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N/A Diesel Exhaust Theory

Discussion in '6.9L IH & 7.3L IDI Diesels' started by Enginerd, Sep 20, 2019.

  1. Enginerd

    Enginerd Registered User

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    Hello everyone, a little intro to me before I ask my question:

    I'm new to the diesel world! A few weeks ago I bought a 1993 F250HD 4x4 Supercab with a newly rebuilt E4od and 318,000 on the odometer. I've been looking for quite a while on what diesel to get and with this truck I got the deal of a lifetime. I wanted this engine specifically because I've gotten comfortable with the design and mindset of gas engines and wanted to transition into diesels. Being N/A, fully mechanical minus some sensors, and IDI, I thought I couldn't find a better transition piece. This truck will be my daily driver once I get the brakes in good working order, get it cleaned, flush the radiator and put in the coolant additive, etc. I'm lucky in that the only thing that looks to have been done to the engine is a straight pipe and turning up the fuel. I turned it back down to clear up all the smoke except near WOT and put a muffler back on for now (boy it sure was loud haha) since I had a good one off a 2013 silverado laying around. I love how simple this engine is!!

    So, now (almost) to my question. I absolutely love designing and building exhaust to manage both the flow and sound/tone, but I've only done gas engines personally, and researched turbocharged diesel engine exhaust. Exhaust is where I'll start on my engine improvements. My goal is not to build a hot rod, I already have one, and it's not to build a grocery getter, I already have one. I want something that has good reserve power for towing but is efficient in its task as a daily driver - essentially a do-it-all workhorse of a truck. So with this exhaust, I'll build a set of headers and more than likely merge through a y-pipe into a single exhaust. I believe I have the math worked out on the primary lengths for optimal timing to produce the best scavenging but this is where I've hit a dilemma. This is a big engine compared to my 350 small block Chevy that I have 1 5/8" primary pipes on, however it is a much lower revving engine. I'm looking to build peak torque at 2,000 rpm instead of 4,000 like on my 350 sbc. Once I build a good exhaust, I'll work the intake side to match. I was thinking with this big of a motor I would start with 1 3/4" primary tubes on my headers, but ran into a question I can't seem to find an answer to.

    My question is about flow vs. scavenging. On a carburated gas engine like my 350, scavenging is just as important as flow-ability when producing torque so the leftover exhaust gasses are cycled out and a potentially larger volume of air/fuel is packed in the cylinder than the displacement itself. Generally speaking, too large of an exhaust kills the ability to scavenge and therefore hurts torque due to the decrease in exhaust velocity. But on an IDI diesel, how am I supposed to look at the balance of flow and torque? A boosted diesel can benefit from large exhaust due to lower egt's and just that you can't really outflow the turbo's flow... But I want to stay naturally aspirated for now and see what I can do.

    That's a terribly long winded piece I know, so in short, can y'all explain to me the effects of scavenging on this N/A 7.3 IDI in relation to the ability to flow a lot of exhaust?

    Thanks in advance for your responses. This group has an amazing wealth of knowledge and has already helped me quite a bit.

    - Levi
     
  2. onetonjohn

    onetonjohn Full Access Member

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    I would think the cam specs would come into play also. There are a lot of things to consider. When you get to this level, I think it's time to break out CFD software. FOAM comes to mind cause it's free. In general, this stuff is way over my head, but I'd like to learn more if anyone has models of our heads (or harley davidson twin cam heads).
     
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  3. FordGuy100

    FordGuy100 Registered User

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    Last edited: Sep 20, 2019
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  4. franklin2

    franklin2 Full Access Member

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    You will find in the diesel world, no one bothers too much with all the exhaust theory. It's too easy and you get too much gain from turbocharging, in which you just pressurize everything, and scavenging and stuff like that are not as critical. You just need it large enough to lower backpressure. Same with the heads, you just blow through them, so valve size and lift are not as critical when you pressurize the engine. If you want real meaningful gains with this engine, put a turbo on it.
     
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  5. 420Bullnose

    420Bullnose Registered User

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    I like the sound of the N/A with the stock exhaust, but if you giver a Turbo, Straight piping has the best tone.
     
  6. Enginerd

    Enginerd Registered User

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    Wow thank you for that Donaldson link, that is interesting stuff in there. I'll take all the literature I can get on that stuff

    I'll see if I can get into the OpenFOAM software and mess with it, thank you onetonjohn. I hadn't found that one before.

    The reason I wanted to avoid a turbo for now is simply because I'd rather have a fresh motor to start with on that, I want to apply a header design approach that came to me the other day, and if I can build something for less than $200 with no moving parts and gain a worthwhile amount of efficiency and power then I'll give it a shot. Everyone and their cousin has headers on a 350 but I guess most don't try on these idi's so I wanted to try it on the 7.3. I know the go-to is a turbo on a diesel, and I don't expect to revolutionize the diesel world, but I would like to try different routes than what everyone else has and see what happens.
     
  7. Thewespaul

    Thewespaul Supporting Vendor Supporting Member

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    First step, get a pyro and take some egt measurements. That will tell you if you are making any improvements. True dual 2.5” or larger exhaust seems to be the best performer out of the various na setups I’ve seen.
     
  8. Enginerd

    Enginerd Registered User

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    Yes a pyro is actually already in my cart and to buy tonight.
    So tell me if I'm correct on this so I can understand the pyro purpose: let's say I run 1,000 degrees on a heavy pull, put headers on and with the same haul run 800. That means I burned more efficiently inside the chamber and that I can turn up the fuel for more power to get back up to 1,000 on that pull. My numbers may be a little off or exaggerated, but is my thinking correct?
    And good point on the dual exhaust - when it comes to a gas engine an x pipe or h pipe produces more power purely because of the scavenging effect (or at least an exhaust pressure balance between cylinder banks). So does anyone know what this actually does to a N/A diesel based on the pyro or dyno? I had found in another forum something about bank crossfire and reversion and true duals being better for that reason? It was just a statement though with nothing really backing it up so I couldn't find the justification in what he said.
     
  9. franklin2

    franklin2 Full Access Member

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    Since you are doing research, I had an idea I never totally worked through.

    The headers you can buy for these engines are pretty expensive, though seeing how many he makes in a year, that is probably a good price. Here's a link. http://www.stans-headers.com/ford_headers.htm

    My idea was to buy cheaper headers for a 460, and see how they fit in the engine bay of the diesel. Both engines are fairly large, and both are going in the same engine bay. If they would clear everything, the next hurdle would be to cut the flange off the 460 headers and weld on a new flange for the diesel engine. Someone makes just the flanges, but I am not sure who it was. If you are going to make your own headers, you probably should start with these flanges if someone remembers who it was that made them.
     
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  10. Enginerd

    Enginerd Registered User

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    Actually that's a pretty good idea come to think of it. I just saw the other day that r&d had just the flanges, but I didn't know if it was $55 each or per set. That's not a bad price for a set. I am also new to the Ford world, so I'll have to look at how the 460 compares in size and firing order
     
  11. Thewespaul

    Thewespaul Supporting Vendor Supporting Member

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    I don’t think these make enough rpms or airflow in na form to merit a crossover but it wouldn’t hurt. I’ve got a banks powerpack intake cover for your truck I can ceramic coat and sell you. You will have a larger filter and the ceramic coating keeps engine heat out of the air for a denser air charge. DA10BF09-2948-4A4D-B9A3-AF34EEFD8CF2.jpeg
     
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  12. IDIBRONCO

    IDIBRONCO IDIBRONCO

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    This is to give you a rough idea on how Stan's full exhaust (if he still offers it) looks. I've taken this off of my truck and will be putting a turbo on when the engine goes back in. In the picture of just the two headers, the top one is the passenger's side. I did have to make one cut in the pipes to get it all out from under the truck so it looks a little funny.

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  13. Macrobb

    Macrobb Full Access Member

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    The most interesting part is that you find that a turbo gives you more (relative) gain on a worn out motor than a fresh one - with a fresh one, you are actually pulling in air, compressing (most of) it. On a worn engine, well, you are leaking a lot more of that past the rings, resulting in lower power and fuel economy.
    With a turbo, well, you shove enough air in there to make power *in spite* of leaky rings.
    What you see, numbers wise, is that a new IDI can make around 125 wheel HP. I've seen worn ones make 85. Add a turbo and crank the fuel and you are at least 190, up to 210 with no other mods(or close to 250 crank HP).

    So... I'm curious what you can do with your engine... But it's going to be nowhere near what you can get with even the worst turbo kit.
     
  14. Enginerd

    Enginerd Registered User

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    Hey IDIBRONCO, thanks for those pictures. It's good to see what else has been done. The posts I've read on those headers make everyone seem unsatisfied with them. I would think you would see at least something positive for the same reason as opening up your exhaust unless your intake is really choked or something.

    Yes this is all very true, and I understand what you're saying. I just don't like the thought of band-aiding worn rings like this. (I actually don't know how my rings are because I haven't done a compression test or driven it enough, but it at least doesn't put out oil smoke). But, if I am to turbo it, I might as well rebuild the motor for basically the same cost. With a good intake and exhaust setup on a fresh motor and good tuning I'm sure you'd be within the ballpark of a worn motor with a turbo right?

    Hey everyone, I want to thank you for your responses! They all are good points because they bring up things I wouldn't think of on my own. I always appreciate input for this reason.
     
  15. franklin2

    franklin2 Full Access Member

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    The diesels are different than a gas engine, but not that much different in many respects. You know yourself a worn out 350 chevy with a turbo or blower on it, is going to make more power than a fresh rebuilt 350.

    I know you do not want to turbo it, we are just trying to lead you to the spot of most power for the money and effort.
     

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