Books

[jogi730]

Some time ago, someone posted a title of a good book to read explaining properly how turbo engines work and differences from N/A engines. I have tried a search but not found anything.
Can someone help please?
Also maybe it would be a good idea to have a list of reference books which would be good/informative to read.
Cheers
Chris

[Graham]

I dont know where it come from but here is the text:
The following excerpts are from Jay Kavanaugh, a turbosystems
engineer at Garret, responding to a thread on www.impreza.net
regarding exhaust design and exhaust theory:
“Howdy,
This thread was brought to my attention by a friend of mine in hopes of shedding
some light on the issue of exhaust size selection for turbocharged vehicles. Most of
the facts have been covered already. FWIW I'm an turbocharger development
engineer for Garrett Engine Boosting Systems.
N/A cars: As most of you know, the design of turbo exhaust systems runs counter to
exhaust design for n/a vehicles. N/A cars utilize exhaust velocity (not backpressure)
in the collector to aid in scavenging other cylinders during the blowdown process. It
just so happens that to get the appropriate velocity, you have to squeeze down the
diameter of the discharge of the collector (aka the exhaust), which also induces
backpressure. The backpressure is an undesirable byproduct of the desire to have a
certain degree of exhaust velocity. Go too big, and you lose velocity and its
associated beneficial scavenging effect. Too small and the backpressure skyrockets,
more than offsetting any gain made by scavenging. There is a happy medium here.
For turbo cars, you throw all that out the window. You want the exhaust velocity to
be high upstream of the turbine (i.e. in the header). You'll notice that primaries of
turbo headers are smaller diameter than those of an n/a car of two-thirds the
horsepower. The idea is to get the exhaust velocity up quickly, to get the turbo
spooling as early as possible. Here, getting the boost up early is a much more
effective way to torque than playing with tuned primary lengths and scavenging. The
scavenging effects are small compared to what you'd get if you just got boost sooner
instead. You have a turbo; you want boost. Just don't go so small on the header's
primary diameter that you choke off the high end.
Downstream of the turbine (aka the turboback exhaust), you want the least
backpressure possible. No ifs, ands, or buts. Stick a Hoover on the tailpipe if you
can. The general rule of "larger is better" (to the point of diminishing returns) of
turboback exhausts is valid. Here, the idea is to minimize the pressure downstream
of the turbine in order to make the most effective use of the pressure that is being
generated upstream of the turbine. Remember, a turbine operates via a pressure
ratio. For a given turbine inlet pressure, you will get the highest pressure ratio
across the turbine when you have the lowest possible discharge pressure. This
means the turbine is able to do the most amount of work possible (i.e. drive the
compressor and make boost) with the available inlet pressure.
Again, less pressure downstream of the turbine is goodness. This approach
minimizes the time-to-boost (maximizes boost response) and will improve engine VE
throughout the rev range.
As for 2.5" vs. 3.0", the "best" turboback exhaust depends on the amount of flow, or
horsepower. At 250 hp, 2.5" is fine. Going to 3" at this power level won't get you
much, if anything, other than a louder exhaust note. 300 hp and you're definitely
suboptimal with 2.5". For 400-450 hp, even 3" is on the small side.”
"As for the geometry of the exhaust at the turbine discharge, the most optimal
configuration would be a gradual increase in diameter from the turbine's exducer to
the desired exhaust diameter-- via a straight conical diffuser of 7-12° included angle
(to minimize flow separation and skin friction losses) mounted right at the turbine
discharge. Many turbochargers found in diesels have this diffuser section cast right
into the turbine housing. A hyperbolic increase in diameter (like a trumpet snorkus)
is theoretically ideal but I've never seen one in use (and doubt it would be
measurably superior to a straight diffuser). The wastegate flow would be via a
completely divorced (separated from the main turbine discharge flow) dumptube.
Due the realities of packaging, cost, and emissions compliance this config is rarely
possible on street cars. You will, however, see this type of layout on dedicated race
vehicles.
A large "bellmouth" config which combines the turbine discharge and wastegate flow
(without a divider between the two) is certainly better than the compromised stock
routing, but not as effective as the above.
If an integrated exhaust (non-divorced wastegate flow) is required, keep the
wastegate flow separate from the main turbine discharge flow for ~12-18" before
reintroducing it. This will minimize the impact on turbine efficiency-- the introduction
of the wastegate flow disrupts the flow field of the main turbine discharge flow.
Necking the exhaust down to a suboptimal diameter is never a good idea, but if it is
necessary, doing it further downstream is better than doing it close to the turbine
discharge since it will minimize the exhaust's contribution to backpressure. Better
yet: don't neck down the exhaust at all.
Also, the temperature of the exhaust coming out of a cat is higher than the inlet
temperature, due to the exothermic oxidation of unburned hydrocarbons in the cat.
So the total heat loss (and density increase) of the gases as it travels down the
exhaust is not as prominent as it seems.
Another thing to keep in mind is that cylinder scavenging takes place where the
flows from separate cylinders merge (i.e. in the collector). There is no such thing as
cylinder scavenging downstream of the turbine, and hence, no reason to desire high
exhaust velocity here. You will only introduce unwanted backpressure.
Other things you can do (in addition to choosing an appropriate diameter) to
minimize exhaust backpressure in a turboback exhaust are: avoid crush-bent tubes
(use mandrel bends); avoid tight-radius turns (keep it as straight as possible); avoid
step changes in diameter; avoid "cheated" radii (cuts that are non-perpendicular);
use a high flow cat; use a straight-thru perforated core muffler... etc.”
"Comparing the two bellmouth designs, I've never seen either one so I can only
speculate. But based on your description, and assuming neither of them have a
divider wall/tongue between the turbine discharge and wg dump, I'd venture that
you'd be hard pressed to measure a difference between the two. The more gradual
taper intuitively appears more desirable, but it's likely that it's beyond the point of
diminishing returns. Either one sounds like it will improve the wastegate's discharge
coefficient over the stock config, which will constitute the single biggest difference.
This will allow more control over boost creep. Neither is as optimal as the divorced
wastegate flow arrangement, however.
There's more to it, though-- if a larger bellmouth is excessively large right at the
turbine discharge (a large step diameter increase), there will be an unrecoverable
dump loss that will contribute to backpressure. This is why a gradual increase in
diameter, like the conical diffuser mentioned earlier, is desirable at the turbine
discharge.
As for primary lengths on turbo headers, it is advantageous to use equal-length
primaries to time the arrival of the pulses at the turbine equally and to keep cylinder
reversion balanced across all cylinders. This will improve boost response and the
engine's VE. Equal-length is often difficult to achieve due to tight packaging,
fabrication difficulty, and the desire to have runners of the shortest possible length.”
"Here's a worked example (simplified) of how larger exhausts help turbo cars:
Say you have a turbo operating at a turbine pressure ratio (aka expansion ratio) of
1.8:1. You have a small turboback exhaust that contributes, say, 10 psig
backpressure at the turbine discharge at redline. The total backpressure seen by the
engine (upstream of the turbine) in this case is:
(14.5 +10)*1.8 = 44.1 psia = 29.6 psig total backpressure
So here, the turbine contributed 19.6 psig of backpressure to the total.
Now you slap on a proper low-backpressure, big turboback exhaust. Same turbo,
same boost, etc. You measure 3 psig backpressure at the turbine discharge. In this
case the engine sees just 17 psig total backpressure! And the turbine's contribution
to the total backpressure is reduced to 14 psig (note: this is 5.6 psig lower than its
contribution in the "small turboback" case).
So in the end, the engine saw a reduction in backpressure of 12.6 psig when you
swapped turbobacks in this example. This reduction in backpressure is where all the
engine's VE gains come from.
This is why larger exhausts make such big gains on nearly all stock turbo cars-- the
turbine compounds the downstream backpressure via its expansion ratio. This is also
why bigger turbos make more power at a given boost level-- they improve engine VE
by operating at lower turbine expansion ratios for a given boost level.
As you can see, the backpressure penalty of running a too-small exhaust (like 2.5"
for 350 hp) will vary depending on the match. At a given power level, a smaller turbo
will generally be operating at a higher turbine pressure ratio and so will actually
make the engine more sensitive to the backpressure downstream of the turbine than
a larger turbine/turbo would. As for output temperatures, I'm not sure I understand
the question. Are you referring to compressor outlet temperatures?
The advantage to the bellmouth setup from the wg's perspective is that it allows a
less torturous path for the bypassed gases to escape. This makes it more effective in
bypassing gases for a given pressure differential and wg valve position. Think of it as
improving the VE of the wastegate. If you have a very compromised wg discharge
routing, under some conditions the wg may not be able bypass enough flow to
control boost, even when wide open. So the gases go through the turbine instead of
the wg, and boost creeps up.
The downside to a bellmouth is that the wg flow still dumps right into the turbine
discharge. A divider wall would be beneficial here. And, as mentioned earlier, if you
go too big on the bellmouth and the turbine discharge flow sees a rapid area change
(regardless of whether the wg flow is being introduced there or not), you will incur a
backpressure penalty right at the site of the step. This is why you want gradual area
changes in your exhaust."

I have hoarded as many titbits of information off the forum since ye olde S2 central days, and my favourites list is Huge as i keep links to all usefull information posts from problems and solutions people have found.

[Nuvo]

Hi Chris, Maximum Boost by Corky Bell is an often cited and recommended reference on here. See this thread for a few other titles that might be of interest to you.

[Smiley]

Very interesting read!
So I need a 3" downpipe and exhaust with a WG pipe from WG to just before the cats..

[S2central.net]

This is useful if you aren't afraid of numbers... http://www.s2forum.com/forum/showthr...ight=turbology

I have rough drafts for new improved versions of this - with fuel injector & pump sizing amongst other things. Could take a while to finish it, but should be good when its done...

[jogi730]

That turbology link does not work
Cheers
Chris

[S2central.net]

Works for me - just tried it...

[spink]

The webpage cannot be found
HTTP 404
Most likely causes:
There might be a typing error in the address.
If you clicked on a link, it may be out of date.

What you can try:
Retype the address.

Go back to the previous page.

Go to s2central.net and look for the information you want.

More information

[denniselbows]

just tryed it doesn't work me either
dennis

[Smiley]

The link to the PDF doesn't work for me: s2central.net/turbology_V4.pdf

[S2central.net]

Its fixed now - honest guv ... sorry !