XR 650 top end up grade-what should I do


5 replies to this topic
  • Travis Newbold

Posted December 27, 2006 - 06:34 PM

#1

My 650 sucked dirt in the baja 1000 and I plan on doing a top end and would like to up performance a little bit, should I go with a high compresion piston, what cam works well? and do I need to install the biger cam chain? if so who makes it. I would like to put reliability before performance but a little more power would be nice down low, I am used to the 450.

  • BWB63

Posted December 28, 2006 - 07:10 AM

#2

The XR650R was first setup to have the HRC cam, 11:1 piston, HRC cam chain, and more free flowing exhaust. In the end to meet air quality standards this changed and Honda put those things in the HRC kit and we got the BRP we have now. The bike really responds to a porformane cam (stage two) that is what it was designed to have. If you get a re-grind you will have to get hardened rockers. The 11:1 compression is a trade off of having to run higher octain fuel for power. If you want to run pump fuel then stay with 10:1. For reliability I would get the better HRC timing chain, and stainless steel valves. The stock intake valve of the XR650R are to soft and dish way fast. That is why they tighten up all the time.

There is info on horsepower gains for different mod's on this sight.
http://xr650r.borynack.com/

Posted Image

  • Potts228

Posted December 28, 2006 - 02:10 PM

#3

The XR650R was first setup to have the HRC cam, 11:1 piston, HRC cam chain, and more free flowing exhaust. In the end to meet air quality standards this changed and Honda put those things in the HRC kit and we got the BRP we have now. The bike really responds to a porformane cam (stage two) that is what it was designed to have. If you get a re-grind you will have to get hardened rockers. The 11:1 compression is a trade off of having to run higher octain fuel for power. If you want to run pump fuel then stay with 10:1. For reliability I would get the better HRC timing chain, and stainless steel valves. The stock intake valve of the XR650R are to soft and dish way fast. That is why they tighten up all the time.

There is info on horsepower gains for different mod's on this sight.
http://xr650r.borynack.com/

Posted Image



I have seen people comment that with the 11:1 Piston you have to run higher octane fuel. If this is true then why can I run regular unleaded in my CRF450 (also 11:1) and even Mexico gas without any obvious pinging? Does anyone know? I'm starting to think this higher octane thing is a wives tail.

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  • cleonard

Posted December 28, 2006 - 02:26 PM

#4

I have seen people comment that with the 11:1 Piston you have to run higher octane fuel. If this is true then why can I run regular unleaded in my CRF450 (also 11:1) and even Mexico gas without any obvious pinging? Does anyone know? I'm starting to think this higher octane thing is a wives tail.


All things being equal the larger the displacement the lower the compression has to be to operate on the same octane. For example the 250 Ninja has 12.4:1 compression and is rated to run on 87 octane. One thing that changes is surface area to volume ratio. The larger the displacement the smaller the surface area gets compared to the volume. This changes the thermal influence of the cylinder and combustion chamber walls.

Now I have no idea, but I bet that things are not equal between the 450 and the 650. I imagine that the combustion chamber is different between the two engines. Combustion chamber design makes a big difference too.

  • Billahjack

Posted January 04, 2007 - 06:51 PM

#5

Octane requirements are a function of a combination of factors which affect detonation in engines. If you have an engine that detonates (the air/fuel charge lights off before the spark plug ignites it), then it undergoes much more stress than normal combustion.

Bore size - The larger the bore, the more chance there is for detonation due to air/fuel mixture inconsistencies in the combustion chamber. Smaller bore engines can run higher compression on pump gas.

Combustion Chamber Shape - The shape has a large impact on detonation. Any little sharp edges get hotter than flat or curved surfaces and can initiate detonation. Carbon deposits on the exhaust valves can cause the same effect. Elimination of hot-spots reduces detonation.

Mixture - If you run a little rich, then you can resolve detonation problems to some extent. Most turbo charged vehicles go heavy on the rich side of the mixture as soon as they start making boost.

Side note -

Octane Rating is the rating that signifies the burn rate of combustion for fuel. The higher the number, the slower the burn rate. Detonation is when the fuel/air mixture ignites in an explosive manner rather than a controlled burn/gas expansion. If you run too high of octane for a motor, then the combustion burn rate is too slow and you lose power. This is because the combustion is not complete before the piston get close to the bottom of the stroke when it starts to slow down and reverse direction.

  • frankstr

Posted January 04, 2007 - 07:02 PM

#6

Welcome to Thumpertalk, Travis Newbold...

:lol: :confused:





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