Modified YZ250F Vs. Big Bore YZ270F

Modified Mid-Sized Comparison

Modified YZ250F Vs. Big Bore YZ270F
Which one would you pick? The Twisted Development modified YZ250F and Twisted Development/ Cylinder Works YZ270F are a blast to ride. Coming from the depths of eBay Jay Clark saved these two Yamahas and totally re-furbed both into these beauties.Photo by Drew Ruiz

We get asked this question a lot in our mailbag: What should I do to my 250 four stroke? Modify the head or drop a big bore in it? Which is better? Well, when we asked Jay Clark to build us two Yamaha YZ250F’s to do this comparison, he already had purchased two beat up 2015 Yamaha YZ250F’s off of eBay to fix up for him and his son to ride. We asked Jay to modify the engine (with reliability in mind) on the standard bore machine and drop a big bore cylinder in the other to see if we could decipher on which engine character would be the most fun to ride/race. Jay trusted Jamie Ellis from Twisted Development to facilitate all the engine work on both machines, but used a Cylinder Works 270cc big bore kit inside the second Yamaha to compare the oversize cylinder against the standard bore. We wanted to do things as equal as possible to both machines (minus the big bore) so Jay and Twisted Development used the following parts on both Yamaha’s to ensure a uniform set up: Vortex ignition, a dual fuel injector system with a YZ450F airboot, FMF 4.1 muffler system, Cycra plastic, Dunlop MX3S tires, Race Tech suspension, DeVol pull rod, stage one Hot Cams, Crankworks rod kit, Hinson clutch kit, Tusk oversize rotor, Renthal bars and sprockets, Works Connection perch, MotoSeat seat cover, ICW radiator strengthening, Scar titanium footpegs, Uni Air Filters, Twin Air Powerflow backfire eliminator, and to make them look race ready Magik Custom Graphics finished off both machines with their customizable graphic kits.

Big Bore YZ270F
If you think that big bores can’t rev think again. With the modifications that Twisted came up with the 270 had plenty of over-rev.Photo by Drew Ruiz

Twisted Development/Cylinder Works Yamaha YZ270F:

The good news about the Cylinder Works YZ270F is that is still looks and sounds like a YZ250F, but the only difference is what you will feel out on the track. Since we are very familiar with a stock YZ250F and how good its stock engine character is, we were hoping adding 20cc’s wouldn’t take away from the easy to ride yet very torquey feel of the standard Yamaha’s character. If you would of saw our smile from underneath our helmet once we started riding the Cylinder Works 270 you would of known right away that adding the big bore kit did nothing but make the power character better. Rolling on the throttle there is even more torque and pulling power out of corners. You can ride this bike very similar to a 350cc or even 450cc machine. With the added power we noticed that the stock 13/50 gearing needed to be changed, as it would go through the power too fast in second and third gears. We switched the gearing to a 13/48 to help improve second and third gear spacing so that it was very useable for a novice to pro level rider through corners. Mid range “meat” as I like to call it, pulls so strong that it doesn’t require you to downshift to second gear in corners and let our vet test rider be lazier around the track. Usually with big bore machines top end is great, but lacks over-rev. With the Vortex ignition installed it made for a pleasant revving experience. The Yamaha pulls strong all the way through the mid range and once you think you have to make a shift you can let it sing for another few hundred feet before having to shift. Rarely do I find myself in fourth gear on 250F’s, but the 270 I found myself in fourth gear on more than a few occasions and it would actually pull that gear and not fall off. This engine configuration kept its excitement down low (throttle response), increased its pulling power and top end/over-rev.

Big Bore YZ270F
Railing corners is one of the best parts about running a big bore engine.Photo by Drew Ruiz

Race Tech Suspension (Both Bikes):

The stock KYB suspension is arguably the best suspension on a production bike today, but can get soft when adding power to the engine, thus making it easier for the rider to push the limit of its intended valving. The Race Tech re-valved suspension helped us with our newfound aggressive riding style (due to the increased horsepower) and kept the comfort of the stock KYB valving set up. The fork had great initial plushness over small acceleration bumps, while on de-cel the fork had better hold up and didn’t dive as much as the stock valving. Mid-stroke feel was firmer and had more damping feel, but still was able to move in the stroke enough without leaving us with a harsh feel on square edge. The shock although not as impressive as the fork was still comfortable enough for us to go fast around a rough chopped out track. The end stroke was a little soft for our liking when landing off of jumps, but kept a high comfort level when exiting turns with holes and acceleration bumps.

Twisted Development modified Yamaha YZ250F
Just because it is only a 250cc’d engine doesn’t mean this Yamaha wasn’t really close in performance to the 270 on the track.Photo by Drew Ruiz

Twisted Development Standard Bore Yamaha YZ250F:

We have ridden plenty of other modified Yamaha 250 engines over the past few years and with some good results. However, the Twisted Development modified engine with its dual fuel injector kit might be one of the best. The dual injector is something that Kawasaki has been doing for a while and Twisted Development is incorporating the second injector in most of its modified bike builds. The second injector gives the YZ250F instant throttle response and if you’re not ready for it can take you by surprise. The roll on throttle delivery is instantaneous and is incredible when railing a soft, loamy berm. Pulling power through the mid range was very broad and had great rpm response, but also left us with plenty of rear wheel traction and minimal wheel spin. Top end and over-rev were very impressive and never left us wanting more pull trying to get over large obstacles. The FI mapping should also be mentioned as well; the Twisted Development engine ran very clean and never once backfired or had a de-cel pop when chopping the throttle. With each throttle turn it felt like our right hand was connected to the rear wheel and that gave us confidence to twist the throttle even more.

Twisted Development modified Yamaha YZ250F
The Twisted Development modified Yamaha YZ250F is a work of art. Dual Fuel Injectors is a unique modification that Jamie Ellis has mastered on the Yamaha’s.Photo by Drew Ruiz
Big Bore YZ270F
The Cylinder Works 270 kit will make any lazy rider feel like a hero out of the corners. It has tons of torque!Photo by Drew Ruiz

Comparing The Two:
It was very close, but at the end of the day we really like the way the Cylinder Works big bore pulls out of corners and can still rev to the moon. The combination of both (bottom end and over-rev) makes it a very fun machine which gave testers confidence in attacking the track. The Twisted development modified standard bore machine has better throttle response and revs out slightly farther, but just lacked a little less bottom end. When we say a "little less" we are talking about riding both bikes back several times in order to really dissect where one was better than the other. It was that close! The good news is the cost of both machines is near the same amount (once added up) and it would really be up to your style of riding on which way you wanted to go. In our opinion neither is a wrong choice as both machines are plenty fast enough for any size and ability of rider. Kudos to Jay and Jamie for building two great Yamaha's that was insanely fun to pound out laps on.

Twisted Development modified Yamaha YZ250F
Twisted Development modified Yamaha YZ250FPhoto by Drew Ruiz

Second Opinion:

Both the standard Twisted Development YZ250F and big bore Cylinder Works YZ270F, race bikes were very impressive. The standard bore showed very noticeable power increase over the stock motor set up (as I own a stock YZ250F). I felt this bike had a very stout bottom end hit that pulled well through the mid-range. Both bikes are equipped with Vortex ignitions and a second fuel injector that I personally have never ridden with. The bottom and mid-range power and torque increase was probably most noticeable for me. Being that both bikes had Vortex ignitions, I felt they revved much further than a stock ignition and re-map combination. The big-bore definitely over shadowed the standard bore in my opinion, overall the big-bore just had more grunt, and a much more aggressive mid-range pull without sacrificing any over rev. I think for bigger riders or those who are more capable would be better off with the big-bore just because of how much more torque and power the rider would have at their disposal. Personally I am not a big guy (155 lbs.), so neither of them lacked power, both of these bikes had zero trouble hitting big jumps right out of corners, and were both a blast to ride. Both of the bikes had suspension work from Race Tech featuring their “Gold Valves” and personally I think both bikes offered great bump absorption, and handled very well, however I feel it was too soft, which left me with a harsh feeling when I was coming into the face of some jumps with more speed. I did notice that both machines were difficult to start and the clutch pull felt slightly harder to pull in than a stock set up. –Matt Bynum 155lb. Intermediate

Twisted Development modified Yamaha YZ250F
Test rider Matt Bynum thought this was the fastest 250F he has ever ridden.Photo by Drew Ruiz
Big Bore YZ270F
Kris Keefer flicking the 270 around Competitive Edge MX Park.Photo by Drew Ruiz

Parts List

Twisted Development 951-698-7222
Engine mods—
Head Porting/Valve seats and deck
Custom cometic gaskets   Stage 1 hot cams degreed  Race Vertex piston set up
Dual injector set up in YZ450 air boot   Vortex ECU X-10 and mapping to operate 2nd injector  Spark plug cap hold down  Balanced Crankshaft from Crank Works with their CWI Rod
Cometic custom gaskets

Cylinder Works 515-251-4070
3mm Big bore Cylinder Kit 269cc
Includes Forged Vertex Piston and Cometic Top-End gasket kit

Cylinder Works 515-251-4070
Standard Big bore Cylinder Kit 250cc
Includes Forged Vertex Piston and Cometic Top-End gasket kit

Hot Cams         515-402-8200
Stage-one camshafts  (Degreed in by Twisted Development)

FMF Racing    310-631-4363
Factory 4.1 RCT Ti Muffler   (no inserts)    Ti Megabomb header

Cycra Racing                                   740-929-0188
Full coverage skid plate  Full Plastic Kit— with max air flow shrouds

Tusk Off road 800-336-5437
270mm Oversized Rotor  Rear Disc
Complete Wheel set with white hubs
Brake bolts kit  Sprocket Bolts kit

Regina Chain
MX Chain

Uni Filter   800-336-5437
Two-stage Air Filter

CV4 800-874-1223
Xceldyne X-2 OEM replacement valves.   Formed Silicone Coolant hoses
High pressure radiator cap

Twin Air
Power Flow kit /Back fire eliminator

Renthal  877-736-8425
Stock front 13T sprocket  Rear 48T sprocket — Also 49T on the std bore  604 fat bars  Dual compound grips

Dunlop Tire 800-845-8378
MX3S  front  80/100-21
MX3S rear   110/90-19

Moto Seat   951-258-5229
Custom Cool seat cover

Magik Custom Graphics   951-654-5550
Full custom bike kit
  Works Connection  800-349-1475
Elite Perch
Rear master Cylinder guard  Front brake cap  Rear Brake cap  Chain blocks   Engine plugs
Oil filler plugs
Hour meter with mount
Bike Stand

Hinson Clutch Components         909-946-2942
Full clutch set up - basket, inner hub and pressure plate.   Fibers, steels, springs and Outer Cover kit.

ICW                               919-795-8084
Radiator straightening and strengthening
  Scar 215-258-5124
Titanium Footpegs

Devol 360-825-2106
Transformer Pull Rod

Race Tech 951-279-6655
Re-valve and set up suspension

Applied Racing 800-853-0555
Triple Clamp with Big Bar mounts