Speed secret # 5

[Moxie]

No flames or disrespect meant here. Pasted below is a viewpoint from Richard Iskendarian of Isky Cams. This article, and the charts mentioned can found at www.iskycams.com, under the Tech Info section.


Rod Lengths/Ratios: Much ado about almost nothing.

Why do people change connecting rod lengths or alter their rod length to stroke ratios? I know why, they think they are changing them. They expect to gain (usually based upon the hype of some magazine article or the sales pitch of someone in the parts business) Torque or Horsepower here or there in rather significant "chunks". Well, they will experience some gains and losses here or there in torque and or H.P., but unfortunately these "chunks" everyone talks about are more like "chips".

To hear the hype about running a longer Rod and making more Torque @ low to mid RPM or mid to high RPM (yes, it is, believe it or not actually pitched both ways) you'd think that there must be a tremendous potential for gain, otherwise, why would anyone even bother? Good question. Let's begin with the basics. The manufacture's (Chevy, Ford, Chrysler etc.) employ automotive engineers and designers to do their best (especially today) in creating engine packages that are both powerful and efficient. They of course, must also consider longevity, for what good would come form designing an engine with say 5% more power at a price of one half the life factor? Obviously none. You usually don't get something for nothing - everything usually has its price. For example: I can design a cam with tremendous high RPM/H.P. potential, but it would be silly of me (not to mention the height of arrogance) to criticize the engineer who designed the stock camshaft. For this engine when I know how poorly this cam would perform at the lower operating RPM range in which this engineer was concerned with as his design objective!

Yet, I read of and hear about people who do this all the time with Rod lengths. They actually speak of the automotive engine designer responsible for running "such a short Rod" as a "stupid SOB." Well, folks I am here to tell you that those who spew such garbage should be ashamed of themselves - and not just because the original designer had different design criteria and objectives. I may shock some of you, but in your wildest dreams you are never going to achieve the level of power increase by changing your connecting rod lengths that you would, say in increasing compression ratio, cam duration or cylinder head flow capacity. To illustrate my point, take a look at the chart below. I have illustrated the crank angles and relative piston positions of today's most popular racing engine, the 3.48" stroke small block 350 V8 Chevy in standard 5.7", 6.00", 6.125" and 6.250" long rod lengths in 5 degree increments. Notice the infinitesimal (look it up in the dictionary) change in piston position for a given crank angle with the 4 different length rods. Not much here folks, but "oh, there must be a big difference in piston velocity, right?" Wrong! Again it's a marginal difference (check the source yourself - its performance calculator).

To hear all this hype about rod lengths I'm sure you were prepared for a nice 30, 40, or 50 HP increase, weren't you? Well its more like a 5-7 HP increase at best, and guess what? It comes at a price. The longer the rod, the closer your wrist pin boss will be to your ring lands. In extreme situations, 6.125" & 6.250" lengths for example, both ring and piston life are affected. The rings get a double whammy affect. First, with the pin boss crowding the rings, the normally designed space between the lands must be reduced to accommodate the higher wrist pin boss. Second, the rings wobble more and lose the seal of their fine edge as the piston rocks. A longer Rod influences the piston to dwell a bit longer at TDC than a shorter rod would and conversely, to dwell somewhat less at BDC. This is another area where people often get the information backwards.

In fact, this may surprise you, but I know of a gentleman who runs a 5.5" Rod in a 350 Small Block Chevy who makes more horsepower (we're talking top end here) than he would with a longer rod. Why? Because with a longer dwell time at BDC the short rod will actually allow you a slightly later intake closing point (about 1 or 2 degrees) in terms of crank angle, with the same piston rise in the cylinder. So in terms of the engines sensitivity to "reversion" with the shorter rod lengths you can run about 2-4 degrees more duration (1-2 degrees on both the opening & closing sides) without suffering this adverse affect! So much for the belief that longer rod's always enhance top end power!

Now to the subject of rod to stroke ratios. People are always looking for the "magic number" here - as if like Pythagoras they could possibly discover a mathematical relationship which would secure them a place in history. Rod to stroke ratios are for the most part the naturally occurring result of other engine design criteria. In other-words, much like with ignition timing (spark advance) they are what they are. In regards to the later, the actual number is not as important as finding the right point for a given engine. Why worry for example that a Chrysler "hemi" needs less spark advance that a Chevrolet "wedge" combustion chamber? The number in and of itself is not important and it is much the same with rod to stroke ratios. Unless you want to completely redesign the engine (including your block deck height etc.) leave your rod lengths alone. Let's not forget after all, most of us are not racing at the Indy 500 but rather are hot rodding stock blocks.

Only professional engine builders who have exhausted every other possible avenue of performance should ever consider a rod length change and even they should exercise care so as not to get caught up in the hype.

[jim_howard_pdx]

Ha

I used to have Ed, Richard's dad, build me custom length pushrods and solid lifters. I wouldn't buy one of his cams though, because they were old hat back in the 70's. Not one of the world class engines I ever wrenched on had an Isky cam. I mean no disrespect, but if you understand the engineering, then you either believe in Richard's voodoo scenario, or you believe the world records you produce throught proper engineering.

Each part in and of itself is totally meaningless. Running a long rod on s h i t heads gives you the same torque as the short rods. So Richard is right in saying that rods alone are never the single answer.

As I said in speed secret number one, it is the burn event that dictates how much horsepower you can make. It is the rod ratio that determines how long the piston is held at tdc and that effects torque. Since horsepower is a MEASUREMENT of torque at rpm divided by a constant, then you cannot get horsepower WITHOUT Torque.

This is what I mean by a speed secret. Guess we will never have to worry about Richard building a world record engine........

[Moxie]

Funny, Jim Oddy and Fred Hahn seem to be doing just fine in IHRA Pro Mod with Isky components, including cams.

[jim_howard_pdx]

Have them send me their cam numbers and I bettcha a dollar to a donut that comp cams was building the same cam over 20 years ago.

Just to let you know, when you race brackets, the closest to their dial in wins. It was extrememly important to me to run within one tenth of a second. So it didn't bother me that the Hemis were crossing the finish line 10-20 mph faster than I was. It didn't matter. I took home the trophies. So I could have run 5 or 6 different cams each season. I usually just ran one or two. And mostly that was to keep up with valve geometry improvements in the "ramps" and so forth.......

Richard's dad, Ed was an interesting guy, but like Smokey Yunich he believed in low valve lift, high duration engines with lots of overlap. Cause back in the 50's, 60's, and 70's, there were a lot of short stroke engines running very high rpms to get to their horsepower peaks. And these cams built big power even though the torque curve was poor.

Like Eric 4 Nitrous said, "who needs a big block anymore" to run 6's and 7's. Now it is high lift, low duration, big flow head engines that take the trophies.

The one thing that Richard said in his article that shows how poorly he understands engineering is that "an extra 5 to 10% power is not worth the engine lasting only half as long." Does an LT1 last half as long as a low power chevy? NO! Because as the engine goes up in power, the reciprocating pieces go up in strength. Often, it is the high power engines that last longer unless they are raced professionally.

A long rod engine typically puts LESS stress on the bearings, rings and piston skirts. WHY, because the burn is more controlled. Less predetonation and detonation makes for longer lasting parts.

You know that the sbc crowd started making a 6 inch rod for the 350's about 15 years ago, because they were being dominated by the 5.0 Liter Ford engines. The 302 has a 3" stroke, a 5.09 inch rod for a 1.699 rod ratio. The 350 chevy had a 5.4 rod, a 3.5 stroke for a 1.54 rod ratio, and IT COULD NOT KEEP UP!.

So in came a 6" rod for the Chevy guys JUST so they could stay EVEN!!!!. So tell Richard to take some engineering courses, take some physics courses, build some engines, and try lengthening the rods by .25 ratio increments. Keeping a tight quench and high compression he will not get a 5% increase in torque, he will get a second off his elapsed times.

Just going from 1.54 rod ratio to a 1.714 rod ratio in an LT1 Impala, we took the et times from 14.3 to 13.3 with NO OTHER CHANGES. No cam work, no intake work, and no ignition work. Just a rod and piston change.

Hope that sheds some light on long rods.

[Moxie]

Jim, I'm not trying to discount your automotive knowledge. Open forums like this just lend themselves to healthy debate. Good reading, no matter what opinions people take. Thanks for your contribution to the board.

[jim_howard_pdx]

No problems Moxie,

If I put 15 engineers in a room to discuss long rods,....
7 would say, "I would rather have more displacement"
7 would say "I would like to study the effects of rod length vs torque on a standard engine" (CLUE LESS).....
and ONE guy like me would stand up and say

"to take advantage of long rods, you need a proper package. This would include 11 to 11.5 to 1 compression, tight quench of .039 to .060 at the deck, and no more than .08 at the wedge and a kernal carrot of .10 to .12 inches.
Light rods and light pistons.
Pistons with low friction rings
and zero gap rings on top of that.
A cam with low overlap, but very high lift.
A dual sequential, MAS controlled EFI set up.

And some beers for the testers!!!

That last point was the most important in this entire thread.
If we are not having fun building speed, then we might as well take up knitting or basket weaving.

Moxie, I like to debate long rods with everyone I know, cause few cars keep up with mine, and everyone wants to know why.

Tight quench, high compression, light reciprocating parts, and LONG RODS. These are things I know make superior torque on any platform.

Let's have a beer sometime!!!!

[gofastmercury]

Originally posted by jim_howard_pdx
g rods.

Hope this helps...

My 428, when it made the finals, only lost two races. One was to a 500 cubic inch chevy, and the other race was to a 426 Hemi. Both beat me by less than 48 inches at the finish line. At this proximity, it is launch quality and reaction time that determines the winner. I ate a fine cuisine of short rod big blocks in my time, and they came up wanting..... Even the 427 Fords couldn't hang with me in my bracket. (short stroke, short rod, huge pistons, this design is called "over square". It does very well on track courses where the rpm capabilities of the 427 allowed it to dominate until the 428 and then the 429 replaced it.

The 428 was a little bit of magic with its 1.88 to 1 stock rod to stroke ratio. There are very few engines before or since with these types of ratios.

Building a 10 second car is getting relatively easy. Like I said in different thread, it is getting to be like building with lego blocks.

Just looked it up.

428
6.488 rod
3.98 stroke
1.630 rod stoke ratio

427 ford
6.488 rod
3.78 stroke
1.716 rod stroke ratio

[jim_howard_pdx]

THANK GOD for go fast mercury.....and moxie.....

Thank you, thank you, thank you.....

I am sorry to bait all of you like this. I am not trying to mislead anyone. And I am not trying to b u l l s h i t anyone. I am trying to get you all to think through the steps to successful power building so you will make good engine building decisions.

Lets really discuss this thread logically, and see why rod ratio does or does not build any extra power. When you see where I end up, the light will click on and you will understand WHY certain engine configurations make hot rod magic, while others just simmer.

My real Speed Secret Number 5 should have read like this.... "If you increase your dynamic swept length, you will increase your horsepower."

Both of Moxie and go fast mercury deserve BIG bonus points on this thread.

I was wondering how long it would be before someone looked up the 428 numbers and called me out on this discussion point. I think some 300+ views took place and GO FAST MERCURY took the prize....at the end. (Like any good racers!!!)

I am not retracting anything I said about the benefits of a long rod ratio.......I am only going to explain the missing link that Richard "Isky" could only hint at!

So HERE is the entire point to my thread......, this is what you need to know when you engineer a world class engine....

Like anything about a basic engine the length of the rod divided by the length of the stroke alone does NOT tell you what horsepower you will derive from your engine. This is also true of static compression ratios. The compression ratio alone does not build horsepower or torque. That is because NO SINGLE FACTOR of your engine is responsible for ALL the horsepower you create.

Looking at static compression ratio as an illustrative example, (something I discussed on a different speed secret) I told you that to win a bracket, you match the maximum static compression ratio of the engine you are running, and then you use overlap and valve lift to create the cylinder pressure you need to power down the strip.

This is what Richard weakly pointed out in his discussion of cam timing, and how 2 degrees more intake event made more horsepower than running a longer physical rod length. SO THE DYNAMIC SWEPT LENGTH Plus proper cam timing combine to create the maximum available horsepower.

I just wondered who could explain to me why he is right???? And thank you Moxie for getting this discussion into this thread......

To simplify this information, just take the total stroke length and add the total rod length together. Compare this new sweep length against where you started. This total swept length is a great prognosticator for maximum power, but it is the rod length that most controls the piston dwell at TDC and this is where an excellent burn event combined with piston surface and stroke and rod length all add together to build the total torque and ultimately the horsepower curve of your engine.

For all of you trying to build the next killer engine, just take the total cylinder height you can play with inside your block. Then plan a way to maximize the total swept length of the stroke and rod together while maintaining the longest possible rod.

Remember Richard's warning about putting the piston pin into the oil or compression ring lands. When you reduce the blow by's ability to push out against the rings you lose piston seal and this dramatically reduces the cylinder pressure and torque production.
You need a good ring land to get good cylinder sealing.

Now look at the cam timing. You may or may not already know that your crank shaft spins the rod cap and bolts extremely close to the crank case on stroker engines. Many times we have to clearance these to clear the reciprocating parts. Did you know the same rod bolt and cap swing dangerously near your high lift, high duration camshaft????

The point Richard made was that there is a maximum lift and duration you can fit in the block with the rod and bolt combination you use. Knowing how to fit in the best cam timing, with the longest rod and the overall longest swept length will help you maximize what you can generate in horsepower.

So I am right, and Richard is right, and this points out why rod length arguments exist in the first place. Just remember that there is no one single factor of an engine that gives it all its magical power. It is really a combination of a lot of small contributing pieces that builds a racer's advantage.

Just to completely finish this thread and I promise you I am done now, unless someone really throws me something I did not expect.....this does in fact happen......Let's take all this new information I have shared and plug it into our first example.

Looking at the STOCK small block chevy we find a 3.5 inch stroke plus a 5.4 inch rod equals 8.9 inches. 8.9 inches is the dynamic swept length of the piston and rod. Now also examine the dynamic stroke ratio of 8.9 inches divided by the 3.5 inch stroke which is 2.5428!

Now look at my 351 Windsor STOCK. 3.5 stroke + 5.95 rod equals 9.45 inches of dynamic swept length, divided by 3.5 yields a 2.700 dynamic stroke ratio. So is it just the 0.16 dynamic stoke ratio that accounts for me taking every chevy 350 off the line? No it is really the 17 percent difference in the total DYNAMIC swept stroke that accounts for my coming out faster...... As long as I am not sleeping at the tree......

Now lets look at the 428 STOCK. 3.98 stroke + 6.488 rod equals 10.46. Divide by 3.98 and you get a 2.68 dynamic stroke ratio.
My favorite Ford engine is the side oiler 427. It has a combined dynamic swept length of 10.268 vs the 428's 10.46. That is why the 428 replaced the 427 wedge in Nascar racing. It was a more effective torque producer!!! The 428's dynamic rod ratio is a little shorter 2.628 to the 427's 2.7164.

That is why we liked to use a 427 side oiler block, add a 428 crank. This ended up supplying 454 cubic inches and I never frankensteined one of these that did not produce in excess of 500 flywheel horsepower with the stock 425 hp 427 cam!!!!!

In summary, it is the rod length PLUS the stroke length of the mill between you and your competitor that you should work to increase. Then make sure you are not giving up too much piston surface because that also effects ET times. Crank throw, plus rod length, plus piston surface area AND DYNAMIC COMPRESSION all combine to deliver your torque curve and horsepower curve. He with the most area under the curve generally wins.

I am sorry if I kept all these details out until the bitter end, but I really expected someone to call me on the hard numbers right away.

I am glad that Moxie and Go Fast Mercury are interested in looking over the details. Thanks guys!!!! Now we can all see how rod length works in the overall scheme of things.