Red Mist Rulz

It is absolutely a limitation with the current block. Sealing off each pair of cylinders with the firing order of a CPC will create massive air pressure differences, both vacuum and high pressure, as the pistons move up and down in their "mini block." It works in an FPC because while one of each pair of cylinders is moving up, the other is moving down, maintaining a constant volume in the chamber. With a CPC, you'd have both pistons moving up or down at the same time in some of the chambers. Firing order is everything.

Ragtop 99

Ideally, but there will be some weight gain. The snails, intercoolers, and plumbing will add weight. I'd assume more cooling too, but I'm not sure what they'll do. Not a lot of space left, so bigger radiators?

RedLS6

I see how you're thinking about it - kindly allow me to point out why the sealed bay mechanics are the same thing with either a CPC or FPC;

In the FPC, each of the two pistons that occupy any given sealed bay, share a common crankpin; this means that their mechanical motion relative to one another, up or down, is phased 90 degrees apart. You can see this in the figure below; shown is the front of the LT6 FPC, with pistons 1 and 2 in the front; these pistons share the same crankpin, and occupy the same sealed bay. Similarly, 3&4 occupy the next sealed bay; 5&6 the next; 7&8 the next.

Now, in a CPC, cylinders 1 & 2 would also share a common crankpin, and a common sealed bay. They would also be phased 90 degrees apart, and would experience the same mechanical motion relative to one another as in a FPC. The difference is in the FPC, all crankpins are in the same plane, whereas in the CPC, crankpins occupy 2 planes.

In both a FPC and CPC, pistons 1&2 (and all other pairs) experience identical relative mechanical motion. The vacuum pump pulls such a high vacuum in these bays though, that the air mass being pumped back and forth is largely reduced.

The firing order has no impact on the sealed bay operation for either.



Here's an animation that shows the "groups of 2" pistons in the sealed bays have the same mechanical motion relative to one another in both a FPC and CPC;




LT6 FPC ; note cylinders 1&2, in the front, have the big end of their rod sharing the same crankpin. Same as in a CPC.



LT6 cutaway showing cylinder 1&2 sealed bay circled in orange; In a CPC, cylinders 1&2 would occupy the same sealed bay, share the same crankpin, and have the same relative piston motion.

GrandSport 2017

No.

But I'd expect the LT7 will have a different block, but for other reasons, like a smaller bore among other things.
In any case the LT6 and LT7 are both part of the larger Gen6 family.

GrandSport 2017

Keep your eyes open, data collection is underway.

Red Mist Rulz

YES. Sorry, but you don't understand how the LT6 block is constructed. Put a CPC crank in it and it would simply be a disaster.

GrandSport 2017

Considering you don't know my background, that's a pretty blind and bold statement.

Please explain exactly why it would "be a disaster" to put a cross plane crank in the Gen6 block?

OnPoint

RedLS6

No - it really isn't difficult.

Jeff V.

Because the vibration and airflow characteristics are totally the same. Just drop a different crank in.

RedLS6

We're talking about crankcase pumping loss differences inside the separate bays here.

But vaulting over onto your subject, the vibration and airflow solutions for both CPC and FPC architectures are well known. My position has always been that they'll push for a FPC TT solution, with a CPC as a strong alternate.

jim2092

To help us laymen take sides in this advanced engine design argument, perhaps you contestants could share your backgrounds.

RedLS6

Engineering; powertrain communications controls, ECU, TCU, AI applications for ICE, EV, and "other" (See avatar)

Also, Grandsport2017 always contributes very good technical insight here, along with a few others.

jim2092

Only one contestant. He wins.

range96

After skimming through this old thread I disagree with half the stuff

I posted!

Nosferatu333

but you can’t replace the displaced displacement.

said differently, you can’t compare a 6.2 vs. 4.8 boosted car. The 6.2 takes the 4.8.

so technically yes you can replace displacement. But what still holds true is that “there is no replacement for displacement”.

glass slipper

​​​​​The 2.7L turbo 4-cylinder GM uses in their full size trucks makes 430 lb-ft of torque from 1500-4000 RPM, scaling that up to 4.4L gives 700 lb-ft. The 2.7L uses a 4.01" stroke, shorten that to the 3.15" stroke of the LT6 and that alone will get you to 720 lb-ft. If we didn't tell you the engine displacement but just told you it makes 720 lb-ft of torque from 2000-5000 RPM, would you be happy? Would you even care about engine displacement at that point?

If scaled up to 5.5L, that's 875 lb-ft!

GrandSport 2017

Chevrolet's past history is full of first innovations, and the "906" is one of them for sure, as was their "D" OHV V8 105 years ago, the SBC 68 years ago, LT5 34 years ago, LS-1 26 years ago and now the LT6.

Its not what can be done, think of it as whats allowed to be done.

range96

I don't think you can scale it up like that, but I rather take 875 over 720.

Red Mist Rulz

Because each pair of cylinders is sealed off from the other 3 pairs. That works because in each sealed chamber of a flat plane crank, one piston is rising while the other is falling, so crankcase volume remains constant. Put a cross plane crank in there, and you're gong to have a constantly changing volume in each crankcase chamber, causing high pressure and low press

Worked on a Ducati motorcycle race team as a mechanic. Rebuilt my first engine at age 18. Have built multiple race engines (mostly motorcycle, but a couple of cars.) Mechanical engineering degree. And paid attention to GM's technical videos and papers on the LT6, describing how the crankcase is built, and why the flat plane crank allowed them to do it that way.ure situations as the engine rotates. There's a reason cross plane engines don't divide the crankcase into multiple sealed chambers.

Frankly, I don't care if you believe me or not. Believe Tadge, who is the one who described how and why the crankcase seals off each pair of cylinders.