Don Terrill’s Speed-Talk

Visualize the rod and stroke the same length.
From 90 to 180 to 270 the piston will not move at all.
The wrist pin will be at the canter line of the crank.
From 270 to 0 to 90 the piston will move the stroke length.

It's all BS of course.

Like lots of things car and engine related.


The difference between a good and a bad rod to stroke ratio is only a few %.
Go too far one way, you get too much side loading, go too far the other way, too long at TDC?

There's always exceptions.


I first came across it reading up about 351Cs and offset grind stroking.


What's that?......5.7 rod with 3.5 stroke?
1.6 to 1 ratio?


A 4G63 is 88 mm stroke, 150 mm rod..........1.7 to 1.
A 6G72 is 76 mm stroke, 140 rod length......1.8 to 1

I've always had the same problem reading tech books......read something, it doesn't make sense.......read it again......still doesn't.
Come back later, still doesn't.

Because it's wrong.

Try this website for the effects of piston stroke:

Piston Motion

joe 90 wrote:It's all BS of course.

Like lots of things car and engine related.


The difference between a good and a bad rod to stroke ratio is only a few %.
Go too far one way, you get too much side loading, go too far the other way, too long at TDC?

There's always exceptions.


I first came across it reading up about 351Cs and offset grind stroking.


What's that?......5.7 rod with 3.5 stroke?
1.6 to 1 ratio?


A 4G63 is 88 mm stroke, 150 mm rod..........1.7 to 1.
A 6G72 is 76 mm stroke, 140 rod length......1.8 to 1

I've always had the same problem reading tech books......read something, it doesn't make sense.......read it again......still doesn't.
Come back later, still doesn't.

Because it's wrong.

BMW/Alpina b4s & b3 3.3; 93.8mm stroke, 135mm rod .......1.44 comes with a warranty.

Circlotron wrote: SNIP

What happens as the crank pin goes from 9 o'clock to 12 o'clock is the pin moves upwards and so the piston does too, but the crank pin also moves toward the centre line and this gives additional upward movement to the piston. (pointy top sine wave). From 3 o'clock to 6 o'clock, though, the crank pin moves downward (and so does the piston of course) but as the crank pin moves toward the centre line it adds a bit of *upward* component to the descending piston, same as when it did going from 9 o'clock to 12 o'clock because again the crank pin is moving toward the centre. The piston doesn't actually move up, it just moves down slower than normal.

On an engine with say a 4 inch stroke, rotate the crank to 90 deg ATDC and you will find that the piston is *more* than 2 inches down the bore. The shorter the rod the further than halfway down it will be. That alone proves that piston movement with realistically sized rods is not actually sinusoidal.

This is the best short explanation I have read. Now visualize that 4 inch stroke engine with ever-shorter rods and you will really see the difference.

JR

Circlotron wrote:...The piston movement is only sinusoidal with an infinite length rod.... That alone proves that piston movement with realistically sized rods is not actually sinusoidal.

Very good explanation. I don't know why, but I especially enjoyed that you used "sinusoidal" without referring to a dingle arm or Turbo Encabulator!

It does make sense now though. Thanks for your detailed explanation!

Circlotron wrote: Close. The piston speed at any crank angle is the same as the piston speed at the crank angle that is a mirror image about the centre line of the first angle. e.g it is the same for 50 deg either side of TDC or BDC.
This is assuming that the main journal, gudgeon pin and bore axis are all in line, but usually at least the gudgeon is offset a little.

How many responses assumed perfect "alignment" and how many didn't?

What of a typically flow limited 2v cylinder versus 4v?

Compared to similar low capacity oem 2v designs, Honda 4v have generous intake port volumes, low rod to stroke ratios, decent street performance AND meet emissions, no doubt part due to their Vtec tecnology

Who'd bet against V8 4V designs adopting similar mechanical ratios and volumes, if exterior constraints such engine bay size weren't limiting factors?

Ignoring specialist aftermarket V8 2v head designs with their focus on 1/4 mile and race performance, for performance 2v street with modified oem parts surely a high rod length to stroke ratio is preferable regardless of cylinder capacity - -

Tuner wrote:Visualize the rod and stroke the same length.
From 90 to 180 to 270 the piston will not move at all.
The wrist pin will be at the canter line of the crank.
From 270 to 0 to 90 the piston will move the stroke length.

Sleep deprived post. ^^^ I meant rod length 1/2 stroke length, such as a 2" rod with 2" crank throw (4" stroke).

In the lower 180 deg, from 90 ATDC to BDC to 90 BTDC, the piston will not move at all because the wrist pin center and crank main center are the same while the rod center line and crank throw center line are parallel during the bottom 180 deg. of crank rotation. The wrist pin has no vertical motion as the rod and crank throw describe the same radius while the wrist pin is on the crank main center. All vertical motion is confined to the upper 180 deg.

Tuner wrote:

Tuner wrote:Visualize the rod and stroke the same length.
From 90 to 180 to 270 the piston will not move at all.
The wrist pin will be at the canter line of the crank.
From 270 to 0 to 90 the piston will move the stroke length.

Sleep deprived post. ^^^ I meant rod length 1/2 stroke length, such as a 2" rod with 2" crank throw (4" stroke).

In the lower 180 deg, from 90 ATDC to BDC to 90 BTDC, the piston will not move at all because the wrist pin center and crank main center are the same while the rod center line and crank throw center line are parallel during the bottom 180 deg. of crank rotation. The wrist pin has no vertical motion as the rod and crank throw describe the same radius while the wrist pin is on the crank main center. All vertical motion is confined to the upper 180 deg.

Although your example, I believe, would be nothing short of physically impossible, I understood what you were saying and it makes perfect sense when you look at it that way. Sometimes you need to take mathematical illustrations to the extreme to quickly show what is happening on a more "easy to see" level. Your example of a 2" rod with a 4" stroke is a good way of describing this phenomenon I'm looking into. Thanks for your input.

That really explains the phenomenon that the author was writing about, and I often find myself reading things that seem to make sense, but I'm not exactly sure why... or vice versa. It'd be nice if authors would sometimes use unrealistic extremes to illustrate the point they are trying to make. In your example of an EXTREMELY short rod, dwell time is 180 degrees at BDC, consequently making dwell time at TDC very short. As the rod is lengthened the values begin to skew toward realism, but this backs up the authors statement that a short rod will be faster at TDC and dwell longer at BDC, and a longer rod will do the opposite.

This makes me wonder, does anyone know what the rod ratio would be where dwell at TDC and BDC are equal?

nickpohlaandp wrote:

Tuner wrote:

Tuner wrote:Visualize the rod and stroke the same length.
From 90 to 180 to 270 the piston will not move at all.
The wrist pin will be at the canter line of the crank.
From 270 to 0 to 90 the piston will move the stroke length.

Sleep deprived post. ^^^ I meant rod length 1/2 stroke length, such as a 2" rod with 2" crank throw (4" stroke).

In the lower 180 deg, from 90 ATDC to BDC to 90 BTDC, the piston will not move at all because the wrist pin center and crank main center are the same while the rod center line and crank throw center line are parallel during the bottom 180 deg. of crank rotation. The wrist pin has no vertical motion as the rod and crank throw describe the same radius while the wrist pin is on the crank main center. All vertical motion is confined to the upper 180 deg.

Although your example, I believe, would be nothing short of physically impossible, I understood what you were saying and it makes perfect sense when you look at it that way. Sometimes you need to take mathematical illustrations to the extreme to quickly show what is happening on a more "easy to see" level. Your example of a 2" rod with a 4" stroke is a good way of describing this phenomenon I'm looking into. Thanks for your input.

That really explains the phenomenon that the author was writing about, and I often find myself reading things that seem to make sense, but I'm not exactly sure why... or vice versa. It'd be nice if authors would sometimes use unrealistic extremes to illustrate the point they are trying to make. In your example of an EXTREMELY short rod, dwell time is 180 degrees at BDC, consequently making dwell time at TDC very short. As the rod is lengthened the values begin to skew toward realism, but this backs up the authors statement that a short rod will be faster at TDC and dwell longer at BDC, and a longer rod will do the opposite.

This makes me wonder, does anyone know what the rod ratio would be where dwell at TDC and BDC are equal?

I think that the only way the motion around TDC would be exactly equal to that around BDC would be if the rod had an infinite length.

DaveMcLain wrote:I think that the only way the motion around TDC would be exactly equal to that around BDC would be if the rod had an infinite length.

I was just doing some sketching on Excel and it seems that's the case. I don't think there'd be any power advantage, I was just curious. Hey, if weight weren't an issue, a 120" deck height would be obnoxiously cool, lol

nickpohlaandp wrote:

DaveMcLain wrote:I think that the only way the motion around TDC would be exactly equal to that around BDC would be if the rod had an infinite length.

I was just doing some sketching on Excel and it seems that's the case. I don't think there'd be any power advantage, I was just curious. Hey, if weight weren't an issue, a 120" deck height would be obnoxiously cool, lol

You have to factor in :
- the effects of a smaller chamber at a longer time when the burning charge is expanding.
- the effects of rod angle due to the longer rods.
- the slower piston speeds that improve breathing up top.

As with everything though there are trade offs.

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.

Cool I guess you got it licked now.
Yeah there isn't a real definite advantage one way or the other, but it can be like a lot of tuning "factors", in that, if it DOES make a big difference it means something.
Like if loose lash works WAY better you probably need less duration.
If a short rod works WAY better then your ports are probably too big, and the opposite.
Take into account other factors which you are stuck with, probably can guess if longer or shorter rod might be preferable.

http://www.camotruck.net/rollins/piston.html

Erland