Is the stock downpipe and factory exhaust diameter a performance bottleneck? - Alfa Romeo 4C Forums
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post #1 of 19 (permalink) Old 03-10-2017, 09:46 AM Thread Starter
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Is the stock downpipe and factory exhaust diameter a performance bottleneck?

As part of my journey of going through my Magnaflow failure experience, I have been reading up on turbo exhaust system designs and my research has spawned more questions than answers ( which is often the case ). One idea that I have found myself wondering is, how much of a performance impediment, if any, is the factory exhaust and down pipe? Lots of people are adding tunes to their 4C's, but is there any headroom available with the factory systems?

Some of the tunes ( like the EC phase 2 ) already recommend an upgraded exhaust to get the most out of the tune. Is there a benefit to be had by simplying upgrading the factory exhaust with everything else remaining stock?

I found the following post to be informative ( original at: http://www.tercelreference.com/terce...exhaust_theory )

-----------

The following excerpts are from Jay Kavanaugh, a turbosystems engineer at Garret, responding to a thread on 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

o 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.

---------------

I would love to hear from some of the experts on whether the factory exhaust is already stretched at the factory power levels and if leaving the factory 1.9" tubing in place is as problematic as I think it is.

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post #2 of 19 (permalink) Old 03-10-2017, 10:00 AM
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I have always been a proponent for a 3" exhaust system especially for a turbo charge engine. The 4C needs a 3" turbo back exhaust system, it would make more power vs 2.5". Unfortunately only 2.5" exhaust systems are only manufactured unless you get a custom one made.
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post #3 of 19 (permalink) Old 03-10-2017, 10:30 AM
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Opening up the exhaust on a turbo engine gives big benefits. On my 1990 turbo Miata (my DD), changes to the exhaust resulted in a 500RPM improvement in reaching peak boost/torque (went to a 2.5" catless system with a separated-gasses type downpipe). I haven't messed with the 4C yet, but it should be similar. And, yes, with a turbo, bigger is always better. Packaging is a compromise though.
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post #4 of 19 (permalink) Old 03-12-2017, 10:53 AM Thread Starter
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While reading up on changing the 4C exhaust system, I ran across some warnings about a potential temperature rise that can cause damage to the engine when altering the downpipe to a larger diameter or going catless without proper support from an aftermarket tune. E.g. the factory tune would not be compatible with such a mod. @André mentions it here and here. @Docron has also included a reference to the need to adjust the EGT support for downpipe changes (can't find the post currently).

What is the significance of this? Some members like @Call me Al have already embarked on exhaust modifications in this area (https://www.4c-forums.com/105-4c-motorspo...t-project.html), so would the EGT issue need to be accomodated in cases like this?

Another point that this brings up, if the tune needs to have support for a different EGT, for those of us that plan on swapping ECU's back to factory for visits to the dealer, does this put the engine at risk due to the lack of support in the factory ECU while it is in the car?

This issue might explain why the Novitec catless downpipe is only available for purchase for customers that have a Novitec ECU. I see that EC is also selling the Novitec catless downpipe, again only for customers that have a Novitec ECU or the EC ECU. @[email protected] can you confirm that the EC tunes have support for avoiding this temperature spike? If so, which phases of tunes are compatible?

My last question is mainly for @André . I noticed that the Novitec Stradale CC exhaust has the cats moved to the rear of the car, with just a straight downpipe at the turbo. Does this mean that this exhaust requires support in the ECU to adjust the EGT and avoid this temperature spike? Again what are the implications of swapping in factory ECU's for dealer visits when using such a setup?

When I hear warnings about possible engine damage it really makes me sit up and pay attention and hopefully we can have some candid discussions so community members can be fully aware when embarking on these sorts of modifications.
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post #5 of 19 (permalink) Old 03-12-2017, 11:41 AM
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Quote:
Originally Posted by MidLifeCrisis View Post
While reading up on changing the 4C exhaust system, I ran across some warnings about a potential temperature rise that can cause damage to the engine when altering the downpipe to a larger diameter or going catless without proper support from an aftermarket tune. E.g. the factory tune would not be compatible with such a mod. @André mentions it here and here. @Docron has also included a reference to the need to adjust the EGT support for downpipe changes (can't find the post currently).

What is the significance of this? Some members like @Call me Al have already embarked on exhaust modifications in this area (https://www.4c-forums.com/105-4c-motorspo...t-project.html), so would the EGT issue need to be accomodated in cases like this?

Another point that this brings up, if the tune needs to have support for a different EGT, for those of us that plan on swapping ECU's back to factory for visits to the dealer, does this put the engine at risk due to the lack of support in the factory ECU while it is in the car?

This issue might explain why the Novitec catless downpipe is only available for purchase for customers that have a Novitec ECU. I see that EC is also selling the Novitec catless downpipe, again only for customers that have a Novitec ECU or the EC ECU. @[email protected] can you confirm that the EC tunes have support for avoiding this temperature spike? If so, which phases of tunes are compatible?

My last question is mainly for @André . I noticed that the Novitec Stradale CC exhaust has the cats moved to the rear of the car, with just a straight downpipe at the turbo. Does this mean that this exhaust requires support in the ECU to adjust the EGT and avoid this temperature spike? Again what are the implications of swapping in factory ECU's for dealer visits when using such a setup?

When I hear warnings about possible engine damage it really makes me sit up and pay attention and hopefully we can have some candid discussions so community members can be fully aware when embarking on these sorts of modifications.

In short there are a handful of tables that need to be modified to avoid high temps and spikes. Andre (Novitec) and I have spoken about this a few times. We (EC & Novitec) were aware of this from the get go.

I am unable to go into detail as a few competitors are not aware of these ill effects that can take place. If the ECU is not calibrated specifically and properly for it you will have issues in the future. Major ones.

We have said the entire time a high-flow cat is a better solution. A cat-less system is really only for a car specifically built for the track. The gains will be nominal 5 to MAYBE 10whp with a cat-less system. Call me Al has a nice setup that is running well.

I apologize for the vagueness of this post but I cant go posting how and why and then come on here tomorrow to see a competitor with a "NEW" discovery. Sorry!
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post #6 of 19 (permalink) Old 03-12-2017, 09:18 PM
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Originally Posted by [email protected] View Post
I am unable to go into detail as a few competitors are not aware of these ill effects that can take place. If the ECU is not calibrated specifically and properly for it you will have issues in the future. Major ones.
Is that to say that an EC phase 2 can handle a 3 inch downpipe or that it would need to be a custom one off tune?
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post #7 of 19 (permalink) Old 03-13-2017, 07:12 AM
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Hi MidLifeCrisis,

sorry, I have not read everything. Honestly I have not the time and feel like to discuss theoretical thinking vs. what we already measured. Maybe I misunderstand it but it sounds that you imply that we are not able to measure the temperature or that we are writing not the truth.

Quote:
Originally Posted by MidLifeCrisis View Post
and hopefully we can have some candid discussions so community members can be fully aware when embarking on these sorts of modifications.
A discussion about is senseless! You have to test it and measure the temperature (incl. the peaks) in detail to get the facts. That's all, very simple

We measured the exhaust temperature in the front of the turbo with OE mapping and downpipe (no catalyst). We recognized temperature peaks with more than 1.000°C (1832°F). Thats all. We have not wasted time to measure/log the details because those values are useless. Why should somebody decat a 4C without to modifiy the ECU also? This is IMHO stupid, it make trouble with the AFR/Lambda-sensor after the catalyst.
So there is a need to modify the ECU in general, otherwise you get error messages every 50-100km (not only an error, you loose also performance until you delete the error message).

A short peak of more than 1.000°C is not a big problem in general. The question is how often it happens, under which conditions/situations and how long. We have not figured this out in detail because it is senseless to waste the time for such things.
Maybe such a modified 4C will hold for some years, maybe not. It is not only the engine which could damage, also the turbo and for sure the wastegate will get a heat damage on mid-/long term. For example: A sticking wastegate by distortion as a result of overheating happens also at untuned Alfa MiTo and Giulietta! It is nothing dramatically, you "only" new/refurbished turbo if it happen.

Quote:
Originally Posted by MidLifeCrisis View Post
Novitec Stradale CC exhaust has the cats moved to the rear of the car, with just a straight downpipe at the turbo. Does this mean that this exhaust requires support in the ECU to adjust the EGT and avoid this temperature spike?
The Stradale CC is a 200 cpi sports catalyst at a much more better mounting position. The advantage of the system is a better cooling situation and a much better "flow". We do not recommend the Stradale CC as single product without a tune (to bad benefit/cost ratio).
It is perfect for the 325 bhp /450 N m tuning if you want to keep a catalyst (of course it is also possible to decat the system but the result is nearly the same).

A sport catalst at the OE position make IMHO no sense, the benefit/cost ratio is terrible. The benefit in performance and torque is very small as Chris already wrote. It is not legal in a lot countries and the temperature hearth is still at the OE place where it make "trouble".
For the German market we are thinking to come with a homologated catalysts at OE position. Decat a car (or move the cat) is a criminal act in Germany because it is a tax evasion.

Quote:
Again what are the implications of swapping in factory ECU's for dealer visits when using such a setup?
Short temperature peaks are not dramatically for a short time. So of course you can drive with OE ECU and downpipe to your dealer. But if the dealer take a closer look in the engine bay? I am not sure if I understand your question.
It make IMHO no sense to decat the 4C in the warranty time if you take care about the warranty. Too much work, too less benefit.
We are speaking about 5-8 bhp and approx. 10-13 N m without a catalyst! With a sport catalyst at OE position less!

BTW: The OE turbo is good for 2,7 bar / 39 psi (we measured this) but the OE mapping is limited to 1,55 bar / 22.48 psi.
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post #8 of 19 (permalink) Old 03-13-2017, 11:49 AM Thread Starter
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Hi André,

Thank you for response, it was very helpful in understanding the issues that you had raised in your other posts.

My apologies if my post sounded like I was doubting you. That was not my intention.

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post #9 of 19 (permalink) Old 03-13-2017, 12:28 PM
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Is that to say that an EC phase 2 can handle a 3 inch downpipe or that it would need to be a custom one off tune?
Yes, and no. We can only account for what we have tested, and Phase 2 is inclusive of those findings. But to be absolutely sure before doing something, it is best to contact us with the details of the exhaust change so that we can assess whether a tune change is necessary or not.

I could not agree more with everything Andre has stated in his post, and I want to highlight something in particular he wrote:

"A short peak of more than 1.000°C is not a big problem in general. The question is how often it happens, under which conditions/situations and how long."

Increasing certain parameters in a tune for this car reveals the need for certain corrections to be made due to scientific events that will absolutely occur in the process (such as temperature spikes, long term and short term effects on VE, etc). Without these corrections, you're running blind with fingers crossed and hoping for the best. Chris is being vague in this regard because you either know what you're looking for, or you don't. It doesn't just pop out at the tuner, but can very quickly lead to adverse effects, as has been already stated.

This is why a tune is required to run a more free flowing exhaust and/or high flow catalyst. These corrections have been measured, accounted for, and safely/effectively maintained without negative effects. Which leads to decatting the 4C in a nutshell:

- tune/exhaust/high flow cat = performance benefit balanced with optimization to stay out of the danger zones

- tune/exhaust/catless = minimal (perhaps nonexistent in most applications) performance benefit over the above, and constant, unnecessary subjection to more intense danger zones.

The 4Cs calculation models and corrections only go so far before physical science starts to overload the electronic science of tuning (the reason I highlighted the quote above). This is why it has been referred to as a "senseless" comparison. What decatting presents to the car overall is just unnecessary due to the performance gained and safety sacrificed in the process. And you will not know the extent of the damage done until it becomes very apparent, and it's too late.

Anyone is free to disagree with this, but we exercise our discretion for a reason.
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post #10 of 19 (permalink) Old 03-13-2017, 03:49 PM
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You are welcome MidLifeCrisis

Quote:
Originally Posted by [email protected] View Post
I could not agree more with everything Andre has stated in his post
My statement was written for those guys who (want to) run OE mapping with decat downpipe.
To make clear: We never would offer a product with crossed fingers and/or praying And we ever take a lot of time for testing detailed.

Our largest "stage" was developed without a catalyst at OE position, we never thought about to keep the catalyst at this terrible position. Alfa has had the need for this position to fulfil Euro6.
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