If a guy does not want an Austrian 350 for one reason or another and a 450 is too much, is a big-bore kit on a 250f worth the cost? I could see the Redmoto 300 kit for the CRF250r/x being a potential worthwhile investment, but they no longer appear to be in stock anywhere from what I can see?
How do the 270/280 kits compare? Seems like a lot of money for not a lot more cylinder volume. I have a hard time believing there would be a significant enough improvement for the cost. Would an ECU, gearing and potential exhaust offer similar performance and/or better value?
I am asking for a riding buddy of mine who is not a member here. He's not much into forums and the internet, so thought I would ask and share some of the opinions or at least try and get him online and send him a link to the thread.
Why does it seem newer Honda's have a lot more big-bore options than newer KX's?
doesnt even come close, i wouldnt even waste my time or money, they actually are weak!
4 stroke heads are designed around the stock bore...
Honestly no. Bikes have so much research and development put into them stock. Some John smith sticking some mods on it will often make it run worse or have a shorter life.
if you were to have a 250 and want to significantly mod it then just get a 350, so on and so forth.
I’ve gone all the way with it in the past and now have totally stopped doing any modifications outside of maybe an exhaust that has factory support. Even that definitely doesn’t have the bang for buck you might expect.
yes you will talk to mouth frothers that rave about the amazing mods. Drag race them on a comparable bike and they will rarely ever pull away from you if at all.
Unless he doesn't have a bike and is asking the general question, it might be more benefiical to ask about a specific year/make/model because some respond well, others not so much and there are physical limitations on some that restrict the maximum displacement due to water jacket thickness, etc.
However, I wouldn't expect anything close to the performance a purpose-bult OE 350 would provide.
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just watch the HP racing video about the aftermarket big bore 250 mod. It was worse than stock 🤭
No to the big bore kit.
If you are saying a 450 is too much then a 250 will be just fine. I ride a newer KX250 and it hauls my ass around with power to spare. Modern 250 four strokes are typically enough for the average rider these days.
"I could see the Redmoto 300 kit for the CRF250r/x being a potential worthwhile investment"
Dirt bikes aren't an investment!
Yes they are.
Very poor ones.
Not sure why all the negative. Sounds like every bike (in this shootout) benefitted. This was the vid that came up first on YouTube, but if you do a “big bore 250f” search there is a ton. Lots of positive reviews.
Here’s one from MXA (since they’re pretty negative most of the time) that’s plain and simple:
Sounds like Cylinder Works plus Twisted Development is the way to go.
My 2009 KXF had an Athena 290cc kit on it, albeit mated to PC cams/valvetrain and modified carb. Put a 270cc kit on a 2015 Husqvarna FC for a friend of mine, with moto tassinari intake and Vortex ignition. Same 270cc kit on a 2018 CRF, but with cams and Vortex.
All of these bikes absolutely ripped and made much broader power than standard, but just bolting the kit on by itself may make it run worse - the extra bits and pieces and tuning on rolling roads is what made the bikes above perform really well. Zero reliability issues either.
I regretted my big bore kits (yam,kaw,hon). In every case I lost over-rev. Purchased a 350 and it ended up being the perfect motor. I can rev it to the moon or lug it like a 450 (especially with a longer header pipe).
So if you have a ‘22 250f with a trade in value of $3500 - 5K, and a ‘24 350 is around $10k, then you’re looking at $5000 - 6K difference to pay.
It sounds like a big bore kit with a company like Twisted Development setting it all up would be the best bet vs just bolting on the parts. So, if you’re willing to spend over $3000 then it might be worth it (Rough estimates). Or plop down the extra $6k for the 350.
What i can gather, doing bolt-on big bore kits kill the top end over-rev which is super important if you ride a 250F somewhat correct and you're a decent rider.
So anyone thinking about a big bore should probably calculate with the other costs of actually making it run well (ECU and a company that knows what they're doing).
I would steer away from big bore kits. Do the exhaust, ECU and basic headwork on the 250cc first.
I've ridden a lot of big bore and stroker 250F's in the early to mid 2000's and I got to ride many back to back and compare to stock bikes on the same track. I was amazed how much more bottom to mid end power just 2-3mm over bore gives a bike. Once you go more than 3-4mm over bore it doesn't actually lose much, if any top end, but because it adds so much more down low, it feels that way. I have not ridden a more current day big bore 250F so maybe things have changed.
Anybody that needs more from a current 250F needs to buy a bigger bike instead of dumping money into a hand grenade. Unless you need a 250F for class racing it's far from financially prudent.
Power spread and location is actually more important than the shear amount. Big bores suffer in this area many times.
Thanks all. I have passed on the link to this thread. I suspect he will drop in over the next few days to read the comments.
Worth the cost? That's a question that can only be answered by 1 person.
I'd expect a little more torque, maybe sacrifice some over-rev with a big bore.
I think a big bore 250 would be a really fun bike to ride. In my experience a pipe doesn't do much, gearing is always a trade off, never had a custom ECU tune.
270 vs 300, I don't know, there's a reason it's not legal to race a 270 in the 250 class.
Pit Row
your shoving a piece of shit recycled pop can aluminum piston in to your piece of shit recycled pop can cylinder on a nice bike... and expecting positive outcome...... Cylinder works, vertex, hot cams chinese bull crap, wiseco is part of that list as well.
best thing you can do to a 250F or 125 Is long rod the little bastard.
Did you have gunpowder with that whiskey cereal for breakfast today?
Years ago when a 250F had a carburetor, I rode a 270 big bore built by a shop in southern California.
I was impressed with how much faster it was than a stock 250F.
Other big bores I've ridden were terrible. The main difference in a good one is that everything needs attention. From the fuel system to the head, compression ratio and ignition timing.
Slapping a big bore cylinder on a bike is a waste of money. It's actually worse than the stock displacement in most cases.
Buying an engine package from a skilled tuner is an upgrade.
Zero chance I'd give up my 350F for a 270 or 290 or 300 big bore.
100% chance I'd buy a Japanese 350F, preferably a Kawasaki or Yamaha.
Depends on who you have do it. I have a 22 KTM 350 My buddy has a 19 YZF 270 that TZR did. While it's not as fast as my 350 It's really damn close I would take that bike over my bike any day just for the fact better suspension and feels lighter
Rode a big bore YZ250F that my local shop built and thought it was great. It had very good power. Im 165 lbs and clear everything but Im not particularly fast.
I remember a couple years ago Keefer recommended the big bore over a 350 for vet riders.
I ve been around many bb 250F's. If setup right they are a blast to ride. There is now a 300CC kit for Honda and Yamaha modern 250F's... as well. Fantic I think is using it to make a 300CC yamaha based bike.
https://www.athena.eu/en-us/big-bore-cylinder-kit-85-mm-300-cc-to-increase-performance-P179195.htm
Video... below.
https://www.athena.eu/en-us/big-bore-cylinder-kit-85-mm-300-cc-to-incre…
Didn't Daniel Blair state privateers in the 250 class all run big bores?
Only ones that can pull holeshots on Tony's bikes.
Fundamentally i agree. But if someone has the time and cash to keep a modded 250F going, do that instead of going the bolt on big bore kit.
Derek at HP Racing put a 276 kit on my 26 250sxf and it made 50hp. It had other stuff done to it as well but it ripped…at least according to the local pro that rode it when I picked it up. I sold it before ever riding it bc I decided to quit the sport.
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