Thread: Breathing

Soooo, which air filter housing are you using? (Possibly in your thread, but I'm lazy).

Your point on top end lacking is interesting, as I have been trying to figure out why the previous owner of mine would have the snorkel plumbed in, but also have an additional 2 inlets in the air filter housing! The snorkel is only 2.5" going into a 3" air filter inlet. Maybe they had the same theory...

Just to be difficult I also have a point in complete contrast to above. I have driven 3 Isuzu 110's (steveG's last weekend! ) and they all accelerate much harder at low revs IMO. Both turbo and NA. That is the complete opposite to most vehicles including my 300TDI (a diesel, but smaller). Have you driven others as a comparison? No point chasing a problem that potentially doesn't exist.

My 2c anyway.

These long stroke engines do taper off pretty quick at high rpm. The pistons start out running the burning diesel and even with automatic timing advance the efficiency and torque drops significantly.

Can you run with the filter housing open to see if it makes any improvement?

The compressor increases the intake pressure by a PR (pressure ratio) depending upon the rpm of the compressor wheel. So the pressure drop in the intake system, before the compressor, reduces the boost pressure.

Say your compressor has a PR of 3 (at a particular engine rpm and load), then for 2 examples you have the following inlet pressures:

14.7 psia (absolute pressure), which is normal atmospheric pressure at sea level, i.e. with no filter loss. Then the absolute pressure out of the compressor will be 44.1 psia (i.e. = 14.7 x 3).
Now convert from absolute pressure to gauge pressure to find the boost pressure, i.e. boost pressure = 44.1 - 14.7 = 29.4 psi.

If inlet pressure is 13.0 psia, i.e. 1.7 psi drop along inlet and filter system. Then absolute pressure out of the compressor will be 39.0 psia (i.e. = 13.0 x 3).
Now convert from absolute pressure to gauge pressure to find the boost pressure, i.e. boost pressure = 39.0 - 14.7 = 24.3 psi.

You can see that the difference in boost pressure is PR x filter/inlet loss.

So an inlet system with minimum pressure drop will make more boost pressure, or will loose less power for the same boost pressure (more efficient).

However don't compromise on air filtration for the sake of a small increase in boost pressure, or power, particularly when normal driving occurs at more modest performance levels.

BTW, the filter system is one of my biggest worries, but I intend to flow more air than you (my injection pump can deliver more fuel).

Thanks guys, all good points. I had to re arrange some things and I'm now maxing out at 12 psi, so it seems I have other work in addition to a potential problem getting air in - even though the red service indicator will be a constant reminder to sort out the air input properly.

Reading around it seems others have had the same problem - did the factory turbo have a different filter? Or is just that more modern turbos move more air?

Out of interest, what rating is the service indicator (they come in a few different ratings - Inches of H2O from memory).

Easiest way to check if its causing you significant issues is to pull the filter/s and take it for a quick drive. The safety filter definitely causes more restriction but is there for a reason.
The other test would be to remove the end cap from the housing (filters still fitted) and see if that improves things. If disconnecting the snorkel didn't improve it then its down to housing size and filters.
What size are the outlet/outlet of your housing? Think my outlet is around 90mm and inlet 75 (long story ).

Steve

The crux is the limited space under the bonnet for an air filter that can provide both a desirable level of filtration and low pressure drop with a performance upgrade of a 3.9 litre turbo diesel.

The modern turbo is not necessarily the issue. The advantage of the better technology is more the reduction in air temperature (adiabatic efficiency) as the air is compressed to the same value.

Air flow will only be increased if the air density is increased, i.e. by increasing boost pressure and/or reducing temperature (intercooler), but that is not dependent upon how modern the turbo is.

Go for a drive with the housing open but filters in place. This will let you know if it's the inlet upstream of the filter causing the problem.

A bit OT, but when we went to Cape York last year I made an interesting observation of the Donaldson bowl style pre-filters.
Two County 4bd1's both turbo'd (mine aftermarket and the other factory) and both running 3" snorkels with pre-filters.

I had the large flow one (think its around 30cm diam) and the other vehicle had the smaller unit - around 20cm diam. I'd chosen the large one as I felt it more matched the airflow requirements of the engine and would give less restriction.

Mine seemed to have consistently less dust in the bowl than the smaller one even though we were driving in the same conditions. Perhaps I was hanging back out of the dust a bit more, but there were a lot of places where that wasn't possible, so I'm more inclined to think it was a case of the bigger bowl having less velocity and therefore less dust separation.

The vehicles without pre-filters had significantly more dust in their main filters than we had, so would likely have had more restriction after driving for a while.

Do the standard round style filters with the swirl vanes that most of us run require a certain amount of airflow to make the dust separation happen effectively, and if so, would ending up with too large a housing (likely not possible due to engine bay physical constraints as John said earlier) actually cause issues with that separation not happening?
Given the environments that our vehicles end up in I'm guessing its not wise to concentrate solely on free breathing.

Steve