Fuel consumption

[Guest Nic-Rebel450CA]

I was just reading a page at my other favorite site:

http://auto.howstuffworks.com/question477.htm

 

Then realized that in an equation to figure out engine load, if the cars were identical, all of the variables would be the same, regardless of engine size. Would this indicate that if you are cruising at the same RPM in the same car that the displacement of the engine would have no bearing on the fuel consumption?

 

Why or why not?

[Guest greimann]

In theory what you say is correct, but reality is all motors have a different brake specific fuel consumption (BSFC). This unit of measure is in pounds of fuel per hour that a motor will use. The lowest BSFC (best economy) is at torque peak. So at a steady cruise where you only need a few HP to keep moving, a 4 cylinder motor will typically give better fuel economy because its BSFC is lower than a V8, which had more internal friction to overcome due to larger displacement. There is also more to ther BSFC rating than just displacement. Induction and exhaust efficiency play a big role. If you were to compare the BSFC of a '70 Pinto four cylinder and a '03 LS1 V8, I believe the V8 would be vastly more efficient because it has benefitted from years of improvements in fuel injection and cylinder flow.

 

The important thing to remember is the best economy is at torque peak. This is why continuously variable transmissions are being developed. The trans can slelect a ratio that keeps the motor at torque peak no matter how fast the car is moving. http://auto.howstuffworks.com/transmission1.htm

[Guest Nic-Rebel450CA]

Ok, so basically the variable would be the amount of trouble the engine has to go through to maintain that power output based on its internals.

 

My reason for asking is because I am trying to select a SBC for my Z. I was going considering selecting a lower displacement SBC to try to help the fuel economy. After doing some thinking though I started wondering if the difference would be so small that I might as well just go for more cubes!

 

Certainly a larger CI engine will gulp more gas when accelerating and racing, but during normal cruising and constant speed driving I would imagine it might be pretty close to the same.

[Guest greimann]

I think a more important question is what kind of fuel system are you going to use? Go back to my Pinto and LS1 comment. A modern fuel injected motor is going to have much better fuel control and thus a lower BSFC throughout the rev range than a carbureted one. Also, if you are building a motor, head and cam selection have a lot to do with economy. In general you want good efficiency at low RPM because that is where the motor spends the most time.

 

Bottom line is to put the money into the best fuel injection most efficient cylinder heads and mild cam.

[baddriver]

In my Chevy TPI and TBI swappers guide, written by the Jags that Run crew, they note that they don't find any difference in the fuel consumption of the 305 cars vs the 350 cars, even though the 350's have significantly more power. I think you will find that this is the case. Even though the size of the engine does have some effect on the efficiency, gearing and aerodynamics play a much larger role. I think that it is possible for the 305 and 350 to both efficiently drive a z car. As long as the gearing is appropriate (i.e. overdive) the engine developes the power needed, and very little energy is wasted.

 

I have a long, drawn out theory as to why, but I am having trouble putting it down in coherent form today, so I'll simply assert that I am correct.

 

Jeff

[Guest Simon]

One of the reasons why diesel engines are so efficient is because the do not have throttle bodies. This means the piston does not have to pull against the TB therefore the vacume is reduced.

 

(DO NOT DO THIS TO A PETROL CAR OR IT WILL BLOW UP)

 

BMW has been developing one though which varies the cam timing to allow the correct amount of air in. SMART

 

Sim

[80LS1T]

Diesels dont produce a vaccum. Thats why they need a vaccum pump to power things like the brake booster.

 

Guy

[Guest Nic-Rebel450CA]

Ok, so I guess I should base my selection on which engine has the best options available for heads and intake. All small block chevys can use the same intake, can they also use the same heads?

[Guest greimann]

The best fuel injection should be #1 consideration. LT1 or LS1 motors will be your best choice.

[A. G. Olphart]

Larger engine displacement brings larger pumping losses, and more friction in general due to increased surface areas, whether swept or rotating.

 

My personal favorite SBC is the early (small journal= less friction) 327. Big enough to do the job, small enough to feed. With a set of Vortec heads and a relatively mild cam (avoiding excessive duration and overlap) this engine should make a pleasant/spunky street Z. Note that the Vortec heads require a special intake manifold, use 'rail' rockers, and must be modified to run wilder cams.

 

Most Chevy heads interchange but I seem to recall that there can be issues with big valve heads and small bore blocks (besides the later generation 'won't fit' stuff). Chamber volume & C/R are obvious considerations.

 

If a person desires to play mix and match this is a handy book: "CHEVROLET SMALL BLOCK Parts Interchange Manual" by Ed Staffel. (There may be better ones, but this happens to be the one on the end table).