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Well, another way to look at it, two valve springs pressing on one lifter is potentially more load than a 1:1 relationship. Who knows?
Or, the two valves are slightly out of phase of each other to induce a swirl effect?
Overall, definitely interested to hear more about this engine if it is the one in the ME. Could also explain the rumored .1L increase in displacement, a basically entirely new engine. Which now that I "say" that outloud, why would they call it the LT2 and not something different? I'd invision an LT2 to be a derivative of the LT1. But if this has 75 degree banks, all new valve train design, etc... it wouldn't be an evolution of the LT1.
Interesting idea. Differential valve lift to induce a swirl effect. I wonder if anyone here can tell if that's happening based on the cam lobes we can see.
Or, the two valves are slightly out of phase of each other to induce a swirl effect?.
Agreed, I can't see any reason why the two valves would need to have the same timing, or even the same lift.
Originally Posted by vndkshn
Overall, definitely interested to hear more about this engine if it is the one in the ME. Could also explain the rumored .1L increase in displacement, a basically entirely new engine. Which now that I "say" that outloud, why would they call it the LT2 and not something different? I'd invision an LT2 to be a derivative of the LT1. But if this has 75 degree banks, all new valve train design, etc... it wouldn't be an evolution of the LT1.
I can't see any reason to go to a 75 degree configuration, unless something about that configuration works out better when it comes to balancing a flat-plane crankshaft.
And I agree that if it was an entirely new engine, it probably wouldn't continue with the LT moniker. A second in-block cam could be done with little more than machining some mounting pads into the lifter valley, bolting in a second cam support structure, and drilling some additional pushrod hole in the head (if ports and water jackets don't get in the way).
Two smaller intake valves can probably be operated at much higher rpms, than one large one.
Interior photo suggests a 6500rpm redline. 3500 marks the top of the tachometer, so it reads to 7000rpm and the last 500 are probably past redline. Pretty much expected for a pushrod engine.
Good call making this a separate thread. I immediately noticed the massive 2 gerotor oil pump in the thread dissecting the marketing video. I can't think of anything a 2nd timing chain would be for other than another camshaft. Neat stuff... very curious what we learn.
Good call making this a separate thread. I immediately noticed the massive 2 gerotor oil pump in the thread dissecting the marketing video. I can't think of anything a 2nd timing chain would be for other than another camshaft. Neat stuff... very curious what we learn.
Only things that come to mind for me are a chain driven water pump, which is unlikely. Or a chain driven fuel pump... also unlikely. This is getting more interesting to look at as time goes on. There are quite a few things in that photo that don't look "right"...
Only things that come to mind for me are a chain driven water pump, which is unlikely. Or a chain driven fuel pump... also unlikely. This is getting more interesting to look at as time goes on. There are quite a few things in that photo that don't look "right"...
Yeah wouldn't make much sense to chain drive those. DI fuel pump works off cam lobe just fine (primary being electric) and, if anything, the water pump may go electric.
Also, note the damper has provisions for 2 belts. Why would you need that if the car is going to be NA or turbo?
The LT2 image from the CAD leak show an extra unknown accessory that seems like it could be driven by a separate belt. Perhaps the hydraulic pump for the front lift system?
It's the top right accessory. Top left is water pump, bottom left is alternator, bottom right is AC compressor.
Edit: another possibility.
The LT2 is still a two valve per cylinder engine, but with two cams in block to permit independent phasing of intake and exhaust.
The bottom cam has two intake profiles of different lift and duration, one profile for low rpm cruising, the other profile for high output. Pushrods follow each lobe, but by using the DFM system's solenoid-controlled hydraulic valve lifters only one pushrod is actuated at a time, or neither if DFM disables the cylinder. The intake valve rocker would be a triangular piece with the valve at one vertex and each pushrod at the others, so the valve could be operated by whichever pushrod is active.
The exhaust could be simply 8 actuators and pushrods for a total of 24 pushrods. The maximum in use at any given time would be 16 pushrods.
Such a design exploits the already developed solenoid-controlled hydraulic pushrod lifters for the DFM system to keep costs low.
Last edited by Zaro Tundov; 04-30-2019 at 04:53 PM.
Front lift pump would be electric rather than belt driven. Especially if it's an optional system.
But that mystery pulley is too big to be an idler. It almost looks like it's got a magnetic clutch too. Maybe some kind of auxiliary cooling pump?
Good point about the lift pump being electric.
I'm stumped. I thought at first it could be related to a 48V mild hybrid system but it doesn't look like any of the motor generator units I've seen and there's already an alternator.
Last edited by Zaro Tundov; 04-30-2019 at 05:24 PM.
This is an example of how much stuff GM has "on the shelf" from which they can choose to accomplish their goal. They can mix and match from technologies that they have tested and are ready to develop for production.
There definitely is something going on with the NA engine that is not obvious. For example, what happened to make the engine a 6.3 instead of 6.2?
IMO, 75 degrees is not likely. Neither is three valves.
But I really don't know what GM is telling us with this photo, especially as we can compare the same view to the new 6.6. That same photo angle was not an accident.
The plot thickens. I love it.
Not to be a wet blanket, but there is a difference between brain storming and choosing. I'm sure the engineers had a ball brainstorming but at some point they had to start eliminating alternatives and make choices.
That would have been the meeting to attend!
When choices were made and directions were set.
Last edited by Kodiak Bear; 04-30-2019 at 05:38 PM.
Interior photo suggests a 6500rpm redline. 3500 marks the top of the tachometer, so it reads to 7000rpm and the last 500 are probably past redline. Pretty much expected for a pushrod engine.
Most dual overhead cam motors, designed with longevity in mind, aren't revving much beyond 6500 rpm anyway. Might as well keep weight and bulk down by using a pushrod motor, since the high rpm advantages of using overhead cams are not realized.
Last edited by Warp Factor; 04-30-2019 at 05:43 PM.
Most dual overhead cam motors, designed with longevity in mind, aren't revving much beyond 6500 rpm anyway. Might as well keep weight and bulk down by using a pushrod motor, since the high rpm advantages of using overhead cams are not realized.
GM's DOHC 3.6L V6 LGX has a 7200 rpm redline.
Ford's Coyote 5.0 V8 is up to a 7400 rpm redline in the Mustang GT.
No reason why a NA DOHC V8 in the C8 couldn't stretch it's legs.
Yes, it can rev that high, but it makes peak power between 6500 and 6800 rpm (easily attainable with pushrods), so the high-revving advantages of overhead cams aren't really coming into play.
The Ford Coyote 5.0 V8 can rev to 7500, but makes peak power at a measly 6500, about where a pushrod LT4 does.
So the overhead cams in these engines are little more than placebo effect, carrying the disadvantages of increased weight and dimensions.
Last edited by Warp Factor; 04-30-2019 at 07:20 PM.
^Not sure it is just "placebo". Up to this point, the only real way to do variable valve timing (especially for both intake and exhaust separately) has been DOHC. So DOHC is about more than just RPM.
04-30-2019, 02:45 PM
