by Hib Halverson
Imagery by GM Communications, Richard Prince, U.S. Air Force and Sharkcom
©2005 Shark Communications
No use without permission
General Motors' 2004 media Product Information Guide, or PIG ("media pig"-hidden meaning, perhaps?) says 93-octane gasoline is "recommended" for use in Corvettes. Owner's Manuals for '04 say to use "...91-octane or higher for best performance." While there seems to be a difference between what GM tells media and what it tells owners (certainly not the first time that's happened), most people will run LS2s on whatever premium unleaded they find. Generally, this means 93- or 94-octane along the East Coast and parts of the midwest and 91-octane in most of the rest of the country.
LS2, like all Corvette engines since 1982, has computer-controlled spark timing and part of that is a feedback loop which eliminates detonation which can damage the engine. When the knock sensors signal the engine controller that detonation is occurring, in milliseconds, the controller retards timing-we call this "knock retard"-until detonation stops and, once it stops; timing is restored. This feedback process goes on constantly such that spark timing is always at the optimum level for best performance, highest fuel economy, lowest emissions and the octane of the gasoline being used.
In nominal atmospheric conditions, 93-octane is adequate for the LS2 to perform optimally with minimal knock retard to control detonation, however, some of my chassis dyno testing of pump gasolines leads me to speculate that, to run detonation-free under the worst conditions, ie: wide-open-throttle, heavy load, low humidity, sea level (or near so) pressure, high intake air temperature and high coolant temperature-the LS2 probably needs about 95-octane
With the engine calibrated for 93-octane, that extra last bit of performance during severe duty requiring perhaps as much as 95-octane, Corvetters in a fair amount of the country stuck with 91 and a few errant owners trying to use regular; feedback spark control and knock retard are what allows the engine to produce highest-performance on good gas but not be damaged by detonation when running on gasoline of less than 93-octane.
Since the early-'90s, advances in digital controls and proportionate increases in the effectiveness of feedback spark control has enabled compression ratios of high performance engines to steadily rise, making them more powerful and fuel efficient. In fact, just going from LS1's 10.1 CR to LS2's 10.9 resulted in a small, half-mpg loss in the car's city/highway average for manuals and a one-mpg. gain for automatics-that from an engine that makes 50 more horsepower and meets ever-tightening exhaust emissions regulations.
For 2005, because the higher CR makes the engine a little more liable to detonate, the knock sensing and control systems were upgraded in speed and accuracy. In addition, more accurate control of air metering, fuel metering and exhaust emissions controls were desired. All this required a more robust engine controller.
Figuring some Corvette Action Center fans would like to know a little of the deep geek stuff, we contacted Steve Bezdek, an engineer at Powertrain responsible for the engine controller.
CAC: What are the main differences between the new controller and the old unit?
Steve Bezdek: The primary reason we went to a new controller for this project was to be able to handle some features that aren't on the Corvette, but I believe your interested specifically in the Corvette?
I'm not sure which processor the "P59" (Gen 3 engine controller) used, but the "E40" is a single-circuit-board controller and a lot more of our integrated circuits are mounted directly to the board instead of being packaged parts, so we've got a lot less complexity.
The overall footprint and package size are smaller. It's got some additional capability. It's running slightly faster. We have two processors. One is on-board, primarily for controlling the engine functions outside of the electronic throttle. The other operates the ETC system.
Processors are Motorola. The main side is an "Excalibur" processor and the secondary side is a "Cerverus". The main side is running at 32 Mhz. We've got one meg of on-board memory for the main side, 256K of flash memory and 34K of RAM.
We have other features. You're probably aware of our Displacement on Demand system that is offered on some of the small-block V8s.
CAC: That would certainly require an increase in controller power.
SB: ...As well as, some additional I/O. We have additional driver transistors to operate the extra solenoids which control the oil flow for the Displacement on Demand system, so we commonized that with the Corvette system.
We made sure we can handle not only the Displacement on Demand for the non-Corvette engines but, also, the additional throughput required for Corvette's higher red line. Our processing speeds have to be fast enough to handle execution time for our code and still be able to accommodate the higher red line. We have to go through a certain number of things every time the engine turns over and we don't want to run out of processing power before the engine runs out of rpm.
CAC: The higher the rpm, the faster those things are happening.
SB: Exactly. Most of them are event-based, so they happen every time the engine is in a certain position and the controller has to be fast enough to keep up. Some of them are time-based and they happen every so many milliseconds regardless of engine speed.
CAC: While we're on controllers, do you have responsibility for the separate transmission controller on automatics?
SB: Yes. It's the "T42" controller used with the rear-wheel drive and what we call the 'step-gear' transmission, starting in 2005.
CAC: Why did they split the transmission and engine controllers?
SB: The primary reason was: we wanted to make sure the functionality of the transmission controllers could evolve at the same rate as their mechanical and hydraulic systems.
There are times when the transmission group would like to introduce new features and functions which would necessitate a redesign to a powertrain control module. That was a new design turn which would take place even though the engine wasn't getting any upgrades or changes.
We felt it better to divorce the two and keep them in lockstep with their respective engine and transmission development programs. Now, if the transmission gets some new characteristics, we can change the controller and it keep instep without tearing up the entire control system.
CAC: Does the manual also have a separate controller for the trans?
SB: No. The limited functions required to be handled electronically on the manual transmission are handled by the engine control module-output speed sensor, reverse gear switch, that type of thing.
CAC: Skip-shift is inside the controller, as well?
CAC: Any other controller issues that are important to note?
SB: They're both underhood-mount-capable. They have to be sealed against the environment. In that respect they're not changed.
CAC: Less mass?
SB: It has a little smaller footprint than the one it replaces. It is packaged in a smaller space, as is the transmission control module, but the two together make up a package that is, perhaps, slightly larger than the controller they replace. That they're two, independent control modules, means they have a little extra room for thermal dissipation and wiring access.
The second segment of the June 4th media ride-and-drive was just about done. It would end back at Portage Lake. Between nearly an hour together in a C6, Jordan Lee and I had become pretty good pals and could have sat another hour talking engines, but all good things must come to an end, so as we rolled down the two-lane road leading from Hell, I asked...
CAC: Any parting shot? Any summary? For instance, it must have a very gratifying program for you to work on. Maybe you could give me a little on how you feel about the engine.
JL: Our philosophy is to put a lot of emphasis on air flow. By doing that we're able to fatten up the torque curve with more conservative cam timing. You end up with an engine that has tremendous low and mid-range but you don't have to worry about losing the top-end-that characteristic trade-off-because you have the air flow.
Some of the engines of our competitors which are deficient on air flow at the top end have to cheat with very aggressive cam timing to get the top-end power numbers they're looking for. But they end up flat in the low- and mid-range, which is not very satisfying.
CAC: So it's not only the quantity of air flow, but also, the quality of it.
JL: Yeah. That's a big part of it because that's the enabler for combustion efficiency. We strive to really refine and massage the details because we think that's what really makes refined engines.
This engine was designed and developed by enthusiastic engineers who not only love engines, but love the Corvette. The opportunity to work on a high-performance engine is always a thrill for our Team. We're all very proud of the LS2. We think it's a remarkable engine. It's refined. It's very high power.
Like Dave Hill keeps saying: it's the only 400 horsepower sports car that you can buy which isn't a gas guzzler. I think that is quite phenomenal.
CAC: And doesn't cost a hell of a lot of money.
JL: That's right.
CAC: Like the Gen 3 was before it, the LS2 is a great engine. You and I talked earlier about how Chrysler kind of stole your design with its Hemi. Imitation is the best form of...umm...
JL ...the best form of flattery.
CAC: Yeah. That's it!
JL: If you look at their bottom end, it's nearly the same size bore and stroke. If you look at their cylinder head-with the exception of the two spark plugs and the hemi chamber-it's very much the same. In fact, the diameters of the intake and exhaust valves are exactly the same as ours. If you flow their head and flow an LS6 head, the lines lie almost right on top of each other.
CAC: Ya did all their work for 'em!
JL: Well...the (Hemi) name helped. Give 'em credit. They've done a great job with the marketing.
CAC: I agree, their marketing is phenomenal. What really counts, though, is your team has done its homework. I mean, your throttle response, the low-end torque and the top-end power, they are really very good. When the LS7 comes along, you guys are probably gonna scare me to death.
JL: That's our intention...and our hope.