Rod Length

plev72

Registered User
As I understand it, longer rods are better because there is less sideways motion (keep it clean!). If you were building a high horsepower engine, do you think there would be a significant difference between going with say the $280 5.9+ inch rods on the GP page, or going with more expensive 6.2" rods? Assuming that the compression ends up being the same, how much difference would rod length itself make in terms of engine longevity and power potential and what not? Is there any disadvantage to going with the longer rod and shorter piston?

(sorry, I'm sure I've asked these questions before, but also fairly certain that I didn't get a detailed response)

Thanks,
Paul
 
Rods, the long and short of it.

The decrease in side loading while an improvement in power and durability is only a minor part of the equation. Increasing the length of the connecting rod will improve the pumping characteristics of any engine and increase its combustion efficiency by extracting more power out of the same amount of fuel and air. The physics of this gets a little complicated, but much of it has to do with the position of the piston relative to the combustion chamber as the crankshaft rotates.

The longer the connecting rod, the longer it takes the piston to reverse directions at either end of its travels. The rod has a wider swing than with a shorter rod so during combustion the piston is higher with more pressure against it when the crank and rod are at a 90* angle to each other when power transfer is most efficient. This extended transition time translates into a greater latitude in ignition timing. You can reduce the timing advance and still be able to achieve complete burn because the piston is traveling slower at the top of the cylinder with the longer rod. Reduced timing generates less negative pressure against the piston before the crank has swung past TDC and started back down. Because of this improvement in piston speed geometry the charge generates it's maximum combustion pressure while the piston is higher, therefore applying it for a longer length of the stroke and generating more power from the same amount of air and fuel.

When combustion efficiency is improved, sensitivity to detonation is also reduced. The advantages above also allow a higher static compression ratio, in fact if you lengthen the rods enough you can increase compression ratio and reduce octane requirements at the same time. This will again increase engine efficiency and reduce timing advance needs allowing each to maximize the other.

At the other end of the stroke, BTC, the slower turn around happens again with more good effects. The piston hangs at the bottom of the cylinder for a longer time, allowing for a more complete cylinder filling on the intake stroke, and on the exhaust stroke it allows for more reduction in cylinder pressure before the piston starts back up which reduces pumping losses.

The more efficient piston movement due to longer rods will improve the flow of gases into and out of the cylinder and will dampen a cam with large amounts of overlap and duration as valve timing events are still happening at crank speed, not piston speed. This makes the engine less sensitive to valve timing and intake and exhaust systems. On the exhaust to intake stroke transition the slower piston forces more exhaust out and allows the exhaust valve to shut and the intake valve to be further open and more clear of shrouding before the piston picks up speed and pulls away. This reduces exhaust gas contamination of the intake charge greatly, especially with high amounts of overlap and exhaust system pressure. The increased piston speed at mid stroke coincides with the intake valve achieving it's maximum lift thereby generating a harder pull on the intake and better cylinder filling with less power used to do so. At the bottom of the stroke the piston slow down allows the intake valve to shut further before the piston reverses direction again, this traps more charge in the cylinder with less intake charge reversion.

Because longer connecting rods get more power out of the same displacement they increase overall efficiency by about 5 to 10 percent at the ground. They will generate more horsepower and torque than the same engine with shorter rods and produce a wider, less peaky, power band. This is how they work and why spare no expense professional racing motors will have the longest rod that can be fitted into its dimensions.

Is it worth it for you and me? I would say yes if you can swing it, but to achieve the greatest gain you need to know what it is you are doing and why, that way you can tune for them accordingly. The specifications that generated maximum power with your shorter rod combination will not carry over. You can and should run more cam and compression and less ignition timing.

Is that detailed enough for you? :D

Vernon
 
That's OUTSTANDING, thanks Vernon :) I think it will probably be worth it then for me to go with the longest rods possible... has anyone used anything longer than a 6.2" or is that pretty much the max we can use? I am most likely going to have a custom ground cam by Dr. Fred as well as a program for him for my chip-tuner. Ideally I'm looking for a pretty high horsepower engine (since I'm spending the money anyway) so it sounds like this is a good start to going about it. The stock '94 SC is 8.4 or 8.5:1 ?? Would it be better to increase the compression of the engine to say 9:1 or just decrease the blower pulley size to increase the blower speed? (realizing that that may exacerbate belt slippage). Thanks a bunch for taking the time to answer my questions on this. I am learning as I go along, but I'm still certainly not anywhere as clueful as I'd like to be (I need performance engine building for dummies or something...) I am going to get the car out to the shop recommended on the SCCOA on Thursday (hopefully!) $1000 labor plus parts and they pull and replace the engine. They were great on the phone and they've done a couple of birds before. Ends up becoming a question of how many of the parts I'm going to secure for myself and how many I'm going to have them get for me.

Just to reiterate the to questions in case they got lost in the paragraph:

any idea what the maximum rod length we can go with is? (I know 6.2" has been done, can we go longer?)

and

how significantly could the compression be safely increased? (or would I be better leaving it alone and thereby nominally increasing engine durability).

Thanks,
Paul
 
I don't actually know how long of a rod we could use, I've seen 347's with the pin behind the oil ring so it's all relative. That question would be better addressed to experianced 3.8 builders like Tom Morana, Dr. Fred, Chris Wise, etc. They have all dabbled in this area.

Compression can be safely bumped, especially with longer rods. On my stock '91 short block I'm running some deeply milled heads that have probably bumped my compression to 9 to 1 at least. So far it has worked great, but this is also with a big lumpy cam that bleeds off some low speed compression. It's all about the combination.

Vernon
 
longer rods

I have a 6.2" rod in my shortblock. I had custom diamond piston made to fit. I think I ended up with a 1.32" deck height or something like that. My oil retention rings sit just on top of my wrist pin hole. I've seen application where the wrist pin hole is inside the oil retention rings. With something like that, you could run a shorter piston and longer rod. If you really wanted to, I bet you could fit a 6.5" rod in there with custom pistons.
My compression is set to be 9.1:1 but it may end up being about 9.4:1 cause I had my block decked to straighten it out.

Another benifit that I found was that my longer rod/shorter piston combo weighs 120g less per cyl. than the stock ones. That's 120g X 6 off the piston/rods, then another 120g X6 off the crank counter balances. Thats 3.24 lbs. off the rotating assembly.
That should help it to rev faster.

I've got 2 extra 6.2" 351W journal sized CAT chromoly H-beam rods if anyones interested. :p Their just paperweights now.

You guys should order 3 sets of rods and end up with 4 sets of 6 as long as their all matched sets and weigh the same. You'll save big money that way.
 
Sorry to beating this one Dan, do you have the link to the previous post, or the link to the website where you got the rods from? Think you said Detroit something?? Maybe I'll move this suggestion over to the group buy page just to present it as an idea. I really have to chill for a bit with my plans until I find out what the actual state of my motor is currently. It'd be pretty cool to go with the super long rods... especially since it implies that there would be even more weight savings going to the even longer rods... have to talk to the shop once they've taken my engine apart (hopefully mid next week) and give them my book (assuming I can get it back from mechanic who has it!) plus the relivent posting from the SCCOA board of course ;-)

Paul

Oh... and with my longer term plans, I should probably have room if I needed to use the thick copper gaskets and raise things abit. I'm not sure the mechanics of all that, but looks like I'll be following a sharp learning curve :)
Paul
 
For long term reliability, having the oil ring span the wrist pin is a bad idea. For short term (10K miles or less) it wouldn't be a problem.

Actually, the major benefit of longer rods is the lighter pistons. That makes the motor spool up faster and spin down faster. Less parasitic drag too. All the stuff Vernon wrote about dwell times and directional changes helps, but the weight is about 5 times more noticable. I've seen charts of 5.7" and 6" SB Chevy's and the dwell time difference is so minute it's not relavant.

It also relates to the stock rod/stroke ratio. Here's a few stock R/S ratio's.

Chevy 350 - 1.638
Ford 302 - 1.697
Ford 351W - 1.702
Ford 460 - 1.716
Chevy 454 - 1.531
Chevy 400 - 1.484

Ford 3.8 - 1.744
Ford 3.8 (6.2") - 1.829

If you look at the chart, you'll see why Chevy 400's and 454's respond so well to longer rods. The stock rods are just too short. A Chevy 400 has a shorter rod than a 350 (5.565" vs 5.7"). Thus, if you build a 6" rod 350 (R/S = 1.724) it will help, but if you build a 6" rod 400 ( R/S = 1.600) it would be a HUGE difference.

It's kinda like a diminishing returns thing. A R/S below 1.5 is wasting significant power pushing the pistons into the side of the block and would never be used by a rebuiler in a performance motor. A 1.6 R/S is considered acceptable. And 1.7 - 1.8 is considered very good. It's been proven that power will continue to grow (for various reasons) until about a 2.0 R/S ratio. Although, you'll have a super hard time ever getting a rod that long in a stock block. Also, the return gets smaller and smaller the higher the R.S gets.

Hope this helps.

Steve

P.S. Little tidbit... a 6" rod Chevy 327 is a killer circletrack motor!!! (1.846) But most classes won't let you run one.
 
I disagree with Steve on his point about the increase in rod lengths effect on piston speeds being relavent. A few lines down he states it's a proven power builder. :confused:

Vernon
 
Bore is 3.81"
Stroke is 3.39"
Rod is 5.9135"
Deck height is 9.232"
Compression distance is (top of piston to pin) 1.602"

In '96 Ford made the rods longer, and not just to save piston wieght.
'96 up rods are 6.091"
'96 compression distance is 1.450"

The 4.2 has the same bore but a 3.74" stroke
it uses the same rod at 6.091"
so the compression distance really shrunk to 1.273"

Don't forget when adding up assembly hieghts to only use half the stroke, (that's as far up as it goes).

Vernon
 
Thanks a bunch for all this info, I should probably take the time to organize it and then get this posted as a faq because I don't think it is listed. When people talk about zeroing the deck, what are they talking about? I can't believe they are talking about shaving 9" ?? or is that the depth of the block without the heads?

Learning curve going up up up :)
 
Zeroing the deck would refer to the process of cutting down the block at the head mating surfaces to the point that the piston and rod assembly will be flush with it at TDC. If you use the figures I gave you for a stock 3.8 you will find that the crank travel at TDC with the rod length and piston hieght will produce a 9.2105" total height. Then the deck hieght it 9.2320", so in a stock combination the piston top surface would sit 0.0215" below deck. So if you then milled that amount off the top of decks (0.0215") you would be zeroing the deck as they would be flush at TDC. This improves charge quench as it forces it into the center of the chamber from where the chamber and piston almost meet. This improvement in quench creates more turbulance so the charge is well mixed and homogeneous. Forcing it all out of the cracks means more of it will be burned. This increases compression slightly without any negative effects, in fact because of the effects power and effeciancy go up. An engine with more quench can run higher compression ratios or boost than one with poor quench. In fact you can actually build your combination so that the pistons will rise slightly above deck to further this tuning tools effect. Remember the head gasket thickness? Without going through a long discussion it depends on several factors as to how close you can safely approach the head as hitting it at speed would be disasterous. As a theretical example if you used a 4.2 crank with a early 3.8 rod and a late 3.8 piston that assembly would rise 0.0015" above stock deck which is very safe and better than below or "zeroed".

Vernon
 
An ideal motor set up is with ,for every 1.0 stroke you want 2.0 rod length.

And I agree with Vernon.The longer dwell at TDC will give more time for flame
travel.

The longer dwell on BDC give more time for the cylinder to fill.

You need a bear minumin of 0.050 clearance for piston tops.

This can increase your piston speeds in the middle of the stroke,
(half way down the cylinder).A stroker motor will increase the piston speed.

On building a motor ,stroker with long rods.You need to keep piston travel
below 6200 fpm so the piston will not melt.

On a long rod & or stroker motor.You need to find the piston speeds.
so you will know what the newer lower safe RPM red line is.
 
Last edited:
Hmm, thought this one could use a raise to the top with all the winter work going on.


Please note this a many year old post but info from Vernon is always welcome.
 
I met that guy...After some of his readings, he is a bigger pimp then I thought before. Some of the things he mentioned I did not know. They make PERFECT sence though. Why wouldnt you run higher compression? The piston stays parked that much longer. With the ability to slow the piston down that much more, you could even rev higher and not suffer any additional ring and bearing wear...Lets keep this one up. Dan, I am interested in some of the other stuff you did to your engine as well....

Chris
 
http://forums.corner-carvers.com/archive/index.php/t-1745.html

Just some good and bad info in there

Going with a longer rod does not mean more HP and in some cases can make less dependant. There is however an optimum rod lenght whatever that may be with a 3.8 I have no idea. Is the stock rod optimum?..Is the 351 rod lenght optimum...Are these rods optimum?? Is there even a way to know? In thsi case longer doesnt always mean better. Long rod motors do tend to rev slower which I wouldnt particularly like in a heavy car. Does a lighter weight piston make up the fact a long rod motor will rev slower? Unsure

I can tell you that it was a big trend with the 5.0's that quickly went out when noone was really making any more power for the hassle..Its generally used in cars that will keep the revs high for long periods of time (various forms of circle track and whatnot). Not drag racing and not for ultimate power.
 
3.24 lbs. @ 6500 rpm

Slysc said:
Another benifit that I found was that my longer rod/shorter piston combo weighs 120g less per cyl. than the stock ones. That's 120g X 6 off the piston/rods, then another 120g X6 off the crank counter balances. Thats 3.24 lbs. off the rotating assembly. That should help it to rev faster.
What does 3.24 lbs @ 6500 rpm weigh? Dunno the correct forumla to figure that out, but it's gonna be a LOT. Also figure that the crank has to STOP 1/6th. that weight (3.24 lbs @ 6500 rpm) everytime a piston reverses direction (@ 6500 rpm). So the long arm combo will be much easier on the crank too.

68COUGAR
 
Re: 347's

Vernon said:
I've seen 347's with the pin behind the oil ring
From what I've read, the rod ratio of the 347 is actually To Long. That's why they wear out much faster than the smaller 302 strokers.

68COUGAR
 
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