Rod Length

68COUGAR said:
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

Marc,

That has been proven false, and the 347 wears just as good as a 331 or a 302. The problem started with Keith Black stroker kits, and where he located the wrist pin with the oil ring intersecting and more importantly that he failed to supply a support ring for it. The rod ratio is no worse that several other performance motors.

People who build these motors say that a current 347 stroker will last just as long as any other motor.

David
 
So long as the wrist pin doesnt interfere with the rings there isnt that problem. They do however say the 331 has a more favorable rod lenght. Ive made plenty of power with a good ol 302 to not worry about strokers..Plsu you can rev the @#&*^#^v Out of em:O)
 
I did not read all of the posts from what you posted Damon, but I did see some snippits that were incorrect. The fact is, that the longer the rod, the longer it stays parked in the TDC and BDC positions. I could NOT find anything that was not true in what I know in Vernons statement, but there are some other things I would like to address. First, the longer rod, will increase bearing life right? Well, in a high RPM engine, Yes. The piston speed and speed of the rings is what wears on a 347. Ask Brian Denton if you dont believe me. His first season before the orange car had the graffix on it went through the engine in one season. Mike (York) and I were talking about it after Brian left the shop. You see, the piston speed on the street car will not overcome the rings there, but it does need to have time to not spin the hell out of it if it is a race engine. Next, there IS a MINIMUM rod ratio for engines. It is supposed to be 1.57 (I think) to 1.6. That is supposed to be the optimim for factroy stuff. In the past I had always heard you could get more HP out of 289 rods in the 5.0 eingine making it a long rod combo. Never did it nor did I ever meet anyone who knew of it or had done it. Lastly, REV slower? Where did that come from? If the weight is down on the combo, there is the same weight on the counter balance. The piston speed its self is slower, BUT THE CRANK IS MOVING THE SAME SPEED. The rod ratio that is short, simply *whips* (for a lack of a better term) the piston/rod. The longer ratio actually has the piston stopped longer since it stops at more of an angle on the crank. If you think you can tell a differenc in the way it revs, check out the SC shootout and watch Dan Sly click off some mid 12 passes.

And 68coug, the larger the displacement the shorter the rod in effect having a short ratio combo.

If anyone wants to add to this or pick it apart, please do. This is not an attempt to belittle anyone either, it is a discussion. Just need to get that said and out of the way first.

Chris
 
Lastly, REV slower? Where did that come from? If the weight is down on the combo, there is the same weight on the counter balance. The piston speed its self is slower, BUT THE CRANK IS MOVING THE SAME SPEED

This comment of mine was actually based on experience and most likely has to do with the inertia of the internal engine parts due to where the actual weight is as well as the piston "hanging up" longer up top. Like I said long rod motors have been used in the circle track/oval track circuit for the way they tend to rev up top. Long rod motors tend to be most benneficial when the rod ratio isnt optimum from the factory such as in the 2.3 I4 motors.

The concensus in the drag racing world has always been its a waste..Stroke it or spend your money somewhere else....Dam that sounded dirty.

But if your getting rods and pistons anyway..I guess why not give it a shot
 
My opinion (hey everyone's got one right?) is that in our application the stock rod length is plenty long and our heads do not have an issue with exhaust ports, (did I say that?) so there is little need to dwell the piston at TDC. Instead, I feel it is beneficial to allow the intake charge more time to get in past those goofy intake ports via a rod length that gets things moving faster in the intake side of things.

Hows that for a counter point? :p
 
Faster Pistons = ?????????????

XR7 Dave said:
I feel it is beneficial to allow the intake charge more time to get in past those goofy intake ports via a rod length that gets things moving faster in the intake side of things.
Dave, I'm hearing 2 things here. Help me out, if either or both are AFU.

(1) We need max intake flow.

(2) It's only 180 degrees from TDC to BDC. The time that it takes a piston to go from TDC to BDC is ENTIRELY rpm dependant. Now in a Long Rod engine, the piston dwells near TDC & BTC longer. Longer dwell at both ends of the crank throw, means that the piston has LESS time to Get from TDC to BDC. Less time = FASTER piston speed.

Here's the question for ya Dave: If the piston is moving faster down the bore, doesn't that increase cyclinder filling speed?

What am I missing here?

68COUGAR
 
Think OUTSIDE the box. Do not confuse actual time that obviously depends on RPM with time...like in degrees. With a longer rod. the piston actually reaches TDC/BDC earlier in the rotation. It slows there because of the angle of the rod to the crank and stops there longer in rotational degres then with a short rod. Understand?

Chris
 
Longer rods favor weak exhaust ports because they dwell longer. Shorter rods favor weak intake ports because they get moving quicker thereby initiating flow in the intake port sooner. It is of reduced value to have the intake valve open if there is no piston movement.

So yes, what I was saying is that piston movement is initiated earlier with a shorter rod which will help out the weak intake ports. In the case of changing rod length, maximum piston speed is not increased, only the acceleration rate. The speeds at different points of rotation is different due to acceleration rates, but peak and average speeds are the same. A longer rod experiences higher G-forces.

I can't speak for all SC heads, but mine have a 95% IE ratio. Not good for any engine. Supercharged engines are supposed to have about an 85% IE ratio. Even in NA applications, some of the new SBF applications have such good exhaust ports that they are starting to run cams with longer intake duration than exhaust duration. In my car I chose a straight pattern cam, but in retrospect I might have done better with a little more intake duration.

Of course none of this applies to stock heads, but I think it is safe to assume anyone contemplating changing rod lengths probably has highly ported heads.
 
XR7 Dave said:
Longer rods favor weak exhaust ports because they dwell longer. Shorter rods favor weak intake ports because they get moving quicker thereby initiating flow in the intake port sooner. It is of reduced value to have the intake valve open if there is no piston movement.

...
Of course none of this applies to stock heads, but I think it is safe to assume anyone contemplating changing rod lengths probably has highly ported heads.

Due to these timing changes some real thought is going to have to into final CAM design. <hint..hint> It seems that some of these understandings for rod length impact on intake and exhaust ports can be optimized for in the CAM.

How about "sufficiently" ported heads? Calling them "Highly ported" sounds too expensive.;)

My main goal with the longer rod (.204") is to improve the quench during combustion increasing efficency thus allowing more power for a given compression ratio and air /fuel ratio. (I'm zeroing the deck as well)
 
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And in the end unles syou totally screw up your rod ratio...It makes little difference either way:O)

Hey Chris..Speaking of long rods..I think I have something cheap for a 4.6 you may want:O)..I'll email ay later
 
Note, Rod Ratio's.

Stock: 1.74
6.2" Rod in a SC block: 1.83

In general, the difference between the stock rod and the longer rod are not too great but there is some. Doing some Piston Kinematics we find:

At 6200RPM
6.2" rod
Max Piston Velocity: 64.83 mph @ 75deg & 284deg of crankshaft rotation
Max Acceleration miles/hour/hour: 186,106,094.45 at 0 deg

Stock Rod
Max Piston Velocity: 65.06mph @ 75deg & 285deg of crankshaft rotation
Max Acceleration miles/hour/hour: 188,052,512.96 at 0 deg
 
Dave, there is one thing that I think is a LARGE component to this equasion. THE BLOWER. Since the ratio has a longer dwell time, it will do better even with crappy inlet ports. The positive pressure adds to this but I dunno how much.

Chris
 
Some people may like these numbers better:

Max Rod Velocity @ 6200rpm
Stock Rod Lengths - 5725.46 Feet Per Minute @ 75.09 degrees
Long Rod Lengths - 5705.40 Feet Per Minute @ 75.67 Degrees

Max Rod Acceleration @ 6200rpm
Stock Rod Lengths - 76613 Feet per second per second @ 0 degrees
Long Rod Lengths - 75821 Feet per second per second @ 0 degrees

Some other interesting numbers

With Stock Rod weight at 662 g and stock piston weight 669g
Max Reciprocating Force: 4670.50 LBS
Max Total Force: 6471.13 LBS
Max Piston Side Force: 598.36 LBS @ 120/240 degrees

My setup with 630g 6.2" rod weight and 591g piston weight.
Max Reciprocating Force: 4161.50 LBS
Max Total Force: 5875.09 LBS
Max Piston Side Force: 506.91 LBS @ 120/240 degrees
 

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Can you change the colors on that graph please? I can only see two lines. I would like to see the comparison too.

Chris
 
Look closer. On the graph the two curves are so close to being one on top of the other that you really can't see the difference regardless of what color is used.
 
That's how really close it is. I'm working with some software right now to model this stuff so I can model up a cam. I took the numbers it spit out and threw them into excel for comparison. Here are some pics that zoom in. I also added piston position.

With the room available, the difference in length from 5.912 to 6.2 is pretty small in the main scheme of things. I feel the duration at TDC will help a little with better combustion, so that's why I'm doing it. The greatest advantage is simply going to be in the lighter weight of non-stock rods and non-stock pistons. That reduced mass is really going to be a bigger benefit than any slight increase in rod/stroke ratio IMHO.
 

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That was what I was afraid of David. Im color blind so I was not sure If I could see it at all. However, the graph does not support your forelisted data.

Chris
 
seawalkersee said:
That was what I was afraid of David. Im color blind so I was not sure If I could see it at all. However, the graph does not support your forelisted data.

Chris
What "forelisted data?" I said the shorter rod starts to move sooner and accelerates quicker. This is shown by the graph, although it appears to be of miniscule amount. I have a feeling that the graph may not have the best resolution to pull relevant data, but it may be that there just isn't that much difference either way.

My main point, in case that was missed, is that there are reasons for choosing one combination over another. In the case of the longer rod, I feel that the benefits include lower reciprocating weight and less angularity. In the case of a shorter rod I feel that better intake flow, reduced pumping losses and a more stable piston due to a longer skirt might be just as important. As Damon said, neither one should prove to be a bad choice.
 
Due to the limitations of Excel, it's hard to see the differences. Thus far I've looked at numbers in 5 degree readings. I'm going to break that down into 1 degree readings where the difference may become more noticable. The key thing is to notice with piston velocity decreases first while movement decreases as well. That would indicate longer dwel.

It is a given of physics that a longer rod will sit at TDC longer than a shorter rod and it will sit at BDC shorter than a Shorter rod.

I'm investigating it purely from a cam selection standpoint. If the durations at TDC and BDC change with the rod change, this will have an impact on how you may want to cam the engine.

What I've posted thus far was simply to show that the difference when we talked about it seemed it may be large, but when doing the math it doesn't seem all that huge. Once I have some time to run the numbers by each degree and do some analysis I'll make the spreadsheet available for folks to look over and to do their own analysis.

Some places where I found info initially:
http://www.wfu.edu/~rollins/piston/offset/
http://victorylibrary.com/mopar/crank-bal-c.htm
http://www.stahlheaders.com/Lit_Rod Length.htm

Page 2 of this one includes an excel spreadsheet that you can modify to play with some rod length numbers. It's measurements are in MM, but the unit of measure has nothing to do with the math. just plug inches in and adjust the RPM down from his 8000.
http://e30m3performance.com/tech_articles/engine-tech/rod-ratio/

http://www.miata.net/garage/KnowYourCar/S11_Piston.html
 
1st. Graph Question

Why is the Rod Acceleration NOT the same for BDC & TDC? Why the long flat spost at BDC but NOT at TDC?

Curious 68COUGAR
 
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