Torque Factory, LLC
Torque Factory, LLC

Y-Pipe Development Notes

By - banking
December 07,2022 11:44 PM

This is intended to explain some of the development details of the Torque Factory 2.7 Y-Pipe and inlet components.

Work started on our current B5 S4 in 2020 and it was intended to be a fast road car that can race ½ mile and some club events.  The car essentially needed a full rebuild and we did a thorough budget build.

  • Engine “BF” short block:

    • H-Beam connecting rods

    • Torque-Factory/ARP main stud kit

    • Fully rebuilt using stock pistons

  • Heads 

    • Stock 2.7 heads fully rebuilt

    • Torque Factory/BC valve spring and titanium retainers

    • 2.8 intake camshafts

    • No porting

    • Stock manifolds and throttle body.

  • Turbo

    • K24/RS6 Hybrids

    • Intercoolers 10” x 8”x 4.5” with 2.5” inlets/outlets.

    • APR bi-pipe 

  • Induction

    • 2.125” inlets

    • S4 Y-Pipe

    • ETSpec 3.5” (85mm) MAF and filter combination.

  • Tuning

    • ETSpec tuning.

    • E85 fuel 

After our initial assembly and tuning the car ran great and made about 585 on a Dynojet a consistent 2.6 FATS.

At this point we were satisfied with the tune and started taking the following measurements:

Restriction (inch/H2O) From Filter


Restriction (inch/H2O) Including Filter and 3.5” MAF/Intake


Restriction (inch/H2O) @ Turbo Inducer (Including Y-Pipe, Inlets, MAF, Filter)


Boost (psi) @ 6500 RPM


Turbine Pressure (psi) @ EGT sensor


Exhaust Pressure (psi) @ 2nd O2


Compressor Pressure Ratio (est)


This data allowed us to utilize the turbo calculation tool provided by Borg Warner.  As seen in the compressor map and details below.

(note: Borg Warner does not have an exact match to our RS6/K24 hybrids, so the closest match was used as a point of reference).

At this point we were in a reasonable range for reliability, but little headroom for additional power.  After review of our logs and BW’s tools, we concluded that more boost was not a smart option. This would result  in a spiral of inefficiency and shortened turbocharger  life.  See pin 2 in the map below for 32 psi, it is not even on the map anymore.  At this point we realized that changes needed to be made.

The area that showed the most potential for improvement was reducing the restriction to the inducer stage of the turbocharger. 

At this point we swapped our s4 Y-Pipe for the larger RS4 unit.  Pin 1 is the S4 Y-Pipe and pin 2 is the RS4.  While the RS4 unit helped it was not enough for a measurable gain.

This validated the direction of our testing and we acquired a “Tuner” Y-pipe which was the largest commercially available option.  We also added a 4” ETSpec MAF and filter.  Although this helped, the restriction at the turbo inducer was still high so we started exploring other options.

We started to design a Y-Pipe that was the largest we could fit under the hood with a 4.5” inlet and 3.5” outlets.  An important consideration is the Y-Pipe had to be flattened and widened for clearance.  This allowed us to keep the large cross section for the best flow..

After our first prototype was complete we contacted Flyboy at and requested he flow bench test the Tuner and our Y-Pipe, 4.5” and 4.0” MAF.  He also had data from the same machine for S4 and RS4 Y-Pipes.

These flow bench results indicated a large improvement and validated our design.

The tests can be viewed at this link:

We then installed our prototype Y-Pipe, MAF, and inlets.  Our tuner (Blake @ ETSpec) was able to tune the 4.5” MAF for smooth idle and driveability.  At the completion of tuning we had 32 psi at peak power and a usable power band up to 8000 rpm.  FATS times had now dropped from 2.6 to 2.2 with similar stress levels on the turbos.

Pin #1 is the original configuration at 26psi and  Pin #2 is with the improved inlet parts and 32 psi of boost.  As you can see the calculated pressure ratio is comparable with much more power and not falling off the map.  This indicates that the increased boost is not generating any more heat, surge or shaft speed.

Torque Factory Induction Section

Science Units - Assume 1000hPa atmospheric pressure

62" = 154hPa depression or 846 hPa absolute inlet pressure

21" = 52hPa depression or 948 hPa absolute inlet pressure

27PSI = 2861hPa absolute boost

32PSI = 3206hPa absolute boost

2861/846=3.38 Pressure ratio

3206/948=3.38 Pressure ratio

Math checks out, 5 extra PSI while keeping the same pressure ratio. Reducing turbo inlet restriction is about as close to a free lunch as you can get especially at high PRs.

(Thanks NIck G)

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