MRN Twin turbo 987
In the last blog post we covered a few options for forced induction and explained our reasons for choosing a twin turbo setup so now we will go into a little more detail on what is involved in pulling a complete system together while trying not to give away too many secrets!! We will assume you know the basics of how a turbo works to improve performance, if you don’t, have a little search on the internet, there is lots of information available.
Spinning the turbo – the hot side
So we need to spin up the turbo with the exhaust gasses, typically a new exhaust manifold would be made to channel the exhaust gasses to the turbo. We explored this in depth because if you can mount the turbos in the same space as the OEM headers we could retain the standard rear exhaust system or the aftermarket one you have spent your hard earned ££ on. Unfortunately the packaging is very tight and although it is possible to fit, there would have to be big compromises made to the air inlet (cold side) of the plumbing so this was ruled out in favour of a better overall packaging solution which is retaining the OEM headers (or your aftermarket ones) and discarding the rear system. We have packaged the exhaust in such a way that the two side of the engine have equal lengths all the way to the exhaust tips.
Creating boost – the cold side
There were a lot of challenges to this, this part of the system encompasses everything from the air filter, MAF sensor (Mass Air Flow), turbo(s), chargecooler, BOV (Blow Off Valve), air inlet plenum. Our target is to retain as much of the OEM car as possible without compromise and to ensure an OEM feel to the kit so as little cutting/splicing/tapping as possible. With that in mind the air filter to turbo inlet via the MAF was the most troublesome to get right, this involves a complete change of direction to get the air to the turbos mounted at the rear of the car next to the gearbox. To get around this we designed an elbow that replaces the OEM MAF tube and fits directly to the outlet of the OEM airbox the same way that the OEM MAF tube fits. This elbow is 3D printed and has connections for the OEM MAF sensor as well as some other connections we will refer to later.
Next up, this air needs to be split into 2 to feed the turbos, we have used CFD (Computational Fluid Dynamics) to work out the optimal place to split the pipes to ensure 50% of the air goes to each turbo so the system is perfectly balanced.
The Garrett Turbos we have selected have a bolt on connection for the in/out connections on the compressor so we need to make an adaptor for this, a pair of CNC aluminium adaptors will be included in each kit, made for MRN to our specifications.
The air exiting the turbo(s) is now pressurised which is good but by pressurising the air we are also adding heat which reduces the density of the air which is not good, we need to cool the air and to do this we use a water/air chargecooler, more detail on that later. We also need to tee the two turbo outlets back into one at some point, for packaging reasons we are doing this as a part of the chargecooler assembly.
The cooled/pressurised air is now ready to go into the engine but to get there we run a charge pipe from the side of the gearbox up to the engine intake. The OEM intake on these cars is pointing to the wrong side of the car for our charge pipe routing and based on the issues we discovered with the OEM inlet when we developed our Carbon Plenum the logical route to correct both of these issues is to supply a Carbon Plenum with the turbo kit.
The last point to mention here from the list above is the BOV, with this type of MAF system the car has metered how much air has been ingested so if we were to vent the spent charge to atmosphere it would not be able to calculate the fuel properly and throw fault codes and/or run very badly. To get around this we added a BOV connection to the 3D printed MAF elbow. A couple of other considerations are the AOS and the fuel tank vent, on the OEM car these are plumbed into the inlet plenum downstream from the throttle body so they only ever see varying levels of vacuum (negative pressure), with a forced induction system there would be positive pressure on boost and negative when on closed throttle. To get around this we added connections for AOS and the fuel tank breather to the MAF elbow, as there is a restriction across the air filter this area will always be in a slight vacuum. We believe that fitting the AOS connection here will actually improve the life of the AOS as it will not see the large vacuum spike of a closed throttle at high RPM which it is exposed to regularly when fitted in the OEM position.
This is often overlooked and just dealt with in an “oil feed in, oil drain out” attitude, which in some cases will work just fine but there are a host of things to consider to fulfil proper oil management. Firstly turbo type, old turbo technology used journal bearing inside the turbo which require a large volume of oil at a relatively high pressure to work properly. Current turbo technology uses ball bearings which are often ceramic, these do not require as much oil flow and pressure, in fact too much oil pressure can blow out the seals. Garrett supply a lot of information on the correct parts/procedures and for this application recommend an inline restrictor to reduce flow/pressure to protect the turbo. Getting oil away from the turbo is a big issue, if excess oil is left in the turbo after engine shut down it can either seep through the seals into the inlet/exhaust or the residual heat from the exhaust side can “cook” the oil and turn it into carbon which can block the oil flow. Porsche turbo models, other aftermarket kits and our kit all use low mounted turbos which brings some other issues. The most obvious one is the turbo oil drain line, other aftermarket kits we are aware of rely on gravity to drain the oil back into the sump of the engine, as long as the turbo oil outlet is above the oil level in the sump this should be ok but what if you park your car on a slope or the oil level is slightly high? The other issue of low mounted systems is the oil that remains in the feed line will drain out when the engine is shut down, this may add to the drain issues as mentioned above but will also mean that when the engine is restarted there will be a delay before the turbo gets a good oil supply. Porsche get around these issues by fitting some additional parts not normally seen on a typical turbo conversion and we have not seen them at all on aftermarket kits for the 987. Firstly they fit a turbo oil sump and a scavenge pumped oil return which takes care of the oil drain issues, secondly they fit a one way “check” valve in the oil feed line to get around the oil feed draining issue. At MRN we have looked in depth at these parts and the pro’s and con’s associated with them but we can’t see a way to get around the issues that could arise without having them…… so we designed/developed our own solutions for both.
The oil feed line check valve could be just an off the shelf item fitted into the oil feed lines but we did nt like this, none of the cost effective valves had the correct fittings so would require adaptors. This would add a further 8 connections into the oil feed line, we see connections as potential leak points so wanted a neater solution. We had been doing some bench testing on the oil restrictor fittings that bolt into the top of the Garrett turbo and wondered if we could incorporate the one way valve into that fitting. To cut a long story short, we did just that! After a lot of testing and development we have a one way valve that opens <10psi and restricts the flow to a level the turbo will operate correctly at without adding any additional connections to the feed lines.
Turbo sumps when used with a scavenge pump system require quite a lot of thought. A single pump serving both turbos will pull oil via the line of least resistance so you will likely bias one side. If the pump capacity is greater than the oil flow through the turbos it will be pulling a vacuum which will overload the pump and reduce its life expectancy and could even pull a vacuum on the compressor seals. The turbo oil sump we developed is (we think) a work of CNC machined art! It secures to the turbo oil drain with an o-ring seal, is reversible so can be fitted in 2 orientations, has a drain connection that is bi-directional and an air breather/vent line to ensure the pump does not overload by pulling against a large vacuum. We will run two separate scavenge pumps, one for each turbo.
Each MRN kit will come with a sump spacer plate, these are recommended for track use, I’ll not go into detail here about that but it is in this spacer plate that the oil will be returned to the engine sump via two separate lines, yes the sump will need to be removed to install our kit but you get a “free” sump spacer and you don’t have to tap a thread into the engine block or any other undesirable oil return method. This plate will increase oil capacity by 826ml (roughly).
If all goes to plan the next update will be some running and dyno results and the much anticipated final pricing information so keep an eye on the blog or our facebook page for updates....