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Old 04-16-2020, 08:39 PM   #826
Datguyjay
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Wow, thank you so much for this! This post cleared up so many questions from hearing back and forth things from everyone.
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Old 06-01-2020, 05:56 PM   #827
Bugeyebluewrx
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Join Date: Dec 2018
Chapter/Region: NWIC
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i think im gonna stick with the BOV i got just because i already have it(even though its the ssqv... i know i know its crap but my kid likes it and it was free) i am ok with the "rich" spike and its a fun car that isnt a power house of perfect engineering.
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Old 06-28-2022, 05:33 PM   #828
Surfingcricket
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Location: Cape Coral, FL
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I won't lie, I had definitely heard that compressor surge would kill your turbo from so many places, and it definitely confused me when I saw the episode of moneypit where Zach installed his turbo on the miata and was making all kinds of compressor surge noises, but he didn't seem concerned at ALL. This makes a lot of sense to me though, and makes me happy that I can tighten up my Perrin BPV some more when I get back from deployment!
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Old 12-21-2022, 03:23 AM   #829
Kingtal0n
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I've been building and tuning turbo vehicles for decades. I've tuned hundreds of sr20, RB, 2J engines and quite a few LS engines, all turbo. With chemistry background, mathematics, biological science, bioengineering, mechanical engineering, PhD MS BS, I'll try to use scientific theory to help explain how and why things happen, best as I can.

Why turbo
The exhaust gas pressure of a turbo engine helps hold the rod to the crankshaft on the exhaust stroke allowing it turn higher RPM with OEM rod cap & hardware than would otherwise be possible, that is, turbochargers relieve rod cap stress and improve engine longevity in performance applications. Turbochargers do not improve economy and they are the least engine efficiency at wide open throttle, worst brake specific fuel consumption, but they offer the highest engine output with least parasitic loss, least wear and tear and the best performance possible per fuel quality at a given compression ratio and combustion chamber design. Provided as tuners we are aware of important variables as IAT and EGT (intake air temp and exhaust gas temp). In OEM piston applications using 'cast' style brittle fracture failure materials, piston temperature is another essential control aspect along with oil temperature. Piston oil squirts help control piston temperature and extremely useful in cast piston turbo applications. All SR20, 2JZ, and skyline turbo engines use piston oil squirts from the factory.

I'll share the details of bypass valves, importance, use, placement, performance, problems, solutions.
Importance & wear, placement
Compressor surge is air flowing backwards through the compressor stage. It can happen at Wide open throttle or when lifting from the throttle, the principle of surge can be evaluated based on a turbo's compressor map vs engine flow rate mathematically if needed on paper before building a combination empirically. Compressor surge applies forces to the tips and edges of the blades which will flex and gradually weaken and break the tips of the wheel. It will gradually damage, deteriorate the brittle materials of aluminum, like bending a paper clip, causing the covalent bonds to break and forming microscopic cracks in the materials which lead to disintegration. It also slams the shaft in the CHRA causing eventual bearing wear and failure. Compressor surge is a serious problem and some modern turbochargers have started integrating bypass valves directly to their compressor housings. The bypass valve should be as close to the compressor as possible. Boost pressure is not only a differential it has a momentum flow, kinetic energy component while flowing through a tube to consider. From an engineering perspective there are a few reasons why it is ideal to bypass near the compressor, directly from the housing if possible. Never bypass after an intercooler as it unnecessarily loads the intercooler, air mass is lost which has been cooled and this raises the temperature of the intercooler with no benefit since the air is ejected and the cooled mass is lost. Always bypass before an intercooler. The closer the bypass is to the compressor the less mass between the bypass and source of kinetic energy (compressor wheel) which can more effectively reduce the pressure thanks to fewer volume (mass) In the pipe between source and exit. The bypass should immediately open to prevent compressor surge and stay open long enough as needed. Some bypass close too early and some open too late. The signal source to open the bypass should be a dedicated vacuum line, short as possible direct to the intake manifold source for ideal response. The same goes for signal to any electronic boost controller and map sensor, each is direct, short, dedicated line.

evaluating performance of bypass
A pressure test shall always be performed for all turbo vehicles. You must pressurize the entire system from compressor cover through to intake manifold. I made a video how to do this because it is difficult to visualize if you've never done it and many users would claim it was impossible to pressurize the intake manifold because of open valves on any engine.

During the pressure test you will evaluate the PCV valve and BYPASS valve for performance issues under pressure. The bypass should remain closed at max pressure otherwise there is some problem. The same with pcv valve, it must be tightly shut with only a minimal tiny bit of seepage if any.
During driving if you hear any compressor surge this is your other indication there is some issue with the bypass in other situations. There is no way to my knowledge, to test a bypass for part throttle condition response time easily other than to actually drive the car and listen carefully.


Bypass use with maf sensors
The maf sensor is extremely sensitive to perturbations. Some turbos are more prone to low speed surge, even at idle, which creates disturbances in the airflow column. Thus, the type of turbo will determine how important maf placement is, and how important having a bypass remain open steady at idle will be. Smaller, highly efficient turbos that move fast near idle and low speeds have more need for open bypass at idle, redirecting kinetic energy to the intake of compressor wheel and preventing it from surging during low speed off-boost operation. The engine may run rough, run extra rich, because of poor maf placement or improper bypass sensitivity setting. Compressor surge as we covered is air flowing backwards and if this perturbation reaches the maf sensor it will buffet the hotwire and cause issues with air/fuel control by interfering with maf output to the ECU. The maf sensor needs to be placed in a position where the airflow can be very smooth. No bends near the maf sensor are tolerated generally. When the bypass opens and releases air this air should not interfere with the maf sensor and the bypass should remain open long enough and open fast enough to prevent air from creating turbulence near the maf sensor, for example a late bypass opening or early bypass closure can interfere with maf signal as it disrupts the airflow moving through the maf. This is why generally all OEM maf equipped turbo vehicles will have recirculated bypasses that open easily and stay open easily as idle speeds and low speeds, to help with smooth flow and smooth transitions at low speed conditions which prevents issues with turbulence of the maf hotwire.
It is extremely important to recognize this issue when it occurs and fix it immediately. If the flow of the intake is interfering with maf operations causing the engine to run excessively rich for any reason- whether the turbo is surging, or the maf is being atmospherically bypassed, or the bypass is closing too soon or opening too late, the rich condition over time will increase wear and tear of the cylinder walls by constantly washing down the oil protective coating. You can identify this situation by watching the wideband sensor, if you see it suddenly dip into the 10's for example during a shift or at low speed throttle transitions, that is an indication there is an issue with maf placement or bypass operation which is dangerous to the health of the engine in the long term, it will gradually cause lost compression in my experience, it may take 10 years but it will generally reduce cranking compression by about 30psi over the course of 100k miles or so.

Bypass use for performance
A bypass may be used to conserve kinetic energy and improve turbo response (reduce turbo lag). To do this, route the bypass back into the intake to the compressor wheel as the OEM does on all turbo engines. Whether there is a maf or not has nothing to do with this. The principle is simple: the compressor wheel imparts kinetic energy to gas molecules, and by re-directing these gas molecules into the compressor wheel we can conserve that energy, as opposed to letting it go with an atmospheric bypass. It will alter the pressure ratio and improve compressor map outlook. This will enhance compressor wheel performance, as faster moving gas molecules coming in reduce the stress and counter torque applied to the wheel which will increase wheel velocity overall.
Here is a picture of what I am talking about




Summary
-Bypass setting and maf placement are 'tuned' features of the engine, just like tuning an ECU these must be tuned for the combination. There is no one size fits all solution, no best bypass brand or setting, no perfect maf location. You must watch wideband and listen for surge.
-All turbo engines must be pressure tested. Leaking air from plumbing causes high EGT, high EGP, high IAT, and may lead to engine failure due to high IAT and EGT. It can also overspeed turbo and explode the turbo into pieces.
-Careful Recirculation of bypass is a performance improving modification when done properly.
-Compressor surge and excessive rich conditions cause wear and tear on parts.
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Old 12-21-2022, 08:13 AM   #830
K3rm1tth3fr0g
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Location: Greater Seattle Area
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Link G4X flex tuned
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Quote:
Originally Posted by Surfingcricket View Post
I won't lie, I had definitely heard that compressor surge would kill your turbo from so many places, and it definitely confused me when I saw the episode of moneypit where Zach installed his turbo on the miata and was making all kinds of compressor surge noises, but he didn't seem concerned at ALL. This makes a lot of sense to me though, and makes me happy that I can tighten up my Perrin BPV some more when I get back from deployment!
Old post but while comp surge doesn't kill turbos it certainly isn't great for them and they comp wheels.
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