to get the engine spinning.

Personally... I like the torque, and smooth running of the valves being closed. On my personal XP 800... I have the RAVE adjustments at full tight. it makes for a nice ride. The only draw back is... it's a very "On-Off" feel. When I get to about 5000 RPM... I get a burst of power.

I don't know if I answered the question or not... but it's just some of my random thoughts.

 

[Guest_User]

Thanks Doc, yes this about answers what I was curious about. Always good to have another perspective on things.
I`m also familiar with port timing, ported jugs, it`s affect on the powerband etc etc, and a few cheater tricks from the old days. Thanks for the lesson!:cheers::thumbsup:

note: I have my raves on both engines set at 2 turns out, or flush with the black cap. this seems to have smoother transition and seems to create a more linear power curve... I still have a "good kick" when they do open.

Thanks...PR...