Exhaust question

[DrMrLordX]

It is my understanding that having a downpipe (or cat-back exhaust piping I guess?) with too wide of a diameter can ruin exhaust scavenging at low rpms producing a loss of torque in that range. However, what happens when you have a downpipe (assume a high-flow cat) connected to an aftermarket cat-back exhaust that is wider than the exhaust system's piping?

For example, let's say you have a 2.5" downpipe leading to a 2.25" cat-back exhaust. Would that flow the same as a 2.25" downpipe leading to the same exhaust?

 

[Zenmervolt]

Overall flow rate (volume per unit time) would be similar, but velocity in the larger downpipe would be lower, which would still reduce the scavenging effect at low RPM. You need to think velocity and not flow rate for scavenging effect.

ZV

 

[alkemyst]

almost all header collectors in a vehicle lineup are going to be the same size. What you bolt it too at the higher performance level will usually be larger than that.

The full exhaust factors into where your powerband sits.

I'd hit up a vehicle specific forum and see what others have dynoed.

On my 240SX many would say a 2.25 or 2.5" exhaust is the limit (2.4L engine).

However, due to it's long stroke the car still makes power with even an 80MM exhaust (a little over 3"), header and hiflow cat/test pipe.

 

[DrMrLordX]

Originally posted by: Zenmervolt
Overall flow rate (volume per unit time) would be similar, but velocity in the larger downpipe would be lower, which would still reduce the scavenging effect at low RPM. You need to think velocity and not flow rate for scavenging effect.

ZV

Okay, you've merely confirmed my suspicions on how it would work. Thanks.

Originally posted by: alkemyst
almost all header collectors in a vehicle lineup are going to be the same size. What you bolt it too at the higher performance level will usually be larger than that.

The full exhaust factors into where your powerband sits.

I'd hit up a vehicle specific forum and see what others have dynoed.

On my 240SX many would say a 2.25 or 2.5" exhaust is the limit (2.4L engine).

However, due to it's long stroke the car still makes power with even an 80MM exhaust (a little over 3"), header and hiflow cat/test pipe.

I have asked around on a forum dedicated to my car (Saturn Ion . . . woop woop!) and have gotten mixed results.

One guy with a 2.4L Ecotec reported that he lost no low-end torque with a 3" downpipe, 1.75" headers and a Tsudo cat-back exhaust (advertised as 2.5", though in reality it is 2.25"). Others warned me to avoid going any wider than 2.25" on the downpipe with my 2.2L Ecotec. The problem is that finding an aftermarket downpipe any smaller than 2.5" for my application is damn near impossible. Oh well, it's no big deal.

 

[alkemyst]

I had a Saturn with the 1.9L...the forums are more or less worthless as I think half are actually driving their parents car. The downpipe isn't so important on a NA car...usually it's refered to more or less a downpipe on a turbo car and a 2piece header on an NA. Either way in NA form the down pipe shouldn't make too much a difference.

I'd look for someone with a dyno of their car. I am going to get one on mine July 12. $75 for three runs to get a baseline. $45-90 to tune it/give me the recommendations if I need to go to a air/fuel control, or something else.

 

[DrMrLordX]

Originally posted by: alkemyst
I had a Saturn with the 1.9L...the forums are more or less worthless as I think half are actually driving their parents car.

Depends on the forum. Saturnfans, yeah, maybe. Ionforums.net and sixth sphere are pretty good.

The downpipe isn't so important on a NA car...usually it's refered to more or less a downpipe on a turbo car and a 2piece header on an NA. Either way in NA form the down pipe shouldn't make too much a difference.

Hmm, interesting. I have noticed that most of the guys going for turbos, superchargers, and/or nitrous favor 3" downpipes.

I'd look for someone with a dyno of their car. I am going to get one on mine July 12. $75 for three runs to get a baseline. $45-90 to tune it/give me the recommendations if I need to go to a air/fuel control, or something else.

$45 for a tune? Damn, that's cheap. Too bad there's no tuning for my car's ECU (not yet anyway).

 

[alkemyst]

it's not a full tune though, I don't have any kind of a/f device. They will only be able to play with what's there and recommend what I need to add if that is the case.

Usually downpipes are turbo only. I know some companies make headers that bolt to turbo down pipes...it's not the same.

 

[fbrdphreak]

Previous posters are correct - up-pipes and down-pipes refer to turbo applications specifically. You can see an example of how an exhaust is setup on a Subaru:
http://www.scoobytuner.com/tuning/?tuningID=15

For your car, you have an exhaust manifold (singular, being an inline motor and all), and piping from there back which includes any number of cats and resonators and/or mufflers.

Reducing any backpressure in an engine exhaust is good. Yes, your power band would be shifted somewhat lower in the RPM range with a smaller diameter exhaust. Also realize that you get diminishing returns with a larger diameter pipe.

For instance, most stock heads/cams 04-06 GTO's (with no FI of course) don't see any benefit by going with a 3" cat-back instead of a 2.5". It isn't until you start flowing serious amounts of air (with aftermarket heads/cam or forced induction) that 3" shows any benefit. 3" exhausts for the GTO's are much more expensive than the 2.5", so there's not much point for most people to get one.

Advice: Leave your Ion alone. You might add 10 crank HP and then just annoy everyone with the buzzy fart can driving around. You will invest hundreds of dollars for a very small real world benefit. Save those hundreds of dollars towards a car that actually has some power behind it and you will be MUCH happier than by putting an exhaust on your low-output four banger economy car.

I don't mean this as insult, I mean this as real advice. Believe me I understand the desire to make your slow car go faster, I had those same temptations with my 97 Subaru wagon. But when you step back and look at the cost/benefit ratio, it just isn't worth it. Not to mention then you're one of those guys that car guys make fun of - the one who puts a wing and fart can on his econo car...don't be that guy :sun:

 

[alkemyst]

Not all import exhausts are fart cans...not all V8 exhausts sound better than fart cans.

Also most of the time adding a catback should add more than 10 crank HP...not sure on the ion, but usually it's not so much a PEAK gain than the overall gain you get.

It's not a terrible car 0-60, I think the non-redline version is in the 7's 0-60.

 

[DrMrLordX]

Originally posted by: alkemyst
it's not a full tune though, I don't have any kind of a/f device. They will only be able to play with what's there and recommend what I need to add if that is the case.

Oh okay, that makes more sense.

Usually downpipes are turbo only. I know some companies make headers that bolt to turbo down pipes...it's not the same.

It's this thing for what it's worth:

http://www.zzperformance.com/c...1.php?id=545&catid=155

Originally posted by: fbrdphreak

Reducing any backpressure in an engine exhaust is good. Yes, your power band would be shifted somewhat lower in the RPM range with a smaller diameter exhaust. Also realize that you get diminishing returns with a larger diameter pipe.

See, that's the problem here . . . on the one hand, going with a mandrel-bent exhaust header combo like the one I linked above would, presumably, reduce backpressure, which is at least theoretically going to help reduce pumping losses which is why guys report picking up a few mpg after modding their exhaust and putting on a CAI or SRI. I think.

However, the best header combo I can find has a "downpipe" (call it what you will) that is entirely too wide for my intended application. The construction is good, the entire product probably weighs less than the stock cast iron (also good for fuel economy), but it's just too darn big. I'd rather get 1.4" headers and a 1.75-2" "downpipe" w/high-flow cat than the available 2.5" "downpipe".

The alternative is to just stick a cat-back exhaust on there without messing with the stock headers/manifold and suffer the unnecessarily restrictions inherent to them (and the stock cat) which will force the engine to do more work to pump out exhaust gasses and hurt fuel economy. Boo.

I don't really care a fig what the exhaust mod does to my power above 3500-4000 RPMs since I don't think I can even get that high in 5th gear before the speed limiter kicks in.


Advice: Leave your Ion alone. You might add 10 crank HP and then just annoy everyone with the buzzy fart can driving around. You will invest hundreds of dollars for a very small real world benefit. Save those hundreds of dollars towards a car that actually has some power behind it and you will be MUCH happier than by putting an exhaust on your low-output four banger economy car.[/quote]

Not after the HP (it's nice, but), more interested in finding ways to reduce pumping losses for increased fuel efficiency using available parts without having to go custom. Keep in mind that exhaust mods are way down the list, I'm just trying to sort things out. Killing low-end torque will not help with fuel economy, I suspect, especially since most fuel-saving cruising is done at fairly low RPMs.

Not to mention then you're one of those guys that car guys make fun of - the one who puts a wing and fart can on his econo car...don't be that guy :sun:

Putting wings on a FWD car? No thanks. Also, most if not all the exhausts available for the Ion don't really fall into fart-can territory. I have seen a guy with a cherry bomb on his Ion though . . .

Originally posted by: alkemyst
Not all import exhausts are fart cans...not all V8 exhausts sound better than fart cans.

Also most of the time adding a catback should add more than 10 crank HP...not sure on the ion, but usually it's not so much a PEAK gain than the overall gain you get.

It's not a terrible car 0-60, I think the non-redline version is in the 7's 0-60.

As much as I love my Ion, and as much respect as it has given me for the 2.2L Ecotec, I can't claim my ride has a 7s 0-60. Most reviews put the 5-speed Aisin automatic version (such as mine) in the 9.9-10.2 range for 0-60 due to the crazy-ass shifting behavior. If you have a manual with the 2.4L engine from later model years, yeah, I could see getting into the 8s maybe.

It's been a rock-solid car though, aside from a few wonky sensors.

 

[alkemyst]

Originally posted by: DrMrLordX

It's this thing for what it's worth:

http://www.zzperformance.com/c...1.php?id=545&catid=155

seems like one of the many 'ebay' type deals. I'd avoid this unless you specifically know of a person that has dynoed it successfully.

A header system should include the recommended downpipe with it. Those claims on power are EXTREMELY optimistic and probably based on much more modifications to the car and include those power gains.

Usually header comes after cat back and intake has been done. A header is worth not so much power, but does help support the other mods. My exhaust adds usually 14-16RWHP on most cars like mine. The header only 5RWHP tops.

I am going to see what I really gained although I only would be going off factory hp rating as I don't have a true baseline dyno on MY car. I do have dynos from cars identical to mine though.

 

[DrMrLordX]

Originally posted by: alkemyst

seems like one of the many 'ebay' type deals. I'd avoid this unless you specifically know of a person that has dynoed it successfully.

It's the only header/downpipe combo out there for the Ion sedan that seems to be worth a darn (highly-recommended by many Ion modders, though many of them bolt on the exhaust from 2.4L Ions and Redlines as a low-cost alternative). Well there's that and the Vibrant system, but the cat-back that goes with the Vibrant header/downpipe combo costs about $300 more than the Tsudo exhaust favored by most of the Ion modders without being demonstrably better. I think the entire Vibrant system features piping with a 2.75" diameter that would make little to no sense for my intended purpose.

A header system should include the recommended downpipe with it. Those claims on power are EXTREMELY optimistic and probably based on much more modifications to the car and include those power gains.

Yeah, 40hp is just not going to happen. 15hp? Maybe with an appropriate cat-back exhaust. Doesn't really matter to me as long as I'm getting better exhaust velocity in the low rpm range.

Usually header comes after cat back and intake has been done. A header is worth not so much power, but does help support the other mods. My exhaust adds usually 14-16RWHP on most cars like mine. The header only 5RWHP tops.

I plan on doing the intake first, then header/downpipe/cat-back in one shot, if/when it gets to that stage.

I am going to see what I really gained although I only would be going off factory hp rating as I don't have a true baseline dyno on MY car. I do have dynos from cars identical to mine though.

As long as your car is in good shape, the existing data should be applicable.

 

[alkemyst]

If someone has dynoed it and can back it up that's good...I will say from many car forums when something is low-cost you will have a ton of people swearing by it...

Also many will claim to have every high end goodie to, while really not.