Zero10

I just had one throught about having the turbo on the P/S, trying to get an external wastegate in there too.

I didn't mean so much about people retrofitting it into a normal 951 as an upgrade, but more about the people who are replicating the callaway setup, or people who have already sunk a lot of money into their engine (S.T.???)
I mean, with the gains of going to a dual scroll turbo, would that counter out the loss of having an internal wastegate?..... If it does, then this could be worth-while I think.
I mean, how many of us have messed up headers and crossover pipes? *raises hand*, how many of us have an old tired turbo? *raises hand again*

You could pipe straight into the center of the turbo, clock it so the discharge is on the bottom, one 45* or 90* bend to go forwards, a 90* bend into a large bottom mounted intercooler, then a 90* bend up into the stock intake and a 45* bend to straighten it out.
I don't think there would be that much more intake piping than the stock setup, but you would take the exhaust piping from like 7' down to 2', and that has to count for something, especially with all these radical ideas coming out to maximize exhaust gas speed.

nine-44

You won't see much lag as a result of a higher volume from a longer intake tract. Lag is more of a result of poor pulse delivery to the turbo to the hot side. Turbulence, excessive length, improper pulse tuning, etc. The sharper the pulse is when it hits the turbine wheel, the harder it will drive the turbo. Because the turbo is regulated by peak boost pressure, it will drive as hard as it can until it reached that point. The split scroll turbo is better for pulse tuning, there are two entry points that end 180 degrees, or opposite from each other. Also, due to the fact that they only have to carry 50% of flow, the ports are smaller, keeping the pulses tightly packaged, loosing less energy. A single scroll has to carry full flow, it's larger at entry and disperses the pulses.

Porschefile

Zero10, having a twin scroll housing won't make up for the problems associated with an internal wastegate. First off, many of the twin scroll housings on the market (such as the GT series housings) are for an external gate. There are a decent amount of twin scroll internal gate turbos from oem stock production cars though. The main issue associated with internal gates is that at higher power levels and flowing a larger volume of airflow, internal gates are just too small and inefficient. You can get issues like boost creep because you are trying to force a large volume of air through a relatively small hole. External gates are generally quite a bit larger. It's the same concept as having a 1" exhaust or a 3" exhaust. This is the main reason for any sort of power "loss" associated between internal and external gates as well.

There are plenty of great turbos out there that no one is using on 951's. There is certainly a HUGE variety to choose from. The nice thing is there are some pretty affordable options that are still very high quality. Take the OEM Nissan S15 Silvia (Japanese "240sx") turbo for example. It's a Garrett Gt28r, it has an Inconnel shaft, it's dual ballbearing, and they typically only retail for ~$650-750 new! That would be a bit small for a 951 (actually maybe similar in size to a 26/6 or /8). There are plenty of other deals like that out there and plenty of affordable turbos using cool exotic stuff. IMO, our stock turbo layout is complicated enough and takes so much time to disassemble that I don't really see too many venturing into the unknown with non-bolton stuff like that around here. I think it would rule if someone would make a p/s mounted turbo setup again, even if it was fairly simple and only consisted of a few components (manifold, custom downpipe, etc). I also think it would be interesting to see some people run an internal gate turbo in the stock location. Internal gate turbos can be made to perform perfectly fine at moderate power levels, and using one would allow you to eliminate the convoluted wastegate section of the exhaust under the car. Just some thoughts. There are obviously plenty of new things left to try with 951's, which is nice considering the age of our cars.

nine-44

Exactly Porschefile! I'm working on a sequential setup with a VNT Garrett as the primary turbo. Alot of cool stuff has arrived since the birth of the 951, we just need to play with it and utilize it on our cars.

TRP951

Hows the progress on that thing I havnt seen a update in a long time, last I saw was the custom widebody pics

Zero10

Wait, so even though so many people are trashing internal wastegates, they would most likely be fine for moderate power levels, what are we taking, 200-350hp?....
You say that the twin scroll won't make up for the internal wastegate loss.
I am not certain I fully understand, will an internal wastegate cause late spool, or excessive boost at high RPMs?
If it is the second, and it will not effect spool-up, then I think it would be just peachy for a setup like what I want. My problem is that boost is falling off at higher RPMs

Porschefile

Yes, internal gates can work just fine. There are alot of factors to take into consideration though. The turbo elbow (pipe coming directly off the turbo) needs to have the internal wastegate flow incorporated into it, which could get a little cramped back there though it shouldn't be too much of a problem. Generally, you can make a turbo elbow that does not have the internal gate flow and exhaust flow separated. Many oem internal gated cars come this way. Ideally though, you want to separate the internal gate flow and join it back into the turbo elbow after not too short of a distance (say at least 8"+, preferrably more). By separating the internal gate flow you reduce the amount of turbulance in exhaust flow directly at the turbine. Without doing this, and without properly shaping this separate internal gate discharge, you can run into problems like boost creep, inability to reach higher boost levels due to extreme backpressure, etc. Shape is also important. If you have a sharp angle (say close to 90deg) immediately after the wastegate discharge, this can cause a reduction in flow due to increased surface friction, and it can lead to boost creep and many of the other same issues as well. Here's a good example of the separated design I'm speaking of (from a Nissan 240sx):


Anyways, the bigger you go in turbo size, the more boost you run, and the more power you make, these internal gate issues will only be amplified until you reach a point that you simply cannot pass due to a lack of flow capacity from the wastegate. There are plenty of non-944 cars running internally gated turbos anywhere from 300-400+rwhp without many issues. Below 300rwhp, you shouldn't really see any of those issues due to the smaller size turbo and lower boost level you'd most likely be running. Generally, I'd tend to believe that when you reach a Gt30r sized turbo or larger making ~350-400+whp, running an internal gate setup is probably going to start having a more significant effect on performance and at that point I'd stick with an external gate. Ultimately, an external gate is going to be superior in flow capability, which in turn will "make" more power as it provides less of a restriction. Most internal gates are significantly smaller in diameter compared to say a Tial 38mm.

To answer your other question Zero10, an internal wastegate can actually affect both spool and excessive boost at high rpm. Generally speaking, internal gated exhaust housings have more turbulent exhaust flow and more exhaust backpressure then non-internal housings do. That's just the nature of it. By running a twin scroll internal gated housing, you aren't causing some serious reduction in turbulence or backpressure so most of the same problems will remain. With a twin scroll what you are doing is using pulse tuning to acheive increased exhaust velocity/efficiency which will have the effect of spooling the turbo earlier. In short, you're speeding up the flow into the exhaust, though you haven't really done anything to "speed" up the flow out of the exhaust so most of the same potential issues could still arise.

Boost falling off at high rpm? What size turbo? Generally, you can see this with too small of an exhaust, too small of an exhaust housing, etc etc. Basically increased backpressure is one factor that could cause this. Though, if you are pushing outside of the turbo's efficiency range then that can be completely normal if you've reached the point where the compressor just can't flow any more.

Zero10

My wastegate is hooped I think, since I hit 12psi by the late 3000's, and by mid 4000's it is down to 6-8psi until red line.
That is my main boost problem, but I also have cracked headers, the tapered seal rings leak by the wastegate and I think there is a crack int he crossover. And I just don't want to bother replacing a wastegate, crossover and headers then still be limited to 250-ish HP on the stock K26/6 if it would be cheaper/easier to go to a turbo like this. I have no plans to try to break 350 or 400hp, that's for sure.
Every day I change my mind on what I want to do with my 951, but this twin scroll turbo idea has really caught my attention.

This thread has been very educational, and I am really starting to wonder, the cost of having some custom headers made, buying a suitably sized twin-scroll turbo, making a new down pipe and intercooler piping, that would probably be on par with replacing my headers, turbo, crossover, wastegate and downpipe (mine is crushed badly), perhaps even cheaper.

Maybe it's not a good idea for my car, maybe it would be better for somebody with a more advanced setup, but I like to learn all I can about things, and I like to really understand why things would/would not work

FSAEracer03

THAT'S what I'm talking about! I've wondered since I got into the 951s why no one has really tried out a sequential setup, and why nearly everyone keeps the poor turbo location. For all those spending ten, twenty, and thirty thousand on building their cars... why not try to utilize the last 20 years of turbo tuning technology?? Fabricate new motor mounts to shift the motor a little. Fabricate new header(s) and a full exhaust, intake, cooling, and lubrication system. Hell... what do you think building a true racecar is all about? Try out a sequential setup. I think a GT28R and a GT35R would make for a killer combination with a high-boost, sleeved motor! Who wants to wait until 4k to make any power anyways?


Keep us posted on the project... I'm very interested to see how it turns out.

Porschefile

Zero10, it would be far cheaper and easier to find some good condition used exhaust/wastegate pieces on classifieds or Ebay and rebuild your stock setup.

As far as twin turbos on a 4cylinder.....completely unnecessary IMO. A single will work just fine. a twin turbo setup would just overcomplicate things in an already cramped and overcomplicated engine bay! Seriously though, from a performance standpoint it's a bit of a waste of time. If someone wants to pursue it more for the "just for the hell of it" factor, that's cool. Though, I'd run a pair of small turbos like Garrett Gt15's, Gt20's, or Gt22's. Much bigger than that and it should start to get a decent amount laggier. Also, running 2 vastly different sized turbos, such as a t28 and 35r, could cause some serious surging issues. The t28 wheels could not come close to keeping up with the 35r wheels. I don't have alot of knowledge of sequential setups unfortunately.

Speaking of new ideas, I think it would be neat if we saw more people move towards doing "vertical" mounted 951 engine setups. I don't know if any of you remember some of the threads awhile back, but there are a handful of people that have done this. Of course there would be alot of custom work involved, however it would simplify things quite a bit. Then you could easily run a manifold mounted turbo setup and try out other cool stuff like twin scroll housings effectively. Ultimately I think I'd like to go this route as having a manifold mounted twin scroll gt35r would be sweet!

Oh, and FSAEracer03, I want to wait 'til 4k to make power! The earlier the party starts the earlier it's over! I want sledgehammer-to-the-chest-big-turbo-top-end-pull!

FSAEracer03

I disagree with your statement that twin-turbocharging a four-banger is a poor choice. The party only ends earlier if you're using a single turbo, or running parallel turbos. With an offset-size, sequential setup, you can have boost at 2500 if you want it, and hold it to redline if the turbos are chosen wisely. That's the whole point of running them in that manner.

EDIT:
With offset sizing, and separate wastegates, you actually utilize the low boost threshold of the smaller turbo to create boost in the lower portion of the powerband, while the larger turbo comes into plat for the latter section. While the smaller turbo would normally be well out of it's efficiency range in this area, it's individual wastegate would be bleeding off some exhaust gases to keep its shaft speed under control.

Porschefile

In theory. In reality it's cost ineffective and most 4 cylinder cars don't tend to have enough room for a twin turbo setup. That's the reason you rarely ever see any 4cyl street cars with a twin turbo setup. Offhand I can't even think of a single TT i4 I've ever seen. If you want a broad powerband, swap an Ls1 in. Or twin charge it with a supercharger and you'll have plenty of low-end. I personally like having a higher boost threshold and having to wait for the boost a bit. For my tastes, the entire appeal of turbochargers is the rush! I could care less about having as much low-end power as possible. You don't win extra points on the street for having 40% more low-end! It makes sense for a race car. Though, for a street car if you are really THAT concerned about having stump pulling low-end then just swap in a v8. It's a 4cylinder for God's sake! Ours already make decent enough low-end torque. I think some people around here are way too focused on low-end torque to the point where their mods to obtain it result in crappy top-end power curves and early power dropoff regardless of what they're peak numbers may be (small exhausts/exhaust housings, small manifolds, etc). Personally I think twin charging with a supercharger is a better route, preferrably with some type of s/c "clutch" setup that will disengage when the turbo starts to spool so as to eliminate extra drag on the motor.

FSAE, I completely understand the basics about twin turbocharging, both sequential and parallel setups. However, the things I am not as knowledgeable about is the general fluid dynamic relationship between both turbochargers, what things can cause surging (I know about a few different surge issues), and other potential issues between the turbos.

FSAEracer03

To each his own. My rule of thumb with building powerbands, though, isn't to have peak numbers, but area under the curve. If I want speed, I don't mind waiting a little for some foreplay-like turbo lag... but if I want a serious speed-demon, I want acceleration, and damnit, I want it 5 seconds ago! There have been a a few production TT 4-cyl. One is still in production, in fact, but we just don't get it here. It's a Subaru, and it uses offset size IHI turbos. Either way, when it comes down to it, I don't follow the norm to build a beast, I follow what works.

As I said, though... to each his own.

PS... when it comes down to the fluids and thermo relationships between the turbos, each is a separate system based off of two differential pressures (four pressures per turbo, defining operating parameters). With sequential setups, they're still separate systems, but they happen to have two of the four pressures linked, only being separated by exhaust gas inlet and outlet pressures as the engine airflow requirements increase.

facboy

if you're talking about a sequential subaru, was it the liberty/legacy gt? the old model used a sequential setup, at least in oz. general opinion was that it was ****...there was a noticeable torque hole around 4k i think. i think the new setup uses a single turbo.

FSAEracer03

I don't remember the model, to be honest. My Subaru-afficianado buddy was indexing the turbos on his buddy's Euro-whatever-model and told me a little about it. Stock it's not all that efficient because it had two undersized IHI turbos on it. It's still in production, and from what he said, the Euro motors still have the setup.