Back pressure and the camaro...

[mikeSS]

Originally Posted by Bama 2SS/RS

Take a look at the links I posted. I learned quite a bit just from those links. It's not an issue of lack of back pressure that kills power down low, it's that the more free-flowing exhaust (usually using larger diameter pipe) kills the velocity of the exhaust flow, and reduces the scavenging effect. The links I posted liken it to a garden hose with and without your finger over the end. When you're at lower RPMs, you have lower air flow in your engine, and lower exhaust flow. With a more open exhaust, it's like your garden hose without your finger on it, the flow rate is too low, and the diameter too large, and you don't get that high-speed flow. When you get up to higher engine speeds, you have more air flow, and thus more exhaust flow, and the same diameter pipe gets more of a nozzle effect (like when you choke down on the end of a hose with your finger) and you get more velocity in your exhaust. This gives you the scavenging effect, where the high-speed exhaust flow draws a slight vacuum (due to the momentum of the fluid) and helps to pull the exhaust from the next cylinder to open it's exhaust valves. Basically, the outward flow of exhaust moving at speed helps to pull the exhaust from the next cylinder easier.

I had always known about the scavenging effect, and in fact, a lot of the design of exhaust headers is to make the pipes equal length to maximize the scavenging effect. But I, too, was under the misconception that backpressure is needed for the engine to run properly. When, in reality, the trick is that it is a balancing act. You need the exhaust large enough to minimize or eliminate backpressure, while also being small enough to maintain exhaust velocity at your desired RPM range. You want it to function similar to a nozzle at your optimum range, but still be large enough to reduce or eliminate backpressure. If you go with larger pipes, you'll reduce the backpressure that is holding you back at higher RPMs, but you can end up sacrificing low-end performance because the pipe is too large and you are reducing your exhaust velocity too much at lower RPMs, thereby eliminating your exhaust scavenging effect.

I guess it's pretty much like anything else high-performance automotive related. You can't gain everything, you have to sacrifice somewhere to make more somewhere else. Just like when you look at axle gearing. You can gear down your differential to get more acceleration, but you're robbing top-end, or you can go with less gear reduction for higher top-end, but then you sacrifice low-end torque and acceleration. You have to find the balance that puts your exhaust system at the optimum set-up for your desired RPM range.

I had always understood the exhaust scavenging effect and the velocity implications of a larger exhaust diameter, I was just personally still misled by the myth that backpressure is necessary. The key point is that freeing up your exhaust with larger pipe benefits you by reducing backpressure on the high-end, but takes away performance on the low end by reducing exhaust velocity. It's not the reduction of backpressure that kills you on the low-end, it's the reduction of velocity.

That's why, going back to the discussion of the EXUP valve on my R1 sportbike, it throttled down at low RPMs. It was throttling down on the exhaust, just like you would throttle down on a hose with your finger to get higher velocity, to keep the exhaust gas velocity up at lower RPM (where the airflow and exhaust flow are less due to lower engine speed). Then, when you ripped on the throttle and got the bike up to higher speeds, the EXUP valve would throttle open to reduce the exhaust restriction and reduce backpressure to keep from robbing the high-end.

Hope that clears everything up. I feel like I built upon my own knowledge, and dispelled an incorrect myth I had believed for all these years.

interesting. thanks for the info

[Bama 2SS/RS]

Originally Posted by mikeSS

interesting. thanks for the info

Yeah, man. I hope you made it all the way through, I know it was a pretty long post.

But in all seriousness, this is a misconception that I've believed for YEARS, until the discussion came up on the forum, and I started reading up on some articles online. I think, once you read some of that information, it just seems (to me) to make so much more sense if you look at the velocity angle of it. I always did think the idea of backpressure helping the engine run better seemed a little counter-intuitive.

[Wgrenon]

Damn good read Gents. Extremely informative.

[Camarowguy]

Originally Posted by Bama 2SS/RS

Yeah, man. I hope you made it all the way through, I know it was a pretty long post.

But in all seriousness, this is a misconception that I've believed for YEARS, until the discussion came up on the forum, and I started reading up on some articles online. I think, once you read some of that information, it just seems (to me) to make so much more sense if you look at the velocity angle of it. I always did think the idea of backpressure helping the engine run better seemed a little counter-intuitive.

I used to argue this. Then I realized that it's best to let people believe what they want. Knowing what I know, I seem to run a little faster than the next guy who spent a bunch more on his mods than me. A highflow, properly designed exhaust system is what you want. Your engine does not want back pressure. Scavenging is what it wants.

[Darth_Emma]

I had 10 more rwhp with cutouts open than closed on my car.

Also, when I deleted the cats, I picked up about 10 rwhp, but I gained about 30 torque.

[mikeSS]

Originally Posted by Darth_Emma

I had 10 more rwhp with cutouts open than closed on my car.

Also, when I deleted the cats, I picked up about 10 rwhp, but I gained about 30 torque.

let me guess the gains were up top right ?

[Darth_Emma]

Originally Posted by mikeSS

let me guess the gains were up top right ?

Nope. Gains were higher down low.
Run 1: No cats
Run 2: With cats

Note the maximum torque gain was 47. The gains were throughout, but more down low (why the facepalm?).

By the way, I just went to a smaller pulley and the torque went up to 527 (a gain of another 31).

[mikeSS]

Originally Posted by Darth_Emma

Nope. Gains were higher down low.
Run 1: No cats
Run 2: With cats

Note the maximum torque gain was 47. The gains were throughout (why the facepalm?).

because this thread is talking about having back pressure down low for better power. and people keep saying " i gain X amount of HP" yes you did but where was it gain ?

maybe i cant read dynos very well, but this dyno starts above 2k RPM ? when most people in a torquey v8 usually do not need to get above 2k RPM or stay around it when driving normal, so we are not even seeing what is happening at the very bottom were most people drive it at.

[Darth_Emma]

Originally Posted by mikeSS

because this thread is talking about having back pressure down low for better power. and people keep saying " i gain X amount of HP" yes you did but where was it gain ?

maybe i cant read dynos very well, but this dyno starts above 2k RPM ? when most people in a torquey v8 usually do not need to get above 2k RPM or stay around it when driving normal, so we are not even seeing what is happening at the very bottom were most people drive it at.

My car has noticeably more torque in the rpm ranges I normally drive in. I have a torque converter which helps.

[mikeSS]

Originally Posted by Darth_Emma

My car has noticeably more torque in the rpm ranges I normally drive in. I have a torque converter which helps.

well geez, a torque converter. that would have been some good info to tell us lol.

but a lot of people report that they "feel" little power gone in the low RPM range after adding a more free flowing exhaust.

but some times people say they gain every where, when really its because the car seems quick with the new sound.

[Darth_Emma]

Originally Posted by mikeSS

well geez, a torque converter. that would have been some good info to tell us lol.

but a lot of people report that they "feel" little power gone in the low RPM range after adding a more free flowing exhaust.

but some times people say they gain every where, when really its because the car seems quick with the new sound.

The dyno runs are a direct comparison. The only difference between them was the cats between those 2. I was just saying that deleting the cats helped the torque.
my turn

[mikeSS]

Originally Posted by Darth_Emma

The dyno runs are a direct comparison. The only difference between them was the cats between those 2. I was just saying that deleting the cats helped the torque.
my turn

if your dyno info went from 1k-6.3K and you gained every where with cat delete then yes i should get a . by you just saying it feels better is not good enough for me.

my only point is the power from 1k - 2.3k. thats what i am looking at. when adding a different exhaust.

[Darth_Emma]

Originally Posted by mikeSS

if your dyno info went from 1k-6.3K and you gained every where with cat delete then yes i should get a . by you just saying it feels better is not good enough for me.

my only point is the power from 1k - 2.3k. thats what i am looking at. when adding a different exhaust.

Maybe you don't find the empirical data on the dyno sheet I provided useful, but I think the OP and others may find it useful information.

[mikeSS]

Originally Posted by Darth_Emma

Maybe you don't find the empirical data on the dyno sheet I provided useful, but I think the OP and others may find it useful information.

quick question, is that dyno with the torque converter ?