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Old 11-28-2005, 01:04 AM
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Rod ratio

Could someone please set me straight on the importance of rod ratio and how it effects performance.

Also what would be considered a desirable ratio to aim for when building a stroker.

Cheers Rob

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Old 11-28-2005, 07:17 AM
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If you`ll look in the knowledge base on this site you`ll find info about it.
Your gonna get various opinions on this. Smokey Yunicks theory was "use the longest damn connecting rod you can fit in the engine" but there`s another side to this, the intake system has to be up to snuff with the longer rod otherwise they`ll be no gain. many professional racers feel the best rod ratio is 1.8:1.
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Old 11-28-2005, 08:19 AM
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From my bench racing point of view is this. I'm a long rod fan. Well over 10 years ago I read an article where a sbc was built using 2 sets of rods & pistons, one long rod and the other short. They swapped the two during dyno tests, keeping everything else the same to keep variables to a minimum. The long rod set up made a modest amount more. But as I said, this was over 10 years ago. I've read so many different point of views, I don't know who to believe. With todays heads, cams and intake systems it may not be such a big deal anymore. Most likely you'd be better of spending money on a custom cam, headers or some dyno time IMO. There comes a pratical limit, is the expense having a custom set of rods and pistons in search for the idea rod ratio worth it over readily and cheaper off the shelf parts? Look athe the Ford 428 and Chevy 454. Two well respected engines know for their power and both have so called lousy rod ratios, something like 1.43 or something like that IIRC. It'll be interesting to hear what a profession engine builder has to say.
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Old 11-28-2005, 08:32 AM
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Regardless of opinions, a few fact's are: delayed TDC dwell time for restricted head and intakes, less thrust wear on cyl. walls, Higher rpm capability. Cam choice is critical to gain the advantage.


My opinion?

The price is the same, the pistons are lighter, they'er many pro's, and not many cons.
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Old 11-28-2005, 08:57 AM
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less rod fatigue with shorter rods, also as far as I see it they have the advantage at higher rpms too.
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Old 11-28-2005, 09:07 AM
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rods

What typ of engine are you building???

drag race, dirt track???

Pump gas,race gas????

400 hp or 1000 hp????

Keith
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Old 11-28-2005, 10:40 AM
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ok, i'm no expert here, but i'll just make a quick couple of quotes:

warren "the professor of pro stock" johnson - a connecting rod is there simply for nothing more than to link the piston to the crankshaft.

reher-morrison - We also wanted to point out some of the common myths and misconceptions about high-performance motors. For example, I've seen dozens of magazine articles on supposedly "magic" connecting rod ratios. If you believe these stories, you would think that the ratio of the connecting rod length to the crankshaft stroke is vitally important to performance. Well, in my view, the most important thing about a connecting rod is whether or not the bolts are torqued! If I had to make a list of the ten most important specifications in a racing engine, connecting rod length would rank about fiftieth. Back in the days when Buddy Morrison and I built dozens of small-block Modified motors, we earnestly believed that an engine needed a 1.9:1 rod/stroke ratio. Today every Pro Stock team uses blocks with super-short deck heights, and we couldn't care less about the rod ratio. A short deck height improves the alignment between the intake manifold runners and the cylinder head intake ports, and helps to stabilize the valvetrain. These are much more important considerations than the rod-to-stroke ratio. There's no magic - a rod's function is to connect the piston to the crankshaft. Period.

ed iskenderian - 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.


sorry for such a long post, but i feel it's worth posting. if you get caught up in all this stuff about rod length in your engine, i honestly believe that you're wasting money and time with something that is pretty much futile. spend your money on high quality parts, and match the parts that count, intake system, camshaft, compression, exhaust, etc...............just my $5.00 (adjusted for inflation, 2 cents just doesn't get what it used to anymore.....)
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Old 11-29-2005, 03:48 AM
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Wow, awsome resposes guys,many many thanks , looks like i may have opened a can of worms here!


DoubleVision, i did that but couldn't find what i was after, best i look again or perhaps get the wife too as she has a knack of finding things.....lol


Huskinhano,Thanks for the info, very interesting.

johnsongrass1, thanks for the reply, higher rev capability would be a bonus forsure as this engine will see 8-8500rpm.

ap72, thanks for your input

k-star, I'm building a boosted Mazda Fe3 2.0L stroked to 2.3-2.4 Ltrs depending on overbore.Its going in a MX5/Miata.



predator carb guru, That reply deserves a medal and certainly clears a few things up for me, big thanks for taking the time to post it all

Cheers Rob
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Old 11-29-2005, 07:33 AM
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Keep in mind that going from 1.55 rod ratio to 1.60 is more dramatic and makes more of a difference then from 1.70 to 1.75. Just like camshaft duration and compression ratio it only helps to a point. Someone mentioned Smokey Yunicks says go for the longest rod. For his type of racing this was probably true. They never saw more then 500miles and probably never saw anything under 5000rpm. For the street this couldnt be more wrong. Johngrass hit it dead on, its not about long or short rods, its about how you match your setup for it. To take advantage of it, you need to match the cam, even more important are the heads. The intake runner size is very important when considering your rod length.

IMO anything above 1.7 is not worth it. The advantages are so small that its not going to make a difference unless your a professional racer limited to a class and every .5hp makes a difference. Your money could be spent somewhere else with probably better gains per dollar.
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Old 11-29-2005, 09:13 AM
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rods

IMO, i think you are going to be very limited with your rod choices with that engine....

At any rate i would be more concerned about getting the bob weight as light as you can. That will get you far more results then any planning of the rod ratio......

There is almost no hard facts on this subject. There are however a ton of opinions on it. Many have been hyjacked from someone elses opinion... Unless you are at the end of your ropes and are looking for that last 1 or 2 hp don't even spend the time on it.......

I assume this is going to be a drag race engine????

keith
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Old 11-29-2005, 09:30 AM
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Quote:
Originally Posted by RobbieRat
Could someone please set me straight on the importance of rod ratio and how it effects performance.

Also what would be considered a desirable ratio to aim for when building a stroker.

Cheers Rob
Both David Freiburger and David Vizard have done extensive back to back tests which proved....... little/no discernable power differences on a dyno.

Theoretical power differences are offset by the additional frictional losses.

As for Smoky Yunick = That is a correct quote that has a distorted meaning......

What he actually meant was.... longer rods flop back and forth less so there is less force trying to break them.... at least that is what he told us.

So Smoky said put as long a rod as possible into it.
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Old 11-29-2005, 02:17 PM
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There was/is an article that was done about this exact subject and the differences are minute. I wish I could find it. Anyway, my "opinion" is that people make much too big of a deal about this subject. The side loading vs. time at TDC, etc.. the difference is so tiny it is really not worth the effort. While I agree in most cases the longer rod will have a lighter piston BUT, you will usually have a heavier rod (since it's longer). Like 99% of hot rodding it is all give and take. Buy what works best for your application and is priced best.

I have built 383's with 5.565" (stock length 400 rods), 5.7" and 6" rods, there really is no difference when it comes to performance.

This is one of those topics that will probably never really be agreed on. Build it either way and it will probably run about the same, so there is no real risk or reward.

Royce
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Old 11-29-2005, 02:40 PM
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A heavier rod will result however, not in direct proportion to the piston weight reduction with longer rods. I agree that with a street engine a long rod doesn't make a lot difference. But it's an additional tuning tool with competition engines.
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Old 11-29-2005, 02:44 PM
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I agree with k-star and camaroman. For most engine builds short of professional circle track or drag racing, rod length is a small issue in a total engine build. I would look at the whole picture. If an engine combination came with only one choice of rod length, your choice is made. If more than one rod length is available, use the cheaper one and use the left over money for something that will make a tangible difference. If the price is the same, flip a coin.

tom
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Old 11-29-2005, 03:29 PM
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There are some basic ideas about rod length.I never gove it much though as a means of increasing performance,but here is what I think about it anyway.

I would never pass up the chance to gain stroke if I needed it just because it would hurt the rod ratio.Of course there are extremes,but in most well rounded combinations this can be taken as fact.I would also never give up piston strength or resort to using a comprimised ring package just to get more rod into an engine.

Short stroke engines can sometime use longer rods as a means of reducing piston weight and improving ring stability,but too much rod can make an induction signal that is too strong on an engine with a limited induction system,so even on an unlimited induction engine,I wouldnt go over about 1.9:1 on the rod ratio.My preference would be to shorten the deck height of the motor and get the rod ratio back down to 1.7:1.

My last theory is the most noteworthy.When in doubt and room permits,build any 90 degree V8 engine with a rod ratio of 1.7:1.I say this not because this is a magic number,but because it is in the middle of where rod ratios typically fall,so matter what anybody else in your class has or claims to have,they will never have any signifigant amount more or less power then you just because of rod ratio.

As a point of interest,my own personal engine is a bigblock chevy built for class racing at 440 cubes,and I use a 3.76" stroke and 6.385 rods with a standard deck height block,and the combination is about as well rounded as it gets,and guess what the rod ratio is.

1.69:1.
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