Divided or non-divided turbo?

As above really. I'm looking into which turbo spec I want to buy and I was wondering what the advantages/disadvantages are of each?

I have the boostlogic T4 manifold which, as far as I am aware, is divided. I think I am correct in saying that means I can go with either style with regards to the turbo?

The different setups come with different ARs but they are close enough for it not to matter in that respect.

Also, am I right in saying that the larger the A/R the more efficient the turbo becomes in the upper end with a reduction in spool? Any ideas how I can work out what A/R would suit me best for a given turbo?

At the moment it's the Billet 6765DBB with ported H cover that I have in my mind. I'm thinking around the 0.80 A/R but not sure if that's overkill or not enough for 700hp. I want to run around 1.7bar max so I'm thinking the A/R will play a huge part in what sort of power I get for that pressure.

Thanks

You can run either Scott as you say. Theory says the divided will spool faster but I've not seen an identical back to back test prove that yet, there have always been other changes that muddy the waters.

If you really want fast spool then you need to step away from the tubular manifold. However if youwant 700hp+ with "as good as you're gonna get" in terms of spool with the stock rev limit then you should probably go for an undivided manifold with a quick spool valve from SP and a divided turbo housing. Next best after that would be one of Mr Whiffins fully divided setups.

Precision turbos are not designed from the outset to be used with divided housings. Apparently the design of their turbines (in general) is relatively insensitive to divided vs undivided. A BW turbo on the otherhand will generally perform better with divided - or as it was put to me.....perform worse with an undivided housing compared to divided. There were a few threads on SF discussing this in detail but I can't find a link now.

I bought two housings for my turbo with a view to experimenting to find which one worked best. IMO, doing it any other way is a bit off the ideal as no two setups are truly identical so you're always going off the back of someone else's opinion and similar(?) setup.

To give that some context - My 0.81 undivided housing on the T67 (allegedly relatively slow spooling) spools faster than anything else I have seen at SRR except for Wez's little PHR61mm tubby. The wall of torque I get is phenomenal.... 590lb.ft @ approx 4600rpm (with over 500lb.ft from 4200-6200rpm) is not to be sniffed at. Most 67mm tubbys are down on that. I'm not looking for an out and out comparison here but I am saying you really need to pick the whole package to work together or you'll lose out. Cams, headwork, turbo design and A/R all link together and you need to get it right to reap the benefits of each component as a whole. I spent ages pondering mine and bought and sold two sets of cams and two turbos plus a full PHR tubular stage one kit before I ended up with what I have now.

Are there any items that are 100% fixed in your spec? You need to nail something down and then start matching the rest of the spec to suit IMO.

The 6765 will have to use a ported compressor intake and if you are using stock cams you may still see a little surge if you go for a 0.81 exhaust housing. If you are using (pretty much) any aftermarket cams then the 0.81 should be fine as it won't shift enough air low down to run into surge issues. The 6765 seems to spool fast anyway so I wouldn't get too hung up over slow spooling...but that depends how you drive too. There seems to be an even split on SF of people running the 0.81 and 0.96 - both undivided.

Hope this helps mate, sorry if I added more questions than answers!

Not at all Dan, very much appreciate the input/discussion.

I've nailed down quite a few parts so I'll try to fill you in.....

I bought JamieP's full turbo kit consisting of:

Boostlogic T4 Manifold

Boostlogic 4" downpipe/midpipe

Tial 44mm Wastegate

900cc Injectors

Twin Feed FIC Fuel rail

GSC 269 Cams

As well as the above I also have...

Brian Crower Springs & retainers

Skimmed Head with Valve Stem Seals refreshed

Full Gasket Kit

Plans are 7250rpm limit and 700hp map with 'low boost' 600hp setting for day to day driving. 700hp being retained for 'special' occasions.

As Jamie has had this entire setup on his car running with a 6765 turbo I want to go down a similar route. The 6765DBB certainly ticks all the boxes for me, other than longevity but I won't be using the car/turbo to anywhere near the extent Jamie did.

Needs are that the Hcover is ported, I want to be hitting the figures with relative ease and I wan't reasonable spool. As I say, at the moment the precision billet certainly ticks all the boxes so was just wanting to see what variant to go for regarding A/R and Divided/Undivided

That manifold has a non divided spacer welded on it as Jamie was running undivided, if you decide on divided you will have to get it modified.

That manifold has a non divided spacer welded on it as Jamie was running undivided, if you decide on divided you will have to get it modified.

Yeah Jamie said to me in PM. I think it will be more hassle than it's worth as I would have to machine off the spacer and weld on a divided spacer to get the Hcover divided 6765 on.

Looks like non-divided it is

Cheers