compound questions

[DVST8R]

In a sequential or compound turbo setup (I hope they are the same or now I will be really confused and bakc to square one) Would the bigger turbo spool up faster or slower then if it was used all by it self (one big turbo).

Second is the big tubo noramlly waste gated or non?

Third if it is waste gated is the max psi for the system setup friom the big turbo or the small turbo, or a combination?

I think that those are all of my remaining questions though with some disscusion here I may have more :wink:

I know we have some turbo guru's on the board so hopfully they will chime in.

The setup that I am going to try is the stock garrett turbo compounded with either a holset hx 35 with the single 9cm2 housing, or a pdr hx35 with the 12cm2 housing. I am also going to go with a custom spearco intercooler (dimentions in the process of being detirmined.)

[web]

Most sequential setups that I've seen, run two roughly equal turbos, first just one, and then both together. No greatly different sizes ; when adding the second turbo you are already doubling capacity!

Your story sounds like you want to attempt switching? So no parallel operation, but first the small one alone and then the big one alone? Or am I reading you wrong.

[QuickTD]

In a regulated 2 stage setup the primary turbo (the first one connected to the intake/exhaust manifolds) would be smaller than what would normally be required for the engine. This turbo will spool very quickly and since it handles air that is already compressed to some extent it does not need to be very large. I would guess that a garret T2 or a KKK K03-003 would be about right. The secondary turbo should sized to deliver the maximum flow/boost that the engine requires at the highest anticipated engine speed. I would use a Garret T3 in a fairly conservative trim and A/R, one of the free turbo calculators should provide reasonable starting points. A wastegate should be used on the primary turbo. Some porting/enlarging of the primary turbo's wastegate would help bypass more of the exhaust gas to the secondary turbo. I would set the primary turbo's wastegate at 7lbs or so referenced to the manifold. The secondary turbo does not need a wastegate if it is sized exactly, but this is a difficult task. I would leave a wastegate on it and set it to the desired boost max level, also referenced  to the intake manifold and not the compressor housing.

 I'm not sure that a true sequential setup would be of much benefit. It is possible to blow any head gasket using a single turbo. If you want to try it though the above setup should also work with just a minor rearranging of the wastegate connections

[fspGTD]

Usually a multiple-staged turbo setup has the smaller turbo (the one that's closest in the air path to the engine) being one that is sized so that it could boost the engine if it were all by itself.  So in theory you could just use the same turbo for that application you would use in a single turbo application for that engine.  The outer turbo is the big one that needs to be sized dependent on both the engine and the small turbo closest to the engine.  And yes, if you ran the outer big turbo without the inner one, it wouldn't work right as it probably would have a severe airflow sizing mismatch.  Since it's designed to flow a lot more airflow than it would be doing in a single turbo application, it might be operating in compressor surge if it developed boost pressure at all, or at least at reduced efficiency.

I think that in general, the small turbo would spool up first and then the big turbo would spool up afterwards, since it would all the sudden "see" the increase in airflow that the small spooled up turbo started to make.  The boost pressure would be a multiplication of the pressure ratio boost offered by each step in the compounding: IE: say both turbos boost by 2:1 pressure ratios (by themselves, this would be 14.7psi gauge pressure.).  The resulting max boost pressure from running them in series would be 2 x 2 = 4:1 pressure ratio.  (44psi gauge pressure)  Strongly recommend the book "Turbochargers" by Hugh MacKinnes as there is a section on multiple turbos including an example and instructions on how to size a system with 2 turbos in series.  They also talk about how to do 3 steps in series, although they don't go into the math.  It also has some fun pictures of multiple-stage turbo diesel motors, such as a tractor-pull diesel engine with 4 turbos in 3 stages that runs well over 200 psi boost pressure.

I think that just like with a single stage turbo, you could run a wastegate if you want or not.

[fspGTD]

It is possible to blow any head gasket using a single turbo.

The max pressure a single stage turbo can give is about a 4:1 pressure ratio (or about 44psi gauge pressure.)

What cases do you know of VW Diesel head gaskets failing as a result of too much cylinder pressure, and how much boost was too much?  I have not heard of a head gasket failure caused by too much boost pressure before.  (And there are a lot of folks on this board running lots of boost, even with full 23:1 compression ratio!)

There are also a number of upgrades that can be made to improve the stock head gasket seal (head studs, switch from fiberous to multi-layer steel head gasket, switch from head gasket to metal o-rings fitting machined-in grooves in the head and block, are a few that come to mind.)

[QuickTD]

I haven't blown a gasket myself, I haven't tried very hard either. I have heard of several cases where running higher than 30psi boost has blown out the coolant due to the head "lifting". Maybe add a bigger catch can to store the blown out coolant.  Using sequentials, even at a very conservative pressure ratios you could get more than 70psi. I'm not sure the mechanical parts would stand it. The TDI folks start bending rods at 35-40psi. Perhaps the IDI rods are stronger or the combustion soft enough to spare them?

 Has anyone here blown a head gasket? Is anyone running more than 30psi reliably?

[Patrick]

Just while we're on turbo's, can anyone tell me what stock boost is on a 1.6? How high can you run without worrying about too much heat? Is the stock wastegate control adjustable?

[vwmike]

In a sequential or compound turbo setup (I hope they are the same or now I will be really confused and bakc to square one) Would the bigger turbo spool up faster or slower then if it was used all by it self (one big turbo).

Sequential and compound are different things. Sequential usually means it runs with both turbos only at high rpm like the RX-7 where it "switches" on the second turbo. Compound forced induction is feeding one compressor outlet into the second turbo inlet. So, your second turbo must flow a lot more volume (lbs/min or CFM) in comparison to it's pressure ratio. Therefore you'd feed a small turbo into a larger turbo. I would tend to think you'd want the big turbo to be first on the turbine side just because the small turbo takes so much less to spool and a lot of the exhaust heat may be lost by the time it gets past the first turbo. I'm sure it would work either way though.

Second is the big tubo noramlly waste gated or non?

I would venture to guess running the wastegate off of the manifold in front of the first turbo would be the best idea. That way overall pressure can be regulated. If you wanted to get tricky you could always run 2 wastegates which allow you to control the speeds of both turbos, but that could be problematic and easily cause you to lose too much heat.

Third if it is waste gated is the max psi for the system setup friom the big turbo or the small turbo, or a combination?

In combination as I suggested, right off the manifold which would regulate overall boost pressure.

[DVST8R]

QuickTD - I belive that Dr. D is running a k24/26 hybrid that pushes 35psi, and no known failuers to this point. I personally havent blown a head gasket, but there are a few here fspGTD has at least once possibly twice tha tI remeber reading about, it seams as though joe GTD form the old board blew one as well, however I don't belive any of these where "presure" related.

Thank you vwmike for clarifing the differnce between compound and sequential, compound is definately what I am looking at doing.

My basis of understanding and ideas are mostly coming from compound setups on cummins diesels, where they take either the stock turbo, or bigger then stock and then compound it with quite a bit bigger turbo yet, and run anywhere from 50 - 90psi and about 500hp - 900hp depending on the application. So what I am tying to do is scale down a proven setup like this for use on the vwtd. I am not looking to go into production, it is purley for research and my own enjoyment, If the motor blows I will rebuild it stronger then b4.

What I am trying to achive is somthing that will alow me to burn as much fuel as I throw at it, with decent driving characteristics I.e not enormous turbo lag, and from having ridden in and dirving both big single and twin turbo setups in dodge diesels, I think that my best way to achive this is through a compoud setup.

This is not ment to be practical, If it doesnt work then I will play with the turbo sizes until it does, and hopefully along the way I will find out usefull info for other people like the limits of the 1.6 ect... and mabey I will have a chance to smoke a few hondas along the way.  

[web]

right... interesting stuff, though only for truly extreme boost levels.

thanks for clearing up compound vs. sequential btw, I was thinking of sequential only.

[dieselpower]

hey,
so far you guys agree that it should be used for makeing extreme amounts of pressure. but what if you use 2 little turbos and then the combined pressure could be like 30 psi but it would practically spool up instantly. thats my reason for wanting to make a twin turbo setup, fast spoolup time and enough flow to run upwards of 30 psi. also would you want the big turbo feeding into the little one or the other way around.

http://www.cardomain.com/memberpage/418501/6

here are some pics of a compound twin turbo setup form a cummins. both turbine housings are the same size, but the one closest to the engine and the one that feeds into the intercooler has a smaller compressor housing.  a truck that had this setup in it dynoed in at 572 hp at the wheels.

thanks for the help

Mark

[web]

Nice pics...
I think I have them figured out though. The turbines ARE different. The bottom turbine is larger than the top one. It just LOOKS the same because its downpipe is bigger, too. Look again, you'll see.

Note that the second turbo (intake wise) is the first turbo in the exhaust stream. You could say they turbocharged a turbo engine, using the intake and exhaust to the engine as a whole, completely ignoring the fact that there's already a turbo in there somewhere. Which could be the key thought in understanding this one. Effectively, what they did is turbocharge a turbo engine!

What if... you would add a bigger turbo external to the "small" one that's already there. The small one will be fed pressurized air instead of atmospheric, but it will still move the same volume of it as it did without a second turbo. It won't care that it's actually operating at a higher pressure as long as it's on both sides. Mind you, flow maps are in pressure RATIO, not absolute pressure, and INLET cfm, not atmospheric cfm. So the small one should still work like it used to, nicely within its flowmap, and multiplying (by pressure ratio) anything it recieves in its inlet (OMG, I'm going to feed it 100 dollar bills!:lol:).

But the exhaust and intake flow that your turbo engine as a whole requires, is much larger than it would in NA form. So to turbocharge it yet again, for compound boost, you need a bigger turbo!

The big one is the one that provides the lot of half-compressed air to the small one. It needs to be big, because in order to supply the small one with ITS full flow demand at full boost, it needs to suck in even more atmospheric air than the small one ever could. Again the flow map story : pressure ratio is "low" but inlet flow is much larger than for the small one! Consequence : need big one!

That, I think, is exactly what they did in the Cummins on the pics.

So you, on the TD, would keep the small one on "as is" and add a big turbo external to it, to its turbine outlet and compressor inlet. Run either one or two intercoolers. Two is probably best... one after each turbo... but a nightmare to plumb

How do I think it will work? Well, the small turbo will spool first (initial flow through both just isn't enough for the big one!), and then the big one. As the big one spools, it will build backpressure. This backpressure will control the small one and keep a healthy ratio so neither overspins.

Make sense? I really don't know. I am just thinking out loud above, but it all does seem to come together nicely. I have some thoughts about boost control and turbo sizing too, but I'll stop here for now and let you shoot me down if I'm on the wrong track entirely

Maybe Corky Bell had something to say about compound boost, in his book "maximum boost".

Marcel

[DVST8R]

Web form my many trips to PDR (piers diesel research) I think that you have got it about right on, or at least my understanding of it anyway . For there towing series twin setup on the cummins 5.9, they use the stock holset hx35, in the stock location, compounded with a ht3b (don't know who makes it or its specs but in person it looks to be almost double the size of the hx35), then they have there steet series that uses the common upgrade for the single hx35 the hx40, compounded with the same ht3b, the max pressure that these are setup to run is 50 psi, and 70psi respectivly, and finally they have there race setup which I have never seen in person, that usess a s440 compounded with the big brother turbo and runs to 90psi. I have ridden in a very similar turcks, with the differnce being one had a single hx40, and one and the towing twins, and the tiwns spooled up slightly quicker and jsut contiued to build (its quite a rush) with almost no smoke, just a slight haze, where the single hx40 built boost to about 43psi and had the smoke rolling out from the time the peddle was matted. This ride along is what has motivated me to try a compound setup on my vw. So I would be greatful for any recomendations.  

[dieselpower]

hey,
WEB, ya you are right. the first turbo is smaller then second. thats what i remember it being when i took the pics, but when i looked at them it seemed the same. anyway obviously not for my car, but do you think a K24 would be a big enough turbo to feed the K03 on a 1.9? and do you think that setup would flow enough air to be worthwhile on a 1.6TD? or would the exhaust be too restrictive and actually loose topend?

O, and DVST8R, i dont know of this personally and i dont know the guy, but one of the guys i worked with said his buddy used to have a little rabbit TD like mine. and the completely disconected the wastegate. and he said it could run 40 psi boost before it blew the head gasket. i guess it could have been for a number of factors, but he said it was from overboosting it.

thanks alot for the help.


Mark

[DVST8R]

well my poor little garrett turbo hit 35psi breifly  :oops:  and the car lived to tell about it. I plan to see how much a stock motor will take b4 it pops, and then rebuild it with raceware or arp studs, 1.9L metal headgasket, possibly the 1.9 head and O ringed block and the head, but until then we will see how far it will go :twisted:. My first priority right now is getting the clutch ready for the power as it won't even hold what I have now, but htere are so many options that my head is jsut a spining, but I have a week off just after x-mas so it will be done and decided then. Then I go back and start redisigning my new header, for both turbo's, I havn't yet decided if I am going to go with a pdr hx35 12cm, or a used hx35 9cm, one is a $1000, the other is $300, I think I might go the cheep route jsut to get it up and going then look at upgrading both turbos with the small one going ot a ball bering gt garret, and the large to a pdr hx35 or equivalent. Lots of fabbing lots of fun, I am excited , but I see a long expensive road ahead of me, as I know that as soon as I get the compoud system up and running I will need more fuel ect.... then things will start to break and so on it will go  :roll: