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Old 03-22-2019, 06:58 PM   #1 (permalink)
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Is a 50 MPG Corvette Possible?

Hey everyone. I'm big into performance and power but I'm also very interested in efficiency. I drove a Chevy Volt for 4 years and now drive a 40 MPG diesel Mercedes. I'd like to hear peoples opinions on building a C5 (1997-2004) Corvette with efficiency in mind.

For those not familiar these cars equipped with the manual transmission were rated at 28 MPG highway with many people reporting over 30 MPG Highway. It's a 3200 lbs car that's very aerodynamic. They come with a 5.7 liter V8 at around 10:1 compression.

Here is what I'm thinking at the moment.

Very high compression by milling the heads or possibly different pistons. So about 13:1.

Designing a camshaft that would make this engine essentially run like an Atkinson engine does.

Lowering the car and using very light wheels/tires.

Free flowing intake and exhaust

Aero mods and some weight reduction.

Gear change to put the car around 1300 RPM at 65 MPH

The goal would be 50 MPG on the highway only, going no more than 65 MPH.

This purpose is just a fun experiment. What do you think?
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Alex

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Old 03-22-2019, 07:35 PM   #2 (permalink)
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I don't think it's possible, but I'm really interested to see what you can do.

The only reason I don't think it's possible is that the Prius gets 50 MPG at 65 MPH, and it's gutless and very aerodynamic.

Here's a chart someone posted with a 7L Corvette, which is still very impressive.



I'd go for a 50 MPG at a lower speed as a goal. Maybe you can do it at 45 MPH.

BTW, I think there are methods to calculate how much power you need to produce at a given speed by doing coastdown tests on your car. You time how long it takes to coast from a starting speed to some lower speed, and by knowing the weight of the vehicle, you can determine with some degree of precision the aerodynamic drag.
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Old 03-22-2019, 07:43 PM   #3 (permalink)
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Thanks for the chart, very cool. Funny you mention the Prius. I was thinking the same thing today. The Prius is actually a tad lighter. No doubt it will be tough or not possible especially at 65. At this point, I'm going to aim high and have fun with it. At the end of the day if i get to 40 or even 45 at a legal and normal highway speed that would be pretty awesome.
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Old 03-22-2019, 07:55 PM   #4 (permalink)
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The LS7 vett is kind of a gas hog.
I would think a normal 5.7L one could do close to 40 mpg if driven at around 55 to 60mph.

Upping the compression to 13 to 1 pretty much rules out lean burn.
Do the gear change.

I found a guy that replaced the 21lb stock wheels on a nissan leaf with used 10 pound wheels. These wheels new are about $620 each, he said they only gave him about a 2 to 3% increase in range. They do work but it's a huge expense for a pretty tiny gain.
If you could go with much narrower tires like a 225mm it might be worth it.
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Old 03-22-2019, 08:06 PM   #5 (permalink)
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I'll start with the easiest things and test after each mod. So a gear change is only about $350 so that's going to be one of the first. Lowering the car is cheap as well and I'll look into some lighter wheels. If they aren't too bad I can sell the zo6 wheels on the car to recoup some money there. But ya the 5.7 does a lot better in the fuel economy department.
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Old 03-22-2019, 08:11 PM   #6 (permalink)
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Welcome!

Hmmm... Lose anything that gives downforce, find some way to get narrower, less grippy tires, and if you can do taller gearing, do it. A powerful engine not working hard can afford the taller gearing, and that'll give good results with the right driving technique (as long as you do get the manual).

I never miss a chance to recommend against modifying the engine innards. Even in a sports car with a manual, there's too much slop between the combustion event and the miles the car travels on a gallon for a custom designed camshaft to be the first, tenth or even fiftieth place I'd look to increase overall efficiency. I'd recommend an engine swap first.

If you want to get amazing mileage in a cool car that nobody thinks of as an mpg machine, do it. That's awesome. If you're an engine nerd and want to design a camshaft that will totally change the character of an engine like this and need to radically modify the rest of the car (and your driving style) to hit a great sounding mpg threshold to show off your engine design skills, do that too. I think that's more awesome.

I'd be tickled to drive a Vette hacked for mileage. That'd be more fun than driving a stock one.
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Quote:
Originally Posted by sheepdog44 View Post
Transmission type Efficiency
Manual neutral engine off.100% @MPG <----- Fun Fact.
Manual 1:1 gear ratio .......98%
CVT belt ............................88%
Automatic .........................86%

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Old 03-22-2019, 08:25 PM   #7 (permalink)
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Thanks for the reply. I'll definitely try out engine mods after everything else. I want to stick with the LS1 V8 engine though. I thinking swapping in anything else would take away from the fact that it's a Corvette. I know that changing the cam and making it run more like an Atkinson would reduce power but I'm curious to how much especially with exhaust mods like headers.

At the end of the day I've gone with the C5 because it's really cheap to modify. If I do a cam swap and have the heads milled and it's doesn't do what I want I can always go back to stock. The heads on this year car sell for a couple of hundred bucks used. I have a lot of LS experience so I'll be doing all the work myself including the tuning.




Quote:
Originally Posted by Fat Charlie View Post
Welcome!

Hmmm... Lose anything that gives downforce, find some way to get narrower, less grippy tires, and if you can do taller gearing, do it. A powerful engine not working hard can afford the taller gearing, and that'll give good results with the right driving technique (as long as you do get the manual).

I never miss a chance to recommend against modifying the engine innards. Even in a sports car with a manual, there's too much slop between the combustion event and the miles the car travels on a gallon for a custom designed camshaft to be the first, tenth or even fiftieth place I'd look to increase overall efficiency. I'd recommend an engine swap first.

If you want to get amazing mileage in a cool car that nobody thinks of as an mpg machine, do it. That's awesome. If you're an engine nerd and want to design a camshaft that will totally change the character of an engine like this and need to radically modify the rest of the car (and your driving style) to hit a great sounding mpg threshold to show off your engine design skills, do that too. I think that's more awesome.

I'd be tickled to drive a Vette hacked for mileage. That'd be more fun than driving a stock one.
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Old 03-22-2019, 08:48 PM   #8 (permalink)
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For a DD, the Corvette is... irresponsibly overpowered. Reducing its power won't change anything but its 0-60 time- and like I always say IRL, I don't drive 60, and I don't live a quarter mile from work.

Hypermilers tend to corner hard, but they don't tend to drive cars that were built to do that. There's one corner in the next town that my daughter claims I take on two wheels. My son, who sits on the side that allegedly lifts off the ground, backs me up when I deny it. If I took them through that just once in a Corvette I'd never get any backtalk about my Honda.

I'm not up on Corvette aftermarket stuff, but since you're DIY wrenching, I'd look into taller gearing. Then aero- lose any downforce and block the grille, and then work on tires.

A high mpg Corvette is exciting, whether or not you hit 50.
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Quote:
Originally Posted by sheepdog44 View Post
Transmission type Efficiency
Manual neutral engine off.100% @MPG <----- Fun Fact.
Manual 1:1 gear ratio .......98%
CVT belt ............................88%
Automatic .........................86%

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Old 03-22-2019, 11:52 PM   #9 (permalink)
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I know it's not apples to apples, but someone on here averaged 44 MPG out of an old Chevrolet Caprice with some ecomods, and what is probably the tallest geared transmission out there. Cruising speeds were between 700 and 1300 RPM in top gear if I remember correctly.

As mentioned lighter, more aerodynamic wheels, skinnier low rolling resistance tires, etc. all help.

Rather than upping the compression I would try to tune the car for lean burn. Lean burn is worth a good 15%-20% improvement in fuel economy, depending on how lean you car can handle.
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Old 03-23-2019, 12:35 PM   #10 (permalink)
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Computer controlled engine management. Howell conversion comes to mind, custom economy map applied to that. Direct sequential fuel injection possibly even a GM 8-6-4-2 cylinder operations.

Depends on your smog control inspections.

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