Things to consider: Sizing compounds!

[-Boostjunky-]

Ok, I hope to spark up an interesting conversation here, and get some good insight from members of the forum that have been down this road, and actually have some factual information (be it derived by personal experience, or by laws of physics). The topic of conversation is, as stated in the title, sizing compound turbo setups.

The way I understand things is that the drive pressures (which will cause high EGTs) are a HUGE concern when running compounds, and this due to the fact that you are running through 2 different turbine housings. One of them is going to be significantly smaller than the other, so this is the one that should be VERY carefully sized.

Now, I've been personally kicking around the idea of running compounds on my truck (it's a virtual certainty that I will be in the near future). In talking with a guy that works at Industrial Injection, I suggested that I would be interested in a final power output of around 800 whp (uncorrected @ 4,500 ft elevation).

In order to accomplish this, I figured an S480 would certainly be up to the task. I would REALLY like to keep the truck driveable and have as little lag as possible. As such, I thought about using a 62/71/14 as the small turbo in the setup.

What I'm hearing from my friend at II, however, is that this is a mis-match and that it will run HOT, and that a better match would be a 64/71/14. This intrigues me and has brought up the following questions/thoughts:

1) The primary determining factor of how hot the EGTs are going to get depends mostly on the backpressure (drive pressure) in the system (not taking into consideration fueling).

If this is true, then a 62/71/14 vs a 64/71/14 should yield the EXACT same drive pressures, being that the turbine wheel is the exact same diameter, and the turbine housing is the same size. I can only assume that the wastegate is also the same size for both turbos, and when it comes right down to it, how much the wastegate flows is going to be critical, since it will be full open once the big guy takes over.

2) The compressor housing size between the two (62 vs 64) is minimal. 2mm on the inlet, and virtually identical on the outlet. So, there should be little to no difference in restriction of the compressor housing.

3) If I can spool up more quickly, that will mean less time for heat to build up due to being overfueled while TRYING to spool up.

So, all variables accounted for, it would seem to me that this would be a win win situation, and that the 64 would only exaggerate lag and initial heat-up (during spool).

Ultimately, the power potential of the setup is determined by the big turbo, being that once you've spooled the big guy, the little guy is just along for the ride! He's no longer contributing to the "power making business end".

Am I correct in my observation, here? Or am I way off base? Can someone set me straight? I'd really appreciate any and all input!

[-Boostjunky-]

Bumpage for exposure!

[LReiff]

I think you got the basic concept!

[nwpadmax]

The point of compounds is to multiply the pressure ratios of each comrpessor and as such take advantage of operating both in the sweet spot of the compressor map.

Therefore your statement about the secondary being "along for the ride" is totally false.

[motoxracer42]

one of my friends is running a 62/71/13 over his s480.

[Big Swole]

I would tend to agree on the DP being the same due to the exhaust flow should be the same with the housings / turbine being the same....The compressor being 2mm bigger isn't going to make that much differnence in you EGT's down low...We're only talkin an extra 100-200 rpm dif. in spool....There are Several here that used to run the 62/71/13 over S480 (Which is what I run) and ungraded to the 64...They say the spool is better! Well, realistically it's probably just the same but they just missed there truck when it was down for the ungrade...lol

And EGT's down low / before spook isn't gonna be the issue...when you start making rpms, thus making high RP and fueling hard for a few seconds (like on the track), then your heat soak and EGT's will be the issue...These CR's are weird...Atomisation of the fuel is so great that it may seem cool by the EGT guage but crazy high inside the cylinder...Remember EGT's are Just that! Exhaust...If it all burnt in the cylinder then the gauge will show lower, but inside it toasty!!!

Nathan at MPI knows his Chit and that's the set I bought...If the 62/480 was a mismatch, he'd have told me....

But, if you looking for all out EGT control and good spool, Go with the 64...I kinda wished I had just for the extra air for future...

my .02

[-Boostjunky-]

Originally Posted by Big Swole

I would tend to agree on the DP being the same due to the exhaust flow should be the same with the housings / turbine being the same....The compressor being 2mm bigger isn't going to make that much differnence in you EGT's down low...We're only talkin an extra 100-200 rpm dif. in spool....There are Several here that used to run the 62/71/13 over S480 (Which is what I run) and ungraded to the 64...They say the spool is better! Well, realistically it's probably just the same but they just missed there truck when it was down for the ungrade...lol

And EGT's down low / before spook isn't gonna be the issue...when you start making rpms, thus making high RP and fueling hard for a few seconds (like on the track), then your heat soak and EGT's will be the issue...These CR's are weird...Atomisation of the fuel is so great that it may seem cool by the EGT guage but crazy high inside the cylinder...Remember EGT's are Just that! Exhaust...If it all burnt in the cylinder then the gauge will show lower, but inside it toasty!!!

Nathan at MPI knows his Chit and that's the set I bought...If the 62/480 was a mismatch, he'd have told me....

But, if you looking for all out EGT control and good spool, Go with the 64...I kinda wished I had just for the extra air for future...

my .02

Thanks for your input!

Originally Posted by nwpadmax

The point of compounds is to multiply the pressure ratios of each comrpessor and as such take advantage of operating both in the sweet spot of the compressor map.

Therefore your statement about the secondary being "along for the ride" is totally false.

Ah Hah! I had wondered about this! In fact, in the beginning, I was dead set on this theory being the case. So, in light of this, I have a few questions.

I know how to read compressor maps, and figure out pressure ratios and lbs/min airflow and such. But, what I Have wondered is the following.

Let's say you are running the 64 as the small turbo, and at a PR of 2.5 (at my elevation, this would be about 18.75 gauge psi rather than the 22psi it would be at sea level). Assuming an RPM of 2500 rpm, a VE of 70%, and taking the low elevation into consideration, the engine alone (without boost) would pump 10.6 lbs/min of air. Multiply that by 2.5 for the PR, and you get 26.7 lbs/min of air. So, at this point (18psi boost), the turbo is supplying enough airflow for about 267 hp, and by all rights, it should be in it's sweet spot of the compressor map. I don't know for certain where the sweet spot really is, as I don't have a compressor map to look at. But let's assume for argument's sake, that it is the sweet spot.

Now, let's add the big guy in the mix, and run him in his sweet spot (let's assume that the sweet spot for the big guy is at a 3.5 PR, or 31.25 psi at my elevation). Now, how do we figure the airflow for the small turbo at this point? By feeding the small turbo with a 3.5 PR, does that multiply it's airflow by 3.5? Or, in other words, the 26.7 lbs/min now becomes 93.45 lbs/min? Or is it not that simple?

[-Boostjunky-]

One other thing that has me a bit confused on this theory is, if your wastegate is set to open at 2.5 bar (in my case, 18.75 psi), once the pressure in the system surpasses that, and I'm seeing upwards of 70 psi by multiplying the 2 turbo PRs, isn't the small turbo practically defeated at that point? I mean, the wastegate is FULL OPEN! I know there will still be some drive pressure for it, since the wastegate obviously won't flow enough to bypass the turbine completely.

But, how do you tune the small turbo to continue running at the desired PR?

That's where I got myself hung up before, and finally dismissed the idea of multiplied PRs from both turbos. But if this is really the case, that the turbos are multiplying PRs and airflow, then it would make total sense why a slightly larger secondary turbo would help, and a smaller one would hurt.

[700rmk]

good stuff here

[Big Swole]

Originally Posted by -Boostjunky-

Thanks for your input!



Ah Hah! I had wondered about this! In fact, in the beginning, I was dead set on this theory being the case. So, in light of this, I have a few questions.

I know how to read compressor maps, and figure out pressure ratios and lbs/min airflow and such. But, what I Have wondered is the following.

Let's say you are running the 64 as the small turbo, and at a PR of 2.5 (at my elevation, this would be about 18.75 gauge psi rather than the 22psi it would be at sea level). Assuming an RPM of 2500 rpm, a VE of 70%, and taking the low elevation into consideration, the engine alone (without boost) would pump 10.6 lbs/min of air. Multiply that by 2.5 for the PR, and you get 26.7 lbs/min of air. So, at this point (18psi boost), the turbo is supplying enough airflow for about 267 hp, and by all rights, it should be in it's sweet spot of the compressor map. I don't know for certain where the sweet spot really is, as I don't have a compressor map to look at. But let's assume for argument's sake, that it is the sweet spot.

Now, let's add the big guy in the mix, and run him in his sweet spot (let's assume that the sweet spot for the big guy is at a 3.5 PR, or 31.25 psi at my elevation). Now, how do we figure the airflow for the small turbo at this point? By feeding the small turbo with a 3.5 PR, does that multiply it's airflow by 3.5? Or, in other words, the 26.7 lbs/min now becomes 93.45 lbs/min? Or is it not that simple?

Dang Dude!! Now your talkin over my head!!!

I musta sounded like a freakin Pre-Schooler to ya with my post!!!

I need to be learning from you I guess........I just know the basics and am trying to absorb all the other high tech stuff....
Sorry bout that!!

I just know that exhaust blows the wheel and then that make the other wheel turn and that wheels blows air into the tube which goes into a cooler thingy and then the air got into a horn thingy and makes the fuel go BOOM!!!


^ Thats my tech stuff!!!!

[White Knight]

Boostjunky you may be interested in reading through this thread: compound numbers - Competition Diesel.Com - Bringing The BEST Together The disscution gets a little jumbled with the arguement over mass flow vs volume flow but Charles, CSM diesel, and Diesel Freak give a lot of good information

The mass flow when everything is up and running is generated by the primary (big) turbo and regulated by the engine.

[lcaad1249]

Originally Posted by -Boostjunky-

One other thing that has me a bit confused on this theory is, if your wastegate is set to open at 2.5 bar (in my case, 18.75 psi), once the pressure in the system surpasses that, and I'm seeing upwards of 70 psi by multiplying the 2 turbo PRs, isn't the small turbo practically defeated at that point? I mean, the wastegate is FULL OPEN! I know there will still be some drive pressure for it, since the wastegate obviously won't flow enough to bypass the turbine completely.

But, how do you tune the small turbo to continue running at the desired PR?

That's where I got myself hung up before, and finally dismissed the idea of multiplied PRs from both turbos. But if this is really the case, that the turbos are multiplying PRs and airflow, then it would make total sense why a slightly larger secondary turbo would help, and a smaller one would hurt.

My understanding of this is, on my setup (MPI) my wastegate on my top turbo is operated solely off of drive pressure. The more pressure, the farther it opens. Now having 2 boost gauges (primary and compounded boost) you can adjust the wastegate so that it opens to the optimal amount under your highest boost situation and achieving a roughly 1 to 2 ratio by bypassing more or less exhaust. Hope this helps.

[-Boostjunky-]

Well guys, I really appreciate the info and input from this thread. It's been highly informative.

Unfortunately, I will be unable to go with compounds due to the cost. I needed a new turbo A.S.A.P. since the one on the truck had lots of shaft play, and the wheel was running into the housing. So, I went ahead and pulled the trigger on a Silver Bullet to hold me over until I can afford (if I ever can...lol) the twins.

I took the truck to the dyno today, also, and it put down 637 whp, and approx. 1400 lbs/ft torque (the dyno tachometer wasn't working, so I had to hand calculate it out). These numbers are UNCORRECTED @4500 ft elevation. The SAE corrected numbers were 732 whp and approx. 1600 lbs/ft torque.

Overall I'm pretty happy with the outcome. I have some sheets from the dyno that I'll have to scan.

Thanks again, everyone!