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Boost, Drive pressure, EGTs...


Cowboy

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I'm not sure if this is the right section to put this. Mods please move it, if it's not. I'm looking for some numbers, Boost, EGTs, Drive pressure, RPM, and Approx outside temp. like when Boost is at X, what is all the others at, and what Turbo your running. I saw this article a little while back, and towards the bottum of it they say 2 things about there twins setup where "Unclear" to them. Well that's got me trying to learn why those things happend, and what is the cause. and that's why I need the numbers, so I can compair my numbers, with real life. Thank you all, I've learned a lot from this forum so far, and hope to be able to contribute my findings in the future. ~Brad

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There are way too many variables to answer that question with a direct answer. But I can tell you this. Drive pressure should be as close to a 1:1 ratio with boost as possible. When your drive pressure starts to exceed boost you start to hinder airflow, performance, increase parasitic drag, and decrease power. Stock 3rd gen turbo's will run as much as 2:1 to keep a quick spool and emissions requirement met. A good aftermarket ball bearing turbo can run as low as .7:1 when towing up a long grade. The following numbers are in terms on towing applications, not racing. Peak boost at 1800-2000 rpms should be very similar to peak boost at 3000 rpms. You may not see it on an empty WOT run because the rpms increase too quickly, but with a load you should have peak boost by 2000 rpms. Drive pressure for peak boost will probably be better at 2000 rpms than 3000, unless you are working with an open wastegate. Remember your airflow will be higher at 3000 rpms than 2000, but the flow and resistance will make for the same pressure. 1250° EGT's is considered max continuous for most motors and timing setups when towing. 1300° is safe for momentary temps, again for most motors and timing. If you are running too big of a turbo for your usage (say you peak at 40psi of boost at 3000, but can only make 25 at 2000) you will run excessive EGT's.Then you have to consider compressor efficiency for airflow at a pressure level, as it may work good WOT at 2000 rpms but be less than efficient at 2500-3000 rpms. This will effect IAT's, boost, exhaust flow, etc.

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That might be tough. Very few people here have both boost and drive pressure gauges.

So I have found, I've been searching the forums for what I can find. Found a few with enough detail to put in the spreadsheet, but not bunch, so far so good though.

There are way too many variables to answer that question with a direct answer. But I can tell you this...

What all factors are you thinking? I know that VGT turbos are going to be everything short of impossible to work with on calculating things. But besides that the list I know is relatively short (I may be missing something), so far I got: Intake Manafold temp, Compressor intake temp, EGTs, Absolute Compressor intake pressure, Absolute Compressor output Pressure (or Pessure ratio), Drive Pressure, Compressor Efficiency, Turbine Efficiency, RPM and a lot of the basic things, like displacment and airfilter vacuum. Obviously theirs little things like engine oil you use is going to effect it some, but I just want to get a idea what the turbo is doing, and how it reacts in different senarios. I've got a spreadsheet going, nothing great, but it would be kinda neat if it could calculate the drive pressure... which I don't know is possible. Thank you ~Brad
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Things like the cam, head, intercooler, valve lash setting, exhaust manifold, turbo housing, wastegate setting, timing, fueling, etc all play a role. There is no way that I know of to calculate drive pressure, as it changes based on fueling, rpm, EGT's, etc. Every turbo is different, even the same compressor with a different turbine housing or application will be different. The best way to get your data is to drive and watch. Get a Ultra Gauge or Scan Gauge and watch all the parameters you are talking about. Get a turbo tachometer as well, otherwise you will never know the true airflow and compressor efficiency, and thus turbine efficiency. But like I said each turbo and application will be slightly different. If you just want to see how things are effected try the Garrett Boost adviser, but it won't show you drive pressure. As for the article there are a few reasons the compounds may have had less peak power. The load from the dyno may not have been enough once they were spooled to sustain full power (not uncommon), or the parasitic drag of the motor pushing exhaust thru 2 turbines was noted. I would be curious what the drive pressure was on the compounds vs big single (it may be in the article, but I didn't see it on a quick glance).

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Things like the cam, head, intercooler, valve lash setting, exhaust manifold, turbo housing, wastegate setting, timing, fueling, etc all play a role. There is no way that I know of to calculate drive pressure, as it changes based on fueling, rpm, EGT's, etc. Every turbo is different, even the same compressor with a different turbine housing or application will be different. The best way to get your data is to drive and watch. Get a Ultra Gauge or Scan Gauge and watch all the parameters you are talking about. Get a turbo tachometer as well, otherwise you will never know the true airflow and compressor efficiency, and thus turbine efficiency. But like I said each turbo and application will be slightly different. If you just want to see how things are effected try the Garrett Boost adviser, but it won't show you drive pressure. As for the article there are a few reasons the compounds may have had less peak power. The load from the dyno may not have been enough once they were spooled to sustain full power (not uncommon), or the parasitic drag of the motor pushing exhaust thru 2 turbines was noted. I would be curious what the drive pressure was on the compounds vs big single (it may be in the article, but I didn't see it on a quick glance).

The drive pressure they said was 77 with the wastgated twins, and they never saw over 1:1 Exhaust to Boost with the single A5000, so that tells us that the DP never went above 60 psi. so the drive pressure was higher with the twins. now here's what I don't understand. The Drive pressure was higher on the twins. Boost was Lower. Compressor output temp was 100* lower on the twins, but they got a B&D aftercooler and they said they never saw over 100* with a input temp of 460* with the single, so 360* with the twins. and EGTs were Lower. How can all that happen with the SAME amount of fuel. They said the max EGTs on the dyno at 3,000 rpm was 1,350* with the single, and max they could hit EVER with the Twins was 1,200 off the dyno, and 1,100 on, so either way they were a bit lower. And the main reason I want to try to understand it better is that I was thinking on putting Compound HX35/HX35 (yes same stock turbo) together. I've always thought that the primary lights the secondary, so how would it act with the same turbo. I know that drive pressure could be a problem, but face it, this isn't going to be a toy, it's a truck, I want the lowend torque, and it's never really going to see 2500 rpm a whole lot. And fuel milage might go up a bit too, for the reasons of the second turbo taking the power out of the exhaust that would otherwise go out the exhaust pipe. Yes it would add more DP so the engine would have to work a little harder, but IDK how much ACTUAL power it take to push it through the extra turbo (reasearching that stuff right now).
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The air with the twins was cooler, and more of it. That's how the EGTS were lower and the benefit to twins. As a single the 5000 was doing all the work, so it puts hotter air out from a single higher pressure compression. With twins each turbo does part of the work, and thus is in the most efficient zone on the map. That high of drive pressure means the secondary needs a bigger wastegate and/or turbine housing, and is probably why the power was a little lower. In addition the added cooler air stayed ahead of the fuel and probably our ran the dyno load. A sequential twin setup with the same turbo is a waste of time and money. As the article said you so sequential twins to get the total flow of a big turbo and the spool of a small one. With the same size you gain no additional flow on the intake size, and like you said you hurt your exhaust flown. The primary does not light the secondary, the secondary lights on its own since its smaller and takes less exhaust flow. As the secondary starts to move more intake air the exhaust flow increases an gets the primary going. At low boost the secondary does the lions share of the work and at high boost the primary does the lions share. When the primary lights the secondary's main job is to increase the flow of the air the primary is pushing. This is why the intake flows from ambient, thru the primary, then secondary, and then to the motor; but the exhaust goes thru the secondary first and then the primary.

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  • 1 month later...

As soon as I get my DP gauge installed, I can give you some numbers on what I'm seeing with my setup, since I'll have overall boost, primary boost, and DP at the exhaust manifold. To be truly consistent, I really should do a second DP gauge, off the secondary exhaust housing too, but that's more money, and to be honest, I'm not sure if I could deal with 6 gauges on my A pillar!:lmao:

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