Part Four: Compression

[Wonkydonkey]

I didn't get the right answer.
I also need to reread that book. Not the calculus one.

Nor did I , maths is not one of my good points

So do I, I got brain ache reading it, but it helped me understand more than I did at the time.

if your figuring out what’s in the book and need to try some maths there are online calculators that can help you, some you have to pay for the whole package thou,
http://torqsoft.net/exhaust-time-area.html
https://www.mopedarmy.com/wiki/Calculating_port_timing
https://lambretta-images.com/tuning/


But as @Ketchup said in post “229” once you understand it you don’t really need to do the maths.
but I don’t think I will ever understand it as there is way too much going on at once .

 

[Stump Shot]

.
but I don’t think I will ever understand it as there is way too much going on at once .

Here's a vidja I did some time ago explaining where we get some more compression from. Maybe it will help.

 

[Ketchup]

So if you cut .030” out of the band and set squish at 0.020” on one saw, and on another of the same model you take 0.060” out of the band and set squish at .028, and they both make 190 psi, will there be a performance difference?

For completely unscientific reasons I want to believe the second one will spin faster. Anybody have experience with this type of experiment?

 

[pbillyi69]

why do you think the second one will spin faster? i think there are many other factors to consider. are they modified in any other way? are the ports all the same?

 

[Moparmyway]

So if you cut .030” out of the band and set squish at 0.020” on one saw, and on another of the same model you take 0.060” out of the band and set squish at .028, and they both make 190 psi, will there be a performance difference?

For completely unscientific reasons I want to believe the second one will spin faster. Anybody have experience with this type of experiment?

Every saw is different, it’s a dangerous place trying to get a blanket statement like that to stick.

 

[huskihl]

If all other things are the same, it should spin faster because the exhaust would need to be 5?- 6?° higher on it to have the same compression

 

[Ketchup]

why do you think the second one will spin faster? i think there are many other factors to consider. are they modified in any other way? are the ports all the same?

It’s more of a theoretical question. Let’s pretend they are identical ported saws except one has a wide band and one has a tight band. Both have the same combustion chamber volume and compression. Is one better?

Every saw is different, it’s a dangerous place trying to get a blanket statement like that to stick.

I hear you. Variables. Every saw gives you some room to work, but not in the same places.

If all other things are the same, it should spin faster because the exhaust would need to be 5?- 6?° higher on it to have the same compression

I just made the numbers up. I’m just trying to get at wide band vs skinny band, all else being equal.

There’s a school of thought that too much compression will hold RPM down. I’m skeptical that’s really true (until the compression is obnoxious). I’ve definitely built some saws that felt held back by compression. But is it related to compression or just the shape of the combustion chamber? Some of my saws weren’t much above 170psi. A really tight squish band and a larger combustion dome seems goofy to me and some of my saws had that. A wider band and smaller dome seems better balanced.

 

[pbillyi69]

i by no means know any of the answers since i havent done the port work to any saws. it seems to me that everything else equal the saw with a bigger band would run better and get better fuel efficiency due to the fact that it has a slightly larger area for burn. and a little bit more room for charge. i think it would just run a little bit better and stronger. having never tried a bunch of different combos of build i am just stating my unfounded thoughts on the scenaro.

 

[Maintenance Chief]

By this rational the 090 ,070 stihl would be a high revving monster.

 

[pbillyi69]

see i dont know chit

 

[huskihl]

It’s more of a theoretical question. Let’s pretend they are identical ported saws except one has a wide band and one has a tight band. Both have the same combustion chamber volume and compression. Is one better?



I hear you. Variables. Every saw gives you some room to work, but not in the same places.



I just made the numbers up. I’m just trying to get at wide band vs skinny band, all else being equal.

There’s a school of thought that too much compression will hold RPM down. I’m skeptical that’s really true (until the compression is obnoxious). I’ve definitely built some saws that felt held back by compression. But is it related to compression or just the shape of the combustion chamber? Some of my saws weren’t much above 170psi. A really tight squish band and a larger combustion dome seems goofy to me and some of my saws had that. A wider band and smaller dome seems better balanced.

Most of the compression argument is spread by builders that don’t do machine work to the base and squish.

 

[Ketchup]

By this rational the 090 ,070 stihl would be a high revving monster.

NAh. I’m not not saying large squish = high rpm. I’m just wondering about modifying squish bands. Simply setting squish as tight as possible doesn’t make the best performance. Why?

And I should have said “will have a high power band” instead of “will spin faster”. I don’t care what a saw piss revs at. I really don’t care if my comp tester says 140 or 220 either if the saw runs right. It’s all just reference points.

 

[pbillyi69]

so here is my question would having the squish band be wider from the factory make the saw perfomd differently?

 

[gduvic]

Just asking: which is better an angle squish band or a flat squish band?

 

[Stump Shot]

so here is my question would having the squish band be wider from the factory make the saw perfomd differently?

Yes and if you make it too wide/tight can be like a hammer and an anvil. AKA popcorn maker.

Just asking: which is better an angle squish band or a flat squish band?

Flat for me and my style of porting is without question bestest.
Others may have a different outlook possibly.

 

[gduvic]

Yes and if you make it too wide/tight can be like a hammer and an anvil. AKA popcorn maker.



Flat for me and my style of porting is without question bestest.
Others may have a different outlook possibly.

Thanks for your reply; as new chainsaw modifier I can try angle or flat for my style and likeing

 

[pbillyi69]

so then my thinking before was correct a wider band will run better and be more efficiant?

 

[farminkarman]

so then my thinking before was correct a wider band will run better and be more efficiant?

There is a point of diminishing returns on the band width, but in general, the combustion process is can happen more efficiently at high rpm when the chamber is smaller.

 

[Dolkitafreak]

I don’t see any reason that a wide squish band would be optimal, just a necessary evil on machined worksaw builds. Need to reduce volume, there’s a few ways to do so, but a band cut makes sense for piston replacement down the road and no worry of crown to ring land issues. Band is going to get wider to make the chamber smaller.

If it were possible I always prefer a narrow band, wide bands trap more fuel and are more prone to detonate.

 

[Ketchup]

Just asking: which is better an angle squish band or a flat squish band?

Flat is simpler. Angled has more surface area for the explosion to effect. The difference in performance seems small.