Husky porting: starting the math

[edisto]

I believe you that Bell's numbers are off for saws, but I think that the reason why might be that his numbers apply to engines with disc-rotary intakes.

Assuming a mean intake port area of 3cm^2, Bell's numbers fall just slightly above the appropriate (according to Jennings) area-time range for rotary intakes, and your numbers fall at the top of the range for piston-ported intakes. Account for the fact less duration=less area, and the number falls in the middle of Jennings' range.

I don't know the mean area, so I'm not saying that your numbers correspond with Jennings', but I am saying that the difference between your recommendations and Bell's recommendations (and yours definitely are better) does correspond to the difference in Jennings' recommendations for disc-rotary intakes and piston ported engines.

I know there are other factors that limit (or preclude) the application of motorcycle strategies to chainsaw mods, but in this case, it seems to me that the problem with Bell's numbers is that they could be based on a different intake system.

I hope you don't think I'm being argumentative, I'm just having fun sorting out what might and might not be applicable, and you are a fantastic resource!

 

[timberwolf]

Good discussion. I played quite a bit with jennings time areas numbers and really never got them to mesh with saws.

#1 calculating time are on exhaust for a saw is of little value, With the oversquare muffler design it's hard to get enough cm2 per cm3. Largyly because the exhaust port must be kept much lower than on pipe.

#2 jennings TA stuff is based on engines that were near square, now I'll see if I can explain this clearly, it's a really importaint concept and also goes to explain why big bores don't have the punch you might think they should.... take a(husky 281) 80.7cc motor 38mm stroke 52mm bore with 70% width exhaust port square (for easy calculation as an example) 36.4 mm wide and 15mm high for an area of 546 mm2 and say we change the shape, one motor with twice the stroke 76mm and a corrisponding 36.77 mm bore =80.7cc, the other design with a 19mm strok and corrisponding 74mm bore 080.77 cc. Now look at the maximum exhaust port widths, the 36.77 mm bore can have a 25.74 mm wide port, but it can be twice as tall (30 mm) to give the same exhaust durration as the stroke is twice as long, this = 772.2 mm2 the other design with 74mm piston can have a 51.8mm wide port but as the stroke is half, the height of the exhaust will be half (7.5mm) = an area of 388.5 mm2. This shows that a motor with twice the stroke length for the same displacement has roughly twice the potential exhaust port area as a motor with half the stroke but a larger bore. Taking this back to Jennings, his sole basis for the time area numbers was reverse engineering bike motors with near square bore to stroke ratios. soon as we go over square with the motor it becomes hard to get the required time area due to port width limitations, and without a pipe exhaust durrations must be kept lower which further hurts time area potential. So the exhaust TA numbers are out the window pretty much.

That then spills over to the transfers, if we know our exhaust durration numbers need to be lower and the port area to displacement ratio will be less than ideal due to the over square design transfers must too be lower to permit adiquate blowdown. I have found good running muffler saws to have slightly less than minimum time area numbers if calculated by Jennings methods.

On the intake the same principals apply that apply to the exhaust, the more oversquare the engine the less availible port area for a given durration. Because we do not have a tuned pipe to aid with crank case induction and the primary base compression numbers of muffler short stroke engines are a lot higher than a longer stroke piped moter the numbers achieved through reverse engineering are also fundimentally flawed.

Looking at time area is no doubt of value, but using Jennings material is just to far out of context to serve as a solid basis for design. I'll see if I can graph this maybe tomorrow to show it in a different way.

 

[Martinm210]

Excellent discussion.

I'm taking notes and appreciate all the insight and experience here..

 

[edisto]

Good explanation Timberwolf...that's what I was looking for. Knowing that bikes and saws are different isn't very helpful when trying to settle on a design, but knowing how they are different is!

I guess I knew target rpm might not be all that useful, because i do not plan to mess with the transfer ports if I can help it, and it seems pretty clear now that they they aren't going to fall into the range of Jennings' recommendations (even though there is a broad range for transfer ports). But I did plan on maintaining the relationship between time-areas of the exhaust, intake, and transfers that Jennings recommends.

I see now why you were so confused as to why I wanted a target rpm. It really was just to see where I was in reference to Jennings, but in the back of my mind (now at the front given the valuable explanations you have provided) I guess I knew that, with not wanting to mess with the transfers, I would be off of Jennings' chart, but if the numbers didn't fit, I still wanted to maintain the same balances recommended by Jennings. Is that a useful approach?

 

[Crofter]

I think it is OK to have a target rpm but it has to be do able. I guess it would give a base to start feasibility calculations from, to see what kind velocities you will have at various points. If you grabbed a figure of say 14,000 in the cut for a 372 you would have venturi velocities that would prevent you from getting sufficient charge density to have useable torque at that rpm.

Maybe it would be better to go the other way and assess some of the physical limitations such as carb or exhaust nozzle size/piston skirt width and let that dictate what the likely operating rpm can be.

 

[timberwolf]

Got some graphs together, first is just the relationship between bore and stroke for 81cc saw. The cross point of a square bore is 47mm bore, 47 mm stroke.

The second graph is the effect of bore to stroke ratio on time area for a port width of 70% of bore diameter 160 deg exhaust duration @ a conservative10,000 RPM. Exhaust port is rectangular with 4mm radius corners, it would be a very radical port to give the maximum time area for a given duration and width limitation.

The third graph is same as second but with 180 deg exhaust duration that would be closer to piped numbers.

It possible to see where with pipe porting numbers and a square bore to stroke good time area up at or above the range Jennings recommended can be achieved, but go to a lower exhaust port design more like what will work with a muffler and the maximum time area that can be achieved drops substantially to about 0.000130 for a typical chainsaw bore to stroke ratio. This is for a conservative RPM and as mentioned with a radical port shape, go to a more conventional oval and TA numbers will drop further.

 

[edisto]

I must be wrongheaded somewhere, but I don't follow your numbers. I remade your first graph because the X-axis was not scaled continuously, and that came out the same:

When I worked the time-area relationship, I just assumed a rectangular port for simplicity (no radius on the corners), and from your numbers it looked like port height was around 160 mm (I used 157). Basically, I got the opposite relationship you did, but mine makes sense to me. For a fixed displacement, an increase in bore/stroke ratio means a larger bore. If port width is fixed at 70% of bore diameter and port height is fixed (because duration is fixed) then port area should increase. For a fixed open time (0.00267 sec), time-area should increase as the ratio of bore to stroke increases (blue line on the following graph).

The result is that, for ratios that you see in saws (let's say 1.5), 70% of bore diameter is way too much. I also ran the 180 degree duration (but forgot to change port area), and it, of course, shifted the line up (red line on the graph). Considering that increased duration means more port height, that line should be shifted up even further for a width that is 70% of the bore.

In other words, to fall within Jennings' guidelines (dashed lines on the graph below) for the 160 degree duration, the width would have to be decreased. The green line on the following graph shows the relationship for 160 degree duration and a port width that is 52% of the bore diameter.

Am I missing something? It seems like less square (higher bore/stroke ratio) just means you have to use a width that is a smaller % of bore diameter and/or a more rounded (oval) shaped port.

 

[timberwolf]

Your last graph looks backwards, if you have less bore and more stroke you will get more time area given the port is kept at the same width % of bore.

I don't follow at all a port height of 160mm, that s longer than the stroke...

Go back and look at what I posted about the availible port width vs height when bore and stroke changes. If stroke is doubled the port height doubles keeping the same port duration, and the width only goes down a little so you get more availible port area. If stroke is shortened in half port height is cut in half but availible port width only increases a bit and you end up with less availible port area. Since I posted that I found it in "High Performance Two Stroke Engine" by John Dixon. This too is part of the reason why more power can be pulled out a 2 cyl 250cc than a single. The combined availible port area in two small cylinders is greater than one large cylinder even keeping the stroke the same.

 

[edisto]

That's my problem! I had port height constant because duration was constant, not increasing as a function of stroke! Let me take another look...

160mm was a typo, I was using just less than 16mm, but I applied it for all strokes.

 

[timberwolf]

Yep, your last graph is stroke to bore ratio not bore to stroke and that make the whole relationship inverse.

Also, you flipped bore and stroke in the first graph, not sure why? I graphed it based on the stroke so it was scaled continiously so that rate of change to stroke was a constant (straight line) and the relationship in chage to bore diamiter could then be seen clearly. If graphed vs bore to stroke ratio both lines are curved and the relationship harder to see.

 

[edisto]

OK...back on track. I didn't flip bore/stroke, our axes just run in opposite directions. The problem was I was keeping port area constant instead of port duration.

Backcalculating mean area from your time-area values, there is a linear relationship between stroke and mean port height. That allowed me to look at mean port area as a function of bore/stroke ratio:

Of course, finding the appropriate mean height then produced the same relationship you had (dashed lines are Jennings' recommendations):

Life is hard, but life is hardest when you're dumb!

Thanks for your patience, but I had to work through it to understand it, and I can be pretty slow.

So...it looks like when I apply Jennings' guidelines, I'm going to run out of room, which brings me back to an earlier question: is it a good idea to maintain the relationships between the time-areas of the intake, exhaust, and transfers that Jennings has?

 

[timberwolf]

That looks better, back in post #27 your time area was increasing with bore size, now it's decreasing which is the way it should be. Thats what made me think you just had the scale front to back. Anyway...

Thats where I was saying the time area on exhaust did not really apply, most cases you can't get the exhaust TA numbers Jennings talks about and they don't really mean anything anyways because of differences in bore to stroke.

So for exhaust you get as much as you can keeping the exhaust down low. I don't think on a work saw motor you could get too much exhaust time area. If you have a really narrow skirt piston and can only get 55-60% of bore diamiter then could look at time area and see how much you would need to raise exhaust to get the same time area as a 70% width port at 160 degs (comparing TA numbers to the saw with it's self just with a theoretical wider port not so much to Jennings). But start heading over 165 deg on smaller work ported saws and about 170 deg on larger saws and torque will start to fall off quickly.

Intake is much the same deal as exhaust you are limited in how wide you can go and also need to look at the intake tube, a 36mm wide port 15 mm high does little good if the intake tube is 15mm in diamiter. Best bet is to widen first then lower to preserve as much base compression as possible.

Transfers, yes just bring the time area in line with the other ports, but keep them as low as possible, wider first before higher, this gives maximum blowdown time for the best cylinder emptying and also pushes base compression up a little higher giving the transfers a little more snap when they do open.

On transfers, look at the duct size too, just making the port window bigger without carrying that change through does little and could hurt scavenging.

This is where a flow bench is handy though, it's often possible to increase flow of the ducts without making the whole duct length larger. You need less time area if the port flows more efficiently. Up to a point anyway. Got to watch port velocity does not get too high.

 

[Farley9n]

272xp pictures

Here's some pictures of a mildly ported 272xp piston and cylinder that are going on my own 61. It already has a stock 272 set up on it and I'm hoping for a bit more. No changes in heigth of ports and no radical widths. Just a little here and there!
I feel a bit silly jumping into this. It's an excellent thread and I have picked up quite a bit from it but here I sat with this set already done and a camera handy. I lost control and something switched the lurk button to post!
I hope I'm not too far out of line!........Bob

 

[edisto]

That looks better, back in post #27 your time area was increasing with bore size, now it's decreasing which is the way it should be. Thats what made me think you just had the scale front to back. Anyway...

Yeah...it was increasing because mean height was (wrongly) held constant, so area increased only as a function of width, which was a function of bore size.

I feel a bit silly jumping into this. It's an excellent thread and I have picked up quite a bit from it but here I sat with this set already done and a camera handy. I lost control and something switched the lurk button to post!
I hope I'm not too far out of line!........Bob

Hey Bob! It's a forum, not a private conversation. I appreciate seeing the pics! Glad you got the urge to post them.

Timberwolf...I figured there was a good reason you went with a duration of 160 and a mean width of 70% of the bore. I'm glad you confirmed those as limits. Now I have some pretty firm guidelines to get started with.

All I need now is the extra cash to get my hands on a 272 cylinder!

 

[timberwolf]

Depending on the brand of piston most likely you will run out of skirt width a couple mm before 70%. On intake you will likely end up a mm or two narrower yet to leave some piston support on the sides of the port.


You can get after market 272s around $100, not a bad option to play with porting, less painful if it does not go perfectly first time.

 

[edisto]

Here's some pictures of a mildly ported 272xp piston and cylinder that are going on my own 61. It already has a stock 272 set up on it and I'm hoping for a bit more. No changes in height of ports and no radical widths. Just a little here and there!

Might just be the light, but I think you'll want to smooth out the work on the piston some to prevent development of any local "hot spots".

 

[Farley9n]

No Sharp Edges!

I finish up with cone shaped cratex burrs/wheels and that leaves a smooth but defused sort of a surface. As a side note though I don't think that underside and window areas of the piston would pose much of a hot spot risk due to the passage of the incoming and transfering fuel mix.





Thank You for your acceptance of my late night intrusion............Bob

 

[edisto]

I finish up with cone shaped cratex burrs/wheels and that leaves a smooth but defused sort of a surface. As a side note though I don't think that underside and window areas of the piston would pose much of a hot spot risk due to the passage of the incoming and transfering fuel mix.

Thank You for your acceptance of my late night intrusion............Bob

Good point...I guess I'm still having a hard time thinking in terms of half the strokes!

Not an intrusion at all...the more the merrier! As you can see, I need a lot of straightening out!