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Thread: Testing wastegate/ boost creep

  1. #1
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    Testing wastegate/ boost creep

    I recently switched to a bottom mount manifold and have been driving for a few months normal city driving. I have been backing off the throttle when i see boost climb past 12lbs as i think it's going to creep past the wastegate spring of 14.5. I am not sure how high its going to creep and my tune for 91 gas may be at its limit past that. Would i be ok to put in 100 unleaded and see how much boost its making without changing my tune?

    She's built like a steakhouse but handles like a bistro

  2. #2
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    If you don't want it to go higher than 14.5 psi, why not switch to a lighter spring?
    get a 7/10psi spring, and use a boost controller to reach your desired boost pressure.
    If you want to know if your current wastegate spring opens your gate at 14.5psi, you can hook up a compressor to your wastegate and see if it opens with the compressor set at 14.5psi.

    with 100 your have less change on knock happening, which is nice.
    but I do not know enough about the rest of your setup to be able to say if you are ok with boosting past 14.5psi.

  3. #3
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    Its not that I don't want to go over the 14.5 spring it's just that I don't think its too good of an idea on 91 gas. Really just want to see what its going to do while being somewhat safe. Motor is built but I don't know if I can just dump higher octane gas and boost a bit higher? Local track has 100 unleaded so I was thinking I can do some testing at the track but not sure if I would need a whole new tune to do so.

    She's built like a steakhouse but handles like a bistro

  4. #4
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    It depends on your tune. You can also mix in some e85 with the 91 to bring it up a bunch (to about 93 octane). I mix in about two gallons of e85 per tank full which allows me to up the boost about 4-5 psi. I had my tuner richer up the tune a bit to give me a bit of head room for AFRs. It runs a bit richer without it but Iím ok with that.
    Last edited by chikinhed; 02-09-2019 at 04:44 PM.

  5. #5
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    Quote Originally Posted by JimsR View Post
    Its not that I don't want to go over the 14.5 spring it's just that I don't think its too good of an idea on 91 gas. Really just want to see what its going to do while being somewhat safe. Motor is built but I don't know if I can just dump higher octane gas and boost a bit higher? Local track has 100 unleaded so I was thinking I can do some testing at the track but not sure if I would need a whole new tune to do so.
    Don't just go adding other fuels and hopping for the best. Many high octane fuels are oxygenated which will cause your engine to run leaner than non oxygenated pump gas. Yes higher octane fuel is good BUT you MUST tune for the fuel and boost the engine will run on/at.
    The way you are wording things it sounds like you have already had the build/set up "tuned" correct?
    What boost pressure what the car tuned for?
    What ECU are you using for tuning? Are you familiar with taking data logs? Do you know what your boost cut or fuel cut is set at?

    Any good tuner should have already set up your car with a safe boost/fuel/ignition cut to protect the engine in the case of an "overboost" situation such as waste gate failure.
    If you are trying to run more boost you need to tune for the extra boost/airflow.******
    If no parts have changed sense you got tuned and the tuning went ok with no over boost problems you should have no issues unless there has been a mechanical change or failure.

    No matter what the case the best thing you can do is start taking some data logs and slowly move up through the RPM range checking each log to make sure the boost is where you expect it to be. example take a log 2000rpm to 3500 rpm in 3rd or 4th gear 100% throttle, review the log and if all is ok do the next log to 4000rpm then 4500 and so on until you have data for the entire RPM range OR until you over boost.

  6. #6
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    Thanks for the advice, I am still on my stock DME tune from the previous top mount setup which was good to 14psi on 91, same turbo but now bottom mount/twin scroll and different turbo exhaust housing. Don't have anything to log with so I will keep driving as a normal citizen until I come up with a proper game plan. So far though driving and letting off at around 12psi car is behaving great and a/f's look good but I can tell their will be boost creep because boost just keeps climbing.

    She's built like a steakhouse but handles like a bistro

  7. #7
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    You could just toss a couple NOS octane boosters in your gas tank on top of your 91 for some easy safety.
    I wouldn't mess with E85 because it has a different stioch ratio so a "good" AFR on gasoline is not a good AFR on eth unless you are watching lambda which I doubt..

    Burn up the fuel in your lines to get that octane booster to your injectors before you try. Do it on a cool day or cool time of day.
    Get yourself cruising in 3rd gear at an RPM a bit higher than your peak torque and start pulling against your brakes.
    Add a little brakes, add a little throttle, add a little brakes, add a little throttle, keeping your speed down and watching your boost gauge and AFR until you are wide open at full boost, unless your boost goes out of control then stop and drop the throttle..

    If you are good at 14psi a little momentary boost spike to 16psi or whatever shouldn't hurt anything especially with extra octane booster and being over peak torque because peak torque is usually the most likely place for detonation.
    Pulling against your breaks is nice because then you have the time to pay attention to your gauges. Just pull harder and harder watching your gauges until you are WOT and your boost control holds or you see your PSI or AFR go too high then get off the throttle.
    If it holds your desired PSI then do some RPM sweeps like that through your rev range and if it holds everywhere a few times then you can be pretty confident it's going to hold and you can just drive it.

    That is how I test when changing my boost control to dial in what boost I want so I can watch carefully.
    I wouldn't be too afraid to go a couple psi over your boost target just for a moment in testing.
    Last edited by fasteddie313; 03-05-2019 at 02:25 PM.
    E36 M3 S50 - E53 X5 M54 - 1980 Porsche 931 - 2001 Impreza RS25

  8. #8
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    I would replace the wastegate spring to limit it to a reasonable boost and be done with it. No need in messing up the engine due to high boost. If you mix in your own ethanol fuel with gasoline be prepare to add more fuel by increasing fuel pressure or injector time but you can see when you have it correctly by looking at the WB and shooting for 11.8 -12.2 when under boost. For gasoline, the lamba is multiplied by 14.7. For E85, you get to the right mixture under boost by running at the same number as gasoline, 12/1, even though the real ratio is different for E85...but remember you are measuring the O2, not the fuel. For different fuels, the Lamda is multiplied by a different number for the gauge to display AFR, Diesel (14.6), Methanol (6.4), Ethanol (9.0), LPG (15.5), CNG (17.2),E85 (9.7).

    The WB measures the O2 content in the gas, when there is no O2 left in the gas it measures a Lamda of 1 which for gasoline was determined to be 14.7/1. Burn any other fuel, when there is no O2 in the gas it will show 14.7 on the gauge but that ratio could be different from gasoline. Target a similar AFR on E85 as if you were on Gasoline. Full boost 11.8:1 (Actual E85 AFR = 7.8:1) 02 sensor is only measuring 02 content and doesn't care what fuel you're using. The controller/gauge will convert the voltage to a value which is displayed as your "AFR" so stoich for E85 or Gas will still read 14.7:1. I did this for years running E85 on the track at boosts of 18- 21 lbs. I could feel a difference on E85 and never had any knock running the stuff. The E85 carries its own oxygen and it cools the compressed air from the turbo better. I can't find E85 local anymore so went a different route.



    Last edited by Adker; 03-08-2019 at 11:03 AM.

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