Exhausts and turbos

[x19Dave]

Guy, Gents
is the manifold set up as in pipe lengths diameters, equal lengths 4-2-1 or 4-1 important on a turbo car or does the back pressure from the turbo make all of it irrelevant?

Thanks
Dave

[Guy Croft]

First of all the static (back) pressure in the pipes is a separate thing from the behaviour of pressure waves. The latter travel thru the gas whatever pressure it's at and they move at the speed of sound. Regarding the influence of pressure waves, this was a question I put to the Senior Applications Engineer when I worked for Napier Turbochargers some years back and his reply was that turbocharged units respond to exhaust manifold (header) design in the same way as normally aspirated ones, which was more or less what I expected him to say. I had asked him because at that time there was a lot of spurious discussion in the tuning business about 'pulse tuning' - trying to get pressure waves to act directly on the turbine. I say spurious because of course the whole idea of the nozzle in a turbine housing is to convert pressure_to_velocity (pressure of course being one of the energy components at the hot end).

Anyway, pressure waves. To quote from a good synopsis at :
http://www.eng.fsu.edu/~shih/eml4421/st ... Theory.htm

"When a wave is confronted by a change in area, two types of waves result: a transmitted wave and a reflected wave. The transmitted waves are always of the same type as the incident wave, be it a compression wave or expansion wave. The reflected wave depends on the area it sees. If the incident wave sees a larger area, then the reflected wave is of the other type"

In the case of the header, when the positive exhaust wave sees a collector (pipe junction - if properly constructed) - 2 things happen (or can)...

1) The pressure wave is reflected as a negative wave (low pressure) which travels back to the exhausting cylinder and can, at some engine speeds - dependent the length of the primary pipe - enhance the cylinder depression beyind BDC and assist cylinder purging.
and...
2) on an 'intereference' layout (eg: 4-2-1 with 1&4, 2&3 primary pipes paired) it can also reflect off the closed ex valve on the linked cylinder and travel back to the cylinder it came from and increase the pressure around BDC. That is a good thing because more pressure at BDC means more pressure to vent the cylinder.

This will work irrespective of whether the engine is turbocharged or not. You can get the same beneficial relection on a 4-1 system - which most aftermarket turbo headers are but designing the layout to get it to happen in the right place (s) in the powerband is impossible without simulation or ahuge amount of trial and error! 4-1 systems are known as 'independence' types and on n/a engines (normally or naturally aspirated) one tries to keep the pipes as long as possible to avoid degrading interference of pressure waves cylinder to cylinder. However, we all know that the available space for headers in turbo installations tends to be very restrictive and thus most setups just use an equal length (and short) 4-1 layout more to ensure that whatever the pressure wave effects - at least they are all the same. The effect of such short 4-1 primary pipes on a n/a engine would wreak havoc on the powerband and they only work at all on turbocharged units because they tend to have short duration low lift ex cams and of course have far higher pressure in the inlet tract.

GC

[x19Dave]

Guy

Many thanks for your reply at this time I have a X1/9 1500cc and have a 1301cc Uno Turbo engine to install at some time when i looked at the exhaust manifold on the turbo to looks very cluttered up and the pipes are not of the same length so I might look at making one up myself, the exhaust sizing thread:

viewtopic.php?f=13&t=1482 from James Bowen looks like a good place to start.

Also I have read that the Uno Turbo 1301cc engine has the same stroke but smaller bore than the 1500cc X1/9 engine from your knowledge would it be possible to bore the 1301 block out to take the 1500 pistons?

Thanks again
Dave

[Guy Croft]

No, as a rule you should avoid ever reboring much more than 1mm oversize. It is fatal to reduce the land betw the bores too much, every bit you lose by boring is less metal for the gasket fire ring to grip to. (To put it simply).

GC