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Engine performance simulations 924 Turbo
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Mike9311  



Joined: 14 Dec 2004
Posts: 669
Location: Chicago-ish

PostPosted: Tue Mar 28, 2017 1:11 am    Post subject: Reply with quote

Carrera RSR wrote:
Mike9311 wrote:
Carrera RSR wrote:
Anyone done away with the recirc valve and no additional BOV?


Isn't that hard on the turbo as well as boost coming in later???


Nope and nope.


Listen to a semi truck going down a hill and engine braking while his turbo bearings take a beating....I respectfully disagree
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1980 931 ...off with the cobwebs
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1979 924 NA M471/M481 / ohne M650 Herausnehmbares Dach
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1982 931 parts car?
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WEASEL149  



Joined: 19 Aug 2005
Posts: 595
Location: UK, Sheffield

PostPosted: Tue Mar 28, 2017 2:18 am    Post subject: Reply with quote

Mike9311 wrote:
Listen to a semi truck going down a hill and engine braking while his turbo bearings take a beating....I respectfully disagree


My Father's truck did this for over a million km's on the same turbo and was still going strong last I heard of it. (The engine was a Cummins, the turbo a Holset).

There are many variables to consider but with the level of boost most on here run (less than 16psi most cases), off-load compressor surge isn't a big deal.

Full-load surge on the other hand is a different beast altogether.
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Mike9311  



Joined: 14 Dec 2004
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PostPosted: Tue Mar 28, 2017 3:21 am    Post subject: Reply with quote

I am pondering my first point that boost comes in faster with a BOV. I now read boost comes back up 'later' with a BOV than without, since the BOV is there 'to slow the wheel down'. However I am not making a decision yet on this and will continue to mull this over

I don't even mind trying a K26/8 on a build without a BOV. Cheaper....and I like to test things. However waiting on me to try will be too late for this discussion
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1980 931 ...off with the cobwebs
1982 931 Entwicklungsfahrzeug
1979 924 NA M471/M481 / ohne M650 Herausnehmbares Dach
0.5 1982 931 smashed parts car
1982 931 parts car?
1980 924 NA (R&D lightweight)
1982 931 WB wana-be GTR race car
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Fasteddie313  



Joined: 29 Sep 2013
Posts: 1556
Location: MI

PostPosted: Tue Mar 28, 2017 6:08 am    Post subject: Reply with quote

A bov, recirculating or not, is going to keep your turbo RPMs up while you're off throttle between shifts..

It lets it keep pumping air freely while the throttle is closed as opposed to trying to pump air into a dead space creating a pressure spike between the compressor and TB..

I like the old F1 method of putting the TB before the turbo so when you close the throttle the compressor just spins in a vacuum but then your entire intake must be vacuum hardened, all solid piping, so nothing gets sucked flat..
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Mike9311  



Joined: 14 Dec 2004
Posts: 669
Location: Chicago-ish

PostPosted: Tue Mar 28, 2017 6:25 am    Post subject: Reply with quote

Fasteddie313 wrote:
A bov, recirculating or not, is going to keep your turbo RPMs up while you're off throttle between shifts..

It lets it keep pumping air freely while the throttle is closed as opposed to trying to pump air into a dead space creating a pressure spike between the compressor and TB..

I like the old F1 method of putting the TB before the turbo so when you close the throttle the compressor just spins in a vacuum but then your entire intake must be vacuum hardened, all solid piping, so nothing gets sucked flat..


Thank you, exactly my point
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1980 931 ...off with the cobwebs
1982 931 Entwicklungsfahrzeug
1979 924 NA M471/M481 / ohne M650 Herausnehmbares Dach
0.5 1982 931 smashed parts car
1982 931 parts car?
1980 924 NA (R&D lightweight)
1982 931 WB wana-be GTR race car
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CÚdric  



Joined: 27 Aug 2004
Posts: 1744
Location: Sweden

PostPosted: Tue Mar 28, 2017 7:13 am    Post subject: Reply with quote

Here we go..

Turbo wise, recirkulation surge is not as bad as you might think when it comes to turbo longetivity. If you drive the latest 13l 500hp Scania (probably the best truck engine you can buy at this moment, not only because im partly responsible of the development ) it can surge during gear changing under extreme loads and very low rpms( sounds like "woff"). Due to the very swift gear changes which requires very quick torque cut off.

I cant of course drop a number in public like this, but a GT45 can take ALOT of stalls/surges or wathever you call them. It doesnt do as much harm as you might think. But of course a throttle closing will be even more abrupt and induce the well known pressure flutter, and passenger cars have weaker turbo bearing set ups, so things can go really bad if you are unlucky. But your boost will weep out regardless, more or less, so I cant see the point of not having a BOV, The drawbacks are very few for cars like ours.

/Senior engineer, heavy duty gas exchange and turbo systems
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WEASEL149  



Joined: 19 Aug 2005
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Location: UK, Sheffield

PostPosted: Tue Mar 28, 2017 9:45 am    Post subject: Reply with quote

Don't believe the hype guys - there's so much internet BS surrounding BOV's it's unbelievable, and listen to CÚdric as he knows what he's talking about.

I like built-in CBV's like the K26/931 and BW solutions. Otherwise IMHO a reliable and tuneable BOV that has plenty of preload to keep it closed at low boost and open only enough to relieve SOME pressure at high boost is the answer.

I've decided not to run one. Why? ... because it's one less failure point and thing to buy. A leaking blow-off is more likely to destroy a turbo due to overspeed than the off-load surge caused by not running one.
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CÚdric  



Joined: 27 Aug 2004
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Location: Sweden

PostPosted: Tue Mar 28, 2017 7:07 pm    Post subject: Reply with quote

WEASEL149 wrote:
Don't believe the hype guys - there's so much internet BS surrounding BOV's it's unbelievable, and listen to CÚdric as he knows what he's talking about.

I like built-in CBV's like the K26/931 and BW solutions. Otherwise IMHO a reliable and tuneable BOV that has plenty of preload to keep it closed at low boost and open only enough to relieve SOME pressure at high boost is the answer.

I've decided not to run one. Why? ... because it's one less failure point and thing to buy. A leaking blow-off is more likely to destroy a turbo due to overspeed than the off-load surge caused by not running one.


On many "tuned" cars overspeeding the turbo is often not so much focus on, which can end up really bad. Leaks can escalate it very quickly for sure.

I agree, the internal BOV is my preferred solution aswell.

You have a holset turbo(if i remember correctly), they have very sturdy bearing systems, it should be able to cope with some abuse, only time will tell if its enough
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Mike9311  



Joined: 14 Dec 2004
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PostPosted: Tue Mar 28, 2017 11:30 pm    Post subject: Reply with quote

CÚdric wrote:
Here we go..



You had me laughing at this one
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1980 931 ...off with the cobwebs
1982 931 Entwicklungsfahrzeug
1979 924 NA M471/M481 / ohne M650 Herausnehmbares Dach
0.5 1982 931 smashed parts car
1982 931 parts car?
1980 924 NA (R&D lightweight)
1982 931 WB wana-be GTR race car
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WEASEL149  



Joined: 19 Aug 2005
Posts: 595
Location: UK, Sheffield

PostPosted: Thu Mar 30, 2017 1:47 am    Post subject: Reply with quote

CÚdric wrote:
Thats pretty easy to check.. Do you have some more detailed specification on that HX35? I happen to have access to hundreds of holset maps at work.

Would be interesting to see response, pumping work and combustion residuals with twinscroll. Probably much better..

But prio1 is to work more on the model itself.


The HX35 I have fitted to the car has an 8 blade compressor with 54mm inducer, 83mm exducer, 4.9mm tip height (42 trim).
The turbine is 70mm inducer, 60mm exducer with 10.5mm tip.
Turbine housing is 12cm twinscroll.
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