Maximum fuel economy when designing engine?

[Hammerhead75]

Hello all--newbie here.

When you are designing an engine, what do you do in order to get good power but also good fuel economy? I purchased Dynomation dyno simulation software to help me design but am at a crossroads.



I hear that if you want good fuel economy, stick with low rpm torque, which basically makes for a tow motor I think? Not a lot of accleration but good throttle response (they say)--whatever that means. I was also told that building a 'red light rocket' will make for very bad MPG. This is for a daily driven street vehicle--I don't race.

Goals are something like 350+ hp, plenty of torque (400+?), 250k miles, and 25 mpg. I'm willing to put on Tri-Y's, a 4L65e so I get overdrive, etc. This is going into a first generation Monte Carlo.

I know this can be done as I've personally ridden in a 350k+ mile 72 Impala convertible that gets 22 mpg with no overdrive, 4-into-1 headers, and highway torque that beats anything I've been in, all out of a large displacement naturally-aspirated small block. Part of the reason for my design is to surprise this person (my mentor) with an engine of my own without letting him know.

My current plan:
small block Chevrolet
4L65e
Tri-Y
Jacobs ignition
Eaton limited slip
3.23 (or so) in the rear
BFG radial T/A tires,
and the list goes on and on, but wanted to give an idea.

Would an engine built to these specifications be more of a highway cruiser or a red light rocket? Given three wishes, one would be torque especially in the 30-90 mpg range but I'm open to discussion.

Any input would be appreciated.

[speedster]

Hey Hammerhead, welcome to the site.

This book is a good place to start:
PERFORMANCE WITH ECONOMY By David Vizard. Detailed guide points the way to top performance with good gas mileage. Includes choosing low-restriction catalytic converters and mufflers, turbocharging for maximum performance, using octane boosters, etc. & lots more. 8 1/2" X 11", 128 pages, 335 photos. Sftbd. Item # SA09...$18.95

Good Luck with your project.

[gen3bu]

teflon/thermal coatings, and roller bearings everywhere. keep the cranking compression in the 180-210 psi range.

[Hammerhead75]

Thanks for the input--I'll have to try and locate that book.

How does the torque curve, manifold vacuum, that sort of thing, determine fuel economy?

[Scott Parkhurst]

The less the engine has to work, the less fuel it will use. If it's got a nice long stroke that makes gobs of torque, it will be able to lug the car along on the freeway in O/D without much effort. Let's say this requires 150 ft-lbs., and a well-built 406 can do this at 1,400 rpm.

A 327 with a shorter stroke may need to go to 1,800 rpm to acheive the same thing...and may use more fuel in the process even though it's a smaller-displacement engine.

Designing the powerplant to run comfortably in overdrive means careful consideration of camshaft, rear axle ratio, and rear tire height to coordinate in a refined package. The transmission's O/D ratio is part of this formula.

mph = (rpm x tire diameter) / (gear ratio x 336)

rpm = (mph x gear ratio x 336) / tire daimeter

gear ratio = (rpm x tire diameter) / (mph x 336)

tire diameter = (mph x gear ratio x 336) / rpm



I hope this helps.

~SP~