Rod ratio and dwell time... doesn't make sense

[Belgian1979]

With my simple brain I think the following :
When offsetting a piston pin towards the opposite of side where the side forces are greatest, you effectively would do the same as using a longer rod, the angle between rod and crank changes. This reduces side forces and thus the friction. It will also have an effect on the distance the piston has travelled.
However with the piston going from BDC to TDC you would have the reverse, the piston would create more friction and it would move the piston faster towards TDC. Of course the movement from BDC to TDC would only take place during the compression stroke and the exhaust stroke, both of which have less force acting on the piston, thus the amount of friction increase overall is lower.

What I would be interesting in seeing is the effect the piston pin offset would have when the pressure in the cylinder is at its greatest, namely around the 15-30° point ATDC. I tend to believe that you would have more side force acting on the side of the cilinder opposite to the normal work side, but that's nothing more than an assumption at this point.

Anyway, offsetting a piston pin is more of a cost reducing feature for me, as it allows you to use conventional crank and rod combo's while reducing side load forces when they are at their greatest.
In a normal engineering situation, when having to use the same deck height, one would not only have to lengthen the rod, but also reduce the crank throw. The effects would then be greatly increased vs just offsetting the piston pin.

In my engine I went from a conventional 4X3.48X5.7 to a 4.125X3.25X6.25 which is worlds apart. With the former you can feel it's not so happy running at high rpm, where the latter is quite happy to do so. The power effect at high rpm is incredible when combined with a short ratio rear end that really pushes the same car forward.

But again, this is my simple non-engineer brain trying to analyse things.

There are advocates of both : long rods and short rods. I believe there is a place for both, depending on what you want to attain. For instance if my goal would have been to build an engine that has great torque between 1000-5000 rpm, I would have stayed with the standard stroke/rod combo and maybe a longer rear end.

[RDY4WAR]

My take on it is this. Figure out your deck height. Determine the crankshaft/stroke you want. Get your desired piston weight and design. Then throw in whatever length of rod will connect the two. Don't "dwell" too much on the rod ratio.

[David Redszus]

My take on it is this. Figure out your deck height. Determine the crankshaft/stroke you want. Get your desired piston weight and design. Then throw in whatever length of rod will connect the two. Don't "dwell" too much on the rod ratio.

Correctomundo!!!

[Belgian1979]

My take on it is this. Figure out your deck height. Determine the crankshaft/stroke you want. Get your desired piston weight and design. Then throw in whatever length of rod will connect the two. Don't "dwell" too much on the rod ratio.

Considering the same deck height and stroke, with a number of choices as far as piston pin height is concerned, what do you choose ? Long or short rod ?

[RevTheory]

Let's say your money tree was well-pruned: custom pistons that match the dish to your combustion chambers, .060 pin offset to the major thrust side and Line2Line coated skirts to reduce skirt/wall clearance down to almost nill for reduced slap and negate any negative friction losses due to the increase in rod angularity on the thrust side, 1.6 rod/stroke ratio?

[Belgian1979]

Maybe some titanium rods to go along ?

[RDY4WAR]

My take on it is this. Figure out your deck height. Determine the crankshaft/stroke you want. Get your desired piston weight and design. Then throw in whatever length of rod will connect the two. Don't "dwell" too much on the rod ratio.

Considering the same deck height and stroke, with a number of choices as far as piston pin height is concerned, what do you choose ? Long or short rod ?

Most likely the longer rod but not for any kind of piston dwell or speed benefit. I'd favor the lighter, thinner c/h piston which would no doubt call for a longer rod.

Let's say though I'm shooting for 362-363ci with a SBF. If given the two options below...

9.5 deck block
4.060" bore
3.500" stroke
1.774" c/h piston
5.956" rod
1.70 r/s ratio

or

8.2 deck block
4.125" bore
3.400" stroke
1.090" c/h piston
5.400" rod
1.58 r/s ratio

I'm going to pick the 8.2 deck block every time for several reasons including less total weight, less piston weight/inertia, shorter stroke / bigger bore, and shorter pushrods for better valvetrain stability.

All I'm saying is that the rod/stroke ratio is pretty much at the bottom of the list of things I care about when spec'ing out an engine. So long as the piston isn't contacting the crank counterweights or coming out of the bore excessively at BDC, run it.

[Erland Cox]

It was pretty sure to me that, compared to TDC, when the piston is near the bottom of the bore, the intake valve is more open and the mixture charge flowing through it is near peak velocity.

For a cam shaft with maximum lift at 110 deg ATC, the lift at BDC is about 75% of max lift.
But the piston velocity at BDC is zero and so is the piston air demand. Allowing about 10 deg for flow lag there could be some air flow but not very much. In fact, reversionary flow is about to begin.

Have you ever heard of back filling David?

Erland

[RevTheory]

Maybe some titanium rods to go along ?

The secret ones that go from 1.5 rod/stroke ratio at TDC to 1.7 at BDC

[Belgian1979]

My take on it is this. Figure out your deck height. Determine the crankshaft/stroke you want. Get your desired piston weight and design. Then throw in whatever length of rod will connect the two. Don't "dwell" too much on the rod ratio.

Considering the same deck height and stroke, with a number of choices as far as piston pin height is concerned, what do you choose ? Long or short rod ?

Most likely the longer rod but not for any kind of piston dwell or speed benefit. I'd favor the lighter, thinner c/h piston which would no doubt call for a longer rod.

Let's say though I'm shooting for 362-363ci with a SBF. If given the two options below...

9.5 deck block
4.060" bore
3.500" stroke
1.774" c/h piston
5.956" rod
1.70 r/s ratio

or

8.2 deck block
4.125" bore
3.400" stroke
1.090" c/h piston
5.400" rod
1.58 r/s ratio

I'm going to pick the 8.2 deck block every time for several reasons including less total weight, less piston weight/inertia, shorter stroke / bigger bore, and shorter pushrods for better valvetrain stability.

All I'm saying is that the rod/stroke ratio is pretty much at the bottom of the list of things I care about when spec'ing out an engine. So long as the piston isn't contacting the crank counterweights or coming out of the bore excessively at BDC, run it.

I agree with this. On my combo my crank counterweight had to be drilled severely to get the whole shebang balanced. I could have asked them to mill the counterweights ( so it would have less windage to cope with)/lighten the rod throws,, but the cost was way to high and not within my budget.
Another thing comes to mind is the wider bores one could employ to end up with the same overall capacity of a given engine would let you deshroud a chamber more or use a larger valve in general.

[ptuomov]

Blast from the past.

What is the curve of the combustion chamber volume as a function of the crank angle from 30 degrees BTDC to 30 degrees ATDC with rod ratio of 1.5 vs rod ratio of 2.0? Let’s say with a geometric compression ratio of 9:1.

Does the more stable combustion chamber volume near the TDC for the long rod engine make it less likely to knock?

The reason why I am asking this is because I just reread this by koenigsegg and their piston design looks like one that is minimizing compression height and maximizing rod length (given stroke and deck height):

“Combustion chamber

Peak pressure is what causes detonation whereas average pressure during the stroke is what gives you power. So in order to create the amount of power we create, you need to have a high average pressure during the power stroke but you also have to keep the peak pressure at TDC as low as possible.

We have a very unusual combustion chamber. We don’t have any ‘squish’ areas. We have created a kind of four-valve hemispherical combustion chamber that avoids speed differentiation to the flame propogation during the combustion process. This is another danger that can lead to knocking when you’ve got such high cylinder pressures. The chamber and the piston are both specially designed to maintain a very even volume in the chamber as combustion takes place. The piston and connecting rod are especially advanced in their design to provide specific geometry to assist with this process.“

[David Redszus]

Blast from the past.

What is the curve of the combustion chamber volume as a function of the crank angle from 30 degrees BTDC to 30 degrees ATDC with rod ratio of 1.5 vs rod ratio of 2.0? Let’s say with a geometric compression ratio of 9:1.

Does the more stable combustion chamber volume near the TDC for the long rod engine make it less likely to knock?

The reason why I am asking this is because I just reread this by koenigsegg and their piston design looks like one that is minimizing compression height and maximizing rod length (given stroke and deck height):

Following are the piston to deck distances for two con rod ratios. Piston distance times area will give volume if needed.

Rod ratio 1.5
angle......piston distance (mm)
0...0.00
5...0.23
10..0.91
15..2.04
20..3.59
25..5.56
30..7.92

Rod ratio 2.0
angle......piston distance (mm)
0...0.00
5...0.21
10..0.85
15..1.91
20..3.37
25..5.22
30..7.44

Note that the longer rod ratio causes the chamber volume to be reduced thus slightly raising temperature.

“Combustion chamber
Peak pressure is what causes detonation whereas average pressure during the stroke is what gives you power. So in order to create the amount of power we create, you need to have a high average pressure during the power stroke but you also have to keep the peak pressure at TDC as low as possible.

Not quite correct. Pressure does NOT cause detonation. Detonation is caused by heat, not pressure.
While it is true that a higher TCR will raise compression temperature, it is the flame front temperature and chamber temperature that causes detonation.

[zums]

Blast from the past.

What is the curve of the combustion chamber volume as a function of the crank angle from 30 degrees BTDC to 30 degrees ATDC with rod ratio of 1.5 vs rod ratio of 2.0? Let’s say with a geometric compression ratio of 9:1.

Does the more stable combustion chamber volume near the TDC for the long rod engine make it less likely to knock?

The reason why I am asking this is because I just reread this by koenigsegg and their piston design looks like one that is minimizing compression height and maximizing rod length (given stroke and deck height):

Following are the piston to deck distances for two con rod ratios. Piston distance times area will give volume if needed.

Rod ratio 1.5
angle......piston distance (mm)
0...0.00
5...0.23
10..0.91
15..2.04
20..3.59
25..5.56
30..7.92

Rod ratio 2.0
angle......piston distance (mm)
0...0.00
5...0.21
10..0.85
15..1.91
20..3.37
25..5.22
30..7.44

Note that the longer rod ratio causes the chamber volume to be reduced thus slightly raising temperature.

“Combustion chamber
Peak pressure is what causes detonation whereas average pressure during the stroke is what gives you power. So in order to create the amount of power we create, you need to have a high average pressure during the power stroke but you also have to keep the peak pressure at TDC as low as possible.

Not quite correct. Pressure does NOT cause detonation. Detonation is caused by heat, not pressure.
While it is true that a higher TCR will raise compression temperature, it is the flame front temperature and chamber temperature that causes detonation.

That depends on water jacket design and cooling pattern, thermal efficiency can go down because of increased dwell

[stealth]

Rod ratio = 8 rods to one crank....all you need to know

Less ratio means you like imports...
More ratio means you have spare parts...

[user-30257]

My take on it is this. Figure out your deck height. Determine the crankshaft/stroke you want. Get your desired piston weight and design. Then throw in whatever length of rod will connect the two. Don't "dwell" too much on the rod ratio.

Considering the same deck height and stroke, with a number of choices as far as piston pin height is concerned, what do you choose ? Long or short rod ?

Whatever one allows you to run the rings on the piston where you want.