Bov Mod

[ENVY-T]

Read this. Must be a load according to you.

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 Late model camira 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

Edited 01/03/07 04:04 AM by ENVY-T

[Ninka]

OK I will try to answer your questions

1 - Simon obviously has his opinions on the BOV, but it is contrary to what the manufacturer recommends.

Not having a BOV will result in the turbo slowing down more than it would, had it been allowed to continue to flow air.

2 - Response is better with the factory BOV as it opens instantly, and remains open when no boost is present, and this allows the turbo to spool up faster. If you fit an after market BOV with a spring loaded piston, the only benefit will be pressure relief when the throttle is closed rapidly, and the pressure exceeds the pre-load on the spring.

I have both the factory BOV and an adjustable V-Port BOV, with the factory unit keeping the car happy at low or no boost, and the V-Port venting when high boost is used.

The setup works extremely well, and keeps the car legal while on low boost, not attracting undue attention for the law.

I have no more to add to this, and am not interested in a slanging match with you, so you can take the information and do with it as you please, it is just my experience.

Edited 01/03/07 04:07 AM by Ninka

[ENVY-T]

As I said a number of posts back, different people, different opinions. I never said you didn't need one or did. You said that it was a necessity. I was just pointing out that there is more colour in this little world than Black and white.

I don't want a slanging match either, cos I respect the fella that has worked on your car, lets save the sliinging for DBIATCH.

[Ninka]

As I said a number of posts back, different people, different opinions. I never said you didn't need one or did. You said that it was a necessity. I was just pointing out that there is more colour in this little world than Black and white. 

I don't want a slanging match either, cos I respect the fella that has worked on your car, lets save the sliinging for DBIATCH.  

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OK ENVY, agreed

[RAP1D]

Do you really think Ford would have fitted a BOV if they could have saved the money

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Well my wifes V40T4 runs 14.7 psi from the factory, and it aint fitted with a BOV and has done 130,000kms. Nizpro also don't fit BOV's, so whats the go then? Emissions? Throttle response?

And it has been running 12 psi for the last 9 months. With factory plumbing, I don't think I would be getting the times I have with only 10psi. But I am only a fool according to you, so what would I know?

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Yeah kind of... Volvo ( V40 T4, V70,T5's, R's ect ) use a recirculating valve that is built into the turbo's compressor housing, it works pretty much the same as a bov... ive seen a few Nizpro cars with bovs, but yeah its no major drama to run without a bov, I did it for years on various cars and never had issues.

I ran a big garret T4 on a ED 4lt for 18months, no bov, 17psi and the turbo was fine.

I get the same noises from mine if I pull off the vac hose so it wont open. Its as loud as hell, louder than whats been discussed here which is why I run it plumbed back, to shut it up.

This is mine plumbed back and as quiet as I can make it...

noises

[ENVY-T]

As I said a number of posts back, different people, different opinions. I never said you didn't need one or did. You said that it was a necessity. I was just pointing out that there is more colour in this little world than Black and white. 

I don't want a slanging match either, cos I respect the fella that has worked on your car, lets save the sliinging for DBIATCH.  

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OK ENVY, agreed

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Good cos I like you,even if you are a sandgroper!

I need a beer!

[Ninka]

The point I was trying to make is this:

A turbocharger is a centrifugal air pump, and ANY centrifugal pump whether it pumps fluid or air will be exposed to shock loadings if the inlet is closed or restricted, and also if the outlet is closed or restricted, though to a lesser degree.

Given that we are talking about a compressible medium (air) here, there is more flexibility, and the application is more forgiving than with fluid, however, there is no doubt that from a mechanical engineering point of view, it is better to avoid having these shock loads in the first place.

Now you may run 999 turbos where this has no impact, but just maybe on car 1000 something will fail as a result, thus the recommendation to have a BOV in the first place.

Another statement from Garret regarding BOV's: "Blow-Off Valves will improve throttle (time to boost) response as well as help relieve the damaging effects of compressor "surge loading".

I just assume that the manufacturer who makes thousands of these things, will have a fair idea

[JimmyXR6T04]

Ninka,

I'm assuming that what I've done to mine is still safe?? as it's still using the BOV but simply venting atmo?? Or is there more to it then that??

[RAP1D]

Something else to think about is the stock bov is fine for stock boost... run more psi and it vents less, so your almost back to having no bov... half a bov maybe lol

[JimmyXR6T04]

Something else to think about is the stock bov is fine for stock boost... run more psi and it vents less, so your almost back to having no bov... half a bov maybe lol

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bugger

but whether its set up to be plumb back, or as mine is, it's still doing the same job isn't it?? or is it more efficient in one form or another?