TOPIC:

Compression Ratio: corrected vs. mechanical

www.bing.com/videos/search?q=Compression...D&first=0&FORM=LKVR2

Good Fortune!

Edit: Or should that be dynamic vs. static.

It'll bring down the dynamic CR sure. Also depends on how you have them adjusted, ie how much overlap you're running.

I always thought pistons were rated for static compression ratio, hence my statements above. If they are rated for dynamic CR then thats different.

No eBay APP ID and/or Cert ID defined in Kunena configurationHere's Pit Stop's ebay ad for these pistons:

If you click it and read his description, he expains his theory in the ad. I always thought like Tyler that "compression ratio" was determined as static. I also agree with Mark and Joe Hooper in that some "compression" is lost in the overlap phase, and so high lift/high duration cams will reduce "compression." Joe advised me that these pistons were designed to work with said cams to provide the type of "compression" that one would achieve with stock cams and 10.5:1 pistons. It's a little bit of a confusing way to advertise pistons, but Joe made sure that I got the right thing before he shipped the pistons. I give him all the credit in the world for clarifying this since I clicked the BIN on his Ebay auction. He could have easily just shipped them and left me to sort it out. I told him specifically what cams I was using and he advised me to use these pistons. I'll trust his advice since he's been in this business longer than I ever thought about it.

Craig asked earlier when I was planning on building this motor, and the answer is this winter. I won't miss the nice riding weather in fall, so I have to stick with the old stock motor until the days get short.

T_Dub wrote: It'll bring down the dynamic CR sure. Also depends on how you have them adjusted, ie how much overlap you're running.

I always thought pistons were rated for static compression ratio, hence my statements above. If they are rated for dynamic CR then thats different.

I agree...that's what throws me for a loop.

However, I think it is just a way for them to make it easier for potential customers to understand.

It's just something that drive me crazy in the eBay adds. Their prices seem fair enough.

Anyway...I think I've gotten us off topic. Sorry bout that.

i just thought your pistons look high and i hope it doesnt cause isues. some guys report pinging with 10.25:1 static compression pistons. here are some pics of mine. i use .425" lift cams.

1200cc J&E 2009


Wiesco 1135cc 1977



1327cc old


pit stop pistons

they remind me of an old style (long skirt) 13:1 piston that has been cut down ?
doent matter as long as they rip.

Mark, I think that your larger bore has an effect on the overall compression ratio as well. More cylinder volume compressed into the same size combustion chamber = higher CR right? That should be the reason for higher domes on the 1075 cc pistons. If I can't make it work with a combination of timing, and super unleaded, then I'll surely report back and make adjustments as necessary. I don't want to have to run this bike on race gas.

You're exactly right Josh about the dome height.

As for the CR crap, seems like Joe and them are doing the right thing, selling you pistons that'll work better with your combination. Manufacturers DO list pistons by static compression (ie, volume ratio), but dynamic compression is more important as it is the actual pressure ratio of TDC cylinder pressure to atmospheric. Dynamic compression is affected primarily by the combination of valve size, porting, and cam overlap. Basically, the higher your dynamic compression ratio, the higher your cylinder pressure at TDC, and the higher your power output.

No eBay APP ID and/or Cert ID defined in Kunena configuration

testarossa wrote: Here's Pit Stop's ebay ad for these pistons:

If you click it and read his description, he expains his theory in the ad. I always thought like Tyler that "compression ratio" was determined as static. I also agree with Mark and Joe Hooper in that some "compression" is lost in the overlap phase, and so high lift/high duration cams will reduce "compression." Joe advised me that these pistons were designed to work with said cams to provide the type of "compression" that one would achieve with stock cams and 10.5:1 pistons. It's a little bit of a confusing way to advertise pistons, but Joe made sure that I got the right thing before he shipped the pistons. I give him all the credit in the world for clarifying this since I clicked the BIN on his Ebay auction. He could have easily just shipped them and left me to sort it out. I told him specifically what cams I was using and he advised me to use these pistons. I'll trust his advice since he's been in this business longer than I ever thought about it.

Craig asked earlier when I was planning on building this motor, and the answer is this winter. I won't miss the nice riding weather in fall, so I have to stick with the old stock motor until the days get short.

good good.....that is when i think i will pull the 900 out of the frame and take it all down too....

maybe have it together and painted by spring and the quail just to ride it to the sunday show and park in the cheap seats.....exciting.....build it tough as you want but by god see that it is ready to ride by spring aye....my worst fear always...MKII and its phase II maiden ride was the quail going 100 and hitting cattle gaurds...see if i can have my spring rides ready in spring from now on......cool pistons and ripper

the static compression ratio is simply the volume of the cyl as it relates to the volume of the chamber, as a ratio. if the cyl volume is 10 times that of the chamber it is 10:1.
if u put the piston at bdc.(grease the rings first) and measure the amount of water it takes to fill it. then put the gasket and head on and torque it . now put the piston at tdc and measure how much it takes to fill it. this is the 2 #'s needed to figure out the static compression.

But, the only thing that matters in the end is that u dont end up with too much cyl psi.
if u get over 200 psi u may need more octane, or more overlap in the cam.
with the cams i have ,i run at can 200psi, with premium gas.

What grind are you running Mark? Also, what octain rating is considered premium in Canada?

531blackbanshee wrote: nice pistons there sir !

that should be a very good running combo.

i cannot believe that craig slipped on getting the evidence on adam .riding in flip flops .

jee wheel lickers wally!!!!!


leon

Awwww crap! Craig's right, I was in flip flops. I hadn't planned on riding a bike that day, let alone stop by Craigs place. But he offered the ride on his beautiful blue MKII, and I couldn't turn it down. I needed an incentive to help me h=get back in the grove with my bike, can you blame me? Craig might be needling me for riding in flip flops (and shorts and a t-shirt as well), but he forgot to mention a few other incidentals about that little ride Thank you for your generous hospitality Craig. The girls needed some time to unwind out of the car, the baby ate her own weight in blackberries, Marina got a handful of awesome produce out of your garden, and I got a much needed shot in the arm on your MKII. Thanks for providing a great little detour and distraction for my family.