I'm in the process of gathering parts for a new motor. After melting down two stock L98 motors, its time for something built to last. I was considering the 383 route, but it just started to get too expensive. So now the plan is to build either a .030" or .060" over 350. I have an unmolested 350 4 bolt main roller block to start with. I have a forged steel crank and new SFI balancer and flexplate to go with it. Now for the tough part. All other things being equal, is it worth the extra cash to go with the long rod setup? I'm hoping someone with some real race motor knowledge can jump in with some wisdom here. I plan on running a supercharger on it, so I'm not sure if a long rod motor is the best bet. With the cost for the longer h-beam rods and short compression height pistons being more, is it worth it? Or am I better off just sticking with 5.7" rods?

 

Have a read on this: http://victorylibrary.com/mopar/rod-tech-c.htm

These guys figure the shorter rod is the way to go for supercharging.

But the longer rod makes the intake porting and manifold 'appear' larger to the engine, so if you're restricted with conservative heads & induction the longer rods allow more time for cylinder filling. If you've already got big-flowing heads then the longer rods can potentially make more power, as you probably already know.

 

That site, along with most others, refers to the 1.75 ratio as "the magic number". I can hit that with a 6.125" rod, which is what I've been looking at. But the other problem is the longer dwell time at TDC. With a supercharger, you really don't want that. Thats why I'm so confused right now. There are pros and cons to both setups, and I'm not sure what way to go. Has anyone used a long rod in a 350? Was it noticeably different?

 

If a 383 is "too expensive", then that sort of thing is pie in the sky. Have you checked the price of pistons for exotic combos like that yet?

Any improvement you get over the 5.7" rod length is going to be very slight. The $$$/HP ratio is probably going to be somewhere in the 100-200 range - as oposed to the $20 limit that most of us consider to be acceptable. That sort of thing is for people who must extract every last possible inch-pound of torque and every single RPM from some otherwise limited motor spec. For a minute fraction of the cost of all that, you can buy everything you need to fine-tune your computer, which will give you many times the difference in performance that the rod length ever will.

Spend no more money than you have to on a bottom end. Build it strong enough to withstand what you intend for it to do, and don't over-spend on things that in the long run make no difference. Put your money into heads and induction. That's where the payoff is.

 

That's the problem right there. It won't be a seat of the pants difference like RB83L69 has brought up. You will be spending up to 5 times the money to build a long rod motor with special pistons, rods, machining costs etc.

The 6 1/8" rod leaves very little piston meat for the ring lands too, which is of concern in a blower motor. You could build a 5.7" rod shortblock with forged TRW blower pistons, now that would be reliable and relatively cheap using off the shelf parts.

Of course I still enjoy reading about the differences between short- and long-rod motors but I think in most cases the difference would only show up on a dyno.

I read an editorial in the National Dragster once and the fellow was saying rod length is item #50 when he builds a race motor. The correct cylinder heads and even headers comes before what rod length to use.

 

I don't want to run actual blower pistons. If I was running a roots style blower that would be fine, but not with a centrifical supercharger. 8.5:1 compression all the time would suck. I was running 10 psi peak boost on a bone stock bottom end, and it lasted an entire season. I'm sure keeping 9.5:1 with an all forged bottom end will be just fine. The only reason I was considering the long rod setup is beacause I have an oppurtunity to pick up the pistons for about the same price as off the shelf TRWs. And the longer rods aren't that much more expensive. So for only a couple of hundred bucks extra, I can do the long rod. I just figure I want to do the bottom end right the first time.

 

I'm only a little confused, but I thought that dwell at TDC was a function of stroke length, not rod length.

 

It's a "Flex Rod"

The faster the engine spins the further it travels.

Kinda cool huh?



Sometimes we get caught up in what sounds kewl or fancy, but not what actually produces more power. The 383 isn't really all of that expensive compared to all of the stuff you're sprechen about.

 

The longer rod will also affect piston dwell.

http://www.macdizzy.com/longrod.htm
http://www.type2.com/library/enginem/rodjohn.htm
http://www.stahlheaders.com/Lit_Rod%20Length.htm
http://www.chevymania.com/rod.htm
http://minimopar.net/perf/longrodengine.html

I'm sure there are many more pages of information but those should answer all your questions.

 

I changed the whole game plan now. I'm not going with the long rod because of the increased cylinder pressure. I don't need that with the supercharger. I went with a forged steel crank, 5.7" h-beam rods, and a dished forged piston. Should put me right around 9:1 compression depending on how I deck the block. Anyone know the optimal quench for Trick Flow Twisted Wedge heads?

 

Thanks for the information, but I understand the theory of altering con rod length to affect power transmission between the piston and crankshaft.

Most of those explanations are from fornicating idiots who have no business trying to explain physical principles. A few of them are legitimate, but are still wrought with improper terms and poor explanations. The only one that was any good is the MiniMopar site (which also happens to have the Fram oil filter study). One went so far as to say the longer rod provides a longer lever for producing torque. That's pure male bovine excrement. True "Leverage" is purely the function of stroke length. If they meant to say "Pressure", then they are using the wrong term. Another even stated that a "Long Rod is faster at BDC range and slower at TDC range". What? How the F*** can the same mechanical device at a given speed spend more time at one end of it's arc than at the other end? GruveB is RIGHT! It's one of those "flex-rods" - the kind that import drivers keep in their pants....

(One site was from an exhaust tube manufacturer, one from a Type 2 Volkswaggen site, one from a Yamaha 250 site, so what can we expect?) The explanation at the Chevymania site was particularly lame.

The rod can only affect the power application angle - and therefore the duration of more efficient power application - and only at a rotational point between about 12° after TDC to a point about 12° before BDC (and vice-versa) with the SBC's typical 3.48" stroke. Within those crankshaft degree points, the durational change is so infinitessimally small that is practiaclly incalculable - and certainly no reason to go hunting for 12" rods for "more torque", because that's not what you'll get from them. Longer rods can change the piston dwell anywhere outside those 12° points, or NEAR TDC and BDC, but not at TDC or BDC.

I'll stand by my original reply, but thanks for the recreational links. I would summarize the theory by saying that longer rods can help create a smoother torque outputat the expense of peak torque - especially at higher RPMs where the longer rod/stroke ratios tend to be detrimental.

Smoother torque output from longer rods is generated by a more efficient transmission angle of the rod at peak piston forces - not from "more leverage" as some of those pages tried to contend. The trade-off is less responsiveness to power increases, and poorer transmission of peak piston forces, not by increased mass, but by design.

I know you unnderstand the theory, but the web pages you listed as examples are not that accurate.

 

I examined the geometry of the parts of the rotating assembly when I was starting to model the engine in my avatar. I don't recall rod length being a term in the equation for the position of the piston.

 

Rod length definitely affects piston motion....

Some time when I have nothing better to do, I'll derive the equation to describe the position of the piston w.r.t. the angular displacement of the crank. You'll see that it makes a difference - not a huge difference on the scale that is possible to vary the length-to-stroke ratio in typical auto engines, but a difference just the same.

 

Using a 6.125" rod with the standard 3.48" stroke of the 350 gives more cylinder pressure. The piston travels away from TDC slower, which allows more cylinder pressure to build. At 90* ATDC the piston w/ 6.125" rod will have travelled .015" less than a piston attached to a 5.7" rod. None of this will equate to tons of power by any means, but if you're looking to build the perfect motor then everything matters.

 

Maybe I need to play on the whiteboard for a while because I'm not picturing how the rod length affects the piston speed.

 

Bleh.. nevermind, I think I've got it. Calculus makes my brain hurt.

 

By the way, I get:

d/d&theta; P( &theta; ) = S cos&theta; + ( R<SUP>2</SUP> - S<SUP>2</SUP> cos<SUP>2</SUP>&theta; )<SUP>-0.5</SUP> S<SUP>2</SUP> cos&theta; sin&theta;

 

Well done!!!! That looks about right.

Noitice there's still an R term in there....

 

I cheated and used this:
http://members.tripod.com/racerjoe/c...od_length.html

 

Yeah, I wasn't thinking of it right. It didn't become apparent until I imagined extreme cases but then I still had to do the geometry for fun. Amazing what you can do with a degree in engineering physics, isn't it?

When I got home I checked the source code for the model too. It wasn't immediately obvious the way I wrote it, but the rod length was in there all along.

 

Ouch. Now my brain hurts. And I thought figuring out a density altitude formula was hard.

The pages I posted were just pulled quickly from a search engine. Everyone seems to have their own theory on rod length and I'm sure if you did a search, many more pages could be found. It was just easier than letting everyone post theories here on what works best and why.

There are pros and cons for using each rod. I've mentioned in other posts that I ran a short rod 383 stroker for 2 years. It was cheaper to build (I never built it) and ran high 11's at 117 mph. It only failed because of the stock rod bolts and cast pistons plus had a lot of street miles from 2 previous cars it was in.

Building my new engine I'm trying to reduce the weight of the rotating mass as much as possible within a budget. A lighter crank wasn't in the budget (twice the price of a regular crank) but going to a longer rod allowing for an even lighter than normal race piston was. Since I spin the engine around 6700 rpm, the lighter the rotating mass is, the better.

 

Building a perfect motor also depends on how long you want it to last. I'd say a 6.125 rod is going to dramatically shorten ring life, from the engines I have seen.

Cant always have your cake and eat it too.

Nice formula. I have that on some notes somewhere here, was a homework problem. Excel is much easier

 

It was a slow day at work today so I did it by hand, but Hewlett Packard has this amazing little calculator called the HP49G that does wonders too.

 

This is probably the best, concise response to the question of how rod length affects mechanical forces in an engine and the resultant power.

I'm not arguing the benefits of longer rods. There have been many times in my life I could have used a slightly longer rod, but that's a different matter entirely. I was simply responding to the statement that rod length affects piston dwell at TDC (or BDC - it doesn't matter). I'll stick by my statement.

From a purely mechanical viewpoint, With a longer stroke crank, longer rods are probably a good investment to reduce radial forces on the pistons and increase the efficiency of the longer stroke. Short rods in a long stroke situation tend to promote taper wear and piston skirt scuffing.

I understand the theory and formula for calculating the power transmission angle/force as a function of rod length, and without testing it, the formula looks close. It should prove that the duration of power at intermediate ROD angles (not crankshaft rotation) will be longer, and also result in lower peak power transmission at any point throught that arc. That should help the longer stroke engine survive better overall.

However, by understanding that combustion dynamics is not an exact science, there is a trade-off at higher RPM due to flame front velocity and the rate of conversion from fuel gasses to combustion gasses (i.e., the pressure in the cylinder is not immediately at peak, and doesn't remain at a constant value at any point in the cycle). Through that, one can see that the force presumed in the calculations is not exactly a constant, and needs to be factored as well.

On the opposite side of the cycle, piston speed at the point approaching the compression TDC is also reduced, so the theories about having a dynamic compression "boost" from a large quench area are negatively affected by longer rods, albeit in an equally small amount.

Throw in a few more tangets, factor for combustion speed, consider flame kernel origination point and propagation, torsional flexure of the crankshaft itself, and throw in harmonics, and you'll have a real solution. You'll also realize that one of the heads on every SBC built it upside down...

Personally, on a 3.76" stroke I'd be thinking about using a 6+ inch rod, too.

 

well i guess i will throw in my .02 since it hasn't been brought up yet. one advantage to running longer rods is that it actually makes for a lighter rotating assymbly. providing you are comparing the same types of rods (6" h-beam to a 5.7" h-beam) the 6" h-beam, by itself, will weigh more than a 5.7" h-beam. BUT considering that the piston for the 6" weighs alot less than the coparable piston for the 5.7" rod, the total weight of the piston/rod assembly is considerably less. this means less stress on the crank and bearings especially at high rpm. i heave heard that 6" rods are better for high rpm simply for the fact of the difference in the total weigh of the rotating assembly. i know i spin mine to the moon sometimes

at least that is my $.02. either that, or every machinist i have talked to, every artical i've read and my scales are all BS.