Damn silly tuning questions...

[SpannerMonkey]

Right, The GN250 is adequate in top speed and accell so dont think I'm just a teenager after speed and straight line thrills, but what I'm after is more power lower in the rev range to pull taller gearing.
The GN250 is a factory custom, and customs shouldn't have to rev to 8,500rpm to get peak power, I want to be able to sit at the 3000-5000 rpm sweet spot as long as possible.

Thing is you see, riding down the motorway at 70mph is 7,500rpm which sounds nasty on a single but the bike barely manages 75 on the flat as it plateaus at around 8,000rpm. What I'd like to do is gain a lil more power in the lower revs to enable me to run taller gearing in normal 40-50mph situations.
I was thinking of ditching the carb that's on there as it is most likely "Adequate for every day use" and finding something a lil more suitable for my needs... but I'm not sure what to do with the air intake. the airbox is massive and restrictive, but also holds the battery in place so I need to decide whether it is even worth ditching the airbox or not... Any smart ideas guys?

Yet another immature post from Spannermonkey 91 =)

[Jamie S]

[chris-red]

It's a 250 you aren't going to get power low down, what you want is a new bike.
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[MarJay]

If tuning was easy, anyone could do it. But it isn't, hence engine manufacturers spend inordinate amounts of money designing engines rather than getting 'Kev' from the scooter forum to fit a pink powerband.
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[Howling Terror]

Do what i do on my infinitely better CB250RSA...Thrash it and change the oil alot.

Pat
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[Blau Zedong]

[SpannerMonkey]

[Blau Zedong]

I'd never of guessed. Still an engine built from the outset for frugality though.
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[N cee thirty]

what you wanna do is buy a turbo then you'll increase performance
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[Kickstart]

Hi

What you need is more torque (power is just torque x rpm, so for more power you either use more revs or produce more torque). However getting that isn't easy.

Simplest way would be a big bore kit. Doubt one is available and not sure how far it could be bored out.

The 4 valve head probably makes things worse. 4 valve heads work well at higher revs, often at the expense of lower revs.

For the more low down power you probably want a larger bore exhaust (lose any tuned effect), small carb (will lose you top end power) and softer cams.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[c_dug]

In fairness smaller bikes have been turbo'd before.
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[SpannerMonkey]

[whitedevil]

What you need is :

https://www.street-dreams.us/images/products/22oznosbottle.jpg

Just pour it in your tank. Instant p0waaaaaaaaaaaaaarrrrrrrrrrrrrrrrrr!!!!!!!!!!!!!!!!!!!!!11111111one

[Kickstart]

Hi

Larger carbs are needed for high gas flow. Trouble is that they tend not to work so well at low revs. They work by having a slight restriction which causes the air to speed up and hence its pressure to drop. This pressure drop draws the fuel from the carb.

Large carb at low revs means very little pressure drop hence they struggle to draw out fuel.

That said the GN carb is probaby pretty small anyway.

All the best

Keith
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[SpannerMonkey]

[SpannerMonkey]

[Shaft]

[Teflon-Mike]

As keith said, power is torque x revs.
A little algebraic expansion of that actually gives:
Power = Cylinder Pressure x Cylinder Displacement x Revs.
So to get more power on, two or all three variables have to go up.
To get more power without a big-bore kit, limits you to pressure and revs.
To get more power at a certain revs, limits you JUST to cylinder pressure.....
Cylinder pressure is a function of how much charge you can get in the pot, and tuning for PEAK power, flow restriction is usually a pretty critical factor, becouse beyond the peak torque rpm, the drop in torque is coming entirely from incomplete cylinder filling due to flow restriction.
UP to peak torque power, the engine has the capability to flow more charge than it can actually cram in the combustion chamber.

Ergo: if you are looking for power gains beneath peak torque, then you wont find any improvement looking for increased flow capability.

Now, if flow were the only factor at work here, the torque trace for an engine would start at its maximum, and slowly drop off as the impediment to flow started to choke the engine. It doesn't. It starts low, ramps, then drops off in a hump. So beneath peak torque, something other than flow capability is robbing power.

YOUR question is 'what'?

And the answer is many things. BUT, burn charecteristics in the combustion chamber is one. Charge burns at a certain rate. Combustion chamber shape, and propagation paterns of how the flame-front progresses from the point of initiation (spark-plug!) within the combustion chamber have influence, which is where you get into things like 'squish' on two strokes or 'swirl-effect' (Heron/Weslake + possibly relevent to you, Suzuki TSCC) combustion chambers. However, those 'dark-arts' asside, (Engineers studying / modeling combustion chamber flow-paterns are like meteorologists and chaos thoery experts!) At a slightly more understandable level.... piston at top dead centre, when spark occurs. Combustion chamber volume, with the piston near the top of its stroke is very small, and with crank turning slowly, its there for quite a while, so an AWFUL lot of charge can be burned and a lot of pressure made, BUT all happens when the crank-shaft is almost virticle, and the effective leverage it has pretty small. Gives big pressure spike, but the agregate 'torque' over the entire cycle, isn't very high. This is where con-rod length and the geometry of the piston to con-rod to crank-shaft can effect where in the rev-range peak torque is made. I think longer rods push peak torque up the rev-range, shorter ones bring it down, angle near TDC giving more effective leverage.

Next up, low revs, you have trapping efficiency. Valves being opened early, and held open longer, for better flow at higher rpm, are allowing charge to be 'lost' at lower rpm, during the over-lap period, when inlet and exhaust valves are open at teh same time, and the venturi effect of exhaust gasses rushing out the exhaust can suck charge through the inlet and out with them, or at the start of the compression stroke, when the piston will be rising before the inlet valve has closed. At higher revs, the momentum of the incoming charge will tend to keep it heading in, while teh valves open, or at least stop it being pushed back out, but at slower speeds, it wont be moving that fast or have that momentum, so can be pushed back out the cylinder.

This is where Chris was touching on. There's some fairly well proven formula for induction tract and exhaust design, which suggests that the 'optimum' volume of the exhaust header should be roughly equal to cylinder displacement. so if you have a 250cc cylinder, exhaust header should also be about 250cc. BUT, for max flow at higher rpm, it needs to be as big in diameter as possible, so necesserily short, but for lower rpm, needs to be smaller in diameter to promote gas velocity and scavenging effects, from the exhaust gas's momentum in the pipe, sucking lingering gas out the cylinder, so long and narrow. Same principle for the inlet tract, and I think overall tract volume ought to be around 1/2 cylinder displacement (Off the top of my head; havent read the books lately!) Again, longer, narrower tracts reduce maximum flow capability, but promote charge velocity at lower flow rates, hence momentum and trapping efficiency.

These are pretty 'broad' generalisations, but where we are getting to is that the engines overall architecture is a compromise between a LOT of conflicting factors.

And going away from what the designers gave us, can be a pretty risky business, and you need to have a pretty good idea of whats going on, and what any change is going to do for you, before you begin.

Tuning for power, is actually one of the easier things to do in tuning... and cramming more charge in the pot at higher and higher revs TENDS to be just a matter of tackling flow restrictions, and getting the engine to breath, by opening up ports and valve sizes, and making the cam timing wilder.....

Tuning for Ecconomy, is a bit more challenging, becouse you tend to be looking to optimise for bottom end torque, and leaning out the mixture.

Tuning for torque, or bulking up the mid-range for driveability is probably one of the hardest tuning excersises, becouse you dont want to loose 'driveability' either from sacrificing top end power OR loosing bottom end efficiency.......

And on a GN250?!?!?

PRACTICALLY?!?!? you AINT oing to be able to do MUCH better than the factory, without a heck of a lot of time, know-how and expense, that the bike probably just doesn't merit!

However, if you are still alive and not drawing a pension, or retired from biking, by the time you have read this far....... two more useful nuggets for you.

1/ Before lookng for MORE than standard, make sure you got what you should AS standard!

Old engine; make sure its as well serviced as you can make it. Look at air-filters, carb condion. Ignition & exhaust.Tappets & cam-chain. Cam chain stretch can retard cam timing by perhaps 10-15 degrees or more, and STILL have the engine running, depending on how soft the cam-profile is. Old Rover V8 I took apart a few years ago, cam chain stretch was the equivilent of slipping two teeth on the timing sprokets! That retarded it by around 20 degrees of cam timing! And it STILL ran! (Old buic block is a very 'soft' motor though) Likewise, wear on the cam peaks can loose you vital valve lift, and worn seats 'shroud' the valves robbing you of even more, as well as knocking the edge off flow performance.

After basic servicing, far more effective, far more often, than 'performance' tuning, is simple reconditioning. You dont need to go to the lengths of 'blue-print' tuning, looking to exploit manufacturing and service tolerences to get performance gains, but a careful, detail top end overhaul, can make a world of difference, and find a big chunk of 'lost' power from when the motor left the factory, and pottentially a bit more on top.

2/ SKIM THE HEAD....
Skimming the head, reduced combustion chamber volume, increases compression ratio. Increased compression ratio, increases cylnder pressure accross the rev range. Obviousely, over do it, you run into risk of detonation problems, but within service tolerences, its a VERY good way to find a bit more 'oomph'.

So.... top end rebuild, get the valves and seats re-ground, and the head skimmed on the hard side. Rebuild, paying as much attension and care as you wish. A little work with a hand file, slotting cam-sprokets to accurately align them to factory spec, setting valve clerances carefully, and possibly using a new cam, will often find far more 'useable' extra power than 'performance' tuning.

Dyno gains, over stock are often miniscule. Over what you started with, often not worth shouting about, but in the power delivery, response and way the motor 'behaves' the improvements often feel an awful lot bigger than power charts suggest.....

Also tends to ADD life and reliability to an engine, rather than remove it, as conventional 'performance' tuning does...

BUT, taking the bike off the road to rebuild its motor..... and it being a GN.... again, cost and effort wise, even though likely to be the most you'll get for either..... probably an awful lot simpler and cheaper and easier to buy a bigger bike!
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[general-lee]

Hey, may seem a bit late but here goes. I had almost the exact idea with my GN 250, I've slapped a pod filter on it, strapped a bungie chord around the battery, and bought a TK speedpump carb. The TK carb apparently throws more power into the engine, how it does it I'mnot entirely sure but it still needs jetting as the carb was designed to run off the original airbox.

After doing some homework I found a bloke with a GN250 ratbike where he used the standard airbox but cut his exhausts, he upgraded his main jet to 135. I'm no expert with carbs at all, however I've using his handy work as a basis for my own. I have bought a 135 jet and the next one up being a 137.5 and a 140, I'm going to see which one will work if any and takeit from there.

In all honesty though the GN 250s we're never designed to impress anybody, they're just city runners. My hat goes off to anybody who wants to tinker with them though, the results are normally far from brilliant, but at least we've had a go