header sizing

[r lane]

For a 3.0, twin plug, 12-1 track motor, what is optimal header sizing, OD. Related thoughts are header style preference. RSR style with a triangle collector or Bursch style with a pyramid collector. How does material figure into this as a resonance effect. Steel or stainless. Any dyno comparisons? Bob ps pictures found in the attic. Andretti at Sebring

[dannobee]

You'll need a lot more info than just displacement to make an educated guess. Cam duration? Cylinder head flow? How long are the intake tracts? What RPM range? Stepped headers? If so, how many and how long? That's just a start.

Most quality racing headers will be 321 stainless. If you want them thinner, lighter and more durable you can $tep up to Inconel 625. Merge collectors work best.

Didn't you ask this same question a few years ago?

[Bill Verburg]

Originally Posted by r lane

For a 3.0, twin plug, 12-1 track motor, what is optimal header sizing, OD. Related thoughts are header style preference. RSR style with a triangle collector or Bursch style with a pyramid collector. How does material figure into this as a resonance effect. Steel or stainless. Any dyno comparisons? Bob ps pictures found in the attic. Andretti at Sebring

you would need at least displacement and rpm to make an educated estimate

but here the theoretical max n/a flow per side vs rpm @100% VE


and some other pertinent info




ti Ys are common 4 and 8s and inline 6s but hard to package on a 911, though it can be done

[Henry Schmidt]

I am not an engineer but I can evaluate trends.
One trend in air cooled development seems to be a resistance to change.
For generations we have seen headers for air-cooled 911 engine basically unchanged with the exception of primary diameter and collector design.
My question has always been, why are so many engine builders sticking to a header design that was conceived for a 2.0, 8000rpm engine? Same basic runner lengths regardless of cam configuration, intake lengths and desired RPM range.
We've been experimenting with runner lengths and collector with some good seat of the pant results. The idea of long primary tubes to create "better" low end torque in engines that never run in what I consider to be "low"end rpm range seems ridiculous.
I no longer have a dyno so I can't produce "real" numbers but customers seem happy with the results.

These headers are not our design and the origin of the design seems to be in question but regardless of the designer, these little guys are showing us a lot of seat of the pants smiles.

[Bill Verburg]

The basic idea of sizing is to balance two things, flow velocity and pressure build up.

Essentially you want as high a non turbulent or laminar flow as possible, but bends merges changes in cross section also affect this

then there is the other aspect, acoustic tuning, A properly designed set of headers can generate a nice strong rarefaction signal at the exhaust valve at TDC compression, w/ the right cams this vacuum signal sucks additional mixture into the combustion chamber yielding extra torque & hp.

The strength of the rarefaction signal is proportional to the change in cross section area of the pipes, there can be be more than one signal if here is more than one change in cross-section area, the bigger the cross-section delta the stronger the signal, to get the best result the collector must be open to atmosphere or to a set of tapering megaphones which add additional tailoring to the signal depending on their geometry. W/ a muffled system most of this acoustic tuning component is lost due to the reduced cross-section area of the transitions from pipe to muffler, the distance from the cross-section delta times the delivery of the rarefaction, hence the concern about pipe length.

unfortunately what works great at low rpm falls on it's face at high rpm and vice versa.

similarly using too small on a big engine works great at low rpm not so much at high rpm

on a muffled system the acoustic tuning aspect is mostly not there so flow is the primary concern, though small gains can be made they are small and hard to come by

an example is seen in 993 mufflers where the factory on the n/s mufflers arranged for s large a cross section change as space allowed inside the muffler



The turbo mufflers on the other hand are solely concerned w/ flow so they use perforated pipe, which is only slightly less efficient in the flow department


the physics of gas flow says that bends and changes in diameter and the geometry of those changes all affect flow velocity and turbulence, hence fewer bends w/ longer radii, and carefully arranged merges, an attempt to retain heat is often implemented as well, higher temp gas flow faster.

911 engines are blessed w/ a favorable firing order which leaves plenty of room between pulses, though residual pressure never goes negative the reduced pressure between pulses does help to speed flow