Ive decided that using the stock turbo manifold on my Z just isn't good enough (the upgrades never end). Ill be making a custom manifold and cannot decide if I should run the .63 ar T3/T04 turbo I have or run two smaller turbos in parallel.
The car is being built for XP class with Scca, general hoonery, track day fun and as an advertising piece for my shop.
I'd go with small twins. For autocrossing and general hoonery, I would want the quicker response and better low-end power. Power under the curve > big peak numbers
Interesting. Prevailing thought seems to say that a single turbo, properly sized and plumbed will spool as fast or faster than two minis in parallel. But I'm no engineer. Sequential turbos spool up fast, but it's real hard not to have a wonky curve when the second one is kicking in.
Turbo selection is a tough one. The internet is so full of misinformation and BS claims. Also 95 percent of discussions are more about peak numbers than quicker spooling.
Fitzauto wrote: The car is being built for XP class with Scca, general hoonery, track day fun and as an advertising piece for my shop.
I think that's your driving parameter. What's going to show off your shop best, an high profile one-off that's potentially too expensive for your customers or a more pedestrian setup that you can sell?
Yeah I'm with Keith on this one. Come up with a really nice and solid single turbo setup.
In reply to clutchsmoke:
That probably the direction I will go as many people have found that trying to weld an external wastegate adapter to the stock cast manifold does not end well.
At the same time twins may draw more people in...
Fitzauto wrote: At the same time twins may draw more people in...
Lookers or buyers?
A twin-scroll, single turbo setup is what I would do.
Thing is, I wasn't taking sides. Sometimes you need a halo car to grab attention. Sometimes you need a car that represents what you can build for your customers. We have both at FM, and they both work.
It depends in large part what kind of shop you are. If you're looking to productionize something like a turbo kit, then definitely your demo car should be representative. Put the parts that customers can buy on the car so they can experience it first-hand. We have several cars like this at FM. They don't get the press, but they sell parts.
If you do more custom work, then go nuts. Build something that will catch the attention of the community as an ad for what you can do. It doesn't have to be ridiculous like a SEMA sticker and rims special, but do something different and do it well. We have these at FM as well, they're what tend to get attention in the enthusiast press at large which lets people know about our company. They find out about us due to some ludicrous vehicle, then they discover that we have a whole bunch of parts.
In reply to Keith Tanner:
This is more what Im going for. While I would love to creat something that could be made in some kind of quantity I don't think that we as a shop could handle that right now.
Ive heard great things about ford sierra cossworth turbine housings on t3's (cossies flow pretty well and are .48 ar so quicker spool).
I vote build a decent manifold and stay single, twins are twice as cool looking but twice the pain in the ass for no real upgrade, at least not worth the pain in the ass I think.
All else being equal, I think twins are supposed to spool faster because each turbo has lower inertia. But that mostly comes from proofreading a turbocharging book, not from actual experience.
Ive always heard the faster spool argument but never seen any hard data on it. I must admit a big draw of doing a twin setup is just for the bragging rights.
Twins will spool faster if the correct sized turbos are used. If you build a engine to say run on 20psi of boost and you use a 76mm turbo in a single configuration to get there. For that turbine/compressor pair to make boost with a given exhaust flow it going to take a given amount of time. Now take the same 20psi and divide that between two smaller say 52mm turbo's. Since they each only need to provide 10psi they will reach that level faster given the same exhaust flow as the single.
Turbo's are a world of fun since you can start mixing and matching parts like the older t3/t4 hybrid turbo's etc. If it's just a wide open throttle car like a drag or king of the dyno a big single is the way to go. You know your going to be giving it max flow. A twin setup will tend to make better power throughout the power band. But it is alot more work to build and package.
In reply to Papabear:
Sounds like twins suits the cars purpose better than a single.
Keith Tanner wrote: All else being equal, I think twins are supposed to spool faster because each turbo has lower inertia. But that mostly comes from proofreading a turbocharging book, not from actual experience.
But they also have to spin FASTER to make the same amount of boost, compared to a single turbo large enough to flow the same amount.
Papabear wrote: Now take the same 20psi and divide that between two smaller say 52mm turbo's. Since they each only need to provide 10psi they will reach that level faster given the same exhaust flow as the single.
Nope nope nope..... You are confusing Pressure and volume(flow). If you want 20Psi each turbo has to produce 20psi in a twin setup vs a single... However, the mass flow each has to move is halved....
oy vey.
In reply to Knurled:
It's a simple work balance equation.. simplified the work going into the exhaust housing = the work going into the compressor housing. Speed is relative.. why does it matter how fast the shaft spins?
Papabear wrote: Now take the same 20psi and divide that between two smaller say 52mm turbo's. Since they each only need to provide 10psi they will reach that level faster given the same exhaust flow as the single.
They both still need to hit 20psi, each only has to flow half as much air at that pressure. My understanding is that it's mostly about the potential for two smaller turbos have a lower total inertia than one larger turbo, due to inertia increasing with the square of radius. Thus two small turbos might be able to reach 20psi more quickly than a larger single. The caveat being that the well selected twins need to actually have a lower combined inertia than an equivalent well selected single. However, the equivalent larger turbo will also typically be more 'efficient' due to a lower ratio of tip clearance leakage. Honestly though, I question how much difference there really would be on equally well implemented (equivalent) twin vs single setups...Especially if limited to using only readily available turbos.
In reality, I'm sure the vast majority of aftermarket twin setups are primarily done for packaging reasons and/or the 'wow' factor, rather than genuine maximization of the underlying scientific principles.
I'll add that I have experienced a big single in a similar layout to the Z31 (72mm turbo mounted on the right exhaust manifold, with the left feeding a crossover to the right) and boost was instant when you applied throttle. It actually would make the blowoff valve chitter at part throttle.
It was a high compression V6 in the mid-4l range and made something like 800whp. The high compression, I think, was key. Need race fuel or E85 to do it. But there was zero lag whatsoever.
In reply to Knurled:
Sounds very exciting. And fragile
In reply to Fitzauto:
Not really. Had pretty much the same pressure-holdin' parts as a 2000hp engine, just a lot milder as far as airflow is concerned.
Besides, IIRC it was only 23 or 26psi boost. Nothing serious. That is another key factor: low boost means you get from zero to max in a hurry.
Compound turbocharging...because measuring in bars is more fun than psi...
High compression and turbocharging is fun. So much more fun than low compression.
In reply to Keith Tanner:
Yep! Sort of regretting the decision for doing a 7:1 turbo engine for my VW. But, the goal was to make the highest HP ten-valve Audi engine in North America, and the combustion chamber is garbage, very detonation prone, so you either drop boost, drop compression, or retard the timing so much that EGTs shoot to the moon, at which point you drop exhaust valves.
I figure Inconel exhaust valves and low compression should allow me to run all the boost... and make almost as much power as an unopened 2.5l Rabbit engine with a modern turbo on pump gas. 
I've never been accused of bein' smart.
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