Boost Rise Trend

Interest in demonstrating a relationship between exhaust system back pressure and boost onset led me to run a test where I measured each while applying a covering over the exhaust pipe outlet.

Exhaust pressure measuring rig attached to 034 motorsport downpipes

The stock MAP sensor was used to record Intake Manifold pressure and a 4 BAR sensor was connected to a fitting that reads exhaust pipe pressure at the secondary O2 sensor bung, which on my car is located in the downpipe prior to the catalytic converter.  The setup for reading the exhaust pressure is similar to what is shown above, though the picture was from a prior configuration, that I have since economized a bit so as not to be hanging down like the picture shows.

exhaust_tip_setup

The cover consisted of a piece of heavy duty aluminum foil doubled over and then a piece of DEI exhaust wrap placed on top of the foil to create the seal with some reinforcement.

I took readings under three conditions; first with the exhaust pipe open as it would normally be, second with the cover obstructing approximately half of the exhaust pipe exit, and last with the exhaust approximately 80% blocked off.

exhaust_tip_blowout

As the above ‘after’ photos show, the coverage of the DEI wrap was not complete, and there was sufficient pressure build up in the exhaust to rupture the foil cover where it was not reinforced.

The occurrence of the ‘blow-out’ is clearly indicated in the exhaust back pressure chart (below) that compares the fully open to 80% covered conditions.  The rising pressure suddenly drops quickly, then continues rising as engine speed increases.

Starting engine speed was 2000 rpm for three sets of measurements and then approximately 2200 rpm was recorded two times.  The intent being to see how boost onset was affected when starting at a higher airflow rate.

TTE550 Boost Rise (70 degF ambient)
TTE550 Boost Rise (70 degF ambient)

Shown above is the boost onset for the six different cases.  The solid dark lines are the first series where the exhaust was open.  The solid red lines are with the exhaust half covered, and the dashed lines are with the exhaust exit 80% covered.  The trend is for the boost onset to slow as the exhaust exit becomes increasingly covered.

ex_pressure

Shown above is the first data series for the two extreme cases, with the exhaust unobstructed represented with the orange line, and the exhaust 80% obstructed represented by the blue line.  The hole forming in the foil takes places around 2750 rpm of the first pull.

Case 1 vs Case 3 Exhaust System Backpressure
Case 1 vs Case 3 Exhaust System Backpressure

Above is the the same exhaust back-pressure data with the addition of the second and third 2000 rpm starting engine speed pulls.

Intake Manifold Boost Onset
Intake Manifold Boost Onset

Finally, shown above is the Intake Manifold Pressure for the two extreme test cases.

Conclusions:

It is apparent that blocking off the exhaust will increase the back pressure in the exhaust system, and the increasing back-pressure causes boost onset to be slowed.

What is unclear is how representative the worst case test may be of alternative exhaust systems.  Without putting different exhaust systems on the car to record the back-pressure that they generate I cannot speculate as to how much back-pressure exists in other systems.

4mm Nozzle Check

4mm_nozzle_check_overview

After installing the Aquamist 4mm nozzles I ran through a series of duty cycle checks to determine the true flow rate through the nozzles, albeit into atmospheric pressure instead of manifold boost.

Aquamist 4mm Nozzles
Aquamist 4mm Nozzles

I found that after approximately 70% duty cycle there was very little increase in flow rate through the nozzles.

4mm Nozzle Datalogging
4mm Nozzle Datalogging
Dual 4mm Nozzle Flow Rate vs Pump Duty Cycle
Dual 4mm Nozzle Flow Rate vs Pump Duty Cycle

With the change in flow rate that the 4mm nozzles caused, capping the maximum flow rate around 375 mL/Min., I updated the Torq Tune Pump Duty Cycle table.

Torq Tune Pump Duty Cycle Table
Torq Tune Pump Duty Cycle Table

The new table values put the pump at up to 100% duty cycle, and do so at 17-19 psi, whereas with the pair of 0.7mm nozzles I was running the pump at a maximum duty cycle of 45% and recording more flow.