Testing exhaust back pressure

[EspenW]

Better late than never:

Promised to make one of these testers a good while back, but finally got it done last weekend:

Parts used:
OXS (lambda) plug
60cm of brake line
Various M10x1.5 fittings
Hose barb

Drilled through the OXS plug with a 4mm, then took the upper part of the hole from 4mm to 9mm, then made M10x1 threads in it.
Inserted the M10x1 brake line fittings on the piece of brake line and flared the ends with my flaring tool.
A little high temperature (and sensor safe) silicone on the threads in order to seal 100%.

How to use:
Unscrew OXS (lambda sensor) and install the tool. Curl the brake line so that it fits under the bonnet. Attach a silicone hose to the barb at the end of the tool and a boost gauge (or other pressure gauge) at the other end.
Test on a dyno or on the road (do it safely).

This can be used to check if the exhaust has excess back pressure, caused by a general lack of flow or things like a plugged cat.
It shall be interesting to see how much a restriction stock RS2 downpipe and stock RS2 cats are at high rpm. with the setup (peak power somewhere in the 530-550 range (440-460 at the wheels)).

The tool that I made looks like this (I will use a much longer piece of silicone hose):

[S2central.net]

ingenious !

[EspenW]

ingenious !

Thanks, Paul

Got the idea after reading about how Dmitri measured off the chart backpressure on one of the cars that he was tuning.

Should be able to do some testing next week.

[A80Avant]

So how much did yours measure?

[EspenW]

So how much did yours measure?

Haven't had the time yet, will do sometime next week

[andy10v]

It would be interesting to see a stock downpipe v 3" etc when somebody is doing an upgrade.

[4WheelDrifter]

Haven't had the time yet, will do sometime next week

Come on, man. You've had almost 2 days with this, Vasilis wants results!

[A80Avant]

Exactly. What's the point in making something so nice then letting it lay on the kitchen table, taking photos of it and drinking beer while discussing it on the forum??? Get it on the car and tell us!

[EspenW]

Week after week after week of torrential rain finally let up, and the combination of almost freezing temperatures and dry roads just begs for testing and WOT pulls.
Got the pressure gauge setup installed. Pulled the stock Lambda sensor (disconnected form the harness too, in order to avoid it reading free air).
60cm of Nickel copper tubing bent into some sweeping curves fit nicely under the bonnet.


Long piece of 4mm silicone tubing into the cabin, attached to a big boost gauge. In order to get a ''log'', I attached a Contour video camera next to the gauge in order to record the action:



First did a 3rd gear pull at 2.3bar peak boost pressure, but turned it down to around 2.0bar for the remaining testing.

Note that this is backpressure after the turbo.
I recorded a peak backpressure of 0.45bar (6.5psi) in both 2nd and 3rd gear. (one usually sees the most in 1st gear, since the gases have less time to escape and ''jam'' in the exhaust, but what happens in 1st gear is of zero interest to me, as I don't do any standing start stuff.
This is on E85, so slightly more exhaust volume than on petrol. In addition, I just installed a turbo blanket that keeps more of the heat in the turbo, increa
sing pressure somewhat too (http://www.s2forum.com/forum/showthr...t=72189&page=3)


At approx. 2.2bar peak boost, this setup produces more than 440hp at the wheels (not hubs), so in the 520-550hp range at the flywheel. Acceleration times are pretty much spot on with the Ferrari 458, McLaren MP4 12C and Porsche 997 Turbo S, so it moves well, especially in the 60-100mph range, which is what I care the about the most, as this is where it really smokes the #1 selling car in this country, the Tesla Model S. A beefy power curve with 620nm at the wheels (likely some 7xxnm at the flywheel) at 4000rpm definitely aids the propulsion in that speed bracket.
The interesting thing here is that it still has a stock 2.75'' RS2 downpipe and stock RS2 cats. The catback exhaust is Milltek 3'' though.
This is why I wanted to test the backpressure post turbine, especially after reading Dmitri mention measuring 29psi or so backpressure (after turbo) on a car that he was working on.

[A80Avant]

That's very interesting...

[speeding-g60]

hmmm.

[EspenW]

That's very interesting...

Yep, I thought so too.
Seeing that Dmitri measured 29psi of backpressure with the (full) 2.5'' exhaust on Andys car, I ''feared'' something like 20+psi peak.
That would have meant potential for easy power, but being able to keep stock parts is a big plus for this RS2. It is more stock now than when I bought it back in the day, but is way faster.
Can't believe that I haven't tested this before, as it is very easy. Very nice to have a baseline for reference in case I suspect a clogged CAT or something at a later time.
Good idea for folks to do an actual test of their current exhaust setup before spending time and money on a different exhaust that might not provide any meaningful gains.

Going to Taiwan for the bike show (real bikes, not ones with motors), but will do some power/acceleration logs when I get back (provided salt free roads) to check spool with the turbo blanket vs. previous logs.

[diesel des]

Would be interesting to put another tap point just after the cats to determine were that restriction is.

[EspenW]

Would be interesting to put another tap point just after the cats to determine were that restriction is.

Yep. The rear box is a straight through 3'', so that probably has little restriction.
The wideband lambda sits in the entrance of the left side CAT, so that is another spot that I can easily measure from. That would measure the restriction of that CAT and the catback.

I see that Dave Vizard, Lingenfelter, etc. claimed that backpressure above 5psi is worth looking into. A reduction in backpressure would typically mean a power increase of 1.25-1.5% per psi reduced.

Still, I was suprised that I saw a much lower number than what I had expected.

[prj]

It depends on the length of the parts too.
There is a huge difference between having a short piece of 2.75" vs having 2.5" on the entire length.
Also, don't forget that 2.75" flows 20% more than 2.5"
I did not measure Andy's car, we measured a car locally with a 2.5" exhaust that had a single cat on it, after replacing it with a 3" exhaust the car picked up 100hp.

There are two types of backpressure you measure. One is the backpressure in the exhaust, which should be as close to 0 as possible - 0.5 bar is no drama.
The other one is backpressure before the turbo, which is the number that is actually important for making power.