Exhaust Pressure Testing

[engineermike]

To clear up any confusion, with the magnaflow street series cat back the pressure profile is:
- Header collector upstream of cat: 11 psi
- Just downstream of cat: 6.5 psi
- Just upstream of resonator, downstream of 2.25” factory-connection: 1.5 psi

Stock:
- Header collector upstream of cat: 15 psi
- Just downstream of cat: 13 psi

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[engineermike]

Post-cat and pre-resonator pressure taps.

Hard to believe there’s 5 psi difference between these two locations.

 

[80FoxCoupe]

@engineermike maybe i need to do this before swapping my exhaust. What are you logging psi with?

I have 2in ARH header's with 3in collectors. 3in xpipe with 3in MBRP race system. With my setup at 1300whp, my thought was going to a 3.5in custom system after the collectors. Ive seen 1000whp cars pick up power with 3.5, but it would be nice to have real pressure data. Dyno? Honestly with the power and RPM involved id rather track tune as the dyno is violent. Track results vary based on a million things...

 

[engineermike]

@engineermike maybe i need to do this before swapping my exhaust. What are you logging psi with?

I’m using this kit:

https://www.ebay.com/itm/332654604203

Earlier, I was videoing the gauge then scanning the video for the peak number. However, I’ve since added a check valve so it captures the max number and I can check the gauge after a pull, which works much better.

 

[80FoxCoupe]

I’m using this kit:

https://www.ebay.com/itm/332654604203

Earlier, I was videoing the gauge then scanning the video for the peak number. However, I’ve since added a check valve so it captures the max number and I can check the gauge after a pull, which works much better.

What about a pressure transducer and pulling the data into Zietronix/ngauge?

 

[GT 550]

@engineermike are you connecting to the pipe with a gas-tight fitting? You probably are but it's hard to tell.

The reduction in pressure between those 2 locations could be due to the cumulative effects of bends, diameter reduction, and gas cooling. A lot of big hp turbos have massive dump pipes but relatively modest exhaust diameters.

 

[engineermike]

What about a pressure transducer and pulling the data into Zietronix/ngauge?

Definitely considered logging, but I would have used vcmscanner pro features a pro link. Eventually I realized a single peak number was all I needed so an elaborate setup wasn’t necessary.

 

[engineermike]

If anyone decides to do their own testing I highly recommend using the nutserts below rather than the ones that come with the kit. The ones that come with it are 2-piece and I had trouble with the internal part coming loose. The attached are 1-piece, much easier to install, and are sturdier once installed.

Astro Pneumatic Tool RN14... https://www.amazon.com/dp/B01J7X4DIK?ref=ppx_pop_mob_ap_share

 

[engineermike]

I deleted the 2.25” factory connection and replaced it with 2.5”. That dropped the post-cat pressure from 6.5 down to about 4.5 psi. Upstream of the cat measured 9-10 now, down from 11 with just the cat-back upgraded.

So the net reduction vs stock with the magnaflow street series cat back and factory-connect delete was from 15 to 9-10 psi.

Based on what I’ve done and measured, I don’t think it’s possible to get below 7-8 psi with factory cats even with a complete 3” system all the way to the cats. The cat dP started at only 2 psi but as I reduced downstream pressure the cat dP increased to 5 psi.

 

[Torinate]

Nice. So deleting the neck down made a big difference you would say? Any pics of you did it? Wonder if there’s any real measurable performance gains. Thoughts?

 

[engineermike]

Not exactly what I had in mind but it works and is sturdy. A drop of 2 psi from the factory connect delete isn’t a ton but should allow a tiny bit more spark, like 1/3-1/2 deg. That could be worth 10 hp. Difference in pumping losses from 2 psi will only be in the 5 hp range. With a centrif or turbo the gains will be greater.

 

[Torinate]

That’s all right though. So say even 10 hp is a gain. Looks fine how it’s done. Functional and sturdy are the main things!

 

[80FoxCoupe]

Gains are always good!

 

[engineermike]

So I sorta got carried away wanting better data than just a peak-hold value from a gauge. It doesn't help any that I accidentally got an instrument engineer involved who egged me along. The following is a bank of four GM 3-bar MAP sensors, a power supply, and an A-D converter. The brass tubes are heat isolators to allow the use of rubber hose to get to the MAP sensor without degrading rapidly.

The 4 pressure locations are just ahead of both cats, just behind the driver's side cat, and just ahead of the resonator. Note that the values in the graph are psia.

Observations:
- Peak pressure was 12.6 psig ahead of the passenger's side cat.
- Delta across the DS cat was only 3.2 psi.
- Delta across the PS cat was about 5.5 psi, depending. I'm not sure how restrictive the flex joint is vs the cat. This cat is the older of the two, so it could be the cat.
- Delta from the downstream side of the cat to the upstream of the resonator was 3 psi. This is most surprising as the only thing between those two locations is 2.5" pipe and a pair of 45 deg mandrel bends.
- Delta imposed by the resonator and dual 3" cat back is 4 psi. I tend to think the lion's share of this is from the 2.5" MF resonator since the rest of the exhaust is dual 3" with straight through mufflers.
- I really wish I had this data set for the stock exhaust system.

 

[80FoxCoupe]

Nice work @engineermike ! Any plans to change hardware based on data?

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