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Thread: Cat Overtemp or Just Turbo Back Pressure?

  1. #1
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    Cat Overtemp or Just Turbo Back Pressure?

    I will post a log later today, but this is more in theory of a discussion

    I put my car on the dyno finally, and hooked up a sensor to log my pre-turbo exhaust pressure. Car was dialed up to 14psi, 19psi, and finally 22psi, 6266 twin turbo, 392 forged motor.

    The exhaust back pressure was pretty much rock steady at right around 2:1, but right near 6200rpms or so I got a big spike of back pressure (went from 35psi to around 41psi), but at the same time my boost pressure started falling off, fading from 19 down to 17.5 to 18psi, but the car also went pig rich going from around 0.72 to 0.66 in a matter of a few hundred rpms.

    I am trying to figure out exactly which is the cart and which is the horse. I know rich means higher exhaust temps and more back pressure, so initially I am trying to figure out if my tune is somehow allowing cat overtemp to kick in (car is catless), and how I can monitor for it. Would the commanded lambda show the change in fueling?

    I could have simply hit the wall on flow of this T3 0.82 AR setup, which I know is small, but is how Hellion sets up the kit. But strangely I got the SAME exact spike in manifold pressure at the same rpm at 14psi, 19psi, and 22psi of boost, always spikes over 2:1 in that spot. Leads me to believe its not really a flow issue, maybe a wastegate issue?

    I didn't get a chance to play with the fueling to see if it would help back pressure, but for sure my VE table doesn't have some big spike there and should be no reason why its going rich. Leads me to believe the rich spot is from high back pressure, gates getting blown open, boost falling, power falling off, way too much fuel.

    Open to thoughts.

    I had posted a few months back about my shitty spool I thought I had. Showed it clearly on the dyno how lazy this setup is. WOT at 2600rpms in 4th, didn't hit peak boost until 5000rpms.

    Diablo Tune 1_3 Bar_v.hpt

  2. #2
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    Dyno picture attached, this is the 19psi, green line is exhaust manifold pressure. I will do an overlay of intake boost pressure later on.

    Dyno TT 6266.jpg

  3. #3
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    Also worth noting, I couldn't get the damn gates to stay closed worth a damn with any other setup other than venting the bottom to atmosphere and applying boost to the top only of the gate. Now I can see even with that setup, a 7psi wastegate spring with even the 19psi on the top of the gate, is struggling to stay closed versus the 35psi of pressure under the valve.

  4. #4
    Well down load the latest beta if its dodge I get the beta. Look at tab fuel then tab temp control. Max all that out to 3k. In diablo atleast in one tune I got unlocked they map the cat and exhaust tables hp some times maps cat temp tables. With all that maxed you should not see throttle torque source say temp I think due to EOT/COT. Bad part about zeroing the enrichment table is triggering COT/EOT triggers other stuff too even if you dont see fuel enrichment.......

    Fuel temp control.jpg

    Read through all the descriptions its very informative now.

  5. #5
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    Attached is the log and dyno plot from the 18psi run (870whp).

    Doesn't appear to be cat over temp, commanded equiv ratio is rock steady and not moving, so its just in the tune.

    Dyno 4 Pull (18psi).JPG
    Dyno Pull 19psi.hpl

  6. #6
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    I realized on this forth and the fifth pull the AIT sensor had blown out based on the IATs not moving at all. If you look at the run prior the IATs drop to below 50F and then climb back up the entire run. In this forth pull and fifth pull, they dip a little but do not move much, since the sensor was laying on top of the motor.

  7. #7
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    Another huge benefit of logging back pressure....I can now see my wastegates blowing open right about 9.5psi or so of backpressure, clear as day in the log, no wonder I have such crappy spool up. I circled it in the dyno lot, you can see the boost to exhaust pressure go to almost 1:1 once the wga opens up.

    I have yet to really find someone with this backpressure level and how they setup their boost controller. If a 7psi spring cracks open at 10psi, I can assume a 10psi spring will still crack open at 13-14psi or so. Still not great. So I need pressure on the top of the gate to keep that sucker closed. Any amount of pressure to the bottom of the gate only lowers to the cracking point, which is how 99% of EBC's instruct you to route the boost. I assume the gain feature on most controllers is exactly that? It applies more and more pressure to the top port based on how much gain is asked for?

    Dyno 5 Pull (WGA Opening).JPG
    Last edited by 06300CSRT8; 06-28-2018 at 10:04 AM.

  8. #8
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    Man, that's a LOT of drive pressure... Have you checked that you don't have a serious backpressure issue downstream of the turbos? You aren't running cats in the exhaust system, are you?

    Out of curiosity, what wastegates are you running on your setup?

    I may be rambling here, and you may already understand this rule. But, in order for a turbo to spool up, there needs to be a pressure differential between turbine wheel inducer and exducer (a pressure drop that occurs across the turbine from inlet to outlet). This is a big part of what generates the necessary rotational force to spool up the turbo. Flow alone wouldn't really be sufficient to get a turbo nice and spooled up. What I mean by that is, without a pressure drop across the turbine wheel (inlet to outlet), you could take 2 identical turbos that have identical amounts of exhaust flow throughput (mass flow), but if one setup has 30 psi of drive pressure pre-turbine and 25 psi pressure measured in the exhaust - downstream of the turbine outlet (or in other words, a 5 psi delta), the turbo would hardly be spooled up and wouldn't be making much boost as a result.

    On the other hand, if the other setup had the same 30 psi of drive pressure pre-turbine, and only 10 psi of pressure measured in the exhaust - downstream of the turbine outlet (or, in other words, a 20 psi delta), that turbo would be cooking right along, producing plenty of boost.

    Now, this is obviously an exaggerated example, and the reality is that you would have very different mass flow rates from the one example to the other due to one of them being spooled up and producing boost to feed the motor. But the point still remains, without sufficient pressure drop across the turbine, you would end up with a very laggy, under-performing turbo setup. And if there were a serious restriction in the exhaust downstream of the turbos, it could explain both the high pressure measured in the exhaust manifold, and the laggy turbos that really shouldn't be so laggy (given the size of the turbos in relation to the engine they're feeding).

  9. #9
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    Interesting thought, I had not really considered the post turbine pressure, it would be pretty easy to measure. Post turbo are catless 3” mid pipes mated to a 2.75” Corsa sport cat back, which are straight through mufflers and resonators. I will hook up a sensor and log it, I have some cutouts that haven’t been installed yet, that would solve the issue if there is one.

  10. #10
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    And to answer your question, I am running 45mm turbosmart gates. I love the add on caps they have with a position sensor of the valve, just very pricey ($4-500 once you order the cap, the sensor, pigtails).
    Last edited by 06300CSRT8; 07-10-2018 at 11:08 AM.

  11. #11
    Just for testing purposes. Take your exhaust off and run it.

  12. #12
    And looking through the log you posted it doesn't appear to me that the car is running at the Commanded EQ ratio at anytime through the log. Once you are in the throttle is is asking for .76 and it's not following that at all.
    Last edited by Moparmatty; 06-29-2018 at 12:03 PM.

  13. #13
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    Quote Originally Posted by Moparmatty View Post
    And looking through the log you posted it doesn't appear to me that the car is running at the Commanded EQ ratio at anytime through the log. Once you are in the throttle is is asking for .76 and it's not following that at all.
    It’s not a normally closed loop setup like 2015+, it doesn’t have factory widebands, I have it forced into open loop and am spraying 14gph of straight meth down it’s throat. So yea, it’s a little richer than commanded on pump gas lol

  14. #14
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    So I took the car back to the dyno, this time with 10psi wastegate springs. Spool was much much better, moved the entire curve over 4-500rpms and at pretty much any given rpm during spool up, torque was up nearly 60-70wtq just from the increase in boost/spool. Indicates I had some wastegate cracking open, so I am happy there.

    But I need some help with this TERRIBLE dip in power I am seeing EVERY single time, same rpm range, no matter what boost, I always get a terrible dip in torque and power. HP drops nearly 70-80whp then jumps back up before the normal rpm taper. (look at my dyno charts above, the latest dyno's looked identical).

    I was picking up a touch of knock, about 0.5 degrees, right at the spot the power starts to dip. Just for testing, I shut off the knock sensors and made a pull. While the dip still happened, it was 80-90% better. So we figured my knock sensors were a bit tight on sensitivity and I was on the other side of timing, it needed more. Next pull I added 1 degree of timing and the larger loss came right back, and I could physically see what was either some knock or blowout coming out the tailpipe right at the rpms the dip happens.

    I have the plugs gapped to .025 now versus 0.28 before and they are brand new 2 steps colder iridiums. Logs will be attached in a bit, as they are back in the garage at home on my laptop. Timing is about 9-10* climbing up to 13* by redline. Running 93 octane and shit ton of straight methanol injection (twin 14gph nozzles).
    Last edited by 06300CSRT8; 07-10-2018 at 08:50 AM.

  15. #15
    when you say you could see Something coming out of the tailpipe what was it? puff of black smoke?

  16. #16
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    Yea, dark dark grey blackish puffs.

  17. #17
    that has to be actual detonation. try pulling a degree or two there.

  18. #18
    as far as boost control I have had similar problems and had to go to a c02 system to hold the gates shut

  19. #19
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    Quote Originally Posted by RickClarke23 View Post
    that has to be actual detonation. try pulling a degree or two there.
    I thought the same thing until I gained 60whp in the middle of the dip by turning off the knock sensors. Prior to turning off the sensors, I pulled out 1 degree and dip got worse.

  20. #20
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    Quote Originally Posted by RickClarke23 View Post
    as far as boost control I have had similar problems and had to go to a c02 system to hold the gates shut
    Yep, exactly the route I am going, I just wanted to get a little stronger base spring in there. With no pressure on top and boost reference to the bottom of the gate, with a 10psi spring I saw 7-8psi, which is plenty low enough for lower gears.