ideal RPM

[zooky]

I installed a 1.6TD in a 2wd toyota pickup. It has a Giles pump and I will be running about 18PSI boost, intercooled. The truck weighs 2600lbs and I will be staying with the stock 25" tires. Right now it has an automatic and I am probally going to swap in a 5-speed. It is going to be a commuter, 100 miles round trip, half of it highway. There is a wide range of gear ratios for the diff, from 3.07 up tp 5.71. What is a good cruising RPM for this engine? I would like good MPG but dont want to kill the "performance"

[Patrick]

My mk2 Jetta weighs in at about 2500 lbs, with a 3.94 final and a .75 overdrive it's still peppy enough, running a 1.9 though. Wouldn't want any more gear than that.........  Ballpark of 2000 rpm @ 80 kmh/50mph, 3000 rpm@120kmh/75mph

[jtanguay]

so judging from that, economy is around 2000-2500. 2000 is pretty low though.. i would not try to accel too hard from that rpm (in a higher gear) too hard or risk snapping something, or roast some bottom end pieces.  however now that you have a Giles pump, that changes everything, unless the pump is relatively stock.  i'm pretty sure the fuel consumption curve will retain its shape, but either increase or decrease depending on how much 'punch' its packing.

[zooky]

good info, I'll shoot for the upper end of the 2000-2500 range. That will give me good mileage and better torque if I have a load in the bed

[burn_your_money]

I'd aim closer to 2700-3000. Most stock mk2s run about 3000 RPM on the highway.
If fourth is close to fifth and you don't mind down shifting to pass and go up hills 2400-2700 probably is what I would aim for
I've never experienced a Giles pump though

[danfromsyr]

in my 4,000# PLUS 1980 vanagon camper. with 25" tires and an aircooled trans I run about 3800~4000 for 70-75mph indicated highway speeds.

sure it brings mpg down to in the mid 20's but thats not bad for a camper.

engine is in general A ok @ those RPMS..  if were a Diesel marine or Generator application 3700~ would be an all day affair.

[Op-Ivy]

I'm wondering why the "spec fuel consumption" is higher at one thousand RPM than at two. Maybe it is compensating for idle? The "thing" that starts fueling the engine once the rpm drops below a certain level while in gear.


Or am I missing something?

[Patrick]

My mk2 Jetta weighs in at about 2500 lbs, with a 3.94 final and a .75 overdrive it's still peppy enough, running a 1.9 though. Wouldn't want any more gear than that.........  Ballpark of 2000 rpm @ 80 kmh/50mph, 3000 rpm@120kmh/75mph

That's an AAZ 1.9, lots more horsepower/torque. I'd aim for more rpm in a truck with the smaller engine, that's for sure!
My golf with an NA motor is geared a lot lower (probably about 20% lower form the sound of it) but it's happy reving.  Sings right along at 120k, not sure what it will run. You'll put less stress on the engine making your horsepower a little higher up the rpm band, it will cool better too. The golf with a 1.6na and short legs will run 100k farther on a tank of fuel than the Jetta with an AAZ and longer legs, go figure.

[jtanguay]

I'm wondering why the "spec fuel consumption" is higher at one thousand RPM than at two. Maybe it is compensating for idle? The "thing" that starts fueling the engine once the rpm drops below a certain level while in gear.


Or am I missing something?

i think the main reasons are heat; the engine is producing more heat and therefore making it more efficient.  plus the fact that everything is rotating which is all momentum.  at idle the compression ratio its a big parasitic loss.  to actually run the engine it only requires about 15:1 ratio, but for cold start purposes it needs 23:1.  this could also explain inefficiencies at the lower rpm.

i know it seems kinda weird, but it does make sense.

[Op-Ivy]

Ahhh! I understand now. That makes perfect sense when you put it that way! Thanks JT!