the lark

The old GM 3.8 motors, like the ones in the grand nationals, had lots of vibration problems. They used a balance shaft above the cam. It sits in the valley under the intake manifold. It helped a lot. Maybe GM did this with the 5.5 motor.
Honda also used a balance shafts.

OnPoint

You are correct. No FPC crank in V6s - my bad. FPC cranks in 4 cylinders. And bicycles (lol). And in marketing departments bent on seducing folks into believing one must have a FPC to have a high/free revving V8.

Rkreigh

grand nationals and turbo Ts with the turbo v6 engine did not use a balance shaft. They had a standard flat tappet hyd cam and solid cast damper too. V6s do have some vibration issues, but not bad

the lark

Does the 911 gt3 rs use a fpc. Because that thing revs to 9,500.

fzust

I believe since it is a boxer engine(horizontally opposed), by definition it is a flat plane crank. To be fair, GM could do a cross-plane with DOHC and get far more revs than they do now. In all honesty I am unsure about the weight and Cg tradeoff. The LS7 was a very light engine for what it was. Mercury Marine took that engine and put DOHC on it and it does 750HP vs 505HP stock on pump gas.

Later motors like LT1 got heavier because of cylinder deactivation etc.

DOHC will add weight up high and take up more space, but free efficiency with 4 Valves / cylinder.

The Ford Mustang BOSS had 7500RPM redline with DOHC and cross-plane. The FPC GT350 is 8250rpm. I think 7500rpm in DOHC corvette would be fine, 8000rpm for bragging rights. Of course 8000rpm FPC would sound SO GOOD!

vndkshn

So really all they did is move the front mounted dampener and move it to the back... semantics IMHO, not sure why you have to come across argumentative about it.

MitchAlsup

Porsche uses a "horizontally opposed" crank shaft.

MitchAlsup

Shaka argued that the vibration damper is "mandatory on the engine".
I agree it is mandatory, but can be placed other than "on the engine"

vndkshn

Again... semantics.... instead of on the front, it is on the back.... Arguing for the sake of arguing.

MitchAlsup

It is more than 1 foot away from the back of the engine.

RJ-92

I would think that's probable. If they are going DOHC, They would likely put a balance shaft in the V where a cam would reside in an OHV motor.

The GM 3.8 v6 was a 90* configuration, like the 4.3 it was basically a V8 with 2 cylinders lopped off. 60* v6's are much smoother.

Ragtop 99

I was under the impression that the LT2 was rev limited by the DOD/AFM system reliability. Is GM just going to punt on any efforts to maintain MPG or is a DOD/AFM system less rev sensitive on a DOHC motor?

auburn2

Correct. None of the GN sor TTypes as far as I know used a balance shaft,and I spent a lot of time working on them as a teen in the late 80s.

Later in the 90s or early 00s the FWD 3800 "Series 2" engines in cars like the Bonneville and Lesabre did get a balance shaft.

Some more trivia - the original Buick 3.8L from the late 60s was an oddfire engine. It basically used a V8 timing scheme with 2 cylinders missing. The engine fired every 90 degrees and "skipped" one firing every rotation. It was extremely rough, literally ran like a V8 that you unplugged 2 sparkplugs. Some time around 1976 they changed to an evenfire system. Since it was a 90-degree V6 they had to use a split pin crank to do that (opposing cylinders are on different journals). Then it fired once every 120 degrees which smoothed it out a lot. I believe every Buick V6 from the ~78 until they stopped building it around 2010 used a split pin crank. I know all the GNs did.

Rinaldo Catria

My 2013 AUDI RS5 4.2L nat asp V8 makes 450 hp @8250 rpm with a cross plane.. good thing it has a DCT behind it because it revs so quick I doubt Id keep up with it with a manual. It sounds wonderful. No vibration either. Flat planes are not mandatory for high rpm or good exhaust note IMO.

vndkshn

Point? It is likely still there to dampen engine vibrations. Go run one without it at high RPM and report back, we'll wait.

Again, you are arguing for the sake of arguing.

Shaka

Can you spot all the trouble that Ferrari has gone thru in order to dampen vibration in their 488 Pista in the video below? Ford has spent about the same amount on their FPC engine as Ferrari has done, using some of the same technology. The Ferrari has a 3.9 liter displacement. We don't know what materials have been used.
There are two major issues. Primary shake or vibration: Some say that there is no need for counter weights on this crankshaft. Consider this: The front two pistons are going down, the rear two are going up. See 1:50 in vid below. Counter Weights are for bending in the crankshaft in this case and not for balance. No one mentions this. They just ignore it. This is not an event that takes place in a 90' V8 crankshaft.
Now, secondry shakes, which is a very destructive force, can only be insulated from the cab and and can't be addressed on the engine itself other than very light rods and pistons and a very short stroke. See .31, .39, 1:40, 1:50, and 3:39.


The theoretical advantages of a flat-plane crank don’t always pan out in the real world. The top race teams in the country already experimented with 180-degree flat-plane cranks many years ago in every form or racing ranging from NASCAR Sprint Cup to NHRA Pro Stock, Top Fuel, Funny Car, and Comp Eliminator. Despite the fact that these V-8 engines turn between 9,000 rpm on the low side (as in Sprint Cup) and 11,000 rpm on the high side (as in Pro Stock), ultimately, any theoretical gains in performance were more than offset by the increase in highly detrimental secondary engine vibrations inherent to the flat-plane crankshaft design. In the upper echelons of racing where cost is no object, an edge as small as two horsepower over the competition is considered a big deal. Even so, at the end of the day, the top race teams in the country stuck with their 90-degree cross-plane cranks and never looked back.

The most significant downside of a flat-plane crank is that they generate some very severe and potentially destructive secondary vibrations(shakes). By definition, these vibrations are produced twice per engine revolution opposed to a primary vibrations that occurs just once per revolution.

Imagine drawing a line at the half-way point of travel as the pistons move from TDC to BDC. Since the wrist pin is positioned slightly below the piston crown, when the crank pin rotates downward to half the length of the stroke, the piston actually travels a distance greater than half the length of the stroke. As a result, the piston accelerates away from TDC toward the halfway point more quickly than it accelerates from the halfway point toward BDC. The same applies as the piston reverses direction back up the bore. The piston’s rate of acceleration increases once it passes the halfway point on its way back up toward TDC. This disparity in piston acceleration creates an upward vibration that occurs twice per crankshaft revolution. Not even balance shafts can completely cancel out these vibrations.

In contrast, since each of the four crankpins in a cross-plane V-8 are phased 90 degrees apart, there are always pairs of pistons moving through different phases of the crankshaft rotation cycle. As the first crank pin (from the front) rotates downward from TDC to 90 degrees after TDC, the third crank pin travels from 90 degrees before TDC to TDC. Likewise, as the second crank pin rotates downward from 90 degrees after TDC to BDC, the fourth crank pin travels upward from BDC to 90 degrees before TDC. Consequently, the fast downward movement of the first crank pin cancels out the fast upward movement of the third crank pin, and the slow downward movement of the second crank pin cancels out the slow upward movement of the fourth crank pin. This effectively cancels out the secondary forces. This excellent secondary balance is why Cadillac invented the cross-plane crank in the first place in the early 1900s. Prior to that, flat-plane cranks were the norm not because of any performance advantages, but simply because they were easier to manufacture.
Only problem is that there are two power stroke on one or the other side of the engine in direct sequence. Heck, one time there were small block Chevys that had one spark plug with a different heat range to the other 7. But I digress.

The secondary vibration inherent to a flat-plane crank isn’t something that should be taken lightly. Careful what you wish for. Many blog entries and articles have suggested that the lighter pistons and rods used in flat-plane crank motors reduce secondary vibrations enough to where they’re no longer a concern. It’s an interesting theory, but that’s not how things pan out in real-world testing. According to Ford Group Vice President of Global Product Development Raj Nair, Ford considered scrapping the flat-plane crank concept entirely due to the vibration issues experienced by early 5.2L prototype engines. Ford’s solution was fitting the engine with a revised crank damper and a dual mass flywheel to quell vibrations, and stiffening up the block, accessory brackets, and exhaust system to survive the vibrations. Other measures may or may not have been taken, but Ford is remaining hush-hush.

When balancing a crank, primary imbalance can be corrected, but secondary imbalance can not. In most applications, lightening up the pistons and rods simply isn't enough to attenuate secondary vibrations.
The FPCrankshaft in the tiny Ferrari 488 3.9 engine is not that light. The counter weights are not for balance, but to counter bending. There is a hell of a price to pay for this design for the sake of faste rev matching and blipping at a show and faster gear changes which has no effect on acceleration. Better combustion sequence, not withstanding, has no real gain. With spark interupt, the LS2 slows done pretty good for fast DCT shifts.

For more demanding environments, such as in the 2.4L Formula One V-8s used from 2006-2013, require far more extreme measures. During the development phase of the Cosworth F1 V-8 prior to the 2006 season, the new flat-plane crank engines vibrated so severely that they broke the bolts holding the scavenge pumps to the block. Consequently, engineers fitted dampers on the back of the crank, on the front and back of all four camshafts, and throughout the valvetrain. In total, 13 dampers were required to get these vibrations under control. Considering that these vibrations increase as rpm and stroke length increase, and the 5.2L Ford V-8 turns a fraction of the rpm but also employs a much longer stroke than a 2.4L F1 motor, this is obviously an apples-to-oranges comparison. Even so, these secondary vibration issues can be a very big deal.
Vibration issues notwithstanding, the fast-revving nature of flat-plane cranks made them the configuration of choice during F1’s most recent V-8 era. Not only did these 18,000-rpm screamers have extremely narrow powerbands, but tight tracks like Monaco require shifting over 60 times per lap. When running through the gears that many times per lap, the ability to rev through the powerband a tiny fraction of a second quicker between each shift can add up to much more substantial chunks of time throughout the course of a race. Furthermore, lighter cranks and rotating assemblies transmit less torsional load through the chassis during acceleration (upshifts) and braking (downshifts), which stabilizes the load on the tires and optimizes grip.
Nevertheless, the needs of an 18,000-rpm Formula One engine that only has to last a few hundred miles are far different than the needs of an 8,250-rpm street engine that must last hundreds of thousands of miles. Furthermore, street engines have much broader powerbands than an F1 engine, and a car like the GT350 only requires a dozen or so shifts to get around the typical 2.5- to 3-mile road course. This begs the question, how much of a performance advantage is a flat-plane crank over a cross-plane crank in a street car?

Do you still want one of these horrible boat anchor things in the back of your brand new C8? It's bad enough with the awful DCT that GM came up with. This is brute force engineering. I discussed this elsewhere.

Please copy and paste so that we don't have to read anymore fake news about FPCs. File this in the 'non existent' reference INDEX . Read it again slowly. I don't have to do this you know?

the lark

Really great write up. And really makes you think is it worth it.
Unless Gm, with it’s huge R&D resources solved these problems?

Shaka

Unless you install huge counter shafts which will defeat the object of the engine itself. Weight and even exhaust pulses for turbos. Turbos don't need high rpm which also defeats the object of a FPC engine. The owners of a McLarens or a Ferraris are not known for putting too many miles on their cars. Corvettes have to pass gruelling endurance tests. Still, I love that 488.

Racer X

Thanks Shaka. It reinforces my thoughts that a FPC is unnecessary, and probably undesireable. One thought is that if one of the solutions to address the secondary vibration it to lighten the rods and pistons, wouldn't the same to a CPC which would allow quicker revving more like a flat plane without all the vibration issues? Besides high RPMS are the a bane to high mileage per gallon (due to increased friction) and longevity for the same reason. That is part of the traditionally long life of a diesel engine. Every indication is that they will turbocharging the engine. As you stated really high RPMS are not needed to get the street level power desired by GM. I will not the appears the racing Ferrari's seem to go to about 7000 RPM based on the BOP post notes. Maybe I read that wrong.

The much touted FPC exhaust note can be replicated with a bundle of snakes exhaust with all the vibration and dampening issues. Granting it creates its own space problems.

FPC is a solution to a non-existent problem.

Shaka

The sweetest sound on earth was the Holman and Moody NASCAR 7 liter engined powered 66 Ford GT 40 down the Mulsanne straight with it's 180' headers. Some Ferrari V12s sounded great and then there is the Packard Merlin powered P51 Mustang with it's Hamilton Standard propeller doing a fly bye, yeah.