5.7 vs 6.0 Rods

[2xmod]

Ive heard (on this forum) of people running near 12:1 on pump gas. Does in fact the long rod play a huge role in this, or is there other "secrets" involved in this. Brad

[bigjoe1]

If anything, it would be the other way around. The short rod would work out better.


JOE SHERMAN RACING

[Warp Speed]

Why such long rods in NASCAR, piston weight?

SKeown

Induction limits/piston side loading.
The reduced piston weight doesn't hurt either!

[Stan Weiss]

This graph shows piston side force. Only the rod length has been changed.

Stan

[falcongeorge]

Ive heard (on this forum) of people running near 12:1 on pump gas. Does in fact the long rod play a huge role in this, or is there other "secrets" involved in this. Brad

I have been well over 12/1, and I usually tend towards longer rod combos, mostly because I hate it when my shit blows up. More important in my eyes re: detonation, is 1) good combustion chamber shape/tight quench 2) keeping the incoming charge cool 3) getting the cam timing right 4) gearing 5) I have better pump gas than most of you! Chevron 94

Some guys say the longer rod is more detonation prone, because the piston moved away from tdc slower, but personally, I dont think this counts for much in the real world, no-where near as much as the stuff mentioned above anyway. But thats just my opinion.

[axegrinder]

Here is three more pages from '04. I think it all still applies now.

viewtopic.php?t=406&highlight=ratio

Denis

[The Dark Side of Will]

I stopped reading when they said 400 block.

Then you *COMPLETELY* missed the point...

UMMM,I got the point.
They want me to shove a HEAVY ROD in a junk 400 block.
How cares about rod ratio when you are putting it into a junk block.
WHY BOTHER!!!!

*sigh*... The point was the geometry of the combo. As I noted above, a similar combo can be built with high end shelf parts, and of course an aftermarket block. I wasn't asking about that specific engine, just the idea.

[Stan Weiss]

Here is three more pages from '04. I think it all still applies now.

viewtopic.php?t=406&highlight=ratio

Denis

This pushes the limits of short and long rod to show the difference. The 5" rod (Red Line) is 1.379:1 rod stroke ratio and the 7.5" rod (Yellow Line) is 2.069:1 rod stroke ratio

But it clearly shows what Jere Stahl was talking about.

I. LONG ROD

A. Intake Stroke -- will draw harder on cyl head from 90-o ATDC to BDC.


II. Short Rod

A. Intake Stroke -- Short rod spends less time near TDC and will suck harder on the cyl head from 10-o ATDC to 90-o ATDC the early part of the stroke, but will not suck as hard from 90-o to BDC as a long rod.

Stan

[falcongeorge]

really neat graphic. Describes (demonstrates??) better than words ever could why short r/s ratio engines tend (please, no hair splitting, this is a generalization) to prefer a tighter lda relative to an otherwise similar high r/s ratio combo.

[bobqzzi]

Why such long rods in NASCAR, piston weight?

SKeown

They have fixed deck height, displacement, and bore sizes so the rod length is only varied by piston compression height and a (properly designed) longer rod/shorter piston combo will always be lighter.

[David Redszus]

The rod ratio argument has been buried long ago. The actual rod length issue continues. Mostly due to my own curiosity, I did some engine simulation calculations for a 5.70" rod vs a 6.0" rod. stoke-90mm, bore-101.4mm, rpm 8000.

While issues such as piston side forces, maximum velocity, crank angle, etc, have been well covered, there is one area that might bear some examination. The combustion pressure at 10deg ATC is the single most important factor in the determination of engine power due to combustion. So how does rod length affect engine factors at 10 degs?

At 8000 rpm, both engines have a mean piston speed of 24.0 m/s.
The short rod (5.7) produces a max piston speed of 39.5 m/s, vs. 39.32 m/s for the long rod. A delta of 0.45%.

At 10 deg (BTC & ATC) the short rod gives a piston speed of 8.57 m/s with a piston acceleration of 4110g. The long rod gives a piston speed of 8.46 m/s with a piston acceleration of 4063g. A delta of 1.3% and 1.15% respectively.

Maximum squish velocity (at 20% SAR and 1.25" clearance) for the short rod is 18.606 m/s (@10.1deg) and for the long rod 18.492 m/s (10.0deg).
A delta of 0.60% and 0.10% respectively.

What does it all mean?
Mean piston speed did not change
Max piston speed was reduced slightly with the long rod.
At 10deg, piston speed was reduced with the long rod.
At 10deg, piston G was reduced with the long rod.
At 10deg, engine squish velocity was reduced with the long rod.

It would seem that the long rod has advantages everywhere but they are very, very small. The area with the greatest sensitivity seems to be at 10 degrees for piston velocity but not for squish.

Since all of the above effects are very small, it would seem that observed performance would be due to other engine factors not related to rod length.

What say ye all?




Those who do not read have no advantage over those who cannot read.

[falcongeorge]

The rod ratio argument has been buried long ago. The actual rod length issue continues. Mostly due to my own curiosity, I did some engine simulation calculations for a 5.70" rod vs a 6.0" rod. stoke-90mm, bore-101.4mm, rpm 8000.

While issues such as piston side forces, maximum velocity, crank angle, etc, have been well covered, there is one area that might bear some examination. The combustion pressure at 10deg ATC is the single most important factor in the determination of engine power due to combustion. So how does rod length affect engine factors at 10 degs?

At 8000 rpm, both engines have a mean piston speed of 24.0 m/s.
The short rod (5.7) produces a max piston speed of 39.5 m/s, vs. 39.32 m/s for the long rod. A delta of 0.45%.

At 10 deg (BTC & ATC) the short rod gives a piston speed of 8.57 m/s with a piston acceleration of 4110g. The long rod gives a piston speed of 8.46 m/s with a piston acceleration of 4063g. A delta of 1.3% and 1.15% respectively.

Maximum squish velocity (at 20% SAR and 1.25" clearance) for the short rod is 18.606 m/s (@10.1deg) and for the long rod 18.492 m/s (10.0deg).
A delta of 0.60% and 0.10% respectively.

What does it all mean?
Mean piston speed did not change
Max piston speed was reduced slightly with the long rod.
At 10deg, piston speed was reduced with the long rod.
At 10deg, piston G was reduced with the long rod.
At 10deg, engine squish velocity was reduced with the long rod.

It would seem that the long rod has advantages everywhere but they are very, very small. The area with the greatest sensitivity seems to be at 10 degrees for piston velocity but not for squish.

Since all of the above effects are very small, it would seem that observed performance would be due to other engine factors not related to rod length.

What say ye all?




Those who do not read have no advantage over those who cannot read.

As far as your first comment, I totally agree, but I never say it, in the interest of avoiding long-winded, meaningless pissing matches with short-rod advocates. I think you can figure out my views on the rest of it.

[BrazilianZ28Camaro]

A longer rod weights more , than it need heavier crank counterweights...

This difference is offset by a slight lighter piston?

[Lazy JW]

A longer rod weights more , than it need heavier crank counterweights...

This difference is offset by a slight lighter piston?

This question came to my mind as well. Perhaps some of the benefit is that the rod is not a purely reciprocating mass but is part rotating/oscillating? Or is the total mass just that much less?
Joe

[levisnteeshirt]

would a longer rod , help start the intake charge more effective since by keeping the piston at TDC longer during overlap , the low pressure area created by the exhaust has a smaller cavity to be expanded accross to initiate intake flow ??