overbore vs. higher compression

[hardcharger]

ok so I have a 2mm over motor, not making bad power. but I am curious the diferent advantages and disadvantages of boring over a motor and building one on just high compression. how are the powerbands and rev characteristics diferent

[MetalScorpio]

oh...BTW, you have high comp. and big bore

[04upon1]

I want one, anyone want to trade

[biger]

higher compression only & over rev.

[MetalScorpio]

I have a 07 engine that is a spare right now. my friend and I were thinking of doing it over and selling it but not sure if i want to do it over size or just high revs.

I'm going to open a pool later to find out what do you guys think will sell.

[diel11]

This could be a long article. 2mm over will give you easy power, lots of torque. Some of the problems I've seen and heard of is nikasil flaking, bores cracking, head gasket leaks. On top of that, unless you use a "special" piston your compression ratio will only get to 13.5:1 (according to JE and theirs is always off). If you want to use any of the "strong" HRC cams that's not enough. If you decide to mill the head to get more compression, you will run out of ptv, which you can fix by machining the valve pockets but then that kills the compression you just got by milling. Depending on the weight of the pistons, then you also have to be careful how you set the pth clearance as you can get rod stretch and the pistons will make contact.
Std bore motors are a bit more susceptible to set up and assembly. Depending on what power output you're looking for, they need a very good head. You can run them all the way up to 15:1 with pump gas (depending on which pistons you use), and rev them up to 13,750 all day long. There's not a lot of milling done or needed, and you can use the full range of HRC cams from 04-07. The main problem with these motors is that although they are very very reliable, their longevity is not anywhere close to a stock motor. They also make a lot of heat. BTW, we are talking about a full blown SBK motor. You can get good power out of one of these motors without going the full blown way.
One last thing to think about is what are you going to use the motor for. The main reason the 06/07 SBK bikes got faster around the track had nothing to do with peak hp. The 05 spec bike was a monster to ride, way too much torque down low and mid. The 06 spec lost a lot of hp and torque down low but gained peak. There's a fine line between lots of torque and ending up on your head, proven by the design change by Honda and TKR. All the new SBK spec stuff is designed to keep the torque down.
They did the same thing with the 08 SBK racer. The works racer will spin up to 15,000rpms and uses cam duration that is even more than the 07 spec. The customer SBK will spin up to 14,500. Main change is the compression. They will run lower compression than 04-07.

[MetalScorpio]

great information shared man. Thanks.

[04upon1]

Thanks Diel, so what should we do if we are looking for the happy medium between sick power and good reliability and longevity. I Have done all I can (except HRC ECU) to the outside over the engine. All that is left is to crack her open. IMO she already runs like a raped ape, but in time I will get accustomed to this power and will crave more. Therefore knowing this will come I will need to set the plans in motion now. All at the same time I don't want a ridiculous motor that for the most part isn't ridable.

[Cmac]

If your running a double duty bike (street / Track) then going SBK route of high rpm and power pushed up high in the rev range does not make sense. The overbore motors make great torque and linear power increases over stock. I used a custom Web cams that stopped producing power just before red line. With an OEM ECU and leaking throttle body boot it put 180 at the wheel before 12,000 rpm. To me that means a lot less valve train abuse.

Now that the new track in NJ is open I may regret limiting my revs and power range, but for street and Laconia's short and tight track the power is perfectly positioned for me.I stiffened up the rear spring some to try and keep the front planted out of the corners but that created more problems then it solved. I may end up ditching the HRC throttle and if I cannnot get better (more fluid) transitions this year.

[04upon1]

Shaty Cmac, I love that throttle, really helped my smoothness and transitions. I am trying to figure this out because I might buy a second bike for street only and turn the '04 to track only. But since motors aren't cheap and I can't seem to win the Lotto I will still have to build a motor that lasts.

[diel11]

See? That's the main thing that you have to look at when building a motor. Cmac has hit the nail in the head. When it comes to building a motor, the first thing that you have to figure out is what you want the motor for, then a quick second is how much money you want to drop. For street, the 04/05 HRC SBK spec cams work great, or some of the Webcams. They will make the bike a freaking monster. On the track, the bike will become a lot easier to ride with peak hp and not a lot of mid torque. And even that depends on the rider. There are guys out there that can ride a moped and still go fast.

I keep harping on this but you can really build a good motor with just 04 cams, some milling and headwork. And the best part is the cams are not super crazy so you can use stock valvetrain other than the intake springs. Best thing about this set up is it will still work well down low and also on top. HRC ecu would be needed and the pricing would be pretty low compared to a full motor build.

[diel11]

Cmac, have someone look at your ignition curve. Might not help with power per say but it can be used to smooth out power delivery. You are most likely seeing a bit of the 2mm over "too much torque" problem. Might need to get an ignition module or HRC ECU.

[Cmac]

Originally Posted by diel11

Cmac, have someone look at your ignition curve. Might not help with power per say but it can be used to smooth out power delivery. You are most likely seeing a bit of the 2mm over "too much torque" problem. Might need to get an ignition module or HRC ECU.

I have not done anything with the ignitioon curve. Believe it or not its more of a problem when you over-estimate the speed of a corner and find yourself lower in the RPMs then you thought. Your brain tells you to grab a handfull of throttle and the bike is a little too happy to comply. I tried using using a 16 tooth front to tame it, but didn't like the way that felt over-all so i went back to 15/42.