I've seen several posts on dirty air filters, and reported increases in maximum boost pressure after installing a new element. There's also a general interest in how air filter restriction effects maximum boost pressure, so I decided to do a quick analysis to quantify that effect.

The bottom line is shown in the graph, which gives the maximum boost pressure, BP, as a function of both air filter restriction, AFR, and the turbo pressure ratio, PR. This graph is for BARO=14.7 psi, and assumes the IC pressure drop varies from 1.2 psi at PR=2.6, to 3.0 psi for PR=4.4. Most upgraded turbos top out at about a PR of 3.4 to 3.6, but I included PR's up to 4.4 to cover compound turbos as well.

On my early 99, the turbo PR tops out at about PR=3.0, which gives a max BP=26 psi at AFR=16" H2O, and that's with a Banks Big Head actuator, which is the same as having the red line disconnected from a stock actuator. If you follow along that curve for PR=3.0, you see that if I ran without a filter element and had AFR=0" H2O, the max BP increases from 26 to 28 psi, and at a max restriction it decreases from 26 to about 25 psi.

With my current setup, AIS and stock actuator with the red line connected, I'm on the bottom curve, and I hit BP=21.5 psi at AFR=10" H2O with a clean element. Again following that curve, removing the element only increases BP to a little over 22 psi, and a very restricted element reduces BP to about 20 psi.

In the 20 psi range for my truck, HP increases by about 4 to 5 HP as BP increases from 20 to 21 psi, but due to the inferior early 99 air inlet system, increasing BP from 26 to 27 psi only increases HP by about 2 HP at most. So for all those years of towing grades at 26 psi, I wasn't getting much more HP, but I was sure wearing out my turbo much faster.

If you look at the top curve, you see that BP deceases from 47 to 42 psi in going from no element to a fully restricted one. That's because the impact of filter restriction depends on PR. After I get to Vegas, I'll post the equations which show how everything plays together to produce this graph, and also do one for Denver.

When you change to a clean element, it reduces restriction which moves you to the left on a given PR curve to increase your max BP. Depending on what's limiting your max turbo PR, the PR might also increase a little, which might move you to a higher PR curve as well. The turbo PR depends on the drive pressure to the turbine, and if it's not waste gate limited, then lower AFR means higher BP, and this typically means a little more fuel to increase the drive pressure to the turbine, and therefore the PR.

 

Well, I am not the most clever fella, but I sure recognize a linear curve when I see one. Have a safe journey to Vegas. I will give you what I can when I get it. I need to reduce my samples for some better results. I watched way too much the first go around slowing down the sample rate.

 

Good post. Im assuming my turbo has a 3.6 PR simply from the fact i could make 35psi with a new one, and 31 with the dirty one.

 

Great job Gene, I guess we all knew in the back of our head that restricted filters rob HP, and now we have the hard proof.

 

Another great post Gene.

Nick

 

Great post! Way too much time. HAHAHAHA

 

Safe yes, uneventful no. Actually, day #1 from Morgan Hill to Bakersfield went smooth as silk. Day #2 from Bakersfield to Vegas was continuous wind and rain. Stopped for fuel at the last exit after leaving Barstow, the "Ghost Town Rd" exit, pulled right up to the pump and dumped in a half bottle of Stanadyne, and the wife returns with the news there'd been a power outage, and they can't get the pumps reset!

Finally got some fuel down the road, but not before a close encounter with a CHP! They have a strict 55 MPH limit for all vehicles towing trailers, and I was doing 75 trying to get around a line of semi's who were throwing water in my Genedad hole. I got lights, then sirens, I turned on my flashers and made my way to the slow lane and was looking for a suitable place to pull over, and the trooper pulls along side, makes a few hand jesters to slow it down, and tears *** on down the road leaving me wondering if he'd read my tag number and CA will be sending a ticket by mail?

Anyway, arrive in Vegas, and have to set up in the rain, the clutch on the front landing jacks starts chattering and I have to finish lifting the 5er off the truck using the emergency hand crank. Then the wife yells that the refrigerator isn't on. Bottom-line is that the breaker was tripped because the compressor is in pieces and rattling around inside its enclosure. Three years ago when the RV refrigerator died, I tried the cheap route of a regular compressor type, and I guess those innards just can't take the pounding they get in an RV!

So here's the equations for boost, a graph for 5.5K ft altitude, and a graph for BARO vs altitude.

PR=[(BP+ICPD+BARO)]/[(BARO-AFPD)], and if you convert the AFPD in psi, to AFR in inches H2O, solve this equation for BP, and rearrange some terms, you get...

BP=[(BARO)(PR-1)]-{[(PR)(AFR)]/[(27.68)]}-(ICPD)

From this equation you see that ICPD trades one-for-one with BP, so each 1 psi reduction in ICPD provides 1 psi of additional BP, and several more HP.

The effect of AFR is magnified by PR, for example at PR=3, an AFR=13.8" H20 (0.5 psi), causes a 1.5 psi reduction in BP.

The effect of BARO is magnified by (PR-1), for example at PR=3, at sea level a BARO=14.7 psi gives a 29.4 psi contribution to BP, but at a 5.5K ft altitude a BARO=12.0 psi gives a 24 psi contribution to BP, or a 5.4 psi reduction. This is why towing at altitude is so hard on your turbo, it has to work at a higher PR to make the same BP that it can more easily generate at sea level.

I experienced this first hand today as I made the 16 mile long ascent up to the 5K ft Mountain Pass Summit going north on I-15 to Vegas! At sea level, I can make BP=21.5 psi at an AFR=10" H20 with a PR=2.6, but at 5.5K ft, it takes a PR=3.0 to make the same BP with the same restriction. That means the turbo is working more than 15% harder.

Well that's it for now, but after I get a new refrigerator installed I'll be adding some more analysis here concerning something else that makes your turbo work harder and run at a higher PR, and unlike climbing a mountain which you've got to do if you want to get to the other side, this other thing is something you can avoid!

 

Welcome back to the Silver State (though we would like to trade Clark County to California for Modoc County, California).

 

Well it turns out we sure took the long route to get here. If I could've predicted the disruption due to the fires, we would've just headed due south from Reno! By the time we leave here, we'll have spent about 1 year in NV out of the past 8+ years on the road!

Did you follow my discussion of the BP equations? I'm working up another interesting chapter to the story on air filters, and their effect on BP and HP. In the meantime, check out this link, and see if you've got any advice for me. If you do, don't type it, just say to give you a call, and when's a good time.

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