Backpressure - Can big wastegates help?
If i were to change my cold side, i would prob increase to 2.25" from the turbo forward for a short distance and keep the 2" to the intercooler and then test it..... Because as i understand it, velocity before the IC should be kept high (within feet per second range) and stepping up the cold side diameter just BEFORE the IC would slow velocity down. Another thought (as i understand it) is the charge side air with a rear mount will start cooling as it progresses down the long cold side pipe and therefore would be more dense and require less diameter pipe to flow. Someone correct me if i'm wrong. Hopefully i answered this correctly.
If i were to change my cold side, i would prob increase to 2.25" from the turbo forward for a short distance and keep the 2" to the intercooler and then test it..... Because as i understand it, velocity before the IC should be kept high (within feet per second range) and stepping up the cold side diameter just BEFORE the IC would slow velocity down. Another thought (as i understand it) is the charge side air with a rear mount will start cooling as it progresses down the long cold side pipe and therefore would be more dense and require less diameter pipe to flow. Someone correct me if i'm wrong. Hopefully i answered this correctly.
i used to do the racecar thing...it was fun but i would end up rubbing on it for weeks at a time and then race one afternoon a month.... Now i get to enjoy this car everyday!!! That is why i'm always smiling in my car....! ha ha
Thanks
Thanks
Launching!
Joined: Feb 2011
Posts: 234
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Actually, I'm not going to do compound turbos. I was just using that as an example to show how regulating the inlet pressure to the turbo(separately from the boost pressure regulating) could(potentially) control the backpressure issue. My actual set up is the single Precision 78mm turbo. The 78mm single turbo is not matched well with the 402 motor(from a hot side flow/pressure perspective). The motors capacity for flow/pressure is much higher than the exhaust housing can handle(causing a butt ton of backpressure). However, it would probably match really well with a 5.3 liter motor. By matching the flow capacities better it will have an exhaust manifold pressure that is not overly restictive to the motor. My problem is : I want to use what I have(ie - don't switch to a 5.3 liter motor or a larger turbo). I think it's feasible to do that if I can control the backpressure(exhaust manifold pressure). This can NOT be done with a bigger wastegate(or even a second one in the same circuit). I understand that point well. However, if a second circuit is set up to look at exhaust manifold pressure and control it to (let's say 1.7 times the peak boost pressure) through a separate wastegate, then the backpressure would be relieved WITHOUT going below the pressure/flow threshold that motor requires to spool properly and maintain boost setpoint through the powerband. The mismatch is so great that I could dump some flow right after the manifolds and still have plenty of flow to run the turbo the same way that a 5.3 liter engine would(at a similar exhaust manifold pressure). I don't think what I'm proposing is much different than some of the set ups that people run in compound turbos. This is my theyory. Tell me what you guys think. Thanks!
Give it a shot and let us know what you find...
Here is the latest... The t4 6765's are on with the spool valves. Early indications are that it is about the same or just a touch softer than the t3/t4 6262's down low..... which is good news because it certainly didn't want a pig down low. Under WOT and tip in it is kinda fat (it goes to 10.0 almost immediately) so i need to tweak the tune a bit. I think i will be able to have the best of both worlds when the tune is dialed in and i start cranking up the boost. The spool valves toned down the exhaust noise a little and also under part throttle the turbo's whistle much sooner. I will log a backpressure/boost test and update soon. thanks
Last edited by TracyRR; Feb 26, 2011 at 08:46 PM.
I went with the SP. It is a nice valve but i may need to play with how soon or late it opens...... I just got them running so i'll have to spend some more time tuning.
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GOING FRONT MOUNT!!!! Actually they will be low mounted by the foot panel just like in the thread below. I removed the sheetmetal on the drivers side on my C5 (foot panel area) and there should be plenty of room for my 6765's.
https://ls1tech.com/forums/forced-in...-76-build.html
Hope to be done in a couple weeks, i'll update with a new thread and progress pics. I had fun the past year with the rear mounted twins but it is time for bigger and better! Thanks for the great idea dustin/josh!!
https://ls1tech.com/forums/forced-in...-76-build.html
Hope to be done in a couple weeks, i'll update with a new thread and progress pics. I had fun the past year with the rear mounted twins but it is time for bigger and better! Thanks for the great idea dustin/josh!!
Last edited by TracyRR; Mar 3, 2011 at 08:05 AM.
Did not read the whole thread, but what is the general consensus, will small wastegate increase backpressure on turbine-wheel choked motor?
Specific example - 383ci, 38mm wastegate, TC78 turbo with 68mm wheel, it all holds boost steady at 14-14.5psi
Specific example - 383ci, 38mm wastegate, TC78 turbo with 68mm wheel, it all holds boost steady at 14-14.5psi
Wastegates dont have anything to do with backpressure....
If the wastegate is too small, boost will creep
if the wastegate is too big, the valve will only open enough to control boost pressure
The turbine housing and wheel control backpressure.
If the wastegate is too small, boost will creep
if the wastegate is too big, the valve will only open enough to control boost pressure
The turbine housing and wheel control backpressure.
OK in my case (383ci with TC78), we have capable compressor wheel BUT choking exhaust wheel. From my research, I assume my backpressure is somewhere near 2.5-3:1. So couldn't we expel a bit more exhaust gases, bypassing turbine wheel, while still having enough pre-turbo pressure to drive the comp wheel?
OK in my case (383ci with TC78), we have capable compressor wheel BUT choking exhaust wheel. From my research, I assume my backpressure is somewhere near 2.5-3:1. So couldn't we expel a bit more exhaust gases, bypassing turbine wheel, while still having enough pre-turbo pressure to drive the comp wheel?
If you let off more exhaust gas, it will lower boost pressure, if the wastegate is not venting enough, you will get boost creep. Your problem is that turbo, not the wastegates. You could put 4 tial 60s on that thing and its still going to have 2.5:1 backpressure.
I see what he is saying . if the turbo needs 20 psi backpressure to hold 10 psi boost and he is getting 40 psi backpressure whouldnt the wastegate bleed off the extra pressure only leaving what the turbo needs to hold boost. ?
of corse the exaust wheel will be the big restriction before the wastegate opens.
seems to me that holding the car wot down the track the wastegate stays open all the way down once up to boost pressure. then it only opens up enuff to hold the preset boost pressure . I see it as the backpressure is dictated by turbo compressor to turbine efficiency. and the rest gets dumped. but I do see where the engine cid plays a big roll in this. jmo.
of corse the exaust wheel will be the big restriction before the wastegate opens.
seems to me that holding the car wot down the track the wastegate stays open all the way down once up to boost pressure. then it only opens up enuff to hold the preset boost pressure . I see it as the backpressure is dictated by turbo compressor to turbine efficiency. and the rest gets dumped. but I do see where the engine cid plays a big roll in this. jmo.
Exactly, I was thinking that larger wastegate capacity would allow engine to produce more exhaust gases that bypass turbo. And in reality of cause I care about more overall power, not about less backpressure
Again, a waste gate can't lower back pressure at a fixed boost pressure. If you are experiencing boost creep the waste gate is too small to control boost at that level or the airflow priority is compromised. If you have 40psi of back pressure at 17psi, only think you can do to change it is to put on a more efficient turbo combination.
Again, a waste gate can't lower back pressure at a fixed boost pressure. If you are experiencing boost creep the waste gate is too small to control boost at that level or the airflow priority is compromised. If you have 40psi of back pressure at 17psi, only think you can do to change it is to put on a more efficient turbo combination.