Another dumb question (sorry) - Exhaust backpressure?

[excelguru]

If you assume that sound levels are not an issue...

From an uninformed point of view (like mine), it would seem that having no backpressure in the exhaust system would be a good thing. But it's my understanding that you actually WANT some backpressure. Why is that? And how much backpressure? Wouldn't that rob power?

Just trying to learn more and more.

Keith

[CobraEd]

I believe that the "need for some backpressure" is an old wives tail than stems from the need to richen the mixture if you open up the exhast. As an exagerated example to make the point: If you take a stock engine designed and jetted for a catalitic converter and single exhast and then put open headers on it, it will break up and misfire all over the place. But this is because given the new free flowing exhast, it will be jetted WAY too lean, not because there is no backpressure. So I think this is how the old wives tale got started. My Cobra has headers going right into the sidepipe with a 3" ID glasspack. From a flow perspective this is like open headers with a collector. When I switched to this configuration I had to richen up the jets to take full advantage of the enhanced flow out of the engine.

If you take that car used in the example that had the single exhast and richen up the jets, it will run great and develop way more power than it had before.

So there you have it, The world according to Ed.

Ed

[CSX 4027]

You will get enough back pressure in a 427 Cobra with the original style header tubes trying to get outside the body. Go srtaight pipes or loose about 70HP.

[CobraEd]

See the "Straight or muffled exhaust?" thread started by RaceDeck. It has some good info related to this.

Ed

[Double Venom]

Excel,

I too thought "back pressure" was a bad thing, Heck it even just sounds like a bad thing! I wish I could get technical for you, but I can't so I wont even try. However I will give you my latest information.

When we started building our first DV"s, we of course wanted every pony that was built into the V-10 that we could get out.
Thusly we were using almost straight through exhausts.

Racing and showing with the Vipers, we have had numerous contacts with some very knowledgeable Viper engine engineers.

The scenario stays pretty much the same, bunch of guys standing around shooting the breeze, when two of these "engineers" mentioned our custom made exhaust. Being pretty darn proud of what we built, I went into great detail about the "Step-up" headers we hand made, a filler block if you will for the 02 sensor, so that it could read all 5 tubes within the recommended 10"s of the heads, etc.

These two guys looked at each other and asked if I had ever put it on a mobile Dyno? answering"YES", I bragged that we were getting more than 412 RWHP! This is almost 15 more HP than any stock Viper was getting at the rear wheels at the time! Both were quite impressed, then stated, close up the "bullitts" a little adding more "backpressure" and you will get another 10 to 20 HP.

End of this tirade, I did. I changed the exhaust centers to what we now use on all our cars-definately adding more back pressure than a straight through system.

New dyno numbers - jumped 15.? RWHP, now reaching over 427 RWHP and the torque took a humongeous jump too!

In my opinion "back-pressure", the right amount at least IS needed!

For what it's worth!
DV

[CobraEd]

Ed, Something about this is not right. If you set the mixture to match the exhast, you will always get more power with less restriction. Now "tuning" an exhast system can produce more power than an untuned system due to proper scavenging of the exhast pulses, but I would not equate that to "backpressure". Proper scavenging (tuning) is why no one runs the exhast right out of the heads. All of the fastest cars around the world in all the various categories run open headers of some sort with no mufflers or restrictions, but they do have their header pipe diameter and length along with the collector size tuned for their particular rpm range and use. Top fuel dragsters are the MOST POWERFUL of all cars (6,000 - 7,500hp) and they run large diameter, short individual open pipes. But again, tuning for scavenging is not the same as backpressure.

Maybe it's all semantics!!??

Ed

[excelguru]

Oh great, now what the heck is scavenging?

"The more you know, the more your realize you don't know."

Keith

[ERP]

This is my understanding of the whole back pressure situation. It could be drivell, but the guy who relayed seemed to know what he was talking about.

The pulse of exhaust gas caused by a previous cylinder firing can actually helps a cylinder expell the exhaust gas, the more restrictive exhaust basically increases this effect, and at low RPM it can result in more torque. In extreme cases reducing back pressure can reduce torque accross the entire rev range.

However with more aggressive cam profiles, the effect is reduced and the backpressure can restrict the engine accross the entire rev range.

A case in point is the post 2000 Viper engines, my understanding is that they are less friendly to exhausts with reduced back pressure (than the pre 2000 engines) largely due to a less aggressive cam profile.

As I said could be drivell but it sounded like a reasonable explanation when it was relayed to me.

[Double Venom]

ERP is absolutely right.
It does have a lot to do with "scavenging", the term used to indicate the maximum vacating of the "old" exhaust gasses from the compression chamber. (Using the previous firing chamber to help the current firing chamber to "pull-vaccum" out the exhaust.)

ERP is holding back on us too Yes, the Vipers have gone through multiple exhaust changes! And it all has to do with scavenging and back pressure. When the new cam profile came out in 2000, they started all over again.

Cobra Ed, I know exactly what you are saying, but those short straight tubes on a full blown "rail" are designed, tuned, right to the emmmpth degree or lenght! They are designed for full out 8K or higher-much higher RPM's.

Then back to ERP's response, cam profiles, mixtures, timing, valve size, exhaust port size, etc., all come into play.

It is semantics, but it is also apples and oranges.

This is a great topic!

[CobraEd]

I agree that the Top Fuel pipes would not be optimal for any street car, that was just an example of tuning for the particular application, but my point is that each system must be tuned for each car. Again, lets not confuse backpressure with proper scavenging, they are not the same thing, and backpressure by itself in pure terms is not a good thing.

For any particular car, given the proper "tuning" of the exhast for optimal pulse scavenging at the desired RPM, with the mixture optimized to the exhast, you want the least backpressure possible.

The reason why some backpressure is sometimes called "needed" or "good" is to compensate for an overall exhast system that is not tuned properly (header pipe diameter/length, collector diameter/length) or more commonly where the mixture is not correct (typically too lean)

If the newer Vipers have less aggressive camshafts, then rather than increase backpressure (a compromise) to work with less overlap and combustion chamber scavenging, the primary pipe length on the headers should have been made longer, and perhaps the diameter should have been made a little smaller. This is standard practice. But again, this is proper exhast tuning, not the adding of backpressure such as making a muffler more restrictive.

Ed I would have to believe, that had you reduced the primary diameter of your header pipes, or just removed the steps where you had them getting wider, and made the primary pipes a little longer instead of restricting your mufflers, you would have made WAY more power. The Viper guys knew that would mean reworking everything, and probable gave you a good compromis solution. If you are talking to them ask them about this. It would be interesting to see what they say.

Ed

[Dave Samson]

Having spent many dyno tuning hours with a good programmer, I'm convinced that the best solution for my supercharged engine, is no back pressure whatsoever. In addition , my exhaust flow problems were only solved after welding in a 2 1/2" balance pipe right after the headers. The affect was almost magical, the engine ran smoother, more hp and quieter.

[Double Venom]

Dave,
Another apple! Do you have pictures of your cross over pipe? I'd love to see that installation. Any chance you dyno'd before and after?

Tell us more please!

(edit ->) CobraEd...I think you are absolutely right! The problem is of course the room available to work with. I'm talking "street use" and your talking "optimum use". With us, building an exhaust system is little more than guessing, I don't have a Dyno to use for testing purposes and the time it would take is absolutely prohibitive. It takes me a full 50 hour week just to build one set of headers for our DV's.

I actually entertained the idea of trying to build equeal lenght type headers for the DV II just to see how much HP would be gained. Roughly three hours later, I trashed what I had and simply gave up.

We're contracting for an entire new type vehicle with a very, very special engine. If this comes to be, I'm going to be knocking on your door along with ERP's. ( I still think he is not letting us in on all his knowledge! )

DV

[excelguru]

Always wanted to ask this one, too... and since I'm in the DQ mood...

What's the deal with wanting equal-length header pipes? Does the difference in back-pressure from one cylinder to another really make that much difference?

BTW, I'm learning a lot from this thread (the above dumb question not withstanding). Thanks guys.

Keith

[ERP]

DV, nope I'm not holding any info back honest
I'm just an armchair mechanic, I write video games (racing games mostly), writing various simulations has given me a pretty good understanding of at the least the basics of most aspects of car design. And generally when I come accross something I don't understand I find someone to ask.
The back pressure question annoyed me for the longest time aswell, I couldn't see how it could be productive, but one of the local tuners gave me the explanation above.
I certainly wouldn't consider myself any sort of expert when it comes to this stuff.
Recently I decided it would be very cool to actually build a real car (it's something I've wanted to do since I was a kid) rather than just simulating one, still haven't decided what I want to or even can build yet.

[Dave Samson]

There must be some truth to the camshaft lift/backpressure theory because I had no exhaust problem until I installed a wilder camshaft 5.5 degrees advanced.
If you think about it, a balance (cross-over) pipe makes good sense for a "V" engine, I was frankly amazed at how it seemed to calm the lumpyness out of the camshaft.
Here in Australia our high performance production cars running GM and Ford V8's all have balance pipes installed as a standard feature. They are simply a pipe that joins the 2 exhaust pipes at the closest point of around 8 inches, usually around the middle of the car. As I run open side pipe, the closest they come together is 36" so that is the length of my balance pipe. It seems to work well and certainly allowed the fuel injection programmer to set the engine running very smoothly throughout the rev. range gaining around 30 hp. Sorry I have no pictures from under the car but you can imagine how it looks.

[niles]

guys:
This is a very complex subject that I can only start to describe the physics going on; few tech tidbit for the soup:
Normally some backpressure is good; on decent exhausts, small pipes dumping into a collector; you need back pressure to set up the standing and reflected waves. These reflected acoustical waves go backwards up the pipe and provide negative pressure when another exh. value opens; hence more flow out. This typically is what is referred to a scavaging.

The top fueler example isnt applicable; because they have such hugh overlaps they just use extreme blower pressure to blast the exhaust out; alone with hugh amounts of excess fuel; hence the roman candel effect on TF exhausts.

The savageing effect is easily demonstrated on a hot two stroke cycle; the effect of coming on the pipe is with this savaging effect actually occurs at the exhaust port.

Cross overs are excellent; just hard to execute with V-8's with the cyclinders seperated by a trannie and drive shaft. Look at all the aftermarket mustang system; crossovers, X-pipes, etc. This is a poor man solution with some benefit.

The real solution is demonstrated on rear enginge F-1's, Indy cars. Nothing to get in the way of the tube benders. You will note that "long tangle of pipes" jointing at diffferent spots, lengths with no apparent reason. What is actually going on is designed savaging at certain rpm's; where the pipe #3 joins pipe #7 such that 3's reflected wave happens to go up 7's pipe at just the right time.
Told you, it was more complicated than mortals want to know and I only know enough to be dangerous.

See Jay Torborgs web page on engine design; he goes over the basics of exhaust and cam design theory.

I really wish some smart people would work this item, Cobra exhaust by and large are terrible and noisey. I've seen tech papers that show a well designed exhaust only give up 1 or 2 %, not the 60 - 100 hp some have seen on dyno's.

The most advanced exhausts are probably formula race cycles; about 4 hp per CI and relatively quiet large diameter collectors. What's inside them are big Honda secrets but they sure work!

gn

gn

[oleyellar]

If you really want to get confused about header sizing and collectors and such, check out the New Dodge Neon Pro Stock cars at the drags. The new hemi motor, with the two center spark plugs real close together, have a wild looking header set up. It is a 4 into 2 , then dumps to what appears to be a really small collector. It sure makes you think, what is right. The 4 into 2 looks the same as the old Doug Thorley headers they used to make, then the small collector looks like 3 inch or so. Lots of engineering going on at Dodge.

[coyled]

Ole yeller, The reason most exhausts go down to a collector and back up is for low end torque. Most cobras in race applications go down to a 3 1/2 inch collector and out to a 4 inch open pipe. This has the effect of sucking the exhaust gas thru a lower rpm's, hence more torque. I left mine at 4 inches from collector thru the open pipes for 2 reasons. I don't need any more low end torque and the slip fit configuration for my street and race pipes was easier. My headers are stepped up 8 inches away from the heads from 1 7/8 to 2 inch. Now, I thought I should have a bigger pipe like 2 1/8 but the builder said in testing they lost low end power and did not pick up top end power with the cup style 358ci motors like mine. Lots of technology there guys, not sure I understand it all. scott

[Dave Samson]

Niles,

Would you please give me Jay's web page address.