Rod to Stroke Ratios
To get the rod to stroke ratio, simply divide the short stroker number in to the longer rod number. 5.700 rod divide by a 3.48 stroke equals 1.6379:1 (1.64:1)
Basically we want a Rod to Stroke ratio of 1.55:1 or higher to reduce side loading force on the thrust side of the piston to minimize friction. This becomes more critical above 5500rpm. The factory Chevy 400 had really short 5.565 rods and a 3.750 stroke having a 1.48:1 ratio. This motor works fine to 5500rpm, then you can experience a hard or harsh feeling for the motor, not vibration but kind of like it. By putting 5.700 rods in this motor it well happily buzz over 6000RPM with a 1.52:1 ratio.
Next we want a piston with better than 1.250 compression height, the distance from wrist pin centerline to piston top.
The greater the compression height meaning the longer the piston skirt, which in addition to transferring heat, keeps the piston vertical in the cylinders.
When we increase stroke length the piston is push up above the top of the block, so we move the wrist pin further up towards the rings, pulling the piston top back below the top of the block. Additionally we often install longer rods with the longer stroke to keep a good rod to stroke ratio, which also requires moving the pin up piston.
The limit is the oil ring. We can go as height as the top of the oil ring groove, and even push the oil ring up toward the top of the piston by reducing the thickness of the top and second ring lands.
But here to there are limits, required to maintain enough strength in the ring lands to support the rings. Placing the wrist pin in the oil ring grooves requires fitting a steel rail under the oil ring in a wider groove than the ring. The rail helps support the oil ring and is fitted after the wrist pin is installed through the rod and the clips are installed.
This set up is more than adequate to allow the oil ring to function as it would in a conventional groove.
I do my best to provide a combination with the best compromise of rod length piston compression height and stroke, all fitting in the block without collision, all focused on long street use life.
When building a performance motor we are constantly challenged with compromises.
In this case as we move the wrist pin up the piston, the piston becomes shorter.
The shorter piston then tends to rock in the cylinder as it moves up and down, because there is less skirt surface area to support the piston.
Further complicating matters is that we typically use a forged piston have 0.004" or more clearance allowing even more rocking, than a cast piston with 0.002" clearance would experience.
This rocking makes it more difficult to keep the rings straight and sealed.
Even though the rocking is not very much it does reduce the effectiveness of the ring seal leading to some increase in oil consumption.
The compromise here is to choose either the longest stroke for more power, or a shorter stroke increase to burn less oil.
This issue is most often discuss in relationship to Ford 331cid verses 347cid strokers.
Will a 347 burn more oil? Very possibly. Using a Keith Black hypereutectic piston with 0.00175 clearance will help, but most people want forged pistons.
Will a 331 last longer than a 347? Maybe, but any difference will not be great.
What are we talking about here, engine life?
A motor may start smoking when the rings wear and loose a percentage of their seal. Worn bearing adds to the oil thrown on to the cylinder wall making a tired ring work even harder. However valve guides tend to wear out sooner than the rings, so likely you will have the heads off for a rebuild before the rings let you down.
I don't hear people talking about other motor relative to this issue, but it does apply equally.
Personally I think this question applied to the 347 is a little over blown.
The 5.400 rod to 3.400 stroke ratio is 1.59:1 very good, but the piston compression height(CH) is 1.09", not so good.
The 331 ratio is 1.66, plenty and the CH 1.175, better but less than ideal.
Probe offers a very useful improved compromise for the 347.
By using the 331 piston with it's 1.175 CH and making a shorter 5.315 rod, the ratio is 1.56, still quite good, but now the piston in the 347 is as at lest good as the 331. The real benefit is the longer piston, not keeping the pin out of the oil ring groove. To do better means giving up stroking.
Small block Chevys are limited by camshaft clearance. A 4.00 stroke crank can be installed, but the rod ratio will limit RPM. Because this results in a lower speed motor, it makes sense to capitalize on this as much as possible. Using a short rod may limit peek RPM, but it increases low speed torque with greater leverage from the piston through the rod to the crankshaft. This is because the rod "lays over" at a greater angle as it descends down the cylinder.
Using a short rod allows us to use a longer piston, helping the piston to stay straight and the rings to remain sealed.
This situation reoccurs in all engines, including monster aftermarket blocks as we install the limits of crankshaft stroke.
Big engines at high RPM are race motors and are not expect to run "forever".
However these mountain motors make for extremely powerful street motors where tall gears can take advantage of the huge torque output, and cruise at high speed, but low RPM.
Traction will be your greatest challenge.
One obvious question might be why not just keep the skirt length long even though the pin has been raised?
The problem is as the pin moves away from the bottom of the piston shirt the leverage is increased. As the piston rocks on the pin centerline the further away the bottom of the shirt is the greater the pressure is on the skirt against the cylinder wall. This increased pressure can bend the shirt tang(very bottom of the skirt) increasing clearance leading to increased rock and noise. The skirt eventually may just break.
We tend to leave it to the piston manufacturer to choose the best skirt length for any particular application. To solve this problem, especially when we want to add as much stroke as possible we must turn to taller aftermarket blocks with increased deck heights, raised camshafts and widen oil pan rails. Dart and World are the most common sources, but there are also Brodix, Donovan, Aries, Bowtie, Eliminator and Ford Racing
Taller blocks allow longer strokes, rods and pistons. However we soon find we want to use all this extra length to stuff in really long stroke crankshafts and we find ourselves back with reduced rod/stroke ratios and short pistons.
Even the extremely tall World Merlin Superblock for big block Chevys with it's 11.625 deck height has a crappy 1.47:1 with 7.80 rods and a 5.30 crank and 1.175 CH, but then with 705cid you don't need to rev it to the moon.
One other consideration is the stroke to bore ratio. A bore of 4.00 and a stroke of 4.00 is said to be "square". A bore of 4.00 and a stroke of 5.00 would be "under square, 4.00 X 3.00 "over square".
Over square tends to rev up quicker and make more horsepower than an under square motor. Having a longer stroke making the motor under square means the pistons and rings have to travel further. This means more fiction and acceleration to maximum piston travel speed. This acceleration like the increased friction takes power away from going out the flywheel. The engine revs up more slowly, but as it produces more torque, until about 4000rpm where the friction and inertia resistance really begins to rob power, the motor makes the weight of the vehicle seem less -power to weight ratio- and so till around 4000rpm the longer stroke, slower to revving up motor will likely accelerate the vehicle faster, than a similar size cubic inch displacement motor have a bigger bore, shorter stroke combination. However this latter motor will eventually catch up an pass as top speeds are achieved.
It is more desirable to have a big cylinder short stroke motor, but using production engines means we can't increase cylinder size in any meaningful way, so we stuff in more stroke.
As I mentioned, increasing stroke lengths can reduce piston and ring life due to the less than ideal rod to stroke ratio and can cause some oil consumption.
These issue are not desirable, but it is the compromise we must make to have the extra power this combination produces.
To minimize problems I focus on the details, such as paying close attention to proper cylinder honing and cleaning.
I also recommend Total Seal Gapless Top Rings.Click Here
While these gapless rings have proven to make more horsepower, torque and vacuum, it is their longer service life that convinces me they are worth the additional expense.
Engines with increased stroke lengths increase piston speed.
All engine have the same time to get the piston from the top of the cylinder to the bottom, half a revolution.
If the piston travels 3" it will not have to accelerate to as high a speed at half stroker, then decelerate to dead stop at the bottom of the cylinder as a piston traveling 4".
These speeds are measured in feet per second.
As RPM is increased, so is this feet per second number.
Therefore a 4" stroke truck motor never revving over 4500rpm would not see as high a feet per second number as a 3" stroke motor revving 6500, or in these two cases 4" @ 4500rpm is 50' per second and 3" 2 6500rpm is 54.2' per second. See Caculator
What this means is, the higher you rev and engine the sooner you will need new rings.
So if you want a motor that last 100,000 miles don't build a motor that makes a ton of power and revs to the moon.
But if you do want a motor that man lesser men weak and causes women to faint, don't expect it to last as long as a stock production motor.
My goal in building high performance street engines is to make the most power in a given RPM range, have the most vacuum for the brakes, and have the longest possible longevity.
Below are some numbers relating to this subject for most of the stroker engines we might build using a factory block. CID is based on a 0.030 over bore.
Very often the distance between the crankshaft and camshaft centerlines restrict stroke increases as the rods begin to interfere with camshaft rotation.
Pushing the stroke limits may result is less than ideal rod to stroke ratios and require using short pistons resulting in a higher level of oil consumption, but the engine will make more torque.
In these application I prefer to choose a compromise, which favors a longer pistons to help with oil control as the engines application is more likely to be used in vehicle revving under 5500rpm.
Ford 302 Block Height 8.20
306 Stroke 3.00 Rod 5.090 Ratio 1.69:1 CH 1.608
306 Stroke 3.00 Rod 5.562 Ratio 1.85:1 CH 1.130
331 Stroke 3.25 Rod 5.400 Ratio 1.66:1 CH 1.170
347 Stroke 3.40 Rod 5.400 Ratio 1.59:1 CH 1.090
347 Stroke 3.40 Rod 5.315 Ratio 1.56:1 CH 1.175
Many people feel the 347 piston is too short for the street,
so Probe offers a 5.315 so you can use the 331 piston,
I recommend their use.
Dart Iron Eagle 4.155 Block Height 8.70
369 3.40 5.700 1.68:1 1.300
380 3.50 5.700 1.62:1 1.250
380 3.50 5.850 1.67:1 1.100
Windsor Block Height 9.50
351 3.50 5.956 1.70:1 1.774
393 3.85 5.956 1.55:1 1.608
393 3.85 6.250 1.62:1 1.325
408 4.00 6.000 1.50:1 1.490
408 4.00 6.125 1.53:1 1.350
408 4.00 6.250 1.56:1 1.250
418 4.10 6.000 1.46:1 1.450
418 4.10 6.125 1.49:1 1.325
418 4.10 6.200 1.51:1 1.250
418 4.10 6.250 1.52:1 1.200
427 4.17 6.200 1.48:1 1.215
434 4.25 6.200 1.45:1 1.175
The 408 and 418 are best for strip & street
The 427 makes torque, great for trucks.
Cleveland Block Height 9.20
356 3.50 5.778 1.65:1 1.672
356 3.50 6.000 1.71:1 1.450
393 3.85 5.956 1.55:1 1.319
393 3.85 6.000 1.56:1 1.275
408 4.00 5.956 1.49:1 1.244
408 4.00 6.000 1.50:1 1.200
400M Block Height 10.30
408 4.00 6.580 1.71:1 1.647 (0.070 below deck) Factory number
408 4.00 6.635 1.66:1 1.670 .005 above deck BBC H-Beam Rod Cleveland KB piston 76 FT 10.6:1
418 4.10 6.700 1.63:1 1.550
427 4.17 6.700 1.60:1 1.515
956 1.55:1 1.300
Ford Big 460 Block Height 10.30
466 3.85 6.605 1.72:1 1.770
520 4.30 6.605 1.54:1 1.545
520 4.30 6.800 1.58:1 1.350
545 4.50 6.700 1.49:1 1.350
545 4.50 6.800 1.51:1 1.250
IDT 11.3 Block (coming soon)
729 5.25 7.550 1.44:1 1.125
IDT 12.0 Block (coming soon)
763 5.500 8.000 1.45:1 1.250
Chevy Small Block Height 9.00
355 3.48 5.700 1.64:1 1.550
355 3.48 6.000 1.72:1 1.250
383 3.75 5.565 1.48:1 1.561
383 3.75 5.700 1.52:1 1.433
383 3.75 6.000 1.60:1 1.125
395 3.87 5.700 1.47:1 1.363
395 3.87 5.850 1.50:1 1.213
395 3.87 6.000 1.55:1 1.063
408 4.00 5.700 1.42:1 1.300
408 4.00 5.850 1.46:1 1.150
408 4.00 6.000 1.50:1 1.000
400 GM Block 4-1/8 Bore
352 3.25 6.000 1.85:1 1.375
377 3.48 6.000 1.72:1 1.250
406 3.75 5.700 1.52:1 1.433
406 3.75 6.000 1.60:1 1.125
420 3.87 5.700 1.47:1 1.363
420 3.87 5.850 1.50:1 1.213
420 3.87 6.000 1.55:1 1.063
434 4.00 5.700 1.42:1 1.300
434 4.00 5.850 1.46:1 1.150
434 4.00 6.000 1.50:1 1.000
You can see that put a 4.00 crank in the 350 & 400 block
is the limit of what is possible, not what is ideal.
Dart 4.20 Iron Eagle 9.320 Height
457 4.125 6.125 1.48:1 1.133
457 4.125 6.000 1.45:1 1.258
471 4.250 6.100 1.43:1 1.095
471 4.250 6.000 1.41:1 1.195
471 4.250 5.850 1.37:1 1.345
Big Block Chevy Deck Height 9.78
460 4.00 6.135 1.53:1 1.645
460 4.00 6.385 1.59:1 1.396
489 4.25 6.135 1.44:1 1.520
489 4.25 6.385 1.50:1 1.270
503 4.37 6.385 1.46:1 1.208
Truck Block Deck Height 10.20
489 4.25 6.535 1.54:1 1.540
489 4.25 6.800 1.60:1 1.275
503 4.37 6.535 1.50:1 1.478
503 4.37 6.700 1.53:1 1.313
503 4.37 6.800 1.55:1 1.212
518 4.50 6.585 1.45:1 1.415
518 4.50 6.700 1.49:1 1.250
518 4.50 6.800 1.51:1 1.150
4.5 & 4.60 Big Bore BBC 10.20
540 4.25 6.535 1.54:1 1.540
540 4.25 6.800 1.60:1 1.275
557 4.37 6.535 1.50:1 1.478
557 4.37 6.800 1.55:1 1.212
572 4.50 6.585 1.45:1 1.415
572 4.50 6.800 1.51:1 1.150
598 4.50 6.535 1.45:1 1.395
632 4.75 6.700 1.41:1 1.125
The 632 doesn't need to rev high, but many are in race cars.
World Merlin III 4.5- 4.680 Bore 11.625 (Oliver Rods)
632 4.75 7.100 1.49:1 2.150
632 4.75 7.750 1.63:1 1.500
632 4.75 8.000 1.68:1 1.250
705 5.30 7.750 1.46:1 1.225
750 5.60 7.650 1.37:1 1.175
805 5.85 7.500 1.28:1 1.200
These monster engine push the limits.
The only way to get a good rod length compromise is with limited lifetime
custom made aluminum connecting rods and pistons.
Aluminum rods on the street require maintenance and replacement.
Deck Height and Compression Distance Data
Motor | Size | Deck Height | Bore | Stroke | Rod Length | Rod Journal | Rod Width | Max. Comp. Dist. |
Buick nail head | 364 | 3.40 | ||||||
401 | 3.64 | 6.219 | 2.2495 | .967 | ||||
425 | 3.64 | 6.219 | 2.2495 | .967 | ||||
Buick | 400 | 10.57 | 3.625 | 6.607 | 2.25 | .928 | 2.151 | |
430 | 4.1875 | 3.900 | 2.151 | |||||
455 | 4.3125 | 3.900 | 2.013 | |||||
Chrysler Slant 6 | 170 | 9.68 | 3.400 | 3.12 | 5.707 | 2.1875 | 1.015 | 2.413 |
198 | 10.68 | 3.64 | 7.006 | 1.854 | ||||
225 | 4.12 | 6.699 | 1.921 | |||||
Chrysler (Dodge) low deck | 241 | 9.29 | 3.250 | |||||
259 | ||||||||
270 | ||||||||
Chrysler (Dodge) raised deck | 315 | 3.800 | ||||||
325 | ||||||||
Chrysler (DeSoto) low deck | 276 | 3.34375 | ||||||
291 | ||||||||
Chrysler (DeSoto) raised deck | 330 | 3.800 | ||||||
341 | ||||||||
345 | ||||||||
Chryslerlow deck | 301 | 3.625 | ||||||
331 | ||||||||
354 | ||||||||
Chrysler raised deck | 392 | 3.906 | ||||||
Chrysler (Plymouth) poly A | 277 | 9.60 | 3.625 | 3.125 | 6.123 | 2.125 | .937 | |
Chrysler (Plymouth)polyA | 301 | 3.91 | 6.123 | 2.125 | .937 | |||
Chrysler (Plymouth)polyA | 303 | 3.31 | 6.123 | 2.125 | .937 | 1.822 | ||
Chrysler (Plymouth) poly A | 318 | 3.91 | 6.123 | 2.125 | .937 | 1.822 | ||
Chrysler (Plymouth) poly A | 326 | 3.95 | 6.123 | 2.125 | .937 | 1.822 | ||
Chrysler LA | 273 | 3.625 | 6.123 | 2.125 | .937 | 1.822 | ||
Chrysler LA | 318 | 3.91 | 6.123 | 2.125 | .937 | 1.822 | ||
Chrysler LA | 340 | 4.04 | 6.123 | 2.125 | .937 | 1.822 | ||
Chrysler LA | 360 | 4.00 | 3.58 | 6.123 | 2.125 | .937 | 1.687 | |
Chrysler LA “R” race block | 340 | 9.56 | 4.125 | 4.00 | 6.123 | 2.125 | .937 | 1.437 |
Chrysler B | 350 | 9.98 | 3.375 | 6.358 | 2.375 | 1.018 | 1.935 | |
Chrysler B | 361 | 1.935 | ||||||
Chrysler B | 383 | 4.250 | 1.935 | |||||
Chrysler B | 400 | 4.340 | 1.935 | |||||
Chrysler B with RB crankshaft & B rods | 451 | 4.340 | 3.75 | 1.747 | ||||
6.768 | 1.337 | |||||||
Chrysler RB | 413 | 10.725 | 4.1875 | 6.768 | 2.082 | |||
426 | 4.250 | |||||||
440 | 4.320 | |||||||
Chrysler hemi | 426 | 4.250 | 6.86 | 1.990 | ||||
Chevrolet SB SB SB *-67 SB 1962-67 SB 1968-* SB 1968-* SB SB SB SB |
265 | 9.025 | 3.750 | 3.00 | 5.70 | 2.00 | .940 | 1.825 |
283 | 3.875 | 1.825 | ||||||
302 | 4.000 | 1.825 | ||||||
327 | 4.000 | 3.25 | 1.700 | |||||
302 | 4.000 | 3.00 | 2.10 | 1.825 | ||||
327 | 4.000 | 3.25 | 1.700 | |||||
350 | 4.000 | 3.48 | 1.585 | |||||
377 | 4.000 | 3.48 | 1.585 | |||||
383 | 4.000 | 3.75 | 1.450 | |||||
400 | 4.125 | 3.75 | 5.45 | 1.700 | ||||
Chevrolet BB | 396 | 9.80 | 3.76 | 6.135 | 2.20 | .990 | 1.785 | |
Chevrolet BB | 402 | 1.785 | ||||||
Chevrolet BB | 427 | 4.251 | 1.785 | |||||
Chevrolet BB | 454 | 4.00 | 1.665 | |||||
Chevrolet BB high deck (truck & bowtie) | 482 | 10.20 | 4.50 | 1.415 | ||||
Ford 6-cyl. | 240 | 10.00 | 3.18 | 6.7947 | 2.123 | .982 | 1.615 | |
300 | 3.98 | 6.2097 | 1.800 | |||||
Ford Windsor low deck | 289 | 8.206 | 4.00 | 2.87 | 5.150 | .830 | 1.621 | |
Ford Windsor; Cleveland & Boss low deck | 302 | 4.00 | 3.00 | 5.090 | 1.616 | |||
Ford Windsor mid deck | 351 | 9.500 | 4.00 | 3.50 | 5.954 | 2.31 | 1.796 | |
Ford Cleveland & Boss mid deck | 351 | 9.200 | 4.00 | 3.50 | 5.780 | 2.31 | 1.670 | |
Ford Cleveland high deck 351M | 351 | 10.297 | 4.00 | 3.50 | 6.580 | 1.967 | ||
Ford Cleveland high deck | 400 | 10.297 | 4.00 | 6.580 | 1.717 | |||
Ford FE | 332 | 10.17 | 3.50 | 6.540 | .875 | 1.880 | ||
352 | 3.50 | 6.540 | .875 | 1.880 | ||||
390 | 3.78 | 6.489 | 2.438 | 870 | 1.791 | |||
406 | 4.13 | 3.78 | 6.489 | 2.438 | 870 | 1.791 | ||
427 | 4.234 | 3.78 | 6.489 | 2.438 | .870 | 1.791 | ||
410 | 4.13 | 3.98 | 6.489 | 2.438 | .870 | 1.691 | ||
428 | 4.13 | 6.489 | 2.438 | .870 | 1.691 | |||
Ford FF “385” | 429 | 10.30 | 4.36 | 3.59 | 6.605 | 2.499 | .989 | 1.900 |
460 | 10.322 | 4.36 | 3.85 | 6.605 | 2.499 | .989 | 1.792 | |
Ford “Modular” V8 | 4.6 | 8.937 | 3.552 | 3.5433 | 5.933 | 2.0863 | .940 | 1.233 |
5.4 | 4.165 | 6.6575 | .940 | |||||
Oldsmobile 1965-67 | 400 | 10.625 | 4.00 | 7.000 | 2.499 | .925 | 1.625 | |
Oldsmobile 1968-* | 400 | 4.00 | 6.735 | 1.890 | ||||
Oldsmobile | 425 | 4.125 | 7.000 | 1.563 | ||||
Oldsmobile | 455 | 4.25 | 6.735 | 1.765 | ||||
Pontiac | 316 | 10.25 | ||||||
347 | 10.25 | |||||||
370 | 10.25 | |||||||
326 | 10.25 | |||||||
350 | 10.25 | |||||||
389 | 10.25 | |||||||
400 | 10.25 | |||||||
421 | 10.25 | 3.75 | 6.625 | 2.249 | .996 | 1.750 | ||
428 | 10.25 | 4.00 | 6.625 | 2.249 | .996 | 1.625 | ||
455 | 10.25 | 4.21 | 6.625 | 2.249 | .996 | 1.520 |
A maximum piston speed of 4500 feet per minute for a well built (internally balanced, 4340NT crank, 4340 rods with cap bolts, steel pins, forged pistons, and a suitable valvetrain) street/strip engine is around the limit for reliability. Production engine's are generally good up to around 3800-4000 feet per minute. Racing engines such Nextel Cup, F1, Indy, and even sportbikes have piston speeds exceeding 4800 feet per minute and maybe exceeding 5000.
Piston Speed in Feet per Minute = (stroke x 2 x rpm)/12
Figure 1 | |
Calculating Maximum Safe RPM | |
Max. Safe RPM = Mean Piston Speed (ft/min) x 6 | |
Divided by Stroke in Inches | |
Example for a budget aftermarket forged crank in a 4-inch stroke small-block Chevy: | |
4,800 x 6 = 7,200 rpm | |
4 | |
Maximum Mean Piston Speeds for Above Formula: | |
Factory cast-iron cranks | 3,750 ft/min |
Aftermarket cast-steel cranks | 4,500 ft/min |
Factory forged cranks | 4,600 ft/min |
Budget aftermarket forged cranks | 4,800 ft/min |
Typical race aftermarket cranks | 5,500 ft/min |
High-dollar custom endurance race cranks | 6,000 ft/min |
ProStock/Mountain Motors | 7,500 ft/min |
Formula One | 7,500+ ft/min |
2618 forged pistons can tolerate more RPM or FPM than 4032
Connecting Rod Length Comparison
By Rick Draganowski
dragan@harborside.com
Piston movement was computed by simulating the crankshaft/connecting rod/piston assembly in several precise engineering drawings (DesignCad) and then determining the exact amount of piston movement for each of 256 divisions of one rotation.
The piston movement data was then used as an input vector in a MathCad program to calculate velocity, acceleration, and dynamic forces.
The simulation of an infinitely long connecting rod, which imparts true harmonic motion to the piston, is the starting point.
The motion generated by a finite length connecting rod is quite distorted by comparison. It has much more velocity and acceleration at the top of the stroke compared to the bottom. A graph of the movement is peaked at the top of each cycle and rounded and flattened at the bottom. This is caused by the rod angle increasing and pulling the piston down and adding to the motion caused by the crankshaft rotating down from top dead center. At the bottom as the rod journal slows the angle decreases. This retards the movement of the piston by subtracting the rod angle component that was added at the top of the stroke from the crankshaft movement component at the bottom of the stroke.
Compression and combustion pressures are in opposition to the inertial forces so the top of exhaust and intake strokes generate the largest forces on the rod.
1) Maximum Piston Acceleration
This table is for a 3.75" stroke used in a 400 0r 383 small block Chevy engine.
------infinite rod--------6.0" rod---5.7" rod---5.565" rod
5000rpm 1332G 1749G 1776G 1790G
6000rpm 1933G 2525G 2558G 2578G
7000rpm 2631G 3437G 3482G 3509G
Percent difference due to rod length in above table.
Difference between 6" rod and 5.565" rod 2.34%
Difference between 6" rod and 5.7" rod 1.54%
Difference between 5.7" rod and 5.565" rod 0.79%
This table is for a 3.48" stroke used in a 350 or 305 small block Chevy engine.
------infinite rod---------6.0" rod---5.7" rod
5000rpm 1240G 1600G 1623G
6000rpm 1786G 2305G 2338G
7000rpm 2432G 3138G 3182G
2) Maximum Connecting Rod Dynamic Load (Tension)
This table is for a 3.75" stroke used in a 400 or 383 small block Chevy engine. The forces are based on the weight of the piston and pin assembly and do not include the percentage of force generated by the acceleration of the end of the connecting rod. The reference piston is the stock replacement Silv-O-Lite piston for a 400 engine.
------infinite rod-----------6.0" rod-----5.7" rod----5.565" rod
5000rpm 2249LBS 2938LBS 2976LBS 3000LBS
6000rpm 3239LBS 4232LBS 4287LBS 4320LBS
7000rpm 4409LBS 5769LBS 5834LBS 5849LBS
Percent difference due to rod length in above table.
Difference between 6" rod and 5.565" rod 2.34%
Difference between 6" rod and 5.7" rod 1.54%
Difference between 5.7" rod and 5.565" rod 0.79%
3) Maximum Rod Angularity
This is the angle the connecting rod makes with the axis of the cylinder bore at 90 degrees after top dead center (maximum excursion from bore axis. This measurement is for the 3.75" stroke of the 400 and 383 only.
6.0" rod-----18.21 degrees
5.7" rod-----19.20 degrees
5.565" rod-19.69 degrees
4) Cylinder Wall Load
Percentage of compression and combustion force against the top of piston transmitted to the major thrust face of the piston and then to the cylinder wall.
This table is for the 3.75" stroke.
6.0" rod----32.89%
5.7" rod----34.83%
5.565" rod-35.64%
This table is for the 3.48" stroke.
6.0" rod---30.31%
5.7" rod---32.05%
5) Piston Speed
Maximum piston speed for the 3.75" stroke at 5000 rpm.
Infinite rod---81.68 feet per second, 55.69 MPH
6.0" rod------85.64 feet per second, 58.4 MPH
5.7" rod------86.01 feet per second, 58.6 MPH
5.565" rod---86.20 feet per second, 58.8 MPH
6) Effective Stroke
Because of the mechanical advantage provide by the toggling effect of the rod the shorter rods act as if they were in a longer stroke engine at the top of the stroke. This effect would make the short rod engine rev faster from 2000 to 4000 rpm and the circle track people claim that acceleration out of the turns is significantly improved with the shorter rod. In all other factors the longer rod comes out superior...
Effective stroke as compared to the infinite rod model for the 3.75" stroke.
infinite rod-=- 3.75"
6.0" rod------- 4.20"
5.7" rod------- 4.23"
5.565" rod---- 4.25"
Note that the differences are subtle...
7) Dwell Time
This measurement is of the number of crankshaft degrees the piston is within 0.250 inches of top dead center. It is the subject of much conjecture and controversy in the automotive literature.
This table is for a 3.75" stroke used in a 400 0r 383 small block Chevy engine.
Infinite rod---59.853 degrees
6.0" rod------52.397 degrees
5.7" rod------52.071 degrees
5.565" rod---51.915 degrees
Percentage difference in dwell time between the 6.0" rod and the 5.7" rod is 0.626%.
Percentage difference in dwell time between the 5.7" rod and the 5.565" rod is 0.3%.
Percentage difference in dwell time between the 6.0" rod and the 5.565" rod is 0.928%. (Still less than 1 percent)
This table is for a 3.48" stroke used in a 350 or 305 small block Chevy engine.
Infinite rod---62.188 degrees
6.0" rod------54.929 degrees
5.7" rod------54.605 degrees
Percentage difference in dwell time between the 6.0" rod and the 5.7" rod is 0.593% at the 3.48" stroke.
8) Author?s comments:
The data in this report seems to indicate that the differences between the rod lengths are exaggerated in the literature. In many (most) cases claims are anecdotal and represent the vested interests of the suppliers. I have seen no objective dyno testing of rod lengths but keep hoping for one.
There are real gains to be had by going to longer rods but they are small, usually a lot less than 2 percent. However, the hard-core racers are grasping at every tiny bit of performance and can justify the expense. For the more average rodder I would suggest staying with the rod length specified by the factory. Money would be far better spent on improving the heads, cam, and induction and exhaust systems. (and perhaps a supercharger..)