Blow-off valves

[WideOpenThrottle]

How critcal is it to have a blow-off valve on a turbo as isnt this the job of the waste-gate?
If i put one on whats the brand i should go for & whats the likly cost? & where's a good place to get one?
1600cc/150 to 200 hp,no intercooler for now[get the picture]-it will happen but not overnight!

[Habanero666]

Nah not necessary, unless you are over 15psi boost.
Anything more than a 30mm BOV is overkill for a 16-18 hundert cc motuer.
PEACE

[SLYDIT]

i just threw on a standard bosch valve from a legacy. cheap and will do the job. see my cardomain page in the sig below

[12a MX5]

If a standard turbo motor comes out with one on (which most factory cars run well under 15psi), then i think it would be a good idea to have one. They help with reducing turbo lag/back spin the buy releasing any pressure trapd between the turbo and throttle body when the throttle is closed (as in shifting between gears). Should be mounted as close to the throttle body as possible.
If you dont want the pssssh noise they can be ventilated back into the intake system. Any Brand will do the job as theres not much to them to go wrong but if you can get a good branded one (hks, Blitz.Greedy) for cheap then Id go for them.
Trademe would be a great place to start looking for one.
Heres a few I found for you .

Listing #: 284533066 .
Listing #: 284563295
Listing #: 285028765
Listing #: 285616684
Listing #: 285840379

Hope this helps.

[Habanero666]

All you need to know, see link below.

http://en.wikipedia.org/wiki/Blowoff_valve

Although, personaly I think the author has misinterpreted the definition of "compressor surge".
It is infact, when the turbo sizing is to large for a given engine capacity, causing the engine to be "over stuffed" with boosted air at a given RPM which results in the compressor impellor "chopping" a compressed air charge with no room to go but backwards.

The reason I don't suggest a bov below 15psi ( although FACTORY turbo cars have them) is in my experience of AFTERMARKET turboed MX5's, the total volume of air below 15psi within the plumbing and intercooler, simply does not carry enough pressure/reverse expansion to cause a detrimental effect on the air leaving the compressor housing at the split second the wastegate slows the turbine wheel and the throttle vale is fully shut.

When a pressure change occurs in the inlet manifold the wastegate signal is changing, decreasing the pressure holding the diaphragm spring back and allowing boost to be reduced.

Car manufacturers use a bov below 15psi because the system is so closely sized ( and economised for mass production) that the system requires one to maintain emission standards and catylitic convertor longevity. But that is a whole other story.......

[WideOpenThrottle]

Once my set-up is up & running may-be then i'll revisit fitting one! baby steps for now-thanks for the info to date!

[Timmo]

The wastegate is a boost limiting/bypass valve so that the exhaust wheel doesn't overspeed (i.e. it diverts exhaust gas)
A blow off valve is a one-way valve to ensure compressed air is all flowing one way (into the engine) and so doesn't cause issues with air flowing other way (shagged compressor wheels and bearings, lag)

And yep, agree about the confusion about compressor surge- Habanaero66 is correct.