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Lets set this straight.
- Thread starter jeepman121
- Start date
BillBraski
NAXJA Forum User
- Location
- Appleton, Wi
If you want a high RPM engine, less restriction. More low torque? More restriction. Its a simple concept once you take overlap and scavenging into account.
BillBraski
NAXJA Forum User
- Location
- Appleton, Wi
mattbred said:
It does great things for both low and high RPM power. It does much, much more for higher RPM power. Low end torque, like has been said, requires less overlap because the engine is moving slower and the pistion pulls more mixture in and in higher RPM engines the exhaust is moving much faster and should ideally assist to pull a new fuel/air mixture in through the intake valve on the intake stroke. But, back pressure has a big effect on torque low in the power band and it is very important there is enough back pressure becuase there needs to be a chain in the exhaust pulses (stuipd example) so too big of a pipe will hurt your low power and mileage due to fact the engine can't fill them and it kinda bounces back.While too small will effect the flow. I don't have all the answers, I just know things from work and the guys I run against in the 1/4 mile.
Heres a guide line for me. A stock chevy (5.7) 350 or (5.0) 305 with 8.5:1 comp. ratio and stock intake and exhaust manifolds will benifit from no larger than 2.5 inch pipes, no smaller than 1 7/8.
What do you mean backpressure has nothing to do with it though?
I mean, if your exhaust was 1" pipe the whole way, then that's obviously going to run worse, since it's working to pump all of that pressure out of the hole. Obviously there's a balance to strike between backpressure and velocity, which is what I was saying.
Also, I must've gotten confused on valve overlap. What is it, exactly? I was thinking of the exhaust valve staying open when the intake valve is opening, but that can't be until the piston is already on the intake-stroke, passed the exhaust-stroke, in which case the exhaust valve would be closed.
Hell I kind of think we lost focus of the thread.
Edit: I'm gonna take a break from this and think it all over.
I mean, if your exhaust was 1" pipe the whole way, then that's obviously going to run worse, since it's working to pump all of that pressure out of the hole. Obviously there's a balance to strike between backpressure and velocity, which is what I was saying.
Also, I must've gotten confused on valve overlap. What is it, exactly? I was thinking of the exhaust valve staying open when the intake valve is opening, but that can't be until the piston is already on the intake-stroke, passed the exhaust-stroke, in which case the exhaust valve would be closed.
Hell I kind of think we lost focus of the thread.
Edit: I'm gonna take a break from this and think it all over.
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BillBraski
NAXJA Forum User
- Location
- Appleton, Wi
Overlap is a camshaft/valve term. All overlap really is, is a term used to describe how long the intake and exhaust valves are open togother (in degrees). When they are open togother, the exhaust has a vacuum (The stronger the better, long tube headers and good mufflers help) and helps draw the intake charge into the combustion chamber when the piston is just starting the intake stroke. So, the result is better cylinder filling, better economy and better power (Mostly at mid-high RPM). I hope your starting to get it because I just got some buffalo wild wings and am getting sauce all over! :doh:mattbred said:
PurpleCherokee
NAXJA Forum User
- Location
- MO
Good grief I'm one of the many that is sick of people saying that backpressure builds torque and very free-flowing systems are killing torque!!! It's simply not true. Think about it? Which requires a higher volume of air, low rpms or high rpms? And which amount do you think a stock exhaust system is better at handling? A higher, or lower volume of gas? Therefore, which is there going to be a more noticeably difference in power? At a higher rpm of course! But I can speak from experience when I say that an extremely unrestrictive exhaust system will give you more power all around. I have a dual outlet SHORTY header, dual 2 1/8" pipes with an x-pipe dual resonators and a dual in/dual out muffler and NO CATS and it is sweet. It's all about exhaust gas velocity. So you want your pipes to be the correct diameter to be able to handle the amount of gas that your motor puts out at its max rpm at the highest velocity. And in order to increase the velocity of the exhaust, you need to have a very free-flowing exhaust set-up (MINIMAL BACKPRESSURE). And o2 sensore have a LOT to do w/performance so of course when you're running open headers it's gunna run like crap!
BillBraski
NAXJA Forum User
- Location
- Appleton, Wi
PurpleCherokee said:
your logic is messed up. High back pressure kills engines, but NO backpressure does NO good.
BillBraski
NAXJA Forum User
- Location
- Appleton, Wi
Purple, you also talking about scavenging. That dual outlet header breaks the exhaust pulse further down stream, so there should be more scavenging.
PurpleCherokee
NAXJA Forum User
- Location
- MO
I dunno what you're talking about. I don't think I said anything about high back-pressure. What you want in an exhaust system really, is NEGATIVE backpressure lol if that makes any sense. You want a vacuum, NOT backpressure. Backpressure = BAD lol. But having restrictions in your exhaust doesn't help ANYTHING.BillBraski said:
BillBraski
NAXJA Forum User
- Location
- Appleton, Wi
Heres a test, go take off your exhaust manifolds and run your engine at 4000 RPM and see how long it takes to burn your exhaust valves up, thats a true "zero backpressure" situation. I've ran a Ford 429 with a really bad header gasket and burned 2 valves that cracked. I had to re-sleeve both cylinders.
BillBraski
NAXJA Forum User
- Location
- Appleton, Wi
PurpleCherokee said:
Sigh...
Ok, do you know what exhaust scavenging is?
High back pressure is like sticking something in a pipe or a bent pipe.
If you had NO BACKPRESSURE all of your exhaust valves would FAIL!
Does a NASCAR engine have backpressure? You bet! Its not much, but they do.
BillBraski
NAXJA Forum User
- Location
- Appleton, Wi
To clarify, NASCAR engines have backpressure at lower RPM, when the exhaust pulses are slower.
Apologies all around.
Apologies all around.
outlander
NAXJA Forum User
- Location
- Columbus,Ohio
THIS THREAD IS WORTHLESS WITHOUT DYNO FIGURES:rtm:
I'm tempted to go down to a speed shop near me tommorrow to see how much they would charge for a few dyno runs....
I'm tempted to go down to a speed shop near me tommorrow to see how much they would charge for a few dyno runs....
wolfpackjeeper
NAXJA Forum User
- Location
- 500' and 500KCAS, RAF lakenheath
Unrelated question:
for those of you talking about putting a dual exhaust system on the cherokee, was it because you wanted the cool v8 look, or just to heat up the gas tank from both sides evenly?
for those of you talking about putting a dual exhaust system on the cherokee, was it because you wanted the cool v8 look, or just to heat up the gas tank from both sides evenly?
PurpleCherokee
NAXJA Forum User
- Location
- MO
BillBraski said:
Holy crap you gotta be kiddin me. Your exhuast valves won't FAIL if you don't have any backpressure... But I'm pretty sure everyone here is running with at least a header or manifold so I'm pretty sure there's gunna be backpressure so stop arguing like we're runnin without even a manifold lol! The goal is to reduce it as much as possible. But if you follow your logic, then the less backpressure you have, the more you're burning your exhaust valves... so those of us with very little backpressure should have some cracked valves very soon... we'll see lol.
- Location
- In the islands
Talk about back pressure and abscense of bak pressure. Two vehicles come to mind. I had a 1st generation Dodge Intrepid with a 3.3 Liter engine. My brother had the Dodge Caravan also with the 3.3 Liter engine. The most noticable thing looking at both engines were that the intake and exhaust were different. Both had the same basic engine. Both tuned to run at different RPM's. While the Caravan would take off screaming with 8 passengers, the Intrepid would struggle too get going. At higher RPM's, the Intrepid would run rings around the Caravan full or empty. Both engines had the air delivery and removal tuned for a different purpose, one for plenty of low end grunt while the other needed a running start to build power. Both vehicles had close the same top speed eventually.
On my 1994 XJ, I changed the OEM manifold to a semi performance/performanance Borla (depends on how one looks at it). It was a much free-er flowing exhaust (at least it sounded like it was). My 0 to 40 (ish) MPH sufferred. I could no longer steal a jump out of a stop turning right if there was a chance another vehicle was coming from the left. It just did not get up to RPM thus MPH as quickly as it did with the OEM manifold. From 2,000 RPM up, it went like a bat out of hell. Highway cruise 2,000 RPM up was noticiably improved. Again, the only thing changed was the manifold. Was it because of more back pressure, ........or less back pressure?
On my 1994 XJ, I changed the OEM manifold to a semi performance/performanance Borla (depends on how one looks at it). It was a much free-er flowing exhaust (at least it sounded like it was). My 0 to 40 (ish) MPH sufferred. I could no longer steal a jump out of a stop turning right if there was a chance another vehicle was coming from the left. It just did not get up to RPM thus MPH as quickly as it did with the OEM manifold. From 2,000 RPM up, it went like a bat out of hell. Highway cruise 2,000 RPM up was noticiably improved. Again, the only thing changed was the manifold. Was it because of more back pressure, ........or less back pressure?
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PurpleCherokee
NAXJA Forum User
- Location
- MO
techno1154 said:
So if it went "like a bat out of hell" from 2 grand on up, then how did your 0-40 suffer? And as far as the whole Intrepid, Caravan analogy goes, there's a hell of a lot more than just manifold differences on those two motors. I'd say the biggest reason the manifolds were different was for clearance reasons. Also gearing, and many other factors.
wolfpackjeeper
NAXJA Forum User
- Location
- 500' and 500KCAS, RAF lakenheath
He did not say less back pressure causes bad valves, he said no backpressure will burn valves. And he is right. If you completely open up the exhaust: 3" all the way, "performance" manifold, no cat, straight shot muffler, few or little bends, you will lose most of your low end torque. Your power band will move farther down the rpm band. And you are effectively moving it to a higher revving application. Congratulations, 4.0's hit the rev limiter right around that magic 5252 number, and unless you balance and blueprint the motor to exacting specs, you will gain absolutely nothing. NO matter what you do, Torque and Horsepower Always intersect at that RPM. Somebody said it before "It is a tractor" Personally I would rather be able to bump the throttle to clear obstacles offroad, than have to run the RPM's Up.
BTW: I am Running a 4.7L with a newer intake manifold. It still has a catless exhaust on it with a glasspack from when I was younger and stupider. It is getting a nice 2.5in downpipe connected to a high flow cat and a superflow muffler with some nice smooth bends in it. I expect a nice pickup in low end power, because what is there just isnt quite like it should be.
BTW: I am Running a 4.7L with a newer intake manifold. It still has a catless exhaust on it with a glasspack from when I was younger and stupider. It is getting a nice 2.5in downpipe connected to a high flow cat and a superflow muffler with some nice smooth bends in it. I expect a nice pickup in low end power, because what is there just isnt quite like it should be.
goodburbon
NAXJA Forum User
- Location
- Westcliffe Colorado.
OMG you're all right, what was I thinking, I have no torque now until 5000 RPM:confused1
http://www.youtube.com/watch?v=EEjDQ5H0bK8
As you can see it still does what I want it to do with header and 3" exhaust with no massive loss of torque at low rpm, and it SCREAMS on the top end.
http://www.youtube.com/watch?v=EEjDQ5H0bK8
As you can see it still does what I want it to do with header and 3" exhaust with no massive loss of torque at low rpm, and it SCREAMS on the top end.
jeepman121
NAXJA Forum User
- Location
- Ohio
So, from what I'm getting is that it probably helps to remove the cat...but not the muffler. Right? When I put that I straight piped I meant that I removed the cat, and put a turbo muffler generic flowmaster 40. Identical baffling inside. Last night I raced a Durango V8 and was ahead 2 cars at 50 mph. So I don't think I lost torque.
