928S4 Twin Turbo "Design Review"

[Herr-Kuhn]

As a follow-up to the recently released dyno sheets on my twin turbo 928S4 I thought I would present what I beleive to be some of the key fundamantals of the design of the 32V twin turbo system.

The car had to retain the basic Porsche elements of clean yet agressive lines coupled with ultimate functionality and reliability. In order to assure maximum radiator airflow on full tilt it was decided to fit twin fender mounted intercoolers to under each fender, not at all unlike the way every new turbocharged Porsche and Audi is configured. Keeping the charge coolers out of the front of the radiator allows for maximum cooling. In line with tuners like RUF and Callaway, tastefully designed vents are fitted to the front bumper cover to allow airflow to each 4.5" thick, state of the art, air to air intercooler (800 HP capacity total for the pair). Venting was designed to match the elements of design employed by Porsche for the front bumper cover of the 928S4, these being a linear, yet streamlined look.



At speed, charge air coolers are rewarded with an abundance of ambient cooling air to allow for efficient cooling of the intake charge after being passed through the compression stage of the turbochargers.




The rear of the car had to give some indication as to what resides under the hood of the vehicle. A carbon fiber Turbo badge with the classic swoop Porsche "S" was chosen as a tasteful upgrade to the rear of the vehicle. All 928 badging was removed from the vehicle.



Interior modifications were kept to an absolute minimum and are simply limited to a pillar mounted vacuum/boost gauge. This simple yet very effective instrumentation allows the driver to monitor vacuum/pressure in the intake manifold at any given time without removing his or her eyes from the road. Elegance in simplicity which ties into an already exceptionally functional interior.



Opening the engine bay had to reveal a "visual feast" of the business end of the system. Piping runs were studied for many months and yielded a plumbing setup which captures many key elements of design including symmetry and balance. Twin compressor bypass valves are used to assure safety of the compressors.



The air filtration system had to meet several criteria, the first of which was to remain out of the 928's engine bay to minimize heat input to the intake charge. Additionally, the air filtration system had to be mounted in such a location to allow for easy inspection and routine maintainence. The area in front of the radiator and condensor was selected to allow for the best balance between cold air intake and easy access for proper maintainence of the filtration system. Aircraft grade hand layed epoxy/fiberglass intake "snouts" were fitted to allow the air to be drawn into the piping system yet still fit between the radiator's stock location and the hood of the vehicle. Several hundred hours were spend perfecting this design.



The 928s cramped engine bay demands two turbochargers for many reasons including, balance cylinder backpressure loads, balanced heat loads, minimum spool times/turbine light-off times, symmetry, maximization of airflow to each turbocharger through medium sized piping. By splitting the entire intake system to the turbochargers into two paths all of the above attributes were acheived and also resulted in the desired level of aesthetics for the build.



Cooled and boosted air from each turbocharger had to be combined before entering the engine. A proprietary plenum box was designed and tested to allow for effective combining of airflows without impeding overall airflow to the engine. An averaged boost pressure signal is delivered to each turbocharger via rigid wastegate lines which tie into the plenum box. In addition, the plenum box was designed to augment the look of the 928s engine bay.



Casual "side-onlookers" had to be aware this was no normal Porsche 928. "Twin Turbo" fender badges were created and installed to let people know that this is a serious piece of machinery.



Driving the twin turbo Porsche 928S4 is nothing short of delight. Positive manifold pressure builds as early as 1,700 RPM and full manifold pressure can be achieved by 3,200 RPM. The fact that the twin turbo system retains the factory intake manifold allows the engine to develop in excess of 550 ft-lbs of torque across a very wide powerband. These high torque numbers not only result in a peak of 550+ Rear Wheel HP, but also develop a super-strong midrange powerband, stronger than any other stock pressure fed 32V 928 engine ever produced on these pressure levels. All of this is accomplished on the totally stock 928S4 engine at a modest 0.8 BAR and can be run on pump fuel doing so. A muffled high-pitched whine is heard as pressure starts to build. Those who continue to hold their foot down are rewarded by being forced firmly into their seat. The vehicle is not for the casual driver or those who are easily startled.

The twin turbo system allows for a true "gentleman's car" when the boost is not required. When off-boost, the turbochargers simply idle along in the background, perhaps helping to erase some vacuum, yet never adding boost until called on to do so. Full manifold pressure is literally a throttle stab away. Downshifting is no longer necessary to quickly close in on 100 MPH passes in top gear with ease. Because the turbochargers rely solely on spent exhaust gasses, which would otherwise be wasted, they are extremely efficient. There is no routine maintainance for the turbocharger beyond being diligent with oil changes on the vehicle. Turbochargers have proven the boost method of choice by top tuners, and the worlds leading high performance automobile manufacturers. Their longevity has been proven for decades on just about every diesel powered18 wheeler running highways around the world today.

To promote ultimate longevity of the turbochargers a run-delay water cooling system is employed which circulates coolant through the turbochargers for a full 5 minutes after shut-down. This keeps the ball bearings cool for maximum turbocharger longevity.

Elegance Through Simplicity.

[lizard928s]

John, I have to admit one feature of your kit I REALLY like is that you used recirculator valves instead of atmo BOV.
This keeps that annoying ricer sound at bay, and actually helps keep the boost level between shifting up.

[Louie928]

Great job overall, John. It was a huge task and your efforts paid off with what appears to be an extremely successful outcome with no downside. Big congrats! Now, the real work begins....

[Herr-Kuhn]

Thanks a lot Louie, the positive comments mean a lot to me right now.

Yes, blow-offs would have worked, but you've hit the nail on the head on why I chose the Bosch recirc valves. I do not like that BOV sound. The recirc valves make it more civilized for sure.

[rhjames]

I really like how you put in the stab at 19 wheeler turbo reliability....


let's see....in the 27 years I've owned and operated over 25 trucks with various engines (brands) in them.

If memory serves me correctly, I've replaced a turbo on at least 75% of them, some of the engines receiving turbos two and three times during their lifespan.....

One of them, a CAT 425 electronic C model, had **** a center section within 56,000 miles since new truck purchase.....in other words, it was less that 4 months old.


Diesel turbo's are not nearly as reliable as one outside the trucking industry would believe.....

CAT is currently having issue with their twin turbo ACCERT engines. It seems that the compound turbo setup they run is building so much heat that it's destroying the bearings prematurely.


But, all of this doesn't really matter in your case John.

Your turbos will never see 40-55 PSI boost, or oil temps near 275 degrees for sustained time periods, with exhaust temps running 1300 + degrees.....


----don't take it personally---your set up is still the the cat's "roar" in the turbo 928 world......

--Russ

[lizard928s]

One other thing that you should mention to everyone who purchases a kit from you.

If you do a hard run and use boost for awhile, do NOT come home and just turn it off!
Then you get oil build up and coking of the oil inside the exhaust housing and this KILLS turbos. So after a hard run, come home and let it idle for about 5 min in the drive to let everything cool down, or 10 min before you get home, drive very civilized and calm not getting on it at all.
This is paramount to the longevity of the turbos life.

Did I mention that I worked in a shop rebuilding the little buggers for awhile, coking was the #1 killer of them, imo. And these little buggers should not be rebuilt at home, as the rebuild kits dont include a few needed parts, and you have no way of balancing them.

[Herr-Kuhn]

The twin turbo system has a water pump which is used to circulate coolant through the turbochargers for 5 (could be any amount desired) minutes after engine shut down . Between using a good synthetic oil and the water cooling with run delay the need for cool down is absolutely minimal. I've already put 7,000+ miles on the system and there is absolutely no evidence of any issues with the turbochargers whatsoever.

When I pulled the KKK K-24 out of my AAN Audi the mileage was about 186,000 and it was on it's original turbocharger. That turbo went onto see more life in a certain Gold Porsche 928. Now the car Audi has 280,000+ miles on the same RS2 turbo I put into the car nearly 5 years ago. It's driven daily to and from work and is pumping 24 psig on top of a nearly 300,000 mile engine.

[Herr-Kuhn]

http://www.bankspower.com/twin-turbo-products.cfm

The above link will take you to one of the authorities on forced induction: Banks Engineering. Of specific interest is the "Twin Turbo Power Guide"...it clearly spells out the benefits of turbocharging on a V8 engine. All of the principles listed can be directly applied to the V8 mounted in the 928.

[rhjames]

just as a footnote to the cool down issue:

Caterpillar had some out with a tech article stating that by the time you exited the (interstate) highway, and pulled into the parking lot (mind you it takes a while longer to park an 18 wheeler than a 928) your turbo was "cool" enough to go ahead and shut down the engine without doing the 5 minute idle time.

this was in an effort to reduce idle time (fuel consumption and wear/tear due to excessive idle time).

but this is worked around by John's water cooling.......

--Russ

[Herr-Kuhn]

I do typically let the car idle for about a minute before shutting it down. If it is cooler weather then I don't even bother with the idle down.

[icemang17]

John
Heres a ?? for ya.....will the Twin Turbo kit fit an automatic 928 with the tranny cooler above the A/C condensor??? It looks pretty tight there??

[Herr-Kuhn]

"Ice"...I realize I will have to rework some things right there for the AT cars, perhaps a different cooler or different snouts. VERY good eye you have!

[icemang17]

Originally Posted by Herr-Kuhn

"Ice"...I realize I will have to rework some things right there for the AT cars, perhaps a different cooler or different snouts. VERY good eye you have!

John
Another odd fact about 928's is ALL the GTS's have the exact same part # cooler in the same place????? Odd but true??? So even the 5 speed and auto's are the same......wierd huh.....so theres another quirk for ya in building a GTS twin turbo (but once you make it work for an S4 auto its all good!)

[928GT]

I'm no artist but KUHN PERFORMANCE looks better than
Kuhn PERFORMANCE.
That's about the only thing I would change.
Jim

[Herr-Kuhn]

Thanks for the feedback Jim.

JK