CASE VOLUME DISPLACEMENT ? / Introduction

[Red97]

If talking volume more than the crankcase alone. Wouldn't you have to measure the voulme at tdc including the entire intake tract?

 

[Mastermind]

If talking volume more than the crankcase alone. Wouldn't you have to measure the voulme at tdc including the entire intake tract?

I would think so. Yeah.

 

[Terry Syd]

Fig 3 in the document shows the relationship of crankcase volume to maximum delivery ratio. The AMOUNT of the 'delivery ratio' is not affected by the crankcase compression ratio, however the RPM that maximum delivery ratio occurs will increase as you increase the crankcase compression ratio.

What is 'maximum delivery ratio'? That is when the transfer ports are just closing as the transfer flow is just coming to a stop. Above that RPM there isn't enough time/area for all the mixture in the crankcase to get into the cylinder. The closing of the transfers stops the flow.

Below that RPM the transfer flow has stopped and is now being pulled back through the transfer ports and down into the crankcase by the rising piston.

If the crankcase volume remains the same and you increase the displacement by putting on a larger jug, then the crankcase compression will increase.

Increasing the crankcase compression ratio moves the powerband up and reducing the ratio will move the powerband down.

Increasing the crankcase compression ratio also tends to NARROW the powerband, and that is why we generally see fairly low crankcase compression ratios, as the lower ratios will give a broader powerband.

 

[Marshy]

Theoretically.....I think so.
You’re adding volume below the intake floor.
Am I correct?

Yeah, relatively small in comparison to the stuffers doh.
It increases the case compression ratio. Stuffers do the same.

 

[SHOVELHEAD]

So by having more case volume you are creating a broader powerband?
&
Reducing volume will put the power @ a higher RPM?

 

[mdavlee]

1.4:1

 

[Deets066]

I look at crankcase vol as everything below the rings at BDC.

Putting a bigger jug on increases that area, but the piston is also larger which decreases vol.

Hard telling until it's measured.

 

[CR888]

Interesting conversation, we see saw makers play with this concept like Husky 545/550 & 555/562 where they detune the models using case compression changing the powerband characteristics. Also 440/460 hybrid builders have enjoyed the benefits of altering case compression/volume. I don't think there is a right or wrong answer it depends on what your trying to achieve but there is definitely a tool for the tuners box of tricks that he may apply.

 

[MustangMike]

That is true, the 440/460 hybrids typically run at higher RPMs than either a 440 or 460, but usually also often have less low end grunt than a ported 460.

I'm heavy handed, so I would personally opt for more case volume to go along with the higher displacement.

 

[mgr1]

Zenoah rc engines: when you replace the crank with a full circle one, the case capacity decrease and the engine gives more top end power.

 

[EvilRoySlade]

1.4:1

I thought I remembered Ed Heard saying 1.2:1 was optimal. But that is almost unrealistic in saws by design, air cooling being #1. Heat and power go hand in hand. Cookie cutter maybe but work saw you’d kill for sure. Also he mentioned most cases would have to be machined to get near that anyway.

 

[drf256]

My vote is the largest case volume on the smaller case one can find to use.

Case in point (as Mustang points out above) is an 044 vs 044/6 hybrid.

The 044 case under a 52mm 046 jug makes the rpm power Range jump about 1500 rpm. The saws are peakier and lose grunt. Not sure if you’ve ever tried one, but the difference between an 044/046H/046 is astounding.

The above example produces a different saw with a 5cc displacement increase. You are increasing yours by 15cc. I’d do all you can to increase case volume.

 

[SHOVELHEAD]

(The above example produces a different saw with a 5cc displacement increase. You are increasing yours by 15cc. I’d do all you can to increase case volume.)

Yes the 272xp @ 72.2 - the 2156 @ 56.5 = 15.7
If using the 2159 that is @ 59.00 = 13.2

The ideal situation is to setup the Buret & CC all 3 Cases 272xp/2156 & 2159 to see the Case volume differences.

 

[Marshy]

(The above example produces a different saw with a 5cc displacement increase. You are increasing yours by 15cc. I’d do all you can to increase case volume.)

Yes the 272xp @ 72.2 - the 2156 @ 56.5 = 15.7
If using the 2159 that is @ 59.00 = 13.2

The ideal situation is to setup the Buret & CC all 3 Cases 272xp/2156 & 2159 to see the Case volume differences.

You would be the man if you could do thst and share your results.

 

[SHOVELHEAD]

What have gotten my self into?
LOL

 

[drf256]

What have gotten my self into?
LOL

Good question.

Not so easy to do accurately.

 

[mdavlee]

Level case where cylinder bolts up. Put piston at TDC and fill case til it overflows. That will give you a general idea.

 

[paragonbuilder]

(The above example produces a different saw with a 5cc displacement increase. You are increasing yours by 15cc. I’d do all you can to increase case volume.)

Yes the 272xp @ 72.2 - the 2156 @ 56.5 = 15.7
If using the 2159 that is @ 59.00 = 13.2

The ideal situation is to setup the Buret & CC all 3 Cases 272xp/2156 & 2159 to see the Case volume differences.

Build it on one, then switch it to the other and see what you like.


Sent from my iPhone using Tapatalk

 

[cus_deluxe]

Fig 3 in the document shows the relationship of crankcase volume to maximum delivery ratio. The AMOUNT of the 'delivery ratio' is not affected by the crankcase compression ratio, however the RPM that maximum delivery ratio occurs will increase as you increase the crankcase compression ratio.

What is 'maximum delivery ratio'? That is when the transfer ports are just closing as the transfer flow is just coming to a stop. Above that RPM there isn't enough time/area for all the mixture in the crankcase to get into the cylinder. The closing of the transfers stops the flow.

Below that RPM the transfer flow has stopped and is now being pulled back through the transfer ports and down into the crankcase by the rising piston.

If the crankcase volume remains the same and you increase the displacement by putting on a larger jug, then the crankcase compression will increase.

Increasing the crankcase compression ratio moves the powerband up and reducing the ratio will move the powerband down.

Increasing the crankcase compression ratio also tends to NARROW the powerband, and that is why we generally see fairly low crankcase compression ratios, as the lower ratios will give a broader powerband.

This poast made a couple light bulbs flicker for me. Gotta read it about 100 more times.

 

[cus_deluxe]

i would think the mods done by @Chainsaw Jim to the 272 jug to turn it into a bottom feeder, as opposed to side fed as it is originally, would add a ton of volume, right?