Turbo Flutter and Blow Off Valves

[JimmyXR6T04]

I'm not sure if the turbosmart website would be totally accurate then?? I dunno, I too read that and I was concerned about my car with the extra boost, but all the advice I've been given from tuners, and every where else seems to suggest that an after market blow off valve isn't really required. So I don't think I'll get one, although I would like to fit one just to see what sort of sound it does make... I might like it after all.. don't wanna fork out for it, and then not like it!!! the standard noise it makes now is pretty tough though

[turbotom]

Stock blow off valve works well, but if you're doing a lot of WOT and lift off (eg tight circuit racing) and running high boost (~15psi), then a larger blow off might be advantagous to control boost spikes in the inlet tract.

On the other side of the turbo, the waste gate manages the huge pressure variations in the exhaust when you back off.

Running high boost pressures causes big pressure spikes in the exhaust manifold too. Hence one of the reasons for the need for stronger valves and/or better free flowing exhaust in higher boost situations, and the need for waste gate mods when chasing power well over the 300 rwkw mark....

It's these little things that not so much give big increases in power, but provide better reliability when pushing the car.

After all, what's the point of having all that dyno chart power if you are too worried about using it.

just my 2.2 c worth.

tom

[Ralph Wiggum]

read this article on the Nizpro Forum Tech Section....this should answer any questions that you have....maybe well clould ask Simon nicely if we could sticky it to try and save some many BOV questions being asked and reasked..... :banghead:

http://forums.nizpro.com.au/phpbb/viewtopic.php?t=764

[81CJH]

read this article on the Nizpro Forum Tech Section....this should answer any questions that you have....maybe well clould ask Simon nicely if we could sticky it to try and save some many BOV questions being asked and reasked..... :banghead:

http://forums.nizpro.com.au/phpbb/viewtopic.php?t=764

←

Umm, yeah that link might work great if you're a nizpro member, not so well for those who aren't.

As XXXRT said, the best way to provoke this noise is by building max boost, then lifting off suddenly. You can hear it better at low engine revs, so try WOT from about 2000RPM in 2nd, then lift off about 3000RPM, or basically as soon as you can hear that the turbo's spooled up.

I'm not particularly worried about the noise, since just about everyone says it happens, and there's no way Ford would be getting it wrong and not addressing it year after year (and model after model) if it was an issue.

[Ralph Wiggum]

Okay my Bad. I've asked Simon about posting the article and im waiting for a reply....The Nizpro forum is worth joining there heaps of good unbiased info on there....

[Adam]

Okay my Bad. I've asked Simon about posting the article and im waiting for a reply.

He said you could post it.

The Nizpro forum is worth joining there heaps of good unbiased info on there....

[Ralph Wiggum]

Posted: Thu Sep 29, 2005 5:15 pm Post subject: Blow Off Valves.. What are they really For ?

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Blow off valves what do they really do?

This is fairly simply if you have a good under standing of what goes on with the engine and how most engine management systems work. It still amazes me how many professionals don’t quite understand, and wont to add them to everything they get their hands on. The bigger and shiner they are the better.

Now there is no hard or fast rule that says you must or must not have one. It greatly depends on your application. There are also many reasons how and what purpose you use them for, so I’ll start with why they were invented in the first place and give some examples of why they were fitted to some vehicles and not others. These examples and the reasons also back up the truth of what they were designed to do.

Firstly they were never designed to keep turbo charger from blowing up on trailing throttle. Example 1: Lets look at the first of the modern mass-produced turbo charged cars that we will all be familiar with, you guess it the VL Commodore turbo. They were also known for having a very reliable turbocharger and yet not a BOV in sight, even with increased boost that 99% of them had the turbo charger gave great life often in the 200,000 k mark.

So why weren't’t they fitted with a BOV? This is simply, it was back in 1986-88 and the emission laws were relatively simple. Hey, I hear you say whats the EPA got to do with it? Well actually every thing. Lets have a look at what happens when the car is driving down the road changing gears accelerating and slowing. Lets also look at how the ECU is determining how much fuel is required with all this going on. The ECU is getting two main signals, one RPM from the crank angle sensor and the other, airflow ,from the air flow meter {AFM}. The afm measure the air entering the engine, it dose this by a heated wirer element like your average light bulb, inside a know diameter hole call it 55 mm. Now the ecu heats this wirer up to lets say 500 deg c with a voltage, normally 0 to 5 volts. The air entering the engine goes pasted the wirer and cools it down, so the ecu puts more voltage into it to restore the 500 deg c. It can now determine how much air is entering the engine by how many volts are required.

So now lets look at just one example of what happen when you decelerate. Your driving down the road accelerating with 5 psi of positive pressure in 3rd gear with an open throttle and are about to change into 4th , as you shut the throttle to push the clutch in and shift gears the incoming air now has no where to go it banks up against the throttle and the pressure risers, at the same time there is no air going into the engine any more so there is very little exiting the exhaust to drive the turbine wheel of the turbo charger. The turbocharger starts to slow, the air that is banked up at the throttle is pressurized and only has one way to go, back out the front of the turbo. Stop Again you say , this is what destroys the turbo. Wait I’ll get to that. Once the air goes past the turbo it then travels up the pipe to the air cleaner on its way passing the afm and at that time the afm measure the air {AGAIN} remember it’s already measured this air on the way in and the afm is not smart enough to know which way the air is traveling so it then tells the ecu to add fuel to the air that’s going in the wrong direction. A-HA VERY MIXTURE ON GEAR CHANGE. EPA says get stuffed go away.

So as emission regulation tightened up car manufactures came up with a devise called the recirculation valve AKA the blow off valve. As these valves are placed between throttle and the AFM with the turbo in the middle They recirculate the pressurized air that we just spoke about on the gear change so the afm dose not see it. No more black smoke.

Now as the EPA laws got tighter engines became smaller [GTR, EVO etc] but more powerful, intercooler became std and turbocharger compressor wheels got bigger this problem became worse as there was more air being banked up, which meant more air traveled the wrong way and was measured. So recirculation valve were fitted to all of them. And that the truth the whole truth and nothing but the truth, and it sorta makes sense don’t ya think.

Then came the aftermarket they made really big shinny ones, didn't’t quite understand what they were for, thought it was to save the turbo from spinning backwards, they normally got you to plumb them up so all the air just blew out to atmosphere again this was measured air so the ecu added fuel although the engine never saw any of it, and it then ran really rich on gear changes but they did make a dunny flush noise that seems to have attracted a cult following. Funny thing is that Indy cars and world rally cars don’t have them. May the EPA isn't on to them yet. They also don’t blow up turbo’s very often. There goes that theory.

So you asked about the BA XR6T. Guess what it hasn't’t got an AFM. It dose its calculation via the manifold pressure sensor [map] and its mounted after the throttle so it never sees the pressure rise on gear changes. So why do they have a BOV STD? Makes you wonder, may be the after market guys are not the only ones that haven’t quite got a handle on it. The other reason would be they properly didn't’t like the dunny flush sound. That my guess.

_________________

Proprietor

Nizpro Turbocharging

Bayswater Victoria.

(03) - 9761 1522

[Ralph Wiggum]

So now we come to compressor surge.

This really isn't the noise you here on decell with no BOV in fact that noise is[THE GOBBLE GOBBLE] the compressed air going passed the compressor wheel of the Turbocharger in the wrong,direction. Dose it hurt the turbo. Not in my experience.Compressor surge is where the turbo is making to much boost pressure for the amount of volume its supplying. In other words the turbo is to big for the engine at the surge point.What can happen at this point is that the turbo tries to to compress the air to a higher level than the compressor can make so it slips or surges. Think of it like when your Tyre's break traction on a wet road the engine speed increases this is effectively what happens to the turbo and at this point the turbo over speeds and can often fail. BOV have absolutely no effect at this point as they are close when all this is happening. So what about throttle res ponce on gear changes. Again the only thing that drives the turbo is the exhaust gas and once the throttle is close for a manual gear change the turbo is slowing very quickly. If the gear change is fast the BOV hasn't got time to open as the inlet manifold needs to go into a state of vacuum to operate the BOV and this is unlikely on a snappy change. Plus on a snappy change as the throttle is closed the boost as we have previously discussed actually increases momentarily and this is then immediately ready to enter back into the engine the moment the throttle is re opened.

So if the gear change is slow and the manifold goes into vacuum and the BOV opens the boost pressure is then vented to atmosphere or if your lucky back to just in front of the turbo, so if your cars fitted with an AFM it doesn't run ridiculously rich for a split second, you have now dumped all the boost pressure in the intercooler and plumbing, you stand on the throttle at this point your car is back in a N/A state so you can now wait for the boost to build up and away you go again. Doesn't sound all that fast when it explained like that. Which is why WRC rally cars don't have them and went to boost enhancement instead.

OK now that you understand what compressor surge is I can now go back to my first comments about the gobble gobble noise not being surge. Well actually it is, But the reason it doesn't damage the turbo is that in that circumstance the throttle is closed so there is no exhaust gas to drive the turbine so it cant over speed. I'll take you back to the car doing a skid. Think of the car up on a trolley jack in gear doing 100 kph steady state throttle. Now lower the jack, yes the car will skid the Tyre's as it hits the floor but with very little accelerate it wont cause the engine to over rev.I hope I've explained it in a way that makes sense

_________________

Proprietor

Nizpro Turbocharging

Bayswater Victoria.

(03) - 9761 1522

[Ralph Wiggum]

So why do you have one?

As in the first post states emissions is the main reason and a must if the ECU is controlled by AFM.Secondly noise as far as a car manufacturer is concerned to get rid of the noise not make it. A well designed factory plumb back valve is almost silent.Thirdly there is some thought that a small increase in fuel economy is gained, as the BOV is opened while in cruise driving as the manifold vacuum is holding it opened the turbo doesn't have to push itself against the small amount of positive pressure it could be making, thus allow the turbine wheel to spin more freely and this then lowers back pressure.

Its a Long read but well worth it...Thanks to Simon from Nizpro for the info and letting me post it here. Adam sticky it if you think its worthwhile

[81CJH]

Thanks Hiddeous, Simon, Adam.

Most informative.