Anyone *know* ex. mani. backpressure number?

[kenamond]

I've heard that a crude estimate for backpressure in the collector or just upwind of the turbine is roughly 2xboost. I have some guesses as to why this might be, but I wonder if anyone knows what it is, either from theory or from direct measurement. I read a post a long time ago and forget where it is saying that folks with EGT gauges would get more valuable info by rigging a pressure gauge to the EGT port (usually #1 runner). If anyone has done this, I'm curious what the backpressure was for different boost levels. That post might have been in a tech article...wish I could remember.

My guess (I'll have to do a bit more research) is that if the turbine and compressor wheels when in their efficiency range are about 75% efficient. I vaguely recall that the efficiency translates directly into energy loss (entropy increase). If you generate 75% of ideal energy at the turbine (which gets transferred to the compressor) and the 75% of the ideal energy at the compressor, you take the product of the efficiencies and get roughly 50%. But like I said, this is a bit hazy and may be flat out wrong.

Anyway, real, observed values are the best, but if there's a theoretical value that's close enough to reality, that'd do as well.

 

[aero_sallee]

I have only measured one turbo for backpressure using a gauge in my EGT bung located in cyl 1 runner.Its an internally gated 8 blade HX40 and internally gated BEP housing. At 17-19lbs of boost, the exhaust pressure in the #1 runner was somewhere around 25-27lbs. That is taking the middle of the needle swing since the exhaust pulses in one runner are less than ideal for measuring pressure and the needle swings rapidly with every exhaust pulse.


I don't think you will find a real answer to this though, they vary too much from everything I've ever read.

I've heard of perfectly matched turbos having more boost than drive pressures in certain areas of engine operation. I've also read reports of the opposite, with massive drive pressure at low boost from a poorly matched compressor and turbine. Then you get the same turbo which in different RPM ranges can have different drive pressure ratios. I know mine did this somewhat when I measured it, but Its been a while so I can't remember what the actual trend was.

You may be able to gain some generalizations that could be useful.

 

[99gst_racer]

Mack,
You might want to send Kiggly a PM. I know he's monitored backpressure each cylinder of his engine in the past. I'm sure he will have some useful information to share (as always).

 

[crimsondragon]

I remember reading a post on how to read it. They hooked up an oil pressure gauge to the bung. I don't remember the specifics.

 

[aero_sallee]

I remember reading a post on how to read it. They hooked up an oil pressure gauge to the bung. I don't remember the specifics.

I used a oil pressure gauge in the EGT bung.

 

[kenamond]

Thanks guys.

I wonder if the issue of mismatched turbin/compressor setups relates to being out of the efficiency range of one or the other of the wheels (why do folks only talk about A/R on the hotside but only compressor maps on the coldside anyway?). I'm one of those geek scientists, so I'd like to know a theoretical explanation if anyone has a link.

As for the backpressure gauge, the thread I referred to mentioned running copper line from the bung out of the heat to a catch can or something that could withstand the backpressure. In the can would be something like steel wool (whool?) or something to damp out the exhaust "putts" as he put it, then on to a pressure gauge. Wonder if a small bleeder valve would work...dial it in so that it responds fast enough, but damps out the pulses.

I'm specifically interested in the evo3b16g and its notorious creep problem. Need to find the "ideal DSMer" with that turbo and a backpressure gauge.

If anyone knows of additional online info about backpressure, I'd appreciate the link.

Thanks again!

 

[pickens]

if somebody can find me a pressure transducer, I drill and tap and then record the data.

 

[DGajre777]

As for the backpressure gauge, the thread I referred to mentioned running copper line from the bung out of the heat to a catch can or something that could withstand the backpressure. In the can would be something like steel wool (whool?) or something to damp out the exhaust "putts" as he put it, then on to a pressure gauge. Wonder if a small bleeder valve would work...dial it in so that it responds fast enough, but damps out the pulses.

You mean this?

"Most people never know what their back pressure is. I mean how many of us really have a pressure transducer tapped in their manifold? I'll tell you, I see it without fail. All these guys have that ridiculous EGT gauge tapped religiously in their #1 runner. I honestly think only the Autometer blinky light A/F meter for a narrow band OEM O2 sensor a more worthless piece of shit gauge to occupy gauge space, but I digress. You don't have to have a gauge permanently mounted in sight for backpressure. Only need to make this measurement after you make a change that affects your engine VE. VE is volumetric efficiency. VE is a measure of your engines ability to move air through it. Head porting, camshafts, intake manifolds, air filters, intercoolers, turbos and exhaust systems all affect your engines VE. Examples of modifications that don't affect your VE are fuel injectors, piggyback ECU controllers, blow-off valves, fuel pressure regulators, fuel pumps, and MSD ignitions. You also don't have to have a fancy stand-alone ECU with an expensive 5 bar MAP to take backpressure measurements. Next time you are in AutoZone, take a look in the hillbilly section. Next to the chrome naked girl silhouette mud flaps you'll find cheap oil pressure gauges. Pull that stupid EGT probe out temporarily and run some copper tubing to the hole with a 1/8 NPT compression fitting. Run enough tubing to protect the gauge from the heat of the exhaust manifold. Then run rubber tubing to a generic fuel filter assembly, then run more tubing to the cockpit to that bitchin oil pressure gauge. The fuel filter acts as a dampener to reduce the pressure oscillations from the exhaust "putts" and allow you to make a cleaner measurement. Get a friend to ride with you to watch the gauge. The gauge may not even flinch till you hit boost, but then should rise quickly with boost and continue climbing with RPM till you reach your engines max VE point then taper off (usually around 5700 with a stock intake manifold).

So now that you have your backpressure reading how much is too much? Generally speaking, you don't want more than a 1:1.5 ratio of boost to backpressure. So if you're at 20psi of boost you should not see more than 30-35psi of backpressure. If you do then you should upgrade the turbine side, you'll make more horsepower for every pound of boost you run."

http://www.dsmtuners.com/forums/1670844-post15.html - Post 15.

 

[kenamond]

Yep, that'd be the one Dee. Thanks! If I had searched on "putts backpressure" instead of "backpressure" it would have been the only match.

 

[GVR4592]

I knew exactly which thread you were referring to when you described it, Dee beat me to it. I just bought a second pressure sensor to log through aem for just this purpose. It should be interesting.

 

[kiggly]

I got a PM about this thread, so I'll jump in and add what I've seen.

Exhaust pressure can be all over the place compared to intake pressure. With a 16g years ago, I saw just below a 1:1 exhaust:intake pressure ratio up to about 18psi before the flow limits of the compressor. As I pushed it, exh pressure went up to the 35-40psi range where an internal wastegate setup blows open.

The Frank3 I had never had below a 1:1 pressure ratio and once at 22-24psi I was knocking on 2:1. This still made power as I cranked it up more, just not a bunch more power.

With the Precision PT74GTS I saw 1:1 during spoolup, but then a little above 1:1 after in boost at 25-30psi or so. By 40psi it couldn't hold boost and I was seeing 80-90psi backpressure.

The turbo I've done a bunch of mapping on is the current borg warner 74mm turbo. This one is much better than 1:1 everywhere so far. At 12-13psi boost, it is about 3-4psi backpressure. As I turn the boost up, it remains at about 10psi more intake pressure than exhaust pressure. The highest it has seen is about 42-43psi. Out there it is about 35psi backpressure if I remember right.

The moral of the story is, exhaust backpressure can be all over the place. It is really a telltale of how efficient all the parts in the system are and also how well an engine breathes. As an engine breathes better, backpressure will go up. This is because it takes more energy to pump more air, so the turbine has to harness more energy in the form of pressure. Backpressure also change A BUNCH with ignition timing. It is a good thing to keep tabs on, but definitely not something you can tune by or put a particular target on via tuning. In my experience, less is always better. Exhaust backpressure can also change optimum cam timing. This usually isn't too dramatic though.

Also, turbine pressure ratio - just like comp wheel pressure ratio - is mostly a function of rpm. As you crank up the shaft speed, the pressure ratio does the same. So if you get into the flow limits of a compressor wheel where it has to speed up to maintain boost, your turbine pressure skyrockets.

Kevin

 

[kenamond]

Excellent!