GO 4LO

I read through the archives and found the old bore/stroke threads, but am still confused. If, say, you're already going to have a custom lighter crankshaft made, even if it's the stock dimensions, is it a good idea to increase or decrease the stroke while you're at it, or is it best left stock?
Going purely by piston speed, you don't seem to lose a whole lot of useable upper rpm when you increase the stroke, and a bigger stroke would theoretically help spool bigger turbos faster.
On the other hand, decreasing the stroke would let you turn a little higher rpm, making big hp numbers more achievable, simply due to spinning faster.
What do you guys think?
Chris

HOHN

The main determinant is going to be rod/stroke ratio. You can go to a longer stroke, but you'll need custom rods (longer) and custom pistons (shorter) to do it right. The B5.9 is already on the short end of rod ratio (IIRC, less than 1.6) and you really want it closer to 1.75.

What the B5.9 Rod length?

If you want to make a high peak number, then yes-- destroking is the answer. But what you REALLY want is area under the curve- how much average TQ you make within a certain RPM range. That's where the 5.9 shines- it has a really flat curve within the diesel's rpm range.

I could spend 8 hours easy going over the variable that are affected when you change the bore/stroke ratio and the rod/stroke ratio. If you want me to type up the book, then i will.... But I won't unless asked because I have gone off on some uber-tech babbling before and some folks didn't like it

Justin

Robert Venable

Don't for get that changing the stroke will effect the compression ratio, so if you are going with supper high boost numbers, then destroking could help, but I would think twice about increasing the stroke AND going with those excessively high boost numbers at the same time.

GO 4LO

Stock stroke is 4.72", and rods are 7.65", meaning rod:stroke is 1.62.
I was hoping that by using custom shorter pistons, you'd be able to drop the compression ratio below stock, even with the extra stroke.
My thinking is that if the stroke can be increased by even .25" and just a slight overbore, you'd add about 20 cubes to the engine, and along with a good cam, you'd be able to spool even something along the lines of an HT3B for your little turbo.
And if the new crank, rods, and pistons were also lighter than stock, then stresses might actually be reduced even though everything is moving faster at any given RPM. Thoughts?
Chris

GO 4LO

If you have the time and don't mind doing this for us, I for one would greatly appreciate it.
Chris

GO 4LO

One other thing - I wanted to double-check a couple things about the stock crank that I've heard:
a) Is it really cast and not forged?
b) What does it weigh? (I've heard ~120 lbs)

Thanks.
Chris

HOHN

AFAIK, the stock crank is indeed cast. And it weighs more like 160lbs! Plenty beefy..

Justin

HOHN

Based on your numbers, that puts the Cummins B5.9 slightly worse than a stock Chevy 350 with a rod ratio of 1.63.

So what happens if you stroke a Cummins to an even 5 inches? It's only .03 more than your proposed 1/4" stroke increase?

Displacement-wise, you gain 20 cubes. Rod ratio now falls to 1.53, which is generally considred poor and not recommended (although many Honda engines uses ratios in this range, as do 454 Chevy engines).

So, aside from gaining 20 cubes, what else happens?

First, you will lose thermal efficiency. This is basically how much combustion heat is bled off into the piston, block, heads, and coolant. The most thermally efficient design is a sphere, because it has the lowest ratio of volumke to surface area. In other words, it can contain the most volume for a given amount of surface area. With less surface area exposed, that means less heat bled off though conduction.
Thusk, the closer to a sphere that the cylinder is, the better. That means that (generally) a more square engine is more thermally efficient. But what happens at the bottom of the stroke doesn't matter as much. What matters is the shape of the combustion space at the point of maximum thermal energy. This is pretty close to the time of injection- a little after.

Next, the engine will breathe less efficiently. This is because the shorter rod ratio causes the engine to have greater rates of acceleration and deceleration as the pistons moves from TDC to BDC. Because air has inertia, it resists sudden changes. The slower acceleration rates of longer rods (higher ratio, more specifically) given the air more of a chance to follow the piston's acceleration rate. Of course, the shorter rods scenario also means that the piston dwells longer at BDC, to it gives the cylinder a little more time to fill. Rod ratio, cylinder head flow, and cam timing must be carefully matched. I'll expand on this later.

A longer stroke will also cause greater wear in bearings and the rotating assembly. Lightweight components would mostly offset this, for the rotating part- not for the reciprocating part. This is because you have to remember the kinetic energy equation 1/2Mass*velocity ^2. Since velocity is squared, it plays a MUCh larger role than mass does. So lightweight components will help reduce the bearing load and inertial loads, but the weight isn't nearly as signficant as velocity, which is RPM in this case. So running a longer stroke means you have to reduce operating RPM to keep bearing loads comparable, unless you can go with SIGNIFICANTLY lighter parts. That means lots of $$, so it's easier to just to turn fewer RPMS.

Rod ratio (and therefore, stroke, if keeping the same rods) affects everything in the engine. A shorter ratio engine will make better use of larger cylinder head ports, a larger cam, and more ignition (or injection) timing. At the same time, a shorter ratio engine NEEDS more cam, bigger heads, more timing etc, because it's less efficient. A shorter ratio engine will wear the bores and rings much faster because of the angularity of the rods.

If you built two engines with identical heads, cams, etc, then the shorter ratio engine would have more "bottom end" and a smoother idle (if using a performance cam) and will "come up on the cam" sooner because of the higher intake velocity. But the short ratio engine will also run out of steam much sooner on the top and, and it quickly falls off its efficiency range.

This is one of the reasons that I favor mopar V-8s. While a Chevy 350 has a 1.63 rod ratio, a Mopar 340 has a 1.85 ratio- MUCH higher thanks to it's stock 6.123" rods and short 3.31" stroke. Mopar 383? 1.88 ratio. Mopar 360 1.71 ratio. Mopar 440? 1.8 ratio. 426 Hemi? 1.83 (the hemi had rods that were .1" longer)

You CAN have a ratio that's too high, as then the powerband gets really peak and narrow. It's all in the combination.

I know I left out a lot, so please ask questions to get more info.

Justin

GO 4LO

Thanks for all the info - that's a great read! Would it be possible to move the wristpin up higher on the piston (w/ custom pistons, I mean) to allow for a longer rod to be fit? Would the block then need to be clearanced?
Chris

GO 4LO

Given that it's stock weight is 160 lbs, how much weight do you think could be dropped off a forged or billet crank while still maintaining at least the stock strength?
Chris

HOHN

You CAN move the wristpin up on the pistons, and indeed, most quality stroker combos do just that to allow longer rods to be used with a longer stroke. Shortening the compression height ( the distance from wristpin centerline to piston crown) is a good thing, to a point. It give you lighter pistons and room for longer rods.

Like everything else in enginebuilding, there's a tradeoff. Once you get the compression height below a certain point, the rate of ring wear will increase drastically-- EVEN if you have long rods for low angularity and tight-fitting (hypereutectic) pistons. As a rule of thumb, the longer the stroke, the taller the compression height needs to be to keep piston stability- regardless of how long the rods are.

All this points in the same direction: deck height (or package height). Eventually, the main limiting factor for how long a stroke should be used becomes the deck height of the block. So to keep things in synch, a longer stroke needs a longer rod AND a taller piston- making you pay a threefold penalty to get that mild increase in stroke.

Some manufacturers (most notably Honda) have really embraced the short rod ratio design in certain applications, and have done it successfully. Honda uses rod ratios in the 1.45-1.55 range, yet they don't seem to have premature ring wear! This is mostly due to the superiority of the Honda oiling system- piston jet oilling, ring scrapers, etc.. Honda engines are VERy well-oiled.

For Honda, the payoff of a short rod engine was worthwhile. First, it gave them a small, compact engine. This is important as people want lower hoodlines and more visibility. Second, the increased piston speed helped to offset the loss of intake velocity (and bottom-end torque) that multi-valve heads are sometimes known for. It's interesting that VTEC engines are set to switch over at the exact RPM when piston speed is exceeding the breathing capability of the first cam profile. The VTEC switchover is at different RPM in different engines. It's curious to note the correlation between higher rod ratios and higher RPM VTEC points. The longer rod engines have higher RPM cutoffs because they run out of air later.

Honda also uses longer rod ratios, up to 1.75. Many people feel that 1.75 is the "ideal" rod ratio, so you can be sure that this is no accident. I've heard of test being run that showed that rod ratios higher than 1.8 only made the powerband narrower and made the longer rods heavier. Since mass (more specifically, reducing mass) is ALWAYS a consideration, this is why rod ratios over 1.8 aren't often used anymore in any form of racing or OEM. On the more performance-oriented honda engines, you'll see the longer rods== Integra Type R, RSX type S, and some od the Del Sol engines. The short rod engines are at home in Accord and Civics.

There's no doubt in my mind that a talented crankshaft specialist (like Winberg) could produce a B5.9 crank of equal strength that weighed WELL under 100 pounds. Through judicious rifle-drilling, knife-edging, and bull-nosing, you can remove a lot of metal that doesn't have a big impact on strength. Account, too, for the incredible advances in metallurgy. A billet 4340 crank could be made that would equal the stock crank's strengh at a MUCH lighter weight.

I'm no expert, but since Winberg has a 426 Hemi crank that weighs 40 lbs (vs stock 70 lbs), I would imagine they could also achieve a near 50% weight reduction on the 5.9

if you're serious about reducing weight on the crank, you're going to need custom rods with a MUCH smaller big end. This will allow you to lose a lot of weight by turning down the rod journals.

You want to see an EXTREMELY lightened crank? You should see the cranks Winberg does for the NASCAR qualifying engines. The counterweigts look like blades!

Justin

HOHN

I forgot-- I'd consider a .25 increase in stroke enough to warrant clearancing the block, but relative to an already huge 4.72" stroke, it might not be needed. There's only one way to know for sure.....

signature600

Anyone else glad that "good enough" gains can be had for most of us without even thinking about this stuff.

And I thought computer lingo was bad, jeesh .

All joking aside, this is a great thread and I'm glad that I get to parrticipate ( If you call this post participating )

Keep it up, Guys

Chris

GO 4LO

Hohn, what do you think about Crower and Scat billet cranks? Do you have a web address for Winberg?
It seems that with a lighter billet crank, lighter rods and pistons, lighter flywheel, lighter driveshafts, lighter wheels and tires, we could drop 300 lbs of rotating weight off of one of these trucks. But no one has done it yet? I'm hoping it's due to cost and not just because it's a stupid idea .
Chris

Mcmopar

Justin (HOHN),

I am very impressed with your "lesson" on strokers!!
What do you do, or better yet where did you get all of your knowledge? I have built a few Mopar strokers (496s) working on a 572 mabye 604 but it has been a lot of reading and getting both good and not so good info from some "self proclaimed" experts.

Nice thread!!