Compounds revisited; Any TMers running them?

[Rrider]

Read through this older thread and it got me curious about compounds.

http://www.turbo-mopar.com/forums/showthread.php?45321

Seems to me like some undeserved bashing in that thread. I agree with this assessment:

I absolutely approve of this just because of the process of creating it. It's more about ability than results unless you are spending an --- load of money. If he had those turbos sitting around and didnt go out and spend $1500 trying to build that than it rocks no matter what. If i had the fab skills id whip up some dual freebie-mitsu turbo piping for a 3.0 motor and not give a ---- what anyone thought of it if it was faster than stock, proved i COULD do it, and cost

If you have the room and the tools to do it.. I'm not seeing much downside? Smallish turbos are at worst cheap, at best free becuase they are laying on your bench.

[turbovanman²]

Read through this older thread and it got me curious about compounds.

http://www.turbo-mopar.com/forums/showthread.php?45321

Seems to me like some undeserved bashing in that thread. I agree with this assessment:



If you have the room and the tools to do it.. I'm not seeing much downside? Smallish turbos are at worst cheap, at best free becuase they are laying on your bench.

So quit flapping and get it done,

Dual Mitsu's on a 3.0L wouldn't be a good idea but honestly, they flow alot of air and don't need help spooling larger turbo's.

[Rrider]

I just found out this is even possible! I need more schooling before I try something like that.. maybe later tho. Lord knows if anything has got the room its a mini bay. What I'm wondering is how "far" you can stray from the ideal size combo before it gets lousy.

[turbovanman²]

Just size them right, I think a GT40 with a stock garret or mitsu would rock,

Hmmmmmmmmmm,

[Rrider]

I was thinking more elCheapo.. TE04 and a chinese T3. And some home depot sewage pipe.

No progress or new developments in the 3 years since the last compound thread? I seen a sweet 300z on youtube nicknamed "streetfighter" with compounds. Looks like hes got it pretty well figured out.

[zin]

I think this is one of those things that looks cooler than it really is... at least for those of use who don't have access to the "parts bin" at a turbo manufacture... If we could mix-in-match as per the engineering dept's suggestions, then try it, go back to the bin and fine tune it... Well, then I think you could have a "best of all worlds" kind of thing!

But, trying to do it with "standard"/used and or cheapo stuff is probably not going to work out so well... that said, if I thought it would be fun, I'd give it a whirl! Sometime not knowing that "you can't do that", leads to some great finds!

Mike

[john1320]

I think if you can do it and you can make room, why not? A stock turbo for fast boost, than a larger frame sized just above your target peak HP. Originally the concept was for insane boost levels, but there's no reason it couldn't be used for efficiently produced moderate boost. (20-30 at lower egt?)

[Vigo]

Well there are several different ways to run two turbos (or one turbo and one supercharger, etc) and not all of them are compound setups. Compound would be one turbo feeding into another one's intake. If both turbos fed into the same charge piping it would just be a 'twins' setup like in my quote up there.

A couple things about a compound setup.

Air flow/paths:

Intake: Filter > big turbo > small turbo > intercooler > engine.
Exhaust: Engine > small turbo > big turbo > exhaust. There are wastegates in there too.

Compound turbocharging CAN be used to achieve insane boost levels but generally it is used to create a more responsive setup that builds boost earlier on a small motor.

For example, if you wanted to build a 700hp 2.0L with one turbo you would be forced to use a turbo that would not spool until 5k+ rpm (im making a generalization for the sake of explanation here).

A compound setup would allow you to build the 700hp 2.0L with boost earlier in the rpm range, and thus a wider powerband and more streetable setup. You use a smaller turbo to make boost earlier, and that boost going through the motor helps it spool the big turbo faster than it normally would as well. So you could conceivably hit full boost at a lower rpm than you would even START to make boost with the large turbo alone.

One thing that people sometimes miss is that in the compound setup ALL the charge air goes through the small turbo's compressor. The small turbo's compressor would not be able to flow enough air to support the peak HP goal (700 in this example) except for the fact that as the large turbo begins to spool it pressurizes the intake of the small turbo. Since the air going into the small turbo is much more dense, the smaller compressor is able to flow enough air to support the 700hp.

However, all of the exhaust does NOT go through the small turbo's turbine. When the small turbo spools to it's setting (each turbo will have individual boost control but both will affect the total boost the engine sees), a wastegate will begin to bypass exhaust past the small turbo, into the larger turbo (the first wastegate bypasses flow back into pressurized piping between the small and large turbines, the 2nd wastegate will bypass to open exhaust just as a normal wastegate usually does). When the large turbo spools and peak boost is made, the MAJORITY of the exhaust flow will bypass the small turbo entirely. This alludes to one major point about compound turbocharging: You basically need to have an exhaust manifold that is 'wastegate biased' or 'wastegate priority'. In other words, the manifold has to be designed with the intention that MOST of the exhaust is going to go out the first wastegate, not the small turbo. So, for example, you couldnt do a proper compound setup with a stock 2.2/2.5 manifold and its internally gated small turbo because you could never get the stock internal gate to bypass the amount of exhaust that would be necessary for the setup to work right.

Another point to make about compound turbocharging is that it's only really practical in cases where a single turbo would spool like crap. For example, if you wanted to build a 500hp 2.4, a compound setup would be unnecessary (but still cool and bragworthy) because you can get a 500hp turbo to spool pretty well on a 2.4. You need big HP targets relative to displacement for the compound setup to be 'necessary' for drivability reasons.

Another thing people usually get wrong about compounds is they tend to guess too small for the larger turbo. For example, if i were going to build a compound setup for a 2.2 id be looking at something bigger than almost anything that even gets talked about on this forum. Something like a holset he351 or hx35 or even the smaller t4 turbines would be too small, imo.

[Ondonti]

I have thought about compounds for low rpm spool but the complexity etc makes no sense because its cheaper and easier to make a motor that can rev higher to take advantage of very very very large turbos then run compound. Creating an extra wide powerband that would not be needed in racing situations is simply a for fun thing or something to boast about.
Compounding to create super high boost levels is great but you need a motor that is willing to see that pressure. Stockish motors and standard headgaskets do not like that kinda stuff. Thats usually O ring territory and a lifetime of race gas or a whole lotta injector(s)/fuel pump for ethanol.

[Tempted]

Not kidding at all, I've been looking at the Mercedes Kompressor pulleys and giving thought to replacing my AC compressor with a small centrifigul. The way the pulley works is identical to an AC compressor pulley. It gives you boost on demand, essentially, and when you don't want the blower spinning you just flip a switch. The click when you turn the AC on is the clutch locking in, and that is how the Kompressor turns on the blower. It also seems like a good way to kill your power steering pump at the drag strip.

[Rrider]

Something like a holset he351 or hx35 or even the smaller t4 turbines would be too small, imo.

Arg. I was thinking like the biggest of the cheap ebay t3/t4 would work ok with the stock mitsu.

I have thought about compounds for low rpm spool but the complexity etc makes no sense because its cheaper and easier to make a motor that can rev higher..

But how do you get an 8v to flow at high revs on the cheap.. head work, 16v hybrid swap?

[chilort]

I have thought about compounds for low rpm spool but the complexity etc makes no sense because its cheaper and easier to make a motor that can rev higher to take advantage of very very very large turbos then run compound. Creating an extra wide powerband that would not be needed in racing situations is simply a for fun thing or something to boast about.
Compounding to create super high boost levels is great but you need a motor that is willing to see that pressure. Stockish motors and standard headgaskets do not like that kinda stuff. Thats usually O ring territory and a lifetime of race gas or a whole lotta injector(s)/fuel pump for ethanol.

Depends on the kind of racing you want to do. If all you are doing is drag racing or roundy-round then sure. If you like to go left and right on a road course then high rpm into some corners can unsettle the car. Having a wider power band can be quite useful.

[zin]

Though I like the engineering challenges of a hybrid turbo set-up, I was just thinking that maybe a more practical arrangement might be a "hybrid" set-up, that is a supercharger and turbo...

My thinking being that the supercharger could be "made to" produce boost where the turbo wasn't quite up to speed since it is coupled to the engine speed rather than exhaust flow... If you matched the maps well, it seems like it would close to ideal for a "broadening the power band" application. Thoughts?

Mike

[Tempted]

That's what I said a little earlier. A small centrifigul at low RPMs using a pulley like the Mercedes Kompressor. After 4k rpms, flip the switch and the blower stops turning.

[Rrider]

Why would you want to turn it off though? You would feed the supercharger with the turbo. Fit a D-valve so the worst the supercharger gets is atmo and you're set.

[shadow88]

When I was around CAT engines, the compound turbo systems they use tend to have an airflow path like

Exhaust manifold - large turbine (with small compressor) - small turbine (with large compressor, tailpipe
Intake - filter, 2nd turbo (large compressor) feeding 1st turbo (small compressor), intercooler, intake. That was the C15

It's a strange mix up to us, but it keeps a tiny turbo from spinning it's arse off. It was supposed to offer a broader torque band.

[Tempted]

Why would you want to turn it off though? You would feed the supercharger with the turbo. Fit a D-valve so the worst the supercharger gets is atmo and you're set.

Because a blower stop producing more power than it consumes at one point. Smaller blowers reach that point early. Leaving it spinning will drain power after that point. It would be like turning an AC compressor on while racing. Using a bigger blower negates the super low RPM benefit and you end up with both turbo and blower lag at the same time.

Volkswagon Twincharger:

Supercharger + Turbo: Volkswagen Twincharger

Everybody knows mechanical superchargers are good for low end output but short of efficiency at high rev, while exhaust turbochargers works strongly at high rev but reluctantly at low rev. For decades engineers dreamed of combining supercharger and turbocharger together. This was tried once in history – the 1985 Lancia Delta S4 rally car. The car was successful in motorracing, but the technology never extended to production.

In 2005, Volkswagen finally introduced a production unit to its Golf 1.4 TSI. Called "Twincharger" system, it is actually developed by supercharger maker Eaton. It connects a supercharger and a turbocharger in series.

At low rev, the supercharger provides most of the boost pressure. The pressure it built up also speeds up the turbocharger so that the latter can run into operating range more quickly.

At 1500 rpm, both chargers contribute about the same boost pressure, with a total of 2.5 bar. (If the turbocharger work alone, it can only provide 1.3 bar at the same rev.)

Then the turbocharger – which is optimized for high-rev power – started taking the lead. The higher the rev, the less efficient the Root-type supercharger becomes (due to its extra friction). Therefore a by-pass valve depressurize the supercharger gradually.

By 3500 rpm, the turbocharger can contribute all the boost pressure, thus the supercharger can be disconnected by an electromagnetic clutch to prevent from eating energy.

[zin]

My thought on the hybrid was for it to function like the CAT set-up, but by using one "supercharger" (I was thinking EATON, not centripetal) coupled to the engine RPM, using the "smaller" to feed the other larger compressor.

In this case I would expect the larger compressor to be the turbo with the "smaller" supercharger being used to boost the bottom end, and using the turbo to "push through" it as the engine demand goes up.

Just another example of how to "skin a cat".

Mike

[Vigo]

I was just thinking that maybe a more practical arrangement might be a "hybrid" set-up, that is a supercharger and turbo...

That's the only thing im even semi-seriously considering (for my van). The downside is you are still stuck with the crappy adiabatic efficiency and power drain of the roots blower because it it's truly a compound setup you cant just de-clutch the roots blower like Tempted is saying. My take on it is leaving it on all the time but if you gear the supercharger to a fairly small PR (and associated rpm) and then use the turbo upstream of it to raise the boost when it spools, you'll have a much more heat efficient and less parasitic setup than if you tried to make all the boost with a roots blower, and you'll still have the 'instant' boost of whatever the supercharger does. So, in my theoretical 3.0L compound m90+turbo build it would still make like 5-7 psi of instant boost which should be 300lb ft whenever i want even if the big turbo doesnt spool til 4k or whatever.

One other thing i forgot to point out is that with the type of compound setup i initially described, the big turbo compressor has to be sized to make the peak HP number. So if you take a 600hp turbo as your 'big' turbo you will not be able to magically make any more than 600 hp because it's a compound setup. The smaller compressor's abilities are amplified by being fed pressures greater than ambient, but the big turbo compressor basically works 'normally' and has to be sized for the peak hp goal.

[Tempted]

My thought on the hybrid was for it to function like the CAT set-up, but by using one "supercharger" (I was thinking EATON, not centripetal) coupled to the engine RPM, using the "smaller" to feed the other larger compressor.

In this case I would expect the larger compressor to be the turbo with the "smaller" supercharger being used to boost the bottom end, and using the turbo to "push through" it as the engine demand goes up.

Just another example of how to "skin a cat".

Mike

I think I see what you are talking about. Essentially running the the blower into the motor, the exhaust spooling the turbo, the turbo into the blower and then the cycle repeats.

My question is how much will the blower impede the airflow once the blower has reached max CFM. I guess you could use a bypass circuit with a valve that opens and allows the turbo boost to go around the blower, similar to the exhaust butterflies on a late model Corvette. You would still be spinning blower and eating power.

---------- Post added at 09:58 PM ---------- Previous post was at 09:49 PM ----------

That's the only thing im even semi-seriously considering (for my van). My take on it is leaving it on all the time but if you gear the supercharger to a fairly small PR (and associated rpm) and then use the turbo upstream of it to raise the boost when it spools, you'll have a much more heat efficient and less parasitic setup than if you tried to make all the boost with a roots blower, and you'll still have the 'instant' boost of whatever the supercharger does.

If you use the turbo upstream of the blower, wouldn't the turbo be choked to whatever the blower is? As in if X cfm can go through the blower, then only X cfm can enter the turbo cold side.