Boost Control Issues With Twinscroll Housing

[CEF11E]

Hey all,

As some may have read I have a HX35W installed on my rb30det. the turbo perfroms very well but boost control has become an issue and i need a bit of advice before i start pulling it off.

The rear housing is a 12cm^ twinscroll housing (approx 86A/R) it has an internal gate and it is bolted to the stock RB25 manifold also a split pulse design.

the problem seems to have started since i have retuned it and added manual boost control. in 1st and 2nd it holds boost fine. in 3rd, 4th and i am sure 5th boost creeps violently well above what it should it would just keep climbing well over 20psi.

The 2 options I have are. 1 port the internal gate, 2 go external gate.

If I port the internal gate I can take it out to 32mm but being a twinscroll housing it will only relieve boost pressure on 1 side. and this will also mean the rear 3 cyl will have far less back pressure than the front 3. is this a bad thing? I was thinking seeming as the rear 3 run hotter than the front it may not be such a bad thing. also will relieving pressure from 1 side of the twinscroll be enough to control boost? this would not be an easy mod nor cheap but would be my preference as it will look standard.

If I go external gate I have no other option but to put a gate on the standard manifold as space is a MAJOR issue. if i get a steam pipe elbow welded to the collector and then install a 35mm tial gate them merging back into the exhaust after the first bend about 600mm after the turbine would it perform well?

I am sure the external will work well but it will stick out like dogs balls and may cause heat issues. cost would be about the same

any advise from those who actually know would be greatly appreciated.

[discopotato03]

I've been wondering for some time when this issue of only venting one side of a twin scroll turbine housing would arise .

The problems going to be that you're attempting to "waste" or bypass the gas from only half of your cylinders so the turbine inlet pressure , back pressure if you like , will be higher in those not plumbed into a wastegate .

I'd also question the ability of half the manifold and half the turbine housing having the volume capacity to control boost pressure .

Imagine for a minute that you had a TT GTR engine but only one of the turbos waste gates was operating (opening at all) . It'd be a grim outlook for the engine I'd reckon .

There really is only one way to have an integral gated turbine housing , a single scroll one needs no explanation but the TS one needs to be able to vent or bypass from both sides . Both Mitsy and Subaru have used twin scroll twin integral gate turbine housings that have separate vent paths from each side of the housings passages . Sometimes they have two outlet holes and sometimes they simplify it to a mirror imaged "D" shape for the flat valve or valves to close on .

I don't know about Holset and Borg Warner but Garrett sometimes use twin scroll twin integral gate turbine housings on Diesel turbos , I have a T4 flanged GT3576 in the cave and it has a HUGE I think 1.19 A/R split T4 flanged GT40 based turbine housing on it .

For the laugh and convenience I call these housings TWIG - twin wastegate internal gig .

Garrett have done a model specific GT3076R (TWIG) for the Mitsy Evo 10 though people want to hold off on this one because it has - issues - Garrett needs to solve in this area . I hear the housing is a really good design but the - well - in time .

A twin scroll system won't work properly unless the volute or internal passages are TOTALLY isolated all the way to the twin nozzles and turbine blades . It's not good enough to have the twin scrolls joined by a common waste gate path if there is communication between them when the gate is shut . This is why the OEM twin integral gate housings use either two separate vent paths and close them with two flat disc shaped valves or one over the mirror imaged D shaped outlets .

The HKS single cast low mount manifolds for RB25 and RB26 do a similar kind of thing but with an external gate parked on top of the double D outlets .

Sorry to say it but if this was simple and cheap everyone'd be doing it .

Cheers A .

Edited July 7, 2009 by discopotato03

[CEF11E]

great reply. thanks Disco.

I did a bot more testing on the way to work and disconnected the actuator leaving the flap open.

I did a full throttle 4th gear pull and it hit 17psi and then tapered back down to 15. this has no real bearing to the gate normally because the turbine speed is not as high when the gate is open from idle as it never gets full cylender pressure pushing the turbine round.

I will add some photos i have found of a modified hx35 gate.

[CEF11E]

Here are the photos of the modded wastegate flap. it is using a 32mm iconnel flap and has been done with a die grinder. really nice job. the guy who did this also took material out of the twinscroll devider so gasses from both sides can go through the gate. so its no longer a twinscroll housing.

as you can see the stock gate is TINY! and the guy that did the mid did a really nice DIY job. I cant find the link to give the guy credit anymore but from memory it was on a volvo.