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High Compression vs. Big Bore

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Doing some research for plans on a winter project. I have an 08 KX250F that needs a little TLC and while I'm in there I want to do it up right. This bike will be mostly trail ridden with an occasion trip to a MX here and there.

Which would be the best in trail/woods riding. A high compression piston or a big bore?

I've noticed the big bores all seem to have lower compression than stock. I know compression can help with bottom end, but so does CCs. I'm not concerned about the need for to run race gas if that's what it will take to achieve what I want. Looking for some more bottom end and lugging ability, but still be able to let it loose in the straights. Technical advice and first hand experience appreciated.

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Big bore kits will make more torque but sign off earlier than the standard bore, High compression pistons make more power but also more heat.

so it's a trade off.

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Big bore will have more bottom end and stall less compared to a high compression setup. Also less strain on the motor with a big bore.  

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Big bore kits will make more torque but sign off earlier than the standard bore, High compression pistons make more power but also more heat.

so it's a trade off.

The question is then how much more heat does HC piston bring along with it in trail riding and not always wide open? How much earlier does a larger piston sign off at? This I'm asking of people who have done one or both mods and rode with them.

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not sure on the heat factor, if you run some race fuel it's marginal. On the big bore kits from the write ups I have seen they sign off 1000-1200 rpm lower than stock ( meaning the power curve )

so they have more torque make more hp but the power range is cut back, ( can be adjusted with gearing )

Let me look up the testing for the big bore setup.

Al

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I read through this thread at length and since I have modified 3 KX250F's to date maybe my input will help. I had a Athena KX290F. That bike did better where more torque was needed but suffered on over heating and it's ability to rev like the stock engine. Since I rode 90% off road that suited me and the fact I was able to add a small fan to the radiator helped some. The KX262F kit I ran on my son's bike for MX Mod Class and Off-Road seemed to be the best of both world's. The bike would still rev out like the stocker and had a little more torque. The best MX setup we ever ran was stock compression OEM piston with a ported head and Del West Titanium valves. That engine would sing and did not break. If you can ride the bike in the upper power band stick to the stock compression with a ported head. If you are a short shifter that likes torque buy a big bore and ride the bike all over the track and shift between 3rd and 4th gear. It is more about your riding style and where you will ride that bike. If you ride more trails the big bore is probably your best bet, but if you relish quick revving power on the track the middle of the road option may be your ticket (a smaller big bore and mod the head).

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Hi, I have a similar, follow-up question. I have a 2011 kx250f that I also ride offroad. I traded a 2011 yz450f for this with the idea it would be lighter and not as much in the woods. I'm finding I'm missing the torque and bottom-end hit of the 450. Now I know 250fs aren't known for bottom-end, but I have a question for this group. I'm looking for increasing the bottom-end, torque, but I really don't want to have to run high-end race gas and I want the bike to still start easy & be reliable. Could I achieve this by simply adding a cyclinder works big bore kit (no stage 1 or 2 cams and keep stock compression)? And would I need an ecu remap? Thx

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I reworked a 2011 kx250f and had some advice.

first I pulled the head and straightened out the ports, Meaning I blended all the port angles to improve flow, ( not gouge out the ports) just take out imperfections that are due to mass production. Polished the exhaust ports. Mainly for carbon to not build up so fast and improve flow out.

installed a Mega bomb head pipe, slightly higher compression piston. (Keeping within the range of Hi oct pump gas) Remapped the ecu with a map from a TT member who is known for remapping. Took his numbers and tweaked them a bit and the bike was a totally different machine. 

plenty of bottom end punch, mid range , and massive over rev. I built it to run right up on the 450s we race against in open class.

gearing we used was 49 rear and stock front countershaft.

Main problem we ran into was this bike ate clutches in no time, even Hinson stuff. Could be rider fault I suppose.

PM me if you need some help on the mapping etc.

Al

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I ran a athena 290 kit in my 07 kx250f with a full fmf 4.1 exhaust. The 290 has tons of bottom end but revs really slow. It threw rocks like 450f, but it had to wind up like a 2stroke that was getting on the pipe. Overheating is a major issue with this kit and for woods riding you would need a high flow water pump and oversized radiators just to keep it from boiling over. Jetting is tough to dial in on the 290 and if you are riding technical trails you have to get the jetting spot on to keep the engine from being unresponsive. jetting also played a key roll in keeping the bike cool. The kx290f was awesome on a wide open mx track, and that was just about the only place it didnt try to overheat. I eventually had my 07 destroy valves due to the constant high temperatures from riding the 290 all summer long when it was 90+ degrees out.  

 

After the 07 died I picked up a 2008 and installed a vertex 13.9:1 Hi-comp piston and my fmf exhaust off the 07. The hi-comp pulls more bottom end than the stock engine but required race fuel. I never had any issues with the hi-comp setup overheating, I'm guessing because i was running 102 octane fuel. 

 

Honestly thought, I feel like reliability went away pretty quick as soon as you start doing these kind of mods. A stock 08 with an exhaust system and team green jetting produces pretty good power. Not to mention adding 2 teeth to the rear sprockets gives it a lot more bottom end and lugging ability. I think the 08 was a seriously under estimated bike, With proper jetting they run better than most of the neglected poorly running 2009/2010 kx250f that i see around.

 

If you have already played with gearing and are set on doing some engine mods i would vote for Hi-comp piston rather than big bore. 

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Thanks for all the replies. I think I'm leaning more towards the HC over the big bore. Mostly because of heat and cost. My current cylinder is good still so I'll prob just through a piston in and do up the head nice.

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if running race gas isn't an issue to you, y not run both. big bore and high compression. my yz400f runs a compression ratio of 14.7:1 with oxygenated race fuel. no pinging, runs smooth and out performs the most modern 4t's.

of course that will require head and or cylinder to be machined.

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ive also been looking at doing a big bore. I would go no bigger the cylinder works 270 kit. for my 2012 i have to remap due to the extra injector. They say atleast on the newer bikes it doesnt steal any revs off the top. I hear cams work best with the bigger kits but it still doesnt rev out as much.Anyway about $350 off ebay and you have a kx270f.....now i dont know how they work on the older kx250f. Dirt rider is building a 2013 online. Id search for your year

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ive also been looking at doing a big bore. I would go no bigger the cylinder works 270 kit. for my 2012 i have to remap due to the extra injector. They say atleast on the newer bikes it doesnt steal any revs off the top. I hear cams work best with the bigger kits but it still doesnt rev out as much.Anyway about $350 off ebay and you have a kx270f.....now i dont know how they work on the older kx250f. Dirt rider is building a 2013 online. Id search for your year

the power gain of a 20cc increase will be so minimal u would need a dyno to pick the difference. although its only $350 its poor bang 4 buck

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the power gain of a 20cc increase will be so minimal u would need a dyno to pick the difference. although its only $350 its poor bang 4 buck

They said it made a big diffrence on the Honda..haha but thats a honda

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They said it made a big diffrence on the Honda..haha but thats a honda

maybe they did say that. I call it the placebo effect. ur talking an 8% increase in displacement. 

spend the money on an aftermarket CDI and ignition coil. makes a huge difference. best bang 4 buck.

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