Thread: 4BD1T Turbo Sizing and Performance Prediction.

Originally Posted by goanna_shire

It has the code 765870-1 which must be one of the superceeded models as all the ones on that page have 765870-00 something.

Cheers,
Brian

It is the numbers to the left of the "-" that are important to us. Those on the right are essentially revision numbers. -00 would be the original version, -1 would be the 1st revision.

In most case revisions don't affect us as the interchangeability is not affected.

This little graph is pleasing to my eye. If I could achieve near this type of figure I will be thrilled.

Cheers,
Brian.

There is supposed to be a graph but the computer is being a pain. Its the one on Dougal's link to the Hino specs. It shows the nice 150KW power and 350 ish nm of torque and that 350 nm is in a fairly straight flat line. We'll see how it goes I suppose.

Cheers,
Brian.

Originally Posted by goanna_shire

There is supposed to be a graph but the computer is being a pain. Its the one on Dougal's link to the Hino specs. It shows the nice 150KW power and 350 ish nm of torque and that 350 nm is in a fairly straight flat line. We'll see how it goes I suppose.

Cheers,
Brian.

To make it flat you've got to manage boost and fuelling quite tightly. Easy on a commonrail engine like that (there is a torque limiting table in the ECU), but harder in a mechanical engine.

The easiest way to make it flat is to make a peak and then saw the top off with the torque limiters. On a mechanical engine, it's easier to just keep and enjoy the peak.

How would you define flat in terms of a torque curve?

If you accept say + or - 3 or 4 Nm, then look at the torque curve for a stock 1988 on 4BD1T - pic below.

Peak torque is 314 Nm at 2200 rpm, but the torque is 310 + or - 4 Nm from 1750 rpm to 2650 rpm - which is not too scruffy.

From 310 Nm to 350 Nm is only a 13 % improvement, which is easy, in fact +50% is easy with a 4BD1T.

The stock 88+ 4BD1T gets a lot of it's flatness from a free-floating turbo that does more and more at higher rpm. It looks quite good at stock levels, but those trying to run that turbo at 2000rpm cruise never seem happy with it.

With EFI you can micro-manage the fuelling to do things like the 4HK1 below.

Here is a 4HK1 for comparison. 2005 NPR.

Though it's more of a "generated" torque curve than a measured torque curve.

Originally Posted by Dougal

The stock 88+ 4BD1T gets a lot of it's flatness from a free-floating turbo that does more and more at higher rpm. It looks quite good at stock levels, but those trying to run that turbo at 2000rpm cruise never seem happy with it.

With EFI you can micro-manage the fuelling to do things like the 4HK1 below.

...

There is no argument that electronics offer more flexibility in the control of fuel and achieving a "flat" torque curve.

I would have thought VE, fuel injection rate and timing and combustion pressure rise are marginally compromised by changing the turbo to one that spins up quicker than the stock full float turbo. The only way I can then see that the stock Garrett full float turbo helps is lower pressure in the exhaust manifold (which is in fact what Isuzu imply as one of the reasons they changed the turbo in the late 88 engine upgrade - see attached pics). I'm assuming the air flow, at higher engine rpm's, from the replacement turbo is up to the task, compared to the old Garrett full floater, i.e. not some small 52mm compressor from a TD5, etc.

We also must remember that engine designers have a goal for the shape of the torque curve, given the vehicle use, number and ratios of the gears, etc.

All the text books I have read on the subject present a strong argument for why the engine of a road driven vehicle should/must have a torque curve which increases as the rpm drop.

One part of this is that when climbing a hill for example , as the load increases the engine slows to increase the torque so the vehicle can proceed.

The full technical argument is more complex than the above.

Taylor in his 2 volume work, Theory and Design of the Internal Combustion Engine, gives a good treatment to this topic, for anyone interested in the subject.

These pics from a document comparing changes to the "existing" 1985 to 1988 4BD1T for the "new" October 88 on 4BD1T

Originally Posted by Bush65

There is no argument that electronics offer more flexibility in the control of fuel and achieving a "flat" torque curve.

I would have thought VE, fuel injection rate and timing and combustion pressure rise are marginally compromised by changing the turbo to one that spins up quicker than the stock full float turbo. The only way I can then see that the stock Garrett full float turbo helps is lower pressure in the exhaust manifold (which is in fact what Isuzu imply as one of the reasons they changed the turbo in the late 88 engine upgrade - see attached pics). I'm assuming the air flow, at higher engine rpm's, from the replacement turbo is up to the task, compared to the old Garrett full floater, i.e. not some small 52mm compressor from a TD5, etc.

The stock free-floater that I have is a 51.3mm compressor. It's garrett T25 but with a very large (0.87) turbine housing and a turbine that is also sized larger to flow all the exhaust (obviously, no wastegate).
I am running this same 51.3mm compressor on my current T25, but with a smaller turbine and much smaller (0.49) A/R turbine housing.
This is the best estimates of compressor demand:



The plot however shows 24psi, I am currently running 20-21psi peak to stay on the map.
It is a very small compressor, but it's also a very fast spooling compressor and right now I like it. It is too small for this same boost pressure but intercooled.

Originally Posted by Bush65

We also must remember that engine designers have a goal for the shape of the torque curve, given the vehicle use, number and ratios of the gears, etc.

I have only ridden in one late 4BD1T powered NPR, it was pulling ~3,100rpm at 100km/h. This is the reason for the large exhaust housing on the freefloat turbo. It is intended to spend much of it's life at 2,800-3000rpm.
It's when silly people like us gear them for 2000rpm that we find the stock turbo woefully inadequate.

Originally Posted by Bush65

All the text books I have read on the subject present a strong argument for why the engine of a road driven vehicle should/must have a torque curve which increases as the rpm drop.

One part of this is that when climbing a hill for example , as the load increases the engine slows to increase the torque so the vehicle can proceed.

Known as "torque rise" and I agree completely. IMO a flat torque curve is almost always done to protect something else. Clutch, drivetrain etc. Having a decent torque peak is preferred for my own vehicles and drivability.

I also think much of the reason the 88 onwards improved so much was the IHI RHB6 turbo used earlier was not a very efficient unit. My backpressure/boost measurements were much higher with the IHI than the little T25 I was running then and am running again now.