turboman1

Thanks guys for the information it has been really helpful

My next question is the wastgate system. The one before the secondary turbo does it need to be piped in to pass the secondary and enter before the primary?

Thanks
Adam

Mike Holmen

If there is a wastegate on the secondary turbo and its internal wastegated, it will bypass the secondary turbine and go into the primary exhaust (transfering energy from one to the other). You can tube in the wastegate and smoothen out the exhaust flow and enlarge too custom tune your ride. At that point its up to the guy's driving style. You have to tweak to understand me more. I like the hard wot pull that the twins give, so I try to work the primary more other would. For me I would run larger wastegate flapper and massage the turbine wastegate some. This will cause the primary to lite faster once the secondary starts to dump exhaust. You would have to play with them until your happy. If you do lots of city lights and tow, you might want something different. Some guys just like single turbo's??

HOHN

One problem I've been wrestling with is the wastegate setup. In particular, what happens when you dump an external gate in front of the big turbo.

The problem with this setup (imo) is that when you dump that wastegate flow in between the turbines, you increase the backpressure on the top turbine, or you might, I should say.

Let's say you have a large external gate set to 25psig. It will start to crack open around 20psi or so.

The more boost you make (total), the farther the gate will open as it tries to maintain the max of 25psi to the small turbine. What happens you you pass 50psi total TIP?

At that point, you now have 25psi going to the small turbine, and 25psi (or more) getting bypassed around it. You can probably see where we are ending up.

With this setup, once you pass 50psi of TIP, you have a negative delta P across the top turbine, and the gas flow will try to reverse flow backwards through the top turbine. Or so it would seem to me. This will cause the top charger to no longer multiply boost, and and the boost pressure will try to drop to where it's equal to the coldpipe pressure.

But it will never get that far, because once the delta P onthe top charger goes back to the right direction (i.e., boost falls below 2x wastegate setting), it will correct itself, then repeat the cycle.

So the lesson I see here is that the wastegate can not be allowed to flow as much as the top charger will at the same pressure. This will always keep the bypassed flow to less than half the total flow.

Either the wastegate can be sized such that it's smaller in area than the turbine (which is more math than I'd like to do), or the wastegate can be plumbed in a way that forces it to be tougher to open.

I'd choose the latter. Instead of venting the "boost control" port to atmosphere, I'd vent it to the cold pipe (assuming a Tial type configuration). This way, the wastegate sees a rising resistance pressure and TIP will always be more than 2x the bypassed flow.


Also in my reading up on this business, I've decided that maybe you really can get away with not using a wastegate at all in a twins setup. But in order to pull it off, you have to have the turbine maps for the hotsides of BOTH turbos with all of the housing options available. Then, just choose the configuration based onthe maps.

If I lost you on this, let me explain. Let's say you are running the top charger at a 3:1 PR and the primary at 2:1 (a good idea, since it helps that delta P thing we talked about earlier. This is total PR of 6:1

If we are running the turbos within their sweet spots, the PRs across the turbines will be roughly the same. So the PRs measured inthe manifold, in the hotpipe, and then in the exhaust would be 6:2:1 (note that 6:2=3:1).

Now, we just have to choose turbos such that the top charger will flow the same at 3:1 as the bottom one does at 2:1.

You're probably thinking this is impossible. Well, the way that the turbine maps are drawn and "corrected gas flow" plays out, it is perfectly possible.

Let's say I have 3:1 PR going into the small turbine at 1100ºF. Let's say that our intercooling isn't all that super, so this amounts to 80 lb/min of gasflow.

When we "correct" this mass flow for the higher pressure, it drops to 45 lb/min. This makes sense because the turbine is restricting flow, so to have a delta P across it, the corrected gas flow must drop. So our top turbine is only letting out 45lb/min, and dropping temp from 1100º to maybe 700º or so.

So the bottom turbo is "seeing" 45lb/min at a 3:1 PR at 700º. When the primary sees this flow, it's dropping flow to 33 lb/min.

Looking at the turbine maps for the GT4202, I see that the smallest housing still wants 36lb/min to deliver a 2:1 PR. So if I use a 4202 here, I am restriction the flow too much up top to get the PR spread I want.

In other words, I need to make the top charger flow a little more so it will deliver more to the bottom charger.

So let's drop the PR of the top turbine down to 2.75. Now, when I feed it 80 lb/min @1100º still running the same total PR of 6:1, the mass flow out of the top charger is almost 49lb/min now, and it will be hotter, like maybe 800º.

So we gain 4lb/min and 100 degrees going to the big charger, both of which help increase its operating PR.

Going to back the the GT4202, this puts out over 36 lb/min, so we now have enough flow to get the bottom gt4202 up to 2:1.

JH


EDIT: after a few more iterations, it's pretty clear that you can't get by without a wastegate. The PRs differential is simply too great. The top charger simply can't flow enough to operate at a low enough PR. Instead, it operates at too high a PR (where it's inefficient) and starves the primary of massflow.

dart3404

so does the boost reference for the top charger internal wastegate come from the intake or the charge pipe from the large charger to the small charger

torqmonster

just want to read in on this

HOHN

Dart, different people have theirs set differently, but I would personally use the charge pipe, assuming the turbos are remotely close in size.

With an external gate, I'd set the gate to "see" the difference between the charge pipe and the high pressure outlet.

dart3404

i have a hx40 and a ht3b thanks 4 u input

dieselcamp

In terms of an ACCERT CAT
The upper turbo is smaller and has a quicker spool and is waste gated,(also callled the hi pressure turbo) the turbine which is driven by exhaust flow is the first turbo after the exhaust manifold. The compressor wheel is second to recieve filtered air.
The lower turbo is larger and is first to recieve filtered air, this turbo turns slower and is not waste gated. The turbine wheel is second in the exhaust flow.
Boost is created by the low pressure turbo compressing the filtered air. The boost air is further compressed by the hi pressure turbo.
Hi boost pressures can be achieved with low turbo speeds. This inturn should increase longer component life.
Turbo speeds are kept under 100,00 rpm to achieve 50 psi boost. I am in a Detroit DD15 course and they are using 1 non waste gated, non VNT, turbo to achieve 50 psi boost at over 120,000 rpm.

NoSeeUm

If I a reading this right, the boost reference typically comes from the intake manifold just like running a single turbo. So if the wastegate is set to open at xx psi this is essentially the combined boost pressure of both turbos added together.

One could think that this would cause the wastegate to open really early, but in reality as long as you are using a boost elbow this delay in wastegate opening allows the top turbo to spool up either before or at the same time as the bottom turbo. So if the wastegate is loose they will spool closer together, if it is tight the top will spool sooner. That said, the bottom charger will always lag a bit behind in spool up. This is really noticeable with an s400 as it is really noisy before it gets spooled and quiets down considerably when spooled. So once boost is built up to the wastegate setting for a sustained period of time, no matter how, the wastegate is always open.

From my experience I can tune my wastegate focusing on one of two goals. The two goals are lowest EGT (faster top turbo spool - for towing) or lowest drive pressure (slower top turbo spool - max Hp). I do this by making it easier or harder for the wastegate to open. Keeping in mind that with sustained boost the wastegate is always open. A faster spool I find is better for EGT's as the boost gets up on top of the EGT's sooner. A slower spool the EGT's will be a quantum step higher and the EGT's will not come down unless the engine is close to WOT. Either way about the same maximum boost.

I would have cholse to have close to a 50/50 split in boost pressures. If I am making 50 psi of boost then the bottom turbo is making about half of that.

What I noticed, like Hohn describes, that once the wastegate opens the drive pressure (TIP) to the top charger rises rapidly. In my case that occured at around 40 psi of boost. So up to 40 psi of boost my TIP equaled very close to boost. Above 40 psi the TIP ratio to boost increased more rapidly. The highest boost I have seen with my set is just over 60 psi, I did not have a gauge high enough to measure the TIP at that boost.

There are others ways to control the wastegate. I don't understand them well enough to really discuss how those guys do it.

Jim

HOHN

I've determined for myself how I would set up the wastegate when I build my twins.

It requires a wastegate like a Tial that has pressure ports on both sides of the diaphragm. One one side of the diaphragm you feed it total boost, like you normally would.

The other side is normally vented to atmosphere. Thus, the wastegate's opening point is determined by an equation that looks like this:

OPENING PRESSURE= atmospheric pressure (PSIA)- exhaust manifold pressure+ (spring force/diaphragm area)- boost pressure

When you vent the wg control port to atmosphere, the wastegate basically sees basically 2x boost pressure on one side of the diaphragm (assuming boost=TIP, and both act the same direction against the spring) and spring pressure and atmosphere on the other. Shen vented to atmosphere, you essntially just balance boost pressure against spring tension. You choose a 15psi spring, you'll get around 15psi opening point on the gate.

But if you plumb the "control" side of the gate to the coldpipe, you have the coldpipe pressure acting to keep the gate closed. This tends to create a desirable (imo) balancing effect between the turbos that shifts more workload to the large turbo as boost pressure rises.

Let me illustrate.

Let's say you have a set of twins with a large external gate that cracks open at 20psi.

As boost rises towards 20psi, the gate sees more and more opening pressure. At 20psi of total boost, it starts to open, providing more drive energy to the primary.

By plumbing the control port to the cold pipe, you provide some pressure to keep the gate from blowing wide open and collapsing all the drive energy to the top turbo. Instead, it stays more proportional.

Best of all, by modulating the pressure between the cold pipe and the control port (via manual boost controller or something), you can dial in the exact balance between the turbos that you would like to have at WOT, giving you the flexibility that Jim describes above.

JMO

XLR8R

What do you guys think of adding an external gate to an internally gated "towing" secondary - in this case an HTT 62/65/13SS - to maintain excellent low RPM spoolup while allowing plenty of exhaust flow cross section "direct" to the primary turbine at high RPM?

Incidentally, aren't HTB2s supposed to have a diaphragm operated wastegate? This one seems to have been converted to coil spring adjustment...

Mike Holmen

How big is the internal flapper of the wastegate? Its basically the samething without welding up a bunch of extra fittings for an external wastegate.

The reason for opening earlier of the wastegate than normal on the secondary, is first you need energy to lite the primary. It will lite some without the wastegate open, but it works so much better with it open. As for the pressure that it opens, there a bunch of variables. I like to use a larger flapper on the internal wastegate (38-40mm) and unshroud the area around flapper for cleaner air flow of the exhaust gases. You then need to pipe the wastegated gases back into exhaust flow to direct the energy into the primary turbo. The exist secondary housing will work without putting in extra piping for the exhaust to flow thru and/or machining the turbine housing, but its restrictive and you'll lose energy (power to lite up the primary). Another reason for openning the wastegate sooner is for keeping the drive pressure low around 1:1. If you keep the secondary wastegate close you're drive pressure will sky rocket and the secondary is doing all of the work.

XLR8R

Not sure of the flapper size - but it's a dual port, likely with good flow characteristics designed in since it's supposed to be high-performance... still going to massage it before the hot sides get ceramic coated.

I'm going to work the secondary up towards the top of it's best efficiency island, so it won't gate to the primary until (i.e.) 45psi. I know the HTB2's 13cm housing isn't optimized to handle 130-140lbs/min produced by the bottom compressor, so methinks more secondary exhaust flow is in order... and I want to keep the low-RPM spoolup.

The additional external gate wouldn't open until the secondary's drive pressure starts to get out of hand (say 60psi) so more of the hot exhaust flows directly to the bottom turbine where it's needed at high RPM.

With the delayed opening, I don't see how the external gate could hurt, but I'm trying to get a better idea of how much it could help, since there'll be a substantial amount of fabricating required... I'll have to kick the secondary up off the exhaust manifold to have room to mount the WG. Planning to use angled outlet/inlet for the flow path from just before secondary foot to short-side radius of hot pipe to primary turbine.

Thoughts?

Mike Holmen

Which primary you using? You might find that once both the external and internal opens most of your energy will be going to the primary. That might be good and bad, with good is it might be the right amount of exhaust to lite up the primary good and hard. The bad its gonna lay down until a wastegate opens to fuel the primary. I would run the internal at 22 psi (if its small flapper) and go higher with the external (if its a large size). Have the external run off a in cab remote pressure adjustment valve. Typically the secondary wastegate are pressured off the compressor housing TIP pressure port. There is tons of stuff you can do. One that I've been thinking about using a couple three-way pneumatic relays that are trigger off other things than just pressure i.e. throttle position and engine rpm. You could also run the relays to balance out the loading on both the primary and secondary, keep the drive pressure down. A poor man's electronic wastegate actuvator.

XLR8R

The primary is an S400 frame with 74mm turbine and generous 1.32 A/R exhaust housing; compressor is only 74mm, so we'll likely swap that for an 80+ or possibly hang on to the 74 if the water/air intercooler design can be packaged in there somewhere.

Good idea on the in-cab WG tuning - I might go with an adjustable boost reference line to the diaphragm.