Back pressure and the camaro...

[SS>GT]

i was told from an old friend the other day that for the best performance, high back pressure was needed for low Rpms and close to none was needed for higher rpms. I didnt believe him, so i removed my exhaust just after the cats and drove around for a bit. Off the line the car was horrible,but once up to speed it felt great!( sounded amazing as well)

So i got to thinking, has anyone developed an exhaust with a valve after the cats that can direct the exhaust to two different channels? One would follow through the resonators and out the muffler (for higher back pressure, lower rpms), the other straight out of the pipe (for lower back pressure, higher rps). Im assuming the valve could work off a vacuum line, just like many of the BMW's have on the mufflers.

Would this have any effect on performance at all, or is it completely a dumb idea?

[csjcsj]

How about something like electric cutouts?

[nancygrl]

ummmm, electric cutouts

[SS>GT]

Originally Posted by nancygrl

ummmm, electric cutouts

Never heard of em...ill google it right now. are there any significant benefits?

[SS>GT]

wait, just did a search but it seems you have to elecronically open the valves...is that correct? and if thats the case, whens the best time to open them?

[Comrando]

Originally Posted by SS>GT

wait, just did a search but it seems you have to elecronically open the valves...is that correct? and if thats the case, whens the best time to open them?

When you would like to attract the highway patrol

[KnightRiderSS]

Corvette comes with a dual mode exhaust...the ZL1 will come with it also...not the same as electric cut outs...which I have and are a lot of fun!

But what u are talking about is dual mode exhaust

[Zach]

electric cutouts are made to bypass the whole exhuast system, so basically you would pull ur exhuast off cut out a section and weld the cutouts on then run the wire to a switch inside that would open and close. you have completely close to no back pressure which gives u less accel and more top speed and its noise right outta the heads... there so loud there illegal, like someone says above.... not really illegal but if a cop is bored he will most likely get ya... but if you have electric cutouts you can just close em.... but yes.. they are DEFINITLY worth it. i spent a few days lookin researchen till i had made my own. there the manual ones but real easy to make with the right tools.

[Carzzy]

Would have been nice if GM offered the dual mode exhaust as a factory option on the Camaro... like they do on the Vettes. It's good for 6 more HP and sounds great when you step on it!

[REDXIIIDave]

Yes, and it's a $1000 + option on the Vettes. For 6hp and some noise.

[mhood]

Originally Posted by REDXIIIDave

Yes, and it's a $1000 + option on the Vettes. For 6hp and some noise.

That's less than my GMPP Axle Back.

[SGOS252382]

Originally Posted by mhood

That's less than my GMPP Axle Back.

If your paid $1000.00 for an axle back exhaust you should hand over your keys.

[mhood]

Originally Posted by SGOS252382

If your paid $1000.00 for an axle back exhaust you should hand over your keys.

My wife surprised me for Christmas by getting it installed at my dealer. I agree...stoopid expensive.

[mikeSS]

Originally Posted by mhood

My wife surprised me for Christmas by getting it installed at my dealer. I agree...stoopid expensive.

its ok your not the only one who bought a over priced axle back + install. what was i thinking

[scottyvegas]

I have the Quick Time Perfromance cut outs.. Love em...Sooooooooooo LOUD!

[edgar3]

these sound amazing! I just put an email into the QTP to find out exactly what I need to have this set up.
Curious, is this a warranty breaker?

[Bama 2SS/RS]

My Yamaha YZF-R1 that I used to have had what I guess you could consider a dual-mode exhaust. Yamaha calls their system EXUP (which I think is supposed to stand for "exhaust ultimate performance"). Basically it was a butterfly valve that would throttle down on your exhaust for more back pressure at low engine speeds and loads, and would open up your exhaust at higher revs and loads. It wasn't a cutout system, because it always went through the full exhaust. But the exhaust was free-flowing enough anyway. It would just throttle to give more back pressure when needed.

I don't have a clue how that compares to dual-mode exhaust in the Vette or the ZL1. But I know that's how Yamaha accomplishes that effect in their sportbikes, and I think other sportbike companies have similar devices.

Anyway, I just figured I'd offer up that little tidbit because it seems like its kind of what the OP was describing or asking about. I'd be interested to hear from someone more knowledgeable about the Vette/ZL1 system(s) to see if the are similar to what my R1 had.

[GEN-IV]

Backpressure is generally always a power loss. Reducing the exhaust restriction without adjusting the tune may result in a power loss or driveability reduction, but that should be the only reason for a power loss on relatively low overlap camshafts like the LS3/L99.

[CamaroSkooter]

I have the noweeds electric cut-outs, but I'll be honest, I only got them for the sound I have tested them at the track, and with them fully open, I lose a few tenths in the 1320.

I don't think anyone (aftermarket) has yet developed a dual-mode exhaust setup for our cars. With electric cut-outs, even though they're electronic, they're not tied into engine speed or anything, so you have to manually push a button to have them open or close.

[Bama 2SS/RS]

Originally Posted by GEN-IV

Backpressure is generally always a power loss. Reducing the exhaust restriction without adjusting the tune may result in a power loss or driveability reduction, but that should be the only reason for a power loss on relatively low overlap camshafts like the LS3/L99.

Backpressure is generally a power loss, however you do have to have SOME backpressure for an engine to run properly. As an extreme example, I was changing out the exhaust on an old motorcycle of mine, and figured, might as well fire it up with absolutely no exhaust on it to see how it sounds and how it runs. So, with nothing connected to the exhaust ports off the cylinder heads (it was a v-twin motorcycle) I fired it up. It made this ridiculous popping bubbles type of sound, blubbering along, and it ran like shit.

I'm not a mechanical engineer, and I don't really know what's going on with some of the finer points of backpressure when it comes to an engine's exhaust system, but I do know that you need some backpressure for it to function properly. Too much backpressure can rob power, but not enough and the engine won't run worth a damn. I think the OP is probably correct in what he heard about needing more backpressure at lower RPM. Again, not being an engineer, I couldn't begin to explain why that is, but I do know that it seems to be the case. The EXUP valve in my R1's exhaust seems to bear that out, because it would throttle closed at lower RPMs, throttling the exhaust and giving it more backpressure, and would open up to free-up the exhaust and reduce backpressure at higher RPMs.

I will definitely agree that anytime you drastically change the restrictions on the intake and the exhaust, you really need to tune to get the proper fuel/air ratio.

[axis]

Originally Posted by SS>GT

i was told from an old friend the other day that for the best performance, high back pressure was needed for low Rpms and close to none was needed for higher rpms. I didnt believe him, so i removed my exhaust just after the cats and drove around for a bit. Off the line the car was horrible,but once up to speed it felt great!( sounded amazing as well)

The big problem here is that you took away the crossover between both sides of the exhaust. It's been shown that true duals, without a crossover, will lose HP/Tq on this car. Your exhaust needs that crossover to help scavenge exhaust from the heads at lower RPM's. I can only assume that at higher RPM's, the crossover affect isn't needed as much.

Originally Posted by Bama 2SS/RS

Backpressure is generally a power loss, however you do have to have SOME backpressure for an engine to run properly. As an extreme example, I was changing out the exhaust on an old motorcycle of mine, and figured, might as well fire it up with absolutely no exhaust on it to see how it sounds and how it runs. So, with nothing connected to the exhaust ports off the cylinder heads (it was a v-twin motorcycle) I fired it up. It made this ridiculous popping bubbles type of sound, blubbering along, and it ran like shit.

The problem here is that your "OLD" motorcycle is most likely carbureted. Carburetors don't have the capacity to change the AF mixture. Taking the exhaust completely off eliminates backpressure, thus you're now pushing more air than fuel, making it run very lean (run like shit). If the motorcycle had a PCM to adjust the fuel mixture, it would have adjusted to fix the changes, as long as it wasn't too far out of whack.

Bottom line is, todays fuel injected engines neither need nor want backpressure. If you want an extreme example, look at NASCAR. They run a massive header into a HUGE exhaust pipe that exits beside the door. Before someone says "well they run carburetors too", they are tuned for the increased air flow. Also note that they combine their exhaust into a single pipe. Not sure if this is a NASCAR rule or if it helps in scavenging. All your "old friends" aren't wrong but you have to look at the technology they were working with. In 20yrs, you'll be telling the newest generation of car nuts what we know now and it will most ikely be WRONG for future car modding.

[blazzin1]

Just FYI....I have a set of the QTP ovals with turndown tips and the digital controller (all brand new stuff, never installed) that I am thinking about selling. Haven't completely made up my mind to sell or not, but if the right offer was to come along.....

[Bama 2SS/RS]

Originally Posted by axis

The problem here is that your "OLD" motorcycle is most likely carbureted.

Bottom line is, todays fuel injected engines neither need nor want backpressure.

Yes, my old motorcycle was carbureted. And yes, I agree that carburetors cannot adapt to changes in intake or exhaust differences as well, which is why I had the carburetor re-jetted when I changed the exhaust. On the same note, even a newer fuel injected car STILL needs a tune when you start making radical changes to intake and exhaust, so they're not as adaptable as you may think to changes like that. Sure, they'll compensate more than a carburetor, but there's still a limit to the changes they can take without needing a tune.

As far as fuel injected engines not needing/wanting back pressure, I'm not sure that is entirely true. In your NASCAR example, you may not realize it, but those massive headers and straight pipe exhaust gives you some amount of back pressure. Any pipe will have some amount of head loss and provide some restriction to flow. Even the most free flowing exhaust system will provide some back pressure, and although you may not need a lot, you DO still need that small amount of back pressure. You don't necessarily need the amount of back pressure the stock exhaust needs, but you need some.

Also, I would argue that a Camaro with a stock tune, which is tuned to run with a restrictive intake and exhaust,will not run at its peak if you threw on a NASCAR style exhaust. Not without a tune.

[Bama 2SS/RS]

well...

I guess we can always learn something new. This discussion lead me to read up on some things. Let it never be said that I don't admit when I'm wrong, and it appears that I've got a bit of a misconception here. Although this does help to clarify what was going on with the EXUP valve in my R1. Basically it changes the exhaust diameter to tune the exhaust for each RPM range. This also leads into what the OP was saying about loosing power at the bottom end, but gaining it on top, because you've effectively tuned your exhaust for a higher RPM range. Like I said, I'm no mechanical engineer.

Anyway, take a look at these links, and you can learn something new.

http://www.uucmotorwerks.com/html_pr...torquemyth.htm

http://www.thumpertalk.com/forum/sho...d.php?t=659727

[axis]

Originally Posted by Bama 2SS/RS

Yes, my old motorcycle was carbureted. And yes, I agree that carburetors cannot adapt to changes in intake or exhaust differences as well, which is why I had the carburetor re-jetted when I changed the exhaust. On the same note, even a newer fuel injected car STILL needs a tune when you start making radical changes to intake and exhaust, so they're not as adaptable as you may think to changes like that. Sure, they'll compensate more than a carburetor, but there's still a limit to the changes they can take without needing a tune.

As far as fuel injected engines not needing/wanting back pressure, I'm not sure that is entirely true. In your NASCAR example, you may not realize it, but those massive headers and straight pipe exhaust gives you some amount of back pressure. Any pipe will have some amount of head loss and provide some restriction to flow. Even the most free flowing exhaust system will provide some back pressure, and although you may not need a lot, you DO still need that small amount of back pressure. You don't necessarily need the amount of back pressure the stock exhaust needs, but you need some.

Also, I would argue that a Camaro with a stock tune, which is tuned to run with a restrictive intake and exhaust,will not run at its peak if you threw on a NASCAR style exhaust. Not without a tune.

Did you not notice that I posted "as long as it's not too far out of whack" in my response to your post? Anyone that has ever tuned a PCM vehicle knows that there are limits to how much a OEM tune will adjust. That's why we get CEL's. Taking a catback off isn't radical. There are ALWAYS exceptions to the rule but the OP wasn't talking about making radical changes.

Anytime you add pipes to your exhaust you'll be producing backpressure, but that doesn't mean your engine NEEDS it. It means you're being nice to your neighbors and you'd like to be able to hear your grandchildren call your name when you're older. It's great if you want an excuse to ignore your wife though.

First off, the OEM Camaro catback isn't that restrictive. It's been shown NUMEROUS times that the only places to make appreciable gains in the exhaust are from the cats and LT's. The only reason that you NEED to tune, is to turn off the rear O2 sensors to keep the CEL off. You will still gain a good amount of HP without a tune. Yes you will gain even more with one but you won't lose anything, as you and OP suggested. This thread isn't about optimizing the tune, it's about the backpressure myth. If a engine really needed backpressure, no amount of tuning would help.