General port timing guidelines?
 

I have the urge to fuss with an engine. I have an ECHO 65cc that I have already converted. I would like to optimize it. I have not measured anything. I am using an RCEXl ignition. I did add a big carb and a free flowing exhaust. If forget the prop and RPM but, it produce about 30# of thrust.

Generally speaking, what should the port timing be for maximum power?

Anyone ever make a tuned pipe from scratch? I was thinking about an aluminum beer bottle or water jug. How do I calculate the volume needed? How important is the angles of the expansion chamber walls and the reducer at the end? Is there a secret to getting the heat resistant tubing to connec the header to the pipe without paying a bunch of money? Do auto parts stores or bike shops have a kind of tubing that will work?

I was hoping to make a trombone like affair that I could adjust and try to see what volume works best.

RE: General port timing guidelines?
 

Here's a little light reading for you

Read all of the pages as each part has an affect on the next part.

I know it doesn't specifically relate to RC enignes but trust me - the information is good for RC ones too :).

[link]http://www.chuckbunnell.com/kart/jennings/jennings.html[/link]

Then when you think you have all of the answers - let us know how you ended up. I personally find experimenting to be the best part of this whole hobby. Even better if the experiments work too :D

Beware that by grinding ports out, you will reduce the life of the plating in the cylinder. Good plating intrudes very slightly into the port area so that the rings don't catch the raw edge of the plating. A good sanding with some fine wet and dry paper (1200 grit or finer is good) around the ground area reduces the issue.

RE: General port timing guidelines?
 

I have read that over an over. It is my waiting on a bench book. He is all over time area curves. Great, I guess...... It still does not tell me what I can apply in real life.

I measured the ports tonight. I got Intake opens 64*BTDC closes 62*ATDC, Exhaust opens 83*ATDC and closes at 87 BTDC. I must have my TDC a little off so call it 63/63 and 85/85.

RE: General port timing guidelines?
 

The way you first stated your numbers before you edited your message came out to 149 degrees exhaust timing, and 125 degrees intake timing. Not bad, though a touch conservative depending on what rpms you want the engine to run at.

After you edited your message, the numbers come out to 126 degrees intake timing, (believeable) and 190 exhaust timing, (I highly doubt it.) Check your numbers.

AV8TOR

RE: General port timing guidelines?
 

It sure does tell you. Time to do some math :)

RE: General port timing guidelines?
 

Doooooh! :eek:
Sorry I was using a drafting protractor that has two scales, one is 90* that repeats four times

Intake
Open 64*BTDC (296*ATDC)
Close 61*ATDC

Exhaust
Open 102* ATDC (78*BBDC)
Close 254* ATDC (106* BTDC) (74* ABDC)

RE: General port timing guidelines?
 

I have been trying to make sense of the Jennings book myself. If I am understanding the part about "mean port opening". The port(s) are essentially smaller than they would appear due to they are only fully open at TDC and BDC. So, in order to compensate for this, the port timing or port area has to be altered for the port(s) to function efficiently for the intended RPM and displacement of the engine.

I may be wrong, but I hope I'm at least missing closer.

RE: General port timing guidelines?
 

In low rpm applications like aircraft, conservative timing is advisable, power gains will come from just making the ports wider where timing changes will move power band up in the rpm range.
Either a timing change or port width change will make an impact on time areas.

If you are using a lawn tool engine the stock timing usually is about right for aircraft.If you want a little more rpm increase exhaust and intake timing a few degrees and try it.
But start with wider ports.Leave the transfer port timing where it is but widen ports.

The exhaust port will give you the single bigest gain.

Pipes, boat guys use silicone tubing to connect pipe to header. Cone angles are important, again conservative is best.Making pipes is a pain in the neck. You may find a usable pipe from the goped hotrod parts that you can adapt.
One of the boat guys made a pipe from an aluminum baseball bat jb welded together for a boat. It may not be big enough in diameter for your size engine.

RE: General port timing guidelines?
 

Tuned Pipe

http://www.mh-aerotools.de/airfoils/javapipe_en.htm

Adrian.

RE: General port timing guidelines?
 

Make sure you check the radio button for gasoline. Temp EGT is in kelvin, which is [K] = [°C] + 273.15.

Some people complain about this pipe calculator but its based on accepted pipe formulas. I think where those individuals went wrong is with the EGT.

A tuned pipe even if the dimensions are off will still make a lot more power than open exhaust port or muffler.

RE: General port timing guidelines?
 

It is a good bit of work to use the Jennings system because you have to map the ports, measure them accurately, and calculate their size. You then have some math to do to figure out the time/areas.

I have had good luck using about 130 to 140 for intake timing, and 150 or so for exhaust timing and leaving the transfer ports alone with the engines we commonly convert for airplanes. This all being for an engine desired to run below 8000 rpms. If you want to run higher rpms, and/or a tuned pipe, you will need to go with higher timing numbers, especially on the exhaust and/or with a tuned pipe. When going much higher on the exhaust timing, you will need to work on the transfer ports as well. It is rather difficult to modify the transfer ports, as they are hard to get to and you want to be very sure to keep their timings equal and at the same angle of exit as original.

AV8TOR

RE: General port timing guidelines?
 

Blairs software does all the number crunching for you.I've used it for years.

RE: General port timing guidelines?
 

Most people, including me sometimes, don't have the patience to properly map the ports. Unless you are racing, is the last ounce of power really worth it?

AV8TOR

RE: General port timing guidelines?
 

It is. Here we have a case where a guy wants to bump up power from a stock engine.
Unless there have been dozens of engine builders modding this particular engine and publishing known timing numbers, you have to come up with a plan on your own and doing it for as few dollars as possible virtually dictates you study it for a while before getting the die grinder out.
Given that its for aircraft use you don't want to go crazy with timing and huge ports. You can do a lot of experimenting and ruin cylinders and so on,where studying the engine and mapping the ports and developing a plan you can make an increase in power wisely without ruining anything.

RE: General port timing guidelines?
 

Well, you're right.... The only thing I might say is that it is often beyond the abilities of the average enthusiast, and more labor intensive than a lot of people care to do.

We know the port widths are safe for the rings as designed/delivered by the engine maker. Cleaning up the ports a bit and raising the exhaust timing to 150 degrees is a pretty safe and proven power booster for people that just want to tinker and gain some power, given that they also pay attention to carb size and muffler flow.

AV8TOR

RE: General port timing guidelines?
 

That works too.:D

RE: General port timing guidelines?
 

On my particular Homie the ex. port timing is 130 and when the piston is at BDC it is almost 3mm above the floor of the exhaust port.

Would it be a bad idea to remove a small amount of material from to piston top just at the exhaust port to increase timing and port area?

RE: General port timing guidelines?
 

a lot of guys do that to make timing changes with out commiting the cylinder to permanent changes.You can change intake timing also using this approach although on the exhaust side you are limited to amount of change you can get because the width of the land between ring and crown.You probably could get a few degrees though.

RE: General port timing guidelines?
 

If you visualize the piston area from the top of the upper ring land to the top edge of the piston itself, I remove half of that. That leaves enough material to support the ring land. I have done this on numerous engines without problems. Chamfer your cut of the piston top, with two things in mind. 1. Good flow out of the port. And 2. Take only as much as absolutely necessary because this lowers the compression ratio.

I don't know why some engines don't completely uncover the exhaust port as in your case, but modifying the piston to allow the port to open fully will definitely gain some power. On one engine that I did this to, it gained 800 rpms!

As a side note, checking to see that the exhaust port is opening fully is the first thing one should do when leaving out the base gasket and/or machining the bottom of the cylinder to increase the compression ratio.

AV8TOR

RE: General port timing guidelines?
 

On a very cold and windy day I had nothing to do so I thought I would check the port timing on my Homie 30 project, to see why it doesn't perform better. I already knew the exhaust timing was 130. I was surprised to find the intake timing was 150 Deg. Isn't that kinda high? How would that effect the performance?

RE: General port timing guidelines?
 

Sounds high, I don't remember what the stock timing is on homies,I thought it was around 140.If its at 150 you rpobably don't need to do anything to it provided it has decent width.

RE: General port timing guidelines?
 

The intake timing, is 150! That is stock, I haven't changed it. The exhaust is only 130. From what av8tor is saying, it should be the other way around. This engine only turns 6600 with a 16x8 prop., gutted muffler and a 11mm carb.

RE: General port timing guidelines?
 

Deja Vu... Didn't we go through this before?

It is possible someone modified the intake timing by either grinding the port or cutting the piston skirt. Don't worry about the intake timing and just get the exhaust timing up to 150 degrees or so.

Your engine is down on power compared even to fairly stock 25/30cc Homelites I have had. It is not hard to get them to turn a 16 x 8 at over 8500 rpms with just a little porting if things are right. Check your ring gap. It should be about the width of a piece of thick paper at the most. I am betting it is much larger than that, and giving away power. .003" is the spec if you have feeler gauges. Frank Bowman's rings are actually set at ZERO ring gap.... Note that you also need at least an 11mm carb and a free breathing muffler to make power.

By the way, I have a fully modified Homelite 30cc that turns an APC 16 x 8 at 9400 rpms. A friend has a 25cc that I did only moderate mods to and it turns the same prop at 8500 plus.

AV8TOR

RE: General port timing guidelines?
 

Yeah we did.

I had gotten frustrated with this engine and set it aside for awhile. But now I have acquired a couple of Ryobi 30s and they have very similar numbers. I was hoping to find a cylinder that would run well enough without changing the ports. I am resolved now that I will have to cut the ports in hopes get better performance.

As far as ring gap. I measured the ring gap on one of the Ryobis and the gap was 35 thou. I'm not sure what the gap is exactly, on my project Homie engine but I know it is similar from what I can recall. I have contacted Frank for a set of rings for the Homie.

RE: General port timing guidelines?
 

Another thought....

Lowering the cylinder by leaving out the base gasket and/or machining the block or cylinder base does several things besides raising the compression ratio. One, it DECREASES exhaust timing. Two, it INCREASES intake timing. Three, it brings about the possibility of the exhaust port not being opened completely by the piston.

Since your engine exhibits all three items, it makes me think that someone has lowered the cylinder....

AV8TOR