Thread: Gear changes at what revs

Hard to say, every veh is different depending on ratio's, fuel, boost. I tend to shift mine about the 2200 mark as this just feels smooth. But, when I have a decent load on the back the engine actually feels smoother throughout the rev range, and I tend to wind it out to 26-2800 before shifting.
Kinda weird huh, the more load the smooher the engine becomes.
Going even further, with a large boat (2-2.5T) on the back it is even smother and I find myself unintentionaly getting ut up to around the 33-3400 mark.

Luv it, I imagine Dougs anwser will be interesting

thats about right, but you drive it by ear dobbo, when it sounds like its gone past the transition point of "pushing" to "spinning" thats when you want to change gears.... if its labouring when you get up off of the clutch and get back on the noise you either need to let it spin on a bit more before changing the gear or hold back on the speed and change if your upshifting on climbs...

In theory, the operating range of an automotive diesel engine is the range from maximum torque to governed speed. In the case of the 4BD1 this from 1900 to 3200, and the 4BD1T, from 1800 to 3000.

Originally Posted by dobbo

ISUZU 4BD1 Diesel Engine For Vehicle Application - FD Power Co.,Ltd


Having a look at the torque curve on a NA 4BD1 at what revs would you change gear to help maintain a nice level of driving, preserving the LT95 and the all important best fuel economy.

My uneducated guess would be at just after or around 2550rpm, am I right or wrong? Why?

HI mate
Around the 2000rpm mark is pretty comfortable with a 4 speed box as these engines deliver max torque in the lower rev range and pull away from 60kph easily in top gear (with an LT95 4 speed anyway). I hardly ever change gear on the highway once I get into top gear. But then I am a lead footed old V8 indocrinated speed fart from the late 60's and 70's but I love these Isuzu motors. Just as well my dad is dead as being a WW2 vet he would never have anything Japanese in the house and I sort of followed his thinking for years before discovering how good these motors were and swallowed my pride and bought one. Regards....Brian

There is no hard and fast rule on what point to change (and in any case the standard landrover installation does not have a tachometer).

But for best performance when pulling hard, I would suggest that a change up should not be made until the peak torque figure is reached for the gear you are changing to. Conversely, a change down should be made at peak torque for the gear you are changing from. But these would only apply when maximum performance is needed, such as heavy towing or hill climbing.

For more general driving, best economy and longest engine and transmission life will normally be attained using the highest possible gear consistent with the engine not hammering the transmission - how low a rpm this entails depends entirely on the throttle opening; at light throttle the rpm may be satisfactorily let to drop to perhaps 800-900, but either completely closing the throttle or substantial throttle opening may require changing down to avoid snatching or hammering the gearbox by hard pulling at low rpm. As an example, it may be best going slowly downhill to drop as slow as 40kph in fourth, but if you need to close the throttle as the downhill steepens, or it changes to level and substantial throttle is needed, a change down to third is indicated.

John

I don't have the LT95 and I also have lots of boost at low revs.

So mine spends almost all it's time between 1500-2000rpm. It's smoothest from 1800-2500rpm.
It uses the least fuel in the highest gear you can pull.