Looking for some info on port timing numbers for piston ported, non piped gas engines any input? What have you guys been finding to work well?

Mark

Look up the thread "Don't forget the Echo..." We covered a lot on engine mods and port timing in that thread.

EDIT: Well, searching is a little complicated in this forum, so I looked it up for you:

http://www.rcuniverse.com/forum/m_69.../tm.htm#695908

Good luck and have fun,
AV8TOR

Hi,
I have measures many engines and found that most chain saws have about the same timing. Trimmer engines vary quite a bit. Here are some numbers. If you want exact numbers for a specific engine, let me know. I will see what I have.

Chain saw:

Intake 140 degrees duration
Exhust 150 degrees duration
Transfer 118 degrees duration


Trimmer:

Intake 110 to 132 degrees duration
Exhust 120 to 150 degrees duration
Transfer 100 to 118 degrees duration

Dave

http://www.rcuniverse.com/forum/m_3497644/tm.htm
TORO HI TECH TERD
I HAVE DONE A LOT OF PORT WORK AND HAVE MY SPECS IN THIS POST. I THINK IM AT THE LIMIT BECAUSE I HAVE SOME INTAKE REVERSION NOW SO I JUST GOT SOME REED VALVE PLATES FROM A SMALL OUTBOARD TO TRY TO BUILD AN INTAKE WITH REEDS IN IT. MAYBE I CAN GO MORE WITH THE PORTS AFTER I DO THAT. IM STARTING TO LOOSE THE LOW IDLE SPEED NOW
FARMER TED

Piston port intake will give high top end power but, as you have found, at the cost of mid and low end. A good reed setup will have the best overall performance. The trick is to contour the interior of the reed box to minimize turbulence. Most don't realize they are dealing with supersonic air flow inside a two cycle.

IF YOU KNOW OF I SITE THAT I CAN FIND SOME TECH INFO ON REED BOXES IT WOULD HELP. THE BOXES USED ON OUTBOARDS ARE PRETTY SIMPLE. I WONDER IF I NEED TO USE 2 REEDS OR IF A 1 SIDED BOX WILL WORK? THERE'S NOT MUCH ROOM INSIDE THE PORT.
FARMER TED

I don't remember the name of the after market supplier I used. I think you can find them by doing a Google search. The most efficient valves I found were conical or 'V' type. The best reeds were fibreglass. I put a high polish on the sealing surface and contoured the internals, this gives a measurable increase. I haven't worked on reeds for anything smaller than 100cc.

I think it would be possible to make a manifold to replace the stock manifold that is just a little taller and install a conical reed valve with four triangular reeds. Cut the bottom of the piston port down as far as possible and possibly shorten the piston skirt. This would also make room for a boost port if the cylinder is thick enough. This is one of the advantages of a reed system. One of the best conical reed valves was made for the Power Products Go-Cart engines in the 60s.

Usually port timing for a particular engine works in conjunction with port area....so we have a parameter known as time-area which determines the port ability to flow within a given time period.

Having done the time area numbers on my 34cc engine I used the following durations (for a power peak of 7500 rpm)

Exhaust: 160 deg

Transfer (twin transfers only): 125 deg

Intake(piston ported): 160 deg

The factory timings (for stock power peak of 5600 rpm) were

Ex: 140 deg

Transfer: 110 deg

Intake: 135 deg.

My timings may seem radical but are necessary because I am also looking for piston & ring life. I could go wider on the port but that then I would be crossing safe limits of about 65% beyond which ring life reduces.

If you have a rotary valve setup then you can safely go to upwards of 210 deg before experiencing significant blowback. This is what I have found on my 125 cc engines. Using a pipe with a rotary is almost always mandatory if you are going to open the port very soon though.


I dont know much about reed valve engines havent worked on them.. Bryan, if you are going to retrofit a reed valve from a larger engine to a smaller one (which is turning many more revs than the larger one) what are the guidelines, or thumb rules if any?

You are looking at flow volume. A 50cc engine at 10,000 would need as much air as a 100cc at 5,000. It will be harder to charge the 50cc because of velocity. I have used a 25mm carb on a 100cc reed valve engine with good results. A good indication of what can be done with carburation is the current 125cc Motocross bikes, they are all reed valve. When you can get 35 HP from 125cc in stock trim, you are pushing the envelope. Metallurgy has been a large factor in the current high performance engines. Reed valves have one advantage over mechanical valves, they do not allow blowback. However, you have the restriction of the reeds and reed cage.

I recently figured the rim velocity of a 3" diameter crankshaft at 10,000 rpm. 10" X 10,000 = 100,000 inches per minute = 8,333 ft. per minute = 138 fps. This is enough to induce considerable inertia into the charge. The better two cycle engines have the transfer ports parallel to the crank and the intake and boost ports in the direction of rotation to use the circulation in the crankcase to assist the boost ports. The intake ports are directed to the rim of the flywheels to assist in charging the crankcase.

Two cycle design has definately changed since the introduction of the Schnuerle system.

WHATS THE FORMULA FOR PORT WIDTH VERSES RING LIFE. HOW WIDE CAN I GO BEFORE I START TO BREAK RINGS? THE REEDS THAT IVE BEEN LOOKING AT ARE STEEL AND SEEM VERY STIFF, WHAT THICKNESS AND WHAT MATERIAL IS BEST SUITED FOR THESE SMALL VOLUME ENGINES. ONE OUTBOARD GUY SAYS TO USE CARBON FIBER BUT AGAIN ITS VERY STIFF. I WOULD SEEM TO ME THAT THE VALVE NEEDS TO OPEN WITH VERY LITTLE RESISTANCE OR THE CHARGE WONT GET THROUGH IN THE SHORT TIME AVAILABLE. IS THERE A FORMULA FOR REED AREA TO ENGINE DISPLACEMENT?
FARMER TED

Ted,

You can go to about 75% of bore width before you notice exponential reduction in ring life (so to speak).

BUT, that assumes you dont have any sharp edges in the port. It has to be a perfectly oval shape with a good chamfer around the edges. Like this intake port:



Once you cross 70% though, symmetricity of the port becomes critical.

If you have limited porting skills or dont have the right equipment, then 65% is VERY safe. But you must still retain the oval shape.


Ananth

The_Pipefather: seems like your timing numbers should be good for more than 7500. Possibly 8500 depending of course on all the other factors involved.

AV8TOR

Take a look at the Boyesen 2 stage reeds. These use a stiff fibreglass reed that is ported to use two flexible carbon fibre reeds. These are available for 50cc engines.

You're right av8tor.....the engine will rev right upto about 9500 rpm before refusing to turn any further...Of course with the pipe we've limited it to about 8500 rpm.

PIPEFATHER
YOUR THEORY IS THAT FOR A 1.OOO BORE DIA. THE PORT WIDTH CAN GO AS FAR AS .750 WIDE. DOES THE OVAL SHAPE NEED TO BE TRUE BECAUSE THESE TORO'S HAVE A RECTANGULAR PORT AND I CAN ONLY RADIUS THE OUTSIDES AND THE TOP AND BOTTOM NEED TO STAY FLAT OR THE TIMING WILL BE TO GREAT. THE PORT IN YOUR PIC DOESNT LOOK TO BE ANY WHERE NEAR THE 75% YOU TALK ABOUT. AS FAR THE WEIGHT GOES IF A 30CC WEIGHTS LESS THAN THIS 25 TORO IN WORKING ON I MUST BE CRAZY FOR WASING TIME ON THIS ENGINE WORK. I SHOULD JUST GET A BIGGER ENGINE TO START WITH AND HAVE THE RELIABILITY OF THE STOCK CONFIGURATION. THIS TORO IS GONNA WEIGHT 3LBS 5OZ WHICH IS A LOT MORE THAN THE HOMILITE 30 THAT AV8TOR TALKS ABOUT.
FARMER TED

Ted,

I never said that the port in the pic is 75%. It was an example of an oval port, that s it.

Regarding your Toro port, this should make things clearer for you:



Well you can double the stock power and still lose very little in terms of reliability. In fact I have seen increases in ring and piston life AFTER porting than before porting. Of course this is during normal usage. I m talking motorcycle engines here.

If you have a bridged port you can even increase the port size to 110% of bore width or even more depending on ring end gap location. This has the advantage of using low timing while allowing awesome amounts of port flow.

Nice drawing. The whole idea of oval porting is to get more area without needing to greatly alter the timing itself, and to protect the ring. As the ring bulges slightly into the port when it passes, it is gently eased back into position by the oval.

TEDDYWILKK: If you have a weight problem, find yourself a Kioritz engine of 23 to 30cc. These engines are very light and they scream. I have a 23.6cc Kioritz all souped up that turns an APC 16 x 8 prop at 9000+ and only weighs 2 lbs. 9 oz. ready to fly!

Also keep in mind that when I quote weights here, it is without the magneto, but with my prop adapter/magnet holder. The first thing I do with these engines is lose the magneto!

AV8TOR

PIPEFATHER
LET ME SEE IF I HAVE THE PERCENTAGE THING RITE HERE. IF WE USE 1 INCH BORE DIA AND WE MADE THE PORT 1 INCH WIDE THEN WE MEASURE THE PORT ON THE CIRCUMFERANCE OF THE BORE WE HAVE 157% PORT???? SOOOO WITH THE HELP OF MY DAUGHTERS MATH CLASSES SHE SAYS THAT THE ARCH LINE OF 110% OF THE BORE DIA. WOULD GIVE ME A PORT WIDTH OF .890. I DO HAVE A BRIDGEPORT AND WORKED AS A TOOL MAKER FOR YEARS BUT ITS NOT A SIMPLE JOB TO MAKE AN OVAL ON IT. SINCE I DONT HAVE A BRIDGED PORT I CAN ONLY GO TO THE 75% WHICH WOULD BE A PORT WIDTH OF .683. TELL ME I HAVE THIS ALL CORRECT?????
FARMER TED

Ted,

This formula should make things easier for you:

Chord Width = (Bore) x [(Arc Width x 180) / (Pi x Bore)]

Arc width means the size of the port as measured along its circumference.

Chord width means the size of the port if you were to take the liner alone and measure it with calipers.

The formula for 75% is for Chord width, NOT arc width.

Anyway on these small engines there is a very small difference between the two.

I suggest you start with 65% if you are not confident of getting a perfect oval shape.



On a different note, I wonder if you have a CAPS LOCK OFF on your keyboard...

[sm=pirate.gif]SORRY
IM ALMOST DEAF SO I NEED TO TALK LOUD SO I KNOW WHAT I SAID AND IM GOIN BLIND TO SO THE CAPS HELP ME READ WHAT I WROTE
FARMER TED

PIPEFATHER
THATS A FANCY FORMULA FOR THIS OLD MAN TO WORK OUT!!!!! IF THE PORT SHOULD BE NO MORE THAN 75% OF THE BORE DIA. WOULD THIS FORMULA WORK FOR SLOW THINKIN CAPS TYPIN OLD FARMER
BORE X .75
WOULD THAT NOT BE THE WIDTH OF THE PORT ON THE CORD LENGTH??
FARMER TED