EVIL ENGINE goes Twin Turbo - Your Chance to Design the 930tt Exhaust System

[DW SD]

Originally Posted by 9dreizig

Brian, I'll have to stock up on leftover turkey at my brothers house!
So if you don't think there will be an advantage why do you think Porsche does it on the OEM lever? Surely not to see if it can be done? Cost ? Not turbo lag ?

There is no shortage of articles on the internet about twin vs. single turbos. This is specifically about V engine configurations, but the principles apply to boxer engines, too.

According to this resource (http://www.airpowersystems.com/ls1/turbo/turbo.htm), there is good reason to use two turbos:

"Whilst turbocharging can be a complex process, there is often a good deal of confusion in the market place regarding the benefits of single versus twin turbochargers - due mainly to the fact that some who dabble in turbocharging do not understand the issues involved.

The following discussion sets out to explain the strengths of each approach so that APS customers can make an informed decision as to which application is appropriate for their requirements.

Turbochargers demand a good deal of high energy exhaust gas to drive the turbocharger turbine. Exhaust gas energy is a function of the mass flow rate of exhaust gas, gas temperature and velocity. The higher each of these exhaust gas parameters, the greater the energy available to spin the turbines. This means that the exhaust manifold design and the proximity of the turbocharger to the exhaust ports is critical in the overall performance of the turbocharger.

A "V8" configuration engine such as LS1 has 4 cylinders on one bank and another 4 on the other bank. This means that for the optimum turbocharger operation in terms of turbocharger response and resultant engine power over the entire RPM range, a turbocharger must be located in close proximity to each bank of cylinders - ie twin turbochargers.

In a twin turbocharger configuration, each turbocharger is located close to the respective cylinder bank for the optimum exhaust gas energy transfer to each turbocharger.

A single turbocharger configuration on the other hand necessitates the exhaust gasses from each bank travel a longer distance than that of twin turbochargers located at each bank. The total distance travelled is determined by the placement of the single turbocharger but in short, exhaust gasses from one bank must travel across the width of the engine bay and merge with the gasses from the other bank before finally entering the turbocharger. This has a negative impact on the total exhaust gas energy available to drive the single turbine.

To offset the affect of lower exhaust gas energy available to drive the single turbine, the size of the single turbocharger must be reduced when compared to the total turbocharger capacity of twin turbochargers in order to achieve similar low to mid RPM engine performance to that of a twin turbocharger configuration.

There are 3 different scenarios to consider:

To achieve similar low to mid RPM power and turbocharger response as the APS Twin Turbochargers one would need to specify a single turbo of around 3/4 the size of the total turbocharger capacity of the twin configuration.
To achieve the same outright horsepower as the APS Twin Turbochargers, one would need to specify a single large turbocharger of equal (or slightly higher) capacity - which is a massive turbocharger for the LS1 engine. The down side is that the low to mid RPM response would be greatly compromised.
To achieve higher horsepower than the APS Twin Turbochargers, one would need to specify an even larger single turbocharger. This turbocharger would have an operational range starting relatively high in the RPM range (no useable power to speak of below that point). In this case, the engine would need to turn out to engine speeds well above the standard RPM limit to have a worthwhile power band. This single turbocharger may be viable in a competition engine which spends little time at low to mid RPM, but be unpleasant on the road in most driving conditions.
There will be a crossover point on the power curve if the single turbocharger is significantly larger than the twin turbochargers - and only in case 3 (where the single turbocharger is larger in total air mass flow rate than the twin turbochargers). This cross over point will be at some point high up in the RPM range.

But getting back to production specification - a single turbocharger (at around 3/4 capacity of the twin turbochargers) - its power curve will always be below that of the twin turbochargers at the same boost level. It's virtually impossible to achieve the same low to mid range power and turbocharger response from a large single turbocharger in a V configuration engine.

If the single turbocharger is matched to produce strong low to mid range performance (which would be the wise choice) then obviously the turbocharger specification will need to be precisely matched to the engine capacity. Bottom line, a large single turbocharger matched for strong low to mid range performance on the LS1 engine will always have a power ceiling of around 3/4 flywheel horsepower of that of the APS Twin Turbo system.

Regardless of the single turbocharger size, the real issue pertaining to high horsepower on a single turbo conversion for the LS1 is the very limited space available to package an exhaust downpipe capable of producing high horsepower. This is where we see the real limitation of the single turbo design for the LS1 in comparison to the twin turbo approach, unless you're prepared to cut the body sheet metal and make some fairly radical mods.

It's all very well to have an large single turbo, but when it's limited to low horsepower due to a restrictive exhaust down pipe (that necks down to around 2.5"), you'll never see the real potential of the large single turbocharger.

Hopefully this helps to put the single turbo in perspective - and to give one an idea of the challenges that are presented to APS as turbocharger system design specialist - who quickly discounted the single turbocharger approach as being fundamentally flawed in terms of packaging a system that produces high outright power and strong low to mid RPM response.

No doubt the twin turbocharger approach is the optimum configuration in terms of overall engine performance, and the slightly higher cost of a total solution twin turbocharger approach (complete with sophisticated fuel system) is a small price to pay for high horsepower, excellent engine response and street legal motoring."

I've also read, there are also arguments about less inertia in the turbos being able to spin up quicker. Of course, much is debatable.

[RarlyL8]

I agree with all of that - if you are turbocharging a 5.7L V8.
The capacity of a 3.3L B6 changes a few things.
Another very important aspect is engine management and component packages. Today you can build a single turbo component package with appropriate engine management that will do everything a twin turbo package will do on the same 3.3L engine.
The lines have been blured.

[copbait73]

Originally Posted by 9dreizig

Brian, I'll have to stock up on leftover turkey at my brothers house!
So if you don't think there will be an advantage why do you think Porsche does it on the OEM lever? Surely not to see if it can be done? Cost ? Not turbo lag ?

Same as drives all OEM decisions: emissions (quicker cat light off), efficiency (power, fuel economy), cost (on their level two can be cheaper than one based on volumes), finally marketing (more is better).
Marty

[Duckworth]

..Curious if you are satisfied having gone the route you've chosen - along with all the persistence needed to work out the various hurdles of the twin system...

Meaning, does all the time involved with this ever start to become a factor ?
If you're having a great time.., then that's a good thing. ..

I guess what I'm asking is, do 'turbo' guys ever get tired of dealing with 'turbo' issues ?

Does a normally aspirated motor - that could deliver some nice numbers - leave many guys 'wanting' for enthusiasm ? You do sometimes read where guys feel in their bones the turbo kicking in...and they are never the same after experiencing a motor -spooling up.


_____________

'82 Targa
'80 928

[Gertvr]

Subscribed, in my future as well.

[EarlySport]

Originally Posted by RarlyL8

I agree with all of that - if you are turbocharging a 5.7L V8.
The capacity of a 3.3L B6 changes a few things.
Another very important aspect is engine management and component packages. Today you can build a single turbo component package with appropriate engine management that will do everything a twin turbo package will do on the same 3.3L engine.
The lines have been blured.

I think the main premise of the article and hence twin turbo's is to position each turbo as close as possible to the exhaust ports for maximum efficiency.

Everything mentioned above applies to both twin and single turbo configurations ( Engine management etc ). I don't see how capacity changes anything other than turbo sizing.

A single turbo on a flat six is has compromised in it's position to all six exhaust ports. How much? I'm not sure. Many factors (including header coatings and design) will make a difference here.

Just IMO.

[RarlyL8]

The reason I am playing with all this is pure enjoyment of the hobbie. With any luck the engine will never be finished.

I started out with a modded normally aspirated 3.0L and then went to the modded 930 engine. No comparison in any way shape of form. My 930 has 2x the torque of the SC engine and feels like 4x. You cannot go back. It would be like getting off a Hyabusa and onto a bicycle.

On paper I can see where twin turbos may have an advantage over one large single. In practical use and with all modern technology in place I seriously doubt if anyone could tell the difference driving a single vs twins.

The SSI development is pretty much done so it's time to dust of the twins and continue on. I've got a few new ideas brewing ...

[Craig 930 RS]

Nice. Timetable for this sucker to run?

[RarlyL8]

Well, it should have been done six months ago. I took off a year and started this little company called RarlyL8 Motorsports, ha!
I do plan to get back to it this spring. The SSI development is wrapping up which will give me the mule motor back. I do have one more product coming out that must be completed first though. It should be done and on my car for testing by the end of February.

[Duckworth]

The reason I am playing with all this is pure enjoyment of the hobbie... I started out with a modded normally aspirated 3.0L and then went to the modded 930 engine. No comparison in any way shape of form.

Thanks....that's how we can benefit.. by asking a few questions.

...A final thought though :

Say you were on a trip in Hawaii and rented a car. They were out of Tercels - but happened to have an SC 3.0 in the lot.. You find out it has 10 : 1, twin plugged, CIS out, 'S' cams, SSI's, and Webers....

Turbo guys would experience (overall) - some excitement, correct ?

..

___________________

'82 Targa
'80 928

[beepbeep]

It's complicated.

Yes, turbos work better the closer they are to heads (and they like heat much more than they like tuned-length headers). Unfortunately, small turbochargers aren't as efficient as bigger ones. It's a matter of tolerances. Both big and small are manufactured with roughly the same tolerances.

If we talk in jet-engine terms, it's like RB211 having better efficiency than JT8D.

There is a sweet-spot. In DIY-world, I believe it's around 1.5L per turbocharger (using new fancy turbochargers). Less than that and you might as well pick up single big one unless single plumbing is a nightmare (and it usually is for V8-engines).

Single turbo = less weight and complexity. I don't see a reason for slapping twins on 3.0L boxer unless engine is built for very high HP-levels.

[RarlyL8]

Don't get me wrong about the N/A cars, one of the most fun cars I ever owned was a widebody '73 with a high strung 2.4L, webers, and open end supertrapps. Total blast drive.

What I'm talking about is power. The 2.4L didn't have it nor did the 3.0 when compared to the 3.3T that came after them. Whole different animal. No longer do I need to "carry speed", this thing makes its own whenever you need it.

[copbait73]

Originally Posted by beepbeep

Yes, turbos work better the closer they are to heads (and they like heat much more than they like tuned-length headers). twins on 3.0L boxer unless engine .

I've read your many postings and you are without a doubt very knowledgable in real world turbo application and modifications. I find your depth of experience and observation very impressive. You must have a fun job.
There is one area I take exception and that is the comment above. I've seen it repeated elsewere on this forum. Yes, turbos love heat, and to a point engines love turbos, but once the turbo is installed it becomes just one component of a dynamic system. It's not always true an engine system will deliver better performance on the street if the focus of your fabrication is placing the turbo closer to the exhaust port at the expense of a proper header system.

[Lukesportsman]

It's all a compromise for sure. BUT when stating "performance" it needs to be defined. Off boost response, turbo lag, rpm to full boost, or max horsepower are all different characteristics that could influence "performance". For street "performance", many focus on turbo lag and spool up.....where I think the short small tube primary headers come into light. With the big heat and strong individual pulses you in theory get faster spool. I certainly on the flip side would theorize that a long larger tubed header COULD yield a better power curve. We must also in a debate or argument discuss power under curve versus dyno numbers.

Race cars are obviously best with good numbers around the operating rpm range, though street cars respond best with overall power under the curve. These tend to muddle the idea of "best" header designs. You might as well throw transmissions in the mix then because it certainly might influence the best performing header in a particular application do to gear spread.

Just another Hoosier's thought.

[mb911]

[QUOTE=Duckworth;3706694]..Curious if you are satisfied having gone the route you've chosen - along with all the persistence needed to work out the various hurdles of the twin system...

Meaning, does all the time involved with this ever start to become a factor ?
If you're having a great time.., then that's a good thing. ..

I guess what I'm asking is, do 'turbo' guys ever get tired of dealing with 'turbo' issues ?

Does a normally aspirated motor - that could deliver some nice numbers - leave many guys 'wanting' for enthusiasm ? You do sometimes read where guys feel in their bones the turbo kicking in...and they are never the same after experiencing a motor -spooling up.



No never tired of it.. it is an awesome feeling when it spools and throws you back in the seat.. Although I do wish I had a 3.5 liter in a light weight mid year just to toss around once in a while. the turbo is a bit different as far as handling because of spool etc. ..

[RarlyL8]

The thing I really liked about my high strung 2.4L was the weber cards and the sound of the supertrapps. The Webers sounded like an Indy car when I started the 911 in the winter time. They have no chokes so I had to rappidly rev the engine as fast as my foot could pump the throttle. Very very cool sounding instant throttle response.
What I didn't like was the lack of torque. That engine was a dog under 3000rpm. The normally aspirated version of turbo lag. My 3.3L turbo actually produces more torque at 2000rpm than a stock SC engine and twice as much as the 2.4 in that range.
So for me the sound and throttle response were the draw. The turbo is no doubt quicker in every way than that fun old 2.4L, even when Auto-Xing.

[copbait73]

Originally Posted by Lukesportsman

It's all a compromise for sure. BUT when stating "performance" it needs to be defined. Off boost response, turbo lag, rpm to full boost, or max horsepower are all different characteristics that could influence "performance". For street "performance", many focus on turbo lag and spool up.....where I think the short small tube primary headers come into light. With the big heat and strong individual pulses you in theory get faster spool. I certainly on the flip side would theorize that a long larger tubed header COULD yield a better power curve. We must also in a debate or argument discuss power under curve versus dyno numbers.

From one Hoosier to another, I don't see any disagreement in "performance" except when it's measured in time to positive boost and boost level. Manifold pressure alone is no overall measure of how the engine is performing. For instance, your delay to spool may be caused by your engine struggling to over come backpressure induced to get that onset.

I didn't say to increase exhaust primary size. Exhaust expansion before the turbine inducer is less desireable. One positive feature of twins is the single large pipe leaving the collector (required of a single turbo) is eliminated.

Maybe my point is better made this way. Each cylinder should operate to the same conditions. On this there is usually agreement? Just as most accept that the intake manifolding should be the same diameter and of some reasonable equal length (we all agree the 930T CIS intake is far from optimal) so should the exhaust. If one is fabricating his exhaust primaries for the shortest route to the turbine, primaries are not equal and this is undesirable. The fact you intend to hang a turbo on the collector flange should make no difference in header design.

One more point that I see RarlyL8 really understands, how often do you drive ON boost vs OFF on the street? Street performance is about moving a free flowing naturally aspirated engine into a boosted one. Twins can make this transition better plus offer more potential at the top.

Your point about transmissions is true. In controlled conditions you can probably achieve better boost onset by reducing tire diameter than shoving the turbo closer to the exhaust port.

[RarlyL8]

Low gears can mask a host of evils.

Here's a picture of my old Banzi Runner. Easy to see why it was so much fun.

[Craig 930 RS]

Had a 79SC with a Varioram 3.6.......talk about throttle response.
It would rev when you ~thought~ of pressing the gas

3.6 Vario vs. 3.0 CIS:

[RarlyL8]

Back to our regularly scheduled program..

This is an interesting out-of-the-box configuration (thank you Sandman). Anybody know anything about this?