Miatapower List Archive

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Mail From: (email redacted)

Yes, I have Corky Bell's book, but I just can't seem to wrap my mind around these concepts...

Can someone please walk me (slowly) through the differences between blow off valves and wastegates (internal and external) and how they relate to our beloved BP engine and Link ECU?

The Garrett turbo that came with my FM2 kit has a flapper valve facing the downpipe that's linked by a rod to a vacuum diaphragm. I understood that to be a wastegate. Does the presence of that type of wastegate system make an external Tial-type wastegate unneccessary? Or would you need to have a whole new turbo without the flapper?

Since FM2 kits include a BOV (I upgraded to the aluminum one) but not a wastegate (external, at least) is there value to an external wastegate like the Tial 38mm ones I see all over eBay? (I notice that FM now offers the Tial--albeit, the obsolete 35mm model--in their online catalog but without any explanation as to how to install it.)

If one were to want to add the Tial-type to an FM2 that has the cast "log" exhaust manifold, how and where would you plumb it in? Into the downpipe? Or would you need a new tubular manifold that the Tial could have a flange attached to?

Thanks for the tutelage!

Bill

This read-only message was archived from a public mail list.
Mail From: Keith Tanner <(email redacted)>

Simple - a wastegate controls the boost level. A blow-off/bypass valve
vents excess pressure under certain conditions.

You're steaming along at full throttle and it's time to shift. You lift off
the throttle - and the throttle plate slams shut in front of 12 psi of
pressure. The intake air bounces off the closed throttle plate and heads
back through the intake system. It hits your turbocharger and stalls the
compressor. It's a little rough on the turbo and also slows the speed down
so it takes longer to rebuilt boost when you put your foot down again. The
blow-off/bypass valve prevents this by dumping that excess pressure. It's
triggered by a diaphragm that's hooked to the intake after the throttle, so
it won't crack open under boost.

The wastegate is very simple. It does crack open under pressure. Once the
turbo makes x psi of boost, it allows exhaust gas to bypass the turbo and
prevents the turbo from making yet more boost. The integral wastegates are
compact, easy to install but don't flow as well as the external ones.

You can convert the FM II to an external wastegate. That's basically what
the FM III was and the FM IV uses one as well. You would need a fitting on
the manifold to fit the wastegate - the FM II one can be modified for this -
and the outlet from the wastegate either has to be plumbed back into the
downpipe (awkward) or vented (really really loud).

Keith Tanner
Flyin' Miata
(email redacted)
flyinmiata.com
1-800-FLY-MX5s (orders)
1-970-242-3800 (tech)


-----Original Message-----
From: (email redacted) [mailto:(email redacted)]
Sent: Thursday, June 10, 2004 11:14 AM
To: (email redacted)
Subject: BOV and wastegate 101

Yes, I have Corky Bell's book, but I just can't seem to wrap my mind around
these concepts...

Can someone please walk me (slowly) through the differences between blow off
valves and wastegates (internal and external) and how they relate to our
beloved BP engine and Link ECU?

The Garrett turbo that came with my FM2 kit has a flapper valve facing the
downpipe that's linked by a rod to a vacuum diaphragm. I understood that to
be a wastegate. Does the presence of that type of wastegate system make an
external Tial-type wastegate unneccessary? Or would you need to have a whole
new turbo without the flapper?

Since FM2 kits include a BOV (I upgraded to the aluminum one) but not a
wastegate (external, at least) is there value to an external wastegate like
the Tial 38mm ones I see all over eBay? (I notice that FM now offers the
Tial--albeit, the obsolete 35mm model--in their online catalog but without
any explanation as to how to install it.)

If one were to want to add the Tial-type to an FM2 that has the cast "log"
exhaust manifold, how and where would you plumb it in? Into the downpipe? Or
would you need a new tubular manifold that the Tial could have a flange
attached to?

Thanks for the tutelage!

Bill