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Old 02-06-2006, 02:09 PM   #1
Freon
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Default Appropriate sensor for exhaust backpressure?

Well, it doesn't seem like it is very popular, but I'd really like to experiment and record exhaust backpressure throughout my car modification process.

I tend to think a big part of EGT is also pressure related. It's not just timing and AF, but a small turbine and forced shut wastegate may be pushing exhaust pressure to 15, 20psi, who knows. I'm very curious to see what it really is, even if it costs me $200 for a gauge.

Any recommendations or ideas on where to start? I think a ~30 psig electronic boost gauge would be a good start, but I'm most worried about getting an electronic sender that will be compatible and withstand the heat of the exhaust manifold. I'm pretty sure I would be placing the sensor in the UP, close to the turbine. Tapping a hole should be no problem.

Alternatively, a simple 5v pressure sensor would work, and I can record it with my LM-1 and go back to look at the logs later. I generally log a lot and don't mind this at all. It's not something I'm worried about looking at while driving around. I just need to be able to get a proper conversion table for voltage to pressure.

Either way, I'm just not sure what a good solution would be, mainly due to concerns over heat.
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Old 02-06-2006, 02:32 PM   #2
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The way most folks do it is to tap off the exhaust manifold with a small diameter steel tube. Coil it a few times so the gases have a chance to cool, then attach a conventional pressure gauge to the end of that steel tube.


On small turbos exhaust gas pressures go up to 2 - 2.5 x boost. Back pressures in the turbo --->cat ----> muffler part of the exhaust can be in the 6 psi range depending on the design of the muffler and pipe diameter.

David Visard has published some back pressure numbers in his book titled "How to Build and tune cars for : Performance with Economy"

305 CID SBC (small block Chevy) stock exhaust at 5000 rpm 9 psi, converted to single free flowing cat and turbo muffler 5 psi, with dual pipes and free flowing cats 2.5 psi.

It would be nice to see some actual numbers off a WRX or STi. Some of the top performance shops have done this but they hold the numbers confidential and have not discussed it much, except in very general terms..

Larry
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Old 02-06-2006, 07:59 PM   #3
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http://www.drivewerks.com/catalog/Sh...CAT447_pg4.htm

You need some sort of isolator from the heat. I found something other just a tube at one time, but I can't dig it up now. The one above I think has an 'isolator' on the end, but it costs some money.
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Old 02-11-2006, 06:30 PM   #4
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Hey Larry,
Have you been interesting in remote mounted turbo systems very much? You bring up one of the points that I think most people overlook. A normal turbo system flows exhaust from ~40psi to ~5psi. Then ~5psi to 0psi from the muffler to atmosphere.
A RMTS should flow that same ~40psi directly to 0psi without a muffler. Same goes for the wastegate when it opens. The long pipework between the engine and the turbo is the only thing that makes the exhaust quiet. Just like a muffler, I think it makes a difference how long the distance is between the exhaust port and the turbo with regards to sound. Something about the exhaust pulses and chopped up reflections bouncing back and forth, I think.

Also, the turbo itself shouldn't be muffled nearly as much. There should be an audible whine almost all the time.
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Old 02-13-2006, 09:27 AM   #5
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Quote:
Originally Posted by aspera
Hey Larry,
Have you been interesting in remote mounted turbo systems very much? You bring up one of the points that I think most people overlook. A normal turbo system flows exhaust from ~40psi to ~5psi. Then ~5psi to 0psi from the muffler to atmosphere.
A RMTS should flow that same ~40psi directly to 0psi without a muffler. Same goes for the wastegate when it opens. The long pipework between the engine and the turbo is the only thing that makes the exhaust quiet. Just like a muffler, I think it makes a difference how long the distance is between the exhaust port and the turbo with regards to sound. Something about the exhaust pulses and chopped up reflections bouncing back and forth, I think.

Also, the turbo itself shouldn't be muffled nearly as much. There should be an audible whine almost all the time.
the only thing that comes to mind is the fact that the pulses of exhaust gas have a lot of kinetic energy, and these spool the turbo quicker. this is why the i4 4g63 can spool up say an 18g almost 1k rpms quicker than our boxers can... they hang the turbine 4" from the head. there's less tubing to pressurize, which means more kinetic energy imparted to turbine, and that means quicker spool.
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Old 02-13-2006, 01:28 PM   #6
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I believe pat at rocketrally in BC has used a backpressure sensor of some kind. Email them at rocketrally.com to pick their brains
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Old 02-13-2006, 04:10 PM   #7
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I use a 4bar map sensor with about a 1.5 feet of steal fuel line then fuel hose going to the sensor.
I just calibrate and log the sensor with my Autronic sm4.

This is with a stock ej257 block and sti ver3 heads. the turbo is very similar to an fpgreen but has the smaller td06(not h) exhaust wheel.
The spikes in back pressure are the wastgate opening and shutting.
The manifold pressure and back pressure use the same scale on the bottom left side.
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Old 02-13-2006, 07:26 PM   #8
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nice setup, rac.
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Old 02-14-2006, 05:07 AM   #9
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Quote:
Originally Posted by ride5000
the only thing that comes to mind is the fact that the pulses of exhaust gas have a lot of kinetic energy, and these spool the turbo quicker. this is why the i4 4g63 can spool up say an 18g almost 1k rpms quicker than our boxers can... they hang the turbine 4" from the head. there's less tubing to pressurize, which means more kinetic energy imparted to turbine, and that means quicker spool.
Right, those are the obvious two points (and they are big ones, don't get me wrong). There's less engergy lost between the engine and the turbo, and the turbo spools up quicker. But I'm trying to point out that there are several other things to consider. Like, "What's the air doing in the pipe?" In a normal exhaust system, say the EVO's, the exhaust exiting the turbine is very turbulent and mixed with wastegate exhaust. It has lost much of it's kinetic energy and heat. Now, it has to flow down a large diameter pipe to the muffler, and then finally out to the atmosphere. Also, the small volume exhaust manifold of the EVO has some fairly tight bends and turns. If it was perfect, then nobody would replace it with headers or other manifolds.

Another question is, "What happens after the wastegate opens?" In a normal turbo system, the wastegate exhaust gets shot back into the downpipe. In a RMTS it goes directly to the atmosphere, like a "screamer pipe". At this point a RMTS has most of the exhaust system filled with high pressure exhaust. The normal turbo system only has the first foot under high pressure, the rest is relatively low pressure. As I understand it, high pressure, hot exhaust flows better than cooler low pressure exhaust.

I really thing that things need to be chopped up in to phases.
First, before the car builds boost it is nearly a normally aspirated car. The normal turbo car will attempt to build boost right away. It has to. The turbo is barely spinning and is restricting the intake and exhaust. Even if the turbo was empty, the whole shape of the exhaust is just wrong for an NA car. The RMTS, on the other hand, has an exhaust nearly identical to an NA car, except for the turbo way out back. It gets a quick chance to build some RPM while gradually spooling the turbo.

Second, the normal turbo car builds boost lower in RPM, but the RMTS car is catching up quick since it doesn't have a muffler. (It IS a muffler)

Third, the wastegate opens and the RMTS is flowing 35psi all the way to the rear of the car where it either goes turbine>atmosphere, or wastegate>atmosphere. The normal turbo car keeps this 35psi in a small volume next to the head. It heats up the turbo and exhaust manifold cherry red. The exhaust has to go turbine>some tight turns>muffler>atmosphere, or wastegate>some tight turns>muffler>atmosphere.
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Old 02-14-2006, 09:39 AM   #10
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I'm not a fan of the RMTS setups, the pumping losses and thermal losses appear to be very excessive.

I have not seen any numbers to tell me that is not the case.

Larry
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Old 02-14-2006, 05:08 PM   #11
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That's fair.

Thermal loss between the exhaust valve and the turbine should be minimized with insulation. There would also be 'thermal loss' of a lot of hot air in the engine bay. The heat lost between the exhaust valve and the turbine doesn't mean that any air is lost. It is still the same 35psi mass of air, hot or cold.

Pumping losses depend on the length and straightness of the pipe. Since the RMTS doesn't need to be squished into the engine bay, there is room for straighter plumbing. The compressor intake *might* have one bend. The 'downpipe' might also have one bend. Both of these could have very large diameters and be quite short. Same goes for the wastegate...maybe no pipe?

Regardless, both types of systems pump the exhaust from the engine to the rear bumper. We also know from large FMICs on WRXs that a small engine can tolerate quite a lot of volume on the cold side. A hard pipe from the Impreza muffler area to the throttle body would likely be shorter and much straighter than some of the ring-around-the-engine FMIC setups out there. Some of the FMIC systems have a 180 degree turn right out of the compressor.
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Old 02-14-2006, 05:26 PM   #12
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Quote:
The heat lost between the exhaust valve and the turbine doesn't mean that any air is lost.

But it does result in pressure loss. Cooler gasses do not expand as much, (bottom line less energy for the turbine to work with). Also colder gases have a lower sonic speed which is what determines maximum flow velocity through the turbine.

Lower pressure due to cooling, and lower choke flow velocity due to the cooler gasses means the turbine cannot flow as much gas as it could with a shorter hotter inlet. Thermal effeciency of the turbo is determined by the pressure and temperature drop as the gasses expand exiting the turbo. Colder inlet temp means less power out.

Larry
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Old 02-15-2006, 12:22 AM   #13
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The pressure loss due to heat loss is surely a problem...until the wastegate opens. You also might have hit on why the RMTS seems to be such a good muffler. There is a temperature gradient throughout the exhaust system. Pressure should be constant the whole length until the turbo, right? Wouldn't the change in temperature along this lengthy pipe cause a change in velocity, and therefore a muffling effect?

STS uses a smaller, quicker spooling turbine in their kits. This would seem to choke the exhaust...until the wastegate opens. Then it doesn't matter.

Pressure drop across the turbo should be more pronounced since there is no muffler, but temperature change would be less as you have pointed out.
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Old 02-15-2006, 12:28 AM   #14
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Oh, and...
The above graph shows how pressure trapped between the engine and turbo is fairly constant after the wastegate starts controlling it. But, would the pressure right at the exhaust port be slightly higher than the pressure right before the turbo? It should, right? On a car like the EVO, there really aren't two different places, but the Subaru engine is different. The turbo is down the pipe pretty far. I think looking at an extreme example might be helpful.
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