Max exhaust port areas

Btw Darrel, did you find a picturen of the KDX 200 cylinder?
I'm curious how it looks.

not found photo yet but will take one for you as i have a friend with a breakers yard / motorcycle parts and spares who will have one on the shelf
they are rotax type with 2 sizeable aux ports either side of the main exhaust port these are also controlled by a kips system powervalve cant remember if its on 1 aux or both
 
from what i can make of it it's got 1 main exhaust port & 4 auxiliiary exhaust ports but if you can get a more decent pic showing all the exhaust ports it would make things much clearer.
 
Just a funny thing; yesterday at work I was reading an article from Frits Overmars (famous dutch 2-stroke man). He was talking about his "perfect" cylinder.

He said that when he started tuning 125cc engines they had around 25 hp at 12500 RPM(30 years ago) and today they are pushing 45 hp at 13000 rpm. He said they improved power with massive figures aswhere the RPM level just raised 500 RPM.

Then he told a story about port timings, that you can better have a large bore instead of a large stroke. His conclusion was that timing should not be bigger, but that cylinders are in need of far lager port area (wider instead of higher).

I will see if I can find that article, so I can translate exactly. Quite intresting and fits this topic perfect.

Just wondering about this this morning. The force on the piston
(pressure x area) goes up with the square of the diameter. Also
bigger bore allows for bigger ports, so all makes sense. Only thing
that doesn't quite fit, is that you would think.. a smaller cylinder
bore would transmit pressure change faster into the case.

Did you find the article? :).
 
Just wondering about this this morning. The force on the piston
(pressure x area) goes up with the square of the diameter. Also
bigger bore allows for bigger ports, so all makes sense. Only thing
that doesn't quite fit, is that you would think.. a smaller cylinder
bore would transmit pressure change faster into the case.

Did you find the article? :).

Yes I did;

85vy8up.jpg


Let me translate;

"In my previous column I promised to tell how I got the idea of a cylinder with FOS-flushing (I don't know where FOS stands for), well here goes;

30 years ago I was working on a 125cc Rotax-engine. You know, the "ancestor" of the Apla-racers where I write about now and then.
That engine delivered 35 HP at 12.500 RPM. I could raise the RPM with bigger ports and shorter exhausts, but that was never good for maximum power.

The Apia RSA125 engine which Faubel, Gadea and Pasini drive with nowadays, deliver allmost 20 hp more at 13000 RPM. The power increased over 50% where as the RPM level where maximum power is deliverd only go up by 4%.

The RPM level allmost didn't change in 30 years!

BIG BANGS
For high maximum power you need 2 things; big bangs in your cylinder and as much as possible in one minute. Why didn't the RPM's go up then in 30 years? Well it issn't like we didn't try or anything. There were built 125cc engines that did 20.000 RPM, but those engines hardly did the 35 hp that I did 30 years ago at 12.500 RPM. For every stroke a 125cc engine makes, it must be filled with 125cc (and preferably more) gasmixture thru the transfers.

If you have a 2-stroke do to much RPM it gets "asthma". The transfers close before the cylinder is properly filled with gasmixture. A short breathing engine produces a lot of bangs per minute, but those bangs aren't that big.

NOT TOO HIGH
If you make the transfers higher, you get bigger holes and they will stay open longer with each stroke. The problem is that the exhaustports needs to be raised alot, because first; old combustiongas needs to flow away thru the upper part of the exhaustport, the part of the exhaustport that opens before the rest of the exhaustport and transfers open and second; because that old gas also needs to be gone in a shorter amount of time if you want to do more RPM.

There is only one thing; the exhaustport cannot be TOO high. I cannot explain this in this column because I don't have enough space for it.

Allright, that's one side of the story; when a 2stroke makes to much RPM, the power gets worse.

Now the other side of this story; 30 years ago there were F1 engines with 3 liter V8 engines, so that makes 375cc per cylinder. Those engines also did 12500 RPM. That made me think; "why can a 375cc 4 stroke cylinder make just as much RPM as my 3 times smaller 2stroke engine?"

BORE
F1 engines have a really big bore and that's for a good reason. If you can make the bore of a 4stroke engine twice the size, you can fit twice the size valves in it, which will give twice the inletsurface, which results in 4 times faster cylinder filling, so 4 times the RPM.
With that you get 4 times more power (well, more or less, not completly true, the combustionchamber gets an ineffective shape).

Now if you want to keep the same CC's with a bore that's twice as big, you need to make the stroke 4 times shorter. That's an advantage, since it will hold up better at high RPM.

KREIDLER
A modern F1 engine has a bore of 98mm (bigger is not allowed by FIA) and a stroke of 39,7mm. That's the same stroke as a 50cc kreidler engine. But you will never get a kreidler engine to make 19000 F1 RPM.

What if we made the bore of the kreidler engine 2x as big and the stroke 4x as small? Then all the ports can become 2x as wide. That's nice. But, the porthightes are a fixed percentage of the stroke and they will be 4x lower as normal. The total portsurface (width x height) will be 2 times smaller as before and the RPM level where the cylinder reaches optimal cylinderfilling will be halved. Same goes for maximum power, so a shortstroke 2stroke engine is not a good idea...

With a 4stroke engine it works the other way around. There, maximumpower, only depends on the valve diameter and so directly from the bore and not the cylindervolume! If you leave the bore unchanged and you make the stroke 2 times as large, the cylindervolume will be twice as large, but that also means it takes 2 times more time to fill the cylinder. The bangs the engine gives will be double as hard, but at a halved RPM level. So the poweroutput stays the same as before, inspite of the bigger cylindervolume.

FOUR VALVES
F1 engines don't have transferports, but they have valvesv (the FIA forbids 2stroke engines, properly because they are affraid it will be to fast).

For those valves you can calculate how big they are and how long they should open in proportion to ports of a 2stroke engine? In other words; what do I have to do to my ports on my 2stroke engine to get just as much RPM as a 4stroke engine without getting the engine asthma?

I did the math (and it was shit) and the outcome was that we need to increase the transferport surface and more important; we need alot more exhaustport surface and to make it more difficult, they cannot be made any higher then they are now...

It's possible, but how would a cylinder like that look like? Nice puzzle he?
I will give you one tip; the difference between a "normal" 2stroke cylinder and mine FOS-flush system is something like 4stroke 2 valve engine and a 4stroke 4valve engine.

Can you picture it?"

I translated by hand, so it might contain some faults in spelling etc. But it's sure is intresting! Also I found a Excel program which let's you calc. time area etc. But all is in Dutch, I have to translate. Will do that later, right now I don't have time.
 
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So here I got the Excel file; I added some of my own formules for timings. So you can calculate for all transfers and exhaustports at the same time. Also it's now possible to have stock timing and adjusted timing in one sheet. To keep track of things;

Enjoy;
 

Attachments

  • 2 stroke tuning eng.rar
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very interesting article
so 30 years on im busy working away on my dyno developing rotax 122 aprilia rs125 engines applying my % of stroke theory widening ports as far as i dare and reducing port durations pushing peak power down to lower rpm (11600)to allow the motor to breathe more efficiently on restricted carb sizes as i found too the motors were getting astmatic at higher rpm
i also found the motor wasnt in need of a high compression ratio as had good corrected cr due to low ex port duration
i feel a longer stroke would also be effective as you can obtain the same port duration with a lower port height
bram what do you think the 2 valve 4 valve comparisson is
i think ill put some extra subs in like the kwak200 design has and see what results i get
 
very nice article! i find his theory about widening the exhaust port is true, I've seen pics of honda rs125 gp bikes' cylinders and the transfers are very wide in comparison with their height. thanks for the excel spreadsheet, very nice stuff!
 
very interesting article, nice work Big B.

the article simply points to how much port area we can cram with a limited bore & stroke, the real high powered 2strokes are almost equal in bore & stroke (square engine) or has a slightly longer stroke (under square). with today's high tech cilinder kits such as the Team 7T, TCR, 2Fast where there is very little wasted space they are making more hp per cc out of the box than a heavily tuned cilinder (Team 6T) & with less port duration too.

oversquare layout works with 4stroke engine's b'coz it is all about how much valve area you can put inside a particular bore size, with a 2stroke you need to make the stroke longer to make significant increases in port area, to get the same gain in area with a bore increase you need to make the bore very big but that also pays some penalty in terms of flame propagation. a bigger bore simply needs more initial advance to get the flame across a bigger piston surface area.

i also have been working on 4stroke engines for a long time now so i understand what Frits is pointing out (2valve vs 4valve), a 4valve head will always outflow a 2valve head but to make more power from a 4stroke other than improving the ports, valves, exhaust pipes & reducing friction you'll need to rev the engine much higher that ever & this is being proved by all the Japanese manufacturers in MotoGP by switching from metal valvesprings to pneumatic, only Ducati is running with a mechanical valve system (desmodromic) which offers more precise valve timing while keeping costs down.
 
I think a cylinder would look something like this;

853drbq.jpg


With extra aux. ports next to the main port (like 2fast, only bigger).
What would that do with the exhaust lenght and shape?
 
very interesting article
so 30 years on im busy working away on my dyno developing rotax 122 aprilia rs125 engines applying my % of stroke theory widening ports as far as i dare and reducing port durations pushing peak power down to lower rpm (11600)to allow the motor to breathe more efficiently on restricted carb sizes as i found too the motors were getting astmatic at higher rpm
i also found the motor wasnt in need of a high compression ratio as had good corrected cr due to low ex port duration
i feel a longer stroke would also be effective as you can obtain the same port duration with a lower port height
bram what do you think the 2 valve 4 valve comparisson is
i think ill put some extra subs in like the kwak200 design has and see what results i get

From what I understand from other articles from Overmars, is that they have a completly different engine as stock. Balans axal at the front of the block instead of between the gears.
That balans axal is driving the disc that's mounted behind the cylinder.
 
well the only thing we can do is to try different pipes to see what the effect would be from increased port area but the exhaust port runner itself must be made a bit bigger to accommodate the extra exhaust ports.
 
I might be the bad guy, but I couldn't find this article interesting. What I don't understand, what you real 2-stroke tuner guys find interesting. You should know these, cause the article doesn't conatain news or innovations!

The first thing I am surprised, the authort is amused the 4-stroke is different from 2-stroke, so why did he think the theories from 4-storke works on 2-stroke. He wrote, he tried to copy some of the the 4-stroke highlights without any luck (comparing the stroke-bore-and rpm possibilities). It's not surprising cause they are far away from each other!

Secondly he talks about asthma and cylinder filling on 2-stroke without mentioning the expansion chamber in this article. In turn 2-stroke is all about many parts working in harmony, we can't talk about cylinder filling on 2-stroke without expansion chamber so talking about transfers sizes/positions only is pointless.
Today technique avaible to design very good expansion chambers to solve the problems, we can play with the pulses etc (simulation softwares, 2-stroke developing softwares).
A very good example the theory changes the crankcase compression decreases at the past few years, because expansion chambers do their jobs much more better than the past, so high primary compression isn't necessary to reach power what's more they are much more powerful.

On 4-stroke we can seperate the parst like valve, cylinder head or exhaust, on 2-stroke we can't do this.

You maybe don't share my view but this is a forum so we can argue! *D
 
No you are not the bad guy; this article is from a motor magazine that's primary read by 4-stroke motor drivers. So it's writen in a language that every will understand. Without getting to much into details. I agree with you that it lacks from theoretical deepness.

Allthough I can see his point, what he means. This man has alot of experience in 2stroke and he absolutly know's his stuff. Maybe his comperession issn't that good, but he has a point, then again; this article is mostly read by 4stroke people, so to make things clear he used this comperession.
 
No you are not the bad guy; this article is from a motor magazine that's primary read by 4-stroke motor drivers. So it's writen in a language that every will understand. Without getting to much into details. I agree with you that it lacks from theoretical deepness.

Allthough I can see his point, what he means. This man has alot of experience in 2stroke and he absolutly know's his stuff. Maybe his comperession issn't that good, but he has a point, then again; this article is mostly read by 4stroke people, so to make things clear he used this comperession.

Ok sorry I misunderstood the target of the article! So it's a general discussion for the readers!
 
Because this discussion is about port areas and I just happen to come accross this article that matches this topic. Just some info to feed the discussion :)
 
I think this one of the reasons why the Malossi 7T cylinder is that fast.
Where we struggled a few years ago with the 6T cylinders to get 20 HP out of them, we now push 21 HP stock with these 7T cylinders and over 25 HP tuned.

Maybe I get wild and drill some extra exhaustports in my cylinder.
 
it is simply all about cramming as much port area into a limited space, like Overmars said it is the difference between a 2V & 4V head but looking at how radical these cilinders nowdays it is becoming much like an 8V head on an NR750.
 
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