Piston Dwell Time

Generally, how does longer dwell time of the piston at TDC affect or enhance the performance capabilities of a given 385 series engine combination.  Is there an advantage to longer dwell time or is dwell just a by product of stroker kits with longer connecting rods. Under boosted conditions?

Or is any of this even relevant?

Thanks

Dwell time is irrelevant. A piston that is not in motion is not transferring power .

Some will say that longer piston dwell time at TDC gives more time for the combustion gasses to exert more pressure on the piston, and thus more torque, than it would if it was already moving away (down) while combustion process is happening.

To slow of a piston at TDC will tend to promote detonation when using marginal gasoline.
The piston NEVER really dwells because it is always moving to or from the top or bottom; the rate simply changes.

When the piston time near TDC is made longer, it gets shorter at BDC and visa-versa.

Hmmm, that is opposite of what those who believe in high rod ratios preach.
How did you come up with different dwell times at tdc vs bdc?

Again thanks for the input.
Did some reading up on the subject. You guys pretty much nailed all the different aspects of piston dwell. It seems that Rod Ratio's have more of an affect than the piston lingering at TDC. Friction, piston thrust loads on cylinder walls, engine durability etc. A lot of engine science has progressed since the 70's. Thanks for your sage advice.

XF-66 wrote:Again thanks for the input.
Did some reading up on the subject. You guys pretty much nailed all the different aspects of piston dwell. It seems that Rod Ratio's have more of an affect than the piston lingering at TDC. Friction, piston thrust loads on cylinder walls, engine durability etc. A lot of engine science has progressed since the 70's. Thanks for your sage advice.  

Yes ... the capabilities to actually measure such things now days, has added enlightenment to a lot of engine dynamics.

6t6mustang wrote:Hmmm, that is opposite of what those who believe in high rod ratios preach.
How did you come up with different dwell times at tdc vs bdc?

"those who believe" can certainly believe whatever they wish; it still does not make it actuality; it's all based upon simple geometry and a piston moving faster away from TDC will exhibit less chance to detonate.

6t6mustang wrote:Hmmm, that is opposite of what those who believe in high rod ratios preach.
How did you come up with different dwell times at tdc vs bdc?

Pretty simple if you speed up one point in an elipitical motion, some point has to slow down.

Supporters of long rods often claim benefits of improved breathing as piston speed is not as great with the long rod. Another cited benefit is longer presence near TDC as the piston resides near TDC longer than a short rod ratios. There is a little truth in these statements, but with modern cylinder heads, compromises elsewhere offset the advantages, which is why modern engines (Pro Stock included) are no longer fixated on rod ratio. I'll try to explain further:

Cylinder pressure decay (CPD) is where it's at. This is the pressure in terms of crank angle and the rate that it decays as volume increases (piston travels down bore). If you model this, you can quantify the decay and the gains are pretty small. To better quantify things, in a Pro Stock engine with 4.730 bore, 3.550 stroke and 16.0:1 compression, between rods 6.103" and 6.500" long (a huge change in rod ratio which can't be contained within the engine package constraints - 1.72 vs 1.83), with 135 bar (1,944psi) peak cylinder pressure, the difference in peak pressure is only 12.4 psi. You have to remember that cylinder pressure between the two combinations is the same at BDC, and the working range is only effective until the exhaust valve opens, so this advantage is a fraction of that. When you do the integration and look at the total work performed under the two curves, the difference is only 0.94% - but don't forget the parasitic loss of the added reciprocating mass.

Shorter rod ratios get the intake charge moving earlier, which is good as peak velocity does not occur at peak piston speed. Additionally, the shorter rod gives you better breathing during overlap as the piston gets out of the way sooner. The short rod provides a shorter deck height and short intake runner - important in a 10,000+rpm engine, lower CG and less weight. For those who might cite about pin angle advantages, the above combination differences equate to 1.04 degrees difference in pin angle - minimal considering 0.400" length difference.

In short, the longer rod had its advantages when cylinder head flow was a problem, but those days are long gone.

Mike Kopmanis

The current trend in "power adder" engines is toward shorter rods too.

Trends come around, then they go away.....then they come around again, usually with different packaging.

A rod is mearly the means to connect the piston to the crankshaft..... the small differences in length are all but irrelevant. Only thing that is relevant is having enough room to fit a good ring package and valve reliefs.

gt350hr wrote:

6t6mustang wrote:Hmmm, that is opposite of what those who believe in high rod ratios preach.
How did you come up with different dwell times at tdc vs bdc?

  Pretty simple if you speed up one point in an elipitical motion, some point has to slow down.

Since when is the motion of a rod throw elliptical?

6t6mustang wrote:

gt350hr wrote:

6t6mustang wrote:Hmmm, that is opposite of what those who believe in high rod ratios preach.
How did you come up with different dwell times at tdc vs bdc?

  Pretty simple if you speed up one point in an elipitical motion, some point has to slow down.

Since when is the motion of a rod throw elliptical?

The motion of a connecting rod center IS an egg shape ... not the crank throw.

To use your words ... "since when is the" rod crank throw and the connecting rod the same thing...? Especially when the discussion was about the reciprocating piston end of the rod.

If you don't wish to learn anything then fine but,
interjecting sarcastic subject change with ignorance into the thread won't help anyone.

At the pin centerline the motion is reciprocating, at the rod throw centerline it is a circle, at rod center beam it is egg shaped? Is that what you are saying? I'm no engine savant, but I'm decent with theory and geometry, explain and ill prolly get it.

Yes that motion is egg shaped and you are correct the big end motion is circular AND constant speed ( whatever rpm it is) yet the pin "stops" at tdc.

6t6mustang wrote:At the pin centerline the motion is reciprocating, at the rod throw centerline it is a circle, at rod center beam it is egg shaped? Is that what you are saying?  I'm no engine savant, but I'm decent with theory and geometry, explain and ill prolly get it.

Draw a diagram and do some math to figure where everything is at TDC, 90, 180, etc (pardon any terminology that is off in that sentence).  I just did the same thing to make sense of it, myself

.

JBR you touched on it there at the end of your post. I'm surprised no one else did. The shorter the rod the sharper the angle it is having to push on the crank to transfer power at the bottom of it's stroke. which in turn creates harmonics and adds stress to the rod and rod bolts. Too long and you sacrifice piston strength. Just like everything else I think you have to find a happy medium. If you're talking a 6.8 rod versus a6.7, 4.5 crank for argument sake. I'd go with a 6.8 because the piston is 100 grams lighter. Probably not enough difference in the rod ratio to make a difference to the average weekend racer. My 2 cents.

We had a discussion years ago on the old network 54 forum regarding the difference in dwell time TDC vs. BDC.
I had to draw this illistration in order to understand the concept. Remember that this was about 2002 and it is somewhat crude...

Looking at the diagram now I agree that the egg shape comparison applies with regard to big end travel.














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