Rejetting an 98 XP?

[Tailgate4jc]

Ok question for the fourm...I like my XP but just curious to know if I can change the jet impellers to make it faster, more responsive and so on.

What does it take? What is there now? Is it the change of the pitch of the blades? Where do you find it?

Just sounds like a good winter project if its doable.

Thanks for your help and good riding.

 

[Strizzo]

bad idea, the 951 is already stressed beyond its reasonable limits in stock form, no need to make it worse unless you're racing.

 

[AdamXP800]

I believe Sqrl recommends Solas Concord 15/20 impeller on all 951 skis

 

[Sportster-2001-951C-Stock]

Nitrous Oxide, baby!

 

[Tailgate4jc]

Love the idea of a Nitrous but ok maybe not that extreme yet. If I did that I think I would need to upgrade for the XP series.

Thanks for all the options. Enjoy always hearing them.

Happy riding...

 

[SabrToothSqrl]

change the prop to the 15/20 and make sure the engine is in perfect shape. Compression, carbs, fuel lines, raves, water regulator, etc.

you don't want to spend the summer working on it every 10 min for an extra 5 mph.

Without changing your MPEM to allow a higher max RPM, you really won't gain anything worth ruining your reliability over.

 

[Tailgate4jc]

So changing the MPEM, is that changing the entire case or a reprogramming? What do you change it with?
Thats great information and really appreciate the patience for more and more questions.

 

[Strizzo]

Here is a copy/paste of a post i made last year some time.

heres an interesting thing, i got to thinking about whether mean piston speed might be a factor in the short life of the 951s, according to that site, to determine max rpm you can divide the engine stroke by the appropriate factor based on engine components.

The formula for determing an engine's maximum rpm is:

Stock - 21,000 divided by the stroke
H.D. - 24,000 divided by the stroke
Race - 30,000 divided by the stroke
the stroke on a 951 according to the 1999 shop manual is 3.079 inches. if you take 21,000 / 3.079 you get 6820.xx... heavy duty (forged pistons, rods, crank etc.) nets 7794.xx...

many people on here advocate a full rebuild any time a top end is needed on a 951, this could well be the reason for such short lifespans of the 951s compared to the other rotax engines. does/did any company ever make aftermarked forged internals for the 951s? i wonder if forged parts in otherwise stock motors could make them a bit more predictable engine life.

also, read this link that I also posted in the same thread as the above post: http://www.mustangsandmore.com/ubb/SteveWmaxpistonspeed.html

The rev limiter on a 951 as set in the MPEM is 7000 +- 50rpm. so every time you jump, or when you're going top speed and unhook the pump a little, the engine is exceeding its "absolute redline"

there is also this, from a site discussing BMW ///M engines:

Mean Piston Speed
The best known "rule of thumb" is mean piston speed (average speed the piston travels at a fixed rpm). It goes like this:

Mean Piston Speed Result
------------------ ------
Under 3,500 ft/min Good reliability
3,500-4,000 ft/min Stressful, needs good design
Over 4,000 ft/min Very short life

Mean piston speed is easy to determine. It's based on the piston stroke and rpm.

Cm = 0.167 x L x N

Cm = mean piston speed in feet per min L = stroke in inches
N = crankshaft speed in rpm

Using the 1988 E28 M5 as an example:
E28 M5, Bore and stroke 3.68 x 3.31 in (93.4 x 84 mm) At 6,900 rpm (factory redline)

Cm = 0.167 x 3.31 x 6900 = 3,814 ft/min

At 7,200 rpm (Dinan chip redline) = 3,980 ft/min At 6,500 rpm (max constant speed) = 3,593 ft/min

Here are some other piston speeds I figured for comparison.

Redline Stroke Piston Speed
Engine (rpm) in,mm (ft/min)
-------- ------- -------- ------------
BMW 2002 6,400 3.15, 80 3,367
BMW 318i 6,500 3.19, 81 3,463
BMW 325i 6,500 2.95, 75 3,202
BMW 3.0 6,400 3.15, 80 3,367
BMW 535i 6,200 3.39, 86 3,510

MM 4000 6,000 3.70, 94 3,707
MM 3500 6,400 3.39, 86 3,623

BMW E36 M3 6,800 3.38, 85.8 3,838

BMW E28 M5 6,900 3.31, 84 3,814
BMW E34 M5 7,200 3.39, 86 4,076

CBR600 13,250 1.78, 45.2 3,939 (motorcycle)

Notice that most production cars stay below 3,500 ft/min, while the E36 M3 has the same 3,800 ft/min piston speed as the E28 M5. With its longer stroke, the E34 M5 piston speed is even higher than the screaming Honda CBR600 motorcycle! The two Metric Mechanic (MM) engines also go above 3,500 ft/min.

>From the MM Catalog Engine Specification table, MM uses
thinner (lighter) 1.5 mm (0.059 in) top rings than the standard BMW 1.75 mm (0.069 in) top rings. Lighter weight reduces vertical ring acceleration force at high rpm. MM talks about less "hammering" of the ring groove from the lighter ring, but it also reduces piston and ring overheating.

http://www.unofficialbmw.com/all/engine/all_redline_rpm_vs_reliability.html

this is why you should simply make sure that your engine is healthy and tuned, put a concord 15/20 in the pump and enjoy the ski while the weather is nice.

 

[Tailgate4jc]

Awesome information.
Tons to look at before bitting this off. Love the information.

Thanks again for your time. Looks like fun.

 

[Sportster-2001-951C-Stock]

Exactly, bore speed plays a huge part in the equation and is just as deadly as EPA-mandated partial seizures despite whether or not the rods are properly shot-peened and made of super-tough forged unobtanium. Run a 2-stroke on the hairy edge of lean and pray, I guess, bore speed is another example for why most big block V8's aren't high-rev engines as well.

 

[SabrToothSqrl]

I'll bottom line it.

DON'T.

If you want a faster ski; buy a faster ski.

You will spend lots of money, time, stress, and frustration to find out that all your efforts result in a small performance increase, while destroying your reliability.

If you want the ski to be it's best; clean everything, inspect everything, get everything back to 100% spec in 100% stock form.

But hey; if you want a money pit, and love to spend more time in the ski than on it, go for it.

However, I'm telling you, save your money and your time.

the MPEM contains your REV limiter, which is set, and not adjustable, to 7150 +- 50 RPM.