HP gain expected

[John(XJ)Jeep]

It's NOT beating the back pressure. It's finally hit the spot where the scavenging is working the best. It also has to do with the intake and the cam.

Well technically I didn't word it properly as I just found out but what I said is what scavenging is. Look, I don't want to argue with you people I just don't agree with you guys are saying. :wave: It just doesn't make sense I guess to me because your pupose of exhaust involves low end and I'm looking for mid to high end power.

I found out what scavenging is exactly and I can tell that I don't want it. Seeing that the torque gets moved to the low end that's where not the horsepower should be in track racing or I should say in that area of performance. In four wheeling you want low end power low speed, low RPM in such. When you pound the pavement you spend most of the time at high speed and high RPM so in turn I want as open an exhaust as possible without being too extreme in my position.

"For virtually all high performance purposes, backpressure in an exhaust system increases engine-pumping losses and decreases available engine power. It is true that some engines are mechanically tuned to "X" amount of backpressure and can show a loss of low-end torque when that backpressure is reduced. It is also true that the same engine that lost low-end torque with reduced back-pressure can be mechanically re-tuned to show an increase of low-end torque with the same reduction of back-pressure. More importantly, maximum mid-to-high RPM power will be achieved with the lowest possible backpressure."

This is what I beleive in

 

[vwkaferman]

The only reason that I see for back-pressure or "scavaging" is for turbo applications. I know that turbo egines really need a certain amount of
back-pressure to run properly.

Whoa there lil horsey! Turbo's ABSOLUTELY do not like backpressure. Ever hear turbo guys talking "spool times". The more backpressure, the SLOWER the turbo "spools up", thus, increasing the time it takes to make max. boost. Conversely, LESS backpressure allows the turbo to spin right up, reaching max boost as quickly as possible.

Notice the "relatively" new Neon SRT-4 has NO muffler, FROM THE FACTORY. Less backpressure.

Not to say normally aspirated cars don't "need" backpressure. They certainly do to an extent, I believe Sarge explained it earlier, scavenging.

And last, something about us contradicting modders and what they do to exhaust, I DEARLY hope you're not talking about kids in their Honda Accords throwing a FART pipe on a car, and calling it "faster". Most of the time, those exhaust do ABSOLUTLEY nothing for perfomance for those bone stock cars.

James

 

[Sarge]

John,

That's cool. However you'll still need some even going for mid to top range. If you were interested in wide open throttle (WOT) only then totally open is most likely the way to go. But with mid to top then you'll need some scaveneging. It will help evacuate the cylinder on the exhaust stroke and depending on overlap help suck the new air/fuel mix in. In other words you're right on one hand as well. Too small exhausts can restrict the engine from a given purpose.

Sarge

 

[John(XJ)Jeep]

Whoa there lil horsey! Turbo's ABSOLUTELY do not like backpressure. Ever hear turbo guys talking "spool times". The more backpressure, the SLOWER the turbo "spools up", thus, increasing the time it takes to make max. boost. Conversely, LESS backpressure allows the turbo to spin right up, reaching max boost as quickly as possible.

Notice the "relatively" new Neon SRT-4 has NO muffler, FROM THE FACTORY. Less backpressure.

Not to say normally aspirated cars don't "need" backpressure. They certainly do to an extent, I believe Sarge explained it earlier, scavenging.

And last, something about us contradicting modders and what they do to exhaust, I DEARLY hope you're not talking about kids in their Honda Accords throwing a FART pipe on a car, and calling it "faster". Most of the time, those exhaust do ABSOLUTLEY nothing for perfomance for those bone stock cars.

James

Those fart cans do something allright but it doesn't mean it always good now.

And for turbos...
It might let it spin up faster but you won't get your peak boost from an open exhaust. The exhaust needs scavenging to help it spin faster letting the exahust escape faster. It's like blowing through a straw vs. blowing through your mouth. The air coming out of the straw is much faster than the air just comming out of your mouth.

Back pressure and scavenging as I found out yesterday are nearly different things.

Basically "back pressure" is when the the gases and energy waves that travel down the pipe hit something then get reflected back. Scavenging "inertial scavenging" is when using the size, length, material of the pipe, the header ect., to adjust the speed of exhuast gases escaping from the combustion chamber. "wave" scavenging is when you tune it to have the valves closed when the energy wave hits back up the exhaust.

And talking about scavenging with valves and such that is called "wave" scavenging. It can be good or very bad.

And those how think that peak power is at WOT only is the result of open exhaust well your about 3/4 right. WOT doesn't mean thats when it only works because you don't have to tromp on it to go throguh the RPM range. I can get up my RPM range without WOT and it still has the same effect. Engine RPM is what effects it not how much or fast you put your foot down.

 

[John(XJ)Jeep]

John,

That's cool. However you'll still need some even going for mid to top range. If you were interested in wide open throttle (WOT) only then totally open is most likely the way to go. But with mid to top then you'll need some scaveneging. It will help evacuate the cylinder on the exhaust stroke and depending on overlap help suck the new air/fuel mix in. In other words you're right on one hand as well. Too small exhausts can restrict the engine from a given purpose.

Sarge

As long as you have an exhaust your always going to have some sort of back pressure, some working scavenging, it all depends on the application. My exhuast on my Jeep can use a more open exhaust I want at least 2 1/2 front to back and hopefully find a true equal length long header with an equallizer to broaden my powerband.

But as I can see your starting to see it my way, but just a little. :laugh3:

 

[JeepSpeed]

There is so much bad information in this thread.

 

[Sarge]

JeepSpeed, it would help if you stated what was the bad info and why. Just a blanket statement really doesn't help much. I'll openly admit most of my knowledge is for bikes but a motor is a motor. I've run a few cars with no pipes or next to it and they ran much better after I added an exhaust system.

Sarge

 

[John(XJ)Jeep]

I don't beleive so much as the information as being bad but I feel that people have different experiences. I beleive what I beleive in because it has worked for me for what I am doing. It all depends on the application.

 

[Cjmartz2k]

Yes, but I think this thread has drifted away from specific 4.0L application to exhuast workings in general, and simple physics don't change from vehicle to vehicle.

 

[John(XJ)Jeep]

Simple physics don't change but mechanical tuning does. Even though this is meant for the 4.0, engines in general all operate the same way using those simple physics. Any engine can be configured to do whatever you want. In this case he just wants to know what kind of gains can be had from a flowmaster and a high-flow cat. All I can say is that its not much, at the most 10HP, a slightly higher powerband and hollow sound comming from the back. If thats what he wants then he's in luck if not then he should read the whole thread carefully and do some research.

 

[phaythe]

Backpressure beyond your collector is sucking power away. Period.

Your header and collector determine your powerband. If you want low end torque, you want short primaries and a long collector to tune it down. If you want high end power, you want the primaries long and collector short to tune you up higher. Beyond the collector, it should look like open atmosphere, lest you are losing torque/power no matter what (even though a dyno will show you have 2 more torque at 1200rpm, you lost 4 at 3600, etc...). Granted, not all of us can run out and have a custom tuned header made for our application, so we do have to utilize the exhaust beyond the collector to do some of our tuning. But, if there is backpressure, there is loss. But you won't be able to get away from it unless you drop a header on. So yes, backpressure is bad, no matter what. In the case of daily driven, non raced engines, it can however be utilized to put the powerband where we want it.

Point in case. Blackdog Motorsports merged headers for the Neon 2.0L DOHC D4RE motor. Gain is around 20hp at the wheels...at 6000rpm. It moves max power up the band about 1000rpm, it works damn well, but only for racing. Most headers for the same motor are tuned such that the powerband is not so high, and only produce about 5-8whp. In my case, I only race the Jeep I currently own, any wheeling I do isn't very heavy, so I don't need loads of low end torque. So I run completely open exhaust (no muffler, gutted cat, yes, I am going deaf). My 4.0 lives in excess of 3500rpm and doesn't see much operation below that. I'm planning on building it to run in the 3500-6000rpm range. I'm going to really love putting in cams and a long-tube header to move my powerband up to that range.

In a perfect world, we'd all be able to afford to get new cams and headers matched for the powerband we want. In the real world, we fudge it and utilize backpressure to our best advantage if we can't afford a header to do it the right way.

Reference the following article, one of the best on the subject.

"No Loss Exhaust" by David Vizard:

http://www.ntpog.org/articles/nolossexhaust/nolossexhaust1.jpg
http://www.ntpog.org/articles/nolossexhaust/nolossexhaust2.jpg
http://www.ntpog.org/articles/nolossexhaust/nolossexhaust3.jpg
http://www.ntpog.org/articles/nolossexhaust/nolossexhaust4.jpg
http://www.ntpog.org/articles/nolossexhaust/nolossexhaust5.jpg

 

[Tenny]

I don't know how any one can live with no muffler / cat... the power at low end is horrible.

I have a '94 Dodge Stealth RT/TT and I have a custom exhaust (no cats) that leads one way to a cut out (electronic switch (so its just straight pipes...)) the other way leads to a muffler. And even with turbos, and the switch open the power down under 3000 rpm is horrible.

If you are only into racing, you really have to consider your 60'. I run 1.6-8's all day (thanks to AWD). Thats why I have mine rigged so that as soon as I launch, I press a button and it opens up while I'm driving. Works out very well, I have the power down low, then by the time I'm open in the rpm it's fully open.

 

[phaythe]

Drag is not the only kind of racing...I'm talking RallyCross, where my engine RPMs just don't get that low. For around town driving, I've still got well more than 100 ft. lbs on the ground under 3000rpm, so I'm still walking away from Hondas with ease...

 

[bajacalal]

I have a '94 Dodge Stealth RT/TT and I have a custom exhaust (no cats) that leads one way to a cut out (electronic switch (so its just straight pipes...)) the other way leads to a muffler. And even with turbos, and the switch open the power down under 3000 rpm is horrible. If you are only into racing, you really have to consider your 60'. I run 1.6-8's all day (thanks to AWD). Thats why I have mine rigged so that as soon as I launch, I press a button and it opens up while I'm driving. Works out very well, I have the power down low, then by the time I'm open in the rpm it's fully open.

Thats awesome. You've got an exhaust bypass. Bet with those turbos it becomes an air pumping machine. Did you build that yourself and how?

 

[phaythe]

Have you ever tried leaving it open and launching your car from 3000rpm? Granted, if you are worried about snapping an axel or something, I wouldn't try it, but if you truely want to race your car, that's about where you should be launching an R/T Turbo...the AWD system in that car will still grip, go and fly launching that high even on crappy tires. Just don't drop the clutch at 5k I watched some little ricer punk do that in his father's R/T T...ouch...bye bye axel...it got towed, and I never saw him out about the local hangouts again, even in his beat up CRX, hopefully his father took it away and said he didn't deserve to be near an engine ever again...damn ricers...

 

[Cjmartz2k]

Yeah, I had a friend with an Eclipse GS-X that tried racing me in my '98 Cobra w/150 shot. He got so pissed off because he kept getting beat that he did one of those 5k RPM clutch drops----bye bye transmission!! :-D

 

[John(XJ)Jeep]

I did one of those 5K drops before and damn! What a horrible noise it makes. Although I did like the fact it nearly went through the first 3 gears smoking like a smog factory at 2 mph. :laugh3:

 

[T&A-XJ]

A little off the original topic but what determines the nessesary pipe size? Have anything to do with engine size? Or just proper scavenging effect per engine/head design/flow? JUst asking as it seems 2.5" seems fine for my low flowing Renix head, wouldnt the higher flowing "HO" benefit from a slightly larger pipe? Or is my 2.5" pipe too big and I just am oblivious. On the other hand those with stock header and a 2.5" exhaust how does a larger diameter exhaust pipe than the collector have any benefit? Restriction in the stock muffler/cat I can see but larger pipe doesnt make sence, in that case.

Just currious....

 

[Panama XJ]

2.25" for stock 4.0L and 2.5" for stroker engines, high flow cat and mandrel bend tubes, the low restriction muffler you like (magnaflow, flowmaster, borla, Dynomax, etc.)

 

[phaythe]

It all depends on what you want to use your engine for, and YOUR engine. For 90% of the people in this forum, having a somewhat restricted exhaust to boost the torque for low end is what is more beneficial. This bit of restriction is simply a jerry rigged way of doing it though, but it does work just fine. And some gain is better than no gain at all.

But to better answer the question. The pipe sizes are determined by the flow characteristics of your engine. Read through that article I posted links to above, buy some of Vizard's books, etc.

My guess is that most of the people here who have ran with an extremely free flow exhaust and thought there was a huge loss in low end had it predetermined in their head that the loss was going to be there. And as a result, "felt" it. Yes, if you are offroad and crawling rocks, you are indeed going to notice a loss of 10wtq below 1500k. But for those of us who run daily drivers, the 130+wtq on the ground at that point is MORE than sufficient to get you beyond 1500k before you even realize it, up to the point where you're in the 3000k range and are now gaining 2-3wtq over what a more restrictive aftermarket exhaust would be giving. I mean hell...our engines can put out more torque at idle than most daily driver engines puts out at peak...

So I really do think that people who complain about the low end power being reduced, really just have it predetermined in their head that they aren't going to like it. At the RPMs it is lost at...you simply are not going to notice the loss unless you are offroad. Butt dynos aren't that sensitive