924Board.org

ESC944 · Joined: 21 May 2004 Posts: 747 Location: FL

I was posting this in response to a recent post about aftermarket turbo kits. http://www.924board.org/viewtopic.php?t=17823

THIS WILL BE LONG AND DRAWN OUT I WILL BE ADDING MORE AND MORE...

HEADER:
1. Fabricate a log style exhaust manifold for the Turbo.
NOTE: THE WINDBLOWN manifold sucked, it was prone to cracking, but could be reinforced. THE BAE IS NOT PRONE TO CRACKS.

ONE OF MY BAE MANIFOLDS:

2. Fabricate an equal length arrangement, good luck with this one, there is not a lot of room to work with, but it can be done. This provides the hanging/suspended arrangement as is typical in aftermarket and stock arrangements. Other variations are possible on this type of header, the idea is it is not a log style.

Also you can or could set it up so that 2 cylinders feed one port and the other 2 feed the second port on a Turbo with a split pulse (dual port) hotside.

Collector Style

Split Port

3. Alright so there are options for the Manifold. Fabricate your own or have one made. Regardless, this is the single most critical piece in setting up a turbo on you car, without it, the turbo can not be installed, will not spin and if done poorly will suck the BIG ONE.

4. Typical cost to self fabricate (Do-it-yourself) is around 150.00 assuming you can weld, and that’s for the bits, besides the exhaust flange, that flange will cost you about 100.00 to have made CNC, Water Jet, etc… assuming you have a template and can not do it yourself.

5. Having someone Else make up the whole thing, expect to pay 400.00 or so, depending on type of unit being made and who you go to.

TURBO HEADER REFERENCE
http://www.not2fast.com/turbo/em/

From My BAE Reference Files:
NON-PORSCHE:
http://www.greasecar.com/projects.cfm
http://members.aol.com/crsmp5/index/crs_cars/truck/bae_turbo/bae_turbo_kit.htm
http://www.4crawler.com/Diesel/BAEturbo/
http://www.buffalochips.org/delorean/turbo.html

Great PICS (non-porsche)
http://www.wheelspecs.com/gallery/BAEturbo12a

PORSCHE:
http://www.alexroy.net/EngineRebuild.html
http://www.alexroy.net/BAEturbo.html
http://www.alexroy.net/Intercooler.html
http://www.alexroy.net/FuelRail.html
http://www.alexroy.net/924home.html

ESC944 · Joined: 21 May 2004 Posts: 747 Location: FL

A log style manifold design works much differently than a “collector style” design. A collector style manifold keeps each cylinder’s exhaust pulse separate until they merge at a single point called a “collector”. (Think 4 into 1 performance header, only smaller).

Note the Porsche 944 Turbo(951) uses a crossover pipe.
This type of arrangement can use either a log or collector style manifold and then routes the pulse under or around the engine to the other side to the turbo. A collector is better, but a log could be used (would look like a big T and would suck performance wise, but is do-able). A cross over can induce some lag, do to the distance involved, if the turbo is less than optimally selected.

There are some tricks to optimizing the exhaust pulse. To increase exhaust energy/velocity to the turbo, however I am not getting into those. It’s not all about heat! I will get into Exhaust and all in my next post.

LOG MANIFOLD:
While compactness and economical manufacturing are advantages to the log style manifold design, there are multiple performance disadvantages.

Turbo response time and high rpm flow is on most engines with a collector tubular exhaust manifold design.

Let’s take a look at the basic layout of a collector style manifold.

On a collector style manifold, the exhaust pulse from each cylinder travels from the cylinder head, down each manifold primary tube, and to the manifold collector. The pulses then merge and travel down the manifold secondary tube to the turbine side of the turbo. This Secondary tube, can be short or long, or in the case of a cross over arrangement, very long.

The basic idea behind keeping each exhaust pulse separate to the collector is to control and minimize the turbulence created when the exhaust pulses merge.

Uncontrolled turbulence equals lost energy, as the exhaust gas pulses lose their direction and cannot deliver their full punch to the turbine. Now of course, this effects lag, both in the onset of boost and reaching Max boost, it can also effect max boost, with regard to turbo sizing.

With a design that creates turbulence inside the manifold, energy is wasted, creating heat instead of spinning the turbine.

HEAT IS BAD< EXCESS HEAT IS NOT SOMETHING YOU WANT.

It bleeds into other things!

In a log style manifold the velocity of the individual exhaust pulse as it travels toward the turbine (turbo hot side) is disrupted by the incoming exhaust pulse from another cylinder, this hurts performance plain and simple.

All due to the turbulence created from the two pulses merging.

Any reduction of turbulence means less wasted energy!

To recap, a collector manifold style is superior to a log manifold style based on the fact that turbulence is limited to one point instead of one per cylinder after the first.

The collector has one, where all points meet in the collector and are fed to the turbine.

The log as stated has several, before even meeting the turbine. So a collector style will maximize the energy potential to the turbo. This results in faster turbo spool up, and potentially more energy to create boost.

Besides the minimization of lost energy, however, reducing turbulence also creates a freer flowing exhaust path for the engine.

Remember all engines are AIR pumps, you want to reduce the restriction in and out of the engine to maximize efficiency.

ESC944 · Joined: 21 May 2004 Posts: 747 Location: FL

Exhaust designs seem more magic than science, more luck than proper execution. They have progressed over the years from theory, but the majority are still being scratch build, test, tried, tune all experimenting.

Only lately have computer or high end engine simulation programs been able to help in this process.

The Basics:

As the piston approaches top dead center, the spark plug fires igniting a fireball just as the piston rocks over into the power stroke.

The piston transfers the energy of the expanding gases to the crankshaft as the exhaust valve starts to open in the last part of the power stroke.
The gas pressure is still high (70 + p.s.i.) causing a rapid escape of the gases (blowdown).

A pressure wave is generated as the valve continues to open. Gases can flow at an average speed of over 350 ft/sec, but the pressure wave travels at the speed of sound (and is dependent on gas temperature).

Expanding exhaust gases rush into the port and down the primary header pipe. At the end of the pipe, the gases and waves converge at the collector. Now remember --- a log style manifold, will induce turbulence, as each wave “crashes”/merges into the next.

In the collector, the gases expand quickly as the waves spread out expanding into all of the available orifices including the other primary pipes. The gases and some of the wave energy flow into the collector outlet and out the tail pipe.

Ok keeping all that in mind, two basic things are at work in the exhaust system: gas particle movement and pressure wave activity. Racing has posted a lot of good stuff on intake design, wave activity, etc on the intake side. Good stuff to read.

The absolute pressure differential between the cylinder and the atmosphere determines gas particle speed. As the gases travel down the pipe and expand, the speed decreases.

The pressure waves speed is based on the speed of sound. While the wave speed also decreases as they travel down the pipe due to gas cooling, the speed will increase again as the wave is reflected back up towards the cylinder.

At all times, the speed of the wave action is much greater than the speed of the gas particles. Waves behave much differently than gas particles when a junction is encountered in the pipe. When two or more pipes come together, as in a collector for example, the waves travel into all of the available pipes - backwards as well as forwards.

Waves are also reflected back up the original pipe, but with a negative pressure.

The strength of the wave reflection is based on the area change compared to the area of the originating pipe.

This reflecting, negative pulse energy is the basis of wave action tuning. – More engine tuning!! Yea so your thinking more crap to sort out, nah… most of the time someone will have done it for u, but remember when you bolt on the aftermarket exhaust, it was designed for a purpose(well supposedly it is tuned and designed for a specific power curve).

The basic idea is to time the negative wave pulse reflection to coincide with the period of overlap - this low pressure helps to pull in a fresh intake charge as the intake valve is opening and helps to remove the residual exhaust gases before the exhaust valve closes.

Typically all this is controlled by the length of the primary header pipe.
Due to the 'critical timing' aspect of this tuning technique, there may be parts of the power curve where more harm than good is done. This is one of the reasons we see people complaining about that aftermarket header SUCKING, or killing low end torque, etc… the other is Gas Speed.

Gas speed is a double edged sword as well, too much gas speed indicates that that the system may be too restrictive hurting top end power, while too little gas speed tends to make the power curve excessively 'peaky' hurting low end torque. Larger diameter pipes allow the gases to expand; this cools the gases, slowing down both the gases and the waves.
Exhaust system design is a balancing act between all of these events and their timing.

Even with compromises of exhaust pipe diameter and length, the collector outlet sizing can make or break the best design. If it’s piss poor then a lot of time and energy was wasted. Keep in mind I am not posting formula, talking just the concepts.

The bottom line on any exhaust system design is to create the best, most useful power curve.

All theory aside, the final judgement is how the engine likes the exhaust tuning on the dyno and on the track. TUNING, TUNING, TUNING!!
When considering a header design, the following things need to be considered:

There are lots of ideas about header pipers and their sizing.

Typically the primary pipe sizing is related to exhaust valve and port size. But in the case of the 924 this is typically fixed (less you pop for head work, bigger valves, etc)

Header pipe length is dependent on wave tuning (or lack of it).

Wave tuning: with longer pipes you are tuning for lower rpm power and the shorter pipes favor high rpm. power.

The collector package is dependent on the number of cylinders, the engine configuration (4, 8, 6, etc. in this case we are talking inline 4), firing order and the basic design objectives (look up interference or independence).

The collector outlet size is determined by primary pipe size and exhaust cam timing. Yet another subject with regard to tuning… not talking that just yet, but the cam can be critical.

In my next post lets talk about NA vs. Turbo exhaust.

ESC944 · Joined: 21 May 2004 Posts: 747 Location: FL

Ok so all this talk about exhaust tuning, lets look at how it applies.

N/A cars: None boosted (N/A – naturally aspirated cars like the 924) utilize exhaust velocity (not backpressure) in the collector to aid in scavenging other cylinders during the blow down process.

To get the right velocity, reduce the diameter of the discharge of the collector (exhaust), which creates backpressure. Do a search on the board about backpressure and performance headers, or the net about exhaust tuning and all that.

Backpressure is an “undesirable” byproduct from trying to have a certain exhaust velocity. If you go too big, you lose velocity and its associated scavenging effect. Go to small and the backpressure gets ugly, more than off setting any gain made by scavenging.

YOU HAVE TO FIND THE HAPPY MEDIUM. This says nothing about exhaust tuning

For turbo cars, what this is all about…

Lets toss all that mess out the window, WHAT?!!

Yea you didn’t have to read all that other stuff I posted, hahaha but your so much smarter now…

With a turbo header we want the exhaust velocity to be high upstream of the turbo hot side (in the header). Take a look you will see that the primaries of turbo headers are smaller diameter than those of an n/a car of about 2/3rd the horsepower.

The idea is simple, we want to get the exhaust velocity up quickly and get the turbo spooling as early as possible.

So getting the boost up early is a 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. This can also happen with the exhaust side (down pipe) of the turbo… it can choke the engine.

With a turbo that is… keep in mind a supercharger can fix a lot on a NA car, but you would get even more if the engine can both ingest and exhale well.

Downstream of the turbo hot side (from the down pipe back), you want as little backpressure as possible.

Simple, no ifs, ands, buts or what abouts, get it done, get it clear and free flowing.

"Larger is better" (to a point). The idea is to minimize the pressure downstream of the turbo hot side in order to make the most effective use of the pressure that is being generated upstream of the turbo hot side.

Remember, a turbo hot side operates via a pressure ratio. For a given turbo hot side inlet pressure, you will get the highest pressure ratio across the turbo hot side when you have the lowest possible discharge pressure.

Now to put it another way for anyone who cant wrap their head around that, you want the finned thingy in the hot side of the turbo to get pushed as hard and fast as possible, without getting choked off by a restriction.

This means the turbo hot side 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 turbo hot side is ALL GOOD. This will minimizes the time-to-boost and maximizes boost response, it will improve engine VE throughout the rev range without question.

Now a note about sizing.

Well as for 2.5" vs. 3.0", the "best" turbo exhaust depends on the amount of flow, or horsepower.

Use this as a rule of thumb, but you can allow a little variance.
¾” -1” per 100 HP is about average (over simplification) so at 250 hp, 2.5" is going to be good. Going any bigger will not get you much, if anything, but might be wickedly loud and/or get you a ticket.

300 hp and you definitely want more than 2.5”, 2.75 - 3”(better) would be good. If SOME HOW YOU ARE PUSHING 400-450 hp, THEN 3" IS ON THE SMALL SIDE OF THINGS.

NOW it really is more complicated than that, and there is something to be said for huge gains made by opening the exhaust up (dependent on HP level and engine configuration) from the turbo back. You also have to consider single or dual exhaust, true or cat back, x pipe, etc…

As for the angle of the discharge pipe from the turbo hot side discharge, the best configuration would be a gradual increase in diameter from the turbo hot side's exducer to the desired exhaust diameter, by way of a straight conical diffuser of 7 to12 degrees included angle. This is to minimize the flow separation and friction losses.

There are lots of thoughts on internal and external wastegates and the effect of integrated exhaust discharge, its worth of discussion, but not here and now.

Necking the exhaust down to a some piss poor to small diameter is never good, but if it is necessary, then you want to do it as far downstream as possible. This is better than doing it close to the turbo hot side discharge, it will minimize the exhaust's contribution to backpressure. THE BEST: 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 process and compounds 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. So--- this in turn leads us to the path that mounting a rear turbo is very feasible, the primary concern would be pressurizing the extended intake pipe, but given the distance and the speed the pressurized air flows at, its hardly noticeable, but for idle and start up, a one way valve would be used to let the engine breath until positive pressure is generated in the intake pipe.

Also the extended intake pipe functions as a improvised Intercooler… you can also feed it through a finned heat exchanger, all this can in turn feed another intercooler, but given the boost levels of a NA conversion, it is hardly likely that other than the extended pipe, you would need intercooling. You just wouldn’t be running enough boost.

But if you built an engine to run high boost, well then one could easily design multiple stages of cooling to generate larges amounts of delta loss(heat loss). In practice Rear mounted turbochargers are not inherently lag prone, not if sized right, keep in mind, you do not need to worry to much about down stream exhaust, since you are at the end of the exhaust most times or right after the cat, you can use conventional back pressure theory and pipe sizing to introduce significant post cat exhaust pressure and pre turbo velocity… start bigger and go smaller as you approach the turbo. Then open it up post turbo.

Another thing to keep in mind is that cylinder scavenging takes place where the flows from separate cylinders merge (in the collector or at the turbo on a log style manifold).

There is no such thing as cylinder scavenging downstream of the turbo hot side, and hence, no reason to desire high exhaust velocity here. You will only introduce unwanted backpressure. So in an rear mount system we can optimize header performance and simply introduce or amplify exhaust pressure and velocity post cat with pipe and turbo hot side sizing.

Other things you can do (in addition to choosing an appropriate diameter) to minimize exhaust backpressure in a turbo exhaust are: use mandrel bends, avoid tight-radius turns – just keep it as straight as possible; avoid step changes in diameter – unless trying to do a rear mount turbo and it’s a little complicated in what is ideal; avoid cuts that are non-perpendicular, use a high flow cat, use a straight-thru perforated core muffler, remember you want exhaust post turbo to be free flowing.

For primary lengths on turbo headers, it is best to use equal-length primaries to time the arrival of the pulses at the turbo hot side equally and to keep cylinder reversion balanced across all cylinders.

Doing so will improve boost response and the engine's VE. Equal-length is however difficult to achieve due to packaging, fabrication difficulty, space, cost, etc and the desire to have runners of the shortest possible length.

ESC944 · Joined: 21 May 2004 Posts: 747 Location: FL

An example of how larger exhausts help turbo cars:

Say you have a turbo operating at a turbo hot side pressure ratio (expansion ratio) of 1.8:1. You have a small turbocharger back exhaust that contributes, 10 psig backpressure at the turbo hot side discharge at redline.

The total backpressure seen by the engine (upstream of the turbo hot side) in this case is:

(14.5 +10)*1.8 = 44.1 psia = 29.6 psig total backpressure

The turbo hot side contributed 19.6 psig of backpressure.

Now toss on a low-backpressure, big turbo back exhaust.

Use the same turbo, same boost, etc. Now you measure 3 psig backpressure at the turbo hot side discharge.

Now the engine sees just 17 psig total backpressure!

Of course something can be said about the nature of the 924 engine and how it responds to back pressure, but thats a different subject and thats what Search is for, its your friend, use it often.

The turbo hot side's contribution to the total backpressure is reduced to 14 psig (note: 5.6 psig lower than in the "small turbo back" case).

So the engine saw a reduction in backpressure of 12.6 psig when you swapped turbo back exhaust 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 turbo hot side compounds the downstream backpressure via its expansion ratio.

This is also why bigger turbochargers make more power at a given boost level-- they improve engine VE by operating at lower turbo hot side expansion ratios for a given boost level.

The backpressure penalty of running a too-small exhaust (like 2.5" for 400 hp) will vary depending on several parameters.

At a given power level, a smaller turbo will generally be operating at a higher turbo hot side pressure ratio and so will actually make the engine more sensitive to the backpressure downstream of the turbo hot side than a larger turbo hot side/turbo would.

Messing with the discharge path of the wastegate will improve the VE of the wastegate. If you have a very compromised wastegate discharge routing, under some conditions the wastegate may not be able bypass enough flow to control boost, even when wide open. Not something you should see on a proper setup on a 924. But if you are going to get into larger turbochargers and proper exhaust, well the gases go through the turbo hot side instead of the wastegate, and boost creeps up. So get it right!

The downside to the common bellmouth discharge is that the wastegate flow still dumps right into the turbo hot side discharge. A divider wall would be beneficial here. This is where aftermarket downpipes can prove very beneficial. All this revolves around turbocharger style and design. Older turbochargers tend to have internal wastegate setups and the wastegate is usually very much a part of the downside.

If you go too big on the bellmouth and the turbo hot side discharge flow sees a rapid area change (regardless of whether the wastegate flow is being introduced here or not), you will generate a backpressure penalty right at the site of the step. This is why you want gradual area changes in your exhaust, without exception.

Keep in mind bellmouth doesn’t not mean and exactly round shape as seen in intake design, it more loosely used to describe the larger rounded opening, oval is more common than a circle.

ESC944 · Joined: 21 May 2004 Posts: 747 Location: FL

Typical internal configuration, note no seperation between the wastegate and the hotside discharge:

Improvised spacer (not my work):

Typical merged collector, the farther down the two join the better, this would be rather short, as note in previous posts, you would want to avoid this for as long as possible.

sequential · Joined: 18 Nov 2005 Posts: 500 Location: BANNED

what the heck ! with my ADD ,I will have to read the whole thing again.
phew i'm tired!
_________________
928 gts prototype
baby blue engine block
steam in 1,2,3,4 sometimes
cold star issues while on stands
112 whp with new 4 valve head and MIS 2

ESC944 · Joined: 21 May 2004 Posts: 747 Location: FL

MAYBE I SHOULD POST A DiScLaImEr At ThE bEgInInG?

924er · Posted: Mon Jan 23, 2006 10:45 am Post subject:

i read the whole thing... interesting : )
_________________
80' Porsche 924 NA - Slower then a S L O W
86' Porsche 951- New toy

ESC944 · Joined: 21 May 2004 Posts: 747 Location: FL

Thanks for reading it all, but its far from done.

I have only covered the exhaust. Thats only one bit of it. A big bit, but only one.

We still have the turbo selection, oil delivery and return, Plumbing the intake pipes, Fuel and timing.

Just have to go over my notes and then copy and paste as I get time.

ESC944 · Joined: 21 May 2004 Posts: 747 Location: FL

From here:
http://www.overboost.com/story.asp?id=101

Do It Yourself Turbocharging Part 1
By: Overboost Staff
10/18/1999
PLEASE READ OUR DISCLAIMER
http://www.overboost.com/disc.htm

ESC944 · Joined: 21 May 2004 Posts: 747 Location: FL

http://www.overboost.com/story.asp?id=1232
Shown without pics - follow link for pics

Turbo System Basics: How Does a Turbocharger Work?
By: Scott Croughwell
3/26/2004
PLEASE READ OUR DISCLAIMER