| Exhaust Theory - Click HERE for Original Thread |
| Ayeso |
We were having a discussion in another thread about exhaust theory and it got rather large, but due to it being in some random thread I doubt anyone will ever read it. Someone suggested making a new thread for it so I will try and summarize the discussion quickly.
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Originally posted by Scourge
About the cat-backs... Even if you're right about the cat-backs, my opinion is that most of the people putting on full catbacks don't know what the hell they're doing or haven't gone much further than exhaust and intake. I'm no exhaust expert but I do know that smaller piping accelerates the gases so they leave the exhaust quicker. With exhaust valve overlap and big piping, the exhaust fumes linger in the system and get sucked back in on the intake stroke with the exhaust still slightly open. At least that's my understanding.
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Yes but then again for SOME cars, there are cat-backks, that just use the factory size of the piping, for instance, for us people with j-bodies there are at least 3-4 companies that dal with just using the factory 2 1/4 piping that is on the car, but then again thats just so its a neutral sizing, most j-bosies have seen best results out of a 2.5-3 inch piping I'm not sure about others, as I am mainly a j-body enthusist( sorry about spelling pretty early in the morning lol). But I am glad that at least one person is not out there to bash a j-body.
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Posted by Scourge on 11-20-2007 12:49 PM:
Wait, how does bigger piping get lead to back pressure?
I left back pressure out because you can't help but have back pressure with smaller piping. But, the gases escape the system faster. It's a tradeoff.
Maybe I'm confused with the term back pressure. I get that high pressure will want to escape to low pressure. You have higher pressure in smaller diameter piping. It wants to escape faster but there is more resistance than with bigger piping because the bigger piping will be of a lower pressure (but higher than outside still) than the smaller pipe, But the air in the piping will be moving faster. still even with the resistance. And the exhaust gases don't linger in the smaller system.
I believe this is why people can run too lean with too big of a piping. The slower velocity and the bigger "chamber" allows exhaust gasses to be sucked back in on the intake stroke (with the exhaust valve open during the overlap). Remember, I'm not claiming to be an exhaust expert. This is just how I'm interpreting it.
I know your examples were just examples. But I wanted to share my thoughts about my setup. I believe 3" piping is acceptable for a K-series. I've heard it makes power naturally aspirated up to 3". I've run a stock K20 to 8600 RPM where I set my rev limiter without hitting the valves. And I believe the K24A2 has a stock rev limiter of 7100 RPM (but can be higher). So, I imagine that they expel gases faster than most 4 cylinders?
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Posted by Ayeso on 11-20-2007 01:12 PM:
BACKPRESSURE IS ALWAYS ALWAYS ALWAYS A BOTTLENECK and should ALWAYS be avoided or minimized as much as possible.
The reason this is so confusing to people is that they are confusing backpressure with turbulence in the exhaust stream, particularly at lower velocities. Lower velocity exhaust columns tend to stagnate near the walls of the exhaust pipe and create swirling effects, which creates high backpressure and lost power. This is what occurs with a larger pipe. If you get a smaller pipe, backpressure increases but backpressure due to turbulence decreases - the exhaust column moves faster and thus has less time to stagnate. This has the effect of boosting torque when the engine is lower in the rev range but chokes it at the top end. Taking this a step further, this is why n/a engines make more power with a nice long smooth connector tube that dumps to atmosphere about 15" down the pipe, rather than dumping to atmosphere straight out of the engine. The dumping of exhaust creates turbulence at the pipe's exiting point that impedes the gas column's flow and hence its velocity - creating backpressure - with shorter tubes. The flow in a longer pipe has a greater chance of becoming laminar and helping with scavenging as well.
The problem is related to the difference between large displacement engines creating high exhaust volumes at low engine speeds and small displacement engines creating high exhaust velocities at high speeds. For a small displacement engine you want high exhaust velocity at high RPM, otherwise the engine will be choked due to turbulence or backpressure. This means smooth bends and a moderate piping size - I recommend 2 1/4" for the average high-revving import that hasn't seen lots of headwork or displacement increases. For these, I would recommend stepping up to 2 1/2" as the extra engine speed and displacement will require larger exhaust capacity to prevent backpressure from occurring.
But back to larger displacement engines... these engines required huge exhausts because even at low engine speeds high exhaust volume was generated and the amount of exhaust generated kept the gas column moving at a regular pace - so really on domestics, you can't go too big in most cases (up to about 3" will produce gains throughout the rev range, top to bottom). Not so with imports. You HAVE to maintain gas column velocity or stagnation and hence backpressure will occur, but you can't choke the top end. On lower-revving 1.5L or 1.6L engines a good mandrel-bent 2" will be best. On high-revving 1.6L engines, mandrel-bent 2 1/4" is best... but be aware that low-speed performance will suffer. It's the wide RPM band that is the beast that makes it tough to choose a good exhaust.
Posted by silent on 11-20-2007 05:28 PM:
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Originally posted by Scourge
The slower velocity and the bigger "chamber" allows exhaust gasses to be sucked back in on the intake stroke (with the exhaust valve open during the overlap).
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I'm pretty sure exhaust doesn't get sucked back through the exhaust when the valves open... It's pretty consistant pressure since the valves don't all open at once therefore there's ALWAYS pressure pushing out your exhaust gasses.
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I know your examples were just examples. But I wanted to share my thoughts about my setup. I believe 3" piping is acceptable for a K-series. I've heard it makes power naturally aspirated up to 3". I've run a stock K20 to 8600 RPM where I set my rev limiter without hitting the valves. And I believe the K24A2 has a stock rev limiter of 7100 RPM (but can be higher). So, I imagine that they expel gases faster than most 4 cylinders?
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My examples were the vehicles I know best and am comfortable with and the numbers are correct for that specific vehicle. If the K24A2 is a 4 cylinder turbocharged engine, then it really only needs a 2.5" exhaust (3" works as well though) based on the stock rev limit. As for your car, I'm not sure since it runs almost 2000RPM higher than the ones I'm comfortable with.
A high rev limit doesn't cause the pistons to smash the valves, its more of a case of causing valve float. Again, this may be different with your line of cars but I know it's the case with mine.
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Posted by Ayeso on 11-20-2007 01:27 PM:
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Originally posted by cavalierguy2006
Well I dont have a turbo on my car yet I am working on that part and building the car up for it (maybe), usually Cavaliers tend to run better on a 2.5 inch Catback(according to dynos), but then again most Cavaliers that run turbo go no higher then 3-3.5 inch with that being said most cars especially N/A cars(generally speaking j-bodies), should run only between 2.25-3 inch exhaust depending on the setup, factory ran vehicle(115-160hp) should be 2 1/4, 160-230hp should be about 2.5 inch and anything over usually is 3 inch, but then again depends what you are going for......sound or power.......
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Physics doesnt lie. 2.5 is not going to gain any benefits on a stock Z24 motor. I dont care what the dyno graphs say.
Chances are it was tested in a different climate, diff day, etc...
Unless the conditions were identical tested with the catback then swapped it REAL fast for the stock exhaust and tested with near identical conditions I wont believe it.
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| Scourge |
| Subscribed! Don't forget to include the graph you had up in the previous thread. :) |
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| Ayeso |
Oh yeah, I posted an excerpt from corky bells "Maximum Boost"
Great book for anyone who hasnt read it and is interested in some basic engine / turbo theory.
Now keep in mind this is a GENERALIZATION there is much more in regards to tuning etc.. different engines.. rpms etc.. but in GENERAL this is a decent guideline.
Now where it says, base the calculations on 250ft/s it is referring to Velocity = Airflow/Area |
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| dc2696 |
Stock motor= who cares about exhaust piping.
Turbo= go bigger
n/a= jump off a bridge before you go bankrupt |
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| Ayeso |
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Originally posted by dc2696
Turbo= go bigger
Only to an extent... biggest isnt always better people always get this wrong, 3 inch charge piping , 4 inch exhaust etc... its WAY overkill and you are actually getting less gains then you should be.
Example:
Say you have a stock turbo car. 2 inch is fine for the power level / turbo size. Lets say you upgrade the turbo to a much larger one with say a 2.25 inch outlet. Now... 2 inch would not be sufficient anymore, so you swap it out for a huge 3.5-4 inch exhaust now YES you will get huge gains over a 2 inch pipe due to 2 inch being much to restrictive BUT if you were to go with a 2.5 or 3 you would have substantially more gains then going with something way bigger.
The problem here is everyone who swaps out a huge exhaust DOES see gains and then makes the assumption that I GOT A 4 INCH and GAINED 40HP!!! THAT MEANS ITS PERFECT and then that continues and everyone believes it. |
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| b18c5crx |
go slap a 5 inch downpipe on a turbo car, it creates a vacuum effect. I had a lengthy discussion with eric @ xs about this one.
he knows too much.
:dunno: |
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| Ayeso |
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Originally posted by b18c5crx
go slap a 5 inch downpipe on a turbo car, it creates a vacuum effect. I had a lengthy discussion with eric @ xs about this one.
he knows too much.
:dunno:
What do you mean? Are you referring to how the air near the walls becomes stagnant and you have a column in the center? |
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| NYGACR |
Here's some evidence I've got. Note the midrange torque and shift in AFR.
Tell me 3.0" is hurting things...
Note: Different cam setups, very similar mods otherwise
N/A 2.4L
3.0" dump (behind door w/ race bullet) tri-y collector vs. 2.25" w/ muffler
N/A 2.4L
3.0" dump from merge collector vs. 2.5" exhaust
The key for naturally aspirated is good collector design. Tri-y or merge is a must to make good power.
I'll stick with my 3.0" dump. |
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| Ayeso |
You never read what i said obviously..
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Taking this a step further, this is why n/a engines make more power with a nice long smooth connector tube that dumps to atmosphere about 15" down the pipe, rather than dumping to atmosphere straight out of the engine. The dumping of exhaust creates turbulence at the pipe's exiting point that impedes the gas column's flow and hence its velocity - creating backpressure - with shorter tubes. The flow in a longer pipe has a greater chance of becoming laminar and helping with scavenging as well.
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| NYGACR |
My evidence contradicts Corky Bell's graph and the notion that a 3" exhaust is too big for NA as velocity slows too much.
Had I posted a dyno of a dump at the collector versus dumping after 15" of pipe you could criticize me. I haven't though. In theory this is true- post a chart to support it. In my opinion, a good merged collector alone will provide ample scavenging- I understand what you're saying, but I'd like to see how much more the pipe length affects things. |
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| Ayeso |
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Originally posted by NYGACR
My evidence contradicts Corky Bell's graph and the notion that a 3" exhaust is too big for NA as velocity slows too much.
Had I posted a dyno of a dump at the collector versus dumping after 15" of pipe you could criticize me. I haven't though. In theory this is true- post a chart to support it. In my opinion, a good merged collector alone will provide ample scavenging- I understand what you're saying, but I'd like to see how much more the pipe length affects things.
This is where we both agree, but you do need SOME length when he says 15inch he means out of the head, not the collector. He is referring to dumping out of a pipe like you have, vs dumping out of the head directly. I agree completely for something such as what you are doing its perfectly acceptable, but on an average street car with a full exhaust the length of pipe has an adverse effect. |
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| NYGACR |
| I thought you were refering to a minimum length following the collector. We agree! Good stuff. |
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