Why are expansion chambers on ski's water cooled ?

[deanz]

Coming from a dirt bike background I do not understand why a jet ski expansion chamber is either water jacked or internally water sprayed......... a dirt bike can run at high load and low speed with a mud caked expansion chamber and not have problems.

Is it a safety issue in a jet ski if it was to get super hot ?

if so could it not just be heat wrapped ?

Dean.

 

[Matt_E]

First, your bike's chamber is aircooled. It has constant air flow over it. The ski has an enclosed bilge. The watercraft engine has quite a bit more load at all times and runs WOT for a majority of its lifetime. It gets hotter than a bike does.

Second, the wet pipes (spray internally) have the injection primarily for performance reasons.
A typical 2-stroke pipe hits at one relatively narrow RPM band. Your bike has a transmission, so it is relatively easy to work within that RPM band.
A ski is direct drive, no transmission.
What makes the pipe hit at a certain RPM is the length of the chamber. Without getting into too much detail, the length of the chamber determines where the motor will hit.
Injecting water into the exhaust stream cools it. This slows down the sonic exhaust wave traveling down the pipe, essentially making the chamber appear longer than it is.
The amount of water injected depends on engine RPM (coolant pressure, really).

The end effect is a variable length pipe.

 

[deanz]

Thanks for that, although I do understand how water is used to effectively lengthen a chamber........ but I am still unsure why a dry pipe is water jacket cooled.

This is what I just thought up, a dry pipe is water jacketed not because it needs cooling in fear of overheating/melting, but it allows a pipe to be made shorter that it would be if not jacket cooled, making it easier to fit into the confined space of a ski.

Although the added layer of the cooling jacket uses up space too. Hmmmm there must be a good reason to go to all the trouble to jack cool a dry pipe......

 

[keefer]

You got it, a dry pipe is better for mid / top end RPM ranges. You have to keep the external part of the chamber cool so it won't melt things in the bilge. They are more popular for race applications or anybody that is more into top speed riding.

 

[SUPERJET-113]

Nice laymans explaination Matt!:biggthumpup:

 

[aqua]

ok....lets think about this for a second.

-dry pipe = no water injection into chamber.
-wet pipe = has water injection into chamber.
-dry pipe is better for top end.
-wetpipe is better for bottom end.
-water in chamber better for bottom end.
-no water in chamber better for top end.

ok this is my question. i understand how water in the chamber will help bottom end but when engine rpms build the pressure in the line builds also...pushing more water into you chamber. how is this added water from the increase in rpms suppose to help the top end performance of the engine?

 

[stanton]

steam control!

 

[#ZERO]

Also water helps with noise reduction and keeps the exhaust hoses from melting.

 

[Mile9c1]

Boats have water cooled manifolds, even though cars with the same engines don't. I think it's a safety thing. Don't want anything to catch on fire you know!