How much can backpressure effect hp and turbo spooling? - LS1TECH - Camaro and Firebird Forum Discussion



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How much can backpressure effect hp and turbo spooling?

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Old 06-10-2017, 03:29 PM   #1
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Default How much can backpressure effect hp and turbo spooling?

Can someone explain how detrimental a backpressure of close to 2/1 of boost could effect hp and how fast turbos spool? I think this may be my issue the whole time not making what I should. I'm currently running twin Borg 76/82s ok a 427 ci motor. Last year I was running twin 6766s on a 387 and made 1200 at 24 psi. I didn't have backpressure readings off that combo to compare though
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Old 06-10-2017, 04:14 PM   #2
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There really is no simple answer as there are so many variables.

You see people with singles making good power etc...with horrific pressure ratios, well over 2:1

Only thing I'd say, is if it is really bad, and you have a large cam with perhaps lots of overlap....that cant be a good thing either off or on boost.

Ive a 383 with a pair of 364's with the smaller 68mm turbine. Even with the stock 0.88 housing I only rose above 1:1 when either over 25psi or 6000rpm. Although even at 29psi and 6700rpm, ie worst case scenario it was still only around 1.3:1

I changed to 0.96 AGP housings over the winter and initial driving, PR's seem to be below 1:1 now, although havent really had a chance to use any decent boost etc yet.

Spool....maybe affected a little, but nothing too dramatic.
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Old 06-10-2017, 05:03 PM   #3
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I should bee making an easy 13-1400 wheel with my combo and I can barely break 1100, and the more boost I put to it the less it makes per pound of boost
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Old 06-10-2017, 05:10 PM   #4
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I should bee making an easy 13-1400 wheel with my combo and I can barely break 1100, and the more boost I put to it the less it makes per pound of boost

Then use less boost, less cam, more timing..etc etc

Find out where it wants to be if it isnt happy the direction you're trying to take it.

And speed sensors on the turbos would be very helpful to see if they are stretched too far.
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Old 06-10-2017, 05:27 PM   #5
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Should be making a crapload of power. Something else is wrong. Have you checked for leaks on the cold side and exhaust side? Do you have back pressure sensors on the car now? Where are they mounted? What are the readings at 10psi?15?20?Compression head intake and specs? What intercooler? Boost loss through intercooler?
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Old 06-10-2017, 05:46 PM   #6
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Yes, I smoke tested exhaust and did a boost leak test. All good, sensor is between the head and turbo, it picks up the valves pulsing too. The highest readings were actually at lower boost numbers. 9.7/1 monster bell intercooler
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Old 06-10-2017, 06:39 PM   #7
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More back pressure at lower boost numbers? That doesnt sound right, does it. It seems like something is holding the compressor side back at lower boost and not allowing the turbines to spool. Could it be something in the cam timing ?
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Old 06-10-2017, 06:42 PM   #8
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Have you tried damping the line to the sensor to get a more stable reading ?

I'd agree with the above, higher EGBP at lower boost doesnt make a lot of sense. Have you any datalogs of the readings ?

Sure both boost pressure sensor and your exhaust sensor are calibrated the same ?
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Old 06-10-2017, 08:01 PM   #9
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Yes but it was on the street with bad wheel spin, I'm going to put it on the dyno this week and get some good data and I think I'm going to undo some v bands and do a pull
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Old 06-13-2017, 11:17 PM   #10
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I got to the track today and made 4 passes ranging from 18psi up to 27 psi. Ran virtually identical passes from 5.75/123 to 5.65/126. Just not making any extra power adding almost 10 psi. Backpressure was around 1.2/1.6 to 1. At higher boost levels it was actually on the lower end. I have no clue why I can't make the power I should. It took 42psi of co2 to make 27 compared to 25 psi of co2 to make 18.
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Old 06-14-2017, 03:56 AM   #11
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Again, the only way to find out what work those units are doing...is a speed sensor.

But you could either have too much cam, a leaking gate, leak elsewhere or out of compressor.
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Old 06-14-2017, 06:30 AM   #12
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Cam is pretty mild, if the gates were leaking I wouldn't see the pressure in the intake though right?
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Old 06-14-2017, 06:54 AM   #13
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What's the converter slip percentage?
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Old 06-14-2017, 07:06 AM   #14
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Not measured but the drop in rpm is between shifts is 7-800, same as last year
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Old 06-14-2017, 12:35 PM   #15
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Cam is pretty mild, if the gates were leaking I wouldn't see the pressure in the intake though right?
I guess saying the gates are maybe leaking...could be misleading.

If they were leaking when closed....that would mostly affect spool. Not overall power.

I guess if one gate was perhaps opening at a vastly different rate than the other for any reason.....that could affect power, because the two turbos would be working at very different rates. Again this would come back to the balance of the overall design, and the speed sensor to verify how it's all working

But once the gates are open, as long as they're fairly even...they probably shouldnt have any real negative impact on power.

I'm sure it's been mentioned, but have you verified cam timing ?
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Old 06-14-2017, 12:41 PM   #16
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Quote:
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Cam is pretty mild, if the gates were leaking I wouldn't see the pressure in the intake though right?
Cam specs? Intake manifold?
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Old 06-14-2017, 04:00 PM   #17
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Bet its the combination or hot side design. You've really stepped up the turbine side but what did you do to compensate for the change.

Typically higher exhaust pressure reduces spool but kills the top end (like a VW jetta). Wastegates will help keep drive pressure inline but there is a point of no return.

Compare MOTOR cfm to compressor map and see where you fall in the efficiency islands. Are your turbos surging?


Ideal drive pressure changes by engine and specific setup. Some like higher, some lower, most like something close to 1:1
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Old 06-14-2017, 07:55 PM   #18
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Quote:
Originally Posted by stevieturbo View Post
Again this would come back to the balance of the overall design, and the speed sensor to verify how it's all working

I think this is the answer to the ops questions. Speed sensors will help you tell where the compressor is working- if the pressure is resulting with increased flow or not. If increasing boost pressure isn't giving more power- then that more or less says the engine isn't flowing more. So its something wrong with the engine, or something wrong with the turbo, in a macroscopic perspective. I really like the theory that one of the turbos isn't moving much and the other is trying to do everything- resulting in one oversped turbo running off it's map. 1k+ setups should be spare no expense anyways, it should have all the goodies, oil pressure logger, fuel pressure logger, compressor wheel speed, backpressure log, etc
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Old 06-14-2017, 08:04 PM   #19
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What is a compressor wheel speed sensor? I've got sensors out the *** right now. Lol. I'm running the holley dominator ecu so what's involved with adding one
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Old 06-14-2017, 11:43 PM   #20
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It's a fast lsx102 intake, cam is a Howard's grind .625/.625 230/232 114 I believe. I have not verified cam timing
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