Thread: cyclonic pre filter conundrum

Hi all,

I was stuck with my build and decided to spend some time on one of the many "todo" items on my list and since I was looking for staino pipe anyway, I thought I'd research the RAI and filter question.

The diameter of the GEMS intake is around 85mm OD so I guess around 80 ID and has a MAF sensor in the intake piping that is I believe even a bit smaller. All in all the nearest widely available tubing I can buy is 76mmx2mm leaving me 74mm of ID which I think would be reasonable. Whilst I was digging up these numbers I decided to double check to see if that diameter is available in cyclonic pre filter units. Of course I could always make an adapter if need be but that's when I noticed that there is a minimum and maximum airflow rating on those units. This goes for the Donaldson as well as the Zy-klone units and I guess for most other brands as well.

After doing some calculations I have come to the conclusion that cyclonic pre filters can't really ever work 100%, certainly on petrol engines that have a broader RPM range. Take my 4.6 V8 for example. If I size the pre filter for maximum airflow, assuming 5500RPM redline I would need almost 13m3 per minute of air. When looking up a suitable unit I get the following specs:

maximum airflow: 13,5 m3/min
minimum airflow: 5,6 m3/min
inner diameter: 96mm

The first thing I am wondering is why the ID of the filter is so much larger than my air intake can actually handle. I would need to make an adapter plate but I would think that the ID of the filter was designed to fit the airflow... puzzling

The other thing is; when I calculate what kind of RPM the minimum airflow requirement represents I end up at around 2500 and up. That might be reasonable when powering through loose sand but under most circumstances I would think that cruising along in a convoy on dirt roads in high range I'd make a lot less RPM. 1600 maybe?

When I look at one size down the figures are quite a lot different:

maximum airflow: 5,6 m3/min
minimum airflow: 2,1 m3/min
inner diameter: 76,7mm

As you can see the max RPM now sits at 2500 and the minimum has dropped to around 900. The ID is nearly perfect though . The conundrum comes down to this: I either have a filter that works well under most circumstances but am restricting my engine badly when I need to punch it (overtaking, tackling a steep hill, whenever) or I have a filter that simply does not work efficiently.

So, the question is: am I overthinking it and is the restriction in the smaller unit not as bad as I think, simply that the cyclonic feature won't work well at high RPM just as the larger unit is not efficient at low RPM or should I always size for the worst case scenario and accept less efficiency at low RPM? Maybe it would even start sucking dirt from the bowl into the engine?

or... forget that type of pre-filter

I guess on diesel engines these babies work a lot better due to the limited RPM range and, in the end, they were designed for mostly stationary engines like diggers I guess.

Thanks for thinking along!

Cheers,
-P

VE is indeed one of those things I forgot to take into account! thanks The stock V8 is indeed not the most efficient of it's day, but then again I have a ported and polished intake and a 285 cam. Perhaps I have improved it somewhat but, indeed, with that in mind a smaller unit should fit the bill. I found another model that runs from 2.8 to 7.8 m3/min and with VE in mind, let's use a figure of 80% I should be good for most use cases.

I do have one of those ram caps in the shed and I will fit that as well and bring the pre-filter in the car. Since I have a LHD model and the air intake is on the left hand side the noise will be close to my head so it's a good thing if we can keep the noise down.

Ron, do you recall what model pre-filter you are using?

Cheers,
-P