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SteveN said:
No, i TOTALLY understand the theory behind it, if you look at it purely from books etc, they are vital.

But NOTHING beats practical testing.

And testing proved to me they not half as vital as they seem on paper.

Paper and reality are two different things, regardless of how complex the books are.

Like said from the start, id never reccomend it as never tried it on GTRs, but tried it on plenty other cars at far higher boost than most GTRs to know that personally id take the risk to see what it was like, but if there no reason to on your car, then dont bother.

From a performance side, many dumpvalves that vent a lot of air can often make throttle response worse rather than better...
Agreed Steve :) .

DV valves are better than vent to air normally so the recir on the GT-R are a better bet and make a lot more sense. Unfortunately there is now a habit to put BOVs are hybrid valves that dump some air to atmos and recir the rest onto cars for "effect".

As you said you have noticed no problems so far but the damage is usually long term and can show up in differing failures than expected (exhaust side problems as the two sides are connected).

The type of turbo and boost levels etc are the main factors in whether you can get away with. If the turbo blades are strong enough and the pressure difference between each side is not enough to break the oil film with the unequal spiralling side forces, the shaft bending/rebound (all materials are like rubber (quoting Royce) i.e. all bend under stress) during this time etc then it will "probably" do no harm at all to remove it.

Also depending on throttle plate opening/closing during shift times etc and how much the DVs leak and energy lost during recir travel etc it may make little difference to the performance or may even help to remove them (less weight also) in some cases.

My problem is I can't see it being a good idea for normal Joe Public to do so, maybe for a tuner on a car that no longer is anywhere close to the original spec it would be of benefit to try it out but it can be a costly exercise in the long run (to the next owner normally) but may benefit in the short term if you are trying to get good 1/4 times etc.
 

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IMO i dont think there an advantage recirc or vent to atmo with regard to performance. Both do nothing. As recircs vent back to the non pressure end theyd not do anything of use really anyhow, not enough air to spin up the turbo or anything, esp as its never angled to shoot direct into the turbo inlet.
Recirc/Atmo is purely a noise/ECU issue in my opinion.

Only failures ive EVER seen that can be associated with no DV are pure shaft failures.

Never seen blades damage barring foriegn object damage, or a dead bearing or shaft snapping causing the wheel to hit the housing.

GT series turbos seem to generally have thinner shafts than T series ones and ive seen shaft fails when running very high boost with no DV.

T series ive only ever seen it on T4 sized turbos, and even then only at 25psi+ sorta boost.

I ran a T34 with no DV at 30psi for 12k very hard miles and it was fine when taken off the car and checked, and thats totally common as most people with T34 cossies dont run DVs these days, as they never seem to do anything of use.
 

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What do you mean by vent to the non-pressure end Steve? I don't know the GT-R setup as I never looked.

The idea of a recir valve is to move the air to the other side of the compressor wheel to even the pressure as surge is caused by pressure difference i.e. the pressure on the throttle side is too great and stalls the blades while trying to get back to the entry side.

A small pressure difference will only slow the blades down and normally glently enough to not cause any problems so some turbo setups don't need a DV. With big pressure difference the recir valve is used to move some of the high pressure air to the other side to equalises it without going through the blades and therefore minimises/stops the stalling.

BOVs and recirs are a totally different ball games unless the GT-Rs are doing something a bit outside the norm?

BOVs were the early idea around 70/80s or so but had a very bad impact on effiency and therefore performance - pressuring air only to dump it to the atmos is a very bad thing while DVs (Diverter Valves) were the new way that avoided the surge and therefore kept the wheel spinning where prolonged throttle closed periods existed at high airflow.

I have a paragraph in an AE somewhere with figures for a test run showing pressure difference etc and graphs for stall points - I'll look it up if I can find it.
 

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Discussion Starter · #25 ·
I think I know where you are coming from SteveN, when the recirc valve does its bit it basically dumps back to atmosphere pressure but keeps the volume of air constant (ie air mass is the same as the AFM's think it is) which helps the ECU fuel correctly.
 

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Books vs real life.

Books are good. I read a ton of books. I read magazines. I read forums. I talk to people. I form my own opinion. I am not always right, and I can and have been proved wrong.

I know that BOV shouldn't leak, but they do. Too me, weighing the cost of BOV vs turbo life and turbo rebuilds, and potential for a failure point means that I don't want to run a BOV.

I have factory turbocharged cars that last more than 100,000 miles on a turbo without a BOV.

I have had turbos fail for all different reasons, I can't put my finger on one that was because I didn't have a BOV.
 

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When the car is pulling a vaccum on cruise, the re-circs are open, allowing air into the engine bypassing the turbos.

Is there an advantage to this that you have overlooked?

Think about it..
 

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carlreeve said:
I think I know where you are coming from SteveN, when the recirc valve does its bit it basically dumps back to atmosphere pressure but keeps the volume of air constant (ie air mass is the same as the AFM's think it is) which helps the ECU fuel correctly.
And by moving the air back to entry of the compressor wheel also stops surge and helps keep the wheel spinning and effiency of the process high.

That's the theory anyhow, if the valve is leaking then that's a different matter lol.

When I get home if I have time (doubtful) I'll scan some of stuff and post it. AEs pro books are well above any mag article to be honest and. Engine Fundamentals is just full of calculus from page 1 to page 1000 :bawling: and the preface recommends having a degree in mathmatics before reading it :mad: .
 

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tyndago said:
I have had turbos fail for all different reasons, I can't put my finger on one that was because I didn't have a BOV.
And in there lies the rub :chairshot - as I have said earlier in the post it is well reported in AE books that surge causes failures that do NOT point directly at it - shaft inbalance failure being one of the main ones, problems also appear at the exhaust side too etc.

Turbos should not fail - period - if they do it is because of something amiss or someone being a muppet. They are built to a high standard and are built to last for the life time of the engine they are fitted to - as long as they are maintained and treated as expected.
 

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I dont see, despite all these words, how recirc valves can help (ignoring AFM issues) anything at all over VTA ones.

A partially leaking DV, which is common and v.hard to spot, will overspeed the turbo and kill it far quicker than not running one.

As far as I can see, the only two people here who have actually had a lot of experience with this tested on a good few cars at decently high boost levels to see the effects of not running one is me and Tyndago (Sean), and funnily enough we both say they a waste of space.

Books say lots of things.
If books were always bob on tuning would be easy, all cars would be as good as the next one with the same money spent, and anyone who has read these books will be the best tuner ever.
How many of those statments are true in reality? None.
 

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SteveN said:
A partially leaking DV, which is common and v.hard to spot, will overspeed the turbo and kill it far quicker than not running one.
That's assuming it is faulty Steve, I'm talking about a working one. How does the leaking DV overspeed the turbo? The turbo speed is defined by the wastegate setting / exhaust gas pressure, the pressure on the compressor/throttle only limits this. The DV's only purpose is to stop surge.
 

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They leak often tho, thats our point, its been mentioned every time Sean posts. Most the time its a tiny amount that you could never detect with the naked eye/ear, but they still do.
So turbo overspeeds trying to make the same boost pressure as its leaking half of it, so loses efficency at least, overspeeds and dies sooner at worst.

The only plausible reason for recirc ones is to "re-use" the boosted air, which is total bollocks as it wont be enough air to be pressurised on the pre-turbo side, will go back to atmo pressure before anyone could ever re-open the throttle, so will be no different in regard to response etc.

And recirc valves will just be putting hotter than ambient inlet air into the turbo, losing efficiency!

Dont see how atmo ones can be any more harmful than recirc, and deffo seen no differences in the all important... reality.
 

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skyline69_uk said:
Ashaft inbalance failure being one of the main ones, problems also appear at the exhaust side too etc.

Turbos should not fail - period - if they do it is because of something amiss or someone being a muppet. T
Should not fail - I will give some reasons I have seen for failure.

1. Ceramic exhaust wheel on a stock GT-R. High boost or detonation causes the exhaust wheel to fall off.

2. Contamination of the oil. Oil contamination normally after engine failure. People don't realize when they spin a bearing, those little bits of metal get pumped all over by the oil pump

3. No air filter. I have run no air filter before which obviously causes some issues.

4. Overspeed on the turbo. Its very possible to overspeed the turbo due to a leak. We had a rear intercooler line pop off on the track. Shortly after repairing the line, the rear turbo failed.

5. Oil coking in the housing. This ones a possible. I had a 2530 that had killed itself. Inside the housing was some left over coking that I couldnt get off. One a 2.2 bar+ run - with nitrous the rear turbo failed. Its possible due to overspeed, possible due to other things. I was drag racing, and I'm a do or die kind of guy. This car still had the stock BOV. 137 mph on 2530's. It was making some power.
 

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All the above is indeed in the books. Also running the incorrect air filter can cause big problems too as if the pressure on the back of filter is too high it starts to strip the material off the back of into the turbo!

They note that oil contamination is the most common reason.

Also said that turbos fitted to diesels last a hell of a lot longer then petrol as they don't have a throttle to cause stall among other reasons (EGT being another main one).
 

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I'm an AE, not in engines but i think the basic principles still apply........

Something that manufacturers have to consider is longevity of the components, especially the ones which are of high cost to replace!

My understanding of any type of BOV or recirc valve was to reduce stalling of the turbine, which i guess would be experienced as a shock load to the turbine!
Over short periods this has minimal effect, but long term it's fatigue which kills parts - the continual exposure to subjective force.

Bare in mind that nissan's test patterns have to replicate worst case endurance over the 3 year warranty and so it will probably be something like follows:
arizona heat
high load
uphill
towing
and possibly the worst - digital throttle control (ie, on or off, no progression)

I can only imagine manufacturers apply them as a belt and braces approach to ensure against recall and retro-fit.........both of which cost companies millions in parts, labour and loss of repeat sales!

Ian
 
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