Measuring boost back pressure

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subcom

subcom

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Let’s have some fun measuring boost back pressure under different scenarios. Step 1, install pressure sensor:
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1/4” OD copper tubing, compression fittings, weld bung, 100psi sensor wired and calibrated in TunerStudio.
 
7875 with .96 AR
Test 1: Running on spring pressure only (5-7psi).
3" downpipe to 3", 2x 90 degree turns to open chamber muffler, dumping before rear axle.
Boost PSI: 5.4-5.5
Back Pressure: 17.7-18.6
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Test 2:
Everything else the same, just dumping the exhaust before muffler with the 3" cutout.
Boost: 7psi
Back Pressure: 12.9-13.0
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Later on I'll turn up the boost and maybe try dropping the exhaust after the downpipe which would give me about 30" of 3" exhaust with no major turns or restrictions.
...to be continued.
 
That’s a lot more back pressure than I would have expected from dual 3”
I’m curious what mine might be with 4” down to dual 2.5…
 
You need a bigger hot side. I like the testing and data collection though. Nice work.
 
DNasty777 said:
I wonder how big a difference a 1.25A/R would make.
Click to expand...
You could probably expect a drop of 4-6 psi of backpressure on a combo like this. At the cost of some low speed turbo response. The never ending controversy of race car or street car and the compromises we have to make. Race car? Huge hot side and low backpressure. Street car? Keep the smaller AR and keep the response for street driving.
 
@subcom great post. As always your content is great. Thanks for this. Real information vs. pure speculation.
 
More data coming in. I dropped the exhaust after the downpipe. It's about 7' of 3" exhaust with a cutout and short open chamber muffler.
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Test 3:
Removed the exhaust after the downpipe.
Still running on spring pressure only (5-7psi). Made 3 runs. This is all easing into the throttle in 3rd gear.
Boost PSI: 7.0-7.2
Backpressure: 9-11.8
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Test 4:
Same as above but turned on the boost controller and set duty cycle to command about 10psi. Made one run.
Boost PSI: 11
Backpressure: 16.7

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I’d say your turbo is a good match for your car if it’s more street than strip. If it’s more strip than street I’d start mathing (I made that word up just now and it’s staying) and come up with a bigger hot side.
 
TT5.9mag said:
I’d say your turbo is a good match for your car if it’s more street than strip. If it’s more strip than street I’d start mathing (I made that word up just now and it’s staying) and come up with a bigger hot side.
Click to expand...
Im more interested in strip performance. The street driving is just for tuning for the track, although the quick spool is fun. I don’t street race or anything. Do you think there are gains to be made by increasing the downpipe from 3 to 4”? I’m thinking the next logical step is an upgrade to the 7875 with the 1.25AR and then a 3.5-4” downpipe. There’s also a good chance that this EBay 7875 just doesn’t perform as well as the VS Racing equivalent and maybe switching to the VS model without any other changes yields better performance.
 
I would build the down pipe the same size as the turbine outlet and no smaller. On my VS 7875s that was 3-1/2. I do think you’ll see an improvement with a 1.25 hot side but only marginal improvements at low boost pressure. Once you get up into the 15-30 psi range it’ll show big improvements as the back pressure will track more in line with boost pressure. I wish you were closer, I have a vs7875 gen2.5 1.25ar sitting here I’d loan you for testing.

On mine I bought the monkey fab vband adapters so I had seamless transitions out of the turbine. V-Band Flange s400 T4 1.25AR (Vs racing 7875, and similar) Turbine; turbo downpipe: 3.5" or 4.0" SS
 
Thanks @TT5.9mag. As always I appreciate and value your feedback.
The exhaust outlet on this eBay model is 3” so that’s where we’ll stay.
I’m not quite ready to throw in the towel on this turbo but the back pressure numbers are informative. I know a 1:1 ratio is ideal and 2:1 is not uncommon at higher PSIs from what I’ve read so I think this turbo still has a little more in it given the back pressure is somewhere in between at 11psi when I just run just the downpipe. Not sure my motor or injectors will scale much higher than 15-16psi anyway. I need to get the tune squared away at higher boost (still a bit rich) and take it to the track and see.
 
Great testing, to keep mine down, I went with 3.5" turbo back to just behind the trans cross member and went to 4" and dumps in front of the rear axle. Wonder what mine would measure lol
 
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