ESE's works engine tuner

[adegnes]

Was thinking this afternoon (I know I shouldn't do such a thing), about CFM vs HP and how this may correlate to the 2 stroke. Now I am NO engineer, scientist, etc. so most of my research is the search button on Google

according to a website, you need 33.31 cfm to make 48hp...now I know there are multiple variables that you need to take into account before assuming such a number, but doesn't 33 cfm seem rather low? I mean a 38mm carb can flow a shit tone more air then that (obviously the test pressure would need to figured). So what am I missing??? Why don't we run smaller carbs and smaller intake ports and ramp up velocity? It would appear from my untrained eye that everything seems to be on the big size.

Please educate me!

Because the intake doesn't flow all the time, and some of the time is does flow, it does it in the wrong direction.
Sorry, no time for a real answer(and someone will probably soon chime in with a better explanation than I can provide anyway...)

[oldjohnno]

why not scale the carb size by area from a known well developed engine.

Using figures from eight engines from Yamaha, Honda and KTM gave an average carb size constant of 33, ie. Throat diameter = √(hpx33). The individual numbers varied from 29.5 to 40, but even after eliminating these two the average remained at around 33, so I think this would be a safe starting point.

I reckon this is a good way to get very close. http://www.underdogsracing.com/fospipe/ And you get a pipe as a bonus.

Input all the relevant bits and it will recommend the exhaust port diameter for the engine. The 2 engines I have at the moment use the crab size that this calculator recommends as exhaust port dia. Good place to start I reckon.

This seems to result in an awfully big carb.

Re carb and reed size for a race engine.

The reed block port dimensions are more important than the petal curtain area, as small petals will self compensate somewhat by staying open longer.

Interesting you should say that - when you look at the evolution of Japanese MX engines from the early days of reeds in the 70's to the current day many of them started off with smallish reeds that grew much bigger in the 80's before reducing in size again later on. The later engines had a much cleaner flow path though.

[wobbly]

One big issue with all the MX engines is that the carb is severely angled ( in both planes ) and this gives hugely asymmetric flow thru the petals.
Thus technically the reed blocks are way too big, but due to this flow limitation,in reality the situation is very complex.
I can pick up 4 Hp ( in 36 ) from a SKUSA CR125 simply by changing the petals and backups, to help even out the flow pattern into the case.
If we were allowed a straight manifold off say the RS125, then another 2 Hp is gained instantly,then another 2 with petals to suit that setup.
So going from 36Hp to 40 then to 44 just by changing the petals and manifold.

[oldjohnno]

...So going from 36Hp to 40 then to 44 just by changing the petals and manifold.

Bloody hell, who wouldn't kill for an easy 22% increase?

Interesting that the engine is so sensitive to the incoming flow bias - I picture the conditions in the crankcase as being like a tornado of turbulence, what with the crank and rod whizzing around so quickly, and would've thought the turbulence would make it relatively indifferent to flow direction. Obviously not...

[wobbly]

Regarding flow bias, there is around 2 Hp in 48 in a KZ10 kart 125 engine by utilizing a reed cage that is angled up toward the piston.
All of the manufacturers use reed petals that are softer on the top, to get this bias to point the incoming flow directly at the transfer entry area.
I have tested this and got an ICC engine over 50 Hp for the first time - with the same petals/backups top and bottom.
Not allowed to use it in the current homologation engine, but that reed block is going with me to the TM factory.
If an RS125 tuner is looking, the first thing you should be into is differing petals and rev plates/backups top and bottom - free Hp in a few minutes, or should I say days,
to get it right.

[DoldGuy]

Wobbly,

Have you seen similar results with the cylinder reed intakes or just case reeds?

[Nath88]

Regarding flow bias, there is around 2 Hp in 48 in a KZ10 kart 125 engine by utilizing a reed cage that is angled up toward the piston.
All of the manufacturers use reed petals that are softer on the top, to get this bias to point the incoming flow directly at the transfer entry area.
I have tested this and got an ICC engine over 50 Hp for the first time - with the same petals/backups top and bottom.
Not allowed to use it in the current homologation engine, but that reed block is going with me to the TM factory.
If an RS125 tuner is looking, the first thing you should be into is differing petals and rev plates/backups top and bottom - free Hp in a few minutes, or should I say days,
to get it right.

For MX bikes with the bent inlet, could you use a divider plate through the bend to keep flow to all petals constant, or bias it if required?
Could the reed cage just have the tip angled like the trailing edge of a wing to guide the air to the transfers?

[richban]

This seems to result in an awfully big carb.

How big is awfully big? My 50 runs a 28mm ok. The 300 runs 2 x 40mm's and My 90 is looking like it will be around 32 / 34mm. Road racing engines that is. Stock YZ250's run 38's


What did you get form the calculations?

[oldjohnno]

How big is awfully big? My 50 runs a 28mm ok. The 300 runs 2 x 40mm's and My 90 is looking like it will be around 32 / 34mm. Road racing engines that is. Stock YZ250's run 38's


What did you get form the calculations?

Seems I may have overstated it. For a mild, 75hp 350 it gives a diameter of 55.3mm, but if you use a constant of 33 (which is in line with the averaged factory sizing) the result is 49.7mm. In other words making the throat diameter the same as the exhaust port area results in a carb with about 23% more throat area than "average". Having said that there are KTMs with a carb size constant of 40, and this gives a size for the same 350 of 54.8mm - virtually the same as exh. port area. I was wrong in saying "awfully big" - what I should have said was that it gave a size that's at the upper end of factory usage.

Out of curiosity I looked at some more stock engines and it seems they all fall into that range from 30 to 40, regardless of whether they are a docile trials engine or a more highly tuned racer. I don't know about disc valved engines but suspect they would be at the upper end of that range or perhaps a little higher.

[husaberg]

Re carb and reed size for a race engine.

Here is a note I had Neels add to the help file of EngMod


" For high performance, high BMEP, applications a quick check through the areas generated by the software will give a good indication of the engines overall efficiency, and thus a in pointer as to where to devote time with changes to an input.

In these cases the carburettor inlet area, the reed effective area, and the total exhaust and transfer areas will converge to equality, as each of these elements is optimized. Using this technique a "too small a reed" for example will quickly become evident – and a change to improve this, will immediately give a power increase when the simulation is re run. "

The reed block port dimensions are more important than the petal curtain area, as small petals will self compensate somewhat by staying open longer.

what is the rule of thumb where did i pluck .8 from?
Side note I have a AC RD350 that i are playing with at the moment.
I was going to use late model YZ85 reed valves rather than RZ350 reed valves but can't really find anything straight to match the NSR250 carbs i was going to recycle. any ideas anyone?
The only thing that seemed to suit was hacking up a RGV intake or a KR1S. Or snowmobile.
Only aiming for 50HP.

[F5 Dave]

what is the rule of thumb where did i pluck .8 from?
Side note I have a AC RD350 that i are playing with at the moment.
I was going to use late model YZ85 reed valves rather than RZ350 reed valves but can't really find anything straight to match the NSR250 carbs i was going to recycle. any ideas anyone?
The only thing that seemed to suit was hacking up a RGV intake or a KR1S. Or snowmobile.
Only aiming for 50HP.

TZR blocks fit in YPVSs, but RDs are probably a size smaller.

[adegnes]

what is the rule of thumb where did i pluck .8 from?
Side note I have a AC RD350 that i are playing with at the moment.
I was going to use late model YZ85 reed valves rather than RZ350 reed valves but can't really find anything straight to match the NSR250 carbs i was going to recycle. any ideas anyone?
The only thing that seemed to suit was hacking up a RGV intake or a KR1S. Or snowmobile.
Only aiming for 50HP.

Maybe you could cast something from suitable rubber compound?

[husaberg]

TZR blocks fit in YPVSs, but RDs are probably a size smaller.

All the TZR blocks i have seen were pretty massive, they looked bigger than the TZ750 which i think are the same as Dt250 or early It175.
I think the reed casing on the rd350 is bigger than the rd400 though.
But its pretty easy to poke a hole fitting the RZ350 reeds. which are the same petal size as a tz750 reed but physically smaller cadge.
edit found this

Maybe you could cast something from suitable rubber compound?

i have seen a couple of threads where people have done that.
One for the KR1s for the 35mm SP version
http://www.kr-1s.co.uk/forum/viewtopic.php?t=5197
one for the RD's but only Angled factory spec ones for the RD's
http://www.precisiontradingsystems.c...et_rubbers.htm
http://www.precisiontradingsystems.com/inlet_videos.htm
I think it would be easier to make something like this

[sispeed]

To prevent pucking a hole in the Cylinder when altering reedcase I do the following things.
Clean the Cylinder on the outside from color and dirt. Plaster the outside with JB-Weld.
If you shave to much from the case inside you see only JB and no light. That is a good thing.
Not so fine may be that you loose some cooling
Regards Siggi

[casal-fan]

The NF4 was really a motocross engine; the slant manifold served to move the carburettor out of the way of the central rear shock absorber. The effect was twofold:
The flow was reluctant to follow the bend; it concentrated on the outside, so the reeds on that side were overloaded and perished, while the reeds on the opposite side could have been much thinner because they hardly saw any flow at all.
The same was true for the crankshaft main bearings: the left-side bearing hardly received any premix lubrication. And boy, were those Honda main bearings expensive.
The short straight manifold worked a lot better, as Wobbly wrote.
So why substitute it with the long straight manifold? My guess is: to better isolate the carburettor from engine vibrations.

I came to think of this post when I found out what a left side roller crankshaft bearing for a GasGas engine costs... those are wayyyy expensive in comparisson with the ball bearing used on the right side.
The intake situation on the std. engine is the same as the one Frits describes above, but as I´m modifying it for road racing, it will get a straight manifold as well as simetrical design left and right Vforce reedcages from an Honda ATV. (usually reedcages for twisted intakes have an asimetric design).

Asking for youguys opinion if this change in setup would open for the possibility of using a ball bearing instead of roller bearing.
The extra cost of the roller covers the expenses of an extra bearing change with some change leftover for a pint... and I like pints...
Just asking for your opinion.
In advance thanks.