High-MPG Corvette

[roflwaffle]

Originally Posted by theunchosen

Cylinder deactivation is not really a good solution.

Even if those cylinders don't have any vacuum. . .have you tried to push a piston in a cylinder? Its not bad if you are just moving one piston through the cylinder, but even then. . .it requires a hammer and a block of wood. To turn the crank with no vacuum on a 4 cylinder I have to have a 2 footer and still pull hard enough that working out later would be a mistake. The engine is fighting that same force about once a second in a v8 if you disengage half the cylinders(by disengage lock the valves open during compression, intake, exhaust and expansion, shut off sparks and injectors).

Compressing the trapped air and the extra friction from the shut off cylinders certainly isn't as good in terms of overall FE as going w/ a properly geared 4cyl, but it's still way better than an improperly geared V8.

My 2008 Chevy Tahoe has the Eaton Active Fuel Management system (internally called "Displacement on Demand"). After 24k miles, and using the ecm provided instant fuel economy, I can see the change from 18 to 26 mpg on a flat road at 55 mph very day, both directions. This includes trailer towing(boat and hay delivery). The ratio is the same when using 85% ethanol.

Originally Posted by theunchosen

If you really wanted to make a vehicle sporty but FE friendly as well you have pretty much 2 options. Forced induction from a supercharger that you can switch on and off with a secondary air intake that doesn't have to flow through the pre-cooler, compressor and extra piping or two small engines with the ability to completely disconnect either from the drivetrain completely and deactivate it.

Option one is obviously more simple. Its also obviously cheaper(especially if you do it yourself).

Option two is much more complicated. Its most likely 3-4x more expensive and you have to do it yourself.

Most people chose the charger and an on off switch, but those who chose the secondary engine have something thats really honestly worth bragging about. If you for example took 2 H22A redtops and dropped one in front one in the rear you'd have more hp than the 08 corvette(430 for the corvette and 437 for the twin engine). IT also still leaves you the option of integrating forced induction on whichever motor you won't be using for the FE runs(or both if you go with a supercharger-switchoff). Another large advantage to this over the standard corvette block. . .they have 2 valves per cylinder whereas the h22 has 4.

It also buys you awd that doesn't get bogged down at launch like every other awd system ever made. The problem with AWD arises with the differentials to split the power. Even the GTR's enormously expensive drivetrain bogs down as it switches power over from RWD at launch to AWD pull, given its much better than anything else but it still happens. The only expensive linkage to make sure each engine is sharing the load equally. . .is the billions of dollars of. . .asphalt.

Theoretically I am a fan of the twin engine idea, but practically. . .its not feasible. The advantages two engines buys couldn't be outweighed by the difficulties in manufacturing it. Yes if something goes wrong you could limp home on one engine without further damaging the other at all, but you would need to build a rear engine mount, move the fuel tank, and build a shift linkage that mated up to the transmission in the front as well.

Iono, both those sound really complex compared to shutting off a couple cylinders and increasing mileage by a good ~50% at low loads. Granted, they might be good for a ~60% increase instead of a ~50% increase since there isn't as much cylinder friction and no heat loss from compressing/expanding air in the close cylinders, but I don't think the extra couple mpg is worth all the effort/complexity.

[evolutionmovement]

Forced induction also usually means a lower CR, which is bad for FE. I lost 3-4 mpg going from 9.5:1 to 8.0.

[Big Dave]

Like any good project, the key is in the upfront costs. This project requires that I find a distressed Corvette I can pick up cheap. Just gotta keep looking.

The 'Vette offers one other thing: I can easily score 2.73 gears for one as all automatic C5 and C6 'Vettes come with 2.73 gears. That gets me down to just under 1100 RPM@70 RPM. Drivetrain nirvana.

The Cummins 4BT3.9 is about 150 lb heavier than a LS2.

Driving slow is a logical paradox. You drive because you want to get somewhere faster than a walking speed. The most energy efficient speed is zero - you simply don't go at all.

I like to maintain at least a foot in the mainstream camp, but I drive so much that anything over the limit for as much as I drive guarantees you a ticket. Bad enough that so many jurisdictions are writing "revenue tickets." I don't wanna give 'em any more, but I do make the trip for a reason.

[jamesqf]

Originally Posted by theunchosen

But I agree with James, smaller lighter and more nimble feels sporty.

Looking from the other side, there are just not a lot of places in this country where you can drive your 400+ HP Corvette at anything approaching its top speed without risking - at the very least - serious speeding tickets. But there are any number of roads where the white speed limit signs say 55 or so, and the yellow signs with the curvy arrows say 20 :-)

[Big Dave]

With 2.73:1 gears you could move to smaller diameter, narrower tires and generate some clearance for fender skirts.

[roflwaffle]

Originally Posted by Big Dave

The Cummins 4BT3.9 is about 150 lb heavier than a LS2.

Supposedly a 4BT is 700lbs dry w/o an intercooler, while an LS2 is supposedly at ~450lbs w/ accessories, so we're looking at a ~250lb more weight at least. There's also going from ~400hp/3000lbs to ~150hp/3250lbs, so we've gone from a two seat sportscar to something w/ about the same power to weight ratio as a new Prius that gets the same or worse BTU adjusted mileage and can only carry a couple people. To each their own I suppose, but if I'm going to the trouble of finding a vette as a donor chasis, I would at least want it to still behave like a sportscar. I think a C6 w/ a blown engine, preferably head issues, along w/ an AFM head, would be the lowest cost/best performance/easiest solution, unless of course the owner wanted to run WVO or something, then they have to go w a diesel.

Originally Posted by Big Dave

Driving slow is a logical paradox. You drive because you want to get somewhere faster than a walking speed. The most energy efficient speed is zero - you simply don't go at all.

Um, at 0 mph, there's no context for energy efficiency, since the vehicle isn't even doing anything, unless of course it's just idling, in which case the driver is seeing 0 mpg, which is as inefficient as we can get. In terms of driving slower than traffic, there's no logical paradox, just a desire for lower transportation costs and/or greater safety in terms of accident avoidance.

[MechEngVT]

Something tells me that you're probably not going to cram a 4BT into a Vette without a dry sump oil system, a huge cowl bubble on the hood, or both.

V8 engines are wide, but quite a bit shorter than straight-up inline engines. The 4BT is straight up, has a pretty long stroke and long rods, and was not designed with anything less than a medium duty truck package in mind.

If you could install it at a 30-45 degree angle a la /6 and make it work it might fit, but good luck getting the oil to drain properly and the cooling system to burp air out.

[Big Dave]

There have been a handful of guys who have stuffed 6 cyl Cummins B engines in older 'Vettes. Whether they used dry sumps, I don't know. Bubble hoods are common among modified 'Vettes and are available off-the-shelf. The four may have some minor clearance issues by the problem is not insurmountable. The Cummins 6 cylinder weighs over a half ton and would make the car too nose heavy. The four weighs about asmuch as a 454 and there is no shortage of 454 'Vettes (C4 and older) running around.

Too bad the GM 4.5 liter diesel is as dead as OJ's acting career.

The bigger problem for me is headroom. "Vettes are designed for 6'1" max guys. I probably should start with a convertible and make a aero but taller coupe top.

Performance is something I don't sweat. The 4BT3.9 can be easily and safely turned up to over 250 HP. A more streetable approach is only turn it up to maybe 150 HP but double bottle (propane/nitrous) the engine for short bursts of fearsome acceleration. The 6b5.9 has been hot-roddedto over 900 HP, and 600 HP versions seem to be lasting, so a short burst (15 sec) double bottle 4BT3.9 should be A-OK at the 400 HP level.

Another option might be a M-B 617.952 engine, but those make very inadequate low end torque. The Cummins is a torque monster and not a bit put out about running at 1100 RPM.

[Frank Lee]

Don't know about the latest gen, but earlier Vettes were pretty junky. I'd start with something else.

[theunchosen]

I also would avoid juice.

The 08s weren't bad especially the high end.

But Juice is. . .a mistake. Too many variables that can easily go wrong. You nitro dump and your sensors are behind just a tenth of a second and you've lean burned your cylinders about 3 and a half times. Not just a little lean either, very. Also the nitrous oxide is much more flammable than gasoline and much more prone to pre-ignition.

A toasty valve head because your coolant is lagging a little is enough to toast the engine.

Those big hp setups are probably steady state. . .so ALL the components are designed for enormous loads to start with. Yours won't be. Their coolant lines are driven by turbo pumps, they are running specialty coolant and they probably brush their cylinders, pistons and valves every few hundred miles to keep residue out(think glow dust).

I've seen a few juiced engines that are junked just because a little bit of carbon residue was warm when they started purging. The first couple of lean strokes got it glowing and then nitrous ignites and bores a hole through the piston.

The pistons afterward make for good conversation pieces. If its going to drop big power all its systems will need to run as if its going to do it all the time otherwise they go bye bye at the wrong time.