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

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David Redszus
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Re: Rod ratio and dwell time... doesn't make sense

Post by David Redszus » Wed Aug 16, 2017 9:03 pm

I am just curious as to why no one seems to address any reasoning for extended time the piston spends near the bottom of the bore when the intake valve is always more open and flowing near maximum mixture velocity.

Well it might be because that is simply not true.

Even with a massive change in rod ratio, there is virtually no change in piston position at BDC.
Neither is there any change in piston velocity at BDC.

There is almost no change in piston acceleration at TDC.
There can be a difference in piston acceleration at BDC covering a crank angle about 35 deg, + or - from BDC.
But while there is some change in acceleration, actual piston acceleration at or near BDC is only one half of what it is at TDC.

The bottom line again is...rod ratio simply does not matter very much.
A longer rod will produce a very slightly lower maximum piston speed at about 76 deg ATC.
A shorter rod will allow a lower block deck height, less engine weight, less vehicle weight, etc, etc.
Look at it from the point of view of how much the combustion chamber volume changes with longer or shorter rod in the top 10 or 20 degrees, + or - 5 or 10 deg. either side of TDC.
The answer is that it doesn't change.

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

Post by Belgian1979 » Thu Aug 17, 2017 7:53 am

less side force on the piston producing less friction.
since the amount of force on the piston remains the same, more is going through the rod to the crank.
if piston speed is slower at 90°ATDC, the force has more time to act on the piston.
If a piston speed is lower at 90 ATDC, the intake charge has more chance on catching up with it, requiring less valve opening after BDC

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

Post by Ron E » Thu Aug 17, 2017 8:37 am

Many favored long rods in the days before adequate cylinder heads were available. I remember Larry Meaux seeing a substantial gain on a (I think) 400 inch SBC that ran mandated OE size heads. Also, with "on the edge" RPM endurance engines, the slightly relaxed extension rate approaching TDC has been mentioned. Once again, mostly before adequate rods, pistons were so commonly available.

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

Post by Walter R. Malik » Thu Aug 17, 2017 9:58 am

David Redszus wrote:
I am just curious as to why no one seems to address any reasoning for extended time the piston spends near the bottom of the bore when the intake valve is always more open and flowing near maximum mixture velocity.


Well it might be because that is simply not true.
Really ...

I must be totally out to lunch.
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. :-k

No matter how hard I try ... I can't visualize it being different.
Greater intake air flow and velocity near overlap ??? #-o
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Re: Rod ratio and dwell time... doesn't make sense

Post by In-Tech » Thu Aug 17, 2017 12:14 pm

Walter R. Malik wrote:
David Redszus wrote:
I am just curious as to why no one seems to address any reasoning for extended time the piston spends near the bottom of the bore when the intake valve is always more open and flowing near maximum mixture velocity.


Well it might be because that is simply not true.
Really ...

I must be totally out to lunch.
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. :-k

No matter how hard I try ... I can't visualize it being different.
Greater intake air flow and velocity near overlap ??? #-o
Walter, with all due respect, do the math. A few posts back Stan showed you a change in rod length of over an inch and it netted ~1 degree.

I'm all for adding some length to a rod if I can, lightens the piston and helps angularity for high rpm...it's just not as much as you would think.
Heat is energy, energy is horsepower...but you gotta control the heat.
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Re: Rod ratio and dwell time... doesn't make sense

Post by David Redszus » Thu Aug 17, 2017 12:23 pm

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. :-k
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.

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

Post by mrriggs » Thu Aug 17, 2017 12:42 pm

Belgian1979 wrote:less side force on the piston producing less friction.
since the amount of force on the piston remains the same, more is going through the rod to the crank.
if piston speed is slower at 90°ATDC, the force has more time to act on the piston.
If a piston speed is lower at 90 ATDC, the intake charge has more chance on catching up with it, requiring less valve opening after BDC
Less side force on the piston also means less side force on the up-and-down crankshaft throw. When the crank is near TDC, pushing straight down on it does not produce the greatest torque. A short rod will push harder on the crank perpendicular to the vertical crank throw than a long rod will.

So a short rod potentially has the advantage but only if the increase in torque is greater than the loss from friction. I don't know what the friction coefficient is of an aluminum piston on an oiled iron bore. When I do the math for my motorcycle engine, plugging in different coefficient values, the tipping point is 0.13. If the actual friction is less than that, the short rod puts out more mean torque.

The short rod always puts out a higher instantaneous torque but the added friction on the upstroke is it's disadvantage when the friction coefficient is higher than 0.13.

Luckily, there is a very simple solution to this. Offset the wrist pin towards the loaded side of the piston (like OEMs have been doing for decades). You get the higher instantaneous torque of a short rod on the down stroke, and the reduced friction of a long rod on the upstroke. It's win-win.

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

Post by Belgian1979 » Thu Aug 17, 2017 1:08 pm

mrriggs wrote:
Belgian1979 wrote:less side force on the piston producing less friction.
since the amount of force on the piston remains the same, more is going through the rod to the crank.
if piston speed is slower at 90°ATDC, the force has more time to act on the piston.
If a piston speed is lower at 90 ATDC, the intake charge has more chance on catching up with it, requiring less valve opening after BDC
Less side force on the piston also means less side force on the up-and-down crankshaft throw. When the crank is near TDC, pushing straight down on it does not produce the greatest torque. A short rod will push harder on the crank perpendicular to the vertical crank throw than a long rod will.

So a short rod potentially has the advantage but only if the increase in torque is greater than the loss from friction. I don't know what the friction coefficient is of an aluminum piston on an oiled iron bore. When I do the math for my motorcycle engine, plugging in different coefficient values, the tipping point is 0.13. If the actual friction is less than that, the short rod puts out more mean torque.

The short rod always puts out a higher instantaneous torque but the added friction on the upstroke is it's disadvantage when the friction coefficient is higher than 0.13.

Luckily, there is a very simple solution to this. Offset the wrist pin towards the loaded side of the piston (like OEMs have been doing for decades). You get the higher instantaneous torque of a short rod on the down stroke, and the reduced friction of a long rod on the upstroke. It's win-win.
I have difficulty with the statement in the first paragraph. If I'm not mistaken, with a long rod, the piston will higher in the bore when the crank and rod angle reaches 90° thus the room above the piston is smaller and the expanding gasses have less room to expand so exert more pressure on the piston. Associated with less side force I would think this would certainly account for bigger torque. Seems like what you describe is contrary to that.

Unfortunately your spreadsheet doesn't show the angle between crank and rod.

After analysis with your spreadsheet there is certainly a piston velocity advantage and I did notice something strange in that the short rod motor (regular SBC) has a bump at BDC in the crank acceleration graph that is not there with the long rodded motor.

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

Post by mrriggs » Thu Aug 17, 2017 1:33 pm

Belgian1979 wrote:Unfortunately your spreadsheet doesn't show the angle between crank and rod.
I'm not sure what spreadsheet you are referring to.

I'm using this one; http://www.gofastforless.com/junk/Pisto ... onTwin.xls

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

Post by David Redszus » Thu Aug 17, 2017 3:14 pm

Let's look at piston side forces.
Given a stroke of 80mm and rod ratios of 1.5 and 2.0, for a given piston weight and 6000 rpm we find:

Rod Ratio = 1.5
Inertia force Thrust side = 510 lbs
w/comb force Thrust side =820 lbs

Rod ratio = 2.0
Inertial force Thrust side =348 lbs
w/comb force Thrust side =555 lbs

With a pin offset of -1.0mm, inertia side forces become 504 and 345 lbs respectively.

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

Post by Walter R. Malik » Thu Aug 17, 2017 7:31 pm

In-Tech wrote:
Walter R. Malik wrote:
David Redszus wrote:

Well it might be because that is simply not true.
Really ...

I must be totally out to lunch.
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. :-k

No matter how hard I try ... I can't visualize it being different.
Greater intake air flow and velocity near overlap ??? #-o
Walter, with all due respect, do the math. A few posts back Stan showed you a change in rod length of over an inch and it netted ~1 degree.

I'm all for adding some length to a rod if I can, lightens the piston and helps angularity for high rpm...it's just not as much as you would think.
That is not the issue I am presenting ... you're missing my point completely.
I have done the math, (years ago and it doesn't change), so, having that measly amount of extra time with the piston near the bottom of the bore when using a shorter rod should affect air flow into the cylinder more than whatever the piston is doing near the top, (if the inertia of the moving weight of the piston assembly is not a concern).

Most everyone here seems to be concerned with what changes may happen at the top of the bore. The very slight change that also happens at the bottom of the bore seems to be the more important phenomena in respect to air flow into the cylinder.
Last edited by Walter R. Malik on Thu Aug 17, 2017 7:36 pm, edited 1 time in total.
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Re: Rod ratio and dwell time... doesn't make sense

Post by exhaustgases » Thu Aug 17, 2017 7:35 pm

nickpohlaandp wrote:Oh, I found this illustration too. I figured I'd post it up in case anyone else was having the same rod ratio/dwell time questions I had. It might help a little

Also on Google I found a post on ST from about 8 years ago talking about a very similar subject.
Tried to delete the post.

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

Post by David Redszus » Thu Aug 17, 2017 8:28 pm

Can someone please define what is meant by having more time at either TDC or BDC as a function of rod ratio?

Running some calcs using increasing rod ratios from 1.5 to 2.0, in increments of .10, we can see the effect of rod ratio on piston position, velocity and acceleration.

Rod ratios from 1.5 to 2.0 represent a change of 33%. Across the entire range;
piston max velocity changes by only 2.3%, which occurs at 73 to 76 degs ATC (a change of only 4%).
piston position difference occurs at 90 deg and ranges by only 4%.

Nothing of any significance occurs at either TDC or BDC.
Small differences in piston speed do occur at max piston speed (73 to 76 deg ATC).
Small differences in piston position do occur at 90 deg ATC.

Conclusion
Rod ratio has absolutely no effect on combustion processes.
Large rod ratio differences do have an effect at the point of peak piston air demand (73 to 90 deg).
Extreme rod ratios can have an impact on inlet air flow at midstroke.

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

Post by Stan Weiss » Thu Aug 17, 2017 9:30 pm

mrriggs wrote:
Belgian1979 wrote:less side force on the piston producing less friction.
since the amount of force on the piston remains the same, more is going through the rod to the crank.
if piston speed is slower at 90°ATDC, the force has more time to act on the piston.
If a piston speed is lower at 90 ATDC, the intake charge has more chance on catching up with it, requiring less valve opening after BDC
Less side force on the piston also means less side force on the up-and-down crankshaft throw. When the crank is near TDC, pushing straight down on it does not produce the greatest torque. A short rod will push harder on the crank perpendicular to the vertical crank throw than a long rod will.

So a short rod potentially has the advantage but only if the increase in torque is greater than the loss from friction. I don't know what the friction coefficient is of an aluminum piston on an oiled iron bore. When I do the math for my motorcycle engine, plugging in different coefficient values, the tipping point is 0.13. If the actual friction is less than that, the short rod puts out more mean torque.

The short rod always puts out a higher instantaneous torque but the added friction on the upstroke is it's disadvantage when the friction coefficient is higher than 0.13.

Luckily, there is a very simple solution to this. Offset the wrist pin towards the loaded side of the piston (like OEMs have been doing for decades). You get the higher instantaneous torque of a short rod on the down stroke, and the reduced friction of a long rod on the upstroke. It's win-win.
And yet years ago when they had to run those piston in the stock classes. There was a performance increase from reversing the direction of the pin offset (installing the piston backwards).

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

Post by RevTheory » Thu Aug 17, 2017 10:10 pm

Wasn't OEM offsetting the pins the opposite direction to reduce piston slap?

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