69427

With every exhaust system iteration I always weld a fitting into the pipes up close to the header collector so I can hook up a pressure gauge to measure the backpressure at WOT. Whenever possible I always like to have real data.

aklim

Where at? Near the 4 to 1 or back at the cat or what? I would think before the cat but I could be wrong.

69427

As far forward as possible. With headers as close to the collector reducer as possible, or if stock up close to the exhaust manifold.

The pressure gauge should be a low range piece, as it's hard to read a wide range gauge at WOT. I use a 0-40" H2O (about 1.5 psi) gauge.

aklim

I'm unclear. If you put this north of the cat, aren't you measuring the system backpressure? If you put this south of the cat, would you not be measuring the exhaust system backpressure which is what you wanted? The way I am looking at it, why do I care about the headers since there is not much I can do unless I change headers and cat. So we have 4 tubes going into 1 on each side. Both sides dumping into a "Y" and that goes to a cat. If I am interested to know the effect of the exhaust, why would I measure so close to the manifold knowing the cat can cause the pressure?

Not doubting you but trying to learn. The crude test I have done is to disconnect the pipe from the cat back and driven the car with the equivalent weight in the trunk.

69427

Your "crude" test is a useful one, but again, it doesn't pinpoint the exact area of the exhaust system that needs improvement (better flowing cat or better flowing mufflers).

C5rider

I like my CORSAs !!

I've got an LT1 in my '85 with a set of CORSA cat-backs. Never had a drone issue or noise issue.

I was driving around and wondered why folks would be looking at my car. Then, I had a friend drive it once. Wow! It sounded MUCH better outside the car! Much more rumble.

I know that they are $$$ but they do what they are supposed to do.

On the other side, I drove an 84 (Crossfire) with a set of Flowmasters. It was like putting your head in a milk tank and beating it fast and repeatedly with a rubber mallet!

Hope you get the drone fixed!

aklim

IF I could find a used one, I might be talked into it if reasonably priced. I could care less what is sounds like outside since I don't drive outside nor do I let anyone but the wife and attending mechanic drive it, let alone sit in it. That goes for all my cars unless there is something special about that person. Big knockers, low cut dress and skirt or a combo of the 3.

GREGGPENN

You know Andrew, I was going to suggest the setup I heard in my first header-equipped LT1. It had EM headers feeding into the stock mufflers (no cats). There was no drone. Until now, I didn't suggest it because you seem very concerned about power loss. Now I feel obliged to do so....

In this post, Will statement(s) suggest any loss with a stock "cat-back" would be minimal...and we all know he's built a lot of engines.

Now that I read this, I feel more obliged to suggest it AND that you consider cut-outs if you're worried about power loss OR...just because it would be cool. (I also think cutouts would drone less via the elimination of your Borla ("box") mufflers....though it would be somewhat louder -- and more satisfying -- when you hit it).

Kingtal0n

If yall are tryna measure what portion of exhaust system pressure differentials needs adjustment in real time you can't do that by looking at backpressure data. Instead, you measure rwhp on a dyno, then make changes, then another pass. If you have a cutout you can open it 1/8" at a time, make a pass each increment, and find the peak performance. The ideal curve would be one where the cutout is mostly shut at the start, then opens gradually to compensate for exhaust gas volume of higher output while maintaining a similar exhaust gas velocity throughout.

sheesh

MatthewMiller

That's not how exhaust systems work after the collector, at all. The idea of keeping gas velocity high with a smaller-diameter pipe is valid for the primary pipes only. It can boost scavenging of the cylinder through the open exhaust valve - not because of backpressure, but because the increased velocity helps the gas column maintain momentum and draw out the next with negative pressure. But again, that's just in the primaries. And it's not a result of backpressure, but rather the exact opposite.

After the collector volume, the best exhaust system (for max power) is the one with the least backpressure, or no exhaust system at all. The whole myth of having a certain amount of restriction in the exhaust plumbing to boost low-end power is just that: a myth. There's no "ideal curve...where the cutout is mostly shut at the start, then opens gradually to compensate for exhaust gas volume of higher output while maintaining a similar exhaust gas velocity throughout." First of all, higher gas velocities after the collectors do nothing to enhance scavenging. Second of all, sticking a restriction in the gas flow to create backpressure doesn't increase gas velocity, it reduces it! That's why you never see any exhaust system at all on any race engine where the rules don't require it, except for the bare minimum required to keep the exhaust gases from entering the cockpit. If there were a magic valve to create some "ideal curve" for best power at all rpms, then every race car would have one. But none do, because that's ridiculous.

Therefore, it is 100% correct to measure exhaust pressure in an effort to minimize it.

Sheesh, indeed!

Tom400CFI

^Totally.

Kingtal0n, Please watch this...

Kingtal0n

If you chop the exhaust off a stock engine, torque drops throughout the RPM range.

Seen it a hundred times. With no exhaust system present, there is an issue clearing the cylinder, despite the exhaust being open and having no "pressure" (its gone, we took it off). Even with perfect camshaft timing the exhaust valve/head cannot control the flow and direction of exhaust gas well enough to run properly without some tube, some designed orifice. There is some need for control, direction, collection, which supersedes what appears on your "backpressure gauge".

If you run a turbocharger the situation becomes far more clear. The engine will run "better" as VE is improving all the while backpressure is climbing, if you are measuring it with a gauge.

MatthewMiller

If you take the headers off so that you just have bare heads on the exhaust side, that could be true. You might also ruin the exhaust valves as the get cooled by slugs of cool air between each combustion cycle. But if you mean running "open headers" with no exhaust after the collectors, you're just wrong. Show me the data that supports this. Any data. And again, if what you say is true, then why does every race team run "open headers" when the rules and safety allow it?

And how does sticking an obstruction in the exhaust path accomplish "control, direction, or collection?"

Well yeah, but this is a whole different story, because the backpressure in this case is doing work to compress the intake air. The backpressure created by a turbine in the exhaust path isn't making the exhaust flow better - it's an impediment to exhaust flow. It's a necessary loss of efficiency in order to cram more air in. But it is a loss and an impediment to exhaust flow: you can either drive a compressor with exhaust gas via a turbine, or with the crankshaft via belt drive. But make no mistake, it's a loss to power, and if engineers could figure out a way to drive a turbine without that loss, they would do it every single time.

SurfnSun

Sorry dude, dead wrong. If that was the case why would all of single turbo Supra guys be running 4+" exhaust systems? Free exhaust = faster spinning turbos. If what you're saying is true a slightly smaller exhaust would make more power on a turbo car. As the former owner of a Callaway TT, I can tell you its just the opposite. My car picked up 30rwhp/40+rwtq by going to a free flowing exhaust.

ddahlgren

This has degenerated into useless in a search about drone.

To clear up 2 myths then ignore this mess
A turbo does not run on pressure it runs on thermal recovery of exhaust heat, the reason it is common to see a turbine housing insulated to maintain heat to drive the turbine with expansion. Back pressure is the enemy of performance in a turbo engine and the closer you can get to having turbine pressure the same as manifold pressure the better.

There is no such thing as a useful exhaust restriction period. yes there are tuned exhaust but back pressure has nothing to do with it.

I find this site very useful for Corvette specific things, engine theory not so much...

69427

I contend that an efficient turbine section runs on momentum transfer (from the high speed exhaust gas onto the turbine blades). Hence the reason for smooth headers feeding the turbine housing. Lesser efficient systems rely strictly on the pressure differential across the turbine ports to get the velocity back up to spin the turbine. Heat by itself doesn't do the job. You need exhaust mass movement impinging on the turbine blades. Thermal blankets keep the heat in (maintaining the inlet pressure and gas speed). Thermal recovery is kind of a broad label to describe what's happening to the EGT in the process.

Kingtal0n

Open header will give the most power, most win races. But not the most area under the curve. They can keep those race engines at top RPM because everything in the car is geared for top RPM. Street cars are not like that. If I give up 30% of the area under my curve for 8% peak power I could win races if the engine sits at peak power all the time, but lose everything under that on the street from stop to stop traffic.


I want you to look at a turbocharger with and without a downpipe attached. With and without an air intake tube attached. The 'secret' of extra power lies in the continuity, the collectibility of the flow of gas. Theres a better word for it.


The atmosphere provides pressure. So does a turbo. The two are identical from the perspective of the engine. If I have 15psi on the intake path and 15psi on the exhaust path I am N/A WOT. Except that now the engine 'breathing' is affecting that original 15psi on both sides, I know that because the exhaust is moving, isn't it? There must be a pressure differential which exists on a running engine. Both turbo and N/A engines have a differential.

People do what other people do without knowing why. The principle of sizing an exhaust system goes beyond simply size. A 3" exhaust tube supports something like 900 horsepower. That doesn't mean 890 horsepower setups won't have 4" downpipes for some reason. Just look at the back of the turbine to decide what size to use, it helps to go based on what the manufacturer intended the turbo for.


Here are some kind words from engineer at Borg Warner,
Notice he says to compare pressures; not to simply measure exhaust gas pressure. Why?

-Exhaust gas pressure can be going up, while engine output is going up.
-Exhaust gas pressure can be going down, while engine output is going down.

Exhaust gas pressure by itself is meaningless. You can lift your foot from the gas pedal and see "less exhaust gas pressure" while also having less engine output. To correlate just the two, output and exhaust pressure, is pure folly.

Kingtal0n

Exactly this. Kinetic energy is imparted to the turbine from collisions with exhaust gas molecules, due to their high temperature and pressure.

Pressure plays a role, and it isn't uncommon to see 30psi of exhaust gas backpressure while the intake side has only 15psi of manifold pressure, all the while engine output nearly is doubled.

SurfnSun

not to mention the amount of gross misinformation contained within.

If you want zero drone buy Corsa.

ddahlgren

Or I fear stock mufflers. Just for fun I am going to see if plugging one side of a Borla kills drone as he engine is stock. I should have just bought a stock set of mufflers but at the time I did have a set of heads cam and headers to put on my 91 but money got short quick and had to sell the prts off for a loss.