235265283...

Disclaimers:

#1. This thread involves C7 speculation. Skip if you think that’s silly.
#2. “A little knowledge can be dangerous” describes me. Corrections and opinions welcome. But I’m not fond of cute flame jobs on Corvettes or forums.
#3. Please don’t misinterpret these remarks as Corvette criticism. I like Corvettes. Proof: I recently bought new 2012 base coupe.

Certain C7 engine features such as direct injection and smaller displacement seem to be fairly certain. Other potential features such as variable valve timing (VVT) are less certain. Fuel economy and to a lesser degree emissions certification requirements are obviously key factors affecting the C7 engine design and features.

I’m particularly curious about potential VVT for the C7, and I suspect perhaps GM had to consider some less-than-ideal choices in this regard. But first some quick background.

I think there are different types and “degrees” of VVT. Oversimplified:

#1. Advance or retard one camshaft (e.g., some LS engine applications such as L99 used in AT Camaros)

#2. Advance or retard two camshafts (Int. & Exh., e.g. some GM 4-Cyl. Ecotec engines)

#3. Variable combinations of advance, retard, lift, and duration of Int. & sometimes Exh. (e.g. Nissan VVEL on Infiniti G37, Porsche, BMW, etc.)

VVT can obviously be used (along with DI and other technologies) to help to flatten the power curve and improve the peak power / fuel economy ratio…which I suspect was a key objective affecting the C7 engine design. And, Category #3 VVT is incredibly more effective with respect to these objectives than Category #1 or even #2.

But, if the C7 has a pushrod V8 that is an LS evolution, I think Category #1 VVT is the only option. Overhead cams are usually required to facilitate Category #2 or #3 VVT, and my guess is that if the C7 engine has OHCs it will not be a LS-derivative. My understanding is that the marginal advantages vs. problems tradeoff of simple hydraulic 1-cam advance-retard system are not favorable for high-performance applications, and that is why VVT hasn’t been previously used on Corvette. There have been published reports (not sure if pure speculation or traceable to GM) that the C7 engine will maintain the same bore center spacing, which obviously suggests a LS series evolution.

My own preference is for the C7 engine to be a V8 version of the Camaro LFS V6 or possibly a V8 based on the 4-Cyl. Ecotec design. This may outrage some of you, but, while the LS3 (my engine, remember) is highly refined, the LFS and Ecotec are far more sophisticated. The 3.6-L, 323-hp LFS has DOHC, 4 valves/cyl, DI, VVT (Category #2, I think). Note that if the C7 base is 5.5 L and has the LFS power/displacement, it will be 493 hp. But my guess is we aren’t buying enough new Corvettes to justify the design and manufacturing change costs that this strategy would require.

JerriVette

I believe you will find the heavy investment to meet and exceed truck buyers expectations in variations of the new Gen 5 V8..

The corvette powerplant always finds most of its research and development paid for by the tremendous truck sales..

Efficiency and power will allow a tenth or two improvement for faster acceleration......ten or 20 percent better fuel efficiency........

I don't know the details of the engine design..yet I do look forward to finding out what lays ahead...

Best of luck with your new ride....

I've got an 08 z51 M6 and its a great ride.

MitchAlsup

Viper has shown that a single cam in block design can have its intake and exhaust cam timings both varry. The intake lobes are on one shaft, the exhaust lobes on another and two variators move them, yet both shafts are co-axial supported by one set of bearings.

One of the big advantages of DI with VVT is that at lower RPMs, one can advance the intake cam and retard the exhaust cam. This moves the exhaust opening later in the power cycle (extracting more power from the combustion heat) and moves the intake closing sooner after BDC (effectively increasing the mass of air in the cylinder). This also greately increases overlap, to the injectors have to avoid spewing gas whiel overlap is present to keep emissions low. DI is great at this, EFI is only pretty good at this. One might get as much as 10% more TQ from idle through about 2500 RPMs using this kind of timing.

At higher RPMs, the exhaust is opened earlier in the power stroke to decrease the power loss of pushing exhaust gasses out of the cylinder on the exhaust stroke. Similarly, the intake is closed later allowing the fast flowing intake gas to pack more mass into the cylinders. Up top, one can get 3%-5% more power than with a cam with one timing for all RPMs.

So, basically, if Viper engines can do dual VVT, so can GM engines.

LS1LT1

Some good info/discussion here.




I think it's almost 'a given' that at least the base C7's (and other future GM model's) V8 engine will be of very similar architecture to the current LSx derivatives...an all aluminum 90 degree OHV design, likely even retaining the current 4.4 inch bore centers.
I think Direct Injection will be a pretty major step forward in both fuel economy and power delivery as well.

Dudeurgettnavette

Stopped at # 1

jackhall99

235265283...

Good observation. I forgot about the VVT system used on the Viper. Illustrates my "Little bit of knowledge..." admission.

Mitch, Since you obviously understand this area, I have a (perhaps esoteric) question for you: It's sometimes claimed that what I refer to as Category #3 VVT can significantly increase efficiency since the low-lift (and perhaps low-duration) intake capability eliminates throttling (pumping) losses. But it seems to me that you are throttling regardless of if through a butterfly plate or a barely open intake valve. Maybe the answer involves the difference between low-lift and short-duration intake events, which starts to borderline on a Miller cycle.

And the most important question for you: What do you think the C7 base engine will be?

Bill Dearborn

The GM small block bore spacing hasn't changed since 1955. Even the 90-95 ZR1 5.7L DOHC engine had the same valve spacing. That engine was a great engine but it showed the problems you get into with DOHC. Too much weight and package volume for no real HP benefit. Right now there are plenty of people replacing 4 and 6 cylinder Turbo engines with NA LS1s because they can get more HP and more reliability from the LS1 with less weight and more room in the engine compartment.

Bill

MitchAlsup

The trick is simple: When a Cat3 intake is open there is no throttle plate obstruction of the free flow path. When a throttle is present 2%-ish is lots as the fully open throttle plate is a non-negligible obstruction. This should be considered "just above the noise level" as lots of other insteresting manufacturing issues dominate.

The Toyota F1 engine (from a few years ago) had a throttle that was unobstructive at WOT, but closed down the intake tract, progressively like cupped hands. At WOT the throttle 'throat' was a perfect straight walled not-quite-round tube.

What you PROBABLY mean is a butterfly throttle plate. This is the thing that can be as good as 2%-ish losses or as bad as 5%-ish losses. You go after small fries like this after you tune the rest of the engine up to peak.

I think 95%-ile of us will adopt it and come to embrace because it provides higher performance than Base and GS do today.

LS1LT1

Racer X

It depends on who owns or has licenses to the patents and how broad they are. GM may not be able to use the technology.

MitchAlsup

In the 1920s the US auto makers got together and created a cross licensing pact so that the originator of a patent would have one model year advantage of other American automakers in the showrooms. Otherwise, some company might invent a new way to meet emissions levels, patent it, and if it was the ONLY way to meet the emission levles, cause everyone else would go out of business or pay such huge licensing and royalty fees that they would go out of business.

So, basically, US automakers signed a compelling cross licensing deal that makes GM's use of Viper's VVT automatic almost 90 years ago.

I know nothing about fees that might be applied to make it happen, nor the manufacturing costs to be incurred.

235265283...

Jerri,

Good point. It's easy to overlook the perhaps paradoxical fact that most of the GM LS series V8 engine development and manufacturing investment was and is justified (i.e., paid for) by truck and some large SUV applications. I hesitate to think what kind of engine my C6 would have if V8 trucks weren't good sellers at GM.

235265283...

Quote:
And the most important question for you: What do you think the C7 base engine will be?

I think 95%-ile of us will adopt it and come to embrace because it provides higher performance than Base and GS do today.

Mitch,

OK, I'm feeling so dumb this morning that I can't tell if you're skirting my question or you misunderstand it. Will the C7 base engine be an LS derivative or new series? Displacement? DI? VVT and if so, type? OHC? Boosted and if so, T or S? Etc.

Racer X

I had not heard that, and have been unable to find a citation. Do you have one? It would strike me as strange that there would be comprehensive agreement, and which companies were a part of the agreement? I understand agreements on packages of patents, but not the whole portfolio. and not inperpetuity.

Forced licensing is not restricted to any industry. I understand that when there are certain patents critical to industry standards that the various technology companies have to cross-lincense and the fees cannot be excessive.

An interesting topic.

LS1LT1

I know you're asking Mitch but I'll offer some opinion on that as well.
I believe it will be both LS derivative and mostly new at the same time.
I'm guessing that the base car's engine will be somewhere between 5.5L and 6.4L but I really don't think it will be any smaller than 6.0L.
Direct Injection is almost a certainty.
Some form of VVT is likely as well but not a definite.
Displacement On Demand/Active Fuel Management or that 'shut it down at a stop light' feature? Not sure there but I hope 'none of the above'.
OHV/single cam.
Pretty sure that the base car will be naturally aspirated but a future ZR1 model will likely be supercharged.

BlueOx

Even Ferrari FF is using "Stop&Start"...with a 12 cylinder...
"This impressive result is attributable to a whole series of factors affecting every area of the car. They include the introduction of the HELE (High Emotions-Low Emissions) system -with Stop&Start, which cuts out the engine during short stops and then restarts it in 230 ms - which includes optimised gear shift pattern, intelligent engine fan control, constant fuel pump capacity control, and electronic air-conditioning compressor displacement control. All of these solutions combine to cut fuel consumption to just 15.4 litres per 100 km (15 mpg) and CO2 emissions to 360 g/km, a 25 per cent reduction on the previous Ferrari V12s."

Here is what was "impressive"...
"Make no mistake about it, the FF has the DNA of a thoroughbred Ferrari. It is not only sporty in the extreme, but also comfortable and easy to drive. It brings a wealth of technology to the mix, starting with a completely new mid-front-mounted 6,262 cc GDI V12 that delivers unprecedented performance and responsiveness at all engine speeds: 660 CV at 8,000 rpm, a specific output of 105 CV/l (77 kW/ cu in), a weight-power ratio of 2.7 kg/CV. The torque is also blistering: 683 Nm at 6, 000 rpm, with over 500 Nm available from 1,000 rpm up to 8,000 rpm (virtually across the entire range), guaranteeing unparalleled flexibility. The FF has a top speed of 335 km/h (208 mph) and sprints from 0 to 100 km/h in just 3.7 seconds.

Those impressive figures are the result of a host of technical solutions which include high pressure GDI (200 bar) and Split Injection Control that guarantee perfect fuel pulverisation and an optimal air-fuel mix up to 8,000 rpm. The FF also has the highest compression ratio (12.3:1) in its segment and a reed valve scavenge system that reduces losses due to windage caused by the pumping action of the pistons. Leading-edge materials have been adopted for the pistons, piston rings and camshaft to minimise friction. The FF has a combustion control system that uses ionisation currents – this boosts the engine’s power output whilst reducing both fuel consumption and CO2 emissions to levels previously unheard of in a car with these characteristics."

I can only hope that GM has taken Corvette engine technology very seriously in the C7 and that the Viper will not get the head start they could with Ferrari's influence.

CPhelps

Note that the 3.6L DOHC v6 in the Camaro (actually called LFX not LFS btw) also weighs within 50 lbs (the older LLT that didn't have the integrated exhaust manifolds was within 20lbs) of the LS3. So the LS3 makes 436hp and weighs 403 lbs (.92 lbs/hp) and the LFX makes 323 hp and weighs 345 lbs (1.07 lbs/hp). On top of that, the LS3 is physically more compact. To me, power to weight is a much more useful metric for judging an engine than power per displacement.

More food for thought: adding DI to the 3.6L V6 GM went from 255hp to 304hp in the CTS, an increase of 17.53%. Granted it is an over simplification, but increasing the LS3's power by 17.53% yields ~512hp.

I do believe the 4.4" bore spacing was actually about the only detail confirmed by GM about the Gen V. The smaller 5.5L displacement may turn out to be true, but was never official information. The fact that the GT2 racing Corvettes were restricted to 5.5L is what lent some credibility to this rumor.

I'm looking forward to seeing what GM Powertrain comes up with for the Gen V, and I'm hopeful they will retain a larger 6.2L displacement, at least on some versions, but confident that even at a smaller displacement they will be able to produce an engine that is more powerful, and likely has less mass, than the LS3, all while having better fuel economy (transmission and overall car mass comes into play significantly here as well).

LS1LT1

235265283...

CP, Good point re power/weight metric compared to power/displacement metric. I obviously hadn't looked up and compared weight, and your comparison is informative. I suspect that weight scaling with number of cylinders is quite subtle and less than linear.

I also suspect that the power and FE increase that can be anticipated from DI on a Corvette-suitable LS engine is also a subtle subject. For example, look at how refined the current generation heads are to achieve the desired flow characteristics. And now we have to fit fuel injectors into these heads (along with the spark plugs, valves, ports, cooling, etc.) without compromising the flow. Easy? This is what makes me uncertain that the C7 base engine will simply be a LS3 with DI heads, less displacement, and maybe the existing GM "Category 1" VVT.

Annd thanks for the LFS - LFX correction.