Chevy Nova Forum banner

Do connecting rod length differences really matter that much?

18K views 79 replies 20 participants last post by  stock z/28 
#1 · (Edited)
Say you could have a 1.655 compression height setup with a 5.7 rod or a 1.34 CH with a 6.00" rod... Is there really an advantage to running the 6" rod?:confused:

I ask because I could run LS pistons(6.00"R/1.34"CH) or 307 pistons(5.7"R/1,655"CH) in my 307 either way:) This is the LS set in question: http://www.jegs.com/webapp/wcs/stores/servlet/product_10001_10002_314074_-1

They aren't cheap, but they are just about the only forged pistons that will work in a 307:) (The keith black ones are hyper. pistons)
 
#6 ·
In response to your direct question about rod length.

yes there is a advantage. And some will argue to the death it does not make difference and want proof that it does or does not. maybe in a normal street car where tires can very more that the advantage the rods may deliver. then point taken.

We all have read the theory of what a longer rod does or does not.

All NASCAR 750hp plus engines have 6inch or longer rods, and they make there HP with flat tappet cams!

Now in a small cu inch engine this could be a great improvement.

Now with the use of the LS based piston in your 307 block, the cylinder quality is much more specific with these pistons. better have a quality block and a "GREAT" machinist to accomplish your goal!

It will have to be balanced!

THE MONEY YOUR FIXING TO LET GO WITH, A 350 BLOCK IS CHEEP!

you will have better result and power by using the money from the 6 inch rod for a block and purchase a set of 350 pistons forged cheaper, even with the 5.7 rod you have, the 350 will make more hp!

Before purchasing the pistons and rods you listed I would purchase a 350 block and a set of pistons for it.

IMO!
 
#7 ·
In response to your direct question about rod length.

yes there is a advantage. And some will argue to the death it does not make difference and want proof that it does or does not. maybe in a normal street car where tires can very more that the advantage the rods may deliver. then point taken.
Jere Stahl seems to think there is an advantage to which rod you choose:
B. Short Rod in Power Stroke -- Piston is higher in the bore when Rod-Crank angle is at 90° even though at any given crank angle the piston is further down. Thus, at any given "time" on the power stroke between a rod to crank pin angle of 10° and ie. 90°, the short rod will generate a greater force on the crank pin which will be in the 70° to 75° ATDC range for most engines we are concerned with.
 
#8 ·
All NASCAR 750hp plus engines have 6 inch or longer rods, and they make there HP with flat tappet cams
They also use the biggest engine they are allowed to start with. I'm not sure what class the Supernovator plans on running. It sure seems smarter to build a 350 than spend nearly 700 dollars on a set of pistons for a 307. What do I know I run a 468 in my street car.
 
#9 · (Edited)
The rod length argument has been debated all over the internet but there's much more to the story than just how long the rod is. Your main problem is finding pistons and you are discovering that there really aren't any easy, cheap choices when working with less than a 4" bore.

I already know that you have these three 307's and you want to make something out of one of them. Plus you've been thinking about this killer 307 project for a long time and seem dead set on making it work.

Do you have a good plan and a budget?
How much power are you hoping to get out of this 307?
How much money do you have to spend?

I can guarantee that it will cost you more money to get the same power as a 350 and a lot more to match a 383.
I can guarantee that you will get less power with the same money spent as building a 350 and a lot less compared to a 383.

I've thought about your engine project and there isn't a cheap solution.
If you don't like my dual 307 engine idea, your best bet is a Weiand 144 supercharger, ported WP torquer "305" heads and a roller cam. You might as well get custom pistons made. As long as you are doing that, you could even use a 6.125" rod....what's next? billet crank? 4 bolt main conversion?

Now imagine you did all that and you have a $10,000 307 that makes maybe 425-450hp.

You pull into the drive-in and show off your "dare to be different" engine. I may be a pessimist but I think the reaction will be "you spent how much on a 307?!"

I was your age once and all I had to work with was 144/170 Ford sixes, BOP 215's, 283's, 305's, 307's and a 318.
While I had big dreams of "giant killer" little engines, the cost and difficulty made it a tremendous waste of resources which is even more a burden when you aren't making good money.

I revisited the little engine, high efficiency idea recently with my Honda 1.6 liter project. It cost a small fortune but it made 125 hp/liter with rule legal production parts. That's "only" 200 hp. About what a decent 307 makes. That's basically 1 reason why V8 owners make fun of Honda racers.

For your engine to make 125 hp per liter you'd have to make over 625 hp.
A 625 Hp 307 would be very impressive, however just imagine how much money it would cost!
Before you blow $750 on a set of pistons you might want to think about this project very carefully.
My signature line is this story in a nutshell.
 
#22 ·
I already know that you have these three 307's and you want to make something out of one of them. Plus you've been thinking about this killer 307 project for a long time and seem dead set on making it work. I am dead set on building a 307, not making it work better than anything else :). Even if it explodes in a fiery ball of death I will be happy because I finally did something I wanted to do

If you don't like my dual 307 engine idea
...I do like it, I love it, but I dont have a reeeeeeeeally long car to do it with:D
Have fun buddy, set your sights high and be prepared to be dissapointed.
no comment:D:D:D
 
#12 ·
Much better buy and would work with any older 350 block.
http://store.summitracing.com/partdetail.asp?part=ESP-B13004E030&autoview=sku

Brand: Eagle Specialty Products
Product Line: Eagle Street and Strip Rotating Assemblies
Part Type: Engine Rotating Kits
Actual Engine CID: 383
Engine Stroke (in): 3.750 in.
Pistons Included: Yes
Piston Rings Included: Yes
Piston and Ring Oversize (in): 0.030 in.
Piston Material: Hypereutectic aluminum
Piston Style: Dish, with a D shaped cup
Piston Head Volume (cc): +12.00cc
Wrist Pin Style: Press fit or floating
Piston Ring Facing Material: Plasmamoly
Crankshaft Included: Yes
Crankshaft Material: Cast steel
Rear Main Seal Style: 1-piece
Balanced: Yes
Engine Balance: External
Balance Style: Neutral
Connecting Rods Included: Yes
Connecting Rod Material: Forged 5140 steel
Connecting Rod Beam Style: I-beam
Rod Bearings Included: Yes
Rod Bearing Undersize (in): Stock
Rod Bearing Undersize (mm): Stock
Main Bearings Included: Yes
Main Bearing Undersize (in): Stock
Main Bearing Undersize (mm): Stock
Cam Bearings Included: No
Gaskets Included: No
Oil Pump Included: No
Camshaft Included: No
Lifters Included: No
Timing Chain and Gears Included: No
Plastigage Included: No
Assembly Lubricant Included: No
Quantity: Sold as a kit.
Notes: Kit includes 153 tooth flexplate for automatic transmissions.

Rotating Assembly, Dish Piston, Plasmamoly Rings, Cast Steel Crank, I-Beam Rods, Chevy, Kit
 
#16 ·
I don't think it's a bad thing, but there is a limit!
A balancing act of what we want the engine to accomplish!

Pull a locomotive? Or accelerate as fast as possible for speed?
 
#14 ·
And I don't care what theory you come up with, in the real world of a running engine, the longer rod has produced more HP!
How much HP are we talking about? When you have a 15,000,000 budget and a team of engineers working on wringing out the last 10-25 Hp out a a 355ci SBC for NASCAR and a 17 year kid building a 307 for a street car. The original question in this thread that makea about a 14,998,000 difference. I'd say he has about a grand to spend on hard parts and wants to waste 700 of it on pistons for an outdated engine. A 383 will make way more power for way less money. It doesn't matter if the 307 had 12" rods in it a 383 with 5.7 rods will run circles around it. I done argueing with him.
If I had decent 307 and wanted to go faster a small cam, headers, intake and other typical stuff makes sense. Buying a 700 set of pistons is a waste of money.
Have fun buddy, set your sights high and be prepared to be dissapointed. I'm done :D
 
#15 · (Edited)
Are you dead set against Hyper pistons? If not and you're looking for the chep way out, do what I did with my 307. You're going to have to deck the block anyhow to make sure it's true, so instead of decking it .010", deck it .020". Now buy the KB pistons for a 283 in what ever over bore you need and get a set of 400 sbc rods (5.565). This still gives you a great rod ratio of 1.71 and a bore/stroke ratio of 1.21 (if I remember correctly on this one). Or deck it .010" and when you have your crank turned, have it offset ground .010"-.015" to get near 0 deck. It works good and is the cheapest way to go. If you deck it .010" and leave the crank along, use a 4.06 x .020 shim style head gasket. This get you to around .035" quench (remember that hypers don't expand like forged do). Do the ported S/R torquer 305 heads, a decent solid flat tappet mechanical cam, and rev the snot out of it. From what the tech guys at KB have told me, you could even throw a 125 shot at it, they're supposed to be able to handle it, just need a little wider top ring gap. This is the cheapest adn most cost effective way to go with the small engine, unless you use a set of 64cc heads to drop compression and add a turbo (which you could also do cheap enough with used turbo parts). I haven't gotten mine done yet (tore it apart to make some changes) so I don't have any dyno numbers, but they should be respectable for such a little engine (316ci). I'm thinking around 450hp@6800-7200rpm's? The only problem is, low torque numbers, so it will have to run some really steep gears, I'm thinking 4.88's, so an overdrive manual trans wouldn't be such a bad idea. I built mine for the same reason you want to build yours, I had it sitting around. I also did one because all my friends said it'll never run good, untill I beat their poorly built, severly mismatched, bottom of the page cammed with stock heads, 350's:beat::D:D. But I used as many cheap and free parts as possible, almost all used swap meet and left overs so the only real money I have in mine is the cam and machine work. I'd say I built mine for probably around $1600.00 give or take a few, so it's possible. My rods, pistons, crank, block, heads, rockers, intake, carb, and headers are all used peices.
 
#21 ·
I'd like to know where all thse cheap 350's are? The last time I looked around here I could only find 1. It was a rebuilder core and it was $450.00. Well, aside form machine work that's all I have in my 307's bottom end, so that freed up a little more money for a decent set of heads. I don't consider $450.00 cheap for a 350.
 
#25 · (Edited)
Hello Shawn,

Are you using a short deck Comp. Elim. style block?
It would seem so, or I would think the piston weight would be very heavy in comparison to a standard deck height type engine.


Reducing the piston weight is one big advantage I see with in a typical engine using a longer rod. Maybe the best advantage.


Most of the circle track (read that as budget) engines I build use a 6 in rod. It allows the crank to be internally balanced with a fairly long stroke with out generally using a bunch of tungsten.

Personally if given the choice between a 5.7 or a 6 in rod I would use the longer rod in a typical high perf application.


I used to have a bunch of data from back in the early 80s (printed material) that some Comp. Elim. guys did testing "spec" style cyl heads on a single four barrel "super mod" style drag car where the actually tried about 4 rod lengths and attempted to access? what the piston speed away from TDC did to induce air flow with various piston dome and/or valve/chamber relationships. It was pretty interesting, and they did find on this combination that a specific rod length did offer an advantage (atleast in their opinion). This data is way outdated Im sure (But im way outdated myself) but it was interesting. And they held a lot of records and won a lot of races, but its been a long time ago.


Im not much into the the 302 style Ford stuff but what rod length to most of these guys use now, with a short deck a longer stroke?


I always find this stuff interesting.

By the way I would have to agree that unless you are required to use the 307 a 350 or 400 or whatever would probably offer a lot cheaper power. BUT if you are really going to build the 307 for what ever reason, think it through a bit, and buy a custom piston from somebody before you try and use an Ls style piston.


I have built quite a few 307s over the years and on occasion I still build a few small journal ones based off of a 283 blocks, generally for early Novas. They usually run pretty good, but a 350 is lot better place to start in my opinion. But theirs nothing wrong with attempting to be a bit different. Just make sure you think it through, and decide its wort the effort and cost for what the end result will be.

One "big" concern in my opinion is generally resale value. If you build a 10,000 dollar 307 or a 10,000- dollar 400 which one do think would be easier to sell "if" you decide to part with it?


Jeff
 
#26 ·
There seems to be quite a few fast Fords with 5.4" rods; a lot more than fast Mopar small blocks that have much longer rods! (probably piss off the Slopar guys)
Like I said, I seriously doubt you'll find more than 5-7 h.p. from the best to the worst rod/stroke ratio, no matter what it's used for.
 
#27 ·
Shawn, I almost forgot to ask, why in the world would you use a big bore short stroke combination to come up with the 358 cid number?:)

Are you not aware that the small bore long stroke, is by far a better combination?:rolleyes:


Oh well it was an attempt at humor.

Failed I guess.

Jeff
 
#36 · (Edited)
Interesting addition age-old debate..
http://www.rehermorrison.com/techTalk/20.htm
http://www.rehermorrison.com/techTalk/53.htm

As for rod length, the best way I've heard it described was "Pick a piston with the best ring package (ring size and overall height from the top of the top ring to the bottom of the oil ring for optimal piston stability) to avoid piston rock and the "BEST" connecting rod length is the one that connects the piston pin to the crankshaft for the given deck height block you're building."...
There's literally thousands of opinions from VERY respected engine builders that go both ways. Winston cup and Nascar like running longer rods where Pro Stock (and a few others) prefer a shorter rod. Personally I'll ride the middle road and shoot for the best possible piston and ring package and choose the rod length that then best fits my application. I'm not crazy about running support rails in the oil rings for street applications even though a lot of people have and don't have any problems. I just don't have the budget to tempt fate with my luck!!
Then there's the cam design to take into account. With the longer rods having a bit more dwell time and faster overall piston speeds, the valve timing events need to be adjusted accordingly, as does the port size of the heads and intake.
I don't believe there's a magical rod/stroke ratio that beats all as every engine combination can literally be infinitely different when it comes to actual running dynamics.
 
#41 ·
I think possibly that fanny pack thing might be in regard to a different forum you may be involved with? Possibly that may explain the apparent "deep" obsession.

I cant say as I blame you for your concern though, in that position stroke depth would potentially be a "big" concern.


I was just visualising an automotive application of the "bore/stroke" combination. I dont even want to see the fanny pack, scenario you are suggesting Mike.


But, please bear in mind I have lead a relatively sheltered life, but I would certainly not condemn someones choices, regarding cranks and/or rods.:eek:

To each his own, I like to think. Please be safe though.;)
 
#42 ·
re

OK im going to bite here.....Theres "dare to different" and "dare to spend alot of money on a turd" Guess wich one a 307 is? If youre hell bent on it for what ever reason, pick youre stroke, find a piston that works and stick the rod in the matches. In your application i dont think it matters at all. I personally run a 6" rod in my 350 but i dont hink it would make much of a difference.
 
#46 ·
Hello Shawn,

Are you using a short deck Comp. Elim. style block?
It would seem so, or I would think the piston weight would be very heavy in comparison to a standard deck height type engine.
Yes. Short deck, 1.150 pin height is what we generally shoot for. Your smart enough to figure it out from there. ;)

Fords under 370" i usually use the 302 deck height stuff, and up to 3.400 stroke, but it gets a little crowed.


4.175 bore??
930hp??????
4.185 bore and being conservative, yes.

shawn
 
#47 ·
I ran across this looking for something else, thought I'd throw it out there.

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.
 
#48 · (Edited)
The whole long rod thing got started by Smokey Yunick who , in his 1983 book, "Power Secrets", said:
Smokey Yunick said:
There are very few secrets to discover about connecting rods, but there is one thing that every racing engine builder must understand: it is essential to use the longest connecting rod you can possibly fit in the engine!
The magazines repeated this (along with the 10 psi for every 1,000 rpm axiom) and the world followed.

Engine builders know more now and Smokey Yunick is dead. His theory's, however, live on. I would guess that if he was still alive, he would change his statement.
 
#49 ·
A lot of John Lingenfelter's 327 super stock engines had 5.4-5.5 rods in them. We had him go through a 350 super stock engine in the mid 80's. It had the 5.7 rods and we asked if we needed the 6 inch rods, absolutely not.

I built 2 identical 481 BBC engines except for rod length. One had the std 6.135 rods the other had .400 long 6.535 rods. The 6.135 engine was about 3 hundredth's faster in the 1/8th mile. Both were 9.8 blocks, same cam, heads, intake etc.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top