Case Compression, Let's Talk About It

[Ketchup]

I’ve been thinking about blowdown a lot lately. I feel like I’ve been doing a lot of saw with very short blowdown (12-18 degrees). I’ve been doing it to compensate for strato tech or for dual/open port transfers. I also like high transfer duration for raising the general power band.

But like everything, there is a line that can be crossed. Some older models seem to lose spunk if you go below 15 or 16 degrees, some of the new stratos perform well down to 12 or even 10 degrees. Weird.

It makes me wonder why we need blowdown at all. My logic is that blowdown durations effect scavenging, fuel economy, and case compression. If the blowdown is too short:
-the combustion chamber has too much pressure and short circuits the scavenge loop
-more fresh charge is lost out the exhaust
-the period between transfer close and intake opening is shortened. Less piston movement during case closure so less pressure (down stroke) and vacuum (up stroke).

Putting epoxy in intakes is seeming like a better and better idea.

 

[Bjorn]

Hello. This Guy is building world strongest 50cc 2-stroke engine. And he explain alot of good fings and problems, one is the crank case compression.

 

[Mastermind]

Hello. This Guy is building world strongest 50cc 2-stroke engine. And he explain alot of good fings and problems, one is the crank case compression.

As he explains, muffler engines, and piped engines have very little in common. When people who are attempting to build a more powerful muffler engine get sucked into piped engine theory, they are leading themselves away from the things they need to better understand to build a stronger muffler engine.

 

[Red97]

And muffler engines are 90% voodoo....

 

[Bjorn]

As he explains, muffler engines, and piped engines have very little in common. When people who are attempting to build a more powerful muffler engine get sucked into piped engine theory, they are leading themselves away from the things they need to better understand to build a stronger muffler engine.

You have a point, the differens is Huge and to many variabler. But I like the way he explane case compression.

 

[Ketchup]

So I’ve been staring at the hybrid project on my bench and drinking too much coffee all morning. Consider yourself warned.

Here’s something to think about:

There are a lot of saws out there with large case volume. What about the ones that are really tight? In a tight case does duration matter more?

In a saw where the case itself has little room the space under the piston will be where more of the fresh charge is held during a cycle. It seems like intake and transfer will be affected because the fresh charge has to travel farther during each cycle. Intake has to make it into the case and then up under the piston, then switch directions and go back down and into the transfer lowers.

On a more open case, more volume would be stored nearer the intake port and the lowers. The volume under the piston could mix and move as a part of multiple revolutions.

In a tight case, more of the volume under the piston has to move through in a single cycle.

Well, more distance takes more time, right? So wouldn’t the end of a duration have more going on and be more important?

And where does duration “end?” I think most of the action happens in the first half of duration (down stroke for Transfer, up stroke for Intake). When the piston changes direction it also switches it’s effect. But there’s a lot of inertia at 9000rpm. The dwell time and some amount after is probably a period where flow hasn’t changed direction even if the piston has. I’m thinking that if charge has to move a longer distance it’s inertia may actually be greater. Transfers will flow more after BDC and Intake more after TDC in a tight case.

Even ignoring the implications on duration, it seems like a windowed piston and side fed transfer might actually lend themselves to a tighter case. Longer intake duration also seems indicated.

I see some flaws in my logic, but it’s an idea worth considering.

 

[Wonkydonkey]

I was/and wanted to add to this thread a while back. But I held back… it would-ave been a plate of spaghetti sort of reply or mess if I did .
Then I found the link that sorts of explain it in words better than I could.

I’ll get this out now “we do not run ex pipes on saws, well only for race stuffs. So we talk std can exhausts”

I guess some of you guys already know this, but for the average guy like me, all then sums make my head hurt and take to much time

http://www.dragonfly75.com/motorbike/PCR.html
And this link from the link above
http://www.dragonfly75.com/motorbike/development.pdf

 

[Ronie]

I'm attempting to do a 440/460 hybrid and have read that most people go with a short blow down, 15-17, and that got me thinking about the reason short blow down works better in dual ports vs quad ports, because the quad has more velocity and the duels are lazy. wouldn't the smaller case capacity in a hybrid make the dual transfers less lazy and more blown could be used?

 

[Bjorn]

My hybrid has 30 degrese blowdown.

 

[Ronie]

My hybrid has 30 degrese blowdown.

Nice, I guess they do run good with a lot of blow down.

 

[Wonkydonkey]

I really don’t know, and those sums.
I only see it as bigger stroke volume in a smaller case (sums/ratios) I know there’s not a lot in it but. As it’s been said before it all works together, just as a lowerexhaust adds torque [107*] ..

there is an optimal timing numbers but then there’s the exhaust back pressure holding back the charge going in, and going out the exhaust

so in short, I’ll wait for peeps that know craploads more than me answer

 

[Wonkydonkey]

My hybrid has 30 degrese blowdown.

Your Exhaust? What is it, and has it been modded etc.. just asking for the info..

 

[Bjorn]

Canadian farmboy have build my hybrid and ported it.

 

[Bjorn]

Here it is With calculated pipe.

 

[Wonkydonkey]

Ahh yeh,,, that’s a pipe

 

[Maintenance Chief]

When I had .040 taken off my 090 cylinder I did not move the exhaust because I wanted all the torque this thing had to offer.
It still makes very good RPMs.

 

[Ronie]

When I had .040 taken off my 090 cylinder I did not move the exhaust because I wanted all the torque this thing had to offer.
It still makes very good RPMs.

I haven't done that my cylinders but of the ones I have done I like the way the ones with lower exhaust run.

 

[Deets066]

Nice, I guess they do run good with a lot of blow down.

I ran 20 BD on my hybrid.
But you have to take into account what your cylinder is giving you for stock timing and adjust from there. Every hybrid won’t like the same porting because your port timing isn’t starting at the same point.

 

[Deets066]

 

[Deets066]

Port work increases volume but can make more power. Larger volume also increases the volume of the charge as well. A balancing act and back to skinning the cat I guess.

Not all port work increases volume.