3 or 4 bolt turbo mount

3 bolt vs 4 bolt. A long debated topic.
In a nut shell, here's the good and bad.

3-bolt style

Garrett 3-bolt turbine housing on a TA-49 turbo.


PTE 3-bolt turbine housing.


Factory located 3-bolt turbo with a 3" Terry Houston downpipe attached.


Factory style 3-bolt turbo with external wastegate mounted off the downpipe.



3-bolt good,
Direct bolt on replacement for all 86-87 Turbo Buicks and TTAs. Largest bolt on 3-bolt turbo available without having to change around a lot of stuff is a PT76 GT-Q. Uses the factory style downpipe flange for stock downpipe, Terry Houston style, or external wastegate mounted onto the downpipe style. Factory heat shield will fit over even the PT76 GT-Q.

3 -bolt bad,
Limited flow capabilities once you start looking to make over 975 fwhp. GT-Q is currently the largest turbine wheel option available. Back pressure can become an issue when trying to hit the elusive 975 fwhp mark. Wastegate hole will need to be ported if wanting to run high hp and high boost levels with a standard Internal wastegate style downpipe, in order to maintain boost control. To anyone that knows how to recognize turbos, the larger 70 series turbos would frighten most.



4-bolt style


PT88 Stage II setup with external wastegate and V-band style downpipe.


GT42 series turbo on Stage II with external wastegate and v-band style downpipe.

4-bolt good,
Tremendous flow capability when using the T4 Tangential style turbine housing. Significantly out flows the 3-bolt T3 style housing on 800+ hp applications. Less back pressure at high boost than the 3-bolt. Turbo selection and hp capabilities far exceed the currently available 3-bolt turbos. You can get a GT4788 with a T4 style turbine housing that will support close to 1300 fwhp.

4-bolt bad,
Requires the use of an external wastegate that will need to be mounted either onto the crossover pipe, or off the up-pipe from the header. Requires specific headers or at least a 3 to 4 bolt adapter flange. Will not work with internally wastegated downpipes. Requires a different style v-band clamped downpipe. If you go larger than a PT76 GT-S, you will need custom air intake piping as most turbos larger than the PT76 have 5" inlets. Higher than 4000 stall converters are required to properly spool up non ball bearing turbos larger than a GT4276, even no Stage II applications. Factory heat shield will not work and on larger Medium framed turbos, and will typically scare the poop out of most on lookers.

Hope this helps a little.

Patrick

Thanks 86brick.
Just trying to help.

Patrick

Technically, they should both seal just fine when used with a copper gasket and some copper high temp rtv. The T4 would have more clamping force due to the 4 bolts vs 3.
Just my O2s worth.

Patrick

Precision should have some left on the shelf.

PTE part number 074-1017, 3-bolt to T4 4-bolt adapter inlet flange. Last price I had was $53.99.

219-996-7832 ask for Pete.

HTH

PatricK