Hohn, get us some part# on the turbos. I want to look into them, as I might try one of them in a set-up, one day, soon.

 

All I have is the Bullseye P/Ns: http://www.bullseyepower.com/S400.asp

 

Wew pricey stuff. I think garrett sells a better turbo for the same $$$. You could even get them in a ball bearing. Thanks for the link. A local turbo shop was gonna sell me a S400 75mm for $625. Looking at these pricey, I should go pick-up a few.

 

It wouldn't respond that well at all, I'd think. Not in the "Street" sense I'm imagining. The vast majority of the time around town, you'd see little to no boost in the cold pipe.

From what I can figure for the hx35/gt45 combo, you'd just be starting to light the GT45 when the top charger hits 30psi and rpm is sweeping past 2100. So you're generally not going to see these conditions until you get out on the hwy. Nor would you need big boost anyway until you put some load in the engine in the upper gears.

How often do you have EGT problems at light loads at 40mph?

If a primary shows any appreciable boost in the cold pipe with the cruise set at 65 running empty, then the primary is way too tight and/or too small-- at least with the low primary PRs I propose. Now, if your yanking a heavy fifth wheel uphill at 65, I'd expect to see some primary boost.

The idea is that the primary doesn't even wake up until about 20psi total boost. As boost comes up from 20-30psi, the primary lights and you see the boost rocket from 25 or so straight up to 50-55psi. With intercooling about as good as OEM, you'll be able to support 600hp with only 50psi of boost. With my proposed setup, you'd never have more the 15psi in the cold pipe. Nor would you need more. With a large primary like an 88mm inducer size charger, you could run a small secondary like a 62mm s300 and have a good towable "Street" twins setup with 15psi in the cold pipe. Simply swapping the top charger to a 66mm size unit will give up to 200 more hp support from the twins setup-- with the same large primary, only it will probably have more like 20psi in the cold pipe. (the larger secondary drives the primary harder and even though the secondary is consuming more air, this is more than offset by the increased drive energy to the primary turbine).

A big primary won't light on a DJ most of the time. Very few shops are equipped with the heavy DJ rollers that have enough inertia to simulate the heavy weight of our trucks. This is part of the reason why nitrous makes such impressive numbers on the dyno. On the street, the nitrous hp isn't as great as what the dyno told you it would be. The light loading of the intertial dyno exaggerates the difference between spray/no spray hp.

The real beauty of a huge primary is not just that they flow more-- but that they have a LOT more useable range. Compare how wide the map is at 2:1-- the 4202 is about 46# wide, the 4508 is about 77# wide, and the GT4718 is over 90lb wide at 2:1. (this is a function of sheer size-- the massive GT60 is over 120lb/min wide-- this is a range of over 600hp, all at the same PR!)

 

We have a similiar philosophy on twins, Justin - a small secondary provides all the air needed to produce the power our trucks need for "normal" use... large primary doesn't do much (or get in the way) until the fueling is set to "kill" for big dyno HP or fast trap speeds.

Mike, we have an S474 "Big Brother" variant, from what I can tell looking at the part # - it'd be a 74/74/26 in nomenclature... A/R is a generous 1.32

I know it's too small for our purposes - if I had the truck 1st I would've spec'd an S48X, maybe even a Phat Bastard or Super Dave. I don't mind using a primary a bit on the small side, since we'll be pushing "high quality" air to run the number while keeping max driveability.

Nitrous will go into the pre-CAC pipe (where we usually install it) for dyno HP only... water/meth will be used for DD duty and towing (yes, this will still be a work truck! ).
I'm spending the $$$ for thermal management on this project - keep the hot stuff hot & the cold stuff cold... I want hot exhaust, cold boost & lots of cool fuel (FASS 250 with -8 SS line through fuel cooler to twin CP3s - injectors will likely be in the Broken Arrow range).

I could wring 800HP out of the current twins, but unfortunately (??) that's a couple hundred short of the goal.

 

some interesting info!!

 

Sweet, what you doing for the head? Which cam you gonna run? Keep us informed. On set-up.

I find that the big primary makes things sluggish, yes once they lite hang on, you can't get one big enough. I bet for drag racing you could launch on the primary then cutting back primary flow, thus balancing out the exhaust flow, pushing up the boost. For racing thats best way to get your primary is working too. As for a street rider, the big primary would just smoke everyone out as nothing lites would sluggish as the the bigger you go, the more hinderance to get the interia rolling to get everything rolling. Most new dynojets I've run, my primary is doing nothing. I want to build a 6.7L twin turbo, twin cp into my 02. Of course its a street driver, you have to use them to have to want to race them.

 

For the pressure I meant using cold pipe pressure and total boost and then correcting it using a TDP/CDP ratio. That should give me hot pipe pressue and total drive pressure. This is because boost pressures are so easy to calculate / measure and EGT is commonly measured.

If you are interested Diesel Freak tossed this tidbit out some time ago. Finding the values for matching PR's.

Total Boost = ((( PR1 ) ^ 2 ) x P1 ) - P1 )

PR1 = Primary Pressure Ratio
P1 = Compressor inlet or Patmos

From there you can figure out everything else. In my spread sheet I just manauly enter PR1, through a trial and error method, until the "Total Boost" calculated by the formula = the target boost value I need to supply the correct lbm/min. The spread sheet also does the PR of the secondary so I can see when they match up.

NOW.....

What can you tell me about corrected air flow? Would it be reasonably close to the same formula to calculate the corrected gas flow?

I am trying to figure that out so I can determine where the secondary would be running on its efficientcy map. Calculating PR2 is easy, but I can't figure out the lbm/min crossing point to plot efficientcy.

Jim

 

You working with actual flow measurements or trying to predict your expect air inlet requirements. You can back calculate the lbs/min from you're expected horsepower. It get actual is more tricky, I saw that garrett sells a turbo tach set-up. The turbo rpm, the pressure ratio from inlet to discharge, and a dyno run, you could almost accurately plot you're position on the compressor map. Thats of course if the compressor maps are accurate to begin with...

 

Mike, it'll have a Helix 2 cam - I'd love to pull the head & work it over, but part of this project's appeal is racking up big #s without "serious" engine work... which I don't consider a cam swap to be. Wish I had the R&D time in the budget to advance it a tooth or two, so we could lean on the motor harder with water/meth and keep a bit of cylinder pressure on the pistons & out of the secondary turbine.

Not sure yet if we'll put our own crankcase evacuation system on or buy one outright; otherwise the truck will also end up with 625+ studs, F1 springs, FluiDampr & one of our intake horns.

 

Jim, I think for our purposes gas flow and air flow are one and the same. The laws of partial pressures apply here, so I can't see why ideal gas calcs wouldn't get you within 99% of actuality. That's good enough for me.

The formula from DF doesn't seem valid to me because the total boost would be a function of PR2 (which isn't in there) as well as intercooling temp drop (which also isn't in there).

Cold pipe pressure (or primary compressor discharge pressure) is somewhat misleading to calculate-- or more specifically, extrapolating calculations or theory into the real world.

For example, let's say you decide (as I have) that a big primary running at 2:1 PR is what you want. HOw do you know that you will get 2:1 and not 1.5, or 2.5?

Well, it's complicated. The primary CDP is partly a function of how big the secondary is-- if the secondary consumes more air, we'd expect the coldpipe pressure to go down. But the bigger top charger also drives the primary harder (turbine side), which will make it pump more air. SO which is it? Does a bigger primary make the cold pipe pressure higher or lower?

And how do we know where on the compressor map our secondary is operating?

Because the primary and secondary have to much interplay, I start my analysis at the engine-- which has known displacement, known VE, known discharge temps, etc.

Then you can move to the secondary and analyze it in light of the engine's knowns.

Lastly, I'd move on to the primary in light of the previous two stages.

At each stage, I'd apply a "fudge factor" to square my calculations with observed results. For example, what if I calculate by a certain turbine map that I should never be able to spool a certain turbo, yet I know anecdotally that the turbo in question will spool well in the intended application. In this scenario, I'd go back and figure where my math was wrong. The real world ALWAYS trumps theory and calculation, imo.

For example, let's calculate the mass flow for my truck at 1800 rpm. Assuming .95 VE, I get 177.6 CFM, which is 13.5lb/min. Can I spool a Stg 3 Garrett GT37 with this setup? The turbine map says the GT37 needs 20 lb/min to even get on the turbine map at a PR of 1.25. It would appear that we can't spool this turbo.

Yet we know from many accounts that that this turbo will very easily spoolup, and much earlier than 1800rpm.

How can this be? Where are my calculations wrong?

First, we didn't "correct" the gas flow to account for EGT. At 800F, our 13.5lb/min turns into 21lb/min.

So what to make of the turbine map? According to my "calculated" spoolup on the turbine map, our corrected 21lb/min of engine-only mass flow should only generate 1.25PR at the turbine, or about 3.5psi of boost onthe cold side, assuming 1:1 for MAP:TIP.

Yet we know anecdotally that we can sure as heck get way more than 3-4psi of boost from a GT37 at 1800rpm and 800F EGT.

Why the disparity? What is missing that "Corrected" flow didn't cover?

First, and biggest imo is the affect of engine load. Load matters because it affects exhaust velocity, which contributes to the drive energy the turbine sees. Why does a turbo spool so much harder in the upper gears when you can't get any boost at WOT in neutral or other low/no load conditions? I can see no way to quantify the affects of engine load on turbo spoolup. All I can think of is the "correct" the calculated results based on observational experience.

The second factor is that the most crucial part of spoolup is what happens before it's actually on the turbine map. Once you've gotten to the beginning of the map, you've initiated the snowball effect that will guarantee spoolup. The turbine map may show you what happens from 1.25PR on up, but it's what happens between 1 and 1.25PR that is absolutely critical and absolutely unknown. So the turbo is already "spooled" at the far left of the plot, typically 1.25PR with the Garretts.

The bottom line is that you can spool a bigger charger than the "calculated" info would show-- at least the way I'm (mis)calculating.

The REAL value of all this calculating is for comparative analysis, not absolute determination. For example, if I compare the turbine map for two turbos and the larger one needs 10% more corrected mass flow, then I can take the known observational results for the smaller one and accurately extrapolate to the larger turbo. So while my math lies to me and tells me that I can't spool a GT37 when I easily could, it tells the truth in when I look at the maps and see that a GT40 with a certain housing will need 10% more flow to spool to the same point. So if I first saw boost with the smaller charger at 1500rpm, I'd now need (1500 +10%)= 1650rpm to get to the same point. *THIS* comparison is valid, imo.

(continued next post)

 

I like the analysis and all the info but what it comes down to is what will be the most driveable turbos for a given hp # . I think most trucks are used for towing at some point and the vast majority of performance turbos do just that but are lacking in " peek towing performance ". The big rigs are "on boost " with no load just driving around . Thay are around 15 lbs at 55 miles per hour or more . So if we can do that with a twins set in are trucks that would be an improvement over the 3 to 6 lbs that the "performance " turbos give us . So just what would you think the most efficient " twins " set up would drive like ?? and dont for get it would have to make 650 to 800 hp of peek flow on the map . My twins [ Bullseye power ]S-300 S-400 type do very well [ stock vp pump 12.05et at 111.29mph 6850lbs ] but if a truck like mine wonts to hook to a 19,000lb RV trailer and tow in over drive with 3;55 gears at say 60 mph wouldnt it be nice to have 25 lbs of boost on flat land just breathing on the fuel pedal. Now that to me would be a 'Performance enhancement"

 

So let's continue with our analysis. We can easily crunch the numbers and determine where on the operating map a single charger might be. For example, if I want enough air for 300hp at 2K rpm, I plug and chug and get an air requirement of 35lb/min. Based on some other calculations, I know that cramming in 35lb/min to this engine will require 27.5psig of boost, assuming I can get intake temps down to 150F (CAC efficiency is big in determining how much pressure is required to force feed a given amount of mass).

So under these conditions, I'd plot ~42psia, or a PR of ~3.8 on the compressor map at 35lb/min. This is how much air I *need*.

But it doesn't tell me how much air I HAVE! This varies a lot with turbo and housing choice. Most stock turbos will drastically overspool and you have way more air than you need. This is evidence partly by the low(ish) EGTs produced at cruise and part throttle.

Predicting how much air you might HAVE is pretty dicey, and I honestly don't have a good idea of how to do this. Measuring the intake temps and pressures, and calculating the mass flow is easy-- the OEMs have been doing this speed-density calculus for fueling calculations for years. But measuring the air you have with a given turbo and *predicting* what you will have with a given turbo are very different matters!

Fortunately, we don't really need to predict what a given secondary will do, because the engine it's bolted to remains the same (ignoring cam swaps, etc).

To large extent, we don't care which turbo is making the boost. For the most part, if we have 30psi at 150F in the intake, that what you have-- the turbo supplying it doesn't matter. The significance only comes in reserve-- at what point does the turbo run out of air?

Running out of air is determined by 1) operating PR 2) RPM range 3)VE.

Since I intend to keep the turbo within its operating PR, I will just calculate the airflow based on the operating PR I can dictate.

Since I don't need to run super high PRs on the secondary, I only need a compressor that can deliver 53lb/min at a PR of 3. (I don't plan for RPM over 2900).

So, what compressor flow will the primary need to deliver at varying PRs to feed this particular secondary?

Oddly enough, the answer to that has a lot to do with your intercooling setup. If your CAC can maintain intake temps with twins that are the same as the intake temps with the single, then you get a true 1:1 return on investment.

For example, let's say you have 150F intake horn temps with a single charger that flows 53lb/min at 2900rpm. If you take that same single turbo, mount it in the upper position of a compound setup, and feed it with a PR of 2, IF you can keep the intake temps in the horn at 150, you will actually DOUBLE the mass flow! That's right, you now have over 100lb/min going into the engine!

Conversely, if your CAC cannot remove any of the extra heat the twins setup introduces to the air charge, then you'll only get a measly 45% more airflow, turning your 53 lb/min into 76lb/min.

So, the question of how much airflow the primary must provide is partly a function of how effective the charge air cooling is. If you don't have very good charge cooling, then a larger turbo might not be worthwhile, because total mass flow will drop rapidly without effective cooling.

Or, you simply have to crank up the operating PR of the primary to deliver the required mass. This is very undesireable-- the higher PR of the primary will reduce the efficiency of the setup and higher PR is always an exercise in diminishing returns.

So, in this scenario, I would choose a primary, if possible, that could cover the whole spectrum from "no additional heat removal" to "complete heat removal" at a PR of 2. This would give us a mass flow range of 76-106lb/min at a PR of 2.

Look familiar?






Who would have thought that a turbo as big as the GT45 would be a great primary for even a lowly stock turbo??

If it will work so well with the stock charger, why would we use something smaller?


That's my analysis-- I'm sure I missed a bunch, but I would love to put my theory to the test.

jmo

 

You can't build an "efficient" set of twins for big power and for great towing. This is simply because the smaller secondary needed for towing response will be too restrictive to let the the primary deliver the goods. If you want a twins setup that will make 800hp on fuel only, you can't do it without inducing some lag.

A twins setup with a smaller secondary and larger primary will give you a little more of each-- faster spoolup when you first get going, lots of air when you need a little more. But it can only deliver so much because it can't push the primary very hard (the small turbo limits flow). So if you want more power with a setup like this, just swap the top charger to something bigger, and now the same primary that made ~600hp (stock charger) can now make ~700 (htt62 on top) or ~800 (htt66 on top)

Your complaints about being "off boost" like most performance turbos are well founded. They simply aren't optimal for towing. They can be used for towing in many cases, but they require some compromises or adjustments.


jmo

 

My set-up works pretty nice for towing. You just have too keep the twins spooled up. I tow a trailer around 12k range and I use OD. What size of injector you running? I like to put in a smaller injector so the truck has to rev (more mass flow), so you have tons of cool air to keep the egt's down. You could also lower your wastegate pressure for the secondary some, to load the primary more. You have to do some tweaking to get everything working right.