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Thread: Exhaust Backpressure vs Boost

  1. #1

    Default Exhaust Backpressure vs Boost

    I've looked around the internet but haven't been able to find the answer to my question. Hoping I can get Bill or Cletus to chime in here with some expert advice I can trust.
    I remember Bill talking about the difference between exhaust backpressure vs boost pressure and the associated thrust on the turbo in his post about the WW2 testing. He had mentioned the ratio between backpressure and boost was much improved with the WW2 and higher boost was possible without causing excessive thrust loading of the turbo. At least that's how I remember it.... But I don't remember ever seeing what an acceptable difference was.

    So what is the maximum suggested difference in boost/backpressure since that seems to be the controlling limit to prevent turbo damage?

    Here's my story(short version?). Bill recommended removing my 38R and replacing with the stock turbo/WW2 combination for my towing setup with stock injectors. I ended up getting the billet wheel from Riffraff due to much lower price and good reviews. So far I've been really happy with going "backwards" by removing the 38R. Now I'm trying to tune in the Wastemaster wastegate controller.

    I watch the backpressure with an Edge Insight. Of course this includes the atmospheric 14psi so it doesn't directly match my boost gauge. I can watch the MAP reading on the Insight up to its limit of around 29 and keep from having to convert numbers, but that doesn't really address the max values that I'm trying to tune.
    So I take the boost reading and add 14 or the backpressure and subtract 14 for conversion.
    I'm getting around 28-29psi max on the boost gauge.
    50-52psi max on the backpressure reading. I've been told 52 it the readable limit of the sensor, it flickers between 50 & 52 so I feel like it's right at the limit but probably not beyond.
    Converting these values I'm seeing around 8-10psi difference between the boost & backpressure. Is this reasonable or should I try to back it down some?

    FYI, recently I've replaced the 6637 filter with the Ford AIS. Loving the new (lack of) sound from my intake suction. It did appear to cut my boost by about 0.5psi at most ranges (cruising & wide open). This intake change did impact the boost/backpressure ratio negatively at least somewhat. Just thought I'd throw that in here for anyone that may be wondering.

    Thanks, Jason
    2001 F250 4x4 Lariat crew cab short bed. PHP Hydra, RRD Billet Wheel, Turbo Master Wastegate Controller, 4" Magnaflow SS exhaust, EBPV delete, DIY 6637, AIH delete, Edge Insight for gauges, RRD Plenum Inserts, Updated Boots, rebuilt Trans, billet TC, Transgo Tugger kit
    +65 Falcon drag car with 289

  2. #2

    Default Re: Exhaust Backpressure vs Boost

    I wish I had a definite answer for you concerning the pressure differential (Delta) between drive pressure and boost pressure but it all comes down to how robust the materials are and the build quality of the turbocharger.

    A lot of people have put a lot of hard miles on a stock turbocharger with larger-than-stock injectors without problems. Some people push stock injectors hard and destroy the thrust bushing in the turbocharger almost instantly.

    One thing for others to keep in mind is that when looking at "boost pressure" which is a gauge reading (PSIG) which factors in the barometric pressure and subtracts it from the final display reading. The exhaust backpressure is (as a default) an "absolute" reading which doesn't automatically subtract barometric pressure. In all actuality, your drive pressure vs. boost pressure delta isn't all that bad because 52 PSI minus roughly 14 PSI becomes 38 PSIG of drive pressure for your 29 PSIG of boost pressure. You already have this figured out and I certainly wouldn't worry too much at all about an 8-10 PSI difference, even with a stock 270 degree thrust bushing.

    I have seen data suggesting that some stock compressor wheels that are badly chewed up and worn out have a pressure differential of almost 20 PSI when pushed hard. I have one of these in my shed. It came off of a 1997 that had 160/30's and a .84 A/R Banks turbine housing....and not much of a compressor wheel left. It made 18 lbs. of boost at 3300 RPM but the drive pressure was maxed-out on the scanner. The turbine wheel and compressor wheel were trashed because the thrust bushing was almost worn through. I wish I kept that shaft because the turbine wheel had points ground on the innermost part of the fins where the interference from the heat shield/cover thingy machined the inducer surface about 0.100".

  3. #3

    Default Re: Exhaust Backpressure vs Boost

    Thank you so much for the answer from a trustworthy source. There’s a lot of “information” on the internet but so much of it is marginally (or worse) true. That’s why I was asking here even though the forum isn’t real active. I’ll leave the wastegate setting where it is and roll with it.

    I’ve also seen a severely damaged turbo. Rebuilt my dad’s this summer. I found by accident he had extreme endplay while trying to compare to check my stock turbo before putting in my billet wheel. Both seal rings were gone, one found in the oil return hole, other one completely missing. The heat shield was toast with holes in it and wear on the turbine. The impeller side had worn a large groove in the housing cover. Ended up rebuilding it instead of buying new center section since he could get the billet wheel for same total cost. My first turbo rebuild, so far so good, I thought it was very simple.
    This is a 300k+ truck but PO said he had the turbo replaced at some point.
    Just to make this entire process more interesting I blew a high pressure oil line during the test run. Then had to rebuild the line fittings. Also add in a rusted through brake line repair. I was never so happy to get a vehicle finished and out of my garage! lol
    2001 F250 4x4 Lariat crew cab short bed. PHP Hydra, RRD Billet Wheel, Turbo Master Wastegate Controller, 4" Magnaflow SS exhaust, EBPV delete, DIY 6637, AIH delete, Edge Insight for gauges, RRD Plenum Inserts, Updated Boots, rebuilt Trans, billet TC, Transgo Tugger kit
    +65 Falcon drag car with 289

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