Reducing lag and making torque peak lower rpm
01-04-2006, 03:11 AM
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Reducing lag and making torque peak lower rpm
I'm now starting my engine tuning project. Also turbos will be changed bigger ones, probably K24/K26..
I haven't planned to change stock headers, cats or mufflers.
Can increased turbo lag reduce by changing some or all of these exhaust parts? I don't want any loud bypass versions but like 130mm cats or something?
Technical/theoretical point of view: is some level of back pressure needed anyway for getting good torque in low and mid rpm range?
I haven't planned to change stock headers, cats or mufflers.
Can increased turbo lag reduce by changing some or all of these exhaust parts? I don't want any loud bypass versions but like 130mm cats or something?
Technical/theoretical point of view: is some level of back pressure needed anyway for getting good torque in low and mid rpm range?
01-04-2006, 03:35 AM
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Originally Posted by js-911
I'm now starting my engine tuning project. Also turbos will be changed bigger ones, probably K24/K26..
I haven't planned to change stock headers, cats or mufflers.
Can increased turbo lag reduce by changing some or all of these exhaust parts? I don't want any loud bypass versions but like 130mm cats or something?
Technical/theoretical point of view: is some level of back pressure needed anyway for getting good torque in low and mid rpm range?
I haven't planned to change stock headers, cats or mufflers.
Can increased turbo lag reduce by changing some or all of these exhaust parts? I don't want any loud bypass versions but like 130mm cats or something?
Technical/theoretical point of view: is some level of back pressure needed anyway for getting good torque in low and mid rpm range?
Here's some good info from a friend of mine. Keep in mind that we were talking about 4 cylinder engines with a single exhaust system. For Porsches and other cars with two separate exhaust systems, pipe diameter requirements will be reduced by 50% to support the same power.
This thread was brought to my attention by a friend of mine [shiv@vishnu] 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.
01-04-2006, 11:14 AM
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shiv
Thanks for you from very extensive theoretical point of view.
I'll read it still few dozen times and try to absord it
But does anyone have good(=tested/dynoed) factory-made solutions for these exhaust parts?
Like we noticed that MOD used Cargraphic's large 130mm cats and mufflers which helped him a lot with wonderful torque curve.
Thanks for you from very extensive theoretical point of view.
I'll read it still few dozen times and try to absord it
But does anyone have good(=tested/dynoed) factory-made solutions for these exhaust parts?
Like we noticed that MOD used Cargraphic's large 130mm cats and mufflers which helped him a lot with wonderful torque curve.
01-04-2006, 12:35 PM
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Shiv,
I found exactly what you have quoted to be true. Removing cats and changing the exhaust to a larger size and a straight through muffler design resulted in sooooo much better spool up on the larger K24 turbos. These things are very very responsive down low yet wail on the top end.
Tom
I found exactly what you have quoted to be true. Removing cats and changing the exhaust to a larger size and a straight through muffler design resulted in sooooo much better spool up on the larger K24 turbos. These things are very very responsive down low yet wail on the top end.
Tom
01-04-2006, 02:10 PM
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Originally Posted by K24madness
Removing cats and changing the exhaust to a larger size and a straight through muffler design resulted in sooooo much better spool up on the larger K24 turbos. These things are very very responsive down low yet wail on the top end.
How about race cats and mufflers, like Cargraphic's?
I like to make my new bigger turbos also spooling up quickly but don't wan't them(exhaust parts) to be too loud, impossible equation ?
01-04-2006, 02:17 PM
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a short novice question:
does turbo LAG describe how much rpm the turbos need to build boost or does it also describe how long (milli seconds) the turbos need to build boost, lets say when you are at 5 or 6K rpm?
I think my muffler bypass reduced both the rpm at which the turbos make full boost but also the response at higher rpm. the digital boost gauge (****, I know) is like dancing around compared with stock mufflers.
does turbo LAG describe how much rpm the turbos need to build boost or does it also describe how long (milli seconds) the turbos need to build boost, lets say when you are at 5 or 6K rpm?
I think my muffler bypass reduced both the rpm at which the turbos make full boost but also the response at higher rpm. the digital boost gauge (****, I know) is like dancing around compared with stock mufflers.
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01-04-2006, 03:04 PM
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Based on everything that I've read and been told (Kevin being a major source here), turbos love a free flowing exhaust. Not only does it minimize lag, but it also affects the pressure buildup all the way back to the cylinders. So, you need to make sure your programming is matched to the back pressure of your exhaust system.
In not quite so terse words, Kevin basically told me that upgrading the turbos without upgrading the exhaust was a *stupid idea* So, I have the 100 cell cats and Cargraphic muffler
My car has minimal lag (K16/K24 hybrids). And it is quite a bit quieter than when I had the muffler bypass pipes on the stock setup.
I'm quite sure Kevin is working on some more exhaust upgrades for his Stage 3; contact him for details.
In not quite so terse words, Kevin basically told me that upgrading the turbos without upgrading the exhaust was a *stupid idea*
So, I have the 100 cell cats and Cargraphic muffler My car has minimal lag (K16/K24 hybrids). And it is quite a bit quieter than when I had the muffler bypass pipes on the stock setup.
I'm quite sure Kevin is working on some more exhaust upgrades for his Stage 3; contact him for details.
01-04-2006, 05:26 PM
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Originally Posted by pstoppani
..Kevin basically told me that upgrading the turbos without upgrading the exhaust was a *stupid idea* ..
..For example, very good 993TT site,
http://www.993tt.com/, tells following about Sport Cats:
"dollars can be better spent elsewhere. These are very expensive, and they will give you a 'little' HP increase"
and
"Don't Bother - Minimal Gain"
Has anybody done dyno test between stock cats and sport cats?
01-04-2006, 06:15 PM
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Hope I'm not stating th e obvious...... if the HP needs (stock) are met reasonably well with the stock cats and there is not significant backpressure of course sport cats will not be needed.. BUT when you increase flow (HP) presumably it is possible to quickly exceed the stock capacity of CATS IMHO.. (maybe someone has dyno results, but few want to go to the trouble of investing $$ in dyno time versus jsut going to high flow cats..or none). if you go to bigger 24's or hybrids it will be difficult to tell if you exceed teh flow capacity of the stock cars unless someone can tell you what the practical limits are.... a seat of pants version would be to run with the cats and then put a bypass in ... if there was a big 'seat' increase then you probably need sport cats.. JMHO if you don't want to invest dollars in dyno runs... or can;t find someone who has... but from years of playing with configs, bigger turbos by themselves seldom achieve desired results, particulairy for rapid spool/torque.... they will sometimes increase max power if other components can support them adequetely....and can actually think you accelerate quicker becuase of teh wait for power....but the clock sometimes tells a different tale... and I am all about low torque for street driving....
01-04-2006, 07:03 PM
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Objectively speaking, I will once again state that Ruf had informed an owner of a turbo-R conversion during install that Ruf would have to "detune" the Ruf ecu if the owner wanted to drive with stock CATS. The conversion assumed "modified exhaust", which was essentially removing the stock cats and replacing with 130mm diameter, 100cell cats in order to work optimally. Leaving the stock CATS with otherwise all Turbo-R mods would have confused the ecu and not run optimally. This infers that the "proper" 100cell CATS are indeed required for the conversion. Mufflers were also changed out to "Ruf" mufflers, which I hear were essentially Techart, or a variation of Techart.....which is the one and only one used by Andial (called them 2mos ago to ask about their favorite muffler for their 3.8 conversions with beacoup horsepower they said they've only used the Techarts).
Hope this helps "piecing together" the puzzle for anyone doing his homework.....I know I did.
Jaime
Hope this helps "piecing together" the puzzle for anyone doing his homework.....I know I did.
Jaime
01-04-2006, 07:37 PM
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Originally Posted by ttAmerica RoadsterAWD
Objectively speaking, I will once again state that Ruf had informed an owner of a turbo-R conversion during install that Ruf would have to "detune" the Ruf ecu if the owner wanted to drive with stock CATS. The conversion assumed "modified exhaust", which was essentially removing the stock cats and replacing with 130mm diameter, 100cell cats in order to work optimally. Leaving the stock CATS with otherwise all Turbo-R mods would have confused the ecu and not run optimally. This infers that the "proper" 100cell CATS are indeed required for the conversion. Mufflers were also changed out to "Ruf" mufflers,
01-04-2006, 07:51 PM
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Originally Posted by js-911
from these updates(muffler and cats), there are several opinions.
For example, very good 993TT site,
http://www.993tt.com/, tells following about Sport Cats:
"dollars can be better spent elsewhere. These are very expensive, and they will give you a 'little' HP increase"
and
"Don't Bother - Minimal Gain"
For example, very good 993TT site,
http://www.993tt.com/, tells following about Sport Cats:
"dollars can be better spent elsewhere. These are very expensive, and they will give you a 'little' HP increase"
and
"Don't Bother - Minimal Gain"
Catalytic Converter Replacement Pipes
Steve in Los Angeles installed a set of pipes that replaced the CATS. He has added the GT2 K-24 Turbos / ECU, and noticed the loss of power below 3,800 RPM. With the replacement of the CATS, Steve has noticed a dramatic power gain at 2,500 RPM and above. These bypass pipes can be bought from Hershel at Car Graphic USA. The following is a quote taken from Steve on the Rennlist 993/996 message board :
"This is in regards to a post I made a few weeks ago about manufacturing CAT bypass pipes for 993TT. I know there are a lot of people who use "sport cats" and like them but, I felt that if sport cats were good NO CATS would be better. The post got so much discussion about the benefits of CAT bypass pipes, so I thought I'd fill you all in. You need to know first that my TT has a modified ECU, big intercooler, .5 bar fuel regulator, GT2 Turbos, etc... The addition of the larger GT2 Turbos gave me the ability to make tons more power on the top end but with a compromised availability in power on the bottom end. With the other modifications I have, the low end wasn't worse than stock but it also wasn't any better. I have now installed bypass pipes on my '96 993tt and it made a HUGE difference! The bypass pipes work KILLER! FULL Boost comes in about 800 rpm earlier than stock under full throttle and around town I get light boost (2-4 tenths) in low rpm ranges (2000-3000) with very little throttle position (1/4 or less). I'm also getting FULL BOOST in the taller gears as low as 2700 rpm. Sounds much more aggressive too... even with stock muffs! No problem so far with check engine light or other weird stuff caused by OBDII system. Looks like it won't need any "trickery" as I was told by some people it may. BTW, power delivery is as smooth as stock except it's just "beefier" everywhere, there's not a huge 930'ish rush like I had with the stock CATS. Power delivery is now so smooth it feels like it could stand a more aggressive ECU!!!
Just remember it is illegal to run without catalytic converters, although the difference is huge with running the bypass pipes. The big power gain has been confirmed by several people on the Rennlist message boards.
Steve in Los Angeles installed a set of pipes that replaced the CATS. He has added the GT2 K-24 Turbos / ECU, and noticed the loss of power below 3,800 RPM. With the replacement of the CATS, Steve has noticed a dramatic power gain at 2,500 RPM and above. These bypass pipes can be bought from Hershel at Car Graphic USA. The following is a quote taken from Steve on the Rennlist 993/996 message board :
"This is in regards to a post I made a few weeks ago about manufacturing CAT bypass pipes for 993TT. I know there are a lot of people who use "sport cats" and like them but, I felt that if sport cats were good NO CATS would be better. The post got so much discussion about the benefits of CAT bypass pipes, so I thought I'd fill you all in. You need to know first that my TT has a modified ECU, big intercooler, .5 bar fuel regulator, GT2 Turbos, etc... The addition of the larger GT2 Turbos gave me the ability to make tons more power on the top end but with a compromised availability in power on the bottom end. With the other modifications I have, the low end wasn't worse than stock but it also wasn't any better. I have now installed bypass pipes on my '96 993tt and it made a HUGE difference! The bypass pipes work KILLER! FULL Boost comes in about 800 rpm earlier than stock under full throttle and around town I get light boost (2-4 tenths) in low rpm ranges (2000-3000) with very little throttle position (1/4 or less). I'm also getting FULL BOOST in the taller gears as low as 2700 rpm. Sounds much more aggressive too... even with stock muffs! No problem so far with check engine light or other weird stuff caused by OBDII system. Looks like it won't need any "trickery" as I was told by some people it may. BTW, power delivery is as smooth as stock except it's just "beefier" everywhere, there's not a huge 930'ish rush like I had with the stock CATS. Power delivery is now so smooth it feels like it could stand a more aggressive ECU!!!
Just remember it is illegal to run without catalytic converters, although the difference is huge with running the bypass pipes. The big power gain has been confirmed by several people on the Rennlist message boards.
01-04-2006, 09:27 PM
#15
Three Wheelin'
I don't think Kevin owns a Turbo R. Actually, the majority was Bill S. (started "60-130 new performance measure" thread). He was the one whom spoke with Ruf and "picked their brains" for info. Also, got lots of info from Weissach and some from Ruf Dallas. Overall. Many inferences through emails. True, Kevin did infrm me/educated me on the 130mm CATS, but you wouldn't think I guy like me would just take his word for it without question....right?? As I posted before in other threads, I too called several vendors of "100cell cats" and the ones I spoke with were smaller in diameter compared to Kevins....including one very popular exhaust company with High Flow systems. True, however, that I did not measure Kevin's CATS as they were welded into my housings by him.....but, I trusted him with my turbo "Ruf clones", why not with his 130mm CATS?
Not affiliated with Kevin in any manner,lah, blah, blah. Just a very happy customer of *several* Rennlist members whom offer services and products.
Not affiliated with Kevin in any manner,lah, blah, blah. Just a very happy customer of *several* Rennlist members whom offer services and products.