Exhaust pipes remain one of the last bastions of hand fabricated metallic parts on an F1 (along with radiators). While we can all marvel at the metal shaping and immaculate welding, the exhaust is also a key tuning aids for the engine and highly influential on aerodynamics too. So here’s a quick dive into some of the unseen technicalities and details of the F1 exhaust from the normally aspirated V8 era.
In the engine tuning 101, its clear exhaust design is a key area of the engine’s performance, this is no less important in F1. From initial 1D simulation to more complicated thermal/CFD analysis in the latter stages, the exhaust’s tuned length is the job of the engine manufacturer. Subsequent dyno testing will confirm the exhaust specification.
Technically the exhaust system starts at the exhaust port inside the cylinder head. From the two exhaust valves, the ports reach out near horizontally to exit the head as two siamesed ports, rarely as a full rounded port, but an open figure of eight.
The exhaust port flange I have is trypical, formed of a flat flange with the exhaust port merging into the rounded primary exhaust section. The flange bolts to the head with four M6 fastneners, although three fasteners is also common. The port at the head is some 70mm wide and 30mm tall, this expands in just 60mm to a round pipe of 50mm.
This port is often surrounded by cooling passages in the head to manage temperatures in the aluminium casting. As the path from valve to head exit is so short the merging of the two ports continues in the initial part of the primary pipe and shortly after the exhaust forms a full rounded pipe.
To package the finished individual exhausts into a complete system the ports are angled to something other than perpendicular to the engine centre line, thus the ports tend to point outwards when viewed from above. This being a gesture towards the chassis designer, to ease final installation of the large diameter pipes, rather than for pure engine performance.
From the exit from the head, the primary pipes are then stepped up in diameter to tune the exhaust pulses back to the exhaust valve. Single, double or sometimes triple steps are employed and the example below shows the more common double primary pipe step. This installation takes the port shape into a 50mm Dia pipe, which then steps up to 57mm pipe after a 135mm long intersection, then after a further 120mm the pipe steps up again to 63mm diameter in a 127mm long transition. This creates a primary pipe of 482mm in total length.
As with all the pipe lengths, the measurement is taken at the pipe’s centerline and all joins are square to the pipe’s centerline.
Then the equally sized primaries enter a collector to merge the four pipes into one. At various points in the 2000’s I have seen F1 pipes with a 4-2-1 format, with the 4-2-1 merging all taking place in the short space of the collector. But typically its 4-1 format used with the latter V8 engines. As with the merging of the exhausts into the primary pipes was done within a designed length, so too is the merging of the pipes into the collector and equally the radius of the collector merging into the tail pipe. Additionally, the order of the pipes joining the collector must be in a specific order of 1-2-4-3 in a clockwise direction.
The 63mm dia primaries enter the collector with a 20mm overlap. The individual pipes all merge inside the collector within a length of 57mm, while the collector is a total of 138mm long. The joint into the 82mm dia tail pipe is subject to a minimum radius of 10mm to keep the gasses flowing smoothly into the tail pipe.
Lastly the collector merges the primaries into the secondary tailpipe. Again of a specific tuned size in terms of length and diameter, being some 82mm dia and 300mm length, with no steps or changes in section. Remembering the latter years of the V8 Era had regulations mandating cylindrical and maximum diameter specification for the last 100mm of the tail pipe. This being done to try to reduce the exhaust blow aerodynamic effects. Before that, the engine designer did allow angled cuts to the tail pipe end, albeit as long as the resulting shortest pipe length met the design length.
For the Exhaust Blown Diffuser era, the teams wanted the exhaust tail pipe to form a nozzle, as wide and flat as possible. An example I have seen takes a 75mm diameter tail pipe and flattens the outlet to form an exit just 40mm tall and 145mm wide.
This helps direct the exhaust gasses tangentially with the floor to improve the blown effect. The later Coanda exhaust exits have already been detailed on this blog http://scarbsf1.com/?p=4548
Attached to the exhaust are several sensors, in each primary pipe is a UEGO sensor (Universal Exhaust Gas Oxygen), which are less sensitive to where along the pipe they are mounted, although they all need to all be the same distance from the exhaust port, with an approximate position halfway along the primary pipe being suggested. Giving the designer some freedom to position them so as not to clash with the rest of the installation. While the fifth UEGO sensor and thermocouple on the tail pipe is more sensitive to positioning and is thus part of the engine designer’s specification.
From this basic tuned length specification, the job of designing the full 3D exhaust system is actually the job of the team. As the Design office needs to trade the ideal exhaust layout for factors such as aero, thermal and general packaging. This contorts the exhaust system into a twisted bunch of pipes, somehow meeting the design specification. For the primary pipes the critical issue is not to bend the pipe into too tight a radius, for the engine designers they’d rather accept a compromise a different length pipe than a tight exhaust bend, with a 10mm Radius being the minimum. Then the secondary pipe is a little more flexible in length, albeit any variance from the ideal will have a small performance impact. The engine manufacturer providing a minimum length of 180mm and a maximum tuned length of 340mm for this pipe, which provides the aero and chassis designers with greater scope for the pipe’s exit position. Furthermore, a reduction in tailpipe diameter is allowed for a different nozzle effects for blown aero so the pipe could be reduced from 82mm to just 70mm with agreement from the engine supplier.
The material of choice for exhausts is Inconel, notably hard to machine and form, but suited to the weight and thermal demands of the F1 designer. Other material choices are titanium and cobalt steel, while Glass ceramic composites such as the brand Pyrosic are capable of withstanding the temperatures and pressures of an F1 exhaust, but regulations preclude this. Thus the use of composites has to be for external heat shielding and areas beyond the tail pipes.
Pre-made short pipes, bends and sheet material is formed to make the individual tubular sections and the collector. With the pipe\sheet fabricated rote being taken to meet the thin wall requirement for lightness. These being hand finished in a jig before being welded in an inert atmosphere and the resulting weld lines ground\polished from the inside surfaces.
One area where the complex shape become arduous for hand fabrication is the exhaust port stubs, these used to be cast until 3D printing became possible with Inconel and now they are similarly thin walled printed items further reducing wall thickness and weight. In fact, one of the differences in weight of older mid 2000’s exhausts compared to modern assemblies is in the weight of these stubs. Typically, 3 or 4 studs mount the stub to the head, depending on the engine manufacturers preference, ease of access to fasten the nuts being a key issue as much as the gas seal.
Indeed, for modern F1 exhausts, even in the current turbo era 3D printing remains a common method for complex sections, in the collector and pipe joins, but a full printed system is yet to be realised.
From my observations most of the V8 exhaust systems were self-supporting, with no obvious struts, cables or stays keeping the assembly in place. Given the temperatures, pressures and vibrations the system is subject, it would seem prudent to support the exhaust to save weight, but perhaps with thermal expansion and inherent stiffness of the curved surfaces its stiff enough. Expansion at the exhaust joints is allowed and the exhausts joining the collector are held in place by being bolted to Inconel plates with a zig-zag expansion joint bent into them.
Heat management is used in two ways in an F1 exhaust firstly to prevent heat radiating for the exhaust being other parts of the car, then coatings to retain heat within the exhaust to maintain gas pressure by preventing the gasses cooling and contracting. Typically, the latter being sought for improving blown aero or in the current era maintaining pressure at the turbo.
Thermal barriers can be formed with various materials. In lower temperature applications gold foil such as that supplied by 3M can be used, this is rarely suited to temperatures produced near the exhaust pipes or plume. Next inline comes carbon fibre heat shields, while a typical carbon material will not be suited to higher temperatures, coating the heated surface with finishes such as metalizing or Zircotech will raise the materials thermal resistance, by reflecting more heat away from the carbon substrate. This is attractive as the carbon can easily be moulded into complex shapes, while still being light. After this special carbon composites such as the aforementioned Pyrosic are resistant enough to be directly blown by the exhaust gasses at over 900c. When blown diffusers were allowed, a large part of the bodywork around the exhaust and the blown surfaces were made from such materials. But these materials are in short supply and come at great cost, hence the ban for them being an exhaust material. Once carbon materials become unsuited its left to metallic materials, so titanium sheet formed to suit the shaped required. When McLaren’s ‘octopus’/fantail exhaust failed to perform, the team were forced to produce the diffuser in titanium to cope with the heat of the exhausts blowing over it.