schoust

For me it will be an s400 under my turbo,I just wish I new somone else who tried this combo? Guy's this is great stuff keep it going very informitive!!

74dart

Im looking in the for a s400 or a HX-65. Is the HX-65 a reverse flow trubo and can somone explain the piping for an ht3b. Ive looked at pictures and i still see the hot pipe going from the small to larger turbo, and the cold pipe goes in the large turbo then out the large and in the smalll then out the small into the intercooler. Both the b1 b2 twins and the hx-35 htb3 twins looked piped the same to me. Am I missing something here.

Mike Holmen

Hohn, any of those big primaries have an internal wastegate? I personally think that the internally wastegated turbo makes a nice clean twins. They just aren't as tunable as the external. I just re-read this entire post. I'm still trying hard to see why the only engineered twins run such high wastegate pressures. How do wastegates work? They open fast or slowly? I drove mine with a frozen wastegate on the HX-40. My spool-up where sluggish. My egt's where sky high. Now my wastegate is set at 22 lbs, it open fully. My spool-ups are almost as fast as the old HY turbo. My egt's aren't that high. Drive pressure is 1:1 ratio until 60psi. Hohn, what you thinking for a the secondary for that primary? I was thinking that the 3788 stage 3 would be a hot ticket. Its internally wasteagated and liquid cooled. The primary doesn't have to be liquid cooled as it doesn't see the heat like the secondary. I'm even thinking that the primary doesn't have to be ball bearing turbo, but it would be nice to have a nice spoolin primary to light fast and hard.

CTD_Neil

Can anybody recommend a good vendor with good prices on the B-D Twins?

HOHN

Almost all of the primaries I've seen lack an internal wastegate-- a very desireable feature for a primary, imo. I'm not sure what is meant by the phrase "running such high wastegate pressures."

A wastegate is just a spring-loaded diaphragm linked to a valve. When the pressure to the diaphragm exceeds spring pressure, it will compress the spring, move the actuating rod and open the valve.

Because of this arrangement, the valve cracks open at a pressure much lower than the "limiting" pressure. In order to limit boost to 40psi (for example) the wg may begin opening as low as 32psi or so because a spring is progressive.

The "bighead" wastegate actuator that Banks sells improves upon this by using a larger diaphragm. A larger diahragm area will exert more force for a given PSI, allowing a stiffer spring to be used and reducing the pressure differential between wg cracked and wg "setting." In the above case, the wg might now only crack at 37psi to limit to 40psi, instead of cracking open at 32psi. This improves tq rise and lowers EGT because you're not prematurely opening the wastegate.

Note that the CTDs beginning with the 04.5 "600" engines and newer now have an electronically activated wastegate to eliminate the comparatively poor performance of the conventional wastegate. Many people have done electronic wastegate conversions using a Hobbs switch and a solenoid valve to accomplish the same thing.

An external wastegate will generally have a much larger diaphragm area than even an "upgraded" internal gate and thus come much closer to the ideal of "instantaneous" opening and closing instead of the spring's progressive action. Also, the typical external gate has a pressure port on BOTH side of the diaphragm, allowing a boost reference to other than atmosphere. For example, an internal gate with a single port can only be setup to pop at a certain value of boost above atmospheric-- say 40psi. It cannot tell the difference between 20psi in the cold pipe and 10psi on the cold pipe-- just when the secondary's output is 40psi, the wg is wide open.

But an external's secondary reference allows you to dial in exact proportion of primary to secondary boost. Instead of just popping at 40psi, I can set it to pop at 40psi MORE THAN THE INLET PRESSURE. Thus, I ensure that my top charger is only making 40psi more the bottom charger, and never more.

The advantage to the external gate setup like this is that as total boost rises, the operating PR of the primary gets lower. Let's say I set it for 30psi on the top charger. With no boost from the bottom turbo, the top charger will operate at a PR of about 3:1 (29.4psi ~=30psi) before the wg opens fully. But as the primary starts to build boost, the operating PR of the top charger gets lower when setup this way. For example, when the primary is making 30psi, the operating PR of the top charger is now only 2:1 (30in:60out). If the bottom charger makes 60psi, the top charger's PR drops to 1.5:1 (60in:90out) This decreasing PR means that as the primary makes more and more boost, the top charger becomes proportionally less and less restrictive-- just what we want.

As for a Secondary, the 3788 would be my first pick for bigger HP. For max spoolup, I'd consider the 3782 stg 2, which has a tighter housing and a smaller compressor to spool faster (about 8% faster, or roughly 130rpm-- barely noticeable). Based on what Hammer says, the stg 3 spools so early and so fast there's almost no reason to go any smaller than the stg 3, but some people could maybe use even more spoolup (tight converter auto, big tires, tall gears, etc).

I agree that the coolant-plumbed turbo on top would be nice, and having a BB turbo on bottom is probably not nearly as significant as on top.

Something I discovered "spreadsheeting" last night is how important cooling the primary's discharge is. With no interstage cooling, I have to TRIPLE the inlet pressure to the secondary to actually DOUBLE the mass flow the secondary spits out.

Say for example, if my top charger flows 40lb/min by itself (sucking atmospheric air), I'd have to run the big charger at 3:1 to get the small charger's output up to 80lb/min. This assumes a typical compressor efficiency of 70% at both stages. Thus, you are only getting 70% of 70% of the increase you want-- about 1.49, so you have to to triple the PR of primary to double the mass flow of the secondary. (3/1.49 ~= 2)

Another interesting factoid: dropping 100º in the cross pipe is the same as dropping almost 150º at the CAC. Perhaps crosspipe is a good water injection location?

Sorry so long--hope I answered all the questions.

Justin

pmtg

its reverse rotation, not reverse flow. when looking at the compressor housing, the air will enter in the center but will be spun in a counter-clockwise direction. other turbos spin in a clockwise direction.

the air travels the same way: in the big, out the big to the small, out the small to the CAC. the difference is in how it all connects together because the housings between the 'counter-clockwise' and 'clockwise' turbos are different.....one will look like a 'd' and the other will look like a 'b'.

Mike Holmen

Hohn, I guess that your talking about balancing the load out in a nut shell. Not too many people know how your wastegate opens, at what pressure, how pressure does it take to fully open the wastegate. I took apart my last HX-40. It took 28 psi to open the wastegate, it took almost 40psi to fully open the gate and it wasn't fully open. My new HX-40, I set it to open at 22 psi and fully open at around 28 psi. I was thinking about build my next set will with electrical wastegates so you can flutter the secondary wastegate so that the primary will lite faster. Once the primary lites, its takes off faster.

AlpineRAM

Did you take the exhaust pressure into account? I think that statically testing a wastegate with compressed air tells only a small part of the story.
There was this device called turbo master that relied only on the backpressure to operate the wastegate. (Turbo-Master IIRC)

The wastegate is closed by spring pressure on the actuator and opended by boost on the diaphragm + pressure on the area of the gate itself (needs to take the leverage and effective area over the hole and flow characteristics in the WG housing into consideration)

Just my 2c

AlpineRAM

HOHN

Yes-- WG pressure is clearly affected by backpressure.

This leads to the question of WG spring tension specs, and whether manufacturers are publishing absolute spring tensions, or if they are assuming a 1:1 boost:backpressure ratio.

So when Tial sells a 15psi wg spring, does the actual SPRING pressure against the diaphragm is 15psi? Will the gate be just cracking open at 15psi, or fully open at 15psi?

I'll email them to find out.

jh

HOHN

Tial says they assume boost=backpressure.

Moreover, the design of their external gates makes it look like exhaust pressure tends to OPEN the wg valve, not close it. This makes sense because you'd want a wg failure mode to fail open, no closed.

So now I'm going to have to re-think how I'd want the wg plumbed. I'm sure it can work as I wish, I'll just have to swap the port assignments or something.

Justin

Mike Holmen

I just found out a cheap digital control for the electronic wastegates. Its programable and hopefully be the cats meow. My brother thinks I'm crazy (he's a duramax boy) Twins, why you need twins??? I tried to explain how it works, he answered "Why do you cummin's guys need two turbo's, when one is good enough" He's a non believer of cummin power, yet!!! His truck barely breaks the 300rwhp barrier.
I'm thinking with the digital controller I'll have the wastegates start to adjust when the drive pressure exceeds 1:1 or try something crazy like bounce the wastegate of the secondary during a 3-7psi boost and the throttle is asking for you to accelerate. It would open the idea to in the truck in cab fine tuning and finding out what would make twin light almost as good as a single could. With proper size turbos, this could be done and then you can bring on the power.

04smoker

I am currently running the bd twins on my 3rd gen and would like to unload them. If anyone is interested in them pm me and ill get back with you as soon as I receive the message. I will say that they are an awesome setup, however, they are just not working out for me at this time.

AlpineRAM

The exhaust pressure on the tial WG is having just the same effects as in the holset WG.
With twins the part that gets really interesting is that the force on the wastegate flapper is determined by the delta P between exhaust manifold and downpipe. For a single you have only minimal backpressure (read 4" pipe, straigh through muffler ) but in twins there is the turbine of the primary that will increase the pressure in the secondaries downpipe (which is manifold pressure for the primary) depending on flow.
The failure mode of the WG depends on the failure- broken spring will result in fail open, torn hose or hole in diaphragm will fail closed. (That's the reason why the stock ECM defuels when it sees an overboost condition - a safeguard for a wastegate failing closed with the resulting increase in backpressure and EGTs that could damage the engine)

As for wastegate design it would be nice to see the relations of the effective areas to see how these influences are weighted. If we use the assumption of boost=backpressure the diaphragm area and the wastegate flapper area being identical would mean that both influences are balanced. So with a wastegate set to fully open at 15 psi boost it would look the same for 30 psi boost and no backpressure and 30 psi backpressure and no boost.
If now the size of the diaphragm is increased the dependency on boost rises and the dependency on backpressure is decreased.

In my opinion an idea that could be followed was that the secondaries WG is actuated mainly by the delta p on the exhaust side since this determines the mass flow through the turbine and then for a high boost situation (total boost not delta p across sec. comp. stage) an additional force that opens the secondaries WG further to increase the flow to the primaries turbine. (keeping the secondary in the high fields of the maps for the delta p on the turbine and the compressor map while being force fed by the primary.)
On the other hand you could also use the diaphragm to determine it's part of the wastegate movement by the pressure difference as Justin has outlined in one of his previous posts about pressure ratios on the secondary compressor in twins.

AlpineRAM

HOHN

Relative diaphragm sizing is something I'd like to know as well. I'd imagine that diaphragm area has to be 20% or more larger than the poppet valve area of the wg--that's just a guess. The more oversized it is, the more control the WG will have because the spring can be stiffer, shifting the weight of functionality to the boost vs the backpressure-- as Alpine mentioned.

I'm not sure I'm following the additional force of opening the secondary's wg even further. Keeping in mind that flow is a zero-sum game, the more I bypass through the wg, the less goes through the secondary turbine. This would lower the operating PR of the secondary.

This is why I view the two-wastegate solution as follows: the SECONDARY wastegate controls the workload between the two turbines, and hence the balance of operating PRs. The PRIMARY wastegate (if so equipped) determines the TOTAL amount of compressor airflow provided.

Now, this isn't perfectly accurate due to the fact that the top wg isn't an either/or choice because all the flow eventually hits the primary's turbine, but it's a useful approach to analyzing the effects of different tuning options.

So that leads me to think that there are generally two competing and somewhat opposite ways to think of twins (compound turbos).

Perspective #1: The Small turbo is the main turbo, and I'm supplementing it (turbocharging it, in effect) with a larger turbo.

Perspective #2: the large turbo is the main turbo, I'm just using a secondary stage to help light it off sooner and share the PR workload a bit.

Your perspective on these setups will greatly determine how you tune and setup your turbos.

Personally, I think that compound turbos should be looked at as one large turbo in two stages. We want to balance the workload across the stages as much as feasible. I'd tend to push a little higher PR on the big charger just because larger turbos are almost always more efficient at higher PRs than smaller ones. But we're talking about a 2.75:2.25 PR balance, or maybe a 2.75:2.5

Unfortunately, this is where some of the myriad compromises come in. The more efficient setup is to run a little higher PR ont he big turbo, but this is also the laggier way to do it. Getting the smaller turbo into the game early will increase its operating PR, but give you a much fatter, "towing twins" type of curve, while living with some lag will give you bigger top end power numbers when the big charger is operating at higher PR.

Sorry, this is already too long.

JH

AlpineRAM

What I meant with the "additional opening" is that if you put the bias on the backpressure of the sec. turbine for tuning the wastegate you can avoid overspooling and kicking the turbine out of it's map. But since the pre-compressed air from the primary will move the island of efficiency for the compressor the total boost should be taken into consideration to bias towards the primaries turbine to make it work where it is the more efficient turbo.

In the interest of efficiency your perspective #2 is the more interesting one. If we use an exhaust routing that has the wastegates built in a way that impedes flow as little as possible and use some way to set our wastegates in a way that depends on load too we can reduce pumping losses and use the right turbine and compressor combo for a very wide range of rpm and load % - which is IMO one of the crucial factors to accept the effort of using twins.
I think that there is a big market for twins that widen the powerband and increase fuel efficiency. If this is combined with better manners (spoolup) it's a killer combo on the market IMO.

AlpineRAM