About backpressure : intake pressure...
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About backpressure : intake pressure...
TED B wrote:
With an NA engine, any residual exhaust backpressure in excess of 1-2 psi creates a pumping loss that reduces power. With a turbo engine, exhaust backpressure tends to be much greater. When the ratio of backpressure:intake pressure is enough to force exhaust gas back into the cylinder during overlap, increasing the boost pressure further isn't going to do any good. A primary cause of excessive backpressure in a turbo engine is a turbo that has a compressor:turbine efficiency ratio that is mismatched for the application.
For example, in his A/B testing of 20G9-6 vs. 20G9-5 turbos, D. Buschur and Robert of F.P. measured exhaust backpressure to compare the P.R. (pressure ratio) of the two turbos, which is the ratio of exhaust pressure to intake pressure. The results were as follows:
20G9-5 - 28 psi intake, 32 psi exhaust 420whp - 475 ft/lbs
20G9-6 - 28 psi intake, 48 psi exhaust 385whp - 397 ft/lbs
If these results are correct, it shows that the 20G9-5 has much better PR, and is better balanced where compressor:turbine efficiency is concerned. This being the case, a cam setup with large overlap (like mine) is not going to work well with the 20G9-6, because as the power levels rise, the high pressure exhaust gas will try to push itself back into the cylinder and compromise VE.
A PR of 2:1 is typical for a street setup where quick spool is preferred over peak hp numbers. A smaller hotside or larger compressor with no change in hotside will drive the PR upward.
Race engines and high rpm, high hp applications want to see a PR as close to 1:1 as possible. They achieve this with larger exhaust systems and hotsides that compromise spool, but allow large volumes of air flow at high rpm and boost pressures.
Obviously, a large overlap cam set works better with a setup that has a PR of closer to 1:1 than 2:1.
Here is my question...
when we are talking about this numbers :
"20G9-5 - 28 psi intake, 32 psi exhaust 420whp - 475 ft/lbs
20G9-6 - 28 psi intake, 48 psi exhaust 385whp - 397 ft/lbs "
are we talking about max intake boost and max backpressure, or backpressure at a certain rpm ?
I am asking because I measured mine, and the backpressure changes with rpm:
4000 rpm: intake 23 psi, exhaust 30 psi
5000 rpm: intake 23 psi, exhaust 33 psi
6000 rpm: intake 23 psi, exhaust 38 psi
6500 rpm: intake 23 psi, exhaust 45 psi
7000 rpm: intake 23 psi, exhaust 57 psi
TIA !
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Originally Posted by ItalianEvo
are we talking about max intake boost and max backpressure, or backpressure at a certain rpm ?
There's also a matter relating to how this measurement is taken: was this a static, or total, pressure measurement? And where was it taken? It matters a LOT when comparing overall pressure ratios.
Also, this is another case where MiVEC would be VERY usefull. More overlap in midrange where the PR is low with less overlap at redline where the PR is high? Sure!
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Originally Posted by SaabTuner
There's also a matter relating to how this measurement is taken: was this a static, or total, pressure measurement? And where was it taken? It matters a LOT when comparing overall pressure ratios.
I got it in a 3rd gear run, I attached a pipe on the exhaust manifold where the 4 runners get into one, just before turbine inlet.
I attached a pressure gauge at this pipe, and put it just beside the boost gauge...
and then I made a video of the 2 gauges...
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Originally Posted by ItalianEvo
I got it in a 3rd gear run, I attached a pipe on the exhaust manifold where the 4 runners get into one, just before turbine inlet.
I attached a pressure gauge at this pipe, and put it just beside the boost gauge...
and then I made a video of the 2 gauges...
I attached a pressure gauge at this pipe, and put it just beside the boost gauge...
and then I made a video of the 2 gauges...
But now we need to know where it was located on the other setups to compare. This is a very interesting thread, btw. I've always wanted to see the respective overall pressure ratio of a number of different turbo setups because it's a great way to compare two setups.
mmm i think you two have kinda answered your own speculations. in teh end it has to be at a certain rpm and the fact is it will be different for every rpm.
with that in mind once you encounter a crisscross, overlap whatever that is indicative of reversion then you have to stop turning up the boost as doing so further is gonna ruin that part of the power band and possibly others.
there is a certain max ve that the engine experiences at a certain rpm range (4-5k i believe) and you would do well to keep that in mind. though the peak exhaust back pressure will always come from the upper rpms as that would be the maximum flow (which is not necessarily where max efficiency is) and the maximum flow and backpressure is a measure of how inefficient the turbine becomes.
with that in mind once you encounter a crisscross, overlap whatever that is indicative of reversion then you have to stop turning up the boost as doing so further is gonna ruin that part of the power band and possibly others.
there is a certain max ve that the engine experiences at a certain rpm range (4-5k i believe) and you would do well to keep that in mind. though the peak exhaust back pressure will always come from the upper rpms as that would be the maximum flow (which is not necessarily where max efficiency is) and the maximum flow and backpressure is a measure of how inefficient the turbine becomes.
Originally Posted by ItalianEvo
I am asking because I measured mine, and the backpressure changes with rpm:
4000 rpm: intake 23 psi, exhaust 30 psi
5000 rpm: intake 23 psi, exhaust 33 psi
6000 rpm: intake 23 psi, exhaust 38 psi
6500 rpm: intake 23 psi, exhaust 45 psi
7000 rpm: intake 23 psi, exhaust 57 psi
4000 rpm: intake 23 psi, exhaust 30 psi
5000 rpm: intake 23 psi, exhaust 33 psi
6000 rpm: intake 23 psi, exhaust 38 psi
6500 rpm: intake 23 psi, exhaust 45 psi
7000 rpm: intake 23 psi, exhaust 57 psi
Did you get your ignition timing map retuned? If not, these readings may very well be misleading.
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I just want to jump in with an **** observation that's been sort of bugging me.
I've seen "pressure ratio" used on this site to describe the difference between exhaust manifold pressure and intake manifold pressure. While it is indeed absolutely technically acceptable to describe this as a ratio of pressures, in the engine/engineering/turbo world this is actually known as "engine delta p." Example: 20 psi boost and 30 psi exhaust manifold pressure (gauge) is 10 psi "negative engine delta p."
"Pressure ratio" is a term almost always associated with describing/determining the operating point of a compressor, or turbine.
It's semantics, I know, but every time I see "pressure ratio" used out of context here I get confused since for me the terms are ingrained differently. Thanks for listening and carry on.
I've seen "pressure ratio" used on this site to describe the difference between exhaust manifold pressure and intake manifold pressure. While it is indeed absolutely technically acceptable to describe this as a ratio of pressures, in the engine/engineering/turbo world this is actually known as "engine delta p." Example: 20 psi boost and 30 psi exhaust manifold pressure (gauge) is 10 psi "negative engine delta p."
"Pressure ratio" is a term almost always associated with describing/determining the operating point of a compressor, or turbine.
It's semantics, I know, but every time I see "pressure ratio" used out of context here I get confused since for me the terms are ingrained differently. Thanks for listening and carry on.
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Please keep in mind i'm not an engineer, and i only love to learn...
Could it be that i always heard and read about "pressure ratio" because this is a way more important value than a delta P?
I mean... Delta P has not a big importance if you don't mention the overall pressure values... For example 10 psi of Delta on 30 psi of boost is acceptable... but it won't if boost was 10 psi.
While backpressure:boost ratio gives a better vision of the situation?
Could it be that i always heard and read about "pressure ratio" because this is a way more important value than a delta P?
I mean... Delta P has not a big importance if you don't mention the overall pressure values... For example 10 psi of Delta on 30 psi of boost is acceptable... but it won't if boost was 10 psi.
While backpressure:boost ratio gives a better vision of the situation?
In your test, how did you maintain 23 psi solid to 7000 RPM? Are you running a closed loop boost controller, or a boost controller that varies its duty cycle as RPM's rise?
I usually see most "stock turbo based" cars fall off in boost pressure. In fact, I have never seen any car hold solid 23 psi peak and maintain 23 psi solid to redline.
57 psi of back pressure in the exhaust should be enough to start overcoming the force applied by the wastegate actuator, and cause the effective intake manifold pressure to fall of some.
Brian
I usually see most "stock turbo based" cars fall off in boost pressure. In fact, I have never seen any car hold solid 23 psi peak and maintain 23 psi solid to redline.
57 psi of back pressure in the exhaust should be enough to start overcoming the force applied by the wastegate actuator, and cause the effective intake manifold pressure to fall of some.
Brian
Saabtuner brought up a good comment about exploiting MIVEC to take advantage of favorable engine delta p. This has been an "enabler" for OEM turbo engines as VVT actuators become more ubiquitous.
As for the escalating exhaust backpressure with engine speed, it is indicative of one or more of the following issues:
1. compressor power requirement is escalating sharply
2. turbine is running out of flow capacity
3. turbine efficiency is plummeting
4. turbine backpressure (downstream of turbine) is escalating
#1 above is due to one of both of the following:
a. compressor pressure ratio is going up (this can happen even if boost remains constant, or even drops)
b. compressor efficiency is plummeting (running deep in choke)
In a. above, there might be a big pressure drop in the intercooler and/or plumbing. And/or a lot of inlet depression (upstream of the compressor inlet) due to a restrictive filter and/or undersized plumbing.
In b., there could be a leak in the intercooler plumbing, compressor could be undersized, or just plain crappy.
Back up to #2, this is obviously affected by any leaks upstream of the turbine (including a wastegate!).
Quantifying/assessing plumbing-related pressures/drops is aided with proper instrumentation. You can use separate gauges (one at the compressor discharge, and one right upstream of the throttle body) and subtract the readings but there's a lot of measurement uncertainty this way. A better way is to use a delta p gauge. Same goes for more accurately measuring engine delta p. For compressor inlet depression measurement you want an inches of water gauge. Using a boost/vac gauge here is like using a claw hammer to remove a splinter.
Once all the plumbing/housekeeping-type issues are identified and addressed you can more accurately determine whether backpressure issues are hardware (turbo)-related. If so, it helps a lot to have representative compressor/turbine maps, which is not as common as you might think.
As for the escalating exhaust backpressure with engine speed, it is indicative of one or more of the following issues:
1. compressor power requirement is escalating sharply
2. turbine is running out of flow capacity
3. turbine efficiency is plummeting
4. turbine backpressure (downstream of turbine) is escalating
#1 above is due to one of both of the following:
a. compressor pressure ratio is going up (this can happen even if boost remains constant, or even drops)
b. compressor efficiency is plummeting (running deep in choke)
In a. above, there might be a big pressure drop in the intercooler and/or plumbing. And/or a lot of inlet depression (upstream of the compressor inlet) due to a restrictive filter and/or undersized plumbing.
In b., there could be a leak in the intercooler plumbing, compressor could be undersized, or just plain crappy.
Back up to #2, this is obviously affected by any leaks upstream of the turbine (including a wastegate!).
Quantifying/assessing plumbing-related pressures/drops is aided with proper instrumentation. You can use separate gauges (one at the compressor discharge, and one right upstream of the throttle body) and subtract the readings but there's a lot of measurement uncertainty this way. A better way is to use a delta p gauge. Same goes for more accurately measuring engine delta p. For compressor inlet depression measurement you want an inches of water gauge. Using a boost/vac gauge here is like using a claw hammer to remove a splinter.
Once all the plumbing/housekeeping-type issues are identified and addressed you can more accurately determine whether backpressure issues are hardware (turbo)-related. If so, it helps a lot to have representative compressor/turbine maps, which is not as common as you might think.