Rod ratio -stroker cranks

[bn69stang]

At the horsepower level i would get from a 463 inch stroker motor , non power adder with a target of 530-540 hp im sure the scat cast crank would be fine , its well below the rpm / hp levels its been pushed to in other builds . And thats 1.16 hp per cubic inch and is certainly a do able package ..

[blykins]

I was speaking in generality, didn't take time to add it up.  It would come down to how much you charge for your stuff and how much I charge for my stuff.  I get a decent break on rpm cranks so even with machine work that they need, I coukd still sell cheaper than what the forged Scats cost.  As for diamond, MWD here, and a change to compression height, volume, etc is just around $40.  Rods are the same price, rings & bearings the same.

You're not really disagreeing with me, nor am I with you, but I don't think the difference is all that big.

[jayb]

One of the points that hasn't come up here is longevity.  A longer rod will load the sides of the cylinder bores less when the engine is running, because of a shallower angle between the rod and the cylinder bore.  Basically this leads to less wear on the bore and a longer lasting engine.  Not a big deal unless you are building 100K mile engines, but something to consider nonetheless. 

I agree with Joe that the longest rod possible should be used in any performance engine, but I also agree with Bob that which rod length to use is pretty far down the list of important decisions.  Given the choice between a smaller stroke and longer rod, or longer stroker and shorter rod (to get more cubic inches), I'd pick the longer stroke every time.

What we really need is a tall deck block with a raised cam, so that we can easily go to a 4.6" stroke and 7.500" rod, for a 559" engine with a 4.4" bore...

[bn69stang]

Jay you need to produce and sell that block  lol , we need more options and new parts and cubic inches are on all our lists .. Bud

[machoneman]

Here's what Darin Morgan and Buddy Reher had to say a few years ago on Speedtalk.com:

rod ratio, who cares??

Postby Darin Morgan » Thu Oct 28, 2004 7:17 pm

shawn wrote:I'm sure Darin Morgan will comment on this.They have a lot of experience wth testing different rod lengths. Here's a quote from Reher-Morrisons site from David Reher and his thoughts on the subject-

" 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."
This is located in the tech article "by the book" if anyone else wants to read the whole thing. Hope Darin does comment on this, he has some deeper thoughts on this with a GM backed test to back them up.
Shawn


DITO shawn.
You pretty much summed it up.
People put WAY to much importance on this preconceived " ideal rod ratio" idea. Rod ratio is not a primary consideration when designing an engine. You get the deck as short as possible so the piston does not come out of the bore. That gives you better manifolding which will make ten times the power any " ideal rod ratio" would net you. You shorten the pushrod and make the valve train stable above 9000rpm. You make the piston ring package as compact as possible to get the pin as high as possible and that will make for a light weight, balanced (not top heavy) piston design, THEN you decide what rod connects the piston to the crank. Its not magic, its simple mechanics. People look at what Smokey yunick said and they take it out of context in my opinion. He said you should put the longest rod YOU CAN not the longest rod YOU CAN CRAM JAM OR MANIPULATE into the engine. I see people all the time screw up the engine combination to facilitate some preconceived ideal rod ratio and they wonder why the thing wont turn up and make power. The difference in the GM 358 NASCAR test engine from 5.250 inch long rods to 6.1 inch long rods was maybe 2ft/lbs and 2 HP. Not much to worry about. That satisfied the GM engineers that there is nothing there. Does a short rod make more TQ? Does a long rod make more top end power? It probably does but its such an insignificant amount, its not even worth messing with! If there was a major advantage or power gain in this, it would have been proven a long time ago and we could all put this to rest but no one has. I wonder why??

Darin Morgan
R&D-Cylinder Head Dept.
Reher-Morrison Racing Engines
1120 Enterprise Place
Arlington Texas 76001
817-467-7171
FAX-468-3147

 Visit our web site at
http://www.rehermorrison.com

[jayb]

That's an interesting post, Bob.  Do you know what they mean by a shorter deck improving the alignment between the cylinder head ports and the intake manifold?  I've got to believe that they are talking about runner length of the port and manifold runner, and how it relates to the plenum.  Higher engine speeds will require shorter runner lengths, and a tall deck would mean that the plenum of a sheet metal intake would be pretty wide.  So maybe the turn of the air/fuel charge would have to go horizontally across the plenum for a ways before it turned down the runner.  Am I on the right track?

[Joe-jdc]

By shortening the deck you effectively change the angle of entry from intake to head.  It is a small difference, but similiar to angle milling in effect.  That is why the 8.900"-9.000" decked blocks make great power with the 9.200" intake manifolds that are machined to fit them.  You straighten out the airflow---seems counterintuitive, but it works.  Joe-JDC

[machoneman]

Your analysis Jay of what they said is exactly right.

I think of it as an upside-down letter "Y" and we are talking custom tunnel ram intakes with carbs here. Giving a straighter shot from the plenum's C/L to the head's intake ports C/L by pulling the heads in closer to one another via a short deck block does increase HP due to maximization of flow. Other benefits include shorter, stiffer pushrods and apparently less drop-out of atomized gas from the now straighter air/fuel charge.

Today, all NHRA legal (500 CID max) Pro Stock engines have really short decks as Joe mentioned.  In match racing such as the IHRA where folks like Jon Kaase's customers have crazy 800 CID+ Boss 429-based Ford engines, the extra tall deck heights do work to negate that straight shot.

This link provide a pic and some details of the GM block that, as supplied, is about 9.5" yet can be machined down to a mere 9.00", shorter than even a 351 Cleveland block at 9.2"! 

http://hotrodenginetech.com/inside-warren-johnsons-pro-stock-engine/

[MeanGene]

I don't focus on rod length more than, as Joe put it, the longest (readily available, reasonably priced, and strong enough) possible, without making a crusade out of it. To put it very simply, a longer rod (which is closer to parallel to the cylinder all through the cycle) transfers energy to the crank, with a little energy pushing out against the cylinder walls due to the leverage. A shorter rod, with greater angularity, pushes against the walls just a bit more- very little, but a bit- so a (very) miniscule amount of power loss and additional wear and tear is just logical- but it would have to get to a pretty severe angle to be worth worrying about, and in the average range of rod ratios, probably not enough to notice or measure. So I go with the longest readily available and proper strength part that will fit, and don't spin about it any more. The 4.25/ 6.7 is a well proven combo that works pretty well (as is the 3.78 or 3.98/ 6.49), and the price is right- move on to @.050's and LSA's where you can REALLY fuss and spin LOL
I see examples of this every day at work, where we use large excavators, CAT 345's and 330's, and most everyone prefers a "long stick"- no pun intended- which is the vertical part that hangs from the boom- the connecting rod between the boom and bucket, as you can just reach farther and do more work with it. A "short stick" will "crowd" stronger horizontally, as in digging and loading the bucket, with more severe angularity (leverage)- like pushing against a cylinder wall. A long stick loses a small amount of horizontal strength, or crowd- which is good in an excavator, but wasted energy in an engine- as in pushing against the cylinder walls. Funny thing, the long stick and short stick lift the same amount of weight vertically- one can nit pick the slight difference in weight of the long stick (or connecting rod) but the total lifting work produced is still the same

[bn69stang]

Thanks Gene  lol , @ 50 and  L S A is will be down the line , and i agree that the 4.25/ 6.7 combo is proven and until i get my hands on a 427 block sometime down the line we will just build probably a 463 , and get on with the rest of the project .. Thanks again to all for the input , info and feedback .. Bud

[Mike Caruso]

As Kasse say's just give me a rod to connect the crank to the piston.

I love that one.

Mike Caruso
Do It Like You Mean It .....or Don't Bother!
www.AERA.org
Engine Professional Magazine Free To All