Fuel Economy, Vac Vs Rpm

[KinLoud]

You are dealing with several things at once when you drive for fuel economy (if you care about it)
Aerodynamic drag, Engine and Rolling drag.

Vacuum gauge (economy gauge) - All that the vacuum reading (sort of) shows you is how much power you are using if you are at a constant speed...
Think of it as RPM x vacuum/boost = power.
So at the same road speed 15" vacuum in 3rd gear = 10" vacuum in 4th gear - same power required to keep the same speed.

Aerodynamic drag - depends on the speed, the frontal area of the car and the drag coefficicient (how slippery the car is... KE70 box vs newest celica)
BUT - the drag force increases as the square of the speed!
Double your speed and get 4 times the drag force!!!!!!
Engine - fuel consumption (brake specific fuel consumption - BSFC) is measured in grams of fuel per kw per hour (or similar). All engine will have a "sweet spot" where the engine is most economic.
Following chart is from here http://www.geocities.com/MotorCity/Pit/ ... ption.html

Brake Specific Fuel Consumption [g/PSh] gramms per PS per hour (PS=japanese horsepower)
The * indicates the most economical rpm
year 1.6k 2k 3k 4k 5k 6k 7k near redline
3M 66-71 268 258 245 245 235* 255 287 (6.
9R 67-68 261 255 244 244 230* 244 267 (6.6)
8R-G 69-72 286 257 232 218 211* 218 246 (6.6)
2T-G 70-74 272 267 244 242 225* 233 267 (6.
3T-GTEU 82-85 212 206* 212 244 279 326 347 (6.4)
5M-GE 82-86 211 208* 208 218 244 288
6M-GE 84-86 211 208* 208 214 233 265
1G-GTE 85? 215 210* 240 265 290 335 370 (6.5)
3S-GTE 86 215* 240 290 307 307 310 330 (6.5)
4A-GZE 86 225* 232 271 279 293 311 339 (6.
7M-GTE 86-92 217 215* 233 256 282 307 340 340 (7.0)
4A-GE20 91-96 220 210 202* 204 208 212 228 242 (8.0)
(Note - these are mostly performance engines, FE engines will have lower bsfc)

So if you have a 20v the most economical rpm is 3000rpm. See how the fuel consumption only increases from 202 at 3000rpm to 212 at 6000rpm. So the rpm is generally not too much of a problem. BUT - if you drive at double the speed the power required will be 4 times as much!! See aerodynamic drag.

Rolling drag - is fairly linear, make sure you tyres are pumped up and cheack you wheel alignment doesn't have too much toe in or out.

So overall - for the most km per litre pump your tyes up and drive slowly in the highest gear. Driving slowly means you will need the least power and use minimum fuel.

End of rave

Ken
Ham
021 408 863

[KinLoud]

Sorry - looks like I was too ubergeek in my reply and killed this thread...

Ken
Ham
021 408 863

[Dirtbag]

thats some motherness stuff there! very informative. i drive a 20v, so that 3k thing is kinda interesting, i drive it around highest gear, 40k + in 5th, 3rd gear for 50kph wud jst feel weird

[KinLoud]

Yeah - what I don't know is if the bsfc is for full throttle/full power or at all any power but at that rpm?
I guess that it is at full throttle as the pumping losses of trying to suck air past a partly closed throttle plate will be less efficient.
Most efficient will be top gear at low speed as I said in my first post.

Ken
Ham
021 408 863