After reading the recent posts about all the piston damage caused by detonation, I've been scared to start my NSR!!! So I checked all round the net about jetting aftermarket system to a two stroke engine.

The first thing that I found was that it most cases jetting will have to go DOWN !! Then that fitting aftermarket chambers etc further increases the compression ratio?? Plus by putting big jets in, you must be upping compression also! This im sure is one of the major causes of detonation in a two stroke engine.

Does this make any sense at all? If so then we are all killing our bikes by adding pipes, upping the jets and doing the cylinder head mod. It seems to make sense now, as it looks like the main probs are with the lower cylinder!

Food for thought, post your opinions up anyway then maybe we can get to the bottom of it!
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NSR250R MC21 (NEARLY SOLD!!)
VFR400R NC30 (SOLD)
Santa, please bring me a NSR500 and a SP-2 next year. Pleeeeease....

Er, not sure I can see that aftermarket pipes and large main jets raise compression ratio.

They will allow more mixture in to the chamber to be burnt, but the ratio for compression will not change. I don't think.

High flow pipes will need bigger jets as they allow more air / fuel mixture to be drawn into the engine. If you don't jet up you go lean and go bang.

Anyone else care to comment?
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MC21SP Plaything
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Well, I suppose you have a point.

As exhaust gas temperature has a direct effect on the speed of the gas pressure wave, I suppose you could say that fitting race pipe and changing the jetting does effect the CR and thus the potential for detonation but the base CR will be the same unless you skim or change the ratios.
For every 1 degree of change in exhaust gas temperature the tuned length of a spannie can vary up to 0.5m - 0.7mm. This is vital on a stroker as the scavenge 'charge is directly effected by the gas temperature and the pipe itself.

Quote - Mark Dent (nice bloke too!)
When designing an exhaust lots of factors come into play, Port timing, Peak RPM, Exhaust port diameter, Horn coefficient and unsteady gas flow to name but a few.
Unquote.

If you change the jetting and the motor runs cooler or hotter you strictly speaking need to have the pipes to match and the porting to match.

You know what I reckon, the NSR engine is in such a high state of tune anyway that changing parts of the bike - pipes, jets and cylinder size etc.. in the quest for more performance, leaves you with an engine that demands a lot more thought and care in setting it up to be reliable . Setting up the jetting right by doing plug chops and listening to the engines beat will give you a basic set up. To really get it right you need to know exactly what is going on and that is beyond may of us, not through lack of ability or understanding but because we just don't have the kit.

E.G. if you go from a 250cc engine that ran well and was reliable to a 300cc, rejet and so on but use the same pipes, the chances are that the exhaust temp ould have gone up. Give a 'guesstimate' of say, 20 degrees which is very conservative (my RG exhaust temp went up by over 35deg from stock to Swarbricks) your pipes are now around 12mm too short.
The hotter the exhaust gas, the faster the return pulse and the longer the pipe needs to be.

That's just one tiny factor on pipes, think of what else needs to be considered like port timings, blowdown, mixture, combustion chmber volume and so on before you start to look at ignition / advance curves etc.. I guess this is part and parcle of tuning an already tuned and relatively high-spec motor ?

As quoted in PB over the stroker hey-days of the 80's, 2 strokes are as reliable or docile as you want them to be. From the factory they are usually rock solid, start playing and fiddling and reliability will suffer.
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These aren't the droids you're looking for.

there are a lot of factors and confusion contributing to the notion that jetting a 2 stroke is a black art! What worked on one bike wont always work on a different 2 stroke.

i do not believe that adding pipes or different jets can change "compression ratio". Which is just a ratio of the engines 2 volumes BDC and TDC

What i think they are trying to say is that by adding pipes and or larger jets, you can increase the "pressure" in your cylinders. The volume of your BDC and TDC are still the same, but now you are sucking in more fuel/air for each stroke. So when your piston moves from BDC to TDC, there is more substance (fuel) in it to compress. Just imagine a cylinder full of water. there would be so much pressure in it that it wold not turn over.

Increased pressure causes increased heat. Thats why turbos use intercoolers to cool down the compressed charge flowing into the engine. So increased heat might now call for different heat range plugs, different air screw settings etc.. So then you have to start retuning. And then after you have retuned, you have made changes, and you could probably retune a bit more.. etc... each time you are theoreticaly getting closer to the best performance. But the closer you are to the perfect tune, the closer you are to failure! So Be Carefull

on some bikes, jetting down with pipes might end up happening. Other times you might just stay stock. These can both be due to the fact that your new pipes do not restrict the motor as much and allow more air flow though your engine. This in turn pulls more fuel through your stock jet.
However it is very unlikely that you will get so lucky.

This is why it is best to jet rich with a new pipe and work your way down.
its a Pain in the ass to buy the jets, plugs, and keep pulling the tank airbox etc and keep testing
But, as you can see from the recent topics of melted pistons, its not such a PITA after all..


I do not claim to be a jetting guru
so its possilbe everything i just wrote is inaccurate
Im sure others on this site can add more information
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Koren
2 Strokes and I'm Off

MAINE, what I'm getting at is the amount of mixture that is allowed to flow into the cylinder at a given point. I know that the squish at TDC is the "true" compression but when there is more fuel/air mix in the cylinder at this point, then surely the compression rises?? My friend has water spray in his car, this is like hitting nitrous oxide on because it puts the compression up! I'm going along these lines.

If you turbo charge an engine, in most cases the compression must be lowered to compensate simply because there is more fuel/air entering the engine per cycle. I am applying the same rule to this topic, in relation to the big jets.

Plus by fitting pipes and going high on fuel etc, the scavange pulse will effectively be mis-timed. Bearing that in mind, the exhaust pulses could end up blowing back into the cylinder which would again put compression through the roof! I would say this applies particularly to the 300 kit using 250 pipes. Surely using 250 pipes on the 300 must seriously compromise power too??

I dont really know much about it, but I'm learning as I read on from here. I just though it would be a good and useful topic of discussion, for our own info and to save a lot of people some money!
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NSR250R MC21 (NEARLY SOLD!!)
VFR400R NC30 (SOLD)
Santa, please bring me a NSR500 and a SP-2 next year. Pleeeeease....

ill agree that due to the increased fuel there is a greater density taken into the cyclinder, and there for when you compress the charge, you will have greater pressure. It seems we are just using the term compression and pressure to mean the same thing.

Now water injection is a whole different story!
it will also increase the pressure due to the same statement as above

but the real power adder is 2 things

1) The latent heat of water will greatly reduce the temperature of the intake charge as it transfers from water to steam (same principle as sweat evaporating to cool down the body). This works a lot like an intercooler mentioned earlier, and is especialy usefull when combined with a turbo. This reduction of intake charge can allow you to boost higher.

2) The water added to the combustion process is like running race gas. People often think that race gas burns faster, when it actually burns slower. Water does the same thing - it gives you a slower, cleaner more controlled burn. This allows you to run more advance on your timing. Or when running a turbo, helps to fight detonation that will occur as you increse the boost.
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Koren
2 Strokes and I'm Off

I suppose your right there, forgive me

In the end it all ends up as the same thing though, increased volumes of fuel = increased cylinder pressure. Put this into the fuel, pipe and big bore kit and you have a very big bang indeed!!!

Perhaps lowering cylinder compression ratios and customising pipes is the safest way to run big bore kits on the NSR's. Then coming to the 250 standard bore engines, reducing jet sizes a bit for hi-flow filters and pipes? I dont mean smaller than stock, but certainly smaller than 160 etc!

With regard to JHA and HRC jet kits, they have done the homework. But you need to be using every single other part the make eg. pipes, HRC carb tray, CDI's and so on.

I know big jets is safer from a temperature point of view, BUT, if you add too much fuel you are weaking the oil concentration. This said you could in turn increase the overall temperature due to further friction instead of burn temperature.

Just a thought, I could be talking crap!
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NSR250R MC21 (NEARLY SOLD!!)
VFR400R NC30 (SOLD)
Santa, please bring me a NSR500 and a SP-2 next year. Pleeeeease....

jeff350lc runs jetts that are smaller that stock,he has gp pipes and a modilied airbox and stephenrc45 has used smaller than stock jetts on his 250.i dont know much about jetting but i understand what you all mean,one point i have is, by going to larger main jetts,as you go up in sizes at some point you arent going to get enought air in through the carbs even with no air box to atommize(sorry if its spelt wrong)the fuel correct,is this right?if it is i guessing that this must cause a problem to the way the oil is mixed and the way the fuel is burned,so thinking about it if you need to use main jetts the sizes stephen did on his 300 then might it be better to use bigger carbs,this will mean you can draw more air across the main jetts meaning more fuel from smaller main jetts and this will stop the problem of the fuel not atommizing correctly,wont it?and wont it mean that as your burning less fuel the exhaust gas temp will be lower and so with the cylinder pressure,as there is less fuel to compress?

Fuel inject it !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
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These aren't the droids you're looking for.

Then stick a turbo on!!!!!!!!!!

wayne, I hear what you are saying and yes I also think that is what would happen eventually with too large mains. PJ38's are the way forward I think!
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NSR250R MC21 (NEARLY SOLD!!)
VFR400R NC30 (SOLD)
Santa, please bring me a NSR500 and a SP-2 next year. Pleeeeease....

dont think the technology is up to par yet

just goin by memory here, but i was running some numbers way back in high school one day on a KX250, and i belive that at 10000 rpm the piston was going up and down 166 times a second.

thats 166 required squirts of fuel per second..
maybe if you have 4 injectors per cyl that would be about 40 per second per injector. not sure of the current rate of injection, but im guessing there is no way to do it with one, or even 2 injectors with current technology.

with a 4 Smoke, you can be revving to 13000, and only need injection at half that rate since it only fires every other time the piston is at TDC. I think the fuel injected 4 Smokes currently use about 2 injectors per cyclinder.

Wasnt that Bimota 500 Vdue supposed to be injected
dont recall that worked out too well
changed to carbs and still didnt sell them..??
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Koren
2 Strokes and I'm Off

I dont think you would have any trouble fuel injecting it, according to my calculations my Mondeo ST200 (Contour SVT in the States!!) injects up to 62 (@ 7500rpm ) times per second per cylinder, thats with a single injector per cylinder too.

So 4 injectors per pot would cover it nicely!!!

So, what size of a turbo are we fitting then? Maybe even two small ones!
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NSR250R MC21 (NEARLY SOLD!!)
VFR400R NC30 (SOLD)
Santa, please bring me a NSR500 and a SP-2 next year. Pleeeeease....

MAINE 2 STROKER wrote:dont think the technology is up to par yet

just goin by memory here, but i was running some numbers way back in high school one day on a KX250, and i belive that at 10000 rpm the piston was going up and down 166 times a second.

thats 166 required squirts of fuel per second..
maybe if you have 4 injectors per cyl that would be about 40 per second per injector. not sure of the current rate of injection, but im guessing there is no way to do it with one, or even 2 injectors with current technology.

with a 4 Smoke, you can be revving to 13000, and only need injection at half that rate since it only fires every other time the piston is at TDC. I think the fuel injected 4 Smokes currently use about 2 injectors per cyclinder.

Wasnt that Bimota 500 Vdue supposed to be injected
dont recall that worked out too well
changed to carbs and still didnt sell them..??

Mr Dent would cringe to read that
Injection is the current area of development for RG's - works very well and extremely reliable but expensive.
The reason the Bimota BB5 not being a success is that too many of them went back to the factory in a dead way. Dodgy autolube kept letting them down.
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These aren't the droids you're looking for.

Just to clear some confusion:

-"compression ratio" is the ratio at which the piston "theoretically" displaces air volume in the cylinder. 10:1 means the cylinder volume with the piston at bdc is 10 times greater than with the piston at tdc.

-"corrected compression ratio" is the ratio between the cylinder volume measured when the exhaust port closes and the cylinder volume at tdc.

-"compression" is just a reading that MANY factors will effect, including corrected compression ratio, static compression ratio, ring wear, etc.

-"volumetric effeciency" is the term I believe most of you are searching for. Pipes, turbos, cams, intake design, and primary compression ratio(2-stroke) are some of the things that effect this. Especially turbos and superchargers. What this term means is a ratio between swept cylinder volume and actual air volume that enters the engine while it is running. Most NA 4-strokes run about 90% VE. In other words, an XR100 will intake approximately 90cc of air at torque peak for every combustion cycle. A good 2-stroke design will run WAY over that due to the tuned pipe ramming air back in the cylinder and the pressurized/forced induction created when the piston pressurizes the crankcases.

The design of the front NSR cylinder allows for a better pressure wave from the intake/crank into the cylinder, or what you can now term "better volumetric effeciency". The HRC 0.6mm base gasket helps this by reducing both the compression ratio and corrected compression ratio by just a tad to help put it more in tune with the other cylinder. It is not the best way to fix this, but it is the easiest and, in some race classes, the only legal route.
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Paul Herr
'88 FZR4/GSXR/YZF Frankenbike
MY BIKE PICS


Last edited by Wrench. on Fri May 06, 2005 2:56 am; edited 1 time in total

Thanks Paul, it's much clearer to me now.
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