Jump to content
Sign in to follow this  

high compression or std compression?

Recommended Posts

What is your guy's thoughts? How much more power does a high compression piston give you? Is it even worth it?

Share this post


Link to post
Share on other sites

What is your guy's thoughts? How much more power does a high compression piston give you? Is it even worth it?t

The higher compression can help build more power in certain places more bottom end response .

Share this post


Link to post
Share on other sites

Well, a factory 250 is in the 12.5:1 - 13.5:1 range, so I'd already call that high compression. If you want to go higher you'll obviously need a higher octane fuel. 

 

Pistons don't really "wear" at all from compression, but all the up and down motions do cause the sides and the internal bore where the wrist pins go to wear down very slightly. For an '04-'09, the service limit of the piston is an OD of 77.940mm and a bore ID of 16.030mm. Pass that point and you may as well toss the thing. This will also wear the cylinder and the wrist pins down, creating for larger clearances between those, but I don't want to go off on a tangent. Anyway, as I understand it, you won't see extra wear from a higher cylinder pressure as a result of a high compression ratio, but you will see a higher likelihood of failure in absolutely everything due to all the added stress. Pistons, rods, and cranks should all be made of forged materials to handle all the extra stress. Don't even think about running a cast piston. Are those even available in race bikes? 

 

It's nothing more than another example of the old saying. Speed costs. How fast do you want to go? You can beef up your top end all you want, but can your bottom end handle it? This is why people sometimes say that big bore kits make the bike unreliable. You're adding all this power up top and not reinforcing the bottom end.

 

Chances are, because of a dirtbike's already high compression, your cranks and rods will already be forged steel and your pistons forged aluminum. Now, your head will still be cast but I've never heard of a factory head cracking from a little more compression. I see people throw on 13.5:1 pistons onto bikes like my '07 which come stock with a 12.9:1 piston, but then again if you buy a new bike it'll already have 13.5:1. Wiseco sells a 14.5:1 forged piston for a whopping $282 for the new bikes. Your only other options are an aftermarket head with a smaller combustion chamber, a thinner head gasket, or start machining the head and/or cylinder to lower that combustion chamber.

 

Your true compression has a lot to due with your camshaft though. The more your intake valve is still open as the piston begins to go up on the compression stroke, the less compression you'll have. The 13.5:1 figure is known as static compression in the performance engine world whereas the figure measured from the moment your intake valve shuts to when the piston reaches TDC is your true compression ratio, or your dynamic compression ratio. Your camshaft controls this figure.

Edited by trozei
  • Like 1

Share this post


Link to post
Share on other sites

I just bought a brand new aftermarket head and camshaft, and it runs pretty strong, but I'm just seeing if I can juice a little more power out of it. And I also don't care how many top ends I blow, I just couldnt stand blowing my bottom end again

Share this post


Link to post
Share on other sites

Does your new head have a factory-sized combustion chamber?

Share this post


Link to post
Share on other sites

A higher compression piston than the one you have in there, now, will increase low-end-mid torque and throttle response, as Ron stated, no matter what's been done to the head. It's worth it if you absolutely must have more, but if you just put a new piston in you might as well just ride it a while and get some use out of the existing piston.

What are you running for a piston, now? Stock stuff? Used or new with the head work you just did?

And yes, a higher-compression piston will wear a bit faster (and scrub the cylinder wall more) than a stock piston, and it will put more stress on your bottom end parts. But that's what "more power" is all about!!! Braaaap.

Edited by Eddie8v

Share this post


Link to post
Share on other sites

i had zero accelerated wear in my 07 with a 13.5:1 piston what year bike do you have?

Share this post


Link to post
Share on other sites

Isn't stock compression 12.9 on that bike? You won't see much of a difference if you're not "using" the power all the time. If you're a more casual rider it won't make much difference, but with the higher cylinder pressure from the higher compression piston there will be more skirt pressure and piston wear, exacerbated by high-rpm and high output situations.

A 13.5 isn't much more compression than a 12.9 and you can still run it on pump unleaded premium auto fuel. A 14 or 14.5 is what I call "hi compression".

Edited by Eddie8v

Share this post


Link to post
Share on other sites

Isn't stock compression 12.9 on that bike?

Correct.

Share this post


Link to post
Share on other sites

I have an 06 and did a top end last year on it. I used a JE high compression piston (13.7:1). Don't listen to the guys that say you won't notice a difference-- I absolutely loved the new piston. Was it faster, yeah a bit. But where you really see the difference is in throttle response and low end torque. It had a 450 feel in the low end-- a blast to ride! 

I'm getting ready to to do a bottom and top end on another 06 and you got it, I'm putting that same piston in. It's my go to from here on out. Here's the piston I used, it's number 274178:

http://www.jepistons.com/Catalogs/Powersports/Offroad-Cycle/Honda/crf_250r-x.aspx

 

Oh, another thing. No, you will not need a higher octane fuel- at least I didn't. Ran fine with Chevron premium and I'm at 4700 elevation.

 

Hope that helps.

Edited by joehicks

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×