17-18 KTM 250SXF big bore owners? 18 CRF250R big bores

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11/13/2017 5:48 PM

Since these 2 bikes have relatively large valve sizes and ports, I’m considering building a 270 or 280cc motor with that head configuration; either may be a nice midrange engine? Anyone have any experience with the KTM? I’d think the Honda is too new for that kit yet...

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11/13/2017 6:22 PM

Harris performance has ktm big bore dynos

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11/13/2017 6:31 PM

Find someone who will stroke it instead. Better all around instead of just the middle. The Honda has to already be pushing it with the piston weight/ stroke length ratio. A 270 stroker would probably be amazing on those two engines in particular.

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when in doubt, pin it out

11/13/2017 6:59 PM

So much more work and money, and I do not even know if people make stroker kits for modern 250f's. It would probably kill the revs so bad that the big valves would not be used to their potential.

Have you considered a tree fiddy?

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Tomac and/or Anderson for 2020.....

11/13/2017 7:20 PM

I have a Harris Performance 270 kit in my 2016 250SXF. Amazing. Really pumps up the low and mid and retains most of the amazing top-end rev of the 250.

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Motocross....a once in a lifetime experience....every weekend.

11/13/2017 7:25 PM

mxjeff575 wrote:

I have a Harris Performance 270 kit in my 2016 250SXF. Amazing. Really pumps up the low and mid and retains most of the amazing top-end rev of the 250.

Is it a big bore or stroker kit?

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11/13/2017 7:28 PM

mxjeff575 wrote:

I have a Harris Performance 270 kit in my 2016 250SXF. Amazing. Really pumps up the low and mid and retains most of the amazing top-end rev of the 250.

Joko wrote:

Is it a big bore or stroker kit?

Bore only. I'm amazed 20cc can add that much to the low and mid.

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Motocross....a once in a lifetime experience....every weekend.

11/13/2017 9:28 PM

gsxr6 wrote:

Find someone who will stroke it instead. Better all around instead of just the middle. The Honda has to already be pushing it with the piston weight/ stroke length ratio. A 270 stroker would probably be amazing on those two engines in particular.

I'd say our ktm big bore isn't "only better in the middle"

[LINK TO IMAGE]

percentage wise - it's equally better everywhere.


KTM's massive intake port, and large valve sizes with cam lobes that are like japense 450's (actually larger) make for an engine that can support more CC's.

The new honda will likely be very similar. The short stroke, large bore, large valve combo from honda is likely going to love a big bore....

The issue with stroking becomes much more challenging too. Pistons already have the wrist pin as high as it can go, nearly into the oil ring.
Rods are generally quite short for packaging purposes and weight...and in the case of the honda, with a stock rev limit of 14500 - adding piston speed due to longer stroke is risky business for the average consumer.
Not the mention the crank will become less supportive of the rod pin, and potentially cases to clear a useful stroke addition to get meaniful CC's. The added piston speed would be great for the large valve/likely large port honda will have for air flow - but the larger piston does the same thing.....

Depending on how the cylinder looks - we may entertain a big bore for the honda. The ktm kit has sold really well to vet customers and offroad guys (even though I totally agree a 350 makes a lot of sense)

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11/13/2017 9:47 PM
Edited Date/Time: 11/13/2017 10:04 PM

Curious, what phenomena causes a dip then a 2.5hp increase between 12,000-14,000 ? Are those numbers on pump gas?

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11/19/2017 10:53 PM

Derek Harris wrote:

I'd say our ktm big bore isn't "only better in the middle"

[LINK TO IMAGE]

percentage wise - it's equally better everywhere.


KTM's massive intake port, and large valve sizes with cam lobes that are like japense 450's (actually larger) make for an engine that can support more CC's.

The new honda will likely be very similar. The short stroke, large bore, large valve combo from honda is likely going to love a big bore....

The issue with stroking becomes much more challenging too. Pistons already have the wrist pin as high as it can go, nearly into the oil ring.
Rods are generally quite short for packaging purposes and weight...and in the case of the honda, with a stock rev limit of 14500 - adding piston speed due to longer stroke is risky business for the average consumer.
Not the mention the crank will become less supportive of the rod pin, and potentially cases to clear a useful stroke addition to get meaniful CC's. The added piston speed would be great for the large valve/likely large port honda will have for air flow - but the larger piston does the same thing.....

Depending on how the cylinder looks - we may entertain a big bore for the honda. The ktm kit has sold really well to vet customers and offroad guys (even though I totally agree a 350 makes a lot of sense)

Derek, can you tell us which head mod or mods have been done to the bike from this dyno chart?

I'm interested in your big bore kit for my 2017 Husky FC250, I see you offer the kit for $700 but then it looks like the head mods can cost twice the amount of the kit.

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11/19/2017 11:18 PM

Twisted Development did a high comp piston with VP T4 and vortex ignition fully mapped on my KTM 250F and is pushing close to 50HP. Pulls until forever! I’m currently at 33hours on my second piston, gonna go in for a rebuild soon.

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11/20/2017 6:08 AM

I have a 2016 250 SXF with Derek's 270 kit and a reworked head. The bike also has an R&D throttle body w Vortex ignition. If I could figure out how to post the pic of the dyno curve I would, but let me just say the gains are pretty incredible. I think peak hp on this bike is right around 53 and the torque jumped up a couple of steps across the whole curve!

So while I was getting the 270 set up, I was riding a 2017 350, which I loved. The motor and handling were very good for me once I geared it down, but since I had the 270 waiting, decided to sell it.

I finally got the fork issue worked out on the 270, it had 4cs but I did a SSS conversion. Only a couple rides now, but I have to say that I may like it more than the 350. Definitely more power and torque across the entire power curve than any 250 I have ridden and in some ways I think it is easier to ride than the 350, because you ride it that extra bit more aggressively. Can also pull tight rutted corners with minimal need to clutch it

Derek knows how to get the most of these bikes and I can recommend his services, although Derek, you still owe me a side case and one of my motor mount bolts has a stripped head!

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11/20/2017 6:24 AM

Save yourself the time and money, just get a 350

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11/20/2017 6:45 AM

gsxr6 wrote:

Find someone who will stroke it instead. Better all around instead of just the middle. The Honda has to already be pushing it with the piston weight/ stroke length ratio. A 270 stroker would probably be amazing on those two engines in particular.

Derek Harris wrote:

I'd say our ktm big bore isn't "only better in the middle"

[LINK TO IMAGE]

percentage wise - it's equally better everywhere.


KTM's massive intake port, and large valve sizes with cam lobes that are like japense 450's (actually larger) make for an engine that can support more CC's.

The new honda will likely be very similar. The short stroke, large bore, large valve combo from honda is likely going to love a big bore....

The issue with stroking becomes much more challenging too. Pistons already have the wrist pin as high as it can go, nearly into the oil ring.
Rods are generally quite short for packaging purposes and weight...and in the case of the honda, with a stock rev limit of 14500 - adding piston speed due to longer stroke is risky business for the average consumer.
Not the mention the crank will become less supportive of the rod pin, and potentially cases to clear a useful stroke addition to get meaniful CC's. The added piston speed would be great for the large valve/likely large port honda will have for air flow - but the larger piston does the same thing.....

Depending on how the cylinder looks - we may entertain a big bore for the honda. The ktm kit has sold really well to vet customers and offroad guys (even though I totally agree a 350 makes a lot of sense)

jbrown15 wrote:

Derek, can you tell us which head mod or mods have been done to the bike from this dyno chart?

I'm interested in your big bore kit for my 2017 Husky FC250, I see you offer the kit for $700 but then it looks like the head mods can cost twice the amount of the kit.

The posted chart is just mapping and big bore with a throttle body kit from twin air

I have also have a chart published with 270 kit with head and cams and a dual injection set up with GET system mapped properly that is well above 50

Proper porting on the ktm requires lots of work and isn't just grinding away material.. if we offer it by hand on a new un used head it's 550, no assembly.

Dollar per dollar the kit is a killer value for the gains, porting and cams are big budget R&D items that I have invested way too much into for the avg joe market (pro race market really only avenue now that can afford and needs high level development on these new engines) luckily a few have invested to get the best they can it allows our r&d process to advance forward every year.

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11/20/2017 6:49 AM

stone881 wrote:

I have a 2016 250 SXF with Derek's 270 kit and a reworked head. The bike also has an R&D throttle body w Vortex ignition. If I could figure out how to post the pic of the dyno curve I would, but let me just say the gains are pretty incredible. I think peak hp on this bike is right around 53 and the torque jumped up a couple of steps across the whole curve!

So while I was getting the 270 set up, I was riding a 2017 350, which I loved. The motor and handling were very good for me once I geared it down, but since I had the 270 waiting, decided to sell it.

I finally got the fork issue worked out on the 270, it had 4cs but I did a SSS conversion. Only a couple rides now, but I have to say that I may like it more than the 350. Definitely more power and torque across the entire power curve than any 250 I have ridden and in some ways I think it is easier to ride than the 350, because you ride it that extra bit more aggressively. Can also pull tight rutted corners with minimal need to clutch it

Derek knows how to get the most of these bikes and I can recommend his services, although Derek, you still owe me a side case and one of my motor mount bolts has a stripped head!

Ah yes let me get that out

On the side case

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11/20/2017 12:38 PM

Derek Harris wrote:

The posted chart is just mapping and big bore with a throttle body kit from twin air

I have also have a chart published with 270 kit with head and cams and a dual injection set up with GET system mapped properly that is well above 50

Proper porting on the ktm requires lots of work and isn't just grinding away material.. if we offer it by hand on a new un used head it's 550, no assembly.

Dollar per dollar the kit is a killer value for the gains, porting and cams are big budget R&D items that I have invested way too much into for the avg joe market (pro race market really only avenue now that can afford and needs high level development on these new engines) luckily a few have invested to get the best they can it allows our r&d process to advance forward every year.

Wow ok, so that posted charted is simply the bolt on big bore kit, ecu and throttle body kit. The kit is $700, what's the cost of the ECU and throttle body kit. I'm really interested in this for the cost.

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11/20/2017 4:23 PM

jbrown15 wrote:

Wow ok, so that posted charted is simply the bolt on big bore kit, ecu and throttle body kit. The kit is $700, what's the cost of the ECU and throttle body kit. I'm really interested in this for the cost.

That chart is still oem ecu just remapped
The throttle body blade kit is like 240 retail and mapping is 100 on oem ecu

It gets pricey past that quickly.


Next level item I think people don't put enough value on is the ecu.... for the little more price of a full ti exhaust system for a honda, we have the GET dual injection set up tuned in really well and is a great addition. Proper ecu tuning from knowledgeable shops such as aforementioned Ellis or dean or PC etc can really do a lot in feel and character of the bike, plus unlock that last bit of power. I'm a GET guy... you kinda choose one or the other from a cost of the tuning software aspect... With Launch control, traction assist, and wifi tuning for simple map changes, it very useful for me.

I've worked with the vortex stuff and find it to be extremely good too

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11/20/2017 5:58 PM

Derek Harris wrote:

That chart is still oem ecu just remapped
The throttle body blade kit is like 240 retail and mapping is 100 on oem ecu

It gets pricey past that quickly.


Next level item I think people don't put enough value on is the ecu.... for the little more price of a full ti exhaust system for a honda, we have the GET dual injection set up tuned in really well and is a great addition. Proper ecu tuning from knowledgeable shops such as aforementioned Ellis or dean or PC etc can really do a lot in feel and character of the bike, plus unlock that last bit of power. I'm a GET guy... you kinda choose one or the other from a cost of the tuning software aspect... With Launch control, traction assist, and wifi tuning for simple map changes, it very useful for me.

I've worked with the vortex stuff and find it to be extremely good too

I actually had a 2016 FC350 but the front fork was terrible and I was having a blast chasing my son around while he was ripping on his 65 2-stroke. So I went and traded the 350 in on a 2017 Husky TC250, but with having a couple arm injuries I realized I couldn't hold onto the TC250 without some series arm pump and a weak grip. It was a bad recipe, I could ride fast for 2-3 laps and then my arms where done.

So I got a really good deal trading in the TC250 on a 2017 FC250, it's a fun bike that I can ride at a good pace for most of the day now without having the same arm problems. I got such a good deal on it that it almost didn't make sense to get another 350. So after seeing your kit it's really peaked my interest. If I could get the power your chart has which looks to give me allot more bottom for just under $1100 I'd definitely be down for that.

I actually love ripping around on the FC250 even though I've always owned 250 2-strokes or 450's.

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3/14/2020 3:48 PM

mxjeff575 wrote:

I have a Harris Performance 270 kit in my 2016 250SXF. Amazing. Really pumps up the low and mid and retains most of the amazing top-end rev of the 250.

I know that this is a really old thread, but after rebuilding my top end I am having trouble with a leaky head. Have checked that the head is not warped, replaced water pump seals and rebuilt it 3 times now. Am thinking that I must have a head gasket that isn't thick enough.

mxjeff575, have you rebuilt your 270 and if so what head gasket did you use?

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3/14/2020 4:25 PM

stone881 wrote:

I know that this is a really old thread, but after rebuilding my top end I am having trouble with a leaky head. Have checked that the head is not warped, replaced water pump seals and rebuilt it 3 times now. Am thinking that I must have a head gasket that isn't thick enough.

mxjeff575, have you rebuilt your 270 and if so what head gasket did you use?

I’ve used both the Harris and cylinder works (when I was in a time crunch) top end gasket kits with good results. Not sure what your problem could be if nothing is warped or torqued properly.

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3/14/2020 4:30 PM

mxjeff575 wrote:

I have a Harris Performance 270 kit in my 2016 250SXF. Amazing. Really pumps up the low and mid and retains most of the amazing top-end rev of the 250.

stone881 wrote:

I know that this is a really old thread, but after rebuilding my top end I am having trouble with a leaky head. Have checked that the head is not warped, replaced water pump seals and rebuilt it 3 times now. Am thinking that I must have a head gasket that isn't thick enough.

mxjeff575, have you rebuilt your 270 and if so what head gasket did you use?

stone881 - I have rebuilt it, when i did i just ordered the piston and gaskets from Harris. You said your head tested flat, did the top of the cylinder test flat? Good luck man!

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Motocross....a once in a lifetime experience....every weekend.