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Old 06-11-2004, 01:25 PM   #1
GeoffSchultz
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Member Since: Dec 2002
Location: Houston TX
Default Building a SBC for maximum fuel efficiency

One of these days I am gonna want to put a small block chevy in a little car (maybe an old 240z, for which there are chevy 350 conversions) and use is as a daily driver. Thus, I would want it to be fun, but get the best gas mileage possible.

Does anybody know how to go about designing a small block chevy (I guess it would apply to just about any engine) for maximum gas mileage?

For example, I guess you would use a smaller carberator, small CC heads, make the rotating assembly as light as possible, use high compression. Not sure about these though, and I'm sure there are all sorts of other considerations.

Would a 383 be less fuel efficient than a 350, all else equal? Are there aftermarket fuel injection systems that can be purchased that give markably better gas mileage than carberators?

Any discussion would be appreciated.
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Old 06-12-2004, 10:31 AM   #2
SWCDuke
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Default Re: Building a SBC for maximum fuel efficiency (GeoffSchultz)

There are three basic strategies whose contribution is cumulative.

Low engine friction
Precise fuel control
maximize low end torque

To achieve these you want:

Short stroke and low backpressure exhaust system
Modern tunable EFI system
Very conservative cam and tall gears

Duke
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Old 06-14-2004, 04:35 PM   #3
GeoffSchultz
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Default Re: Building a SBC for maximum fuel efficiency (SWCDuke)

Thanks Duke.

Question about the length of the stroke---I guess you say to use a short stroke because that means the pistons travel less distance and therefore create less friction.

However, doesn't using a longer stroke mean you can keep your RPM's lower on average (i.e., same horsepower with lower RPMs)? Doesn't the lower RPM's offset this somewhat?

Thanks again.
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Old 06-16-2004, 07:51 PM   #4
GeoffSchultz
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Default Re: Building a SBC for maximum fuel efficiency (GeoffSchultz)

bump
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Old 06-16-2004, 10:58 PM   #5
69stingray
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Default Re: Building a SBC for maximum fuel efficiency (GeoffSchultz)

Efficiency is the key word. There is a book out there...forget what it is called, something like Engine Design for Fuel Efficiency.....or something like that. Forget where I saw it.

You do not want to was fuel. Fuel injection allows the computer to control the fuel, this is good. Efficiency exhaust is required. You may want to Look into Holley Projection-4 system, which is a "4-barrel" TBI. You can put this on top of an Edelbrock performer (Vortec style, see below) manifold.

Complete and efficiency combustion is required. High-quench design would be helpful. Flat-top pistons with zero deck clearance. This will drive your compression up, therefore you may want aluminum cylinder heads, like Edelbrock E-170's.

Edelbrock E-170 have a good combustion chamber design, excellent exhaust port and good intake port velocity. From Edelbrock website...."They incorporate Edelbrock’s new high-quench combustion chamber which promotes a more uniform atomization of the air/fuel mixture for more power. Spark plugs have been positioned closer to the center of the cylinder for more efficient combustion./"

Lower your engine speed, the less fuel you use, therefore, you will need a "low" duration cam. An overdrive transmission will keep your RPM low during cruising, you want to ensure you have plenty of torque for when you step on the pedal at low RPM. A retro-fit roller would be ideal to kept the friction low, you will also get more power from the greater lift. Something along the lines of 200/206 duration @ 0.050" lift.

You may also want to look at 4.3l V6 options. Plenty of aftermarket parts, except cylinder heads. Stock Vortec 4.3 are plenty strong for a small car!

Maybe someone will chirm on the turbo/supercharger options. I think if an engine is design correctly, you can have a fuel efficcinecy engine with plenty of power.
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Old 06-16-2004, 11:46 PM   #6
SWCDuke
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Default Re: Building a SBC for maximum fuel efficiency (GeoffSchultz)

Quote:
Thanks Duke.

Question about the length of the stroke---I guess you say to use a short stroke because that means the pistons travel less distance and therefore create less friction.

However, doesn't using a longer stroke mean you can keep your RPM's lower on average (i.e., same horsepower with lower RPMs)? Doesn't the lower RPM's offset this somewhat?

Thanks again.

Everything is a compromise. For the same gearing a longer stroke will provide better performance but less fuel ecomony. For maximum economy you have to sacrifice performance or make some reasonable tradeoff between the two.

At some point as you attempt to lower revs both fuel efficiency and driveability are lost. Low revs and high load yield best fuel efficiency for spark ignition engines to minimize pumping losses.

In the case of the C5 manual, sixth gear yields about 1500 revs at 60 MPH and they will deliver about 30 MPG - about the same as a sedan of the same weight with a V-6, but the C5 delivers poorer city mileage because the engine is operating at high average internal friction eventhough for the EPA test the speed-time profile is the same for all cars.

The extreme case is current hybrids. They yield the best fuel economy, though most users are achieving much lower than the EPA numbers. That's because in real world driving you have to beat them with a whip to keep up with traffic, and on a long grade their performance drops off drastically once the battery energy is consumed.

Modern truck HD diesel engines are usually governed at somewhere between 1800 and 2100. Hilgh boost pressure is used to get high torque from the smallest displacement to minimize friction losses. They will make greater power at higher revs, but are governed at relatively low speed to increase fuel efficiency and longevity. Diesels achieve better less than full load fuel economy than spark igntion engines partlly because they are not throttled and don't have the associated pumping losses.

Duke

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Old 06-26-2004, 09:35 AM   #7
Ganey
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Default Re: Building a SBC for maximum fuel efficiency (GeoffSchultz)

An interesting question. Into perf. instead of fuel efficiency. It's easy:
With a light car like that a 350 makes sense for fuel efficiency. You might be very surprised what you could get w/ a 350, headers, duals, O/D even w/ a carb. Then you could go to a factory LT1/4 FI.
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Old 06-26-2004, 09:35 AM
 
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