Rod Length Discussion - Engine Masters

[mbrunson427]

I'm sure some of you have Motor Trend On Demand. The only reason I pay for it is the engine masters show. Most recent episode is one that we have kicked around on this forum quite a bit.... rod length.

I found it very interesting. The one thing I never heard stated is what the bobweight of each piston/rod assembly was. I'm curious if some of the effects seen were a product of bobweight rather than rod length?

If you haven't seen it, spoiler.......there was darn near zero difference in engine performance.

[gt350hr]

  I have ( in the last 17 years) helped several competitors including a couple of winners. The "test RPM"  has caused some to re evaluate their combinations when it comes to rod length and camshafts. If the test RPM stops at 6,500 you don't need a combination that makes peak power beyond that. "Some" combinations can show bigger low RPM numbers with a specific rod length versus a length that would favor 8,000 + drag strip operation. Same for cams. "I" don't feel the bob weight is a critical factor since the competition is not about acceleration on the dyno , just HP along the way to the RPM limit.
    Randy

[cjshaker]

Just to bench race....

Since the longer rod has a longer dwell at TDC, it makes sense that it would produce more torque through the lower RPM ranges. Longer dwell makes for more time for the fuel mixture to ignite and expand, placing more force on the piston. At higher RPMs, I'm guessing the length of time isn't long enough to make a difference.

In a nutshell, it would appear that long rod combos are better for any street, hot street, mild competition or engine built for durability. Short rod combos would be reserved for all out competition engines where longevity isn't the main goal. All "in general" statements, of course. Although I don't really see an up-side to the short rod combo, as long as the longer rod doesn't get into the oil rings.

[blykins]

It all also depends on what engine it is.  I don't know what engine they used to do the test, but a SBC won't want what a Cleveland wants.  There's a lot to be accomplished with a particular rod length on a big ole fat intake port. 

Also, as an aside, I know that the "wrist pin in the oil rails" deal seems to get a bad rap, but in all actuality, it works.   You don't get much shorter than the piston on a 347 SBF and I know of many non-oil-consuming 347's out there with very high mileage on them.  I was speaking with one of the writers for Modified Mustangs several years ago and he had a Fox Mustang with a 347 that had 100k miles on it.  No oil usage.

I think the bad rap came from very preliminary piston offerings from Probe/Coast High, which had the wrong shape and caused lots of issues. 

[Barry_R]

I've never been that fixated on rod length numbers.
If it will package up I will prefer the get the pin up as high as I can without piercing the oil ring land.
Advantages are a lighter piston and a reduced roll/rock at direction change relative to the bore - better ring sealing if the ring pack is lower to the pivot.
I am not really convinced that the rest of the package sees much from minor rod length variables - feeling its more of a "volume change" reaction rather than a rod length specific deal - volume is volume no matter how you achieve it.  An instantaneous gain at one spot in the cycle is largely if not completely offset by a comparable disadvantage in another one.

The pin hole through the oil ring land is obviously accepted in the race world, and "good enough" for most performance cars, but the fact that exactly zero OEM pistons are made that way should serve as an indicator of its desirability in a true street application.  Acceptable oil consumption in a daily driver is way different than acceptable control in something that sees constant maintenance and far lower accumulated miles between oil changes.  In terms of risk versus reward rod length variation in reasonably comparable engines seems not a big win - a "wash" at best.  Other stuff is much more important.

I have built a couple EMC entries that were really similar in displacement, but really different in bore vs stroke because of the rules each year.
One had a 4.350 bore and a 3.64 stroke, with a 6.700 rod. 
One had a 4.250 bore and a 3.78 stroke, with a 6.490 rod.
By the time they got fully sorted out the score numbers got startlingly close to each other.
Enough that you could swap dyno sheets around and they'd be convincing.
That 4000 wide RPM range deal really limited some of the advantages/disadvantages associated with other factors.

Spend your money on heads, cams, and intakes....

[blykins]

My guess on the OEM stuff is that it’s not really a case of oil consumption, but it’s an expense and assembly step that they can’t account for.  A $1 part over millions of engines adds up, and its an extra “eyeball” step to make sure the dimples are in the right spots on assembly.

With the sheer numbers of 4.375” stroke FEs and Windsors/Cleveland’s I’ve done with them, I’ve never been able to draw a correlation between the two factors.

[mbrunson427]

I became really interested in this discussion a couple years ago, pulled out my Internal Combustion Engines book from Engineering School, and made a pretty in-depth spreadsheet with data that carries out quite a few decimal places. When comparing the graph data, I have to make the graph full screen, pick a 10 degree window of data, and then zoom in to find one line straying from another line (graphing piston position, velocity, and acceleration). This is on a connecting rod length difference of anywhere between .1" and .5".

After building that whole thing and playing around with it for a bit I kind of let the idea go that there were any breakthrough advantages to be found there. I think that 1 degree adjustment of any cam parameter can find more horsepower than any rod length adjustment can. I'd say this episode of Engine Masters further solidified this for me.

If you're competitive enough to be searching for 1 single horsepower, this might be worthwhile to dig into.

[My427stang]

I became really interested in this discussion a couple years ago, pulled out my Internal Combustion Engines book from Engineering School, and made a pretty in-depth spreadsheet with data that carries out quite a few decimal places. When comparing the graph data, I have to make the graph full screen, pick a 10 degree window of data, and then zoom in to find one line straying from another line (graphing piston position, velocity, and acceleration). This is on a connecting rod length difference of anywhere between .1" and .5".

After building that whole thing and playing around with it for a bit I kind of let the idea go that there were any breakthrough advantages to be found there. I think that 1 degree adjustment of any cam parameter can find more horsepower than any rod length adjustment can. I'd say this episode of Engine Masters further solidified this for me.

If you're competitive enough to be searching for 1 single horsepower, this might be worthwhile to dig into.

Overall, I agree with you completely.  However,  I also think there may be some available trades with longevity/wear/packaging etc, but again, likely greater gains had elswwhere until exhausted.  In the end, when all of our "rules of thunb" don't perform THAT much differently across different rod ratios, we have yet another indicator

However, in the end, a few HP may be what it takes to win a race, and if it does, then it does

[blykins]

I became really interested in this discussion a couple years ago, pulled out my Internal Combustion Engines book from Engineering School, and made a pretty in-depth spreadsheet with data that carries out quite a few decimal places. When comparing the graph data, I have to make the graph full screen, pick a 10 degree window of data, and then zoom in to find one line straying from another line (graphing piston position, velocity, and acceleration). This is on a connecting rod length difference of anywhere between .1" and .5".

After building that whole thing and playing around with it for a bit I kind of let the idea go that there were any breakthrough advantages to be found there. I think that 1 degree adjustment of any cam parameter can find more horsepower than any rod length adjustment can. I'd say this episode of Engine Masters further solidified this for me.

If you're competitive enough to be searching for 1 single horsepower, this might be worthwhile to dig into.

Overall, I agree with you completely.  However,  I also think there may be some available trades with longevity/wear/packaging etc, but again, likely greater gains had elswwhere until exhausted.  In the end, when all of our "rules of thunb" don't perform THAT much differently across different rod ratios, we have yet another indicator

However, in the end, a few HP may be what it takes to win a race, and if it does, then it does

I agree....if we all do something differently and it works, then that means lots of different things work. 

[gt350hr]

     Rod length ( FE specifically) is not a critical factor (IMHO) because the flow capabilities of current heads and attending engine size are fairly well matched. It would be a factor with a 500CFM head on a 427CI combination (again IMHO) because the "balance" is off. Various engine combinations not known to many here have some specific criteria for rod to stroke. I don't expect guys to know Bowtie combinations but they are different. For example a 4.250 stroke BBC works VERY well with a 6.385 or 6.535 rod when our FEs use 6.700. The cylinder heads are vastly different and the likely reason why the short rods work. There are many other similar situations.
    Dwell time. This (IMHO) is in line with the rail support myth. According to renowned cylinder head expert Darrin Morgan the "normal" flame travel speed is about 65MPH but in some cases can go to as much as 5 times that in certain situations.That breaks down to about 90ft per second and turns changes in burn time to nano seconds. Since we still fire the plug BEFORE  TDC , a change in dwell time ( where the piston is NOT moving) should have little to no affect on power output compared to a change in ignition timing.
     Pin in the ring. This WAS a myth created by Probe. Virtually every other custom piston manufacturer was scratching their heads when this came out. What made the 347 Ford the ONLY engine to have this problem?  The internet and ragazines blew up with anti rail support information and Probe advertised to the MAX that their design ( going to a .085 shorter rod and .070 thinner oil ring) was "the cure" and "the sheep" followed in mass. Guess what , Probe  is out of business now! Mark is still in business under another name but buys pistons from other companies and THEY use RAIL SUPPORTS regularly.
      Bottom line . "Some" engines respond to rod length changes , others don't. An FE is one of those that doesn't care.
     Randy

[jayb]

To me the issue is always about rod length vs. cubic inch displacement.  Whenever I've gone to a shorter rod in an engine, it is to accomodate a bigger stroke and pick up cubes.  Any potential power loss from a shorter rod is more than offset (WAY more) with the added cubes.  My 585" SOHC has a horrible rod ratio, with a 6.700" rod and 4.600" stroke, of 1.456:1.  And my latest iteration of that engine, which will be 605", is even worse with a 6.625" rod and 4.75" stroke (1.395:1 rod ratio).  The 585" engine made just over 1000 HP naturally aspirated, so picking up 20 cubic inches will likely result in an additional 30+ horsepower.  No way the reduction in rod ratio is going to offset that. 

If you are limited by a displacement rule then fooling around with rod ratios certainly makes some sense.  If not, cubic inches always wins over rod ratio.  I spoke with a guy about a year ago who went with a 4.125" stroke, instead of a 4.25" stroke, just because of the rod ratio difference.  Not the right move, IMO.

[gt350hr]

  Exactly right Jay. Jon Kaase said when you get SO big , the rod is simply a "link" between the crank and piston , length doesn't matter. He was speaking about one of his 5.3 stroke 800+ CI engines.
   Randy

[plovett]

So when does a piston become too short for you guys?  1"?  Less?

Forget ring support rails and rod/stroke ratios for a minute.  Is a 1" piston as stable and seal as well as a 1.5" tall piston? 

[blykins]

So when does a piston become too short for you guys?  1"?  Less?

Forget ring support rails and rod/stroke ratios for a minute.  Is a 1" piston as stable and seal as well as a 1.5" tall piston?

A piston starts getting short to me at around 1.100".  You can see some skirt wear on them from where they rock at BDC, although I think it depends on the design and the application.  I tore down a 347 that I built that ran for 8 years in a street/strip application.  The only reason it came apart was because he blew a power valve and didn't know it.....washed the cylinders down.  The skirts and cylinders on that engine looked excellent, but compared to a 100% street engine or daily driver, it didn't see much action. 

I've also ran some 1.040" pistons and have some .990" pistons here for a SBF build.   These are race only instances and can be justified.   The 1.040" piston was in a Cleveland block that was filled and had a couple thou more clearance cold than a normal engine.  I could tell it on tear-down.

This is just my opinion, but I really like a 1.300-1.400" piston with a thin ring pack.  I feel that the pistons are stable enough to not move around. 

[gt350hr]

     "My" personal comfort zone is in the 1.200-1.400 range. Shorter pistons can be made more stable by carefully changing dimensions like the skirt shape and diameter of the ring lands. Some of the problem comes from cylinder length ( as in the 347 small block) because the piston is pulled out of the bottom of the cylinder due to the longer stroke and limited rod length. ANY time a piston comes out of the bottom of the bore , stability is compromised regardless of the piston manufacturer. This is because of the different temperatures in the piston and the undersizing of the area of highest heat , the top and ring land areas. The temperature difference is typically 400*s higher than the temperature of the skirt sometimes more. "Heat management" is another critical factor in short compression height pistons. Thermal barrier coatings "can" be employed as a band aid but the rest of the piston must be changed to adjust for the lower expansion. Short compression heights also bring the top of the rod closer to the higher heat and have been known to heat oi in the pin bores to the point where the "hydrodynamic boundry layer" ( oil film strength) is lost and piston/pin damage occurs. One of my small block engines has a 1.050 C/H but it is not a "highway" engine by any means.
    I welcome additional comments/ questions.
       Randy