compression vs. displacement

[yo2001]

If you had a choice between little less displacement with say 20psi more compression and little more displacement with 20psi compression which one would you choose?

Here is what I just went through. I guess It's my opinion but here is goes.

1) 044 with BB kit 76cc with 155psi cold
2) same saw with stihl jug (71cc) and aftermarket piston kit (kept the stihl wrist pin since it was noticeably lighter.) compression at 175psi (with carb off, with carb on 165psi??? mystery to me.)

Both were ported about the same with base gasket removed. May be I might have raise the exhaust port too high causing the compression to be lower on BB kit. (when I touch the exhaust port, I lost about 10-15psi)

Anyway, both cut well but I did notice I'm getting more rpm in cut from smaller 71cc with higher compression and I can lean on it pretty hard cutting 18" dry white oak with no bog. So my conclusion was that I made up the difference in displacement with more compression to equalize or surpass the torque of displacement.

It just might have been the fluke or difference in porting, port timing etc but any professional opinion welcome.

 

[tek9tim]

I'm certainly no expert, but I like the looks of the stock combustion chambers better than the BB kits. Seems like you'd get a better burn out of them. Dunno other than that. I for sure got better results with a ported stock jug on my 460 than with a ported big bore.

 

[Crofter]

If you had a choice between little less displacement with say 20psi more compression and little more displacement with 20psi compression which one would you choose?

Here is what I just went through. I guess It's my opinion but here is goes.

1) 044 with BB kit 76cc with 155psi cold
2) same saw with stihl jug (71cc) and aftermarket piston kit (kept the stihl wrist pin since it was noticeably lighter.) compression at 175psi (with carb off, with carb on 165psi??? mystery to me.)


Both were ported about the same with base gasket removed. May be I might have raise the exhaust port too high causing the compression to be lower on BB kit. (when I touch the exhaust port, I lost about 10-15psi)

Anyway, both cut well but I did notice I'm getting more rpm in cut from smaller 71cc with higher compression and I can lean on it pretty hard cutting 18" dry white oak with no bog. So my conclusion was that I made up the difference in displacement with more compression to equalize or surpass the torque of displacement.

It just might have been the fluke or difference in porting, port timing etc but any professional opinion welcome.

You say both were ported "about the same" How did you establish this? Also you suggest that you might have raised the exhaust too high. How did the exhaust duration differ from the OEM cylinder. What I am getting at is that it seems possible there may be more factors affecting the outcome than just the compression. I think a rough figure is 3% increase output for each 10 psi increase in compression. The advantage for displacement is approximately equal to the percentage change of displacement. The porting though would have to be such that flow velocities were proportional. That would take a fair bit of calculating. timberwolf is the guy for that.

If they indeed had the same port timing, shape and proportional areas, at a certain compression the lower compression but larger displacement would be even with the other. The difference in base compression ratios could be a small factor but that should favor the larger displacement one a bit as far as torque is concerned, other things being equal.

 

[yo2001]

Well, I know my comparison is not good between my ported BB kit and ported OEM P&C since variable is not consistent. What about MS 460 and MS440? 440 with higher compression without changing other variables? (if possible)

I have done alot more of rebuilding car engine and found oversize pistons do not do as much as actually changing the compression or changing stroke.

 

[Crofter]

I think compression is always the cheapest horsepower but it does have a few drawbacks such as smoothness of idle, and danger of detonation if lugged, starting a bit harder. This pretty much applies either 2 or 4 stroke. Sure you can make a 440 run with a stock 460 but you will have some trade offs. It will be way louder, use more fuel and wear out a bit quicker and be just a tad harder to pull. Course you can do the same to a 460 and then compare it to an 660.

Whether it is cars or saws, given the same tuning and compression the bigger displacement wins (unless you go to the extremes of bore to stroke ratio)

The bib bore kits for the saws will do real well; you just have to be a bit more creative to get the compression you want while still arriving at the timing you need. It would be nice if they had just a tad smaller combustion chamber volume but I am sure they will sell more cylinders for customers that just want bolt on dependability rather than competition performance. Readers on this forum probably the exception though!

 

[Haywire Haywood]

Didn't the second generation of Bailey's BB kits have a smaller combustion chamber? I think they did that when they went to the new piston and cylinder coatings.

Ian

 

[yo2001]

It will be way louder, use more fuel and wear out a bit quicker and be just a tad harder to pull.

A word loud doesn't describe the loudness of my 044. :greenchainsaw: I got DP cover and a big port muffler that's ported. It's right around 125% of the exhaust opening. The saw does bite me every now and then when I'm trying to crank specially when I'm not committed to the pull. It will drink some fuel if I'm on the throttle alot. The carb adjustment isn't much further than stock 1 full CCW turn though.

 

[yo2001]

Didn't the second generation of Bailey's BB kits have a smaller combustion chamber? I think they did that when they went to the new piston and cylinder coatings.

Ian

I'm not sure but the bore was on the loose side on mine (small amount of piston slap marking on the bottom of the skirts) which was probably why my compression was never up to par with my Stihl OEM which was marked "A"

During preassemble, I did compression test with no rings on both. BB kit, none to less than 25psi (gauge starts at 25psi) OEM at 25psi.

 

[scootr]

The lower compression reading with carb on, is because the butterfly is closed. Hold the throttle open while cranking, and I bet you get the same psi.

As for the other? I think I would go with larger displacement and lower compression. Prolly about the same hp, but the lower comp would be less problems. It would last longer, start easier and fuel would be no prob. I had a Harley with high comp pistons, fuel was a pain.

 

[yo2001]

The lower compression reading with carb on, is because the butterfly is closed. Hold the throttle open while cranking, and I bet you get the same psi.

As for the other? I think I would go with larger displacement and lower compression. Prolly about the same hp, but the lower comp would be less problems. It would last longer, start easier and fuel would be no prob. I had a Harley with high comp pistons, fuel was a pain.

The weird part is that I always set the carb on second choke position (choke off with butterfly simi-open) and have had a consistent result on my other saw and my 044 before I went back to OEM jug. Seems like I can't get enough rpm with the carb on for some reason.

 

[drunkenredneck]

just an idiot thinking here......

I think why everyone has problems with making power on the big bore kits is simple. A two stroke chainsaw motor forces the air from the crankcase into the cylinder, still with me?, so when you create a larger cylinder volume you in turn have to enlarge the crankcase volume. Plain and simple the saw which is basically a glorified air pump doesn't have the crankcase volume to fill the demand of the cylinder, thus you have lower compression. Stihl saws generally already have a rather small crankcase volume for the oem cylinder so going bigger isn't helping. Now you can either do a couple of things really easily. One get out the die grinder and open up the case and massage it, it helps to match the case to the transfer ports if they are opened to the base of the cylinder. The second easy solution is to take a ton of material off those big bore pistons because they are pigs. Think about it every bit of material that you take off below the top of the piston is going to add to the crankcase volume, because where the aluminum was air can now go. Walla!!! now it will push enough air up to charge the cylinder. There is your raise in compression and power.:greenchainsaw:

 

[yo2001]

I think you explained that pretty well. To me, the 2 stroke need so many things to consider even though, like you said, it's a simpler air pump of the two. I guess that is what's interesting about the whole thing. Plus, never can't complain about able to cut a tree down faster.