Max exhaust port areas

[Browni]

:winner:

The weird thing is, that allmost every racer with 125cc or up uses a single port + aux. ports. You never see T-shaped exhaustports on that.

A fact is, the middle of the exhaustport (where the bridge is) is the most effective part of the port.

If you want power, it's better to adjust angles then adjust timings. The timing affects when it starts flushing, but the angle affects how it flushes and since where are talking about gas dynamica, we are talking about the "HOW" and in less way the "WHEN"

Not only the flowing is the main reason why rotax style ports (one oval + 2 auxiliary) are used on race engines. Sure they usually flows better, but the other important reason is the piston and ring life is much more better, also the reliability. The bridge is the critical part of the exhaust side, because it's in the most hottest place (in the middle of the port), and has a very intensive dilatation, that's why you need to grind down the bridge a few houndreds, but it will be still dangeres for high rpm engines like karts, or GP bikes. Rotax style exhaust have more support for the ring, and much more safe. This is important for a circuit racing engine!

 

[StepVino]

Aloha, I have read all the previous posts here and have comments and maybe some questions...

1: I have seen that an exhaust port that is widest and flatest on the top give the strongest exhaust pulse into the expansion chamber and more power.

2: The strongest cylinders seem to have the most exhaust escaping at the top of the port, ie Evo, wide bridge port. No single rounded exhaust port will produce much power. Of you look at the carbon buildup in the exhaust port, you will find the cleanest area on the top and so the fastest exhaust escaping there.

3: My Malossi Replica with single exhaust port and two auxillary ports gives much better power than a single exhaust port because of the larger exhaust area opening. (time/area). My Metrakit sp2 is mostly the same as the Malossi Replica, but has a larger main port and much larger auxillary side ports and gives more power than Malossi.

But I think were it gets very difficult when you make the transfer angles different with larger exhaust port, you will loose the charge short circuiting into the exhaust and get less power. This is where a lot of research and development and several ruined cylinders will come into play.

frank

Looking for a mechanism for why a too big exhaust is bad, I'm wondering
if a wide lower part of port might drop cylinder pressure too fast. A little
bit of pressure should help the flow out of the transfers develop better,
not wonder towards exhaust port, so by the time the negative pulse comes
from the pipe, there is a nice pattern with cleaner boundary between fresh
mixture and exhaust... So correct area first, then as much of it at the top
as possible = good initial pulse, then some restriction to further flow till
negative exhaust pulse arrives.

PS I need a favor if possible..

I'm working on a simple cad program that would draw out a simple
single hole exhaust port. I have it converted to a curved barrel projection,
so if printed same size, it can be stuck inside the barrel, against cylinder
wall, outline drawn, and hey presto.. ready for grinding. Program makes
a PNG image, that can be printed actual size from windows paint program
IF properties of paint image is 96pixels per inch.

Question is, does the paint program in Windows xp European version use
96pixels/inch. or is it pixels/mm? I have to know this to make the correct
png image. Resolution spec should be available in the paint program
menu, under image-properties. Thanks!

 

[DRXRACING]

That depends on what kinda engine you are running I think. On scooters I haven't seen negative results in bottom end power. Maybe with automatic (chain driven) engines.

Or do you mean it has "trouble" getting up to revs. That I agree.

I first measure all timings stock, then do some calculations to timing and blowdown and count in the parts that will be used (carb/exhaust/cartervolume), also I take in to matter for what purpose the engine will be used (cross/race/sprint/street).

Your thoughts are correct, but you don't need me to tell you that.
And yes all of the engines I currently work on are the Jog engines for the ATV Chasis so the load is a bit more for the engine to pull.
For other information on transfers, I always make sure the transfers are directed at the front half (Intake side) of the piston. It makes a difference . Stagger is also important. Always leave the hooks in the transfers near the windows.
My description of an exhaust port being to wide is when it is already raised to 197 to 202 degrees.

 

[DRXRACING]

i totally agree with you, 4strokes are an evolutionary dead end, it is the poppet valves that limit it's potential. there is only so much you can play with valve angles, sizes, ports etc. the only way for a 4stroke to make more power is by revving it higher which shortens it's service life really quick.

Then you take friction and heat away from the engine for even more power.

 

[DRXRACING]

A few thoughts:

The top of the exhaust port stays open for the longest period of time, hence that is where the most flow happens.
Rectangular exhaust ports make the harder hit of power. (Narrower / peakier). Oval exhaust ports tend to promote a broader delivery of power.
If you have exhaust port over transfers , have it controlled by powervalve. Metra kit pro race cylinders for jogs are Honda NSR 250 powervalve cylinders scaled down with out powervalve by design. (Port lay out).
Your expansion chamber is sucking fresh unburnt fuel into the combustion chamber area from the cases thru the transfer ports. any pressure residing in the combustion area is momentarily entering the first port that opens. Then the rest open and flow into the cylinder.

 

[Big B NRG]

Another reason for power being broader is that single port + aux. port exhaust layout have different timings.

The aux. exhaustport is lower as the main port and opens later. If you raise the aux. port you get the same effect as a narrow/peaky port.

Because scooter engines only do 1 constant RPM, you can have as liltte as 200 rpm powerband.

Just for general info, not telling anybody anything

 

[StepVino]

Are there any more up to date books that cover some of
this material? I'm hoping to get Blair's book sometime, it's now
up to almost $400!! Though from the reviews it seems that
it's full of equations.. That's ok, but it would be nice if it would
also cover what is being discussed here.

 

[tiba_karotsu]

A few thoughts:

The top of the exhaust port stays open for the longest period of time, hence that is where the most flow happens.
Rectangular exhaust ports make the harder hit of power. (Narrower / peakier). Oval exhaust ports tend to promote a broader delivery of power.
If you have exhaust port over transfers , have it controlled by powervalve. Metra kit pro race cylinders for jogs are Honda NSR 250 powervalve cylinders scaled down with out powervalve by design. (Port lay out).
Your expansion chamber is sucking fresh unburnt fuel into the combustion chamber area from the cases thru the transfer ports. any pressure residing in the combustion area is momentarily entering the first port that opens. Then the rest open and flow into the cylinder.

metrakit prorace kits are rotax type exhaust ports not butterfly (bridged) exhaust ports (TCR, 2Fast, MHR, Evo1), i have tuned NSR250's too & even the early MC16 has butterfly exhaust port.

it is a well known fact why the reason there are no powervalves for these kits is b'coz of the CV tranny, if a geared bike would have it's powervalve open permanently (NSR250) that thing will struggle to pull itself from idle to 8000rpm.

i for one advocate the staggered port layout especially on the transfers, opening the transfers all at once wastes the primary compression & the charge quickly loses it's energy for scavenging the cilinder. 2stroke tuning has come full circle when the latest cilinder kits & even some factory tuned kits have the roof angles pointing more upward reminiscent of the old GP bikes.

i have the opportunity to put this into practice once again as i have another project in the works, all we need now is a crankshaft & carters so i can begin the work. i can't reveal all the details now as there are spies lurking around here but the new cilinder kit we will be using has rotax type transfer ports.

 

[50cczip racer]

Hi, just wondering if there are any up/down sides to making
an exhaust port bigger than it needs to be. Assuming
you have already met timing and area specs, and extra
width would still be ok as far as rings are concerned.

Thanks!

there are great benefits to widening ex ports when done correctly and this really is the way forward in 2 stroke tuning that ive discovered and developed in the past year it requires lateral thinking
ive applied it to rotax 122 motors in the aprilia rs125 with huge success getting power from 29 up to 43bhp thats 1bhp more than a honda rs125gp gives on my dyno from a cylinder reed engine with stock generator ignition with light coils and only a 34mm carb on pump fuel on a road legal exhaust system

 

[StepVino]

Do you follow mean area guidelines, or just raise the port and
widen as much as possible?

I went the last route on my 50cc and got (seemingly) great results.
Didn't know anything about time areas at that time, just the name.
The last widening session on it made no difference though. Next
one will be strictly by the book.

 

[50cczip racer]

i have tsr but instead use an extremely simple formula i work to get durations measured in mm = ex ..% stroke / prim transfer % stroke then stagger back dont see short circuiting out of the exhaust as a problem when primary transfers are aimed back and boost left low and the exhaust does its job well so also works as a transfer port cramming it all back in whatever escapes out
Ex port then opened up to max area above transfer height only,the trick is when ex port area is increased then the port duration can be lowered an avenue of tuning left uncovered by most.Years ago when many 2t tuning books/theorys were formulated ex design was never as efficient as it is now and most porting was on single oval ex port designs so in order to get sufficient port area once 70/75% bore width was reached port duration was used at 200+degrees ,now bridged and rotax ports are common the area is coming from width and therefore durations reduced considerably to low 190,s, we may see in the future a design that uses a full cylinder wall of transfers like 7t then above it a full wall of 5/6/7 ex ports with a matching letterbox ex outlet+manifold+serpent header pipe ,i love the kawasaki kdx 200cc cylinder design that uses 5 ex ports,ill try to find a photo if your not familiar
if you get chance and have a late race cylinder like 7t in hand try pushing the piston in from both ends and look at the wall of transfers from the bottom of cylinder and how little ex port there is and then again how much there is above transfer height youll see its maxed out on area to max safe bore width and right up till closing time so adding sub boost as 2 fast is the next evolutionary step

 

[StepVino]

Thanks for the detailed reply. So time areas do work, trading port
height for more area. It just seems like a very loose concept, it's
hard to feel totally comfortable with it.

My current thought is to satisfy both height and area, only because
it still easier to grasp the concept of height vs rpm, and it seems
like if you can satisfy both schools of thought, then the chances
of getting good results would be better

The problem I have with area vs rpm, is that I haven't seen any
mention of how blow down time factors in, almost like it wasn't
relevant.
....

Just went back and looked at an angle area graph. Maybe an
equivalent blodown area can be had from the difference between the
intake and exhaust lines.

 

[tiba_karotsu]

blowdown is becoming less of an issue b'coz of the pipes, they are getting more efficient at sucking out the exhaust & fresh air/fuel from the ports.

 

[50cczip racer]

i find transfer timings of 128 to 132 cover almost all my high end road race set up requirements the higher figures giving a broader spread of power, on mini moto and auto mx 50 fixed gear motors ive gone to 140 when ive gone too far(190) on ex port duration and lost usable spread for the single gear
step vino i really like the program ur working on but due to my lack of mathmatical and computer programming knowhow havent been able to produce or work with myself,my own formula,s are based on known values that work for me every time and take me a minute to calc out prior to marking up a cylinder b4 porting ,fitting testing and evaluating results
personally i would design your program around the best cylinders currently in use ,get hold of a few and calc out port areas.timings,angles etc and use them as base values as without these your values will be unfounded as theyll lack the concrete test of whether or not they produce the desired results as a conclusion to your experiment

 

[Big B NRG]

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

 

[tiba_karotsu]

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

me too, never seen a 5-port exhaust cilinder but there are a few KDX200's running around here.

 

[StepVino]

I tried a search, but didn't find any picks either.

50cczipracer, I do need to pick up some cylinders
(used, seized..) and study them. Right now I'm carefull not
to use anything but known formulas.

Thanks Tiba. Bell mentions that the best performance can
only be had by doing the engine porting first, testing with
a near zero restiction stub exhaust, then making a pipe
to match the engine. I guess pipe design is so advanced now,
that if you have a perfect combination of pipe and engine,
for max power, the engine wouldn't run at all without the
pipe?

 

[tiba_karotsu]

I tried a search, but didn't find any picks either.

50cczipracer, I do need to pick up some cylinders
(used, seized..) and study them. Right now I'm carefull not
to use anything but known formulas.

Thanks Tiba. Bell mentions that the best performance can
only be had by doing the engine porting first, testing with
a near zero restiction stub exhaust, then making a pipe
to match the engine. I guess pipe design is so advanced now,
that if you have a perfect combination of pipe and engine,
for max power, the engine wouldn't run at all without the
pipe?

that is true, a couple of months ago when i was riding my aerox home from testing the header pipe broke off from the flange & it barely ran, it lost 90% of it's power & that is with just a mildly tuned midrace kit, if this should happen to a well tuned race engine i don't think it would even continue running at all.

 

[Big B NRG]

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.

 

[Egebo]

It is very difficult to say weather you will gain more hp from oversizin your exhaust port. It depends on 1000 variables.

But generaly, when you got the port area and shape (time/area) right, to effeciently scavenge (massflow) the cylinder, at your given rpm, that is a good starting point.
Altering areas and shape from here, can result i two things.

1. More energy delivered to the piston
2. More energy delivered to the expansion-chamber

For maximum energy delivering to the piston, you keep port shape as low as possible, to keep the expanding gasses working on the piston, a long as possible.

For max energy delivery to the expansion-chamber, you want a high port shape, to release more energy to the expansion-chamber.
That will result in stronger exhaust/boost pulses.

So results will be defined by how you expand your ports.
Making more energy release to the expansion champer will certanly lower overall power.