Turbo spool time debate and manifold design

[Shad]

We all are tired of reading people discusing about wich spools faster: T-anything or GT-anything. But something I never read is the most important thing: manifold design.

Almost every BT setup uses ATP's manifold, with that horrible design that causes too much interference in between consecutive exhaust ports, thus reducing power and creating a system prone to knocking and bad spool up times.

To me it's obvius a Ball bearing turbo can't show its juice when coupled with such a manifold. The reason why spool times are so similar is that they spool when it's simply impossible to be still un-spooled: up the tach. Low end response can't exist when the manifold is a flow-wise nightmare.

That said, is there anyone out there who did a BT install using a real manifold, with individual runners, of not necessarily equal lengths? Or will everybody wait till APR shows up a new kit, with a good manifold, that spools earlier and makes more power with the same turbo everybody is talking about?

We need to ask for better manifolds cuz it's easier to sell crap when everyone seems to be happy. Sorry for my english.

[fast_a2_20v]

lol like the full race manifold? or mine? or whatever whoever custom built.

They don't help spoolup a whole ton, tuning and engine V.E. helps more then a manifold will.

[MRP2001GTi]

Tuning has dropped 400 to 600 rpm off my spool time. Next I am trying a manifold of "better" design. We shall see how much more this gets me. In all honesty my spool is fine where it is. I really wouldnt want mush more power sooner than I already have it.

[Shad]

Perhaps. But you won't have much VE with turbo cams, since overlap should be kept to a minimum. So, head work anyone?

[TBT-Syncro]

Since you've stated that it has a 'horrible design' perhaps you could back that up with some cold hard facts, or are you just a keyboard warrior with nothing to back it up? Do a search for the fastest VW in the world and then check out what manifolds they're running.

[fast_a2_20v]

who are you talking to?

how about this, the fastest vw's in the world aren't that fast. Try looking at what the fastest honda 4cyls are runnig.

Oh and the vw's that actually ARE fast, are only fast because they weigh 2lbs. Were not even coming close to the power output they are. So don't say its all in the manifold. Its all in the hole saw :-D

PS you can run a pretty freaking hot cam on turbo, a lot of guys run ITR cams which are way the hell hotter then anything our stock hydraulic lifters will even allow us to run.

[Shad]

Quote, originally posted by TBT-PassatG60 »

Since you've stated that it has a 'horrible design' perhaps you could back that up with some cold hard facts, or are you just a keyboard warrior with nothing to back it up? Do a search for the fastest VW in the world and then check out what manifolds they're running.

I don't think I need to do a search when I got a bunch of books over my table, that I've already read. Most of them are about good tuning, like developing turbo systems, exhaust and intake systems, chassis engineering, engine building, etc. The others are for "bad" tuning, like car stereo desing

What I'm doing is starting to make some use of that.

[Shad]

Quote, originally posted by fast_a2_20v »

PS you can run a pretty freaking hot cam on turbo, a lot of guys run ITR cams which are way the hell hotter then anything our stock hydraulic lifters will even allow us to run.

Our knock sensor can deal with mech lifters?

[nico18t]

The following excerpts are from Jay Kavanaugh, a turbosystems engineer at Garret, responding to a thread on http://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."

[blackmkIII]

well alright.....

[polov8]

Simple as that aye?

Seriously useful info in that!

[Stroked1.8t]

Quote, originally posted by Shad »

That said, is there anyone out there who did a BT install using a real manifold, with individual runners, of not necessarily equal lengths? Or will everybody wait till APR shows up a new kit, with a good manifold, that spools earlier and makes more power with the same turbo everybody is talking about? We need to ask for better manifolds cuz it's easier to sell crap when everyone seems to be happy. Sorry for my english.

Dude you are a moron.

Chris will you put this guy in his place.

Oh, please tell us what is wrong with the current APR mani?

[Shad]

Wrong?! Didn't I make myself clear or you didn't read? There's nothing wrong, and I think APR manifold is awesome!

..oh... I see...

"It's easier to sell crap when everyone seems to be happy...." Dude, go back do school and learn to read! crap = log style manifolds =/ APR individual runners manifold

*edited for poor english


Modified by Shad at 5:21 AM 10-17-2004

[blackmkIII]

i hope you spelled school wrong on purpose just f---ing around, cause if not than you need to go back to school yourself to learn how to spell correctly.

[blackmkIII]

now that looks better.

[18bora]

Quote, originally posted by nico18t »

The following excerpts are from Jay Kavanaugh, a turbosystems engineer at Garret, responding to a thread on http://www.impreza.net regarding exhaust design and exhaust theory: 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" "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)

conical diffuser of 7-12° ??
Is it like this one?.....................
.

Or this?
.

[Stroked1.8t]

Quote, originally posted by Shad »

Wrong?! Didn't I make myself clear or you didn't read? There's nothing wrong, and I think APR manifold is awesome! ..oh... I see... "It's easier to sell crap when everyone seems to be happy...." Dude, go back do school and learn to read! crap = log style manifolds =/ APR individual runners manifold *edited for poor english Modified by Shad at 5:21 AM 10-17-2004

forgive me for not being able to read your mind. you said will everyone wait until apr comes out with a good manifold. I don't know any other way to take that except you are saying that the current mani the APR has produced is not good enough, in your words, crap.

probably less then 1% of us who will be building a BT kit will use it for competion. so the cost vs. performance, ATP makes a mani that works, fits, accomedated a WG, its already made, ready to order, and it can support plently of power.

[Shad]

Quote, originally posted by blackmkIII »

now that looks better.

[Shad]

Ok, don't want to fight here. So, let's get it clear.

I asked if people will wait till APR shows up a good manifold, making the GT28RS spool ealier, to realize that ATP's manifold is holding down the turbo's performance.

Sorry.

[fast_a2_20v]

Shad

nobody is going to run a freakin mechanical lifter setup on the stock ecu, so the knock sensors are a non issue.

once you get into the 400-450 whp range on hyrdo lifters you are kind of out of options with chip tuning and you pretty much have to go to a standalone.

and yes, people have built non log manifolds.

pictures of the one i built for my setup are here

http://forums.vwvortex.com/zerothread?id=1620067

[Shad]

You got some skills

Already tested your engine?

[VDUBNDizzy]

I've seen this ass clown driving around Atlanta several times. LMAO.

[fast_a2_20v]

nah its more then just building up that engine becaues it is going into a mk2 and i'm a college student so i can only do it a little here and a little there.

working on the intake manifold and it is as trick if not more so then that exhaust manifold, but its taking some time.

[2002GTI]

Is it a joke that hes making, or does he really think thats hot ****?

Quote, originally posted by VDUBNDizzy »

I've seen this ass clown driving around Atlanta several times. LMAO.

[fast_a2_20v]

i dunno but you could store extra t3's in that exhaust. lol.