11:1 stock bore... is it worth it?

[grassman]

Im constantly flip flopping what I want to do to my engine. Right now it is stock w/ +6 key, jet kit, uni filter, and a hmf slip on. I have a stage 1 hc, and just bought a crf timing chain and a set of cometic gaskets. Would it be worth getting a 11:1 piston to go along with the cam and ditch the +6 key or get the 10:1 piston and keep the key... Ive read through alot of threads but they didnt answer my question. Which set up would give me the most power without losing reliability... any info would help.

[slightlybent47]

That’s a very good question. I have never used the timing key so how dose the key affect starting? Can you run both? Where is the cut off point regarding the key verses higher compression? I guess these are the questions you are looking for? I would go for the 11:1 over the key because of the cam you are running. I think the timing trick is just a bandage for the real thing. but this is just my guess.

[duroc825]

I think I've read around here somewhere that the key is not recommended with Hot Cam. I do know with the 11:1 ya don't get true 11:1 without decking head or using xr400 head gasket.
http://www.exriders.com/vbb/showthread.php?threadid=8316&perpage=10&highlight=have%20you%20decked&pagenumber=1

More reading info....quoted from Mickey Dunlap's tech article...

I prefer to build a motor that pulls quick and hard through the RPM range and the piston with higher compression will build more cylinder pressure which develops torque and will rev quicker and run faster through the RPM range. On most after-market pistons, the pin to deck height is the same as stock. You should have a minimum of .040 clearance between the piston and head with everything torqued down. Right now the piston is about .043 too low and you are not getting a true compression ratio as advertised by most piston companies. For instance, a 440 11:1 flat top piston advertised is really only about 9.75:1 if you measured the volume correctly. In order to get the deck heights better, I use Honda HT silicone seal on the cylinder base and an XR400 gasket if you are running an 80 over piston or smaller. This will get you close to what your deck height should be. Keeping the squish clearance down to .040 gives a better quench area around the outside of the piston keeping it cool and less chance for detonation. You will be able to run an 11:1 motor on pump gas in most cases and have a more efficient flame travel and quicker burn rate across the top of the piston. I have read on the forums here how everyone thinks a 416 is faster and revs quicker than a 440 kit with the same compression. I think most people are not running as high a compression ratio with the 440 kits as they think they are and when you put a cam with it, you end up with lower cylinder pressure making the machine rev slower than the higher compression 416 motor. Just my thoughts anyway. Without talking to each individual, it's hard to figure out why people think the 416 is faster than the 440. Any time you put a longer duration cam in, you have less time to build cylinder pressure. This is why it's important to put a higher compression piston in when you install a longer duration cam. You can get more torque the higher you go on the compression ration and your cam will give you the mid-range to top-end without losing low-end.

[DragonGunner]

I don't think the key is recommend for higher comprssion pistons...The advantage with a high compression, smaller bore psiton is it well rev. faster...I've read results that a stock bore with High compression was actually faster than a 440....because of the lighter weight piston....cams can change things as the above poster stated as well though...so going to cam it, I would go 11:1 for sure...Thats what I have in a 406 with web cam.

[grassman]

Looks like Im gonna ditch the key and go with stock bore 11:1 with a stage 1 hc... That should get me through the trails pretty good...:macho

[killer454]

if your going to do this i would suggest having your cylinder bored and buy a new piston to fit the new bore.this is what i did

bored .020 this will give you a brand new top end and fitted to the piston perfectly.
11-1 namara piston
hotcam stage1 cam
170 main jet
stock pilot
3rd notch needle
procircuit exhaust
uni filter
no lid

you can go to youtube and type in philipjd1 and see this 400ex in action if you want.

this thing kills the trails the best lowend hill killer i have ever rode.wheelies in 1st,2nd,3rd no problem.cant keep the front end down after this rebuild.

[bkb88]

How long have you been running on that Namura piston? I have only read bad things about them on this forum. Not really sure how much of it was from actual experience or from unjustified prejudice towards them. I have wondered if they were so bad, how would they be able to stay in business?

[PaRedneckRiders]

Theres nothing wrong with a stock bore 11:1, I loved mine.
It had
A stage one cam
Port and polish
11:1 stock bore wiseco
K@N filter and a White bros MX4 exhaust

That thing ran dam good for being what it was, i loved it but then i sold it to fund my 426 racer.

[killer454]

the namura piston has worked perfect for around 65 hours of ride time i would guess, with no problems.

i would buy another namura piston again.