Hi, checked the manual and noticed that radiator cap pressure varies from 1.1 to 1.4 kpf/cm. However stock cap is at lower end - 1.1; any idea why?

 

I don't know. I've seen this same kind of data on cooling systems in the past and never really thought about it or considered it a problem...as long as the cap is within the factory recommendation. Also, it may be an issue of the cooling system itself having a higher potential range of operating temp and the cap being intentionally at the low end for safety purposes. Or the whole issue may be a non-issue. The cap is within the recommended range, so...

 

HINT: SOMETIMES you can use an old Honda Civic replacement rad cap.... Probably not on the newer bikes, though.

 

So... just curios. I sought high pressure caps for KLX and it made me check what I have on my bike.

 

higher pressure cap = higher boiling point for the coolant

 

Of course...but if I was reading the OP correctly, he was curious as to why the OEM rad cap delivered on the bike was rated lower than the maximum allowable for the radiator cap. I would speculate that the cooling system requires a cap within the range listed in the manual just as guidance if you use some other cap. I'd rather have my cap on the low end of the recommended range to provide a level of protection for the rest of the cooling system...hoses and the rad core itself. If an engine has an overheating issue, a higher rated cap obviously won't fix the problem. In fact, using a higher than suggested rad cap could result in a blown hose. You're right that a higher pressure cap equals a higher boiling point, but IMO you'd be better served in addressing why the proper cap at the low end of the pressure scale isn't getting the job done. The correct cap at the lower pressure range is still providing some extremely high boiling point control.

Cooling systems are interesting in internal combustion applications. Water is a better cooling medium than just about anything out there, but it has a low boiling point. Antifreeze raises the boiling point, lowers the freezing point, and has corrosion protection, but doesn't cool quite as well as pure water. Water-wetters improve the cooling properties of coolant by improving heat transfer, but they are have a short life in a cooling system. I run a 60% distilled water/40% antifreeze mix where I live, along with a manual fan switch. The KLX has a very good cooling system compared to many other bikes, so it usually doesn't need much help unless something else in the package isn't operating optimally.

 

(Even though a cap may be "rated" a certain pressure, doesn't always mean it FUNCTIONS properly. Test your cap at a local rad shop. I've seen "bad" caps before.)

 

Good comment...
another point why to get high pressure cap would be that stock is tuned up to sea level. Higher you go - boiling temperature decreases.

 

After my big bore installation/carb rejet I would find the overflow bottle almost full and a mist of coolant underneath the rear fender. I installed a kx75 rad cap and the problem stopped.

That was 3 years ago, still going strong

 

Different pressure on the KX cap or a defective KLX cap?