Cylinder filling is a problem even with a Turbo, i haven't got the paper work with me at the moment so don't hold this as gospel.
It takes from memory 150cfm of air for 100hp so if we just look at the "Theoretical CFM" for this engine it is roughly.
Approximate only:
3.9ltr = 238cu/in & just say 3500rpm at the engine
238cu/in X 3500rpm/ 3456 = 241cfm
241cfm equates to around 160hp.
This is for a Normally aspirated engine & not Turbo.
Unfortunately everything you've written above is straight from the petrol engine school of thought.
Petrols run a 10lb/min of air = 100hp rule of thumb. At atmospheric pressure that's approx 150 cuft/min.
It's irrelevant to a diesel as excess air is already present. Even at peak power they're still shifting between 1.5 to 2 times the petrol volume of air.
At cruise my 4BD1T is processing enough air for 150hp in a petrol engine, but is only producing about 40hp.
It produces 40hp because that's what my right foot has dictated the fuel pump to deliver. Not because the VE is poor.
In stock form a 4BD1T has exhaust temps of around 400 deg C pre turbo. It is not air flow restricting power, it is fuel.
You can add roughly 50% more fuel without running out of air (but you will hit smoke).
VE is not the problem restricting power. These engines are massively detuned so they'll last for 500,000km between rebuilds.
As you can see even with a Turbo the Actual VE is much less & the power figures show this at around 120hp or so. A cam & a port job done correctly will not only increase hp but also improved torque where they so badly need it.
A cam and port job might increase VE from 80% to 85%. A 7% increase in boost (3psi) will give the same air volume increase in the cylinders and will not weaken the head.
Have you had a look into the intake ports on this engine? There are no big gains to be had.
Increasing boost is easy but the results won't change much due to many factors. To increase boost we need the exhaust gas to drive it as well, before we increase boost we should increase the engines airflow capability & configure the ports accordingly to do so.
This is wrong.
Extra fuel and the boost required to burn it are the main factors in diesel tuning.
Extra boost simply requires a wastegate modification. If you're running a freefloat exhaust housing then more fuel will automatically give you more boost.
In a diesel more fuel = bigger exhaust volume = more power spinning the turbo.
Again the VE is not the problem. It is not VE restricting the power of the NA version, it is fuel atomisation.
Modern common rail engines get much higher power from the same airflow because they can burn more fuel without smoke.
The 4BD1T injector pump will handle a lot more than the engine can deliver std, normally you could expect this to be able to cope with at least the Theoretical CFM figure of the engine & then some. It wouldn't make sense for the factory to make something that only just does the job.
In fact if you could see the black smoke pouring out the exhausts of some Isuzu's up long hills etc here in Australia you could see the pumps are wound up & far to much fuel being delivered. Don't make sense really!!
Blowing black smoke can be for many reasons, a clogged air filter is usually the main culprit, a malfunctioning turbo is another. None of which indicate the pump can deliver more fuel than the healthy engine and turbo can burn cleanly.
I'm still hoping some one can tell me what the max fuel delivery of the pump is (per cylinder, per injection).
There is already a lot of experience on this forum in diesel engines and turbocharging. We get a lot of people here trying to apply petrol engine tuning theories to diesels without success.
There is a long thread about cams in the cummins performance section which I think you should read.
A long badly written analogy on boost pressure is not any help. But earlier you mentioned oiling issues which we would be greatly interested in learning more about.
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I hope this resource is useful to you modifying your engine and I hope you will help add to the knowledge base here. But please take the time to read and research first.