Not sure what else to call this little idea. Anyway, everyone knows that scavenging helps create a vacuum effect between exhaust pulses creating higher exhaust velocity than if there was no vacuum. What dictates the amount of this scavenging is the velocity of the previous pulse and how it's joined up to the pipes leading to the next so the vacuum can be created. (think of it like how perfume mister works by sucking up fluid as high velocity air passes across the tube). As each pulse passes the opening to the next it creates a vacuum etc. Anyway, what I'm getting at is the loss of power some people see when going to true dual setups would not necessarily be due to less backpressure. Backpressure is bad _all_ the time. What the loss of power seen is likely due to is a loss in exhaust velocity due to both increased volume available for it to travel in and much more rapid cooling due to doubling the surface area exposed to outside air via the second pipe. Cooling of exhaust gas greatly reduces it's velocity. And if we lose velocity, we lose the scavenging effect. You would only see a pick up of performance when the volume of exhaust gas from the engine is large enough to bring the velocity back up to previous levels (before the move to TD). Thus the dyno charts would show an increase in performance of top end but a decrease in low end, where velocity of the exhaust is too low for scavenging.


Now, there are multiple factors that can change exhaust velocity. So there are multiple solutions to fixing the issue if anything less than WOT is a concern for you.

So finally, the idea I want to throw around and ask about is if anyone has tried thermally insulating (via ceramic coating or other means like wrap) their entire exhaust to reduce heat loss. This would benefit any car regardless of needed pipe diameter and length. Theoretically anyway. Would be interested to see if any measurable gain would be made. Assuming a lot of heat is kept in the exhaust I dont see how it couldn't. I dont plan on ever making more than 400HP so when i go TD i plan on going with 2.5" piping to not give the exhaust excessive volume, but it will still be substantially more than a single setup. So the heat loss reduction was my idea of reducing any loss of scavenging in the low end.

I'm not concerned with the look of 3" pipes vs 2.5" or the sound one or the other produces. I'm just looking for the most efficient exhaust setup for my <400hp car across the power band (on average). I understand that will mean a sacrifice at the very top end. That's how everything works.

Plus, setting things on fire that come too close to the rear of the car would be a bonus. And sorry if the post is long and tedious. I just didn't want to re-hash what scavenging is and how it works and x vs h pipes and no crossovers at all etc etc. Just interested in if anyone has tried really reducing heat loss in the exhaust system to some noticeable effect like it's should theoretically.

 

I speed read most of this, so excuse me if I miss something..

Scavenging is caused by the collector, IIRC.. It's a restricted opening in the colllector that opens up to a larger section of piping which creates a bit of a vacuum or pressure drop.. (Shouldn't be a VACUUM persay, but just a lower pressure area). A lower air speed is caused directly after the area of restriction when the piping diameter is increased.

There are also two basic designs to "headers." Equal length headers and tuned length. Same goes with a turbo header. The equal length are not designed to have the engine pulses "follow" each other, they just spit whatever out at whatever time. The Tuned length headers are designed with the engine's pulses which are calculated into the length of the tubes.

You do't find many people insulate wrapping their exhausts unless they are concerned about it or they are running a turbo. And I don't much care for people saying "Back-pressure is bad." Tell that to the boosted crew here and zip up that flame suit. Why do you think their TQ numbers are so high? You need some degree of back pressure to "keep the flame in the hole" or you will be risking burnt valves and other issues. Someone told me that a long time ago when I first bought my headers back in 04 or so and I never heard of any problems with this, so I forgot about it.

You CAN run a 1 7/8" or 2" primary and people do without issues. There will be some degree of backpressure. It's just not gonna dissapoear. Some like to argue that BP is a myth, and sometimes I do agree with them. BP will prolly be non existent on a full open LT, ORY and LM1 setup (for instance) but there is SOME there. it is a good thing in small or moderate numbers. Such as also true when you install or uninstall cats. It moves your low end TQ numbers around. No cats = roughly higher RPM torque and a slight loss of low RPM torque and the opposite would apply WITH cats. They're not TERRIBLY restrictive as most think they are. Try driving a compressor wheel with the exhaust. The pumping and back pressure losses from a turbo are FAR LESS than the parasitic drive losses of a blower setup. You do not lose that much power, if any, by adding a restriction, but in THAT case, the restriction is also helping to cram more air down your throat!

The Flowmaster Merge commonly seen here is also a scavenging effect, but in the Y-pipe. Slamming two pipes together creates rasp and unneeded exhaust "conflict" with the pulses of the exhaust and creates some BP but not enough to SERIOUSLY choke a setup. The BP some people think a "crappy" system has could be compared to having a rats nest in your exhaust. Now THAT kind of stoppage is NOT good.

The X-Pipes in the dual exhausts are supposed to be a Siamese design, not just slammed together. The opening FOR the ex should be close to the size of the pipe but not a full O. They work best when they're ovaled out. It just evens the pressures in the two pipes.

Not sure how heat retention plays into all this unless you're running a rear mount turbo, or a turbo in general. But the rear mounts require as much heat retention as possible to maintain the air density. If you're concerned about a loss of BP which would cause a loss of low end TQ, then just stick some race cats in there. That's what I'm going to be doing when I redo the entire exhaust some time this year. (HOPEFULLY this spring)

 

Rear mount turbos require the exhaust to flow fast, hence the need for hot exhaust. Cooled exhaust is more dense and given the same volume, will thus be moving at a slower rate. In that case, backpressure isn't so much what is required, but rather the side-effect of what is required, which is exhaust kinetic energy. If you allow it to expand (with oversized pipes) then you lose density and so the motion of exhaust is spread over a wider area and slows forward motion. If you allow it to cool then you lower the kinetic energy, causing it to slow. In both cases, the less energy still in the exhaust by the time it hits a turbo the less energy there is to spin the turbo. and so the less power it will make. The existance of high backpressure just means a lot of the kinetic energy is still doing work in the exhaust by driving the turbo fan. It's the necessary evil. If you could somehow drive the turbo to create the same compression charge without creating as much backpressure then the efficiency of the turbo would increase. backpressure is parasitic to power, but an optimal setup will always have some amount of it because it's created alongside effects that greatly improve performance far more than backpressure hurts it.


So no, i'm not concerned with a lack of back pressure. I'm just wondering if saving that much energy by reducing heat loss can reverse the effect of losing the scavenging effect at low rpm caused by having much larger exhaust volume in a dual exhaust setup.

It may be able to, or it may not be enough. It all depends on just how much faster the exhaust flows when it's insulated from headers back vs not. Heat may tend to not be the limiting factor here as much as volume, that's what i'm wondering and trying to see if anyone has before and after's.

 

I think the idea that backpressure gets confused with alot stems from what is causing it. Backpressure is "good" when it's caused by things that create power greater than the energy sapped by backpressure. Backpressure is bad when it's simply the result of obstructions. Though the absence of it is not necessarily good either, as you can't regain any power from the exhaust without there being any backpressure, since it's a byproduct of directing exhaust flow.

scavenging is taking back some of the lost power in exhaust. Turbos take back a lot more. a lack of backpressure means these are likely not functioning. but this doesn't mean throwing a potato in your tail pipe to create backpressure (which it will) will help. It means you probably need to either produce more power, or decrease the volume the exhaust is flowing in order to regain velocity. creating backpressure again.

I dont think the turbo guys are confused when someone says backpressure is bad. But i think some people dont understand that "backpressure is bad" doesn't not mean "no backpressure is good" while at the same time still meaning "backpressure is not good". In some magical land of super fluidity and perfect gasses and frictionless surfaces, having no backpressure while directing and focusing exhaust gas would be perfect. In the real world that's not possible, so it's existence is a marker that things are working so long as it's not too high and it's absence is a sign that things aren't working correctly (because friction exists and we want to actually make some use of exhaust rather than just waste it all).

 

^ I think you are mistaken on the first part of your first reply. Turbos need the hotter air and it being more dense is needed to more efficiently spin the impeller. Hotter air is MORE dense, but cooler air contains more free air molecules. It's odd, cold air in is better and hot exhaust out is good. I would think that hotter exhaust would aid the scavenging effect. But the same amount over a larger pipe would be a slightly less dense exhaust with the same given temperature.

 

Can we say, OFF in a tangent!! LOL His original post had NOTHING to do with a turbo. He is simply wanting to know if coating or wrapping your whole exhaust to keep the velocity of the exhaust fumes high will increase power or if it doesn't make a difference. Back to the question people.

 

^ You're right, but the "back pressure" argument was relative, and I was using the hair dryers as an example of how BP isn't exactly a bad thing.

 

hot air is not more dense. hot air balloons would not be able to get off the ground if that was so. What hot air does have is a lot more motion. Heat makes gas molecules move more, this in turn causes them to bounce off eachother more and push outwards. Take a gas in a balloon for instance and heat it up. It expands, this is due to the molecules moving more, and this causes an increase in pressure.

In exhaust this would be what leads to backpressure. This is not an increase in density.

But yea, back to the topic. I'm not sure i'll find anyone who has attempted to fix the loss of scavenging effect using any method other than increasing engine output. But i suppose it wouldn't be too hard to coat mine with something prior to installing it when i get the parts. Maybe a decent layer or two of some ceramic paint. reducing heat loss through the pipes is always good. Just not sure it's enough to counter the increase in volume. I'm going with 2.5" pipes (if possible, everything is 3" for sale) so it shouldn't be as bad as some people see with 3" duals to start with.

 

My duals are 2.25". they do just fine

 

Ahh whatever the hell... I don't know.. I asked someone and I got the wrong answer. Maybe this is why I've heard cooler exhaust isn't a problem.