Big bore or High comp. piston?

The ol' XR4 is about due for some new life. I'd like to hear some recommendations on what to do. I don't want a 440 but would bore to a 416 or 426 and a higher compression piston. I would like a bit more power, but don't want to mess (too much) with the durability I've enjoyed over the years. What's worked well for you guys?

I'm in the same shoes, done a lot of reading back in the forums, and decided I'll go with a 426 11:1. I already have a stage 1 cam - the PO set the valves too tight, damaging the stock cam.

As far as I've researched, everybody was happy with the 426. Also don't forget 440 is actually 435cc.

I'm about to order from XRsOnly - JE piston kit and Cometic gasket kit, 213 USD:

http://www.xrsonly.com/dirt-bike-parts-1/honda-dirt-bike-parts/honda-xr-models/honda-xr400r/je-pistons-honda-xr400r-piston-kit-426cc-88mm-11-1-compression

http://www.xrsonly.com/dirt-bike-parts-1/honda-dirt-bike-parts/honda-xr-models/honda-xr400r/cometic-top-end-gasket-kit-honda-xr400r-trx400ex-s-s-head-88mm-bore

Or Wiseco from Ebay? 200 USD.

http://www.ebay.com/itm/XR400-400EX-88mm-4mm-426cc-11-1-Wiseco-Piston-w-Gasket-/140537383002?pt=Motorcycles_Parts_Accessories&vxp=mtr&hash=item20b8ae485a

Any other, better suggestions please? Many thanks.

I have Xrs with all bores,stay away from hi comp.That is what can kill your xr.All of our big bore are trouble free even my own two 440s.Just the pumper carb alone made more power than big bore without carb.

I'd contact CW about the piston and your build. The 426 piston kit with about 10.5 to 1 would be my choice. A cam is a great complement for it as is some port work. That will make a real difference without hurting reliability.

Please clarify hi comp, 11:1 and above, or 11:1 is ok to go? Lowest fuel octane is 95 here, 98 costs almost the same.

I have Xrs with all bores,stay away from hi comp.That is what can kill your xr.All of our big bore are trouble free even my own two 440s.Just the pumper carb alone made more power than big bore without carb.
Please clarify hi comp, 11:1 and above, or 11:1 is ok to go? Lowest fuel octane is 95 here, 98 costs almost the same.

With that fuel 11 to 1 should be fine. A pumper carb will make about the same overall power but the throttle response is worth every penney. It will wheelie on comand.

Thanks, I forgot to write but I already have a pumper.

So with the correct fuel should I be worrying installing an 11:1? I'm not really to get the last bits of juice out of the bike, I would install then a 435.

As I can see from their websites, for 426 JE has 11:1 and 13:1, Wiseco has 10:1 and 11:1.

426 10:1 w/ a pumper or even better 42mm bsr is the ticket!

10:1 tech is high compression for a xr4 considering coming up from stock compression.

426 10:1 w/ a pumper or even better 42mm bsr is the ticket!

10:1 tech is high compression for a xr4 considering coming up from stock compression.

He gets fuel good enough to run an advertised 11:1 piston no problem. :lol:

The true ratio will be slightly lower. A nice cam also works great with high compression pistons because the increased duration will lower the cylinder pressure to about what a stock cam and an advertised 10:1 piston would have.

This combo has even more power and torque, the higher compression-cam combo is the way to go. Add some portwork as the icing on the cake. :bonk:

Durability will be still be great and you'll love the extra power. :busted:

:busted:

Bore or compression???? A little more of both. Too much of either is where you run into complications. Otherwise go stock bore with 12=1 or 426 10-1 compression. Most all pistons are highly overated on compression. We have researched them all over on the XR600 forum and found that they can be as much as a full point lower than advertised.

426 with a 10:1 compression sounds like what I want. But now I'm thinking I could put off the rebuild untill next winter and start with getting a pumper carb. It would be nice to lose that little bog that only seems to show up at the worst times. Like when cracking the throttle to pop the front wheel over a ditch/washout only end up with the front wheel in in the ditch and a gas cap in the....:busted:

I've just ordered a 11:1 426 Wössner from the local distributor, along with a Cometic gasket set. Should be here in 2 weeks, I'll report back.

While I'm at it, should I be looking at doing some port job? +1mm valves? For both intake and exhaust? Springs? What else here?

I've just ordered a 11:1 426 Wössner from the local distributor, along with a Cometic gasket set. Should be here in 2 weeks, I'll report back.

While I'm at it, should I be looking at doing some port job? +1mm valves? For both intake and exhaust? Springs? What else here?

With the cam you have i would replace the valve springs.

+1 valves with porting and a good 5 angle valve job will make it a beast. It will make some very nice HP numbers and have a wicked torque curve.

You might replace the keepers and maybe the retainers for lighter ones so all the valvetrain is new.

Otherwise just make sure all the parts you are reusing are in great condition.

I beleive in doing right or not doing at all. :busted:

Thanks, I forgot to write but I already have a pumper.

So with the correct fuel should I be worrying installing an 11:1? I'm not really to get the last bits of juice out of the bike, I would install then a 435.

As I can see from their websites, for 426 JE has 11:1 and 13:1, Wiseco has 10:1 and 11:1.

11:1 is getting up there on an XR.

I'd stay with 10.5 as a top end, 10.0 to be 'safe safe'. :lol:

(don't think euro gas uses the same octane ratings as us)

TNTsXLR, can you please explain this a bit, many thanks.

The true ratio will be slightly lower. A nice cam also works great with high compression pistons because the increased duration will lower the cylinder pressure to about what a stock cam and an advertised 10:1 piston would have.

Aftermarket pistons rated compression seems to run about .5:1 lower then they advertise.

A larger camshaft has increased duration. This increase causes the intake valve to stay open longer which effectively shortens the duration of the compresson stroke. Which reduces the cylinder pressure. This is why big cams need higher compression pistons to offset the pressure lost due to the cams longer duration profile.

With the correct cam an 11:1 piston has the same cylinder pressure as a 10

:1 piston and stock or small camshaft. This said it will make more power since the cylinder is filled with a larger amount of air and fuel. Both engines will run on the same fuel, one will just be faster.

Thanks, make sense, I just wasn't expecting the intake to remain open even a bit during the compression stroke.

11:1 is getting up there on an XR.

I'd stay with 10.5 as a top end, 10.0 to be 'safe safe'. :lol:

:)

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