Boring out ZXR250?

[outtalive]

This might be a crazy idea but would it be viable to bore a ZXR250 into a 400cc or higher? And if so what kind of costs are we talking?

[omen_child]

Too much. Look into purchasing a larger capacity machine.

[mike-s]

Uhm firstly i admit i've only read about boring things up.

i don't think you could get 400cc out of a 250cc bike, that's a 60% increase in capacity. Generally the walls of the cylenders are 3-5 maybe up to 8mm thick in some cases. Say you bore this hypothetical 250 out the 1.5mm that they say you can safely do (without risking the engine exploding/melting on you). you might be lucky to get 10-15cc out of the entire motor.

so if a 400cc engine can be shoehorned into the chassis, do that, else just wait until you can legally get a bigger bike.

besides the time, cost and agony of trying to get it working would be a veritable nightmare

[Felix]

Yep, very uneconomical.

Usually it isn't actually the boring alone that gives the capacity, but the stroke. It isn't too bad with cars, where an engine might be designed to be modular, such as a lot of the older mazda engines - not a lot of difference between 1300 - 1800cc variants (blocks and heads essentially the same). I knew a guy who put 1500cc internals into a 1300cc 323 motor - that thing went harder then you'd think possible. But he was a mechanic and knew what he was doing. I am not sure motorcycle engines are quite so interchangeable.

It requires some knowledge of cams and carbs and ignition to get it all working nicely. By the time you've paid for the dyno to tune it, you've bought something else that is decent to start with.

[NovaCoder]

buy yourself a second-hand ZXR400 lump and shoe-horn the bugger in (frame mods may be necessary), that would be a better bet

[I-K]

buy yourself a second-hand ZXR400 lump and shoe-horn the bugger in (frame mods may be necessary), that would be a better bet

Wouldn't work. The 250 and the 400 are_completely_different. On the 250, the block slant is of the order of 30-35 degrees, enabling the engine to hang completely beneath the frame beams. On the 400, the block is almost upright and the frame beams wrap right around the head, like on the ZXR750.

The only thing there'd be room for in a ZXR250 chassis would be a big single.

[Nanna10r]

Yep total waste of time. Be less drama robbing a bank to upgarde to a bigge3r scoot like a zepher & GETTING CAUGHT ...... besides who in thier Right mind would destroy a zxr400 to do up a ZZR250. Like trashing a Ferrari to update a Lexus. No offence to the ZZR250 Riders but The ZXR4 is a pretty edgie bit of kit; upside down forks, slipper clutch, over sized rims, huge brakes, Air Ram etc etc,
Cheers Brett

[icebreaker]

your not talking boring.. your talking stroking!


BIG difference...

talking new crank and pistons, as you'd most likely have to bore the block.
then you'd have to do head work to make up for the increased capacity.

ALOT of $$$ assuming you can get the crank.. And really a total waste of time, cause you'll spend as $2k+ plus tuning etc. just an utter waste IMO.. get a 2 stroke if you need more power out of a 250cc..

[Tack]

The only thing I can think of to increase performance without a huge amount of hassle is to turbo the thing....butttt...you would have reduce the compression ratio...which means taking some meat off the pistons or getting custom ones made (or putting an alloy decompression plate in but that stretches the timing chain...don't like that way much)....and also you should have fuel injection (although it could be done with carbies its not the best way to do it) which means finding throttle bodies of the right size to suit and having a tunable engine management (fuel only is all thats necessary).

Again it would mean $2000 or so I reckon....I was wondering though if it would be legal??? For a restricted rider or a p plate rider...that is??

I mean I could do this....I've done this sort of stuff before (built my own supercharger once) and I have to laugh cause you could get some real serious horsepower out of this 250 (compared to other bikes that is...big increase in torque as well). A camshaft regrind may also be a source of xtra power on a turbo as well

The only other thing is you might start breaking gearboxes and chains etc etc and the brakes might fade because your going so fast and it might be a real handfull to ride and pop monos in every gear...lol.... and I dunno if anyone would insure it either.

so is there a law stopping restricted riders from doing this...it is after all still only 250cc...or do they have a multiplication factor for turbos???

[99coconuts]

you might be onto something there matey. in vic the restrictions only state that the bike must be less than 260cc, theres no power restrictions in there at all. so if you had a 400hp 250, you'd be ok to ride it cos that'd be much safer than say a 50hp 400

vics laws are stoopid, give us the NSW set....

[mike-s]

i think they do have a multiplication factor in there of 1.4 or so.
30 seconds of googling gave results that confirmed this.
http://www.xs4all.nl/~robdebie/models/911-935.htm

But there are other places that have 1.6 or even 1.7 such as WRC
http://www.motorsportireland.com/App_Q.pdf

Cams uses the following.
http://www.southernsportingcarclub.com. ... 210105.pdf
MULTIPLICATION FACTORS FOR CAPACITY
For all Vehicle Types, capacity must be calculated on the following basis:
Rotary Engines Multiply displacement by 1.8
Forced Induction Multiply displacement by 1.7
•Forced Induction Rotary Multiply displacement by 3.06

Either way, it ain't going to be legal for a learner.

[99coconuts]

but, does the multiplication thing apply to us? thats the thing, racing rules arent road laws. with cars, you have to pass both power to weight and litre to weight restrictions to drive them. with bikes, its just a litre thing, they dont even take weight into consideration, let alone power. while a turbo DOES increase power output, it wont actually increase the size of the motor. i mean there might be more to it, but i've never seen anything that states that a LP platers are subject to anything more than a cc restriction.....

be interested to find out a definate answer. not that learners should be riding turbo acrosses or anything, just reckon it would be funny if they've completely overlooked the potential of a turbo

[mike-s]

i had a hunt around on the RTA's website and that turned up f'all as to be expected. i called their technical enquiries line (1300 137 302) and was told that there is a formula they use for calculating the performance boost (no pun intended) that a turbo will give. However this varies based on both the engines capacity and the boost levels the turbo is capable of.

He also mentioned that there are limits that they have to impose on the performance increases permitted, i.e. a 18psi boost turbo may be legal (i.e. pass an engineers report) on a vl commodore with a smallblock chevvy motor, however the same turbo may give too much boost for a datto 180b with a stock motor (my example, not theirs).

Either way, turbo'ing a bike would be a long and costly road.

[zbeast]

I thought about the 400 conversion for my ZXR250 when the little end siezed but was told by a number of people that it wouldn't fit so I had to put up with the 250.

If you really want to turbo the 250 give S & R Pro in Penrith NSW a call. They turbocharged a zeal or something and it had 80 or 90 hp at the wheel. I called them and they reckoned they'd get more out of the ZXR motor and they only wanted $10k to do the work . You could sell your bike and put that with the $10k and pick up a second hand ZX10 which is designed to do the sort of speeds that you would end up doing on the turbo 250.

[mike-s]

Uhm, unless you get the gearbox recut to take advantage of the extra HP then the zx10 would whoop the arse of the turbo'd 250 as it would kind of peter out at about 180km/h or so. Stock it'd be hell fast, but you'd be kicking through the gears up & down like a madman.

Even if you turbo'd the 250, the 10 would still kick it's arse anyhow, but 10k for the conversion is obscene.

10k, fark it, go get some wiseco pistons, steel tubing, a turbo from a charade & lots of beer for a mate with an oxy and you'd be set . (all said with a shitload of sarcasm)