I'm getting my engine built in about 2 weeks and my engine builder tells me too just go with the stock bore. He said your getting your power from the higher compression piston and not the bigger bore. Can anyone tell me what the difference is and if it's really more power? Even if you do get the more power from the bigger bore is it enough to tell? Or is just as good with stock bore for reliability purposes?

 

Getting power out of a 4 stroke is all about the accumulation of power derived from many properly matched changes. The bore itself is one of those changes. Some mods make a slightly bigger difference than others, compression is probably one of those.

If you are looking to generate some good power, you need to focus on making many small changes & shoot for a cumulative total, instead of expecting to get power out of only one change.

 

Would you say that if I keep stock bore, but drop in a higher compression piston, and different cam, I would be able to tell a significant difference? Will it make my 400EX a lot more competitive on the MX track? Thanks.

 

i would say just up the compression on a stock bore. i had a 400 foreman that was bored out a hundred over and could hardly tell a diffrence. Now i know that the motors in a foreman and ex are different but in my 400ex I just upped the compression and put in a race cam and rev limiter and to quote one of my budddies "that thing is wicked" so go with the stock and then go race and beat some 440s and 416s

 

I totally agree with Couga21. Get an 11:1 piston and then get a 450/451 Web-Cam. I have these in addition to a DMC pipe, and my 400EX is "way" faster than stock. It runs with my cousins piped Banshee.

 

As I said before....

If you just increase the compression, or if you just put in a cam, or if you just bore the engine, or just bolt on an exhaust, or just Port the head, just change the carburetor, etc..., etc..., you are going to see nothing more than possibly a slight improvement or a shift in the powerband on a 4 stroke engine.

Now, put all these things together and you should have a very nice running machine.

Chances are, you'll be disappointed with any single modification you make to the engine. Set your budget, choose your parts & mods wisely, make the most of your money.... For instance - why not bore the engine a bit while you're in there? Take it to a 416, you won't need to sleeve, the piston won't cost you any more, there will be a minimal boring charge from the machinist. Just one way to make your money work a little harder while you're in there already.

 

Well here's my mods Gabe. You think stock or 416?

Je piston
WB rev kit
xr400 cam
Lectron Carb
3 angle valve job
Port and Polish
I'm also running a k&n filter and LRD exhuast system.

 

Well here's my mods Gabe. You think stock or 416?

Je piston
WB rev kit
xr400 cam
Lectron Carb
3 angle valve job
Port and Polish
I'm also running a k&n filter and LRD exhuast system.

 

Here's my engine mods:

10.8:1 JE 87mm piston (we lightened it to within 10g of stock - the biggest portion of this weight comes from the non-tapered wrist pin JE uses, if they offer a tapered unit, that's the one I'd get next time)

XR400 cam - (Probably soon to change out to a GT Thunder cam, depending on how a buddy's buildup goes, his mods are identical to mine except he used the GT Thunder cam)

Port work on the head, polishing of the combustion chamber, and your typical 3-angle valve job.

Modified stock exhaust header & silencer

Modified air box

Keihin FCR 39mm carb

XR250 CDI box (currently removed for cold weather riding)

What I was saying is, it most likely wouldn't be worth changing the bore size if you weren't already going to have the engine torn down. But since you will have the engine torn that far down anyway, why not give yourself a few extra cc's at a cost of almost nothing? The only negative aspect is that next time the engine needs bored, it will need to be sleeved (either back to stock or to 89mm). For me that wasn't a concern as it's not that big of an expense when the time comes.

Like I said, don't expect it to make THE difference, heck, I've outrun my share of 440's even without my carb or silencer mods... Its all about a cumulative effect & matching of parts. It's just one of those little things that adds up, plus, a larger engine volume should match up well with a bigger carb...

 

Gabe thanks for all the info. I have one more question though. I ride every Sunday. On every Saturday i race TT. My friends that have 2 strokes all freshin there's up with a new piston every 8 races and they do not ride these race bikes. I race and ride, so how often probably would I have to or need to put a new piston in? Also if I ever did put a piston in b/c it blew or just to freshin it up, do I have to go to a bigger bore or is there something that must happen to make me have to go to a bigger bore? Here's the thing. I want the 416 but if I have to go to a bigger bore after that with a new sleeve and all i know it's going to get a little bit expensive. So I would like to stay away from that as much as possible but if you think it's going to be a noticable difference then i think i will go ahead and go with the 416 and then if it's has to be bored after that maybe the 426 or 440.