Hot Rods So, let's talk reciprocating weight on a crankshaft rotating assembly

Let's say you have a typical SBC being assembled for a lightweight drag car.
Would it be better to have the lightest weight rotating assembly you can afford or would it be better to spend money elsewhere?

I don't know that lightening up will give an added advantage for HP, although response time might be better. For instance in a dirt track car....

What do you think? A crank/rods/pistons built out of unaffordium or spend the cash on a CNC port job?

Squirrel, you have any thoughts?

 

When I building race engines years ago we did all we could to lighten the rotating mass. Lighten the flywheel, find the lightest rods we could, use lighter crankshafts from earlier model engines, forged pistons with lightened wrist pins, chrome moly push rods, lightened lifter......you name it. If we could reduce weight we did. We also had the most DYNO proven horsepower in our limited class and won a crap ton of races.

But if you don't have a cheap option to obtain those items...…….money is on the intake and exhaust.

 

I just wonder if the cost associated with titanium this or that pays divedends? Know what I mean?

The cost difference between a lightened crank and a standard weight crank may be close to a grand.

 

It takes energy to spin up the rotating assembly. That energy is not being used to accelerate the car.

What is benefit, on a dollar per second ET basis? that's a tough thing to calculate...

 

For a drag car, the launch is critical, and a lightweight rotating assembly isn't generally what you want. That mass stores energy between firing pulses. Heavy car and/or small motor combos generally use heavier than stock flywheels because of this. As with everything, there's trade-offs involved. Roundy-round or road racing is different, there throttle response off corners is important.

 

Road race and roundly pounders typically want everything super light weight, faster accel and decel (while moving) most run a flexplate and mini-clutch not flywheel(when allowed) Drag you would want heavier rotating, light reciprocating (properly balanced of course). As stated above your money would best be spent on air flow, (heads, cam, ect.)


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We had to use stock rods but we were a rebuilder...we weighed each one for basic balance. The lighter ones were set aside and then balanced end to end. We did the same with the pistons too.

Is there another forged crank that is lighter and will fit your block? Another year of engine? I guess its thinking like that is what set us apart.

 

One consideration is that more rotating mass will have higher forces at a given rpm. Or put another way, if you want to turn higher rpms, lighter internal parts are better. Assuming you have the cam and head capability, higher rpms will typically give more power. Lower internal forces just seem better for long term reliability.

It is a good question though as to where the most cost effective money is spent.

 

A tangentially related issue to think about, especially on high RPM engines...
I saw a GM study on connecting ros stress...probably via Smokey Yunick discussions back when Circle Track magazine was a hotbed of engine tech discussions. The tests had been done as part of the Z28 302 development, I think. The revelation was that power related stress was much lower than the stress on rods going up without resistance...the exhaust stroke!
That ties in well with Sm0key's thoughts on rod failure caused by locked in bearings.
I suspect this area of weakness probably applies to all or most "normal" auto engines, meaning to me not Formula One, blown nitro dragsters, etc.

 

I have always wondered. Dad was drag racing back when things were just evolving. He ran a 265 Chevy with a 38 lb Scheiffer (SP?) flywheel to help the mass keep rolling. I would think on a drag car engine more mass might be beneficial? You have time to spin it up vs a oval track car that is on and off the throttle.

 

Had a friend relate this to me years go.

Years before bracket racing existed. Joe was always losing to the same guy at the drags. Joe had the latest and greatest but the other guy seemed to be a step ahead. Joe was standing there talking to the dude one day and the guy ended their conversation with "wrong cam" and walked away. Joe did a lot of cam research (before internet) and came across what he thought the other dude was running. Go on and put it in. He ran the best ever but still lost. That night as they were talking the dude ended their conversation with "wrong gear". Joe researched gears and found one he didn't know existed between 2 he already had. In went the new gears. Outcome the same. Last race of the season Joe looses to the other dude again. After the races dude ends the conversation with "wrong flywheel". Joe was running what all the suppliers were pushing (aluminum with steel insert). During the off season Joe stopped by the other dude's shop to get some machine work done. There laid "the flywheel". A 70# steel flywheel.

For drag, lighten the rods and pistons. The crank doesn't make much difference so get a sturdy one. Flywheel weight is good.

In circle track we lightened everything that bolted to the crank. Everything. 38# transmissions with internal clutches, lightened rods, cranks, etc.

JMO
SPark

 

On a drag motor, the two places less weight is beneficial is the rods and pistons. Pistons are easy; lightweight forgings are almost standard these days, even the factories are using them. Rods are another story. Any sort of 'standard' I-beam rod isn't going to show much weight reduction, you have to step up H-beams to get even a small savings, and beyond that you're into exotic metals.

 

In the 60's, the materials they used for pistons and the piston design was extremely heavy. The idea that a short stroke engine revved better was a fallacy. The truth was, the heavy pistons couldn't handle the piston speeds of a longer stroke. As far as other components being lighter... the benefits of lightening engine parts lie within the parts themselves. For instance, rods... and pistons... are "bob weight" being swung by the crank. Lighten these and the engine will be able to last longer at higher speeds, and be able to "rev quicker" to a certain degree... but nothing really measureable, imho.
Crankshafts are very similar... but because they are so heavy, measureable gains can be found in lightening crankshafts concerning reliability at high RPM... and to a little more degree than the pistons/rods... the acceleration, or revability (I just invented a word) will be better. But you also have to consider the materials and the design, and the torsion of the crank... lots to consider that is way over my head and pocket book.
Flywheels... I can remember talking with my engine building mentor... a gentleman who was good friends and industry partners with the likes of Byron Blair, Bruce Crower, Leonard Abbot etc... telling me that there were two different schools of thought in drag racing on light flywheels. One, was that a heavy flywheel will help with the inertia needed to get the car off of the line... and other being that once the car hooks, and starts accelerating, all of the extra weight is hurting acceleration. He tended to like the later.
Valve train components... this is where EVERY engine that spins over 6,000 rpm needs lightened components. The higher you go, the lighter they need to be.
The thing I think about is how the philosophies have changed over the years. Because horsepower increases with RPM, twisting an engine higher increases horsepower. It was common for short stroke small blocks to turn 10,000 rpm plus. However, as the volumetric efficiency improved with different heads, pistons and camshafts (and other parts, but mostly these three) there was no longer a need to rev engines so high.
Also, another technological advancement happened... the 400 small block Chevy. With this engine came the almighty 383 SBC. Also, a few years after 383s were being built with stock parts... then came cheaper custom pistons that allowed the use of the longer 5.7" and then 6" rod. After that came cheaper stroker cranks, and then finally... aftermarket blocks that would allow the little SBC to grow.
The other thing that paralleled the technology of aftermarket engine design growth... was the death blow to class racing in drag racing and limits in cubic inches in local stock car racing. I've heard guys say that drag racing died with the advent of bracket racing. But this couldn't be the furthest thing from the truth. What was happening was that drag racers were finding out, very quickly, that in class racing where car classes are based on cubic inches and weight... the cost to win was astronomical. Racing against the clock, afforded the racer to go as fast as his budget allowed. Win, win... win.
In a nut shell, it's all about challenging yourself. Everyone is made up of time, money, knowledge and drive. You may be driven to challenge yourself by trying to go as FAST as you can with a certain amount of money, certain parts... a certain type of genre of hot rod... or maybe a certain type of engine...

The good news is, however you choose to challenge yourself to go fast... you can buy the parts, build the motor and car... and go racing!

Sam

 

You wrote this while I was typing... good stuff!
Sam

 

You're all wrong: Years ago, think it was Popular Hot Rodding, did a comparison between a '40 Chevy coup and a slope-nosed Vega, same engine went several tenths faster in the Vega. Forget about motor, find a sleeker body!

 

If there are class restrictions on tires and you are blowing off the tires less flywheel is probally a good thing.

 

Sleeker than a model a with a flat windshield and cowl? Nah.....

 

Running a power glide with a 5500 stall converter, switched from a std s.f.i. flex plate to a 4 lb heavier flex plate called (The Wheel) for bracket racing. With no other changes 60 ft time improved but even better is the car became more consistent. Often thought about drilling a heavy s.f.i. flywheel for the converter bolt pattern.
Somethings gonna happen when you let you finger off the trans brake button @ 5500 rpms.

.

 

Thats the exact setup I have, other than the heavy flywheel.

 

Actually it makes it worse! Back in the day we drag raced a KZ 900. Then later they came out with the KZ1000 which had the same stroke, but a heavier crank.

We discovered this by accident when we needed a crank and had to use a KZ 1000 crank one time. The bike normally ran low 10's. After we put the KZ 1000 crank in it, it quit burning the tire off and ran high 9's with the heavier crank. This was in the days when tires were limited and chasing the HP game.

Our synopsis was the heavy crank with more inertia, slowed the tire spin and caused it to hooked better. It also didn't spin in 2nd gear either. We were very happy to discover this!

 

Are you thinking about building another engine?

 

Sam, when I built the Fly years ago, I was on a shoestring budget. Lots of things I wished I would have done different. I have picked up some parts over the years. The Fly is in pretty sad shape. It's up on blocks and the Injectors are off.

I would like to get back on it. Dad isn't getting any younger and he really loves to go to the races. The car was built to get our family through a dark time after my sister was killed and give him a focal point. We have spent a lot of good time wrenching on it. Memories are what it is all about.

 

The flywheel is Rotating weight, not reciprocating weight.
The heavier flywheel stores kinetic energy ,which helps with the launch.
Too heavy and something gives , and breaking traction is cheaper than breaking driveline components.[you can tune the launch with flywheel mass]

With reciprocating weight, the combustion energy must push the piston down. and the rotating crank must push the rod back up.
A heavy reciprocating weight will require more energy to push the piston down, leaving less surplus energy at the crankshaft.
And the crankshaft also requires more energy to push the rod back up.
The net result is less output.

With oval cars, [eg: sprints and old VW era midgets] they sometimes removed the flywheel, because the mass off the moving vehicle is the flywheel.

 

Good explanation.

 

Two scenarios could be possible here.
1: the later crank had most of the mass closer to the centreline [low inertia]
or
2: the riders skills could take advantage of the heavier crank and feed the clutch better

I would place my money on the Rider

 

The early, lighter crank has a "pork chop" shape counterweight and the later, heavier crank was a full circle. I do know the heavier crank was much smoother out of the gate and the lighter crank reved too quickly, taking the tire with it. Sometimes it would even bog if you didn't feather the clutch just right.

WHERE'S FALCON GEORGE WHEN YOU NEED HIM?!

 

Speaking of light weight.....A current day Formula One engine stops turning in less than a 1/10 of a second after being turned off. It ides at 4000RPM

 

It's the starting and stopping that's the killer a 4 inch stroke motor at 6000 rpm you have a piston moving 66.7 feet per second changing direction 200 times per second.

 

BTW that technique works the same way in fishing. When I was still guiding, the answers I used to give were equally cryptic.
In the mid sixties, I had an O/T sports car, where all the moving parts, except for the crank, were lightened, and the flywheel was shaved. There was an easily observable improvement in performance.
Bob

 

QUOTE="Roothawg, post: 12846156, member: 160"]Thats the exact setup I have, other than the heavy flywheel.[/QUOTE]
Also the heavy flywheel helps keep the rpm up during the power glide 1-2 shift