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T3 Divided vs T4 Divided - Worth it?

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Good evening gents,

      I'm currently running a 6boost t3 twin scroll manifold with dual gates on an RB20. I'm in the middle of building a stroked RB25 that I would like to make ~700whp(520wkw) with. Looking into turbo selections, I'm noticing most turbo's in that size range are either T3 open or T4 divided but rarely T3 divided. 

Is there a reason for this? Will I be limiting myself?  I'd love to keep this manifold as it was a pain fitting up/fabing it for twin gates with AC lines and I'd rather avoid doing it all over again.


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The biggest advantage of going t4 is not only the bigger range of turbos that use it but also the range of turbine housing sizes as well. Also things like the gt/x35 range and can also have a t4 turbine housing made to fit. 

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You can buy t3 divided housings up to A/R 1.25 for turbos up to EFR 9280, S372 SXE, GTX3584RS, GTW3884, and 6766, from AGP Turbo for usd 245-305 + shipping. Those turbos and housings should be enough for 700whp. If you decide to use t4 divided turbos, you can buy t3 divided to t4 divided adapter-spacer from customfabshop for usd 90 + shipping. This way you can reuse your t3 manifold, but will need mods to the piping as the turbo sits higher due to the adapter-spacer.

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On 22/08/2021 at 12:35 PM, hypergear said:

The turbine housing didn't make a great deal of differences. but the twin pulse manifold with twin gate did. 

That's because most manifolds are not true split pulse - I don't think I've ever seen a "proper" pulse convertor manifold ever used on an RB.  Every aftermarket twin scroll manifold I've seen for RBs are constant pressure (ie, designed to work with an open housing) but divided into two - which means they are effectively just operating as two single scroll turbos, but at least have the advantage of having no high pressure collisions and having a slightly smaller volume than a single 6>1 collector manifold would have... all which is good stuff, but not really stuff that is properly focussed on exploiting a divided turbine housing.

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On 8/23/2021 at 3:40 PM, Lithium said:

That's because most manifolds are not true split pulse - I don't think I've ever seen a "proper" pulse convertor manifold ever used on an RB.  Every aftermarket twin scroll manifold I've seen for RBs are constant pressure (ie, designed to work with an open housing) but divided into two - which means they are effectively just operating as two single scroll turbos, but at least have the advantage of having no high pressure collisions and having a slightly smaller volume than a single 6>1 collector manifold would have... all which is good stuff, but not really stuff that is properly focussed on exploiting a divided turbine housing.

can you elaborate on what a proper pulse converter manifold looks like?

my manifold is cyl 1,2,3 -> one scroll  4,5,6 -> other scroll. the first place the gas paths meet is just before they hit the turbine blade.

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concept being even pulses?

I can see two differing approaches for an I6 engine with regular cylinder pairing (1-6,2-5,3-4)

equal length runners into a split T04 collector with seperate wastegates (lets assume each runner in this example is equal length and same cross sectional area):

See the source image


vs a concept like this (shared flow path to collector):


See the source image


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Yeah, so the second one is what (I think) 6Boost first started out with (many many years ago), claiming correct spacing of the consecutive entries into the  runner meant that the pulse train was built up nicely and there was no need to increase the diameter of the runner as the additional cylinders' flows were added because the pulses were in train with each other, it wasn't like simply adding continuous flows to each other. Which is true, and the arguments made for it were all convincing - but this concept was dropped after a couple of years in favour of more conventional banana bunch manifolds like the first one. So while the concept seems to be right - it was dropped by the guy who was pushing it hardest, which might tell us something. I haven't done much thinking on the matter myself.

The first one is essentially "modern 6Boost". It's almost impossible to obtain proper equal length along with a good collector along with packaging against the engine and the car. So most people don't even try, or just assume that it is equal length because they can't measure any differently anyway. And I think the majority opinion since a long time ago is that the gain from a true equal length manifold, laid out as ideally as possible, against what is possible in reality, is small enough to just give it away and not think about it.

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Ok, I'm no engineer so I'm not going to claim my understanding is 100% correct but this is my general understanding on the difference between the two major approaches.

Constant pressure

This approach essentially is about smoothing out spikes and drops in pressure and making the gas flow through to the turbine run at a more linear rate.  Basically they need to have a nice even smooth collection of the pulses from all cylinders, and rely on a reasonable volume to ensure the pulses dissipate and even out.   

The idea here is instead of each pulse driving the turbine individually, instead the drive pressure gradually builds up and then keeps a constant amount of force driving the turbine.   This means that there is a natural transient lag when exhaust pulses get stronger as the drive to the turbine doesn't really increase until the whole manifold has increased in pressure to match the higher "load" the engine is under, but on the flipside it means that once the drive pressure has built up to a useful level the turbine is kept operating quite efficiently.   

They're at their relative happiest at full boost/higher rpm situations, they are at their saddest going from low rpm/low load to high load.

This is a textbook constant pressure manifold, x2.  3x even length runners of reasonable length and diameter feeding into a CNC collector with all runners facing the turbine at a similar angle.   The volume that each set of 3 cylinders feed into are significantly higher than the volume of the cylinders that are feeding the manifold, so when exhaust blow down is finished for a cylinder the runners are fairly well far from saturated so instead of all the energy from that pulse driving the turbine... much of it basically expands out in that side of the manifold, at least until the pressure has reached a point where the turbine is still effectively being driven by residual drive pressure (remember that there are gaps in the pulses when you have just the front or rear 3 feeding one side).    


Pulse Convertor

This is basically what twin scroll exhaust housings were designed to work with, and they swing the other way.  Basically the idea is the front and rear 3 cylinders from an I6 engine feed alternating scrolls on a common turbo.  The general idea is keep the volume down and the path from each cylinder to the turbine as efficient as possible, meaning the least messy path and the least opportunity to expand along the way as possible.   Each pulse should bring the pressure of it's side of the turbine up rapidly, then die down again - with the other side of the turbine getting drive by the following pulse on the other side of the engine.

Each side of the manifold should have fairly dramatic spikes in pressure and it will drop (relatively speaking) for when the next cylinder blow down event is due in that side of the manifold.  You don't run a collector, or even length runners - you make sure that each pulse is able to drive the turbine as effectively as possible.  If the manifold takes less time to pressurise, it also takes less time to empty - there will not be direct pulse collision as there is a gap in the blow down events between the sides of the manifold to allow it to clear a bit before the next cylinder gets a go.  

This is a more of a nice "split pulse" type manifold:

N54: M18 Performance N54 cast stainless steel single turbo manifold

The design isn't trying to set up for the pressure to build up evenly so much as trying to set up as there it's just a pair of exhaust energy sources alternately blasting at the turbine.  The idea is that the drive pressure builds up FAST which results in more spool from low rpm and generally a much faster reaction at the turbine to an increase in engine load.   

Basically turbine efficiency is MUCH higher than with a constant pressure turbo configuration (merge collector style) going from low/mid rpm or lower load to higher load, but then when the pulses become fast enough the constant pressure method becomes more efficient - and they start coming into their own when you start trying to really push the limits with a given size turbo, or generally turn high rpm and make max power.

Seeing what happens with cars which often run pulse collector manifolds stock or aftermarket, realistically if someone made them for RBs I'd suspect they'd be ideal for what a lot of the folks here would be normally talking about.   Anecdotally speaking I'd pick them as awesome in the responsive/torquey 400-750hp @ hubs area with mid sized turbos, but not what you'd use for roll/drag racing or very high power circuit/time attack cars.  

TL;DR: Basically all manifolds designed for RBs are a design which is optimal for steady state and max power, pulse convertor manifolds (stock single turbo and many aftermarket BMW manifolds) are optimal for quick spool and transient response.    It CLEARLY doesn't mean one type doesn't perform well where the other one performs best, just more pointing out what each are "optimal" at.

An amusing example of what I consider a "missing the point" of how pulse convertor manifolds work is people freaking out when B58 Supras came out with "two runner" manifolds, thinking it was way too small for a 3litre... not realising that those two pipes are just the last pit of a pulse convertor manifold pairing cast into the head, and they are not actually the kind of choke point it would seem because they're not operating in the same way.   If you've ever seen a dyno plot for a A90 Supra making solid power with a highflow stock turbo, you'd know that they are not laggy at all and they CAN make 700+whp/do 9s reliably with this configuration.  

Please don't confuse this kind of design with a generic log manifold which has just been made to be small and cheap to make.  There are some rubbish ones out there, which do NOT use the exhaust energy well at all and just result in restriction and sometimes even lag despite the small volume.   There are obviously draw backs, I'm just explaining because I was asked... but they 100% have their place imho.

I would argue that the T4 divided tubular manifold above is not a split pulse manifold in the sense that it combines the pulses to provide a steady drive pressure, as opposed to separating the pulses to the turbine.

Edited by Lithium
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Going back to the OPs thoughts , T3 Open vs T4 Divided - and why you don't see many divided T3 systems .

Firstly need to go back to when these flange sizes (footprints) were first made and why .

T4 started out as a truck diesel thing from memory back in the 1960s , open and divided . Garrett was producing turbos in different locations around the world and there a a few variations of what people like to call T4 . To keep it simple I'll say the big size is T4 , and what people like to call divided T3 was actually "T4 International" . These may have the same external dimensions and stud pattern as the generic T3s but the total distance across the divided ports is approx 10mm wider than the T3s single port . Anyone that bolted say a GT30 on an RB25 would have noticed that the turbine housing inlet isn't as wide as the exhaust manifold ports and it forms a step in the wrong direction . Truthfully the only thing T3 about an OE 20/25 turbo is that stud pattern and external flange dimensions .

Garrett T3s I think came about in the early 1970s , I believe it was because T4 was really too big and heavy externally for passenger cars especially turbines and their housings . I think N trim was the smallest of the original "T4" turbines and these are not exactly small light or responsive for smallish passenger car petrol engines . Anyway the T3 centre housing was the same as T4 as was the turbine shaft diameter , but the turbine wheels were smaller and lighter and the compressor wheels and their housings and backplates . T3 compressor wheels where also a more modern design than the agricultural T04B ones so basically the T3 was smaller lighter cheaper and more responsive than most T04B units .

Turbocharged production cars really took off in the early 1980s mainly from Japan and Europe . This is where your Nissan Z18s FJ20s 280ZX and eventually RB30ETs actually used small series T3 cartridges in Nissan supplied turbine housings , note these were the small T3 turbine series units not the larger T3 turbine series cores found in say Buick grand Nationals . These turbos and engines were not brilliant by todays standards but they opened eyes back in the day when the usual roads to decent power was more cubic inches .

Now to twin scroll systems . These were originally developed for big truck diesels to give them more torque at low revs and wider power range . It was a lot easier to do this with a diesel because you could size the turbine/housing to suit the engines power and power range without a wastegate . With a petrol engine in a road car the turbo needs to be sized so that it wakes up reasonably early and doesn't over boost or over speed in the upper half of the engines rev range , hence he waste gate . Also because mechanical/thermal/detonation limitations cars back in the day road cars didn't run very high boost pressures and generally had lowish compression ratios . As for the split pulsed manifolds and divided turbine housings go I think the gains are mainly from making our humble piston pump work better , or maybe I should lose less for having restrictions in the exhaust paths . Many of you won't remember the days when general purpose cars had pretty woeful OE exhaust manifolds , literally a hole for each exhaust port on one side and one out the bottom for a single engine pipe . Higher performance designs were basically split pulsed ones with two outlets to try and get some scavenging happening so hot spent exhaust gas didn't reverse back into the cylinders before the exhaust valves closed . It's not a very difficult concept to understand . Its obvious that gas under pressure will always travel towards the least path of resistance . so you have your power stroke and towards the bottom of the piston travel the exhaust valves begin to open . The combustion temp drove the cylinder pressure up so when the exhaust valves open there is a path to lower pressure beyond so out the exhaust gas flows . The theory is that when the cylinder blows down the velocity of the gas can actually leave a lower pressure in the exhausting cylinder than than down the exhaust tract . If there is a sufficient pressure rise in the exhaust tract then some of the exhaust gas WILL change direction and flow back into the cylinder - this is known as reversion . When you have pulse divided manifolds and divided turbine housings each half of that turbine housing is only seeing half as many exhaust pulses - so the time between pulses is double of what a single scroll system see's . That extra time before the next cylinder blows down gives the present one a better chance t get most of the hot stuff out before the exhaust valves fully close and seal .   

Now from the turbo/turbines perspective . To make lots of power (torque) you have to be able to move a lot of exhaust gas or the engine will choke (massive reversion) . So luckily for our twin scroll system we need to have reasonably larger turbine housing volutes because our exhaust pulses like to expand into a reasonable volume and vent allowing the pressure to drop ahead of the next pulse . I reckon it's the opposite for the T housing and turbine . I believe the turbine needs short high pressure/flow spikes to accelerate it to the speed where the turbo starts to pump usefully . So I guess in a nut shell twin scrolling done properly means exhaust manifold pressure is low when it needs to be low (for the engine) and high when it needs to be high (across the turbine blades) .

Single scroll means higher than optimal pressure across the exhaust ports and lower if more consistent across the single volutes nozzle and turbine blades . 

The down side of twin scroll systems is that they are a bit more complex and expensive to manufacture . The manifold has to group the exhaust ports in the correct order and is a bit more expensive to cast . Turbine housings are  bastard to make because the divider at the nozzle cops a thermal thrashing and there isn't a lot of material in this area to conduct the heat away or just wear it . So expensive difficult to machine materials are needed to make them reliable . 

And then you need a control mechanism which is usually a wastegate . This is also difficult to arrange if the sides of the system are kept isolated . It's generally easier in a production car to make the wastegate integral which is what Mitsubishi did with their Evos 4-10 . Still not easy to make a shared or paired flat valve seal long term and their seats to not crack or warp. External gates are probably to expensive to use unless you are Porsche or some other very expensive low volume exotic brand .

I should mention the parallel twins vs single twin scroll setup on things like RB26s here . Always remember that these were developed in the 1987-88 era with the technology and materials of 32 years ago . Two smallish twin turbos on short manifolds with separate waste gates for each bank of 3 cylinders , and fitted up close to the head so the body shell could drop over it on the production line . Maybe not ideal by todays standards but I can't imagine Nissan doing a twin scroll single with twin ext gates etc in such a low volume car . History has proved that GTR was a pretty successful concept and probably too good in some ways .

Lastly T3 divided (really T4 International) vs T4 divided turbine housings . Imo the issue here is that it's a big ask to merge 6 exhaust manifold runners into the the two ports of the divided "T3" sized turbine housing . With the larger T4 divided size ports you are less likely to get a pressure rise ahead of the volutes and dual turbine nozzles . The whole idea is to keep the energy/velocity in the exhaust pulse as high as possible to give it the chance to vent its maximum into the turbine blades .  This isn't as much of an issue when you have a single scroll turbine housing downstream of say an RB25s split exhaust manifold - particularly when its ceramic turbine is so so small as is its turbine nozzle . The thing no one eve talks about is that those RB20/25 turbine housings (as I mentioned earlier) are wider across their inlet ports that a real T3 housing is so would potentially have a bit less pressure rise at that point .

My 2c spent , cheers DP03 .     



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