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FzerozeroT
12th March 2005, 09:52
Hollowmen asked so lets figure this one out:

INCREASING COMPRESSION
borexstroke = 72x62.1
so displacement = 252.8cc
divide by 8.9 = combustion chamber volume = 2.8cc
we want 10:1 so ideal combustion chamber volume = 2.5cc
difference of 0.3cc is taken out of barrel so as to not change combution chamber shape.
So i ended up with 0.767mm shave to achieve 10:1, because of increased compression 96 would be used, flame propagation would be faster so therefore more controlled
Because of the head being lower the slack would have to be taken out of the cam chain as well.

ADVANCE IGNITION
simple as making the woodruff key thinner and rotate the flywheel, would need to be done on dyno.

EXHAUST
Ditch it from the footpegs back for some straight pipes. Probably lose power but the diapproving looks from old ladies makes it worth it.

CAMS
throw a few $$ at a cam grinder and see what he comes up with, I'm thinking you want stage3+ and you'll need to replace the valve springs at the same time with something a bit more heavy duty.

CARBS
Will need some serious work with these mods, specialist work with that high tech system.

TURBO/NOS?
You know you want it.

Dr Bob
12th March 2005, 09:59
Why would you want to?

thehollowmen
12th March 2005, 10:06
Why would you want to?

well I was just wondering how to get it to 120 easily to pass those twits at 90, but without taking up the whole passing lane getting up to speed...

although it would be good for a laugh...

Although I've heard of someone down here who raced GN125 by shaving the head and putting GN250 carbs on it...

sedge
12th March 2005, 10:08
I reckon a small turbo would be the go, if you're half arsing it anyway just chuck in a decompression plate, I guess you'd need a suck through system and some hefty jets, do they have cams or push rods ? coz you'd probably want to change the overlap a bit... might get you all of 60hp on boost and about 20hp off ( till it popped) :(... seriously though, why would you do it ?

Just switch a GN400 engine in if you really have to ?

Sigh... suggestion 2 is to get a GN250 and set fire to it.

Sedge.

zooter
16th April 2005, 19:47
How much slack would a 0.76mm shave off the block make anyway?

FzerozeroT
16th April 2005, 20:40
12.3mm..............

Kwaka-Kid
16th April 2005, 22:32
CARBS
Will need some serious work with these mods, specialist work with that high tech system.
Just biff a Mikuni competition 36 odd mm (or stock XR500 sucker if ur poor) flatslide, smoothbore, pumper carb


EXHAUST
Ditch it from the footpegs back for some straight pipes. Probably lose power but the diapproving looks from old ladies makes it worth it.

Nah dude itll ruin all the work youve done!.. Must get the exhaust just as precise as the rest of the beast. rip out the old exhaust chart and work some fine magic!

FzerozeroT
17th April 2005, 01:33
best would be tuned length straight-pipe (x2 one for each valve same as standard), I don't have a 2T tuners handbook handy, but you just need to figure out the speed of sound in hot air and it has to travel to the end of the pipe (+ a little extra travel to suck the next lot through) and back in 8.5 x10^-3 secs (7,000 rpm?), 330m/s at 25degrees, 1.414 m, so probably start off at 1.6M and cut off 2cm lengths. Of course the diameter of the pipe will come into play as well , guess it would be 1&1/4"? that seems awfully long though :( faster sound in hotter air might bring it back under 1m though?

Guess when you take the pipes off you just use the same diameter as the manifold/port?

Has anyone ridden both a (1981)CB250RS and a GN250, whats the power difference like? I might have to sort out a dyno :D

Kwaka-Kid
17th April 2005, 08:48
dead stock CB250RS is up on the GN250.

I had a fully worked/tuned CB250RS :) thats where all my info comes from. And yeah about the pipes... but to save weight (altho make calculations harder) your better off just joining the 2 header pipes straight together ASAP after exhaust ports, without disrupting flow too much... its not like they are individual cylinders and their timing is going to affect one anothers backpressure... so the only reason they made them twin outlet i beleive was to get more exhaust port area to let it out.

i think. And 1981 CB250RS is 1978 XL250S motor :) from memory 30odd HP? ProX piston kit number 01.1500.000 std, same as XR250RB.

valves 622.1014EX and 1014IN from memory? Should you want to rebuild one :D

Shade
17th April 2005, 09:21
ProX piston kit number 01.1500.000 std, same as XR250RB.

valves 622.1014EX and 1014IN from memory? Should you want to rebuild one :D

Nerd! Shamed!

Kwaka-Kid
17th April 2005, 09:50
no no shade. nerd would then be if i told you the Superin Gasket set number was 19-59444, that they have the same rings as a TRX300, number 624.060R std... and have a 74mm bore stock (i THINK, or 74.5?), Rodkit 03.1255(i know thats wrong, its close to that though.. 1285?)

Not to mention they use SHKIT-1 SH bearings, FSK-003 fork seals... then the easier stuff like tubes/520 chain etc etc, rear brake shoes 92-39150 superin number (i suspect) front brake pads are possibly GF020?(thats a hard one, cant fully remember what they look like. Master cylinder rebuild kit MSB-102, camchain is 82RH102L? ahh crap hes rambling again.

The better info anyway is that the CBX250RS 6 speed gearbox goes straight in with a little shaving off the casing. :)

ohh myyybad on this post.

gman
17th April 2005, 11:58
i still cant beleive that some one with way too much time on their hands would want to even do this let alone admit to wanting to!!! :lol:

crazylittleshit
17th April 2005, 12:25
just get a faster bike

FROSTY
17th April 2005, 12:26
I'd suggest going for 28mm Id headers of the same curve /length as stock and feed it back into reverse cone megaphone

DEATH_INC.
17th April 2005, 12:28
Best mod for the buck is a 280 big bore if you can still get 'em.On a single a megaphone is the best thing you can do on the exhaust,with a reverse cone,way better than a straight pipe,and twice as loud.Bigger carb and a hotter cam. :banana:

Coyote
17th April 2005, 13:10
Rip the motor out, replace it with a GSX1300R motor, then add a twin turbo and intercooler, nitrous, and fill the tank with a kerosene/nitro methane mix

Coyote
17th April 2005, 13:12
Oh, and also add 20" Chrome spinners and a big bore exhaust, cause obviously, they are the only things needed to deem it a high performance bike

Ixion
17th April 2005, 13:36
I'd suggest going for 28mm Id headers of the same curve /length as stock and feed it back into reverse cone megaphone
Yep, 4 stroke single, reverse cone mega has always been the way to go. Just be aware that depending on the reset of the setup (cams, carbs, timing) you may find it's not much cop at lower revs. Which sort of defeats what a GN250 is all about. Sounds nice though.

cowpoos
22nd September 2005, 00:16
exhaust science huh....well....why has know one asked the obvious questions an exhaut tuner would ask or find....

what Cfm will the head flow at peak rpm...with airbox/filter/carb attached
what is peak engine rpm
what are the cams profiles
dialed is at?
engine working temp range/peramiters
CR?

nudemetalz
23rd September 2005, 10:37
Why don't you get an NZ250 engine for it?
Puts about 34hp, 6-8 more than the GN already, before you even think about tuning it.
Based on a DR so very bulletproof. The only thing you'd need to do is hook up an oil cooler for the cooling, but that shouldn't be a big deal.

The NZ's no slug for a 250 single, so would think that all that tuning would only bring it up to NZ-250's levels anyway.

Cheers
Chris

ducatilover
15th October 2005, 23:42
Why don't you get an NZ250 engine for it?
Puts about 34hp, 6-8 more than the GN already, before you even think about tuning it.
Based on a DR so very bulletproof. The only thing you'd need to do is hook up an oil cooler for the cooling, but that shouldn't be a big deal.

The NZ's no slug for a 250 single, so would think that all that tuning would only bring it up to NZ-250's levels anyway.

Cheers
Chris
if your lucky. why dont i just stick my fucken gpz400r enjine in mine? i fixed its gear box today :niceone:

The Pastor
6th May 2006, 23:05
Just nos the basterd, cheap and easy to do, and biggest hp gain, albeit for a short time but you did just want it to pass the slow twits right.

peanuteater
26th June 2006, 20:23
From memory, just shove in a suzuki dr 350 motor, just pops straight in-cheap, quick and easy !!, then add nos, turbo etc-seriously tho, they fit the mounts etc !

Titanium
26th June 2006, 20:36
All sounds fantastic if you want a lifetime of wrenching on a bike instead of riding it.......

Why not just get a bigger bike?

degrom
25th July 2006, 10:54
Best mod for the buck is a 280 big bore if you can still get 'em.On a single a megaphone is the best thing you can do on the exhaust,with a reverse cone,way better than a straight pipe,and twice as loud.Bigger carb and a hotter cam. :banana:

Is this true? Can you get this? And what is part of the kit?

I just hope that it won't replace the 250cc indicator in the motor for L-Plate riders... LOL

kro
2nd August 2006, 06:49
Don't mock the mighty GN, a large portion of its engine parts can be used in a DR 250.

MikeyG
2nd August 2006, 08:00
Barrel and piston from a GN400. The 249cc stamping is on the block so with the new barrel and piston the it will still look like a 250

degrom
2nd August 2006, 18:15
Barrel and piston from a GN400. The 249cc stamping is on the block so with the new barrel and piston the it will still look like a 250

I think you might need a GN400 head then?!?

degrom
12th September 2006, 18:49
What kind of improvement will you see if you make the cam a bit hotter?

mokr
29th April 2007, 19:59
a gn250 should do 140 down hill.
and at least 110-120 on the flat.
or the engine is stuffed.

i wouldnt bother putting cash/effort into the engine unless ure really bored
its air cooled so there are big gaps in the engine for the heat expansion
so its never gonna be a big rever. a 500 or 600 air cooled is still not big horeses but they do haul ass, my tt600 single pulls hard up till over 180

if u want a fast bike get something else. youll spend heaps of cash on it and it will just cook the engine.

keep checking the oil and be happy with the gn for a cool cheap runner.....maybee do the pipe ;)

ready4whatever
23rd July 2009, 14:35
put a massive spoiler on the back it'll go hard

vifferman
23rd July 2009, 14:39
another dredged thread
What is it with you?
Do you have shares in a trawler or summat? :confused:

The Pastor
23rd July 2009, 14:45
i think a cam regrind would and a big bore kit would work wonders on a gn250

Brian d marge
23rd July 2009, 16:53
Power =( nf Nv N Vd Qhv p ( F/a ) ) / 2

Nf = fuel conversion efficiency ( V good is 30% my enfield is 18% )
nv =Volumetric eff
N = Rps

Vd = volume displacement

p = density inlet

f/a = fuel air ratio

there are different versions of this , but basically the answers are there

becareful with cams, they aint the panacia of engines

Stephen

also if for some reason u get huge power from the engine , are the othe parts rated for the increase in power

got some money?

The Pastor
23rd July 2009, 17:23
power =( nf nv n vd qhv p ( f/a ) ) / 2

nf = fuel conversion efficiency ( v good is 30% my enfield is 18% )
nv =volumetric eff
n = rps

vd = volume displacement

p = density inlet

f/a = fuel air ratio

there are different versions of this , but basically the answers are there

becareful with cams, they aint the panacia of engines

stephen

also if for some reason u get huge power from the engine , are the othe parts rated for the increase in power

got some money?
so what would you suggest for gn250 power mods?

Is ur engfield modified?

Rodney007
23rd July 2009, 17:41
Hollowmen asked so lets figure this one out:

INCREASING COMPRESSION
borexstroke = 72x62.1
so displacement = 252.8cc
divide by 8.9 = combustion chamber volume = 2.8cc
we want 10:1 so ideal combustion chamber volume = 2.5cc
difference of 0.3cc is taken out of barrel so as to not change combution chamber shape.
So i ended up with 0.767mm shave to achieve 10:1, because of increased compression 96 would be used, flame propagation would be faster so therefore more controlled
Because of the head being lower the slack would have to be taken out of the cam chain as well.

ADVANCE IGNITION
simple as making the woodruff key thinner and rotate the flywheel, would need to be done on dyno.

EXHAUST
Ditch it from the footpegs back for some straight pipes. Probably lose power but the diapproving looks from old ladies makes it worth it.

CAMS
throw a few $$ at a cam grinder and see what he comes up with, I'm thinking you want stage3+ and you'll need to replace the valve springs at the same time with something a bit more heavy duty.

CARBS
Will need some serious work with these mods, specialist work with that high tech system.

TURBO/NOS?
You know you want it.

sounds like a whole bunch of bs to me, ...

any pics yet?

Pedrostt500
23rd July 2009, 18:03
I think just junk the GN motor and slot in a SV 1000 donkey.

SARGE
23rd July 2009, 18:07
cheaper, more effective and heaps more fun....

Brian d marge
23rd July 2009, 18:11
so what would you suggest for gn250 power mods?

Is ur engfield modified?

very modified I race it ........ try increasing the fuel conversion efficiency after all that's where the process starts, ( but if you release heat ,,,you must control it !)

if you have 1 gram of fuel...............


Stephen

FzerozeroT
23rd July 2009, 21:54
Well, this thread just came back from the dead, good thing Jackos casket is sealed with concrete.

I don't think that anyone around KB now has appreciation for these quality threads, they were from a time when KB posts were free from trolling, input was constructive, and those who posted actually rode bikes.

gwigs
24th July 2009, 10:43
Just smoke a shit load of weed before you ride it ...wont be any quicker but it will certainly feel like it :done:

The Pastor
24th July 2009, 10:56
just smoke a shit load of weed before you ride it ...wont be any quicker but it will certainly feel like it :done:
typicall comment from someone who lives in the haven

The Pastor
24th July 2009, 10:57
very modified i race it ........ Try increasing the fuel conversion efficiency after all that's where the process starts, ( but if you release heat ,,,you must control it !)

if you have 1 gram of fuel...............


Stephen
do you have a thread or blog or something about ur mods? You sound like you have a bit of knowledege in making things go faster

tychver
24th July 2009, 11:19
A properly jetted modern motorcross carb in the 35mm range and a decently flowing exhaust would make a fair bit of difference. Whether it's worth the $1200 dollars is a different matter!

gwigs
24th July 2009, 11:46
typicall comment from someone who lives in the haven

Its spelled typical by the way....
and it didn,t take long for you to bite :nya:
Get a sense of humour...

Brian d marge
24th July 2009, 15:52
do you have a thread or blog or something about ur mods? You sound like you have a bit of knowledege in making things go faster

no, I have a blog about racing and some stuff , but as to making things go faster

20 odd years of reading/studying and working in the trade ( talking to people much smarter than me )

but there is a torrent

internal combustion engine fundamentals by J b Heywood

a bit academic but ... its a start

Get a engine that is easy and cheap to rebiuld , Cg XL125 etc

measure , think measure again and ask LOTS of silly Questions

there is a computer software that you program the engines specs and it will act like a dyno and produce a graph ( its not too bad ).. i think its about 160 us dollars

but you can try different things such as valve lift openings etc

its a usefull guess thats all

Stephen

Stephen

http://477racing.blogspot.com (http://477racing.blogspot.com/)

rok-the-boat
25th July 2009, 20:34
Set it on fire ...

davebullet
25th July 2009, 21:45
Why would you want to?

There's gotta be a use somewhere for a GN? Maybe this is it?