Nissan Juke : Juke Forums banner
1 - 5 of 5 Posts

·
Vendor
Joined
·
1,107 Posts
Discussion Starter · #1 ·
Exhaust back pressure is an often talked about concept with tuners and car builders. The following is a VERY simplified discussion on turbo back pressure and how it effects MR16DDT engines. We will not be discussing things like AR ratio and compressor CFM. The goal of this is to explain why larger turbochargers are better for your goals, even if the stock sized turbos are capable of “making” the horsepower goal you have related to the exhaust side of the turbocharger.


The higher your exhaust back pressure, the less air is able to escape the combustion chamber during the exhaust stroke. This un-released air is considered “dirty”, as it has components not ideal for combustion in it. Such as: heat, co2, nitrogen, and the ability to expand more as the intake stoke occurs. If the back-pressure is higher than the intake pressure, this can lead to a lack of combustible air entering the chamber ,meaning higher engine pressures with less horsepower/torque being made. An ideal Boost to Back Pressure ratio is 1:1. Anything <1:1 is considered EXTREMELY efficient, and anything 1:>1 is considered inefficient.


As important as a compressor side on a turbo is, not many people understand the importance of the turbine housing/wastegate in regards to efficient performance. The larger the turbine housing, the slower the turbo is to react but on the flip side, the larger the housing, the less back pressure the system has to contend with, which means you make more power on less boost.

We conducted the test as follows:

We took a full bolt on 2014 Nismo Juke Six Speed car installed a stock turbo manifold with added sensors and strapped it on the dyno. We then hooked up 3 external gauges to the car: Boost, Exhaust Pressure, Exhaust Temperature.

We conducted 3 pulls back to back to back with the stock tune. All three pulls where recorded on video for later analysis. We then custom tuned the car to ~20psi, as we would any stock turbo car. 20Psi will be shown as the magic max number shortly. We then replaced the stock turbo with a Turbobay Stage 2 Turbo (comparable in size to a Mamba 19t) and tuned that to 17psi. This boost pressure will make sense later as well. All data is measured from 2psi to peak boost. We did not graph out the data after peak boost, or to red-line. This would require graphs by RPM, which will not accurately show how the back-pressure is effected by the boost pressure when compared at a 1:1 scale.

Here is the data we ended up with:


Lets analyze this data:

Stock Tune Stock Turbo:

As expected, Nissan/Renault designed and paired this turbo specifically to this engine at this boost for a reason. The turbine housing flows just enough air to maintain a 1:1 ratio. We can see, however, that at peak boost, when the wastegate opens, that the back pressure continues past the 1:1 ratio. This tells us that the wastegate hole is not large enough or efficiently placed to let out enough air to mitigate back pressure, even at stock levels. This tells us that modifying the wastegate hole COULD provide a more stable torque curve, as the back pressure would be reduced, allowing more “clean” air into the engine as RPM's rise. We will be testing this using a custom tubular manifold on a stock turbo with a 38mm wastegate at a later date.



Custom Tune Stock Turbo:

When tuning a modern turbocharged vehicle one of the first things you do is apply cam phasing to increase cylinder pressures to help spool the turbo up faster than stock. The effects of this are shown in a very obvious manner here. You can see that at 2psi of boost we are already making 5psi of back pressure. This increase in back pressure is what spools the turbo up faster, and in a backwards sort of way is what you want to happen to spool the turbo up. BUT. As your RPMS and boost climb and you reduce the cam phasing, you want to see that back pressure move back to a 1:1 ratio with the boost when doing this. As you can see, as the boost approaches 12-13psi, our back pressure comes down slightly and gets closer to the boost pressure, but is still quite a bit more than it, right up to around 17psi where the back pressure starts to skyrocket. This is where the turbine housings size really comes into play. We are now at higher than stock boost levels, that the housing was not designed for. At peak boost of 20psi you can see we are near a 1:1.75 ratio. This means we are basically choking the engine. Even with the wastegate now opening, the back pressure is climbing. If you look at a stock turbo graph with a tune on it, this point right here is where you see the torque start to tank and where you see the boost start to die off. The turbo cannot maintain this flow of air. This is why 20psi is the MAX you should ever see a stock turbo make. Any more than that and your hurting the engine for the sake of a boost number or a spike in torque on spool. You also are increasing the chance of knock occurring. As the “dirty” air being trapped inside the combustion chamber gets more and more as RPM's climb. This turns into hotter and hotter combustion temps, and less and less cool air during combustion. This is ONE of several reasons why stock turbo tuned cars struggle to make power at such high boost levels. Combine this with compressor sizes and you get an inefficient system.

Turbobay Stage 2 Turbo With Custom Tune:
Turbobay and Mamba both offer a similar sized turbo. So this data can more or less be applied to the TB stage 2 Turbo and the Mamba 19t Turbo. Both companies put larger exhaust wheels into a bored out stock turbine housing. This larger exhaust wheel helps reduced back pressure (more air can flow in and around it) and helps keep turbo RPM's up to maintain boost better. BUT. What this does not help is the over-all size restrictions of the stock housing, nor does it help make the wastegate more efficient.

As you can see, even when cam phasing starts to come off, the back pressure continues to rise at the same rate through the pull, right up to peak boost. You will also notice that at ~15-16psi when the wastegate starts to open, the back pressure skyrockets. It is no confidence that this starts to happen around the same pressure that the car has stock. You are now jamming a lot more air into the engine than it was deigned for, and using a turbo that has very little head room for more cfm out of the exhaust. At peak boost of 17psi, you are already at a 1:1.88 ratio. Any more boost than that, you start choking the engine even worse. Why? The turbine housing is far too small. Perhaps a larger wastegate opening would help alleviate this problem, again, this is something we plan on testing at some point. But as a stock bolt on option, yes you make more power. Yes, you hold torque better. But you are still over-stressing the engine.


In our next round of tests we will be putting a 16g BPT turbo on a stock Juke manifold and conducting the same tests.


We will then be making a custom manifold to mount a stock turbo on to see if we can alleviate top end back pressure.
 
  • Like
Reactions: MambaJuke

·
Vendor
Joined
·
1,107 Posts
Discussion Starter · #4 ·
On these tunes we ran the vvt to its limits, but limited the boost to what you saw. If you kept the boost climbing to 21+psi, the back pressure would be insane. You can clearly see on the graph that after 17psi its an exponential increase with boost. If you add in the pressure to redline it gets pretty nuts. If you watch the video, you will see 40+psi of back pressure on the tuned turbos at redline.
 

·
Vendor
Joined
·
1,107 Posts
Discussion Starter · #6 ·
My guess is the 16G will have less BP and more power so its less strain on the motor. My Juke made 310whp on your dyno which is more like 325whp on a DynoJet. Stress to stress. My old 19T engine was more stressed than the present 16G setup ?
When we do the test with the 16g we are expecting to see <1:1 until higher in the rev range when using a internal WG. at the most we are expecting a 1:1.2 at 7000 rpm, as the WG hole on the bigger turbos is massive compared to stock. We will be doing the test using one of our most powerful car builds yet shortly. But on that application we are expecting a terrible ratio, as we have to run loads of boost to make the power its going to make. Once we have the big turbo data we will be breaking it down by RPM (or time whichever is easier to graph out).
 

·
Vendor
Joined
·
1,107 Posts
Discussion Starter · #8 ·
Cam phasing, heaps of boost target. And to edit your question: gets 50 more PEAK WTQ, on an OEM housing.

At redline those tunes are not making any more torque than that of any other tuner out there. Because of the OEM housing size. The housing doesn't become a restriction until x amount of cfm or pressure is trying to fit through it.
 
1 - 5 of 5 Posts
Top