C6, Naked and Exposed:  Corvette Action Center's First Look at the 2005 Corvette - The Sequel: Finally, We Drive It! - Page 6 of 12

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C6 Naked and Exposed - The Sequel: Finally...We Drive It!!! - Page 6 of 12

And, Talk to Some of the People who Built It

by Hib Halverson
Imagery by GM Communications, Richard Prince, U.S. Air Force and Sharkcom
©2005 Shark Communications
No use without permission

The Mystery of Generation Four

I'll bet you're wondering:  why do they call it the "Gen Four?" That terminology was originally an internal designation which enabled GMPT to differentiate between the existing "Gen 3" Small-Block V8 engines, variously used in cars and light trucks, and an evolution of that engine family which would power the same vehicles from mid-decade, on. Generally, Powertrain has designated an engine "generation", according to Jordan Lee, "...if the architecture is significantly different enough. If it's a refinement of the existing powertrain, we just note it with a new name. The Gen 4 is kind of internal to General Motors, but apparently it's become external, as well."

While the Gen 3 engine was a clean-sheet-of-paper design, and certainly qualifies as a "generation", other, previous instances of this policy were the "Gen 1E" (original "Vortec" light truck engines) and the Gen 2 (the '92-'97 L99/LT1/LT4, Impala/Camaro/Corvette) Small-Blocks.

The reason the LS2 is a fourth generation small-block is its cylinder block is quite different. It's still an aluminum, six-bolt main, deep-skirted block having cast-in-place iron liners and is still made with the semi-permanent mold process; but that's where similarities end. Two major changes were to accommodate the larger displacement and the hardware for Displacement on Demand, GM's cylinder deactivation system to be used in light trucks and utilities.

During a second interview with Jordan Lee a couple of weeks after the media ride, joining us was GMPT's communications representative for V8 engines, Randy Fox, and GMPT Staff Engineer, Mark Damico, who works with the "basic engine", or what most of us call a "short-block." Jordan, Mark and Randy explained the architecture changes.

MD:  We increased the bore size from ninety-nine millimeters even, to one-oh-one-six, which is common with our six-liter, truck. We, also, took some mass out of the block.

CAC:  How did you accomplish that?

MD:  Further study of the block structure and working with the foundry on where metal can be removed and still provide a good casting. It was a large number of minor changes.

JL:  The bigger bore removes metal, which is always a good thing for mass reduction, and gives us the ability to make more power.

CAC:  Can you quantify the mass reduction?

JL:  The engine is 7 kilograms (15.4 lbs) lighter, compared to the LS1.

MD:  The other changes to the block-most of them, other than the barrel core, are common with the 5300 mid-sized (utility) block. The 5300 block has added bosses and machining for the DoD lifters and the Corvette block has those features as well, as part of our cost-reduction via deproliferation.

CAC:  But DoD is not used on the LS2?

MD:  Correct. Those bosses are in the valley of the block so the knock sensors had to move from the valley to the outside of the block, down by the oil pan rail.

CAC:  Where they used to be on Gen 2, down low on the sides of the block?

MD:  I'd say "similar locations" because this is a deep-skirt block and those weren't.

JL:  The location of those knock sensors was chosen, based on a lot of analysis, to give us the best sensing of detonation from all cylinders, so we can improve the spark retard system.

MD:  It's one of the enablers of the higher compression ratio.

CAC:  So, when you bump the compression, you need better sensitivity on your knock sensors.

JL:  We need to do a few things. When you bump the compression, you need better combustion efficiency, so you're less prone to knock; but if you do get knock; you need a control system which can react very quickly so that it's unobtrusive to the driver.

The block has siamesed bores. Do we want to talk about that?

CAC:  Go for it.

JL:  (On the Gen 3) The bores were siamesed with the exception of the last 20 millimeters or so which had a little slot to allow water to pass through. That has been eliminated for Gen 4. There was a lot of development in the water jacket design to get good heat rejection without the need for the water between the bores.

We've made advances in analysis tools. CFD really helps us do a better job with water flow, in this instance, and air flow in other instances, to optimize efficiency. We applied that tool when we developed the water jacket and we applied that tool when we developed the cylinder head.

CAC:  With Gen 3, that slot was there because the thinking was, at the top of the bore, where the most heat's gonna be; you needed that little bit of extra cooling, but then, with improvements in Gen 4, that's no longer necessary?

JL:  Well...correct, but probably the better way to think about it is: you want the bore and the piston temperatures as consistent and as cool as possible. If the bore has some hot spots in it, it can cause bore distortion, which isn't good for oil control or for friction. We have internal guidelines for allowed bore distortion and one method to reduce it on the earlier design was to put water between the bores. That causes some manufacturing complexity and cores can be somewhat weak.

By eliminating the slots, we get a more robust cylinder block but still meet the same bore distortion requirement. We were able to use CFD to help us design a water jacket with heat rejection such that we're able to do the same job with cooling and maintain low bore distortion.

CAC:  Could you state that name again?

JL:  It's "CFD, computational fluid dynamics." It's an analysis code. We model the water jacket or a duct and can analytically project what the water or air flow through that water jacket or duct will be.

With CFD, we can examine things like water flow profiles, the movement of particles in the water to see where there are eddies, or divergent flow. CFD helps you shape-you're actually looking at flow (on a computer display)-the water jacket so that you maximize or improve the thing you want to improve.

We can analytically model the engine which helps us make design changes, quickly evaluate them for effectiveness, then apply them to the actual hardware.

CAC:  How much can the engine be overbored?

MD:  Ok, the LS1/LS6 was restricted to half as much as what we'd like to do, a quarter of a millimeter oversize. The LS2 can go a half-millimeter oversize.

JL:  But after 300,000 miles you still shouldn't need an overbore.

CAC:  Of course not. Those babies are going a million miles! (all laugh)

MD:  Well, I've got parts that went 500,000-mi. but I haven't seen any million mile engines.

CAC:  Really! Five hundred thousand! That was with an LS2 or an LS1?

MD:  It was actually a truck engine.

CAC:  That would make an interesting article for a truck magazine.

JL:  Do you have a web site called "TrailBlazer Action Center", by chance.

CAC:  (laughs) The 500,000-mile, ah...was it a six-liter, a five-three or a four-eight?

MD:  It was a five-three. Actually, we captured two vehicles.

CAC:  "Captured?" You mean you just went out and found some owner who'd driven a Silverdo that far?

MD:  Oh yeah. We had a hard time buying his trucks (there were two) from him. He didn't want to sell 'em.

JL:  Before we leave the block-and this is part of cylinder heads, too-I think it's worth noting the compression ratio increased from ten-one to ten-point-nine. Due to improvement in combustion efficiency as well as in the control system; we're able to tolerate that. It has helped, not only in improving power, but in fuel economy, too. Compared to Gen 3, our brake-specific fuel consumption (BSFC), on average, between the two test points we look at for most city and highway driving, went up about 3%.

CAC:  What was done to get the increase

JL:  A bigger bore-a bigger displacement cylinder- and a flat-top piston yielded 10.9:1.

MD:  The LS6 head with a 99 (millimeter) bore and a flat-top piston is 10.5. The (LS2's) 101.6 bore and a flat-top is 10.87:1.

Some moving parts of the short block are changed but some are not. The crankshaft design is similar to that of the LS1/6 piece except its counter weights were altered to rebalance the engine for the higher mass of the piston/rod assembly. The connecting rod is basically the same-6.1-in., sintered, forged, shotpeened and net shape-except it, now, has a slightly larger, small-end which is bushed for a floating pin, a change said to have been made to reduce cold piston knock at startup. We can't help but believing it'll also be a reliability/durability enhancement for the soon-to-come, higher-performance, LS7.

The piston is still cast from a eutectic, aluminum/silicon alloy which contains traces of copper and nickel but, obviously, it's got a larger bore size. It has the anodized surfaces either side of the top ring groove and the polymer, antifriction coating on the skirts introduced for '02 on the LS6 and now used in all Gen 3s and 4s.

New Rings and Revised Oil Control

The ring package has, once again, been changed. The top ring is still steel with a moly face and the second ring is still cast iron with a Napier face and they both are now 1.2-mm. wide, down from 1.5. The oil ring is still three-piece, two oil rails and an expander. The tension of all three rings is less than that used with the LS1/LS6 and the rings are more flexible.

The discussion with Jordan Lee and Mark Damico got interesting when I asked a question about the lower-tension rings. C5ers will recall the LS1/LS6 oil consumption fiasco that ended in a technical service bulletin fix.

CAC:  Now, you guys went through the low-tension thing once before and then had to have a service bulletin fix for the ring flutter issue. How are you going to avoid that same scenario?

MD:  Bore distortion is improved and the oil rails are thinner, so they're more compliant, so it takes less tension to get the same sealing.

CAC:  How does bore distortion affect ring flutter?

MD:  I don't know that it affects the flutter, but it's harder for the rings to seal when the shape's not round. Because we made the rings thinner, they're lighter, so they won't flutter until a higher engine speed.

CAC:  My understanding is that with the later LS1 and LS6, the Napier-face, second improved oil control and eliminated that ring flutter problem.

MD:  I don't know if I'd call it a "ring flutter problem.

CAC:  This is an important point because John Juriga (Asst. Chief Engineer for passenger car Gen 3 engines from 1995-2003) was on record with media in May of 2001 stating that the reason the LS6 had an control problem was due to ring flutter.

MD:  The issue was high engine speed at low MAP (manifold absolute pressure)...

CAC:  And that was causing ring flutter. That was the explanation given. That's what I've published both in print and on the Internet. Was that not the problem?

MD:  I don't remember it being "ring flutter" per se. We measure a characteristic called "blow-by", which is how much of the what you're putting into the cylinder to burn goes by the rings.

If blow-by goes up dramatically at a slight change in engine speed or load-everything's fine, then all of a sudden, it (ring seal) goes out of control-that's usually attributed to what's called "flutter." I don't remember us having that problem, but we definitely had increased oil consumption at high engine speeds and low MAP.

The other thing we ought to point out on the LS2 is the PCV system. There are two sides to it. There's the "fresh side", where air goes into the crankcase and then the "foul side" where air comes out.

You have oil separators on both sides. The separator on the foul side is the one that's the most important because that's where you have oil in the air. Blow-by goes in the crankcase then through the foul-side separator. The LS2 has a different design for that separator. It shares the same location (as Gen 3 parts) but internally, it's significantly different. Back to our CFD analysis and, actually, there's a spin on that that I think is proprietary to GM: analysis with droplets.

JL:  "Rain drop analysis". During that separator design, there was a lot of sophisticated analysis done with a proprietary code. We were able to model the air and the oil through that chamber and develop baffles that would separate the oil from the air. It was quite a new technique and there aren't many companies that utilize it, today.

MD:  It really helped us design the separator which, I think, was submitted for a patent.

JL:  Yeah. The oil/air separator design is patented. When we talk about oil control, it's not only oil control in the pan to keep from sucking air-keep the pickup covered-we need oil control through the ventilation system.

You always want to separate the oil and drop it back into the crankcase and only burn the air. If you can't do that adequately, you're going to have high oil consumption. In the Gen 3s, when we had a lot of air moving through the PVC system at high speed, light load; the ventilation system didn't do a great job in separating the oil from the air so we ended-up burning some of the oil. That would manifest itself in higher oil consumption.

With Gen 4, we made significant improvements to that oil/air separator. We've also made significant improvements in lowering the amount of blow-by air under those conditions that caused the problems which Juriga referred to.

So we have two benefits here: 1) less blow-by through the crankcase under those conditions and 2) our separator is much more effective in separating the oil from the air, so we don't burn the oil. Our oil consumption is less as a result.

CAC:  Why do you need an oil separator on the clean side of the system?

JL:  On the clean side, in many instances, you have high blow-by at wide open throttle.

CAC:  That air flow reverses?

JL:  Yeah. You reach the capacity of the dirty air side to consume the air. You don't want to make the dirty air side so large you don't have to worry about the pressure side, but you don't want to consume a lot of crankcase air through the intake side of the engine all the time, either. You consume just enough that you're constantly purging the crankcase vapors adequately.

At light load, you have a little bit of help from manifold vacuum. You create that vacuum in the crankcase and it constantly purges the air and reduces sludge formation and it burns those hydrocarbons, which is a good thing.

When you go wide-open-throttle, high engine speed; you no longer have that vacuum to help pull the air from the crankcase and you, also, have more blow-by, so you may exceed the capacity of the dirty air side, which is sized for most normal light load engine operation.

You have to make sure your fresh air side, which starts to reverse-flow, does not blow oil and air out (into the intake). That's why we end-up having oil/air separation on both inlet and foul air sides.

CAC:  In summary, it sounds like a lot of work was done on PCV, crankcase ventilation and oil control.

JL:  That's right.

MD:  Yeah.

A challenge during the mid-'90s, Gen 3 development was getting the LS1's oiling to work right during high lateral acceleration maneuvers typical of Corvettes being driven hard. The result was the now-famous, "batwing" oil pan used from '97 to '04.

For Gen 4, Powertrain decided that a new oil pan was necessary to reduce weight and cut cost. Obviously, this new pan would have to be capable of providing consistent oil pressure in the C6 which can generate a little more "lat" than could C5. Indeed, this was quite a tall order. How GM met the challenge was another subject Jordan Lee, Mark Damico and Randy Fox discussed.

JL:  The oil pan is all new. It's different than the wing design.

CAC:  Is it still cast aluminum?

MD:  Yeah. Internally, it has patented features which provide the function of the gull-wing pan, in terms of oil storage when going around turns at high-g, but without the added mass and at significantly less cost.

CAC:  I'm interested in the specifics of how you accomplished that.

JL:  An extensive amount of dynamometer and race track development and defining the baffling inside the pan to give maximum oil control under high-g maneuvers-the pickup is never uncovered. With Gen 3, a lot of those functions required those little extensions on the oil pan. By being more scientific and more diligent in development, we were able to get rid of those appendages and some mass-which is always a nice thing-and offer better oil control.

CAC:  There was a reduction in oil capacity, too?

JL:  Yeah. It went from six-and-a-half to five-and-a-half quarts, in the pan.

CAC:  Up until '04 you guys recommended to those running their Corvettes in road racing events to overfill by a quart. Is that still recommended?

MD:  Yeah.

CAC:  Any change to the windage tray?

JL:  No.The windage tray that was developed for the Gen 3 is very good.

CAC:  This oil pan contributed to less cost and less mass?

JL:  Well, definitely less mass-reduced by about 20%.

MD:  Definitely less cost.

CAC:  Reducing cost is always a good thing, when possible.

JL:  It is.