Author Topic: Turbo Exhaust Back Pressure  (Read 270 times)

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Offline Scoobum

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Turbo Exhaust Back Pressure
« on: June 03 2018, 06:45:22 AM »
Chuck Leeper posted this on the other board...and it's a good read. Note what he says about valve spring pressure.

http://www.yellowbullet.com/forum/showthread.php?t=2386450
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Offline Scoobum

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Re: Turbo Exhaust Back Pressure
« Reply #1 on: June 03 2018, 07:56:12 AM »
I'm oversimplifyin g this...and there's others on the board here that are far smarter than I and can explain things in detail. This is my single digit IQ of my take on things.

Turbos don't like back pressure...it makes them work harder...highe r boost pressure.

Two things I noticed. When I swapped from a pair of heads a local does...to Joes heads...the boost pressure dropped. When I swapped from the GBODYPARTS SLIC I was running...to the FMIC I went to...the boost pressure dropped.

One thing I wouldn't do...is go to a larger diameter crossover pipe. My thinking is it would lower the exhaust gas velocity going to the turbo. The RJC DP with its long easy bend away from the turbo helps along with the ATR exhaust and Pitbulls.

I tested back to back with the stock appearing race intake and the race intake from Champion. The car 60 footed slower with the full race intake compared to the stock appearing intake. Larger ports...for what I'm doing...weren't better.



« Last Edit: June 03 2018, 08:06:13 AM by Scoobum »
109 235 Cubic Inch Stock Stroke Balanced Rotating Assembly
Stock Crank
TRW .030 Pistons
Stock Rods
Stock Timing Chain/Gears
212/212 Flat Tappet Cam
JB Racing Heads
Stock Rockers
5.7 TT Race Chip
Built 2004R
AC 16930
6262 JB

6.72 at 104.67-25 PSI

Offline Steve Wood

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Re: Turbo Exhaust Back Pressure
« Reply #2 on: June 03 2018, 11:02:40 AM »
I am too old to think about stuff too hard and stay awake.

But, I don't disagree with anything you say, Brad.

In the very early days, it was common to see boost come up a bit when the car was taken off-less back pressure.

In the very early days, I bought a beautiful set of hooker headers.  they fit well and did not leak.  they also did not want to make any boost against the foot brake or at low rpm.  The individual tubes were much bigger than factory.  Anderson and others used them on race cars with loose converters and high rpm.   The larger cross over pipe kills the velocity from the drivers side and can hurt spool a bit as you say. Race car that leaves at higher rpm may well be okay with it.

Jason had a 5" downpipe..or at least it started off that way.  Idea is to reduce back pressure at the turbine wheel and allow the gas to expand as it comes out of the turbine.  Expansion of a gas creates cooling.  Cooling increases density which as the pipe necks down, picks up velocity and may tend to provide a scavenging effect...I said May.

Larger turbines or less blades can decrease backpressure to the gasses coming out of the engine and allow higher boost before power starts to fall off.  It also can make it harder to spool at low boost so then more converter might be needed.  All these ratios are way beyond me and I would not be surprised to discover that about 5% of the experts that talk about this stuff actually know what they are talking about and not parroting what they think they learned from reading someone else's comment :D

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Offline motorhead

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Re: Turbo Exhaust Back Pressure
« Reply #3 on: June 05 2018, 11:32:52 AM »
It is pretty common for a 2" diameter crossover pipe/mid pipe to support ~1000hp.  You have to consider volume and velocity against the goals/application.

Personally I am building my turbo LS hotside with 2.5" pipes because it is just cheaper/easier based on what is on the market in stainless j-bends/reducers; not to mention goofy long tube upswept Chinese headers.  The mid-pipes are going to merge into a single 3" pipe into a T4 flange, and be vented by a 60mm waste gate.  I am running a 4" downpipe off the turbine housing which will connect to a single 3" exhaust (hopefully made out of aluminum), and will have a 4" dump.  On the intake side I am not running an intercooler as I plan to point the compressor right at the throttle body with a short length of 4" pipe and a 50mm BOV in between.  Obviously I am running it all on E85.  I figure adding a pressure transducer/MAP to measure the backpressure is pretty straightforwar d - could even run the math so it auto calculates the BP ratio in the scanner.

So while I am making a few compromises here and there, the ultimate goal is to minimize that variables that will influence spool time, hot and cold side restrictions, and backpressure.  Plus, lighter is better.  I've always wanted to go sans intercooler since I saw Bruce Plecan do it over a decade ago.

At the end of the day this "kit" should support a 4.8L engine as well as 454 stroker monster (which I will never be able to afford).  But 4.8s, 5.3s, 5.7s, 6.0s, and 6.2s from the yard should be plenty for my needs/goals for years to come.

Essentially, any restriction you remove will reduce your "boost" - which is just a measure of restriction anyway and is going to introduce more heat into the intake charge as it works against itself.  IMO, the best way to measure power is through mass air flow - it becomes pretty evident when your boost goes down but your calculated air flow goes up.  When running a MAF it is a more linear and logical air flow model to follow than a VE table.  My TBSS has a functioning MAF on it even though it is Speed Density tuned vehicle for this very reason.  Less heat is more better too.

I've recently considered "building" a VE table using my MAF.  Essentially I just need to log the MAF flow Hz against RPM and kpa and it should fall into place pretty easy.  I am interested to see what the predicted model of the VE (uses baro, IAT, CTS and some other crap IIRC) versus the measured air flow of the MAF, and if they overlay equally.

GM uses a blended or hybrid Operating System (OS) where it will validate itself using the MAF and VE below an engine speed threshold (say 4000rpm) for all fueling calculations (and will default to the VE table if the MAF fails), and will go "MAF-only" above that RPM threshold for all remaining fuel calculations.  Damn shame it isn't a Closed Loop OS.
« Last Edit: June 05 2018, 04:33:57 PM by motorhead »
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Offline Forzfed

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Re: Turbo Exhaust Back Pressure
« Reply #4 on: June 05 2018, 03:15:24 PM »
My buddy clipped the exhaust wheel on his turbo in his 2.2 liter 8 valve motor and runs almost 150mph in the 1/4!

Offline Scoobum

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Re: Turbo Exhaust Back Pressure
« Reply #5 on: June 09 2018, 09:30:21 AM »
I bought a pair of TA aluminum heads from Norbs a few years back...street version. Nick Micale had done a ton more porting on them. You could run the titanic through the ports. After testing the Champion race intake against the Champion stock appearing intake, I concluded that I'd need a bigger turbo and lots more boost to get any benefit from the TA heads. Ended up selling them to one of the Buick guys from Huntsville...h is name escapes me. Daveismissing mite know the gent. Funny side story. His friend is looking under the hood of my GN while I'm talking to the gent about the heads. He asks what fuel management system I'm using. I tell him to pop the passengers door and look at the kickpad. He opens the door...looks in...shakes his head...and walks back to the car. Scoob don't need no steenking XFI!
109 235 Cubic Inch Stock Stroke Balanced Rotating Assembly
Stock Crank
TRW .030 Pistons
Stock Rods
Stock Timing Chain/Gears
212/212 Flat Tappet Cam
JB Racing Heads
Stock Rockers
5.7 TT Race Chip
Built 2004R
AC 16930
6262 JB

6.72 at 104.67-25 PSI


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