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Originally posted by Knightrous
From my understanding (I'm happy to be proved wrong here), fuel usage is more related to engine load then engine rpm.

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Fuel usage is very much related to engine load AND RPM, however contrary to your thoughts SPECIFIC fuel consumption decreases with increasing engine load. By specific fuel consumption I am referring to the fuel flow rate per kilowatt of engine power produced, ie. it could be expressed by the unit l/s.kw (litres per second per kilowatt).

So why is that? - I'm glad you asked! (any of you olds out there remember The Curiosity Show??)

Firstly, there's the matter of internal friction in the engine. An engine running under no load (free revving) is still burning fuel but producing no usable power - the specific fuel consumption is therefore infinite. All the power the engine is producing is being used to overcome its own friction. Now from this point if you start to increase throttle but maintain RPM by putting load on the engine you start to bring down the specific fuel consumption. As load increases more the proportion of engine power overcoming friction reduces, further reducing specific fuel consumption. Once under full load the engine's friction only consumes a small percentage of the total power produced. Higher RPMs also equate to more friction, and more power required to overcome this before any useful power is available.

Secondly, there's the fact that fuel mix compressed to a higher pressure prior to combustion produces usable pressure in the cylinder for a higher proportion of the expansion stroke. This is why diesel engines are so fuel efficient, and also why high compression petrol engines return better economy than equivalent low compression engines. Cylinder pressures increase with increasing engine load, which due to the more efficient extraction of energy from the burning fuel results in lower specific fuel consumption too.

Now to throw a big spanner in the works, specific fuel consumption under constant load also tends to follow a similar curve to the engine's torque vs. RPM curve. As volumetric efficiency increases so too does the engine's ability to extract the available energy from the burning fuel.

The net effect of these factors is that the engine will have an RPM point where specific fuel consumption is at its best for cruise, and this will be very low - close to the point where the torque curve starts to flatten after its initial rise from zero. It may take 35kw to maintain a steady 100km/h cruise, and unless it has wild cams any engine will use less fuel producing the required 35kw at 2000RPM than it will at 3000RPM, because it is fighting less internal engine friction and burning fuel more efficiently at higher pressure.

I'd also like to add my 2c to the E153 vs. S54 debate in regards to fitting a 3SGTE. You don't HAVE to use an E153, and indeed a V6 will be just as capable of breaking an S54 as a 3SGTE. Breaking these boxes happens as a result of abuse, so which box you choose depends on how you intend to drive. If you want to dump the clutch at 6000RPM you're gonna break things eventually, but an S54 will take the torque of a mildly tuned 3SGTE forever if it's treated with respect.

Interesting comments in there Cogs, some real food for thought. Will see how I go with the fuel economy

Im sure we dont do all this modding to get better fuel economy, you'll be giving crap all the time anyway!

wow, very interesting indeed..

Gave her a quick blast on the dyno tonight...

Standard Bigport 4AGE with a pod filter. 258000km, no rebuilds. Standard 4AGE Ecu with no Oxygen sensor (running rich...)
Red = 89.9HP, this was run as how it turned up at the workshop.
Green = 86.7HP, this was with 5 degrees more timing wound in on the dizzy, it reduced the peak HP but the mid range picked up around 10-15%. It feels a little better around town now on the butt dyno, but I don't notice the missing 3.2HP...

Next time I'll take it in with the[url='http://www.youtube.com/watch?v=BBsyJoXajOI']2" straight through exhaust[/url] and the megasquirt in it for a blast.
These dyno runs are base line to compare/highlight the future improvements of the car.

I'm aiming for a conservative 150RWHP with the 1MZ at the beginning, pushing for 180RWHP with ITB's and tuned exhaust. Then to see if I can push it to 190RWHP maybe 200RWHP with staged injection and variable length intake and possibly some 2mz/3mz cams. For the staged injection, I'm looking at 4AFE 135cc injectors mounted right up close to the ports to provide good down low response and atomization, then have the standard 285cc 1MZ injectors about 10-12" up the intake to kick in up high. From all the articles I've read, 5-10% increase in HP due to a longer atomization time for higher intake velocities. Also a reduction in intake temps is a bonus side affect. The variable length intake is something I've wanted to play with for ages, I have ideas on how to drive this from the MegaSquirt, or a possible stand alone controller. The idea has been used before and was very successful in the Mazda 787B LeMans car. If I can get a 5-10HP gain by chasing the correct runner length across the rev range, I'll be happy!

If Adam is chasing 87+ rear struts, Randomist should have a pair from his wreck.
I can bring a compressor and rattle gun with me to speed up removing parts off your wreck.
Give me your E153 for a few weeks after I get the S54+1MZ sorted and I'll make up a jig for you in the shell. It might be useful for me also if I start breaking S54's and need to upgrade to the E153

Nice work I like a good project thread; your neighbours must hate you with that straight pipe!