Rod Lenght?

[toad490]

I've bought a KB 4.25 stroke crank for my 440 build, it looks like it means custom pistons, so I have a choice of rods. What would be the difference between hemi rods and stock lenght rods, in engine characteristics?

I hope everybodies not sick of my questions. I'm relying alot on the advice I get here, the last time I was down at NAPA I asked for valve cover gaskets for a 440 and the kid had to ask who makes a 440. Nothing in town resembling a speed shop.

[TK]

toad, for my experience with the brand X'es, and other things in life, the longer rod the better, think of it as being on the short end of a prybar!

[DartGT66]

Shouldn't be much difference wether you use a 6.76" or 6.86" rod. Going with a 7.1" rod would dive you a slightly shorter piston with less weight, but the rod will be heavier.

[djswwg]

Put the longest rods possible, there's lots of good reasons why! Cat makes a decent h-beam that is 7.1" long. With a stock deck height of 10.725" you still have 1.50" left for piston height. Ross makes a piston that is 1.471" (#99538)compression distance. That means if you wanted the piston flush with the deck you would cut the block .029", should be a real tork hog!.........djs

[cutting torch]

Here's my take on why long rods work better...... With a long rod, the crank can turn a few more degrees towards and away from TDC and BDC before the piston moves very far up or down. This essentially lets the piston "park" at TDC for a little longer than it could with a short rod. Since it sits there a little longer, with the crank (and cam) still turning, there is some extra time for the intake valve to open. Since the intake valve is opened to a higher lift BEFORE the piston is speeding down the bore, you get better cylinder filling. With a short rod motor, the piston has moved down the bore further by the time you have any effective valve lift, wasting some of your intake stroke. I hope this explains it well enough!

torch

[DartGT66]

I'm not a big believer of the importance of rod/stroke ratio, but I guess that's my problem. Same thing with the "mopar cams", that take the advantage of the big lifter diameter. I know that there are some big and small block chevys that run decently without those advantages. So, if they can do it without them, what do we have worse so that we need them?

[toad490]

Now that makes sense Torch. Long rod it is. I found a set 7.1 ally rods. Can anybody talk me into using them in a strip/street engine.........please?

[DartGT66]

It's propably true, that a rod/stroke length of around 1.8 1 is good, but there are more important things to consider than that, that have even more benefit. If you look for example at Pro Stock engines, they do not build them to get a certain rod/stroke ratio. For them, it's more important to use a very low block (I think they are below 9" at the moment, over 1/2" less than in a SB mopar) so the rod ratio will fall in to whatever it takes to connect the crank & pistons. Mopars have such a high blocks, that you can easily shoot for the "optimal" rod ratio in most cases without harming anything.

[cutting torch]

In a street-strip engine, longevity is a concern, and long rods side load the piston less than short rods do. That will help piston life. On the other hand, if a stroker/long rod piston is really short, it can wear faster by rocking in the bore. You need to stay away from super-light, short skirt pistons for the street. Luckily, piston companies realize that not all stroker engines are balls-out race engines anymore.

Toad490, I would not advise aluminum rods for the street. For your application, the fat big end of the rod may cause clearance problems with the block, or cam, or windage tray, or oil pickup tube. Disclaimer: I have not stroked a B or RB engine, so those could be a non-issue. Since AL rods grow a lot more than steel ones, you would need more piston-to-head clearance. If you were planning on getting close piston-to-head clearance for a good quench area, that goes out the window with AL rods. Also, a street driven engine will usually take your oil temp higher than those rods would like, so an oil cooler would be in order.

torch

[rumblefish360]

I could talk you into makeing use of them allky rods, only to scoop up the mess you'll have later for cheap....Ha ha ha, (Clearing the throat sound,,ah-hack-hem)

Seriously folks, a longer rod is nice for many reasons. However, going out of your way for a longer rod on your everyday ride is really not worth the hassle. Heck, if the masters and great gurus are still out to lunch if it's realllly worth it and dynos show no super real advantage on it at our level, why kill yourself on big spendy parts?
Other engines of brand C-F-X-Y&Z are making good power with stumpy rods, theres no reason your engine can't do it and live a long life.

Shorter pistons are in the same boat. There nice, but theres limits. If your driving this on the street 40% of the time, like to and from the track and then some, I really can see a reason to push the piston to thinner than thin levels. The rings get up into the pin area after awhile. Thats not good for the street.

If you can use the stock rods without creating other problems, then use them. Shot peen and polish the rods after there magnafluxed for cracks and resized/straightened out.

By the way, Alum. rods have a shorter life span.

What we have that is worse is parts developed after they were developed for Chevy.

[toad490]

Originally Posted by rumblefish360

)

Seriously folks, a longer rod is nice for many reasons. However, going out of your way for a longer rod on your everyday ride is really not worth the hassle. By the way, Alum. rods have a shorter life span.

If I'd gone with the 4.15 stroke I wouldn't of considered longer rods. The 4.25 stroke requires custom pistons anyways, so I thought I'd ask.
Ya, the ally rods are out. I'll go with 4340 H beams.

Torchs reply made sense to me. So far nobody has given an argument like that for short rods.

[rumblefish360]

Heres a counter thought, A shorter rod may load the cyl. wall more, but the piston dosent sit @ TDC as long. This should require less timing. (I think I got that right. However, I myself would not do that. Theres actually more to this, but I ain't going through the printing of it.)

[djswwg]

you are right on this one Rumble, the shorter rod does create a shorter piston "dwell" time at top (and bottom) dead centre, which gives a slightly better idle. for my money I'll go the longer rod every time, but the short rod engine is not "wrong" either, it really depends what you want versus what you are willing to give up to get it.
As for Moparhounds rant, he brings up a lot of good points, and some which aren't so valid. Here's a good rule of thumb; up to 470 cubes, a 400 block is usually the best way to fly, for more cubes use a 440 block. A 440 block is a waist of space for under a 470" motor. As for block toughness, a sonic check of each blocks individual cylinders is the only real proof. As for main web, just have a look, the 400 is way beefier. 440 source website has a great article with pictures showing all kinds of info about the B/RB blocks. I recommend it. Cheers............djs

[rumblefish360]

LOL @ djswwg. I see your on top of things. I think theres another thread I asked a bunch of questions on this. But thats another thread. Which I'm enjoying. http://www.moparchat.com/forums/showthread.php?t=88516

[cutting torch]

Originally Posted by rumblefish360

Heres a counter thought, A shorter rod may load the cyl. wall more, but the piston dosent sit @ TDC as long. This should require less timing. (I think I got that right. However, I myself would not do that. Theres actually more to this, but I ain't going through the printing of it.)

That's true, but the engine doesn't really care when the spark happens, as long as it is timed to provide peak cylinder pressure at 20 degrees (I think) after TDC. That can happen quick (less advance), in a hemi with flat top pistons (smooth, easy flame travel). Or, it can happen slow (more advance), like in a BB Chevy with a big dome piston.

torch

[rumblefish360]

Heres another. (LOL) Smaller piston bore size. Less flame travel. Easier to ignite and continue burning and flame propergation. (ouch, sp) Not to be taken till theres valve shrouding extremes of course.

All pointless for Toad. Just good food for thought and to create more wrinkles in the brain.
Gald to see thinking minds. Never hurts to throw in a twist. So long as we don't get carried away.
I think toad will make out well myself.

[cutting torch]

Originally Posted by rumblefish360

Heres another. (LOL) Smaller piston bore size. Less flame travel. Easier to ignite and continue burning and flame propergation. (ouch, sp) Not to be taken till theres valve shrouding extremes of course.

That is the reason Mopar stopped making the 400. By the time the flame front gets out past a 4 inch bore, it starts to cool off, raising hydrocarbon emissions. Also why they made a V10 - lots of cubes with an emissions-friendly bore size.

torch

[rumblefish360]

I think thats also true with the 440. It's that 4 inch magic bore size thing. Viper engines are like a 2cyl add on to the 360 kind of thing right? I never really looked into the Vipers engine that hard. Thing being it might be another decade before I consider a purchase of one.......

[toad490]

Originally Posted by DartGT66

If you look for example at Pro Stock engines, they do not build them to get a certain rod/stroke ratio. For them, it's more important to use a very low block (I think they are below 9" at the moment, over 1/2" less than in a SB mopar) so the rod ratio will fall in to whatever it takes to connect the crank & pistons. Mopars have such a high blocks, that you can easily shoot for the "optimal" rod ratio in most cases without harming anything.

D66, I seen a piture of a Chevy Indy piston, and that thing didn't look more than 2" top to bottom. Is the idea of the low deck a light rotating assembly?

Playing devils advocate... I looked at whats available for Chevy rods and didn't find very many shorter than stock, but lots longer.

[sanborn]

This has been a very good thread. Lots of good comments.

Longer rods are good, have less wall loading and are less stressful on pins and accel/decel loadings of the rods. But don't spend big $$ on getting longer rods. The benefits just aren't there to warrant big $$.

Race engines want the lightest piston possible. This is not practical on a street engine. On the street you want a light piston---but also one that will live many miles on the street. Try to get a street piston with a long skirt---this will help to keep the piston square in the bore. On a side note! Nascar engines are frequently "rehoned" after one 500 mile race. They are rehoned .002", fitted with new pistons, etc. High $$$ drag engine are done the same after a few passes. You can't afford to do this on a street engine.

The comment about the 4" bore and flame travel is absolutedly true. And the smaller bore allows less ignition advance. Modern wedge race engines use very small combustion chambers(less than 45CCs), flat top pistons(where possible-dish is even better) and spark plugs as close to center as possible. This lets them get max power with 32 degrees advance---even with large bores(4.200"). But they also use serious ignitions.

Another comment often overlooked---modern race engines use the best metallurgy possible. Serious race cranks/rods/pistons/pins use the best there is----and the cost reflects it. There are real reasons why the latest race crank costs $3K+, but weighs only 40 (or less) lbs. and will take the abuse of 800HP. It's not necessarily the metal, it's the heat treating, development cost, etc. The same is true for that $2K set of rods, that $1K set of pistons. Street components don't cost that much but aren't produced to the same specs either.

If you wanted a street engine with all the latest race components, you can easily spend over $20K just for parts, no machine work. And sadly to say, most of the really good stuff has been developed for the small blocks(Nascar engine budgets just dwarf everything else combined). Finally, the really trick stuff is not listed in any well known catalog---but produced by companies/divisions not well known.

Hope my ranting wasn't too far off the subject. If it was, please forgive me, I'm old, bald, and my Bassetts got me up too early this AM---and the coffee hasn't got my batteries charged up yet.

I think I will just sit down and ZZZZZZZZZZZZZZZZZZZ

[fox]

Another area to think about is the port sizes. This is also something to match with the Rod/stroke ratio.
Ever compare a 440 head to a 454 chevy?
The 440 has smaller ports and likes velocity the chevy is huge and porters I know don't like the huge ports.
But, when the piston is able to move away from top D C fast then the big gulp is ok. Of course, this also steels the TDC pressure, so it is less efficient.
A longer rod, with all other things constant should show better fuel economy.
So, what heads are you planning?

[toad490]

My wife just bought a Dakota R/T so I'll be towing the Cuda to the strip, so the street part of my build will be much less. I've gone with 7.135" ally rod.
I talked to R&R about their aluminum rods in a strip/street engine. They said that there was no problem using their rods on the street, but should be replaced after 7500 miles (they knew I already had the rods) and for me thats acceptalble. Now that we have a truck I wont put more that 1500 miles a year on them. I got new R&R rods for $335 with shipping, on eBay. For that price I figured it was worth while, at full price probably not.

Fox, I have a set of 906s that flow not bad, so I'll use them to start. I see on another thread that eddelbrock may be coming out with new heads, I'd like to see what they're coming up with before I buy head.

[MitchB]

Originally Posted by rumblefish360

Heres a counter thought, A shorter rod may load the cyl. wall more, but the piston dosent sit @ TDC as long. This should require less timing. (I think I got that right. However, I myself would not do that. Theres actually more to this, but I ain't going through the printing of it.)

No, this is wrong. Excuse my being direct. With a shorter rod, you have to start spark sooner because the piston accelerates away faster from TDC with a shorter rod than with a longer one. The chamber volume grows faster for a given crankshaft rotation, but with all else being equal, the flame front still travels at the same speed. If you do not start ignition earlier, therefore, you will build lower peak cylinder pressure and at a point later in the cycle.

Mitch

[MitchB]

Originally Posted by djswwg

the shorter rod does create a shorter piston "dwell" time at top (and bottom) dead centre

This is incorrect. With a shorter rod there is less dwell at TDC, but there is increased dwell at BDC. There is a fixed amount of time the piston has to complete one 360 degree cycle. If you increase dwell anywhere in the cycle, you must necessarily decrease it somewhere else. Shorter rods increase dwell time at BDC.

Mitch

[toad490]

Originally Posted by MitchB

This is incorrect. With a shorter rod there is less dwell at TDC, but there is increased dwell at BDC. There is a fixed amount of time the piston has to complete one 360 degree cycle. If you increase dwell anywhere in the cycle, you must necessarily decrease it somewhere else. Shorter rods increase dwell time at BDC. Mitch

I have enough trouble figuring how rod lenght effects dwell time. Trying to figure out how it could be different at the top than the bottom is beyond me. How can this be possible? You have a reasonably perfect circle (crank) and a piston connected by a pretty solid rod.

The only thing I could come up with, that could make the dwell time different, is rod stretch at the top of the stroke and none at the bottom of the stroke (metal doesn't compress much). If thats what it is, wouldn't the stretch be more in a long rod, increasing dwell time at the top not bottom?

[djswwg]

MitchB, re-read my post, that's exactly what I said!..........djs

[rumblefish360]

Originally Posted by MitchB

No, this is wrong. Excuse my being direct.
Mitch

Wrong, ok, direct, good, excuse you, no, please butt in. Weather wrong or right.

[djswwg]

duh :blast:

[rumblefish360]

No, no duh's. He did address you. I'm trying to find the article on what I said. I think I was right, but will have to prove it.