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Old 09-16-2007, 03:49 PM
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6in rods in a 327?

would 6 in rods be a good adder to a 327?what about any other power adders for the STREET not the strip.love this 327, decided to keep this little booger! what about rear end ratios?got a 700r4, would it be better than a th350?or would the th350 be a better street tranny?

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Old 09-16-2007, 03:58 PM
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Read this tech tip from Iskenderian......


"Tech Tip - 2005

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."
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Old 09-16-2007, 04:04 PM
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thanks man,,alot of info here!
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Old 09-16-2007, 04:34 PM
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I'd be more apt to use the 700R4. You need help getting power to the tires because of the rather small displacement.

Don't get me wrong, I think the 327 is a fantastic motor and have long thought that the bore to stroke ratio on it is optimum for an internal combustion motor. It just hit the "sweet spot" in my opinion. Chevy claimed 375 hp from the little jewel back in the 60's, STOCK from the factory with technology which is now 40 years old.

As I posted on another thread concerning the 700R4, using a rear gear somewhere around 3.70:1 will give a very low first gear to get the vehicle moving (11.32:1 first gear final drive ratio) and a 2.59:1 top gear final drive ratio.

As far as more poop for the motor, I'd concentrate on the cylinder heads, keeping the intake runner size around 160-170cc's for street use. Research to find heads that flow the best at 0.400" valve lift at that runner size.

Don't make a camshaft selection until you get the final static compression ratio sorted out and determine the rpm range you will operate the motor in.

Last edited by techinspector1; 09-16-2007 at 04:42 PM.
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Old 09-16-2007, 06:40 PM
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If you're buying rods anyway, there's really no reason not to use 6" rods in an engine with as short a stroke as the 327 has. Your pin location won't be high enough to compromise good ring sealing caused by piston rock. The 273, 318 and 340 Mopar small blocks all shared the same stroke, a very short one similar to the 327, I think perhaps just a bit less, w/o looking up the numbers. Anyhow, they used rods of a nearly 6 & 1/8" length from the factory and certainly have never been known for any such problems. I use 6" rods in my 388 Chevy stroker which has a 3.80" stroke. I actually used pistons with a compression height for a 3.75" stroke but that's another conversation. Anyhow, the pin is indeed into the bottom (oil) ring land but using ring supports for the oil ring, I don't consider this a problem. It's preferable to squeezing the rings closer as techinspector made mention of. In using 6" rods in your combination, I can say with surety that your pin location would be considerably lower than mine and would not encroach into the bottom ring land.
I like the 6" rods for my purposes because they help bring the torque on just a little earlier in the RPM band. I considered this especially beneficial in a 3400# car with a 3.07 gear. Not a big gain but every little bit helps if you're building for a specific purpose and goal. You're doing as you should, seeking info, opinions and the experience of others. Once you develop a broad picture in this or any other matter, in the end only you can decide exactly what is right for your goals. Everything you consider must be weighed as part of a total package with an end result in mind. Everything in engine design and modification is a compromise.
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Old 09-16-2007, 07:21 PM
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There is power in longer rods, but the builder must know how to change the breathing characteristics of the engine to take advantage of them.
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