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So as part of the next batch of mods I'll be doing I plan to replace the factory BOV as, from my reading, it can't really cope when you start to increase the airflow.
I suppose the first question is.... Is this the case or will the factory BOV be fine up to ~12 PSI boost?

It seems the Turbosmart Kompact is a popular choice but I have a couple of questions....

I want to keep the plumb-back set up but the size of the pipes and/or connections is a bit confusing.
- The inlet elbow (MX5 Mania) I have has a 34mm opening for the BOV
- The pipework from the BOV to the Inlet duct is 20mm
- The opening on the inlet duct (AEM) is 20mm

The BOV is available with 20mm, 25mm and 34mm openings. (Factory is 20mm).

So which one do I get? The 20mm version or 34mm version?
Is it available with a 34mm inlet and 20mm outlet and would this be a problem for it's operation? (can't really tell from their website)
I know it will have to be reduced at some point and I have the bits to do that with but am I better off with a larger capacity BOV or smaller?
Does anyone know what size MX5 Mania use in their kit? (tried calling but no-one was answering)

Just to confuse matters further the Turbosmart website recommends the 25mm unit as the direct replacement, despite the factory unit being 20mm.....

The Mania Stage 1 kit I purchased secondhand was using the factory BOV so I'll just use that until I can sort this out.

Hey Nev,

My BOV started farting/failing as soon as I stuck on the AEM intake. That was on "stock" boost too!
I bought this: http://www.ebay.com.au/itm/New-Genuine-TURBOSMART-Kompact-20mm-Universal-Plumb-Back-Blow-Off-Valve-BOV-/150856269338?ssPageName=ADME:L:OC:AU:3160

i.e. 20mm. Fits up to all the stock pipes etc. I was also confused like you so busted out the ruler/string.

I would get the mania tube re welded to have a 20mm barb, so you can easily use the stock bov and all the stock sized recirc piping.
Or a 20mm aftermarket one of your choice.


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Thanks for fast replies, 20mm it is...... just ordered this one.

http://cgi.ebay.com.au/ws/eBayISAPI.dll ... 1220718646

emily_mx5 wrote:I would get the mania tube re welded to have a 20mm barb, so you can easily use the stock bov and all the stock sized recirc piping.
Or a 20mm aftermarket one of your choice.

No need to alter the Mania tube as it currently has a factory BOV fitted so I'll just reuse the adapters they had to fit the 20mm inlet to the 34mm opening......

Yup the Turbosmart can have a optional 34mm fitting on the inlet and 20mm on the outlet.

You should be able to order a 34mm (or 35mm - the custom FM Turbosmart Kompact Bypass valve has a 1 3/8"=35mm inlet and 3/4"=19mm outlet) screw-in barb separately from TurboSmart.

I originally had the GoFastBits BOV and ordered a 35mm barb to fit the FM TB elbow - hose reducers look "fugly".