28s vs 35s

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500bernie
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Re: 28s vs 35s

Post by 500bernie » Sat Apr 16, 2016 9:20 am

Hi James
The thing that keeps niggling away in my head is;
The maximum volume of fuel air mix is limited by the displacement (which correct me if I am wrong, is the same for all the KRs). The 28s max flow rate limits the top end revs.
So I still see the same volume of fuel air mix being drawn in per cycle at WOT @ 9000rpm as does at 11000rpm. The 28s being almost at their max flow rate. But the bike with the 35s should still be pulling through the same volume, so I am struggling with why it needs a different oil flow rate (I presume it would be higher, if it is different from the standard).
And then my headache gets worse, as I think the increased ratio of oil in the same volume fuel air mix would lean off the mix, then it would need bigger jets, which would help at the top end, but I still keep coming back to the displacement volume #-o

Cheers
Bernie :)
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Re: 28s vs 35s

Post by Luders » Sat Apr 16, 2016 10:27 am

Power jets are the way forward Bernie :-)

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Re: 28s vs 35s

Post by JanBros » Sat Apr 16, 2016 1:15 pm

Luders wrote:Power jets are the way forward Bernie :-)
oh now, you have made his brain explode, 'cause powerjets give more fuel but the carb is still limited because of it's max flow :mrgreen:
Last edited by JanBros on Sat Apr 16, 2016 3:18 pm, edited 1 time in total.
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Re: 28s vs 35s

Post by Luders » Sat Apr 16, 2016 1:48 pm

:lol:

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Re: 28s vs 35s

Post by 500bernie » Sat Apr 16, 2016 2:31 pm

Doctor my brain hurts :shock:
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Re: 28s vs 35s

Post by James P » Sun Apr 17, 2016 6:02 am

500bernie wrote:Hi James
The thing that keeps niggling away in my head is;
The maximum volume of fuel air mix is limited by the displacement (which correct me if I am wrong, is the same for all the KRs). The 28s max flow rate limits the top end revs.
So I still see the same volume of fuel air mix being drawn in per cycle at WOT @ 9000rpm as does at 11000rpm. The 28s being almost at their max flow rate. But the bike with the 35s should still be pulling through the same volume, so I am struggling with why it needs a different oil flow rate (I presume it would be higher, if it is different from the standard).
And then my headache gets worse, as I think the increased ratio of oil in the same volume fuel air mix would lean off the mix, then it would need bigger jets, which would help at the top end, but I still keep coming back to the displacement volume #-o

Cheers
Bernie :)
I think I see your point Bernie and I can't say you're wrong - in truth, I don't know :| . I can only offer the following ramblings :lol: :

For the same engine, bigger carbs most often need bigger jets (at least for the larger throttle openings), which means more petrol (and hence more oil if one subscribes to the notion that the oil should be a certain percentage of the fuel).
Can we assume that the KR-1/1S is "restricted" by its 28mm carbs and that there is some "spare volume" which can be used to good effect if larger carbs are fitted? This view is perhaps a little simplistic, but if available volume is fully utilised with 28mm carbs, there should surely be no advantage in fitting larger ones.
I'm not sure how much effect the slightly longer exhaust duration of the KR-1R has on its ability to use the 35mm carbs (I would think not a great deal, as the KR-1R uses the same crankcase, cylinder head, exhausts and ignition as the KR-1S). Has anyone tried 35mm carbs on an otherwise standard KR-1 or KR-1S?

I daresay that volumes of air and fuel will be similar for 28mm and 35mm carbs at lower speeds and smaller throttle openings. However, a 35mm carb at full throttle is theoretically capable of flowing more air and more fuel than a 28mm carb at full throttle. I would therefore assume that the volume of oil for a 35mm carb at full throttle should be greater, so as to maintain the same petrol:oil ratio. As a consequence, I would assume that the performance of KR-1S and KR-1R oil pumps is similar/identical up to the point which is full throttle on a 28mm carb...and that the KR-1R oil pump then continues increasing the volume of oil until it reaches full throttle on its 35mm carbs.

On the topic of additional oil causing air:fuel leanness; I'll defer to anyone who has actually proved this. However, my own experience has shown this effect to be negligible. I would imagine it to be a problem only if jetting is absolutely spot-on in the first place (i.e. not erring on the side of richness). How much bigger a jet is required if changing from 3% oil to 4% oil? Is the required increase greater or lesser than one jet size??

Irrespective of all of the above, I prefer smaller carbs - they seem to offer broader power bands, better tractability and slightly lesser fuel consumption. For me, it is not worth sacrificing these features for the high-speed increases which larger carbs may offer.

Regards,
James

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Re: 28s vs 35s

Post by JanBros » Sun Apr 17, 2016 10:40 am

the 28's ain't at their limit on the KR1, as making about 20% more power is achievable with the 28's.
the 35's however can deliver the same amount of mixture faster at WOT, so there is time left to produce even more mixure each intake and that's where the benefits come from.
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Re: 28s vs 35s

Post by mellorp » Sun Apr 17, 2016 3:19 pm

This will really give you a headache, I'd stick with my tap explanation but here goes...

You are assuming an unknown quantity so it will never make sense.

Think of it mathematically.
The max volume of fuel/air the engine can consume is X cubic mm
The max amount of oil the pump can deliver and the engine needs is Y cubic mm
The max amount of fuel/air mixture the ideal carb can deliver is Z,
The max amount of fuel the exhaust can get rid of is W

W, X and Y don't change, regardless of how much or how little mixture flows. The engine always needs Y amount of oil. Put 100cl of oil in a bucket of petrol or a bottle, it's still 100cl. Y is only a variable if you use premix where it become s a %of the mixture delivered to the engine.

Volume X, which is dependent of inlet tract diameter and length, crank case volume, stroke, transfer port size and shape, squish and piston crown
Volume of W is dependent of the resonant frequency of the pipe

To calculate the correct value of Z you take X and W into consideration. This will determine the ideal velocity of the mixture as well as the volume. If may be that there is no ideal physical solution to the mathematical answer, however 28mm might be the answer to the ideal velocity and 35mm the ideal solution to the volume

Now this is a massive over simplification of the actual way you calculate the theoretical size of things, and the physical implementation is always more difficult. Air solenoids and Power jets artificially alter the volume of fuel the carb delivers whilst altering the volume and the velocity, Power valves or KIPS artificially alter the exhaust, CDI's alter timing and what ever else it looks after to match the spark advance, kips, air jets and electronic power jets.

Who would be an engine designer as everyone else knows better.

Stick to solvol :-)

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Re: 28s vs 35s

Post by 500bernie » Sun Apr 17, 2016 6:48 pm

I like maths =P~
So other than calling all of the major variables a designated letter, is there anywhere I could see the formula or calculations?

I am under strict instructions not to get the Solvol out on the new bike, so my Lenny Henry time could be well spent improving my understanding of the infernal combustion engine (two stroke only).
Cheers
Bernie :)
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Re: 28s vs 35s

Post by JanBros » Sun Apr 17, 2016 7:32 pm

On pitlane or ESE topic, this was Frits' answer on how to determine the carb size. keep in mind this is for racing !

Rule of thumb:

Diameter = SQRT ( cylinder capacity x rpm of maximum power / 900 )
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Re: 28s vs 35s

Post by 500bernie » Sun Apr 17, 2016 7:49 pm

Thanks Jan,
That should keep me occupied for a while.
Cheers
Bernie :)
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Re: 28s vs 35s

Post by maccas » Mon Apr 18, 2016 12:51 pm

This is my understanding.

35mm carbs need larger main jets than 28mm carbs because the airspeed through the carb bore is lower, for a fixed mass/volume flow rate of air.

Volume flow rate (m^3/s) = mass flow rate (kg/s) / density (kg/m^3)

Volume flow rate (m^3/s) = AVERAGE velocity (m/s) X cross sectional flow area of pipe (m^2)

So it is clear to see that if you increase the cross sectional flow area of the carb, the velocity through the carb bore will decrease.

The pressure within the carb bore is proportional to the velocity of air through it. The higher the velocity, the lower the pressure in the carb bore.

As the float bowl is vented to atmospheric pressure through the breathers, you get a pressure difference between the fuel in the float bowl, and the air flowing through the carb bore. As the float bowl is connected to the carb bore via the main jet and emulsion tube, this pressure difference (high pressure float bowl, lower pressure carb bore) forces fuel to flow into the carb bore. The greater the pressure difference between the float bowl and the carb bore, the higher the fuel flow for a fixed size of main jet.

So the 35mm carbs need larger main jets in order to flow the same amount of fuel as a 28mm carb for a given air to fuel ratio and volume/mass flow rate of air/fuel mix.

Right, next, one way to improve the efficiency and power output of the engine is to increase it's volumetric efficiency.

100% volumetric efficiency means the engine is consuming 250cc of atmospheric air (atmospheric temperature, pressure, and density) with every intake cycle.

I don't think a KR-1 will manage 100% volumetric efficiency at peak torque. Probably more like 80%. Turbochargers and superchargers can increase the volumetric efficiency above 100%, by increasing the density of the air entering the engine. Cram more oxygen into the cylinder (with an appropriately mixed amount of fuel) and you get more power.

Now, we come back to the difference in carb size.

The airspeed in the 28mm carb is higher than the 35mm carb; thus, it's pressure is lower. This also means it's density is lower. Increase the carb size and I think you can improve the volumetric efficiency, by getting denser air into the cylinder.

Maximum power is made when all the oxygen is used in combustion, this requires excess fuel to make sure all the oxygen is involved in combustion. This is why the stoichiometric air to fuel ratio does not give best power output.

The required air to fuel ratio for best power should be the same regardless of the size of carb. To get more power, you need a greater amount of this mixture.

I'll add more later, got a meeting now lol.

Dan

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Re: 28s vs 35s

Post by Top-shaggy » Sat Apr 23, 2016 6:59 pm

Well I'm puzzled...

Part numbers and flow rates for oil pumps as follows:


KR-1 16082-1093 3.0-3.7mL
KR-1S 16082-1103 3.6-4.2mL
KR-1R 16082-1104 3.6-4.2mL

The tech specs show KR-1 oil pump flows less oil than KR-1S
The R pump appears to flow the same as the S? - think I need to investigate this further...hmmm

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Re: 28s vs 35s

Post by 500bernie » Sun Apr 24, 2016 3:13 pm

Thanks Shaun
Very interesting.
Cheers
Bernie :)
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