More of a book than a post (BB TWINS!)
08-10-2007, 01:50 PM
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More of a book than a post (BB TWINS!)
[SIZE="3"][SIZE="4"][SIZE="3"]OK fellas, brace yourselves. I’ve been thinking—scary thought—about twin turbos. I recently built a spreadsheet that would calculate some aspects of turbo selection, and starting applying them to various scenarios on the mighty 5.9 CTD.
Bottom line up front: for even a mild performance application (say 450hp) where you want to be able to *continuously* use your power, twins are a must. Not a nice-to-have, but really a must. Yes, that’s a weird thing to say in the day and age where single turbos have become better than ever, and many guys are running them in lieu of twins. But I want a truck where EGT is NEVER an issue- even towing at 6000 feet on a warm day. I want the truck to run CLEAN and LEAN- even with big injectors and power modules and all that. I want perhaps a tiny little puff of smoke at launch and be nearly instantly into the boost. Hence, twins are mandatory for the following reasons:
The first, biggest reason is pressure ratio. A centrifugal compressor simply can’t compress to high ratios efficiently with only one stage. Yes, a large single like the SPS66 or Silver Bullet can make power. But that’s like saying that you can do dental work with a die grinder—it’s simply crude and inefficient. I want to avail myself of modern technology to the greatest extent.
Secondly, the nature of my 24V HO is not conducive to high performance with a single turbo (keeping in mind Hohn’s definition of performance: clean and lean, nary an EGT issue, broad area under the curve, and zero reliance on water, meth, or nitrous). The torque rise of the 5.9 is insane, meaning that the airflow demands change WILDLY in a very narrow RPM range.
Referring to a rather suspiciously inflated dyno run I once made, I observed that the torque curve (not the HP curve) was basically a big mountain with 2000 rpm at the peak. Torque rose incredibly quickly from 1400-2000 rpm, but FELL OFF FAST AS WELL. The 900+ lb ft peak at 2K rpm corresponds to about 340hp. If I could have held on to that 900lb-ft up to 2800 rpm, I would have had 462hp! Instead, my torque fell off quickly, and HP barely squeaked another 30-40hp higher over the range of 2000-2800rpm. The reason for this is beside the point (camshaft grind, imo).
The lesson from this is that you need to preserve the bottom of your torque curve to have true drivability. A large single that doesn’t spool until 2000rpm is completely out of the question if performance (as I define it) matters. With my 6-speed transmission, I simply cannot live with only 800rpm of useful powerband. I need to be able to roll into it at 1600rpm or so, and instantly have brutal torque and little to no smoke.
[FONT=Calibri][SIZE=3]The rapid tq rise on bottom is mostly what rules out a single. Plotting the HP requirements and calculating air demands tells the story. Keep in mind that I’ve assumed you want low EGT and essentially zero smoke.
Bottom line up front: for even a mild performance application (say 450hp) where you want to be able to *continuously* use your power, twins are a must. Not a nice-to-have, but really a must. Yes, that’s a weird thing to say in the day and age where single turbos have become better than ever, and many guys are running them in lieu of twins. But I want a truck where EGT is NEVER an issue- even towing at 6000 feet on a warm day. I want the truck to run CLEAN and LEAN- even with big injectors and power modules and all that. I want perhaps a tiny little puff of smoke at launch and be nearly instantly into the boost. Hence, twins are mandatory for the following reasons:
The first, biggest reason is pressure ratio. A centrifugal compressor simply can’t compress to high ratios efficiently with only one stage. Yes, a large single like the SPS66 or Silver Bullet can make power. But that’s like saying that you can do dental work with a die grinder—it’s simply crude and inefficient. I want to avail myself of modern technology to the greatest extent.
Secondly, the nature of my 24V HO is not conducive to high performance with a single turbo (keeping in mind Hohn’s definition of performance: clean and lean, nary an EGT issue, broad area under the curve, and zero reliance on water, meth, or nitrous). The torque rise of the 5.9 is insane, meaning that the airflow demands change WILDLY in a very narrow RPM range.
Referring to a rather suspiciously inflated dyno run I once made, I observed that the torque curve (not the HP curve) was basically a big mountain with 2000 rpm at the peak. Torque rose incredibly quickly from 1400-2000 rpm, but FELL OFF FAST AS WELL. The 900+ lb ft peak at 2K rpm corresponds to about 340hp. If I could have held on to that 900lb-ft up to 2800 rpm, I would have had 462hp! Instead, my torque fell off quickly, and HP barely squeaked another 30-40hp higher over the range of 2000-2800rpm. The reason for this is beside the point (camshaft grind, imo).
The lesson from this is that you need to preserve the bottom of your torque curve to have true drivability. A large single that doesn’t spool until 2000rpm is completely out of the question if performance (as I define it) matters. With my 6-speed transmission, I simply cannot live with only 800rpm of useful powerband. I need to be able to roll into it at 1600rpm or so, and instantly have brutal torque and little to no smoke.
[FONT=Calibri][SIZE=3]The rapid tq rise on bottom is mostly what rules out a single. Plotting the HP requirements and calculating air demands tells the story. Keep in mind that I’ve assumed you want low EGT and essentially zero smoke.
08-10-2007, 01:51 PM
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Here’s where we start to get into some numbers, so I’ll try to make it relatively painless.
We start by asking ourselves how much HP we want, and WHERE we want to make it. Making big power at lower (read: streetable/towable) RPM is VERY tricky, and is further aggravated by the small 5.9L displacement. My dyno run told me that if I make peak tq at 2000rpm, and my overall HP peak won’t be much higher than the HP I made at 2k rpm.
So, we “center” ourselves around this RPM. It just so happens that we end up with about 700rpm of useful range on either side (1300-2700rpm). For my personal goal, I want to make 500hp at 2000rpm. Making more at higher RPM would be nice, but that’s just gravy. Remember, I’m focusing on area under the curve and building the broadest possible powerband. 500hp at only 2K rpm is very ambitious for a 24V HO truck, as that means making over 1300lb-ft at this rpm!
Making 500hp and doing it with no smoke and perfectly manageable/sustainable EGT takes a lot of air-- 73lb/minute to be exact. Cramming 73LB/min into a 5.9L engine at only 2000rpm is not easily done. In fact, it’s going to take around 70psig of boost to do so (assuming you can get intake temps down to 130°F).
A single turbo would have to operate at a pressure ratio over 7:1! Any single turbo operating at that high of a pressure ratio is going to be terribly inefficient—60% at the very best. This means that our poor single turbo is beating itself up making 70psi, and it’s spitting out air that’s over 700°. Now, can you expect an intercooler to cool 70 POUNDS of air a minute from 750 degrees down to 130 degrees? Never going to happen!
Now, at higher RPM, the job of a single turbo gets a little easier. At 2800rpm, it only takes 47psi of boost (assuming again you can get intake temps down to 130°). Still, the turbo is operating at a PR of over 5.2:1, and at a charitable 65% efficiency, compressor outlet temps are still 530°. Can we cool 73lb/min from 530° down to 130°? It’s still pretty unlikely, even with an upgraded aftermarket CAC.
We start by asking ourselves how much HP we want, and WHERE we want to make it. Making big power at lower (read: streetable/towable) RPM is VERY tricky, and is further aggravated by the small 5.9L displacement. My dyno run told me that if I make peak tq at 2000rpm, and my overall HP peak won’t be much higher than the HP I made at 2k rpm.
So, we “center” ourselves around this RPM. It just so happens that we end up with about 700rpm of useful range on either side (1300-2700rpm). For my personal goal, I want to make 500hp at 2000rpm. Making more at higher RPM would be nice, but that’s just gravy. Remember, I’m focusing on area under the curve and building the broadest possible powerband. 500hp at only 2K rpm is very ambitious for a 24V HO truck, as that means making over 1300lb-ft at this rpm!
Making 500hp and doing it with no smoke and perfectly manageable/sustainable EGT takes a lot of air-- 73lb/minute to be exact. Cramming 73LB/min into a 5.9L engine at only 2000rpm is not easily done. In fact, it’s going to take around 70psig of boost to do so (assuming you can get intake temps down to 130°F).
A single turbo would have to operate at a pressure ratio over 7:1! Any single turbo operating at that high of a pressure ratio is going to be terribly inefficient—60% at the very best. This means that our poor single turbo is beating itself up making 70psi, and it’s spitting out air that’s over 700°. Now, can you expect an intercooler to cool 70 POUNDS of air a minute from 750 degrees down to 130 degrees? Never going to happen!
Now, at higher RPM, the job of a single turbo gets a little easier. At 2800rpm, it only takes 47psi of boost (assuming again you can get intake temps down to 130°). Still, the turbo is operating at a PR of over 5.2:1, and at a charitable 65% efficiency, compressor outlet temps are still 530°. Can we cool 73lb/min from 530° down to 130°? It’s still pretty unlikely, even with an upgraded aftermarket CAC.
08-10-2007, 01:52 PM
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Since higher RPM is an easier problem to solve, we need to look at the lower RPM scenario to see what we’re up against. For ease of illustration, let’s say we have 500 lb-ft at 1500rpm, and ramp up linearly to the 1300+ we need at 2K rpm. Considering just the range from 1500-2000rpm:
1500rpm= 500 lb-ft= 143hp= 21 lb/min of air
1600rpm= 663 lb-ft= 202hp= 30 lb/min
1700rpm= 825 lb-ft= 267hp= 39 lb/min
1800rpm= 989 lb-ft= 339hp= 50 lb/min
1900rpm=1149 lb-ft= 416hp= 61lb/min
2000rpm=1313lb-ft= 500hp= 73lb/min
In the span of only 500 rpm , we have more than TRIPLED the HP and the amount of air we require. Not only this, but we need to have boost pressures of 20psig at 1500rpm and 70psi at 2K rpm! No single turbo is going to do this. I suspect very few twins setups can do this.
A-HA! But there *is* a way to do this! Garrett has been kind enough to develop dual ball-bearing chargers that can spool rapidly while still delivering good airflow and reasonable compressor efficiency.
Let’s start by selecting the large primary.
In their catalog, Garrett recommends the GT4718R, a positively enormous turbo that will move 120lb/min at 4:1 PR with better than 70% efficiency! With its 88mm inducer 56 trim compressor, it moves a lot of air. It will support well over 750hp with very good efficiency. Unfortunately, even with the tightest A/R turbine housing (.96), it still wants over 40lb/min before it’s going to make any real boost. T
That means our small turbo will have to “go it alone” well past 40-lb/min if we select this turbo, and we just don’t need this much compressor. Instead, we need more spoolup.
If we go down a size to the GT4508R, we find a turbo that’s almost a PERFECT match for what we want. First, the required 73lb/min we want is smack in the middle of its center efficiency island. Second, it can still go up to 100lb/min and be efficient at PRs up to 4! Finally, the 1.01A/R turbine housing on this turbo will actually spool a lot sooner than the .96A/R on the bigger GT4718R, and be more efficient due to the higher A/R. So we’ve found the perfect primary for an HO truck seeking 500hp of street usability: the Garrett GT4518R with 1.01A/R housing.
But there’s still a problem. Yes, we’ve got our 73lb/min, but at what pressure from the big charger? The compressor map tells us we want a 2.5PR on the big turbo. We know from our math that we need a total PR of 7.2 at the worst-case peak of 2K rpm. Running the big charger at 2.5PR means the little one has to operate at (7.2/2.5=) 2.88. Now the “perfect” setup for twins is the PRs for the respective chargers that let them operate well within their efficiency range. Normally, you’d just take the square root of the total PR and split the workload 50/50 this way. But differently sized turbos are differently efficient. Bigger turbos are generally more efficient at higher PRs than smaller turbos are. So maybe we’ll shift the big turbo to 2.88PR and let the little one operate at 2.5PR.
1500rpm= 500 lb-ft= 143hp= 21 lb/min of air
1600rpm= 663 lb-ft= 202hp= 30 lb/min
1700rpm= 825 lb-ft= 267hp= 39 lb/min
1800rpm= 989 lb-ft= 339hp= 50 lb/min
1900rpm=1149 lb-ft= 416hp= 61lb/min
2000rpm=1313lb-ft= 500hp= 73lb/min
In the span of only 500 rpm , we have more than TRIPLED the HP and the amount of air we require. Not only this, but we need to have boost pressures of 20psig at 1500rpm and 70psi at 2K rpm! No single turbo is going to do this. I suspect very few twins setups can do this.
A-HA! But there *is* a way to do this! Garrett has been kind enough to develop dual ball-bearing chargers that can spool rapidly while still delivering good airflow and reasonable compressor efficiency.
Let’s start by selecting the large primary.
In their catalog, Garrett recommends the GT4718R, a positively enormous turbo that will move 120lb/min at 4:1 PR with better than 70% efficiency! With its 88mm inducer 56 trim compressor, it moves a lot of air. It will support well over 750hp with very good efficiency. Unfortunately, even with the tightest A/R turbine housing (.96), it still wants over 40lb/min before it’s going to make any real boost. T
That means our small turbo will have to “go it alone” well past 40-lb/min if we select this turbo, and we just don’t need this much compressor. Instead, we need more spoolup.
If we go down a size to the GT4508R, we find a turbo that’s almost a PERFECT match for what we want. First, the required 73lb/min we want is smack in the middle of its center efficiency island. Second, it can still go up to 100lb/min and be efficient at PRs up to 4! Finally, the 1.01A/R turbine housing on this turbo will actually spool a lot sooner than the .96A/R on the bigger GT4718R, and be more efficient due to the higher A/R. So we’ve found the perfect primary for an HO truck seeking 500hp of street usability: the Garrett GT4518R with 1.01A/R housing.
But there’s still a problem. Yes, we’ve got our 73lb/min, but at what pressure from the big charger? The compressor map tells us we want a 2.5PR on the big turbo. We know from our math that we need a total PR of 7.2 at the worst-case peak of 2K rpm. Running the big charger at 2.5PR means the little one has to operate at (7.2/2.5=) 2.88. Now the “perfect” setup for twins is the PRs for the respective chargers that let them operate well within their efficiency range. Normally, you’d just take the square root of the total PR and split the workload 50/50 this way. But differently sized turbos are differently efficient. Bigger turbos are generally more efficient at higher PRs than smaller turbos are. So maybe we’ll shift the big turbo to 2.88PR and let the little one operate at 2.5PR.
08-10-2007, 01:53 PM
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One SMALL little complication here. Our calculations have assumed (mistakenly) that our engine and our small turbo are both drawing in air that’s 130°. In other words, our calculations have assumed that we can take the hot air from the big charger and cool it down to 130° before it gets into the small charger, and then again that we can further cool the hot air coming out of the small charger back down 130° again. NO biggie, right?
The way twins are “normally” done, our big charger is actually discharging air that’s over 330° into the small turbo. Let’s say we run our turbos at 2.88 primary PR and 2.5 secondary PR. Our big turbo has compressed 100° air into 340°, and after the small turbo is done with it, that air is now over 590°, even with 75% compressor efficiency on both stages (center of maps). Still better than the single turbo, but we have a CAC challenge ahead of us for sure. If we don’t find a way to cool things off a bit, the amount of boost required will go way up, and things just get much worse much faster.
We simply don’t have the space for successive layers of air-to-air CACs, given the presence of the A/C condenser, engine radiator, and so forth up front. So yes, we’ll definitely upgrade the outclassed OEM CAC to the most modern, efficient CAC we can find (cool twist? Spearco?). Even then, we won’t be able to dump the heat needed to get 73lb/min from 590° down to 130 or as close to ambient temps as we can get. Further compounding the problem, we’ve self-imposed the complication that we can’t use any consumable method to bring temps down: no nitrous, and no water injection. This system has to be able to stand on its own.
We’ve talked about true INTERcooling before, trying to cool the intake between stages of compression. This no doubt greatly helps the efficiency of the second compression stage, though we’ve already given it the biggest help possible by sharing the workload with another compressor. Still the intercooling is the toughest challenge to make work in this application.
In the absence of true intercooling between stages, we end up with a huge skewing of the pressure ratios.
For example, we’ve been wanting to run our big charger at 2.88PR and little one at 2.5. But because the little charger is compressing HOT air, it ends up having to run at a MUCH higher PR to overcome the lack of inlet density. So even though we WANT to run the little charger at 2.5PR, we find that in reality it has to be running closer to 3.8PR to produce as much outlet air as if it had been running at 2.5PR ingesting nice cool air. This higher PR drops efficiency (compressor are less efficient at higher PRs) and also cranks up the heating of the outlet air.
The solution, then is to shift even MORE workload to the primary turbo. This will raise outlet temperatures, and in this case that’s a good thing. Why? Because we’re going to install a small water-to-air intercooler on the cold pipe, and the hotter the “cold” pipe, the more effective this intercooler will be. An air-to-water-CAC has engine coolant running through it at 180° or so, so while it can absorb a LOT of heat, it won’t be very effective at dropping temps of the inlet charge down below 200 or so. Still, this little cooler will end up dropping almost 270° off the small turbo’s discharge temps and reduce the PR that the small turbo must operate at.
So after we’ve added the intercooling, we see that we have the following:
Big charger takes in air at 100°, compresses it 3.5PR and discharges air at 380°. The water-to-air intercooler drops that 380 down to 200°. Our little turbo take in 200° air, compresses it 3.2PR and discharges air that’s 514°. Notice that our overall PR is much higher after we’ve accounted for the inefficiency of compressing hotter air. Our “real world” PR is almost 11:1, though on paper we only needed 7.3:1. This is with efficient turbos and interstage cooling!
Still, we find that we still have air that’s 514° coming out of the small charger. In this case, it’s looking like a good idea to use another water-to-air cooler. That will absorb a lot of the heat down to 240-250 or so, then after that the air will proceed to the air-to-air CAC mounted in the grille and hopefully dump enough heat to get down to near ambient temps.
As we add the additional intercooling, we’ll realize that the restriction posed by the coolers FURTHER raises the pressure the turbos have to deliver, raising the PR they operate at and increasing the heat that the coolers must dump.
I forgot to mention that the small charger in this twins setup is a Garrett GT3788R designed for our trucks and sold as the Stage 3 powermax kit. It has a map nearly ideal for this twins application, and compliments the larger GT4508R very nicely.
There you have my concept of “perfect” twins for a 24V HO truck: Garrett turbos, GT3788R over GT4508R and TONS of intercooling capacity. For you big-hp guys that don’t need instant spoolup, try a GT4088R over a GT4718R.
Gee, my head hurts.
The way twins are “normally” done, our big charger is actually discharging air that’s over 330° into the small turbo. Let’s say we run our turbos at 2.88 primary PR and 2.5 secondary PR. Our big turbo has compressed 100° air into 340°, and after the small turbo is done with it, that air is now over 590°, even with 75% compressor efficiency on both stages (center of maps). Still better than the single turbo, but we have a CAC challenge ahead of us for sure. If we don’t find a way to cool things off a bit, the amount of boost required will go way up, and things just get much worse much faster.
We simply don’t have the space for successive layers of air-to-air CACs, given the presence of the A/C condenser, engine radiator, and so forth up front. So yes, we’ll definitely upgrade the outclassed OEM CAC to the most modern, efficient CAC we can find (cool twist? Spearco?). Even then, we won’t be able to dump the heat needed to get 73lb/min from 590° down to 130 or as close to ambient temps as we can get. Further compounding the problem, we’ve self-imposed the complication that we can’t use any consumable method to bring temps down: no nitrous, and no water injection. This system has to be able to stand on its own.
We’ve talked about true INTERcooling before, trying to cool the intake between stages of compression. This no doubt greatly helps the efficiency of the second compression stage, though we’ve already given it the biggest help possible by sharing the workload with another compressor. Still the intercooling is the toughest challenge to make work in this application.
In the absence of true intercooling between stages, we end up with a huge skewing of the pressure ratios.
For example, we’ve been wanting to run our big charger at 2.88PR and little one at 2.5. But because the little charger is compressing HOT air, it ends up having to run at a MUCH higher PR to overcome the lack of inlet density. So even though we WANT to run the little charger at 2.5PR, we find that in reality it has to be running closer to 3.8PR to produce as much outlet air as if it had been running at 2.5PR ingesting nice cool air. This higher PR drops efficiency (compressor are less efficient at higher PRs) and also cranks up the heating of the outlet air.
The solution, then is to shift even MORE workload to the primary turbo. This will raise outlet temperatures, and in this case that’s a good thing. Why? Because we’re going to install a small water-to-air intercooler on the cold pipe, and the hotter the “cold” pipe, the more effective this intercooler will be. An air-to-water-CAC has engine coolant running through it at 180° or so, so while it can absorb a LOT of heat, it won’t be very effective at dropping temps of the inlet charge down below 200 or so. Still, this little cooler will end up dropping almost 270° off the small turbo’s discharge temps and reduce the PR that the small turbo must operate at.
So after we’ve added the intercooling, we see that we have the following:
Big charger takes in air at 100°, compresses it 3.5PR and discharges air at 380°. The water-to-air intercooler drops that 380 down to 200°. Our little turbo take in 200° air, compresses it 3.2PR and discharges air that’s 514°. Notice that our overall PR is much higher after we’ve accounted for the inefficiency of compressing hotter air. Our “real world” PR is almost 11:1, though on paper we only needed 7.3:1. This is with efficient turbos and interstage cooling!
Still, we find that we still have air that’s 514° coming out of the small charger. In this case, it’s looking like a good idea to use another water-to-air cooler. That will absorb a lot of the heat down to 240-250 or so, then after that the air will proceed to the air-to-air CAC mounted in the grille and hopefully dump enough heat to get down to near ambient temps.
As we add the additional intercooling, we’ll realize that the restriction posed by the coolers FURTHER raises the pressure the turbos have to deliver, raising the PR they operate at and increasing the heat that the coolers must dump.
I forgot to mention that the small charger in this twins setup is a Garrett GT3788R designed for our trucks and sold as the Stage 3 powermax kit. It has a map nearly ideal for this twins application, and compliments the larger GT4508R very nicely.
There you have my concept of “perfect” twins for a 24V HO truck: Garrett turbos, GT3788R over GT4508R and TONS of intercooling capacity. For you big-hp guys that don’t need instant spoolup, try a GT4088R over a GT4718R.
Gee, my head hurts.
08-10-2007, 02:07 PM
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Wow lots of thinking went into that!
Yet it still made sense to me for some reason? I would as you how good a setup it would be with my stock hx-35 and a b-2 steriod but i dont want your brain to hurt to bad lol.
Yet it still made sense to me for some reason? I would as you how good a setup it would be with my stock hx-35 and a b-2 steriod but i dont want your brain to hurt to bad lol.
08-10-2007, 03:07 PM
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I fixed some of the formatting. I wrote this in a Word doc and pasting it over made if very tough to read. Hope this is better.
**back to researching how I'm going to make water-to-air setup work**
**back to researching how I'm going to make water-to-air setup work**
08-10-2007, 03:22 PM
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Great read Justin!
Without the same thought process as you, I've arrived at the same conclusions...GT3788R/GT4508R would be better for driveability than my planned setup, but for a 12v where RPM is not a problem, It think I can live with the GT4088R/GT4508R setup without the fancy intercooling...I'll try chemical intercooling if needed!
Chris
Without the same thought process as you, I've arrived at the same conclusions...GT3788R/GT4508R would be better for driveability than my planned setup, but for a 12v where RPM is not a problem, It think I can live with the GT4088R/GT4508R setup without the fancy intercooling...I'll try chemical intercooling if needed!
Chris
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08-10-2007, 04:10 PM
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But I got hung up at the above! And while admittedly I only had a chance to read through once.... I think these #'s are almost impossible.
Hope I haven't said something stupid and missed where you address this difficulty? Will reread tonight!
Thanks for your efforts.
RJ
08-10-2007, 04:15 PM
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Hey, you already bought them, so who cares what I think? NO doubt using the stocker saves a lot of money..
S400 is proven, it's just the wrong animal for a performance application. The s400 is designed for heavy machinery, and spoolup is given very little consideration.
Far better (and more expensive) to start with a turbo designed for performance from the beginning (like a Garrett or Turbonetics).
But the S400 works well enough, and delivers good bang for the buck, no doubt.
JH
S400 is proven, it's just the wrong animal for a performance application. The s400 is designed for heavy machinery, and spoolup is given very little consideration.
Far better (and more expensive) to start with a turbo designed for performance from the beginning (like a Garrett or Turbonetics).
But the S400 works well enough, and delivers good bang for the buck, no doubt.
JH
08-10-2007, 04:24 PM
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Good read, Justin!
But I got hung up at the above! And while admittedly I only had a chance to read through once.... I think these #'s are almost impossible.
Hope I haven't said something stupid and missed where you address this difficulty? Will reread tonight!
Thanks for your efforts.
RJ
But I got hung up at the above! And while admittedly I only had a chance to read through once.... I think these #'s are almost impossible.
Hope I haven't said something stupid and missed where you address this difficulty? Will reread tonight!
Thanks for your efforts.
RJ
If you can't get the air cooled enough to get it down to 130º, then the amount of boost you have to have to get the 73lb/min goes up. For example, if you can only get temps down to 200º, then it's going to take 83psig to give you the same 73 lb/min. If you can only get temps down to 250º, then you need 90psi and so on. It's easy to see how you can end up chasing your tail, because the hotter temps not only need more boost, but more boost makes even more heat!
If you have a killer intercooling setup and can achieve ambient temps (underhood, let's say 100º), then you only need 68psi of boost.
If it sounds like this is a LOT of boost for the HP level-- well, it is. This is calculated using a lean fuel ratio that will be smokeless and have EGTs in the 1150 range. In addition, I'm calculating at 2000rpm, where the engine is swallowing air a lot slower than at 2800rpm or so-- hence you need more boost to cram the air in when RPM isn't helping you.
Justin
08-10-2007, 04:57 PM
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Have you given any thought to running a supercharger for the primary stage as opposed to the big turbo? You want instant spool up, and thats not going to happen with two turbos. Might be quick spool in comparison to others, but if you have the fuel for 1300 lb-ft, you're gonna have more than a puff getting on the throttle.
08-10-2007, 09:52 PM
#13
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For a 500rwhp rig, its been done to death. Buy the BD twins, they work well and low spool-up. If your thinking about supercharger, I would like to try a GMC 8-71 sized roots. Lots of air flow, low boost pressure though. If you run the super at 5 psi at idle, a 2-3 pressure ratio turbo will get you to 70psi+ range. I would go with a small a/r ratio on the turbo. Fast spools and you get to 50-60psi. I didn't read the stuff, its too long. Building the twin stuff is way harder than thinking about the prefect turbo. I just re-built my turbos, tons of work tweaking this and that. Imagine welding hot pipes and the air tubes. Lots of playing around. I could build one but it would take time to get it right. I would also keep you cold air tube small as posible. It takes time to fill-up the tubes. I was talking to PDR about building a twin BB garrett. I liked the super idea to but that more work than twins.
08-10-2007, 10:06 PM
#15
Registered User
Nothing new under the sun, Justin - your intercooling thoughts hearken back to last year's air-water heat exchanger using the existing water/meth system's pump, plumbing & reservoir... 
You're on the right track!
You're on the right track!