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I have a 4" stroke with 6.123 rods in a smallblock. The pistons are shorter, not the rod.

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Don't most stroked motors use a longer rod.

I'm in the camp whereby compression height rules over rod length , get ring placement right then get a rod to fit.

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The difference is 2.5 thousandths, or 1/16th of 1% of the stroke. BFD.

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Agreed. BFD.


lighter piston probably only real benefit.

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The rod's job is to connect the piston to the crank. Build the stroke you want for the cubes oyu need, find a piston CD that is liveable and order the rod accordingly. Nuff said

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Right, those annoying auto manufacturers have no idea what they're doing!
Why don't they follow your advice, and all use whatever rod they have lying around...

Hint: when designing a new engine, the deck height is frequently the last choice. Does that tell you something?

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Funny I had no idea we were talking about 200,000+ mile production engines. Apples to Oranges, we are discussing RACE engines since this is a RACE forum. Not production based engines

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Right, those annoying auto manufacturers have no idea what they're doing!
Why don't they follow your advice, and all use whatever rod they have lying around...

Hint: when designing a new engine, the deck height is frequently the last choice. Does that tell you something?

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Right, those annoying auto manufacturers have no idea what they're doing!
Why don't they follow your advice, and all use whatever rod they have lying around...

Hint: when designing a new engine, the deck height is frequently the last choice. Does that tell you something?

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Funny I had no idea we were talking about 200,000+ mile production engines. Apples to Oranges, we are discussing RACE engines since this is a RACE forum. Not production based engines

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Maximize the stroke and bore, then make the compression height large enough to keep the wrist pin out of the oil groove

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So how does the rod ratio affect your 60' it HAS to somehow!

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Long rods give more and longer torque curve... torque
is what moves a item

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so when you stroke a mopar,, you just made it a chevy..

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looks most guys on here have gone chevy then.

Good post BTW.

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The difference in piston position B/ATDC with even large changes in rod ratio are very, very small. I have yet to read an analysis that explains the changes in engine function.

Example 1: 4" stroke, 8" rod = 2:1.
In 10° rotation, the piston moves down .0379".

Example 2: 4" stroke, 6" rod = 1.5:1 (25% lower ratio).
In 10° rotation, the piston moves down .0404".

The difference is 2.5 thousandths, or 1/16th of 1% of the stroke. BFD.

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The problem with this statement is the shown difference at Only one point/perimeter of a 10* rotation.

While in reality its not the same difference at every 10* of crank rotation. How about the difference between the other "35" 10* points of reference.


And nope, I Still cant fire up that deqree symbol on my home computer. But at the shop pute, no problem. mike

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I don't really care about the length of the rod all that much so I'm not sure why my name is in this discussion.

I'm more interested in combinations that go together easily because lots of people have trouble with anything more than a basic combination. My recommendations tend towards whatever is commercially available that actually works.

So I tend to recommend things such as the 4.250 stroke with a 6.800 long rod in the RB motor. That combination works, there is plenty of room for a good ring pack, the piston is tall enough to be stable, the rod ratio is good enough to work with off the shelf heads, etc.

Most people on here will never build a motor where the rod ratio actually matters while almost everyone on here has to work within a budget. So my advice is directed towards the budget aspect of engine building rather than finding the last hp in a build.

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I can agree with this! Without being arguementitive mike

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... ill use a ss hemi reference again,,, cause thats what i run,, and i run the [Edited by Moparts - Family Friendly Site - Keep it clean] out of them on the dyno,, if i advance the cam youd think it would gain tq always once you are in your ideal position due to the intake valve closing earlier,, but i can advance the cam and loose tq,, maybe it made more at 5000, but when i peak at 7000,, it lost

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Sorry for going a bit , but Jeff's comments on how advancing the cam doesn't necessarily improve peak torque fits in w/ something I was looking into myself earlier this week. I was trying to decide whether I should follow the advice of a well-known "tech expert / writer" to advance my cam 6 degrees due to it fitting right into his definition of an "undervalved" wedge engine (2.14" intake for 4.38" bore x 3.75" stroke).

First, my Engine Analyzer Pro simulation, in which I've spec'd out a baseline program that mimics the real dyno results of my combination pretty closely, predicted that would be the wrong move, as it showed a loss of about 13 HP and 6 Tq compared to being advanced only 2 degrees.

Then, I came across a cam drive comparison test that Steve Dulcich did that also included testing one of his 440 dyno mules at 0, 2, 4 and 6 degrees advanced. For his particular test, the results for 6 degrees advanced vs. 2 were... a loss of 11 HP and 4 Tq. My EA Pro results didn't look as odd as I'd thought they were after seeing a "real world" example that basically mirrored them.

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... ill use a ss hemi reference again,,, cause thats what i run,, and i run the [Edited by Moparts - Family Friendly Site - Keep it clean] out of them on the dyno,, if i advance the cam youd think it would gain tq always once you are in your ideal position due to the intake valve closing earlier,, but i can advance the cam and loose tq,, maybe it made more at 5000, but when i peak at 7000,, it lost

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Sorry for going a bit , but Jeff's comments on how advancing the cam doesn't necessarily improve peak torque fits in w/ something I was looking into myself earlier this week. I was trying to decide whether I should follow the advice of a well-known "tech expert / writer" to advance my cam 6 degrees due to it fitting right into his definition of an "undervalved" wedge engine (2.14" intake for 4.38" bore x 3.75" stroke).

First, my Engine Analyzer Pro simulation, in which I've spec'd out a baseline program that mimics the real dyno results of my combination pretty closely, predicted that would be the wrong move, as it showed a loss of about 13 HP and 6 Tq compared to being advanced only 2 degrees.

Then, I came across a cam drive comparison test that Steve Dulcich did that also included testing one of his 440 dyno mules at 0, 2, 4 and 6 degrees advanced. For his particular test, the results for 6 degrees advanced vs. 2 were... a loss of 11 HP and 4 Tq. My EA Pro results didn't look as odd as I'd thought they were after seeing a "real world" example that basically mirrored them.

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I dont think you are looking at this in the right
direction(JMO) but if you advance the cam you move
the torque lower in the rpm range and most likely
it did increase at that rpm BUT it lost some on the
higher rpm so it did increase it at the lower rpm
but it needs to be in the working range of the engine
and drive line set up.... JMO

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Short Rod is slower at BDC range and faster at TDC range.

Long Rod is faster at BDC range and slower at TDC range.

I. LONG ROD

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

B. Compression Stroke -- Piston travels from BDC to 90-o BTDC faster than short rod. Goes slower from 90-o BTDC to TDC--may change ign timing requirement versus short rod as piston spends more time at top. However; if flame travel were too fast, detonation could occur. Is it possible the long rod could have more cyl pressure at ie. 30-o ATDC but less crankpin force at 70-o ATDC. Does a long rod produce more efficient combustion at high RPM--measure CO, CO2? Find out!!

C. Power Stroke -- Piston is further down in bore for any given rod/crank pin angle and thus, at any crank angle from 20 to 75 ATDC less force is exerted on the crank pin than a shorter rod. However, the piston will be higher in the bore for any given crank angle from 90-o BTDC to 90-o ATDC and thus cylinder pressure could be higher. Long rod will spend less time from 90-o ATDC to BDC--allows less time for exhaust to escape on power stroke and will force more exhaust out from BDC to 90-o BTDC. Could have more pumping loss! Could be if exhaust port is poor, a long rod will help peak power.

D. Exhaust Stroke -- see above.

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. Will require a better cyl head than long rod to produce same peak HP. Short rod may work better for a IR or Tuned runner system that would probably have more inertia cyl filling than a short runner system as piston passes BDC. Will require stronger wrist pins, piston pin bosses, and connecting rods than a long rod.

B. Compression Stroke -- Piston moves slower from BDC to 90-o BTDC; faster from 90-o BTDC to TDC than long rod. Thus, with same ign timing short rod will create less cyl compression for any given crank angle from 90-o BTDC to 90-o ATDC except at TDC. As piston comes down, it will have moved further; thus, from a "time" standpoint, the short rod may be less prone to detonation and may permit higher comp ratios. Short rod spends more time at the bottom which may reduce intake charge being pumped back out intake tract as valve closes--ie. may permit longer intake lobe and/or later intake closing than a long rod.

C. Power Stroke -- Short rod exerts more force to the crank pin at any crank angle that counts ie.--20-o ATDC to 70-o ATDC. Also side loads cyl walls more than long rod. Will probably be more critical of piston design and cyl wall rigidity.

D. Exhaust Stroke -- Stroke starts anywhere from 80-o to 110-o BBDC in race engines due to exhaust valve opening. Permits earlier exhaust opening due to cyl pressure/force being delivered to crank pin sooner with short rod. Requires a better exhaust port as it will not pump like a long rod. Short rod has less pumping loss ABDC up to 90-o BTDC and has more pumping loss from 90-o BTDC as it approaches TDC, and may cause more reversion.

III. NOTES

A. Rod Length Changes -- Appears a length change of 2-1/2% is necessary to perceive a change was made. For R & D purposes it appears a 5% change should be made. Perhaps any change should be 2 to 3%--ie. Ignition timing, header tube area, pipe length, cam shaft valve event area, cyl head flow change, etc.

B. Short Rod in Power Stroke -- Piston is higher in the bore when Rod-Crank angle is at 90-o even though at any given crank angle the piston is further down. Thus, at any given "time" on the power stroke between a rod to crank pin angle of 10o and ie. 90-o, the short rod will generate a greater force on the crank pin which will be in the 70-o to 75-o ATDC range for most engines we are concerned with.

C. Stroke -- Trend of OEM engine mfgs to go to longer stroke and/or less over square (bore numerically higher than stroke) may be a function of L/R. Being that at slower engine speeds the effect of a short rod on Intake causes few problems. Compression/Power Stroke should produce different emissions than a long rod. Short rod Exhaust Stroke may create more reversion--EGR on a street engine.

D. More exhaust lobe or a earlier exhaust opening may defeat a longer rod. I am saying that a shorter rod allows a earlier exhaust opening. A better exhaust port allows a earlier exhaust opening.

E. Definition of poor exhaust port. Becomes turbulent at lower velocity than a better port. Flow curve will flatten out at a lower lift than a good port. A good exhaust port will tolerate more exhaust lobe and the engine will like it. Presuming the engine has adequate throttle area (so as not to cause more than 1" Hg depression below inlet throttle at peak power); then the better the exhaust port is, the greater the differential between optimum intake lobe duration and exhaust lobe duration will be--ie. exh 10-o or more longer than intake Carbon buildup will be minimal if cyl is dry.

IV. DEFINITIONS

Short Rod -- Min Rod/Stroke Ratio -- 1.60 Max Rod/Stroke Ratio -- 1.80

Long Rod -- Min Rod/Stroke Ratio -- 1.81 Max Rod/Stroke Ratio -- 2.00

Any ratio's exceeding these boundaries are at this moment labeled "design screw-ups" and not worth considering until valid data supports it.

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Don't most stroked motors use a longer rod.

I'm in the camp whereby compression height rules over rod length , get ring placement right then get a rod to fit.

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Don't most stroked motors use a longer rod.

I'm in the camp whereby compression height rules over rod length , get ring placement right then get a rod to fit.

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That's because you have an actual race engine. Rod ratio is the most overstated and unimportant aspect of an engine build. The rod connects the piston to the crank, period. Guess how BAE adjusts for compression? They lengthen or shorten the rod.

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Must be something to it..........



Ok , it's already been said a short rod will pull harder on head from TDC to around 75* ATDC.............seems a longer rod may have it's advantages on power stroke......considering the longer rod pulls away from TDC slower than shorter rod it will 'harness' peak pressure for a longer period of time , in doing so will have a greater force applied to piston earlier in power stroke , think of a fire cracker held in a clenched fist (long rod) , light it off & it can blow a finger off , hold it in the palm of you're hand (short rod) & you may suffer a few burns.

If i was to build a stroker motor for the street then i would use the long rod over the short rod (cylinder loading) ..............if you feel rod lengths only impotance is to connect piston to crank then all is good.

An engine built for nitrous then i stand by my above post , ring placement is more important than rod ratio.

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How about the difference between the other "35" 10* points of reference
They get smaller.

Please: I hope we can all agree that "dwell" (meaning literally to reside, stay, remain, rest) only means "move very slowly vs. motion at other points of rotation"?
The piston dwells at TDC exactly this long: .00000"
By comparison, it dwells at BDC this long: .00000".

This also gets repeated way too many times: "a long rod motor would dwell for a longer period of time & TDC & a short rod @ BDC , this was'nt so , dwell time is EXACTLY the same for a short & long rod @ TDC/BDC".
This is never, ever true in any engine (unless you have one of those very rare engines with infinite rod length - not really long like 12" - infinite like light years).
The zero change observed is actually so small that it's completely masked by stacked bearing clearances.

HTML note: degree sign is made by
Hold ALT down, then press 0, 1, 7, 6

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Jeff, I agree ATDC and a ABDC the dwell between the short/long rod is .00000

Its the Dwell degrees Near TDC that is being referenced to not just "Only TDC" witch backs your argument.


Now for my question about the other "35" 10* points of reference from your other post.

You say Smaller, I say Bigger

The differences only grow, IMO and after some limited measuring tests.

Off the Cuff, I have Vizard stating that the difference in a 3.75 stroke 1.48 RR vs a 1.63 RR is about 2* in crank position between the two before they both reach peak piston speed @ around 74* or so.

Was he correct, I dont know?? If he was what would that 2* difference equate to??

2* out of 360* BFD, right

Coming off of TDC the difference of 2* would be squat, .008 piston height maybe

If they were truely "2* apart" What would the difference in piston height be at say 60* ATDC between the two?

""Its as far down as my dial guage could measure easily"

With a 3.31 stroke the difference 2* crank changed the piston height was .063

A 3.75 stroke would see a slightly higher difference.


IMO, on the power stroke a longer rod sees more psi pressure on the piston because its higher up in the bore.

But, the shorter rod sees a better leverage angle quicker then the longer rod. so maybe the above is a wash.


But theres the intake stroke to consider too.

All in all, in the scheme of things, for a race motor Id want the longer rod if theres a choice and a shorter rod for the street.

On some strokers you dont have much of a choice, I would also never comprimise the ring pack and piston stability to get any extra rod length on a race motor.

No doubt the rod connects the piston, but in the same token it can be used to a slight advantage, if you have a choice one way or another. Lots of variables. mike