I saw a post the other day about the rod length to stroke ratio of a 427 small-block being 1.5 and causing some drag on the engine. And, of course, that is very true. Everybody knows the a 355 with 6” rods will out run a 355 with 5.7” rods if they’re both built the same way. However, I just wanted to put it into perspective with some numbers. Before I get started, I’ll admit I’m not an automotive engineer and there are probably all kinds of dumb things in my calculations, but I think the CONCEPT is accurate, if not perfect. Here goes: A stock 350 has 5.7” rods and a 3.48” stroke. When the crank throw is perpendicular to the cylinder the angle is 17.77 degrees. (I know that is not the maximum angle, but it’s close and easier to calculate.) For a 427, the angle is 19.47 degrees (6.00” rods and 4.00” stroke). Let’s assume 70ft-lb per hole. That’s 420 pounds of force 2 inches out for the 427. For the 350, let’s use the same 420 pounds. (Of course the engine would be less powerful, but that’s accurate.) (420lb)*{sin(17.77)}=128lbs on the cylinder wall for the 350 and (420lb)*{sin(19.47)}=140lbs on the cylinder wall for the 427. NOW the question is, what is a typical coefficient of kinetic friction for the piston to cylinder wall? It’s going to be real low. Since I’m not a mechanical engineer, I’ll just make a wild guess and say it’s like 1%. That would mean the 350 would have 1.28 lbs of drag due to the rod:stroke ratio and the 427 would have 1.4 lbs of drag due to the rod:stroke ratio (per hole). For all 8 cylinder that would not add up to a pound of difference.

Of course, it’s not that simple. It’s a dynamic system and the longer the stroke the shorter the piston and the more piston rock you get. On the other hand a 427 is usually built using an aftermarket block and there are advantages to that, thicker cylinder walls for one thing.

Here’s another way to look at it: The factory 400 small-block engine that Chevy built had 3.75” stroke and 5.625” rods. That is a 1.5:1 rod:stroke ratio – exactly the same as a 427 with 4.00” stroke and 6.00” rods. If The General :chevy thought it was OK to build the small-block with a 1.5:1 rod:stroke ratio then it couldn’t have been too bad. When I was “a kid” I pulled a 400 small-block out of a '71 Caprice and dropped it in a ’77 Monza Spyder that came from the factory with a 262 V8. It was a straight bolt-in swap. That Monza ran pretty darn good and it was bone stock. I never worried about winding it up and it never blew up.

“Food for thought”

 

I'm not sure I buy all the "therory". I am not saying it's wrong, I just haven't digested it all. Because of the excessive stroke, *I* think the longest rod you can economically use, is the best. I bought one with 5.85" rods. If *I* had been building it, I would have used, 6.00"ers. These rods/pistons DO carry the advantage of the oil ring NOT encroaching inti the pin bore.
Make that 5.565" rods for the 400. Time to get the "slip stick" out again. Good luck, and...

RACE ON!!!

 

You have done your homework. That kind of food for thought is actually a feast. :thumbs:

 

I did not think you could build a 427 small block with 6 inch rods unless you used a tall Rocket block. The stock 9 inch deck height will not accomodate rods much longer than 5.85inch when the stroke is 4 inch. The Rod/Stroke Ratio drops accordingly.

Calculating:
427 CID with a 4.155 Bore (.030 over) needs a stroke of 3.94

Minimal practical cumulative "piston height"(everything except the rod length and stroke) can be assumed by using 6" rods in a 350 CID (3.48 stroke)
This calculates to 1.26" (9" Deck- 6" Rod- 1/2 Stroke(3.48/2).

Stretch the limit to 1.2" and get a perfect block with the design 9.025" height.
With a 3.94 Stroke for a 427CID the max rod length comes to 5.86"

The Rod/Stroke Ratio is then 1.49 (5.86/3.94). This is about the same as a stock 400CID with stock 5.56" Rods.

To get a more acceptable 1.64 Ratio in the 427CID (same as a Stock 350CID with 5.7" Rods) you would need 6.45" Rods a block with 9.6" deck height. The only tall blocks available from Chevy are the iron Rocket block with 9.35" and the aluminum block with 9.5", so no go.

If you use 6" rods in a 427CID with 3.94" stroke, you need a 9.17" deck height block, so either Chevy tall block will work.

After all that calculating, the point I think is that small block Chevy's with strokes over 3.75" are challenged from the ideal rod/stroke ratio perspective. As such, they will not make as much HP at high RPM as the CID increase would suggest. The additional piston friction effectively eats up the hp gain from the increased displacement at high rpm. The motor will be a great low rpm torquer, however.

If I could afford it I would build a 427 with the aluminum 9.5" tall block and max out with custom 6.3" rods. Then, of course, a custom intake manifold to get everything to fit. For about $50K you would have a hell of an engine. I just need a sponsor.

 

My 427 has a 4.125" bore and a 4.00" stroke. 2" crank throw + 5.85" rod leaves (9.025 - 7.85") 1.175" for piston deck height. The 6.00" rod combo only allows for 1.025" deck height. These pistons ARE available. Even this tight, I had to have my block decked to get the quench I wanted. I can still bore to 4.155" (.030 over) for 434 cid, if I want.

 

Yea, the 6" rods are available. Eagle actually has a "canned" rotating assembly with them. I know this because I'm putting a 427 in my Vette. It has a standard height block and 6" rods. If you want to go exotic, you could actually build one with longer rods. The extra money would not be justified considering the minimal improvement.

Based on what CFI-EFI said above (I was incorrect about the factory 400's rod length) that means a 427 actually has a better rod:stroke ratio than the factory 400 did. Of course, the short rods were always considered to be one of the problems with the factory 400.

No doubt I'd like to have a better rod:stroke ratio. My point is that it's not that bad.

 

Can make good Hp as well as torque if you have the money.
Lingenfelter built a 427ci sb in a '95 coupe for Car and Driver mag.

4.13 x 4.00 in; 427ci alloy block and heads.Type not noted.

603 bhp @ 6200 rpm
554 lb-ft @ 5100 rpm REdline 7000rpm

Ran 11.5 @129 and ran up to 212 mph on a 6 speed with a 3.30 rearend.
3340 lb car with all items including cats and EGR.

0-60 3.6 sec

Only cost $51,350 for the engine !!!

 

One of the biggest advantages to a 6.00 rod is the added dwell time on at TDC and BDC you get from them. [helps to cut down on detonation effects] Unfortunately....the benefits don't always outweight the downfalls of the longer rod. And that is the encroaching on the oil ring. In order to keep the piston in the block, the wrist pin is placed up higher in the piston and this typically interferes with oil management.

I see many many people having problems with the 6" rods. That is why I steered clear of them. :leaving:

 

With the size of the counter weights on the Eagle crank, you've got no other options than the 6" rod. Counterweight to piston skirt clearance problem.

The counterweights on the Eagle crank make it easy to internally balance though.

 

Yea, isn't that price actually for them to do the install? I can hardly believe anybody would pay that much just for a 600hp engine. Mine is costing me about 1/5 that much (just for the engine).

 

The real problem with small rod-stroke ratios is really two-fold. Yes the side loading caused by the increased angle between bore centerline and rod DOES reduce the lifespan of the motor, but that's not the major problem. I don't have it in front of me, but if you figure out the mathematical equation for the piston motion as a function of rod-stroke ratio and crank angle, what you'll see is that the higher the rod-stroke ratio, the more sinusoidal the piston motion. The more sinusoidal the piston motion, the smoother the acceleration is. The smoother the acceleration is, the lower the overall peak acceleration is. The lower the peak acceleration is, the lower the peak load tensile load...F=ma (or compressive, but we don't care about that with rods...they can handle the compression...that's why many rod failures happen when the driver LIFTS...rod goes in tension every time past TDC, whereas under power, it's under tension at the end of the exhaust stroke, and compression at the end of the compression/beginning of power stroke). If you lower the loading on the rod, you lower the probability that it's going to break. Also, for a given deck height/stroke combination, the longer the rod, the lighter the piston is. You have now combined lower acceleration (and thus force) due to the longer rod, AND the longer rod has reduced the mass of the piston that you're accelerating, which FURTHER reduces the load on the rod. On the acceleration issue, if you look at the piston acceleration vs. crank angle for a 1.3:1 R/S motor, you will see that the piston's acceleration actually changes direction 4 times during a given cycle (3 times between the 90 and 270 degree crank positions, once at TDC), whereas longer rod motors only change piston acceleration directions twice per crank revolution.

Now, that being said, running a longer rod will reduce the compression height of the piston (distance from wrist pin centerline to piston deck)..this is where the weight reduction comes from. On MOST SBC pistons, going under ~1.15" or so on the compression height will result in a notch in the oil ring land to allow the wrist pin to be installed. There are piston ring sets made with oil ring land supports specifically for dealing with this issue. They do help, but they don't solve the problem. If I were building a street motor, I'd say stay away from this scenario. The oil rings life is reduced resulting in more frequent rebuilds to keep from burning oil. On a street motor, we care about this. On a race motor, we don't....run the longer rod.

As far as power gains go, it is a myth that a longer rod motor will produce significantly more HP than a short rod motor. There is likely an effect (the engineering and physics show us this), but most dynos are not repeatable enough to statistically prove this theory. Yes, I have seen the magazine articles showing that long rod motors make more power. My response to those? The author of the Circle Track article from the mid '90s was fired after the editors found out he had the dyno operator change the dyno calibration when the dyno results came back inconclusive. Until I see a real test with statistical analysis of the results done here at the engine lab or by someone I trust (this also includes atmospheric corrections, etc, etc, etc), I will stand by my position that the major benefit of long rod motors is lower cylinder wall loadings and lower rod tensile loading.

 

lol- this subject keeps magazine editors employed, and that is about all-

the reality is, the rod angularity affects the torque curve, and little else- it's a tuning measure, which is one reason why you see top fuel teams tune with rod lengths-

the block is not going to be incrementally "side-loaded" to any significant extent, one way or the other, simple because the barrier of lubrication prevents any real issues- if there was truly side load as people like to wax about, these components would quickly wear and fail- they don't.

A 1/2 inch difference on a six inch rod just isn't going to be a big deal- just a different torque curve.

 

Wow. Like I said in my original post, I'm not an automotive engineer or a mechanical engineer. I mean no disrespect to you with the "blah blah". It was just too much to post again. I enjoyed reading it. I had actually never heard it explained that way. It is interesting. So, what you're saying is the common idea that you're better with a longer rod is correct, but not for the reason(s) most people have in mind. Is that called serendipity? Something like that.
;)