Turned down crank journals 1

[turbododge]

On another board there is an ongoing discussion about extra power and response being gained by using undersized crankpins and rod bearings(.020"). Everyone agrees that the smaller diameter will very slightly reduce bearing speed and possibly reduce friction or increase rpm range for the engine.

What is being batted back and forth by a few is that the undersized crank and bearing set will reduce the inetia and make the engine respond quicker to the throttle, due to the fact the crank weighs less with the undersized journals. The other faction says that as long as you use the same rods, the removed crank material is replaced by added bearing thickness with the same center of mass, so there is no net change.

Anybody got any good reference material that I could send these guys to? They don't seem to want to pick up a physics book. They are convincing folks that they should be reducing the crankpin size (but use the stock rods) for more response, which seems pretty silly, unless you are doing it to get to a smaller, lighter, bearing and rod size, like a Honda rod in an American V8.

 

[Metalguy]

Tell them that going on diets will make their cars faster than by uselessly trying to lighten the rod big-ends that way.

 

[ShaunT]

Way to go METALGUY!!! SOme folks are so trick hey out trick themselves. It is pretty easy to spot the ignorant ones. They are the ones who will try anything. Why don't they play with a cam and maybe a head or two? I guess you could get a really good education about cranks by turning to Callies Crankshafts.

Shaun TiedeULTRADYNE/LUNATI Austin,TX(stiede@ev1.net)

 

[enginesrus]

If you want light weight there are better ways to accomplish it. In some instances grinding off that much material will just remove the case hardening on the journal. It also is a waste of crankshaft. That is if you ever NEED to grind it then you have already lost the material to do such. And as you go smaller it gets weaker.

 

[patprimmer]

The NASCAR guys run 1.88" Honda journals, for gains from reduced peripheral speed of the bearing, and reduces losses from displacing oil from the bearing (smaller bearing, same clearance, less oil pushed around as the rod bearing rotates in a concentric manner around the big end journal).

There will only be a mass reduction if a rod specifically designed for the smaller journal is used.

A gram or two, 1.5 to 2 inches from the crank centreline will have an effect on inertia that is so small, it might be calculated, but will never be measured with anything less than the resources of NASA. It will be right up there with the gains from the weight loss if the driver breaks wind before entering the car.

A few grams of a piston or the little end is another story, as it is reciprocating rather than rotating mass, but really I'm with metalguy. Send the driver to the Gym for a much more significant result

Regards
pat

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[Chumley]

On engines stressed to their limits, I would be more concerned with the increased crankshaft flexure caused by the loss in moment of inertia due to the smaller crankshaft journal diameters. On stock factory engines, it's probably not a big deal either way.

At higher loads and RPM's, crankshaft flexure is a huge waste of power.

Chumley

 

[patprimmer]

Chumley

The other concern re flex and cracks is the reduced journal overlap with smaller journals.

The flex and fatigue cracks can be suppressed significantly by crank arm and counterweight design, as well as by extra large radii on the journal to arm junction, or at least that is what the only FEA I have seen on the subject indicates.

These guys are chasing response as well as power, and the response can be improved by:-
1) Increasing power in the rpm range being used.
2) Reducing inertia in the driveline.

In my experience, when these are in conflict, the best compromise is mostly found by the suck it and see method.

I expect that the crank could also be made more rigid in torsion by increasing the main journal dia. I would speculate that the main journal would remain more central in the bearing than a big end does, and therefore not absorb quite as much power at the same periferal speed as a big end, so a net power gain might be had by increasing the main and decreasing the big end.

Huge (5mm?) radii might further increase durability and power, by increasing rigidity for minimal weight gain

Regards
pat

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[Chumley]

Hi Pat:

I was thinking more about the load path, say from the #1
crank journal through the crank all the way back to the
flywheel. As it transfers from journal to journal its
transferred as shear accross all the rearward rod journals.
The less meat on the journals there is, the more they are
going to deflect under dynamic loads especially with
designs that have two rods mounted on each rod journal.
(granted, very small deflections, but significant since
they would be reversing 50 times per second on a 4 stroke
engine @ 6000 rpm)

I suspect that's one reason why most crank bearings, both
rod and main journals limit their undersizes to about .030"
for stock applications. Any wear reducing surface hardening
treatment would surely be compromised as material is
removed also.


Chumley

 

[unclesyd]

We used to bore and stroke engines with no problem. I don't remember the numbers but it was considerably more than what you are talking about. You can have the crank journals renitrided for not much money. Using high dome pistons we sometimes blew the spark plugs out.

Here is a website that might help, at least their software.

http://www.auto-ware.com/index.htm

 

[patprimmer]

Chumley

So you are thinking of losses due to hysteresis in the steel.

Do you have any feeling for how great this is compared to the shear by the journal acting as the rotor in an eccentric pump and pumping oil around the journal. I see these 2 in conflict and have always considered the hydraulic action to be the greater.

I have absolutely no evidence to support this, but I am interested as I am about to specify journal size on a new crank I am having made for a drag race car I am currently doing an engine for.

I am probably already locked in, as I have specified the longest stroke that will fit with a 2.00" journal. and not interfere with the cam lobes nor the crankcase.

Also it is a SB Chev and is supercharged, and should make about 1600 HP, so durability is more an issue than marginal power gains.

I went for the maximum possible size on the mains.

I went for a big end size that would allow the stroke I wanted without removing metal from rods or block.

I tried to retain a reasonable journal overlap for strength

I have asked the crank grinder if he can give me bigger radii.

I have asked him if he can roll finish the radii.

I guess a narrow journal would help with hysteresis and hydraulic losses, but bearing area might become dangerously low.

Regards
pat

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[Chumley]

unclesyd: Yeah, thanks for that, but:

I know how cranks are stroked, BTDT. Nitriding is done in the first place because it works well and because of the relative low cost, so that's not a problem to redo.

I'm thinking more in terms of building high time engines
with minimal vibration and very high reliability factors
rather than racing applications.

When engines are expected to last for several thousand hours at 75% rated power, cycles to failure become much more important than HP per cubic inch and winning a race.

Those turbo piston powered twin engine airplane pilots who fly along for hours at 20,000 feet MSL in the clouds at 325 mph with zero visibility tend to get a little nervous about tweaky engines because they can't just push in the clutch then pull over and stop when/if a crank lets go.

I won't mention all the discussion between the passengers other than to say, "Hey Captain, isn't that prop out there supposed to be spinning? What are you going to do about that and why are we descending so fast? We can't be anywhere near New York yet and I've got a stockholders meeting in two hours."

Chumley

 

[Chumley]

Sorry Pat, I didn't see your post.

Hysteresis? Nahh. Non elastic deformation and stress reversals.

According to Corky Bell and much more significant others, in most V and I type engines the highest deflection (power wasting) loads are unopposed and inertial because of the basic engine configuration, and based on the piston and rod weights, not the power generated.

You've blown up a few VW engines. (who hasn't) Look at the crankshaft design of a horizontally opposed or radial piston engine. Minimal counter weights! Were, Pratt and Whitney, Wright, Continental, Lycoming and Porsche wrong? I don't think so.

I and V type engines suffer. They sacrifice weight, and the engineered optimum in favor of cost and dimensional desirability as though they should in automotive applications. Make everything as strong and as stiff as possible using minimal material. That's the engineering challenge!

Best wishes!

Chumley

 

[mAJORD]

you forgot subaru! , ok they got the original EA design off porsche/vw , but EJs are a differnt story