1HDT maximum efficiency? (1 Viewer)

[TZ62]

Hi all,

I DD a 1990 HDJ81 with the 1HDT and 5-speed manual. The truck is largely stock (small lift, bullbar and roof rack, BFG AT 275s), and I have no additional gauges installed. On average, I get about 450-475 km from a full stock tank of diesel in town, and 600 km+ on the road and in the bush, depending on conditions. That averages out to about 6.93 km/L (16.3 mpg) for the past year.

I'm trying to figure out where the fuel efficiency 'sweet spot' is for this engine. At what RPMs does the 1HDT perform best? Mine seems to like the 90 km/hr range (2800-2900 rpm) and again at the 110 km/hr range (3300rpm), but fuel use rises between those speeds.

I've looked at a bunch of sources and can find things about maximum power and maximum boost, but not about efficiency. I recognize that 1) driving a beast of an 80-series TLC will never yield good fuel economy, 2) various combinations of tire size and gear ratios influence overall fuel use, and 3) driver habits also significantly impact fuel use.

cheers

 

[Locked79]

Someone has regeared your rig sometime in the past. Sweet spot is around 2000-2200 rpm for economy. Standard gearing is around 2400 rpm @ 100kph with 4.1s. You can regear to 3.73s and be around 2150 rpm @ 100kph.

 

[AussieHJCruza]

X2 on someone having messed with the gearing, that's definitely not stock.

Ideally, I'd like to be able to gear mine, without losing the low gearing, ie, with a different transmission, not diffs or tyres or whatever, to turn at 2200 rpm at 120 km/h but that's not likely to happen...

 

[TZ62]

Someone has regeared your rig sometime in the past. Sweet spot is around 2000-2200 rpm for economy. Standard gearing is around 2400 rpm @ 100kph with 4.1s. You can regear to 3.73s and be around 2150 rpm @ 100kph.

I certainly haven't messed with the gear ratios. What would be the purpose of that regearing? I mean, why would the PO set it up that way? Better low-end torque? Better towing capacity? And you think from the numbers I gave you that it's diffs, not the transmission?

Thanks.

 

[Locked79]

People normally regear when running larger tires 35"+ or for rock crawling. Maybe in your case for towing very heavy items.

Changing gearbox ratios is nearly impossible and transfer gearing can only drop about 8% in high range if you have the hf2a full time transfer. Hf1a part time have no reduction gears avaliable AFAIK.

By the rpm your giving above, I'd say you have a set of 5.29s in your diffs. It works out about perfect.

To check this, you can jack up the wheels and count the number of tail shaft revolutions it takes to spin the wheels one revolution.

Eg. About 4 revolutions of the tail shaft to 1 wheel revolution, 4.1 gearset.
About 5 1/3 revolutions, 5.29 gearset and so on.

 

[Locked79]

Also, almost all 80s were 4.1's. Some European 80s came with 3.73's. There were supposedly some 3.54's out there, but I have never seen an OEM gearset.

Aftermarket low range gearsets come in 4.56, 4.88 and 5.29.

All these are for the diffs.

 

[torqueluvr]

Yep, someone must have changed the gears. Mine is an auto and with stock tires 265/75/15 I was at about 2400RPM @ 100km/hr, a little high in my opinion. I just put 33s on and it has dropped it to 2200 RPM @ 100km/hr. With the 265s I could get 400km to half tank on the gauge. I know that seems unreal, but I got it 4 times on 4 different tanks of fuel on the highway. That's about 10L/100km by odo km's and Litres filled. With the 33s (only had them for two weeks) I'm at about 375km for a half tank, up at about 12L/100km now. But thats all city driving. I think on the highway I can get it down to 11L/100km. I can tell it takes longer to get going with the 33s (and braking) but once you're at highway speeds it seems to hold it nicer being down 200 RPM to 2200 from 2400.

I accelerate slow, and never go over 110 on the highway. Hit the right lane and enjoy the drive.

**** To be fair, I do have the Marks Part Time 4wd kit installed, which helps with the fuel economy too****

PPS I've read the 5 speeds in these 81's rev really high on the highway and are not known for their fuel economy feats.

 

[MonsterCruiser]

If your gauge is anything like mine, half a tank on the gauge isn't half a tank. If it shows 1/2 tank it will take about 50 liters to fill which is more than half a tank.

 

[torqueluvr]

If your gauge is anything like mine, half a tank on the gauge isn't half a tank. If it shows 1/2 tank it will take about 50 liters to fill which is more than half a tank.

Mine is around 45 at half. However, I did calculate with litres filled against km gained on the odo with the stock tires.

 

[bugsnbikes]

You need to look at the rpm area under where both curves intersect. 2800+ rpms is way too high for a 1hdt in terms of max efficiency, The diesel should enjoy nice torque and hp at much lower rpms. My 1fz is getting that type(slightly better actually) of mpg on 35s- although im alot lighter and geared appropriately 35s- 4.11s and 4300lbs.

i will see if i can find a ho/ torque graph for that engine. what weight is your rig?

 

[bugsnbikes]

so after looking at the hp/tq graphs the real issue with the 1hdt fuel efficiency is that it makes so little hp off idle when the torque curve is in its flat (360nm) line from 1400-2800rpms. I would hazard a guess that switching turbos to one that is geared to ramping up much sooner would yield some very noticeable mpg improvements.

I was always curious why the 1hdt was such a thirsty diesel and after looking more closely at the anemic hp curves relation to torques nice low rpm/off idle torque- it starts to become a bit clearer.

 

[mudgudgeon]

Yep, someone must have changed the gears. Mine is an auto and with stock tires 265/75/15 I was at about 2400RPM @ 100km/hr, a little high in my opinion. I just put 33s on and it has dropped it to 2200 RPM @ 100km/hr. With the 265s I could get 400km to half tank on the gauge. I know that seems unreal, but I got it 4 times on 4 different tanks of fuel on the highway. That's about 10L/100km by odo km's and Litres filled. With the 33s (only had them for two weeks) I'm at about 375km for a half tank, up at about 12L/100km now. But thats all city driving. I think on the highway I can get it down to 11L/100km. I can tell it takes longer to get going with the 33s (and braking) but once you're at highway speeds it seems to hold it nicer being down 200 RPM to 2200 from 2400.

I accelerate slow, and never go over 110 on the highway. Hit the right lane and enjoy the drive.

**** To be fair, I do have the Marks Part Time 4wd kit installed, which helps with the fuel economy too****

PPS I've read the 5 speeds in these 81's rev really high on the highway and are not known for their fuel economy feats.

2200rpm cruising rpm is too low for the manual IMHO
it leaves you running too close to peak torque. Hit a hill that drops rpm a couple of hundred rpm and you'll be dropping torque and down shifting too often.

I found mine ran at around 2500 rpm (or a little higher) at 110km/hr on 35s and stock gearing.
Never drove mine looking for economy, so NFI what rpm gives best economy.

 

[bugsnbikes]

imho- Toyota likely designed it to run at that 2800 rpm sweet spot for the higher total power produced at higher rpm and torque for things like towing and extra capacity. Running at lower than designed peak power isnt going to yield better efficiency due to the way that turbo ramps up on the slow side. Toyota likely designed that turbo to be more durable at that rpm and the tradeoff is a slightly slower ramp up in power.

What kind of longevity are people seeing with these turbos before needing service/rebuild?

 

[Locked79]

We're seeing about 300k miles, 500k km on the 1hd-fte turbos over here before they start getting sad. Basically same as 1hd-t but is better sized.

 

[bugsnbikes]

Not the engine......the actual turbo. Are those stock turbos getting over 300k km before needing rebuild?

 

[AussieHJCruza]

Mine was starting to show signs of wear at 300,000 km, so good excuse to go to a GTurbo!

 

[TZ62]

Yep, someone must have changed the gears. Mine is an auto and with stock tires 265/75/15 I was at about 2400RPM @ 100km/hr, a little high in my opinion. I just put 33s on and it has dropped it to 2200 RPM @ 100km/hr. With the 265s I could get 400km to half tank on the gauge. I know that seems unreal, but I got it 4 times on 4 different tanks of fuel on the highway. That's about 10L/100km by odo km's and Litres filled. With the 33s (only had them for two weeks) I'm at about 375km for a half tank, up at about 12L/100km now. But thats all city driving. I think on the highway I can get it down to 11L/100km. I can tell it takes longer to get going with the 33s (and braking) but once you're at highway speeds it seems to hold it nicer being down 200 RPM to 2200 from 2400.

I accelerate slow, and never go over 110 on the highway. Hit the right lane and enjoy the drive.

**** To be fair, I do have the Marks Part Time 4wd kit installed, which helps with the fuel economy too****

PPS I've read the 5 speeds in these 81's rev really high on the highway and are not known for their fuel economy feats.

I've got the part-time 4wd as well, and I cruise around 105 most of the time. When I bought the truck in 2014, the first thing I did was a 3,500-km trip through southwestern Tanzania with a buddy and his petrol 80. His chewed fuel almost twice as fast as my diesel. On our longest highway stretch of the trip, I ran at 115km/hr and about 3,400rpm. Engine stayed cool, purred like a kitten, and got 11km/L that day - still the best mileage I've ever made with the 80.

 

[bugsnbikes]

Thats pretty damn durable for a turbo which leads me to believe its working real easy at that rpm.

 

[Locked79]

Not the engine......the actual turbo. Are those stock turbos getting over 300k km before needing rebuild?

Yeah, on the standard turbo. They nearly last as long as the engine provided they always have fresh, clean oil. This also assumes a standard tune. When the boost is increased above around 14-15psi, the lifespan of the turbo is reduced. 300k kms is relatively easy.

 

[TZ62]

You need to look at the rpm area under where both curves intersect. 2800+ rpms is way too high for a 1hdt in terms of max efficiency, The diesel should enjoy nice torque and hp at much lower rpms. My 1fz is getting that type(slightly better actually) of mpg on 35s- although im alot lighter and geared appropriately 35s- 4.11s and 4300lbs.

i will see if i can find a ho/ torque graph for that engine. what weight is your rig?

No idea about weight. The commercial scales in Tanzania aren't too welcoming of 'little' trucks like the 80. I have an ARB bullbar and Aisin winch in the front, the stock swing-out tire carrier in back, and a custom rooftop platform that weighs about 100kg. I tend to travel pretty light in terms of camping gear.

My best mileage is consistently at lower speeds, 40-50km/hr on dirt roads, running 4th gear and 1600-1800rpm. Yesterday on pavement I was turning just over 2800rpm at 90km/hr with AC on high.

Can you screen shot or scan one of those graphs? I'm a science teacher and like looking at data!

Thanks for the feedback.