jimculp

So why are unequal length headers not good for track use? Do they heat up too much?

kellcats521

In the 'perfect' world of track use, you keep the motor in a fairly narrow RPM range which will be in the upper RPM range of the motor, where the equal length gives you the most benefit. Low RPM grunt and quick spool up at low RPM are not important in this 'perfect' world. Heat is not an issue WRT primary length - any differences would not be measurable.

In reality, DonE is on target - the shorter, un-equal length header is the way to go for 99.99999999999% of 911 Turbos. Even most of these track cars will not be set up ala 934/935 with the huge turbo and very narrow RPM range - modern turbos and fuel injection allow for much broader power curves and way better drivability across a bigger RPM range, both on and off of the track.

nathanUK '81 930 G50

Because equal length will make more power, but at the expense of not spooling the turbo as quickly.

jimculp

Thanks Nathan, but I still don't get it.
Are you saying that on the track, the car is generally run at higher sustained rpm's making the loss of spool time insignificant?

underpsi

Not if they are short tube equal length headers.

nathanUK '81 930 G50

Hehehe, this is getting confusing.

Basically tuned headers (meaning the lengths are played with from where they bolt onto the head to where they join into the next pipe or collector) will give more bhp at a certain speed of gas flow (rpm/bhp). Then there is also the diameter that could be tuned too...
Just like a cam is of a certain lift/duration it can be chosen (tuned) to operate better at a certain rpm.

The headers can get the gases out of the engine better allowing the engine to give more bhp. Possibly at a certain flow of gas (rpm/bhp) there will be room for a cylinder to empty into the tube, if however one cylinder emptying crashes into another at the collector there will be a restriction and bhp will be down.

Another tuning idea is the variable intake length (sometimes known as G). Porsche called theirs varioram I believe but I don't know much about the later engines.

On N/A cars sometimes the headers will be tuned to give better lower rpm power to help a cam that is tuned for high rpm.

I think also it is the same for port sizes in cylinder heads, smaller ports means more low rpm power at a expense of reducing high rpm power.

It also comes back to a large Vs a small turbocharger. The small will spool earlier but the larger will not run out of air at high rpm/bhp.

There are many options when tuning an engine, I would just take Stephens advice as he has access to his own dyno and lots of experience with our cars.

DonE

So, there you go. Shut down the list - we only need to make one call....

NineMeister

I doubt that there is any real advantage of equal length headers over unequal length with respect to tuning the average 930, in the real world the header pipe diameter, collector diameter and turbo nozzle play a far more influential role in the operation of the engine as a whole rather than the length of the header pipes.

The theory for equal length is to match the resonance & flow between the cylinders, the result being more power where all these effects have a positive influence on the cylinder filling. However the fundamental problem with this approach is that the effects will only work at certain rpms (roughly equivalent to the harmonic lengths of the gas in the pipe) and at all other rpm points could actually work against the engine, so the result is usually seen as a fluctuating torque curve. Now, as far as I am concerned, a properly tuned engine has as wide & flat a torque curve as possible, so we take a lot of trouble to tune out these undulations to gain torque in the dips and increase the overall area under the curve. We achieve this using unequal length headers.

My conclusion w.r.t. the average 930/965 at up to 550hp is that although equal length headers are capable of making a peak hp gain, unless the design is perfect you lose elsewhere in the curve and the nett result will be that you are driving a slower car.

Falk930

Ok, sounds understandable.
Which pipe diamter recommend the experts?

Falk

NineMeister

For up to 500hp we use a 1.5" primary pipes to maximise gas velocity which increases the scavenge efficiency on overlap.

Here is an example of maximising the area under the curve incomparison to a standard 930. Jon's engine would be excellent on track, boosts early, strong midrange and hanging on to the redline with a power peak above 6000. This engine runs 1.5" unequal length primaries.

bogey1

Equal length is going to hurt spool times and create a dead lower end of the RPM range. If your cars are not upgraded to EFI and pushing in excess of 500rwhp, the end result will be dissappointment in the vast majority of driving conditions. Track use may be another story, but if its a short - tight track I still think most owners will be going back to the B&B. It sure was the case on my 3.6. I do agree Marco does great work, but life without heater boxes is no treat when you need to defrost.

LukeSportsman

OT:
Colin, are the 930 heads on the market yet?

pzull

Exhaust tuning is mainly two fold: Wave tuning (sound wave pressures) and Scavenging (exhaust gas flow). For turbo cars tuning will mainly benefit from scavenging not wave tuning

Primary pipe size determines which rpm peak scavenging effect is at while pipe length shifts the tuning effect to below or above that peak point at the expense of the other

Larger pipe sizes shifts peak tuning point to higher rpms. Longer pipes shifts tuning to rpms below peak rpm at expense of lowering tuning at rpms above peak.

Imagine a bell curve. The apex of the bell is shifted higher rpm as pipe size increases and the bell is tilted to lower rpms with increased pipe lengths with the pivot point being the apex of the bell.

Max non-turbulent exhaust gas velocity is theoreticall about 240 ft per sec at peak tuning rpm. Above that it becomes turbulent and power is lost. Below that gas flow is slower and scavenging effect is less. Assuming most of our cars have peak torque at about 4k-4.5k rpm and turbo typically spools up from 2k rpm and assuming gas flow velocity at 2k rpm is about 100 ft per sec, an extra foor of primary pipe will only cause a delay of 10 ms to reach the turbo.

If however that "delayed" exhaust pulse is stronger on the average due to less loss from clashing of exhaust gas pulses due to equal length headers then turbo may actually spool up faster.

If we drive the car like a sedan on the road then I think the short B&B non equal length headers will feel perkier but if we drive is a spirited fashion where gear changes result in rpms seldom falling below 2.5k-3k, I believe a short equal length headers will be beneficial (Marcos dont seem to have optimised to be shortest possible)

Of course I am only theorising and have no real world proof.......but may soon have.

jimculp

Very interesting post pzull.

pzull

saying it in pictures....