So whos got the longest con-rod?

[Big_Daddy_T]

Ive been doin some reading and came across this.







I did a pair of in-depth and somewhat controversial articles in Hot Rod magazine that explained the basics of torque versus horsepower and argued that higher torque at low RPM, using a longer stroke and shorter connecting rods, was preferable over high-RPM horsepower for street engines. The common thinking in any racing circles is that you want maximum horsepower, period.



its on the banks site.



So what are the dimensions of the conrods compared to the competition. Ive seen the pics.

 

[HVAC]

The Cummins has a larger bottom end then anything I've seen in its class. The rods are huge! That's why we can power these things up without going through the bottom end. Try to find a 800 horse small block with $0. 00 invested in the lower end.

 

[Big_Daddy_T]

What is the length of the rods?

The largness of the crank and wrist pin is what makes it difficult for me to eyeball from that pic.



The bottom end is definitely a stout one. Well the top end compared to the comp is very stout too.

Can the comp hit 550-600 hp without touching bottom or top end? I know ford cant.

 

[HoleshotHolset]

The competition can hit that HP mark - but will NOT have the durability of a I6 with such a huge lower end that we have.



I'll take 7 main bearing caps and one rod journal per rod anyday over the competition's offerings.



Our I6's have been produced for over 20 years with very little changes - look what F*rd and Chevy have had to do with their V8 platforms just to keep up with us.



Matt

 

[Grey Wolf]

I don't know what the competitions lengths are but I just put a tape on a "B" and it measures about 7 1/2" center to center

 

[Vaughn MacKenzie]

You want 'em long

Longer conrods are definitely preferable over short ones, for a couple reasons:



1) Cylinder wall loading. The shorter the rod, the sharper the angle between the piston and crank throw, resulting in the piston pushing much harder on the cylinder wall. Of course this leads to greater wear. Advantage: Cummins



2)TDC dwell time. With a long rod, the piston spends more time at TDC which lengthens the period of time the cylinder sees maximum pressure. This contributes to better economy because the fuel has more time to ignite/burn. Advantage: Cummins



#1 is partly due to why a Slant Six would last a very long time with proper maintenance. The rods on a SS are veeery long. Knew a guy who had 422,000 on an untouched Slant Six (it helps too that they don't have much power )



Vaughn

 

[catoiler]

Vaughn,What do you mean they don't have no power??How does 10. 18@126 mph sound?another one in a wagon went 11. 90,ours went 13. 20 with the motor going at half track(to much drugs

)ain't nothing like watching the faces on them ferd boys when they get beat by a old slant six!!:{







Mike

 

[Hohn]

Rod length comments

It's not so much the overall rod length as it is the RATIO of rod length to stroke. The higher the ratio (generally) the more peak power.



There are some circumstances where a short rod (ratio~1. 5-1. 7) will make more power. They generally make more torque at lower rpm.



Longer rods (ratio 1. 7 and up) favor higher rpm hp. NASCAR engines run ratios of around 1. 8.



It's interesting to compare the rod ratios of common v-8s:



Chevy 350:1. 63

MOpar 340:1. 85

mopar 383:1. 88

Chevy 454: 1. 53

Ford 302:1. 69

Ford 351W:1. 7

Ford 351C:1. 65



All things considered, if a CTD rod is ~7. 5 inches long, then the rod ratio comes in at ~(7. 5/4. 72=) 1. 58, which is fairly short.



Also, remember that the higher the ratio, the less effect a difference in ratio has. For example, the jump from a 1. 4 to a 1. 5 ratio is a large difference. The increase from a 1. 8 to a 1. 9 ratio is almost nothing. Even though both cases the ratio difference was only . 1, it had a different effect.



You can easily visualize this by picturing the angularity of the rod with the crank at 90°. Hypothetically, it might be at a 17° angle from vertical (rod to bore centerline). To cut this angle to 16° we would have to make the rod longer by a given amount. To make the angle 15° we would have to make it longer by a greater amount than before. Even an infinitely long rod won't make the angle zero. Since this is an exponential relationship, you can see where longer rods QUICKLY become a point of diminishing returns.



SO what Vaughn said is right about long rods (more accurately, high rod ratios). But the CTD doesn't have a high ratio because of it's long stroke. It's all about the rod angularity, and the rod length and stroke both determine this value.



More specifically regarding Vaughn's post:

1)cylinder wall loading is less with longer rods, subject to the "diminishing returns" described above.

2) While longer rods increase dwell at TDC, they decrease the time a BDC, which can hurt the breathing of the engine (less time to fill cylinder).



Shorter rods have more rapid piston acceleration, which is bad at high rpm, but at low rpm, helps the engine "come up on the cam" better.



A shorter rod engine also needs better breathing to function right, even with identical displacement. That's one reason that restrictor plate NASCAR motors use VERY high rod ratios (1. 9 sometimes)-- it makes the most of restrictive breathing.



There's a lot more to be read on this topic, so I will post a linkhere\



Jeff's site has all kinds of great info. It's worth spending the time to read his writeups. I learned a lot from him.



HOHN

 

[bighammer]

Thanks Hohn, for the informative post. Very interesting stuff.

 

[HoleshotHolset]

catoiler: I wanna hear more about the slant-6's..... PM or email me if necessary.



Matt

 

[roadranger]

Re: Rod length comments

Originally posted by Hohn

More specifically regarding Vaughn's post:



2) While longer rods increase dwell at TDC, they decrease the time a BDC, which can hurt the breathing of the engine (less time to fill cylinder).



HOHN

Huh??

How does this happen?

The crank is sweeping thru it's arc the same at the top as at the bottom.



Please explain...

 

[Vaughn MacKenzie]

Well I'll try to explain it and make sense. . .



The angle of the rod on the piston affects the speed the piston approaches/leaves TDC and BDC. On engines with short rods but equal stroke length, since the rod has to angle out further as the throw continues through its arc, it begins to pull the piston down from TDC sooner. Same deal approaching TDC, as the throw is reaching the vertical position, the short rod is quickly "standing up" and causes the piston to approach TDC at a faster rate.



At mid-stroke, the piston speed on a long-rod motor is faster than the short stroker (which makes up for the slower speeds at TDC and BDC.



Does anyone follow? It would be interesting to have piston-speed charts comparing two engines with exact equal dimensions except for rod length. The two engines' average piston speed would be equal, it's just that they go different speeds during the same position of the cycle.



Vaughn

 

[Vaughn MacKenzie]

Catoiler, guess I better not call a Slant Six slow, eh?



I had my dad's '73 Duster in mind



I did see a radical SS once though when I was in my teens. Dual header, 600 or 750cfm, major cam, very lopey idle. Sounded really mean, one of the coolest sounding engines I've ever heard!



Vaughn

 

[Mark_Kendrick]

It is physically impossible for a piston to have less dwell at TDC than BDC.



The crank moves in a circle the top and the bottom are symetrical. . there fore the same.



I agree with the rest.

 

[HoleshotHolset]

Email me with more slant-6 stuff!



One of the 1st cars I ever drove was a '70 Duster. 3-speed auto, 4-wheel drum brakes (only the LF worked!... . jam on the brakes and you'd change lanes!! :-laf ), no power anything, no power (engine!)... .



We beat that poor thing to death - it survived through it all.



Matt

 

[Un-Hemi]

SSs aren't indestructable.

Broke a ring and holed a piston in one back in prehistoric times after a 19-hour crash and burn to the Winternats. Replaced it with a junk yard core in an underground parking lot with a crescent wrench and a pair of pliers (insignificant exaggeration). The guy that sold me the core didn't/couldn't tell me what it was. I couldn't tell if it was a 225 or a 170 until I tried to bolt up the exhaust. That 170 was sure smooth running, but it was slower than molasses in a Yukon blizzard!



I bought a basic '70 Dustpan SS 3-speed from my uncle 'way back when. Billious green, it was. transmission never had any oil in it, (probably since new, since it was as dry on the outside as it was on the inside) and finally started to chatter and self-destruct near Wolf Creek, Oregon whilst on the way to the Winternats (note the trend?). Replaced it with a 3. 09 low gear A-833 4-speed and supershifter after the epic journey back home. Was fun to drive with the 4sp. Duster got snuffed when my X got rear-ended by some road barge. She drove it home after the wreck and I had to strip it before it was turned in to the insurance co.



6-banger racers have my sincerest respect. Quick 6-bangers always seem to be operating on the hairy edge of survivability. There used to be a record-holding M/P Maverick up here in the NW that used to bustify flywheel bolts on a regular basis. Something about crankshaft harmonics, I believe. We used to make bets on whether it would complete the next run without scattering expen$ive parts from one end of the track to the other.



Ah, to reminisce!



Unless it's a Cummins, I'll take a V8 any day. Ive had my fill of gasser 6s, thanx.

 

[HoleshotHolset]

TommyTurbosaurus: Great stories!



I've almost had my fill of V-6 gassers..... I'd love to get my hands on a gasser I-6.



A Toyota 2JZ-GTE would suffice.



Matt

 

[Vaughn MacKenzie]

Originally posted by Mark_Kendrick

It is physically impossible for a piston to have less dwell at TDC than BDC.



The crank moves in a circle the top and the bottom are symetrical. . there fore the same.

Exactly, that's what I was implying (if this comment was directed toward me)



Vaughn

 

[ToolManTimTaylor]

Originally posted by HoleshotHolset





A Toyota 2JZ-GTE would suffice.



Matt

MMmmmm Jun

#ad

 

[Hohn]

Vaughn, Mark, et. all:



IN EVERY crankshaft position, the piston is lower in the bore with a shorter rod.



Technically, there is NO dwell at TDC OR BDC, since these are by definition 0° and 180° points of crankshaft rotation.



The idea of "dwell" as I understood you to mean is the time a piston spends NEAR TDC or BDC. "Near" must be defined relative to a constant point (for example) within 1 inch of either extreme.



For example, if a long rod motor has to the piston 1" below TDC @44°, the short rod motor will hit that point earlier, say 42° of rotation. Again, the short rod motor has the piston lower in every crankshaft position.



So if we define "dwell" as being within 1" of TDC, the long rod motor in this example would have 88° of "dwell" while the short rod motor would only have 84° of "dwell". The long rod has more dwell at TDC.



Remember: a short rod motor always has the piston lower in the bore.



Thus, we can see that the long rod motor has more "dwell" relative to Top Dead Center.



But what about "dwell" at BOTTOM dead Center? Because the piston is lower in the bore with the short rod, it hits the point of "dwell" sooner. So if "dwell" is relative to 1" above BDC, then the short rod motor will hit it first on the way down. While the short rod motor will get there at roughly 121°, the long rod motor doesn't hit that point until 126°. So the dwell relative to BDC is [(180-121)*2]= 118° for the SHORT rod motor, but only [(180-126)*2]=108° for the LONG rod.



So we can see that a longer rod has MORE dwell at TDC, and LESS dwell at BDC relative to a shorter connecting rod. This is because a shorter connecting rod has the piston lower in the bore for EVERY crankshaft position except TDC and BDC themselves (0° and 180°).



Vaughn, Mark, you said that:



"It is physically impossible for a piston to have less dwell at TDC than at BDC. The crank moves in a circle, and the top and bottom are symmetrical and therefore the same. "



While this would SEEM to be logical enough, but the fact that the piston is connected to the connecting rod NOT the crank itself is what keeps this from being true. The geometry of the linkage throws it all off.



Moreover, piston motion is RECIPROCATING, while crankshaft motion is ROTATING. This alone rules out a linear relationship like the one you described.





I know this is techno-geek stype stuff, so if I didn't do a good enough job explaining it, then I have an Excel sheet i made that shows (graphically) the piston travel and how it is affected by rod ratio. It's basically just a bunch of sine waves, with the shorter rods being more vertical and having more area under the curve (which is why a shorter rod engine needs more air).



Hope this was helpful in explaining something that's hard to visualize.





HOHN



(PM me if you want the Excel sheet)