Twin turbo configuration questions: Theory & Application
 

From what I understand the two (993/996) Porsche factory twin turbo set-ups have the turbos running concurrently.

Why do they not go with sequential operation. It would seem most benificial to have one "smaller" turbo hand off the task of compression to the larger "turbo" at the right time. Lag would be minimized, but at what price, if any?

I also recall Protomotive taking quite a while to develop and synchronize the twin configurations for their aftermarket offerings. Is it a matter of mixing/matching turbo components/characteristics and endless dyno runs to get the optimum combo or are there other factors that aren't as obvious?

Probably because it just isn't that easy to get it to work right.
Many companies have tried but the compromises are just too great and usually outweigh the benefits. it can be done and would work better in theory.
I believe Mazda and toyota have pulled it off with limited success.
Most will agree twin non-sequencial turbos spool up almost as quickly, provide greater gains at the top end, and are more efficient with less backpressure that anything but the very best engineered sequencial system, which can't be tampered with very much.

From what I have read, sequential turbos are very complex compared to an equal-sized twin turbo setup. I would also imagine that the 911 engine bay and the flat-6 configuration make it a challenging plumbing exercise. That said, it was done on the 959 so it's not impossible.

What porsche did learn with the 959(sequential turbos) is that is was not cost effective for production. Twin turbos are pretty effective as is without the added headache of sequential.

I would think complex plumbing and limited space would be the main obstacles.

As JP911 said, it's not impossible since Porsche did it with the 959.

Anyway, why re-invent the wheel? Parallel twin turbo's work great with the Porsche engine's layout ;)

Thanks for the answer guys.....

So, if designing a 3.0 or 3.2 system would it be of benefit to go twin or single? I'd guess if not lowereing compression and doing other internal upgrades then single would make sense. Unless there are plans to modify further in the future.

Also, since the plumbing is so complex on a sequential would it make sense for designers to make a two or three "stage" turbo? Or is it already out there?

A twin turbo setup yeilds some benefit for engines with displacement of 180 ci and up. Your decision should be based on a few factors, most importantly budget and driving style. Most single turbos come up on boost around 3000 rpm.

I assume that you are thinking of building your own system, whats the scoop?

Actually, I have most of a Protomotive 3.2 stage-one side mount system (used). I figure if I jump in and start fabricating additional pieces such as headers, intercooler and other plumbing why not consider other options. Although the install can be done with the engine in I'll be dropping it for better access.

I'll most likely go with mild boost and intercooling. Future plans will most likely include an engine redo with upgrades/mods to handle additional boost.

I was also curious about Porsche's thinking behind sequential vs matched turbos...I'm not considering the build of a seq system.

You will have fun. I recommend "maximum boost" by Corky Bell. Good stuff in that book. I picked up an 87 930 engine/tranny for my 914. I have decided on twin turbos to make my install more efficient.

Sequential turbos were an attempt to cover all of the rpm range and have big horsepower #s too, so no lag yet the turbo isnt maxed out at 5000 rpm. Maybe someone can chime in on it. Matched turbos require you to decide where you want power to be at. Small turbos for quick spool larger for upper rpm range and bigger horsepower . The GT2 turbos are a good example, they have good upper end but are not that great for street driving.

The compromises that you mention are what led me to ask if there was a "multi-stage turbo" in a single unit. Or perhaps an adaptive/progressive type of operation. You'd have to think someone has tried to design this.

BTW, any pics of your project?

I'm not sure, you'd have to talk to a garrett dealer, but they offer some option on their turbos that variate the turbine of the turbo. I'm not sure if it's a varition of exhaust size, or AR ratio, but it's an expensive option. I also believe the new evo has something similar which enable it to boost just after 2k rpm all the way to 7k with no drop in pressure at high rev. That book "maximum boost" i read and second the opinion of it being a very good book, they also show how this turbo called an "aero" turbo works, which changes AR ratio of the turbine constantly to make it a virtually no lag system.

Are you taking about variable a/r turbos? If so those are neat. I have heard of overboost problems when you attempt higher boost levels though. If not tell me more.

I dont have pics right now, I am slowly getting the parts together. I need intercoolers(water) and start to fab brackets, headers, plumbing for the intercoolers. The budget is definitley getting a little heavy, but it is no where near what most people spend on their rides.

Will you put the system on your 85?

Yes, the plan is for my '85. The only thing to prevent me is if I stumble onto a low mile '89 930 :D

I'm obviously in no rush...just trying to absorb as much as possible before I commit.

How do you guys fabricate your intake plumbing? Send it out to get mandrel bent or weld cut pieces together? Could one use some of that flexible exhaust pipe to get the complex shape and sent it somewhere for duplication in a superior material and form?

Last month Bruce Canepa gave a group of us a tour of his facility. We were allowed to see some of the 959's he was working on up close. He is replacing the original turbos with new custom ones. The new ones operate nonsequentially. He said the original setup, with equal sized sequential turbos demonstrated a large dip in power when the second turbo kicked in. This was due (if I remember correctly) to the pressure drop when the second turbo kicked in. His new dual setup develops over 1 bar at 1500 RPM.
See photos here, scroll to the bottom: http://www.montereydiving.com/BAR/020704bar.html

-Brad

My buddy is big into drag racing and street racing turbo Buicks and he says they all run big single turbos, because they generate more power with less headache. His car is unbelievably fast, like a 9 second street car. That's Buick and not Porsche, though. Just my $.02.

My piping should be straight forward so I will be piecing and welding then powder coating. For exhaust I will make them unequal and as short and straight to the turbos as I can. There is debate on tuned versus untuned, I would like to hear other opinions on it. I feel with such mild cams tuning is a waste, what do you think?

Sequential turbos is not DIY-thing. Most manufacturers that went that route (Mazda RX7, Porsche 959 and Toyota Supra MKIV) had problems with boost dips and sticking exhaust valves. Also, when Supra guys want big power they go for one big turbo, due to better efficiency and aerodynamics of bigger turbos.

As i said before, i very much doubt you could fabricate sequential turbo setup w/o hughe amount of money and expirience.

My advice to you is to go single if you don't have (lot's) of expirience. Use two if you are confident that you can fabricate custom headers and intercooler, altough i doubt you would benefit that much from dual setup on engine that small.

Good luck!

Traditionally the large turbos afforded one the air volume required at higher rpms. However, spooling up took quite a bit of time...and there's your lag.

Seems like the newer units are more efficient (Garret GT series)but I can't see overcoming the lag unless using a unit as described by 1fast and Burn-Bro.

Goran,

As mentioned I have no plans on a sequential setup. You obviously have quite a bit of turbocharging experience....what did you think of my header fabrication question?

Thanks for the feedback.

The hot setup where money is no object is a turbo with variable inlet vanes. That effectively changes the loading on the compressor so it spools up quickly like a small turbo but can handle higher pressures and flows like a larger turbo. Basically an adjustable A/R ratio.
Problems are higher cost, higher complexity, and potentially higher maintenance.