In general most mechanical engineers will design systems so that they should be able to tolerate 200% of the intended load without catastrophic failure. I would imagine that Porsche's engineers are of that mindset. Now, in our engines 200% of load is not equivalent to 200% HP, since engine loads increase in a non-linear fashion.

 

Really ..................

 

I have been told by my tuner that Porsche engineers usually over engineer by 30%... that is why we are getting away with 600 rwhp without a problem.
I ran my car at 1.3+ bars of boost for 50K miles no problems....

 

Hey if crankcase is still intact - I'd call that a success!!


Seriously though, 30% added hp is probably on the order of close to 100% increase in engine load, especially if the rev limit has been raised.

The twisted/thrown rods are usually secondary to detonation/boost spikes/lean outs which realistically place a momentary load well in excess of whatever headroom the engineers could spec - i.e. much more than 100% excess.

Realistically though, I really wonder what kind of margins they are using nowadays. When I changed the exhaust on my TT I was extremely disappointed in the cheap steel they used on the exhaust studs. I snapped 2/6 turbine outlet studs on a 1000 mile car. By comparison, my old 944 turbo rusty 21 year old studs were very tolerant of R&R even with air tools.

 

I'd be willing to bet I could wear out your motor in less than 40 hours at any of my favorite tracks. A built motor on the street will last me 7K to 15K miles before rebuild.

 

Here's one of mine:


GM factory rods have there limits, too.

Jeff

 

I dont think that is hard to do... my point was that our cars stock are over engineered as is so we can get away with a lot more...
Just about every cup motor gets torn down every second race... You can't get around HEAT.....
Drive smart, use good gas, check for boost leaks and monitor ur AFRs... simple basic precaustions that go a long way...

 

100% so true, Sonny.

 

There are trade offs with any change to the exhaust manifold. Larger diameter piping reduces back-pressure, so as to ensure that the burnt air/fuel mixture is able to escape your engine smoothly, without choking the engine. If the pipe diameter is not able to adequately flow all of the burnt air/fuel out of the engine, the resulting back pressure will impair performance, particularly on the top end. On the other hand, larger diameter piping also reduces the velocity of the exhaust gases, thereby increasing turbo lag (among other things). Thus, any increase in the diameter of the header piping, whether through porting the OEM manifold or purchasing an after-market header, will generally reduce back-pressure, but increase lag. The key is to properly balance these two considerations, so as to prevent back-pressure but keep velocity high.

The OEM piping was designed to flow enough burnt air/fuel for the OEM power, without back-pressure, while maximizing velocity for turbo spooling. The OEM headers were NOT designed to support higher HP applications, which involve a greater amount of burnt air/fuel escaping the engine. That being said, the OEM headers, with or without porting, will permit a greater amount of burnt air/fuel to escape the engine than created by the OEM parameters, without unduly restricting pressure and impairing high end performance. In other words, you can use the OEM headers for power above the stock level, without excessive back-pressure. Moreover, it is obviously desirable to push the OEM headers to their limits, before increasing pipe diameter, because increased pipe diameter will reduce velocity and increase turbo lag. For example, if one is merely doing an ECU tune and exhaust, the OEM headers can likely flow the additional burnt air/fuel without creating back-pressure and impairing performance, and by retaining the OEM headers, your retain the velocity of the smaller diameter piping.

There is a point, however, where the OEM headers, even ported, will not flow sufficient burnt air/fuel out of the engine, and will therefore create back-pressure and impair performance. For the record, I have no idea where that point is. I only know that it exists. At that point, if you retain the OEM headers, you will have excessive back pressure that will impair your ability to make top end power. Every tuner has a different theory as to where that point is in the power range. Some say 600 fwhp. Others say it is higher. Most tuners concur that the really high HP 996TTs NEED larger diameter headers to properly expel the additional burnt air/fuel generated by their engines. Whether certain other lesser powered 996TTs will benefit from larger diameter piping depends on the particular power level in question and is subject to debate. That being said, the more power you are generating, the more likely it is that you will benefit from larger diameter headers.

There has been a lot of discussion of particular headers (e.g., EVOMS, Fabspeed, Protomotive, etc.), but no mention of the actual diameter of those particular headers. You cannot discuss after-market headers without reference to pipe diameter. For example, EVOMS offers at least two different after-market headers, each with a different diameter piping. They offer a header that is marginally larger than OEM piping, so as to permit greater flow while not reducing velocity too much. These are the headers they recommend for most applications where additional flow is needed. Unfortunately, I do not recall the precise diameter of these headers. EVOMS also offers even larger diameter headers for very high HP applications (44.5 mm). These headers would be inappropriate for lesser HP cars, but are necessary to prevent back-pressure in very high HP applications.

There has been discussion in the thread regarding whether the benefits of larger headers are sufficiently powerful to register on the proverbial butt dyno. I suspect the answer is no with respect to cars in the middle HP range, but yes with respect to higher HP cars. The greater the restriction in flow and resulting back-pressure created by the OEM header, the greater the improvement that will be achieved by relieving that pressure through larger diameter headers, and the greater the resulting increase in performance that translates to seat of the pants feel. If one did an A/ B test with JoeTwinT’s car, comparing OEM headers to his considerably larger EVOMS headers, I have no doubt that the difference would register on all of our butt dynos . . . in other words, we would all feel the reduction in top end power resulting from the restrictive OEM headers. On the other hand, I would not be surprised to hear that one does not feel the gains achieved in a 600 fwhp car. However, the fact that one does not feel the gains/benefits/improvements does NOT mean they do not exist. It simply means the gain is not so significant as to translate to the butt dyno. I don’t know if anyone has ever tested just how sensitive the average butt dyno is, but I suspect 20 HP changes in power on a 500-600 hp car are very difficult for most people to feel (the percentage delta is less than 5%). The fact is that MANY after-market modifications that create verifiably increases in performance are not individually negligible to the typical butt-dyno. That does not mean the particular modification does not work . . . it simply means you cannot feel the impact of that particular mod, isolated from all other mods. On the other hand, if you aggregate the collective impact of several different mods, the results are most certainly negligible to all butt-dynos, and that is the fundamental goal of car modding.

There are several other considerations visa-vie headers beyond simply pipe diameter. For example, the design of the collector is also important. You want a collector that ensures smooth flow of the exhaust gases and does not disrupt or deflect flow so as to create back-pressure. Frankly, the OEM collector is not ideal in this regard, and a properly designed after-market header will utilize a collector that is considerably better than the OEM collector.

Another relevant consideration is how low the header protrudes underneath the car. I initially used an S-Car-Go header. It sat very low underneath my car and constantly scrapped the ground. My current EVOMS headers do not sit as low and, therefore, scrape much less.

There has been some discussion of what certain Euro-tuners do visa-vie headers. I researched this issue a while back. Several well-respected Euro-tuners do use larger diameter headers for their higher-HP applications . . . some Euro-tuners offer different diameter headers for different applications, just like EVOMS (9ff is one that comes to mind). Therefore, the notion that no Euro-tuners use after-market headers is simply not true. On the other hand, if a particular Euro-tuner does not offer a package with greater than 650 hp, they might well stick with the OEM headers.

That’s my 2 cents. Headers can and will improve performance under the right circumstances. As I said above, I have no opinion as to the dividing line in terms of when one should upgrade their headers. Al I know is that, at some point, it is a worthwhile investment that will translate to more power (with the proper tuning and complimentary modifications).

Regards,

Craig