Crankvolume , how much is it related to everything ?

[Bart Drieghe]

This might be pretty technical but , how important is crankvolume ? Is it related to anything ?

Any possible explanation for a rookie ??

Nitro And /or Petrol/gas

Thanks already ,

B

 

[dave roach]

The smaller the crankcase volume the better the performace because you can stuff the crankcase better with fuel.

 

[Terry Keeley]

Take a look at this pic in Jim Allen's gallery:

https://www.intlwaters.com/gallery/displayimage...album=364&pos=6

 

[chunk t]

Bart,

Crank case volume is important.

The fuel/air mixture is moved from the carb, through the block, into the cylindar bore and finaly out of the exhaust due to the changing pressue in the engine (and other things happening in there). The smaller the volume the more dramatic the change in pressure. This good because it increases velocity and creates greater turbulance. More turbulance, more even mixture of air & fuel. A greater velocity of the mixture entering the combustion chamber also helps in the scavenging aspect of a 2-cycle engine.

Hopefully that's enough information without being too confusing :blink:

chunk t

 

[lohring]

It is not true that smaller crankcase volumes are always better. Because modern engines depend on tuned pipes, not crankcase compression, for much of the scavenging, flow through the engine can be more important than absolute minimum crankcase volume. Most of the engine rework actually increases the crankcase volume by removing material to improve flow.

My personal experience with dyno testing of two Zenoahs using the same cylinder but different crankcases with different volumes showed no difference in power. Similar results were found with Quickdraw engines, one with a full circle crankshaft and one with a cutout crankshaft. The gas record of over 109 mph is held by the Quickdraw with the highest crankcase volume.

Lohring Miller

 

[Joe Warren]

I have never seen an Nitro engine that does not benefit from added case volume....... If the case volume is increased the Intake duration will also have to be increased to pak the case with fuel. But this is a Speed secret most do not want to talk about..... it does not matter if you mod the sleeve to increase the volume or the case. Volume inside the engine is Added Fuel. Added avail Fuel is Power....

 

[Don Ferrette]

I have never seen an Nitro engine that does not benefit from added case volume....... If the case volume is increased the Intake duration will also have to be increased to pak the case with fuel. But this is a Speed secret most do not want to talk about..... it does not matter if you mod the sleeve to increase the volume or the case. Volume inside the engine is Added Fuel. Added avail Fuel is Power....

This is true and along with the added volume is the need to be able to flow that increase so special attention to how the mix gets to it's ultimate destination is also a big factor. And a little tip- although there are those who might argue this, don't waste your time polishing intake case or drum housing port work you do. The finish from a fine grade stone actually helps keep the mixture atomized by inducing a minute amount of turbulence close to the port surface as the mixture passes. Keep in mind I said the finish of a fine grade stone, not something that looks like it was done with a hammer & chisel. Now on the exhaust side a polished exhaust port can be a good thing if you want something in there to be polished & shiny.

 

[Mike Broad]

It's quite interesting to note that the MAC engines have a relatively large crankcase volume and large flat topped transfer ports. This seems to mirror recent thinking in the full size engine world, where the tuned pipe creates most of the pumping action in the engine, and then stuffs the fuel air mix dragged thro' the engine and into the pipe back in the combustion chamber, thereby cramming more fuel/air in to be burnt.

Mike Broad

 

[JKramer]

Both high and low volume is correct. This is depending weather you want a high RPM motor or a torque motor. If you want high RPM's increase volume, if you want greater torque decrease the volume.

Right Greg!!!!

 

[Joe Warren]

Some engines like CMB have small case volumes. You will not benefit from increase case volume. Unless you change the overall intake duration to pak the larger case with fuel. Most engines that you thrown in & have to run hard for 2-3 laps before needling. To prevent the fear of Going thru the needle after Warm Up . These same engines are small on case volume. There are small .21 & Larger .90 engines that have these Peaky - Querks... Larger Boost transfers help. But Overall Case Volume is HP... Now.... Light those dremels UP.... :unsure:

 

[Brad Christy]

Bart,

Just remember this: What we are dealing with is an air pump that operates on a volumetric ratio between TDC and BDC. The greater the ratio, the more efficient the pump will be, thus the more air it will move in a given amount of time at a given RPM. If you do some math, you will read into this what I am saying........

Thanks. Brad.

Titan Racing Components

BlackJack Hydros

 

[lohring]

The crankcase is a really bad pump, no matter what the compression ratio. Tuned pipes have improved engine performance to the point where power in a tuned pipe engine is more than double that of an open exhaust engine. As an example, the Dooling 61, a 1950s engine, was rated at about 1 hp. Today .12 engines can develop this much power. All the crankcase stuffing in the world can't increase power that much. Scavenging improvements made possible by tuned pipe design did.

Lohring Miller

 

[Terry Keeley]

The crankcase is a really bad pump, no matter what the compression ratio. Tuned pipes have improved engine performance to the point where power in a tuned pipe engine is more than double that of an open exhaust engine. As an example, the Dooling 61, a 1950s engine, was rated at about 1 hp. Today .12 engines can develop this much power. All the crankcase stuffing in the world can't increase power that much. Scavenging improvements made possible by tuned pipe design did.

Lohring Miller

Agreed, big time!

But you still need bottom end "grunt" to get to the point where the pipe starts workin'.

 

[The Grinch]

A smaller volume (relatively) crankcase only really helps on lower RPM motors or On the bottom end of Screamers. Crankcase Pumping really becomes insignificant at revs over 18K +/-…in higher RPM regimes, wave timing and flow is more at play. High RPM motors work more on wave interactions within the case, so shape is very important…there isn’t enough “open time” on the port openings for crankcase pressure (due to crankcase pumping) to move gasses into the ports for High RPM motors.

I would say that larger displacement motors( with lower revs) should minimize the case volume to feed the ports better….I’ve found that it doesn’t matter much on .12’s and .21’s

Adios and Merry Christmas,

The Grinch

No Real Name cause I'm no idiot....this is the internet with lots of Locos

see yas next month...LOL

:lol: :lol:

 

[Mark]

just remember flow is not all about the size but rather the shape of the port

ask anyone with a gas flow bench ( 1 secret out of the bag)

How is this so with an exhaust port?

 

[lohring]

Exhaust port shape has a lot to do with the type of pulse that gets to the pipe. An exhaust port that has a lot of area immediately upon opening generates a strong, short pulse. A constant width port generates a gentler pulse. Since the trend in larger engines is toward a divided exhaust port with the maximum area when the exhaust opens, I think this strong pulse into the pipe develops more power but over a narrow rpm range.

Lohring Miller

 

[Jim Allen]

Bart,

From my bench testing it would appear that large or small crankcase volumes, when used with an effective tuned exhaust & an effective tuned intake, both give a good charge delivery, independent of crankcase volume. The effects on HP of a properly designed tuned pipe & intake are substantial when compared to the effects of reductions or increases in crankcase volume.

Only through bench testing can answers to questions like this be answered for the particular engine in question. Suggested reading; The Two-Stroke Cycle Engine by Heywood & Sher, chapter 5-5-3, page 199; Two Stroke Performance Tunning (second edition) by Blair, page 48-50.

Jim

 

[steve speas]

This might be pretty technical but , how important is crankvolume ? Is it related to anything ?

Any possible explanation for a rookie ??

Nitro And /or Petrol/gas

Thanks already ,

B

bart, all i can tell you is the best running engine i have ever had, was cut out so bad that i had to use j b weld on it to seal the case up.ask don he has seen it,,, it was a black case cmb 67 the one with the carb built on to the back plate.

im no engineer but it seems to me when the pipe kicks in you better have the extra fuel in the crank case!

steve

 

[Joe Warren]

Steve Hit on it. When the pipe comes in & You are hauling down the back stretch. You better have enough fuel flowing thru the case to keep this High Rpm & Load Happy....Once the fuel & Air are inside the engine & the intake valve is Again closed. This is what you have to work with..... As fast as you can you want it On top of the piston. Some, Loop port the transfers to help out on this... The Barrel effect the piston has on the fuel while it is traveling down Will move the fuel to the top of the cyl. IF You loose the barrel effect by cutting too much skirt off You will not move the fuel towards the top of the cyl.. Frank O has a Cyl Mod that works like a Poor Mans Supercharger. His Mods are Not radical But help flow the fuel smoother thru the entire engine.... This is desired on High RPM....

 

[Mark]

Exhaust port shape has a lot to do with the type of pulse that gets to the pipe. An exhaust port that has a lot of area immediately upon opening generates a strong, short pulse. A constant width port generates a gentler pulse. Since the trend in larger engines is toward a divided exhaust port with the maximum area when the exhaust opens, I think this strong pulse into the pipe develops more power but over a narrow rpm range.

Lohring Miller

Thanks mate. In my superkart racing days the bridge in the port was to help keep the ring from poping out (something nitro guys don't have to deal with) being able to widen the port as much as possible but maybe the effect was 2 fold.