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)

cnc_rod_lrg.gifultra_lite_rod_lrg.gif    Scat_H-Beam_Rod.jpg    

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.

See what Hastings Manufacturing has to say on the subject.

 

 

 

 

 So is a 347 or 331 better. A 347 being 16cid bigger, a strong running motor making say 1.3 foot pounds of torque per cubic inches the 347 will make another twenty foot pounds than the 331. Peek horsepower will be much the same, but the 347 will make it at a slightly lower RPM, say 200rpm.
So if you want minimal oil consumption and guessing now, another 10,000 miles before needing to replace the rings and possible the pistons, give up a little power an build a 331.
I can build a 331 to make 440bhp, which is quite a bit for such a small motor.

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.

trw-l2491f30_(resized).jpgKB_408_piston.gifmahle2.gifmahle1.jpg tfs-51404111_xsm.jpgKB734.gif

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.

Comparing Rod Lengths 383

MORE ON THE SUBJECT OF ROD/STROKE RATIOS

 

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

 Eliminator Blocks Click Here

IDT Block 4.700 Bore
624  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

SBCScat9000.jpgBBC_crank_1_sm.jpgSCAT-SBF351-4340_Crank.jpg

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.

 SEE BLOCK DECK HEIGHT TABLES

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.Aluminum_Rods_sm.jpg

 

 

 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

 

COMPRESSION

 

CONTINUE

   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