mahle 18:1 pistons discontinued- how much will 10 thou drop my comp ratio?

[chevyinlinesix]

As always my goal is to correctly understand things, & spatial orientation isn't my strong suit, so I draw things out.

Here's a drawing of how I understand changing rod length, piston pin height, or piston crown height affects compression ratio. Granted I pictured cyl area instead of volume, but as the bore stays the same.....

I believe the comp ratio calculator on the Silvo-lite site only includes rod length in the dynamic comp ratio calculation.

That's because it doesn't affect static compression ratio.

The only thing rod length is changing is the distance between the crankshaft journal and the piston pin. It does not change the total swept volume. All it does is move the pistons location in the bore. Take 1/2" off the connecting rod, and it will make the top of the piston 1/2" farther from the deck of the block (which makes it seem like the compression should drop because there is now more chamber volume) however, remember that when the piston is at BTC it is also 1/2" further down in the bore.

Here is a simple test: Lay a ruler down on your desk (beside your beer ):h ) and grab a good length pencil. Hold the pencil at midpoint with your pointer finger and your thumb, and put the tip of the pencil at the 1" mark on the ruler (hold the pencil parallel with the ruler) Slide the pencil down the ruler to the 4" mark and then back up to the 1" mark.

Now, grab the pencil at the eraser end put the tip of the pencil at the 1" mark on the ruler (holding the pencil parallel with the ruler), slide the pencil down to the 4" mark, and then back up to the 1" mark.

In both cases you have a 3" stroke, and in both cases you have a 4:1 compression ratio. Even though rod length (represented by the pencil) has changed, neither compression ratio, nor stroke has changed.

 

[JiFaire]

I went out to do some work on my Daytona, and as usual, having a wrench in my hand affects my thinking.

What I'm thinking is that I messed up my last post... (one of those 'I know what I meant to say, just didn't say it' things...).

Let's try that again...

Static Compression ratio = (Volume of cylinder at BDC)/(Volume of cylinder at TDC) expressed as a unit ratio (~x:1)

Shortening the rods does not change the stroke volume (the volume between TDC and BDC)... stroke volume is determined by crank aspect and piston diameter, ergo static compression stays constant (more volume into more space)

However, shortening rods DOES lower effective compression ratio by increasing the compressed volume (the space between the top of the piston and the head, at TDC). Ergo, same stroke volume is compressed into a larger 'squished' volume = lower compression ratio. That's assuming you cannot effectively move (in/out) more than the stroke volume, leaving you with 'dead air' in the cylinder.

Shaving the top of the pistons would give the same result - all it's doing is changing the overall length of the piston/rod combo. So would thicker head gaskets, again by increasing the 'squished' volume.

'Dynamic' or Effective compression ratio, is influenced by cam overlap, boost pressure, altitude, scavenging efficiency, and other factors. As always, then, airflow (CFM) rather than boost pressure (PSI) needs to be a factor.

In a N/A engine (like most gassers), increasing the 'squish' volume while maintaining the stroke volume would give you a big drop in power because you would get the same amount of fuel:air into the cylinder as before, with a 'dead' space that didn't scavenge properly, being compressed less. In a turbo- or supercharged engine, especially one that is EFI or on drugs, you should be able to compensate for that by increasing the air volume while upping the fuel (or adding drugs) in ratio... in effect bringing the compression ratio back up to what it was, while also increasing the fuel load.

To really make this work well, you need to empty the cylinder right out and more than fill it right up, every time... THAT will give you more HP.

My, I hope that made more sense. Sorry about that.

Here's another calculator for Static and Dynamic compression ratio. Not a better one, just another one.

on edit: Found another one, has a useful diagram.

 

[SmithvilleD]

Your pencil example would be a combustion chamber volume of zero. Hence my comment on the other thread about trying to compress into zero volume resulting in infinite pressure.

You're missing the point that there is still volume above the piston at TDC.

Yes, moving the top of the piston down .010" adds the 0.13 cubic inches to to the volume at both TDC & BDC.

Look as how significantly the addition of 0.13 cubic inches changes the volume above the piston at TDC.

Next, Look as how significantly the addition of 0.13 cubic inches changes the volume above the piston at BDC.

I'm all for the beer!!! and if I'm wrong, I certainly want to understand how so.

Believe they include the rod length on the dynamic ratio calculator because changing rod length does impact piston speed.

 

[buddy]

Your pencil example would be a combustion chamber volume of zero. Hence my comment on the other thread about trying to compress into zero volume resulting in infinite pressure.

You're missing the point that there is still volume above the piston at TDC.

Yes, moving the top of the piston down .010" adds the 0.13 cubic inches to to the volume at both TDC & BDC.

Look as how significantly the addition of 0.13 cubic inches changes the volume above the piston at TDC.

Next, Look as how significantly the addition of 0.13 cubic inches changes the volume above the piston at BDC.

I'm all for the beer!!! and if I'm wrong, I certainly want to understand how so.

Believe they include the rod length on the dynamic ratio calculator because changing rod length does impact piston speed.

yep, the displacement does not take into account the left over volume. so shortening the rod we have not changed displacement, but comp ratio is affected.

yes more air can get in to fill the space but air compresses on itself.

now I can hardly believe we can affect cylinder scavenging in this way and the engine still works....j/k

 

[chevyinlinesix]

I'm all for the beer!!! and if I'm wrong, I certainly want to understand how so.

Well, I thought I had a solid grasp on this whole concept, but now this thread is making me wonder if we are both talking about the same thing and are just not good at explaining it :cheers2:

I am going to call my crankshaft/connecting rod builder later this week and make sure I get it right, phone conversations are always easier than internet ones.

 

[Matt Bachand]

opcorn:

Good readings guys... this is the type of stuff that helps all of us better understand things.

When I'm figuring something out, I usually take it to the extremes, and then the opposites.

Say you're making the connecting rod LONGER, Therefore making the 'squish pocket' smaller, now the finial compression movement before TDC is that much tighter, most defanately resulting in higher compression, of the same volume.

...

How about this question, when doing a compression test on a 19:1 engine vs a 22:1 engine, the numbers would be lower on the 19:1, right?

 

[Diesel Tech]

How about thinking about it just a little different. Compression ratio is the total volume of the cylinder, head gasket, and head at bottom dead center versus the same at top dead center. Anything you do to change that relationship changes the compression ratio. So shortening the rod increases the volume at top dead center thus lowering the compression ratio.
Also most people forget to use the area between the piston wall and the top ring. While it's a small amount at 20:1 it more than most think. Down at 10:1 for a normal gas motor it's effect is much less.

 

[SmithvilleD]

Yes. Less pressure; therefore less temp increase of the charge, tougher starting.

All these parts (piston.cyl walls, head surface, valves, & prechamber) are at ambient temp during a cold start. So they're far colder than the compressed air charge. That temp difference makes for heat transfer out of the compressed air charge into surrounding parts - lowering the charge temp closer to the temp ultimately needed to initiate (compression) ignition.

As parts surrounding the chamber/cylinder get to operating temp, they're not pulling as much heat out of the air charge, so compressed charge temps are higher - more easily exceeding the temp needed to ignite the diesel/air charge.

The faster you can spin it starting, the less leakage past the rings, so slightly higher cylinder pressures - more heat - higher temps. Also spinning faster means more compression heatings/unit time, so you're putting heat into the chamber at a proportionally higher rate.

 

[JiFaire]

As CIL6 indicated, changing the rod length just moves the piston stroke further down in the cylinder... the stroke remains unchanged. What DOES change is the ratio between total cylinder volume (max) and total cylinder volume (min), which is what SmithvilleD was getting at, I think.

Using made-up numbers, in a normal engine (4" stroke) we might have 5" of (max) volume being compressed into 1" of (min) volume (I'm leaving the pi*r^2 out because it cancels in the ratio, anyway), which gives you a static ratio of 5:1

If you shorten the rod by 1", you move the stroke 1" down in the cylinder, which gives you 6" (max) compressed into 2" (min), or 6:2, or 3:1

The effective static ratio changed, even though the stroke volume remains the same.

The dynamic stuff is much more involved, and is 'way above my pay grade, but the static part is reasonably simple.

Although, as Will indicates, we're probably all busy confusing ourselves here and not saying what we mean.

Which is part of the fun. Or the fun of the beer. Something like that.

 

[SmithvilleD]

How about thinking about it just a little different. Compression ratio is the total volume of the cylinder, head gasket, and head at bottom dead center versus the same at top dead center. Anything you do to change that relationship changes the compression ratio. So shortening the rod increases the volume at top dead center thus lowering the compression ratio.
Also most people forget to use the area between the piston wall and the top ring. While it's a small amount at 20:1 it more than most think. Down at 10:1 for a normal gas motor it's effect is much less.

Yes, I posted the simplified drawing so folks visualize the concept before getting caught up in the math adding up the volumes. Putting it on graph paper makes it easy to count the squares & see the proportions.

Essentially, put the piston at TDC, then add up the volumes (prechamber, piston top flow pattern, compressed gasket volume, cyl wall to piston volume down to the top ring) = compressed volume (small volume)

Compressed volume + 1 cylinder's displacement = BDC volume = Non compressed volume (big volume)

It really is just the big volume number divided by the small volume number.