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So I have gt4202r for 20b with large street port. I'm getting boost taper as I go up in RPM. I'm looking at this in the turbine map, and if you count 20 b as a 4 liter engine, the turbo is running out of air anything above 7000 RPM and over two bar which is what I'm experiencing.
this is the real kicker: when I set the boost LOW for street driving, GT42 is way out of efficiency range beyond choke line. When I look at the compressor maps only, the only turbos that will be in efficiency range at LOW boost would be something huge like GTX50, and it will also not run out of air at high RPM at say 30 PSI. This sounds little backward but that's what compressor map dictates.
now my question is this. will my car be just as driveable with the larger turbo? i know it sounds way wrong, and i'm not sure what to make of this data. for those of you who have bigger than GT-47 sized turbo.. did you find it as responsive in low boost as the smaller sized turbo? if the GT42 is actually undersized for 3 rotor, I would assume getting a proper sized turbo will not affect lag too considerably..?
i'm not sure whether to chase after compressor map or just max out what i have. i'm not even sure whether we can put down over 1000whp on street with DOT legal tires with RWD chassis even with traction control if I go with GTX50. I want to maximize the whp and torque area under the curve, and not sure how you do that with the compressor map data without being on the dyno. Thanks in advance.
Last edited by stickmantijuana; 07-25-18 at 08:20 PM.
well, let me ask in another way. for those with actually running street ported 20b, which turbo are you using that doesn't run out of air at over 24psi? and when does it hit 0psi and full boost during spool? thanks!
Yes, either gtx47 or gtx50 would be a good choice if you do not want to run out of air going by compressor map. I was curious about their streetability. I want to punch it in a corner, and not wait for the turbo to kick in and not have it kick in so abruptly to make handling impossible... do you know anybody running big turbos for 3 rotor engines on street or in circuit racing?
Ive run a 1300 hp precision pt76 sportsman, made 916 at 25 psi, and still had the same issue. Even when set at 700 hp. We think its the wastegate dump going into the downpipe. and the backpressure/ wg flow fighting each other. I say try a 5 inch downpipe and external dump of the wastegate .
thanks man. well i spent the last few minutes looking for compressor map for that turbo and realized precision don't publish them? well anyway, i think that's a very similar sized turbo to gtx45 maybe? so that's a bummer to hear man.
however! a couple of us have a similar suspicion as yours: backpressure may be pushing the wg open at high rpm which causes the boost taper rather than turbo running out of air... i think also because we run such large wg's in general, our problem is only exacerbated. i think once it cracks open, we lose too much manifold pressure and we can't regain the boost.. i'm not sure whether post-turbo backpressure is the problem though. i think it's backpressure within the manifold that's giving us problems, no?
anyway we thought about that... so what's the fix? giant exhaust turbine and housing to decrease backpressure which in turn will lead to horrible lag. for some reason playing with solenoid has not been ideal for many of us. i'm going to install EMAP sensor soon to see what's actually going on. if backpressure is actually pushing open the wg valve, i'm not sure what the solution may be. install a smaller wg? maybe a restrictor plate between the manifold and wg? i don't want to go up in spring rate because i don't want to drive around with 800whp+ all the time. i've also looking at wg offered by synapse engineering. it looks like it may better address bypassing too much air when we don't want it to. i'll talk with them tomorrow.
thanks man. well i spent the last few minutes looking for compressor map for that turbo and realized precision don't publish them? well anyway, i think that's a very similar sized turbo to gtx45 maybe? so that's a bummer to hear man.
however! a couple of us have a similar suspicion as yours: backpressure may be pushing the wg open at high rpm which causes the boost taper rather than turbo running out of air... i think also because we run such large wg's in general, our problem is only exacerbated. i think once it cracks open, we lose too much manifold pressure and we can't regain the boost.. i'm not sure whether post-turbo backpressure is the problem though. i think it's backpressure within the manifold that's giving us problems, no?
anyway we thought about that... so what's the fix? giant exhaust turbine and housing to decrease backpressure which in turn will lead to horrible lag. for some reason playing with solenoid has not been ideal for many of us. i'm going to install EMAP sensor soon to see what's actually going on. if backpressure is actually pushing open the wg valve, i'm not sure what the solution may be. install a smaller wg? maybe a restrictor plate between the manifold and wg? i don't want to go up in spring rate because i don't want to drive around with 800whp+ all the time. i've also looking at wg offered by synapse engineering. it looks like it may better address bypassing too much air when we don't want it to. i'll talk with them tomorrow.
Let me hit up a buddy who just ran through a backpressure calculation for 2-rotors on optimal turbo configurations. We'll do our best to model a 3-rotor and I'll post back some of the results (with his approval first). I think this is why multiple, smaller wastegates are generally seen as a better option for control on higher boost applications. Dual 3 Port MAC Solenoids in Push/Pull configuration should definitely be on your to-do list if you're not already setup that way. Nearly eliminates that as being an issue (wastegate cracking open).
There's a recent thread about Turbine Housing/Manifold design, which I'll try to find and link to in which a few of the members are talking about high and low wastegate priority and how it effects drivability. Good thread, I'll try to find it.
with a compressor that flows efficiently, while delivering your desired lbs/min at your desired pressure, you shouldn't have much of an issue with a standard 3-port. You should be able to run around at least 1.5 to 2x your spring pressure. At least my 3 rotor with 87mm T4 1.15 turbine on 80mm s480 compressor (smaller version of s480) allowed 12psi spring to hit 24psi, with only a slight trail off past 7500 rpm. From this, I gather that adequately placed compressor map is priority.
My 26b, with s480 (full size) dropped boost extremely aggressively, requiring 100% duty cycle just to maintain 1psi above spring pressure, and falling short at 9k rpm. Horsepower = same as the 3 rotor. Yes, back pressure is higher because of the extra rotor/porting, if you look at the compressor map, the demanded flow falls extremely far outside the efficiency range per pressure ratio.
So in my opinion, compressor efficiency will reflect back pressure greatly.
You shouldn't need fancy boost controller methods as long as you're happy with 1.5-2x you spring being your maximum
I think best option for a 3 rotor would be an s480, it falls dead on the island of efficiency for a street port 3 rotor, revving to 8.5k. Choose the larger t6 turbine 96mm. If you're planning to run lower boost, you may benefit from smaller A/R for quicker spool at expense of needing more duty cycle to maintain your desired pressure. They make a v-band housing at 1.0 which would be choice.
07-25-18, 02:19 PM
