CRF's Only Forums

I'm planning on rebuilding my sons 2010 250R this spring,and was thinking about installing a aftermarket high compression piston.Are there any other mods that need to be done to go with this? It already has a leo vince full exhaust and a tokyo mods ecu.I was also thinkin of sending in the head for kibblewhite valves.I did this with his 05 and was thrilled with the results and longevity. Will this shorten the life of the bottom end or cause any other ill effects? Any recommendations and advice would be appreciated.

when ever you increase the compression ratio just make sure you know what kind of octane will be required for the increased compression ratio. you won't shorten the life of the bottom end. with the ignition you should be fine with the mapping. switching to KW stainless will provide the same level of valve durability as your other head. with our performance CNC valve service you'll get improved flow above the valve guide.

He's been running race fuel his last few races and plans on only running it if i install the high compression piston.Any review i've read on these has been increased power over the entire rpm range,but no one will address longevity or durability.My other concern is with the fuel injection and whether it needs to be addressed like jetting with a carb.Being as his fuel injection has been adjusted for his exhaust,i thought this would be an easy mod for a little bump in power.

increasing compression is an easy way to get more power. you may have to have your ECU reprogrammed but you can test it with the new piston safely as long as you are using the correct octane for your compression ratio. the bottom end will be just as durable. they are built to withstand these types of modifications.

Thanks for the knowledge.It's greatly appreciated.

Every parts who will add power will reduce realibility, exhaust, camshaft, piston.

DarkCRF wrote:Every parts who will add power will reduce realibility, exhaust, camshaft, piston.

If you truely believe that statement, you need a reliable mechanic.

DarkCRF wrote:Every parts who will add power will reduce realibility, exhaust, camshaft, piston.

We could get into a long discussion about what "reliability" means. However, for the purposes of my reply I'll take it at face value and assume it means both engine failure and longevity of the parts. These two are not the same however. An unreliable motor is one that will either not predictably start and operate and/or is likely to have some minor or major engine failure. I'm assuming you intended reliability to mean durability/longevity of parts.

While it is theoretically or generally true that engine parts that create more power will increase forces and friction on engine components and thus wear them faster than their stock counterparts. However, some parts are over designed and will not show significant reduction in durability/longevity. Also, some performance parts should never be installed without also replacing supporting parts and/or changes designed to support the new performance part. I would generally accept your statement if engine performance modifications were combined and those modifications were significant. However, the question here was increase piston compression alone.

Exhaust: An aftermarket exhaust will not reduce motor durability/longevity. The only way an exhaust could reduce motor reliability is if it leaned out the A/F mixture and this condition was not addressed during the installation of the exhaust.

Camshaft: Depending on how radical the profile an aftermarket camshaft could reduce durability/longevity. However, if the cam uses the stock springs it will likely not have any negative effect on motor reliability. If the profile of the cam requires new valve springs this is an indication the cam design will be opening the valves quicker and/or further than the stock cam. This can have a negative effect on valve train reliability. However, if installed correctly with the correct valves and valve seats the durability/longevity of these parts can be extended to stock levels or beyond.

Piston: Increasing compression with most commercially available pistons, by themselves, will not significantly decrease reliability (motor failure or longevity of the bottom end). If increasing compression to it's absolute maximum then yes, durability and longevity could be reduced and then I would recommend replacing other components with the piston to overcome this reduced durability and longevity.

However, as a single modification of increasing the compression ratio from stock ratio will not significantly decrease reliability, durability or longevity.

I was mostly concerned it would over stress the bottom end causing catastrophic engine failure. Just as adding a performance exhaust can cause a lean condition to not only reduce power, but shorten engine life, too. When i was a kid i swapped my good snowmobile engine into a better chassis not knowing that old gas would cause the piston to come back through the carb in tiny pieces on my first test ride .Hauled butt for awhile,then i was without a snowmobile,period .Tryin to avoid that kind of learning.

I don't say this would dramaticaly reduce realibity, I get a 43 hp 250R and my engine still realiable enough to rebuild it every 40 hours without any stress, but the fact is here, every horse power you get, the stress on all component will increase.

This is a quote from a post about high compression piston on another site, from a big reference in motocross engine development,

http://www.thumpertalk.com/topic/897998 ... on-piston/



now...to debunk the RELIABILITY VS HIGH COMPRESSION myth...hopefully for the last time....

horsepower and torque are a direct reflection of the combustion pressures seen inside an engine......

ANYTHING that makes your engine have a higher output is a result of it creating more combustion pressure within your engine...... whether the power came from a jet kit...pipe...cam...special fuel...etc...etc...

as far as the stress on your engine components....they have not the slightest idea wher the pressure comes from...and they wouldn't really care either...more pressure = more power = more stress on everything...

a 50 hp pump gas setup ..... is putting out more stress on the engine components ....than a high compression engine delivering 47hp.... the compression isn't what is the stress...the actual pressure from combustion is.... and combustion pressure is MANY times greater than cranking compression in any event....

increased power = stress and accelerated wear.... that is the bottom line....it doesn't have anything to do with what compression you have..aside from the actual power it adds to the engine..

and BTW....on the piston reliability thing...compression notwithstanding... there are design and material components that will make one piston\ ring setup better in the reliability and longevity arena's

i think we are in agreement. when building an engine for increased power the engine builder considers the additional stresses and where these are concentrated and replaces the stock parts with parts designed to handle this added stress. in the case of this question from the OP the stock Honda lower end is designed to handle an increase in compression in the range the OP is considering without any negative effects on durability/longevity.