Shaka

You brought up counter shafts, you ...I'm done with you. Nobody here knows over here that you don't know what you are talking about, except me. Try this crap at the SAE Forum. Quit pretending. Change in the CofG, yeah sure. You don't even know the correct language.

RedLS6

Haha, I got a pretty good laugh out of that one. But I still like you.

Back to the second order shake. If you consider an inline 4, the center of (gravity, mass) of the reciprocating assembly is the highest when the crank is at 0 and 180 degrees. The center of mass is lowest when the crank is at 90 and 270 degrees. This creates the second-order shake that is primarily directed up and down in relation to the cylinders. These are the green and blue vectors in the plot below.

When you combine 2 inline 4s into a 90-degree flat V8, the primary component of the second order shake vectors reach their peaks 90 degrees of crank angle opposite of each other. When the left bank of cylinders is creating an upwards shaking force, the right bank is creating a downwards shaking force. 90 crank degrees later, the left bank of cylinders is creating a downwards shaking force, the right bank is creating an upwards shaking force.

If you do a vector phasor addition of these two forces like I did below, you get the yellow resultant below that I superimposed on the cylinder picture.

The resultant shaking force (yellow vectors below) for the flat-plane V8 is root2-greater than the shaking force from either bank. It is also directed sideways, back and forth, twice per crank revolution. It tries to shake the engine towards each side of the car, back and forth. There are other fourier components that are outside this analysis.

These yellow sideways vectors are the subject of previous efforts, like Lanchester balancers, to counter the second-order shake over the years. I agree with part of what you said, in that other second order components or higher fourier terms are too complex to try to counter, but they're usually much smaller in magnitude as well.

Any vibration system designed to counter this second order shake component needs to take into consideration this direction. Or they can just let it go and design the rest of the car to be able to handle it.

Racer X

RedLS6,

You have forgotten that you are speaking to the second smartest engineer on the planet (2nd only to my father-in-law, who is a chemical engineer).

Most really smart people I know, understand the topic so well they are able to put things in a a way that smart people, but not experts in the field, are able grasp the topic. Shaka is unable to do that anymore. When he first came to the board, I found him to be helpful, and able explain things in a way that people that understood Newtonian physics could understand. I look for his answers on topics, because I knew they would on point and helpful. I want away from being on the boards, as I moved away from Corvettes and sports cars.

When came back a couple years ago, he seemed to have changed. He knew more than everyone, Corvette engineers included, and would only say people were wrong and he wasn’t going to explain. He demanded others explain the topics they were talking about, and how they should explain what he knew was wrong. He has been very obtuse. Much like my father-in-law that has become much more difficult to have a civil and interesting conversation with. This happened as my FIL went from his mid 60s to his mid 70s. It is sad because my FIL used to interesting to talk to, even if he went into way too much detail. I feel the same about Shaka. Going from someone whose knowledge admired and posts I looked forward to, to someone whose posts I no longer look for, and sometimes find sad to read.

What all of us non-automotive engineers really want to know is will a 5.5 liter flat plane crank engine with small block Chevy bore centers be practical on the street with 100,000 + mile life? Is to even necessary to produce 850 HP that GM indicated in earlier leaked documents?

Kodiak Bear

The context is why did GM go to a flat plane, DOHC, NA in the C8R? And having done so, how did they get it to live at 5.5lts when all others usually limit them selves to about 4lts with that crank? Given that GM did use a flat in the C8R, , a MB GT Black TT version which peaks at ~6800 rpm with a limit of ~7200 rpm makes a LOT of sense for the street version. Bottom line, having chosen to race a flat plane DOHC, GM must meet some production number units and weight of the crank in the DOHC street engine.

Racer X

The rules don’t require the street engine have the same firing order or design of crank. It only addresses the weight of the crank. The race crank crank can no more than 10% lighter. The race block must be derived from the production block, so it needs the same bore centers. And the race heads and street heads need to be of the same design, so the street engine will need to be DOHC 4v.

Kodiak Bear

Precisely. Therefore can you build a cross crank with the proper weight? And if you're using a very short stroke in the 5.5 lt can you stroke the DOHC engine to get the displacement/power you want?

So, back to question one, why the 5.5lt race engine in the C8R? And if you did that and for some reason wanted/had to stick with the flat crank, why not do a MB and put TT on it? What's the advantage pf a flat crank NA street engine?

RedLS6

We go back a ways on these arguments. Sad thing is we agree 99 percent of the time....when we don't it ends up like this. You can either check your ego at the door, talk through your technical differences and everyone wins, or not.

fzust

More I think about it, it seems like a correctly built 8,000 rpm short stroke/long rod crossplane crank with DOHC VVT would meet the design goals. It seems impossible for me to believe that between the C8R race engine, single-cylinder prototypes, and simulations GM could run that they wouldn't converge on a solution fairly quickly.

RedLS6

Quote:

Originally Posted by Grand Sport 2017

The OHC Gen VI engine will have a damper system in the valley where the camshaft usually resides on the push-rod engines that will likely spin at twice crankshaft speed.

GS 2017, you had posted this a while back in another thread, do you still believe this to be the case?

Racer X

Isn't this how they dealt with the issue on large 2.5+ liter 4 cylinders to mitigate vibration issues?

RedLS6

What you're thinking of are probably the counterrotating balancers used on some inline 4's. These spin at twice crank speed, are usually located directly above or below the crank centerline and counteract the second-order vertical shaking force that's caused by the movement of the center-of-mass of the reciprocating assembly twice per crank rev. This is the vibration that I mentioned above in the pissing contest that is relatively easy to model mathematically.

What Grand Sport 2017 described is a torsional vibration dampener. Specifically, he mentioned the firing pulses, which create a 4th order torsional vibration. There's another 4th-order torsional due to piston kinetic energy, and lots of other frequency content. Very likely they could do this without issue -

Could they also locate a second-order shake absorber (dual 2X crank-speed shaft system used in Inline 4s) into the valley where the cam used to go? Maybe but while this location works for a torsional absorber, it's not optimal for a second-order shake absorber on a flat V8, since it would create a rocking couple. These shafts are located above or below the crank in an Inline 4, directly in line with the second order shaking force. On a flatplane V8, the shaking force is transposed from vertical to horizontal, and these shafts need to be located on a horizontal line going through the crank centerline to avoid any rocking couple.

If they did use the shaft system in the old cam valley and replace the second order shaking force with a rocking couple, they could potentially counter the rocking couple with active mounts mounted in the same direction that they are now. Now this is a whole lot of power robbing band-aids. yuk.

Racer X

Ultimately, I think it is an unnecessary complication, for performance goals that can be met by a cross plane crank. If they feel compelled to get the sound, I would rather they did a bundle of snakes header, without all the NVH issues.

GrandSport 2017

I think they are going to address the "Vibration" issues with 3 separate solutions. I don't know how they will address each one but have ideas. I'm not convinced that the 5.5 will have a FPC.
I'm under the belief that the smaller TT engine with short stroke will be FPC. I'm speculating thou I don't know. But it would be easier to manage lower amplitudes.
They have a lot of room in the valley. And a few mounting bosses on the side of the engine. Lucky I can speculate on what i don't know.

Racer X

A 4.4 liter DOHC TT should have no problems meeting a 675-750 HP goal. Moving up to a 5.5 liter DOHC TT CPC should have no problem meeting the 850 HP 720 lb/ft design maximum mentioned in the North American Engine Plan that was leaked over 2 years ago (not that error filled spreadsheet floating around).

Kodiak Bear

If you go TT, the entire ball game changes for the flat plane. You're no longer going for extreme RPMs but rather dealing with how the exhaust drives the TTs. The MB AMG GT Black peaks at ~6800 rpm with a red line at 7200 rpm.

RedLS6

/\

One guess for the dimensions on a 5.5L flat-plane engine for the C8Z, are a 4.1" bore with a 3.2" stroke; Big bore and short stroke; Maybe a 6" rod as a total guess.

Assuming they destroked it to arrive at 4.4L, then the new shorter stroke would be about 2.55". Maybe they also use a longer rod here, longer by half the stroke difference; maybe a 6.325" rod. The connecting rod length impacts the secondary but not the primary.

If you run both of these hypothetical flat-plane engines at 8250rpm, the 4.4L has a peak secondary imbalance which is 60% of the 5.5L's peak secondary.
If you then drop the 4.4L's max rpm down to 7000, its peak secondary is about 43% of the higher-rpm 5.5L. That's a significant reduction, but it's still there in the "noticeable" category.

I didn't account for lighter pistons or lighter reciprocating parts. These linearly impact the secondary.

boogie00

this is what I’m hoping for. Gt3 owner currently. Watching this zo6 development closely. If it’s in the low 3,000 lb territory with a high revving flat plane I’ll buy one. It’s honestly the way to go, less displacement, less weight. 600 hp is more than adequate if it’s in the right chassis, could you imagine a gt4 with 600hp?!?! Mid to upper 3,000lb weight and it’s more of a stoplight to stoplight car than a true track car and it will need gobs of power.

Dominic Sorresso

I recall driving the 3Gen RX-7. I believe it was 2800lbs. Tight cockpit for a 6’2” guy, but what a blast to drive. Stopping was terrific and steering really light. You simply can’t overcome the effect of mass w electronic wizardry. Lighten it up!

TB427Z06

Well it's definitely a flat plane crank engine. The mule video's including the latest confirm that it's definitely a flat plane crank engine. We just don't know the displacement of the engine.
https://www.motortrend.com/news/2022...ype-spy-video/

Rkreigh

fpc seems to be the new darling. And if the car is TT, the FPC offers more even pulses I've heard to better drive the turbos (I don't get this either)

Fords pulled hard to 8250, but just like my old C5 Lt5, windage and oil control becomes and issue.

And the internet amplifies any failure orders of magnitude beyond what it really is.

Seeing Mickeys, AMG, Ford, Ferrari and others go FPC with screamin memees with no torq, high maint, and for my $ not enough spank

Vs v8 thumpers that run out of wind by 7k. I prefer the broader powerband and "pulling down a mineshaft" feeling that the higher rpm provides

for sure it requires a very well built engine, or shorter service intervals. As evidenced by the old LT5 even with all its foibles, you can have a very reliable 7k engine

Now take away the constraints that gm handed lotus, design it properly, and 8k should be very doable with DOHC, and the right parts.

GM will be again dipping it's toe where it hasn't been, and if they can do it on a working Joe's budget like they have, it will be quite an achievement.