The message from "scott" contains these words:
Out of interest - why should combustion be inefficient at low revs. I'd have thought that the longer time available would have improved matters, not worsened 'em.
Of course, there's longer for the gases to lose heat to the bore and head and piston crown...
I haven't a clue! But something must be making the engine less efficient at lower speeds, and inefficient combustion was the only think I could think of!
Yeh, probably, I don't know enough about the exact process of combustion to comment!
It's the 1.9, and it's certainly good for more than 100... tops out at bout 110, but it's geared for 135.
Paul Cummins ( snipped-for-privacy@vlaad.co.uk) gurgled happily, sounding much like they were saying :
I think your speedo might just be lying worse than a politician.
" Tim (Remove NOSPAM." wrote
...when producing a high value of torque. When that torque is not needed, then the most efficient operation is at the lowest possible revs.
Engine "efficiency" is only one part of the equation - how much energy an engine can deliver from a quanity of fuel. The other half is power demand by the car. Combined they make "economy".
At low speeds (sub 40 mph, say), the power demand at the wheels is probably less than under bonnet friction (engine, alternator, pumps, geartrain). Economy stops improving below a certain speed because some of these energy drains are time dependant, not distance dependant. They are also rev dependant so keeping that down with the highest reasonable gear helps too.
Answered in my previous post. Cam timing and valve overlap optimised for higher rpm.
Car works, is taxed and legal, and gets me where I want to go. If the speedo is gererous, so what...
Similar economy & performance to an older 250
You've got to qualify that "max torque rule" - drive at max torque at a speed where wind resistance isn't too high. So, if your gearing in 5th translates max torque to 90mph, it isn't going to do you much good as you drive into a brickwall of air. Nor perhaps the notional 72mph in 4th gear, but maybe 3rd gear at 56mph?? ;-))
Playing devil's advocate a bit there, as clearly driving in 3rd gear at 56 mph is *not* as economical as 5th gear at 56mph - so where's your "max torque rule" then?
Is this rule perhaps more useful in larger capacity engines, or diesels, with fairly flat torque curves at lower revs?
I think I would like wise, in top.
Have you looked at whether Toyota ST185 GT4 injectors would fit, the=20 ST185 are 440cc's and the ST185 and the generation4 Supra are 550CC.
--=20 "Sorry Sir, the meatballs are Orf" The poster formerly known as Skodapilot.
Got it wrong I thought the chip wasn't included, Stage 3 chip, 300ZX AFM and injectors is £375.
ST185/205 are side feed, I need end feed like ST165. 550cc would easily allow over 400bhp, just needs a really big turbo, tubular turbo manifold, 3" drainpipe, custom map.
One person on SXOC has welded a set of injector bosses on his inlet manifold so he has 8 injectors, to run 2x370cc = 740cc per port. He put 2 stock AFM's on a Y pipe at the inlet but only wired one up so the ECU sees 1/2 the air and gives 2 * 1/2 the fuel. Stock AFM's max at about 290bhp so he's limited by AFM to about 580bhp. Stock parts are quite cheap secondhand, complete cars with knocking big ends and/or rotten sills and chassis rails are about £150.
-- Peter Hill Spamtrap reply domain as per NNTP-Posting-Host in header Can of worms - what every fisherman wants. Can of worms - what every PC owner gets!
Excellent link - thanks Nick.
Indeed. Most road vehicles have a most efficient speed between 40 and 60mph. Modern and well designed cars tend towards the top of this range, so 56mph will be very close to the most efficient speed for most modern cars, particularly as it will be a very shallow curve at this point, so a few mph will matter little. Vans and lorries, and older cars with poor aerodynamic qualities will have a lower most efficient speed and a sharper drop off above the speed.
This is based upon pure aerodynamic and engine properties and assumes a gear is available that capitalises on the most efficient speed and engine revolutions. The gearing may skew the actual most efficient speed somewhat if not optimised for this parameter.
Other parameters may affect the most efficient speed. Opening the window will likely reduce the most efficient speed, as it disproportionately affects the air resistance, which is approximately related to the square of speed. Turning on air conditioning will likely increase the most efficient speed, as it provides a relatively constant load, so less energy will be used on a faster run of constant distance. It will also increase engine efficiency, as engines are most efficient run near 100% load. (For the hard of thinking, that is very different to saying cars are most efficient when run near 100% load).
Christian.
The aeodynamic resistance to motion (force) is proportional to the square of the speed - so the *power* required is proportional to the cube of the speed. In addition, there is rolling resistance - which is more or less constant, but increases slightly with speed. But above about 50mph the rolling resistance is fairly insignificant compared with the aerodynaimic resistance.
Whenever the engine is required to produce an amount of power which is less than its maximum power, it can do this at any number of different speed/torque combinations - some of which are more efficient in terms of mechanical energy produced per unit of fuel consumed than others. Engine manufacturers map these characteristics, having carried out numerous dynamometer tests. In general, an engine can produce a specified amount of power most efficiently when it is running at low speed and high torque rather than vice versa.
A vehicle can be made as efficient as possible if the gearing is such that the engine can be made to run at the most efficient point on its map for all road operating conditions. This requires infinitely variable gearing rather than a small number of stepped ratios. Although several motor manufactures (e.g. Daf) have tried this, none have been totally successful. Right back in the 1920's there was an Austin Hayes with an infinitely variable transmission - using tilting discs running between two hemi-spherical cups. This concept was developed further by Rover/Perbury in the 1970's but unfortunately never reached production. The idea was that the highest gear would be very much higher than normal top gear - maybe 60mph per 1000 rpm, so that the engine would only be doing 500rpm at a steady 30mph - but with the ability to change down very rapidly when acceleration was required.
If this could be made to work, it could result in some incredibly high mpg figures at low road speeds - which would certainly reduce the max efficiency speed to considerably less than the speeds mentioned elsewhere in this thread.
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