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  #16 (permalink)  
Old 02-22-2011, 09:15 AM
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Quote:
Originally Posted by ericnova72
Backpressure is never a desirable thing to have.
There is no benefit to trying to hold the exhaust into the engine. You can't burn it again, so it doesn't help in any way except emissions compliance(EGR valve).
If you want a BIG flat spot coming out of the corners on a road course, go ahead and eliminate all back pressure.
Some back pressure is desirable in certain racing situations, not all across the board. In one way your right and in another, your wrong. I can't expain it all but I've heard it talked about many times in road racing.
If you want to debate the fact's, you'll have to talk to someone more knowledgeable on the subject than I am. Not all racing is WOT.

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Old 02-22-2011, 10:29 AM
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-0.01cents

install a 12" ID exhaust pipe.
still have 14.7psi exhaust pipe back pressure at sea level.
install a pump on end of exhaust pipe to achieve 0 psi back pressure.
heck, turn up the exhaust pipe pump to achieve -100 psi back press.
i think many could agree the reduced ex pipe back pressure can rob incompletely combusted mix from cylinder when ex vlv opens b4 ex cycle.
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Old 02-22-2011, 12:02 PM
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Quote:
Originally Posted by gow589
If the back pressure is not at least in the appropriate range to keep the intake charge in long enough

Quote:
Originally Posted by gow589
not enough backpressure and the combustion is not completed before exiting into the header


I have read and re-read these posts and I still can not see the mechanics involved in exhaust backpressure holding a cylinder charge until combustion is complete.
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Old 02-22-2011, 12:56 PM
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well.

what is the cylinder pressure during power stroke when exhaust valve opens?

combustion efficiency will increase with residence time in the hot cylinder.
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Old 02-22-2011, 01:25 PM
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Here is a very basic article on exhaust:here
This has been a very interesting discussion, let's keep it civil and have some respect for opinions, right or wrong. It's ok to disagree.
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Old 02-22-2011, 01:28 PM
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Quote:
Originally Posted by oldred
I have read and re-read these posts and I still can not see the mechanics involved in exhaust backpressure holding a cylinder charge until combustion is complete.
With what I have read, this is mainly a 2 stroke issue. http://en.wikipedia.org/wiki/Kadenacy_effect
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Old 02-22-2011, 04:28 PM
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My Two Cents

Quote:
Originally Posted by DanielC
This is a theoretical question.

I have heard that you can go too big on the exhaust pipes. Not the headers, coming off the heads, but the rest of the piping aft of the headers. Some people telling me this say the engine needs a slight bit of back pressure.

I believe you can have some benefits for having the headers tuned to a certain length, long headers favor lower RPM running, shorter headers favor higher RPM.

I realize that there is a point of diminishing returns, but would a small block engine in a pickup for example, make more horse power. with pipes that are "too big"?
What runs the quarter mile faster? Your truck with open headers or your truck with the full exhaust system?

If your answer is faster with open headers then you realize that there is no such thing as too big an exhaust system.

Exhaust systems can be tuned to produce more torque in a given RPM band area, such as to produce extra low end, but generally at the expense of high end power.

I think that the too big pipe theory can apply to a stock factory vehicle that is smog controlled/computer controlled because it throws the tune off base and makes it run like a sack of poo poo. If the engine/computer was retuned (fuel/timing) to run with the low restriction exhaust, then you make more power. Even with a carbureted engine, when you add headers and low restriction exhaust, you have to go back and retune(fuel/timing) to gain maximum power.

I think some of the "pipes too big" crowd just don't know how to hotrod tune an engine. I'm talking about hotrod engines (WOT) mind you. I'm sure that engines for other forms of racing could be exhaust tuned to gain a bit of extra torque at a specific area in the RPM band. Just as todays computer optimized vehicles produce as much average torque under the curve as possible. It is an engineered package, exhaust included. With a given engine/drivetrain package, I'm sure they try bigger exhaust until they get to the point of "diminishing return" as Daniel put it.


Okay, Thats my two cents. olnolan
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  #23 (permalink)  
Old 02-22-2011, 04:36 PM
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that sounds right.

how about a typical street situation where exhaust side is over sized compared to intake side because no hole in hood to put proper high flow intake?
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Old 02-22-2011, 04:40 PM
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If you run open exhaust the cam timing is adjusted to compensate.
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Old 02-22-2011, 04:50 PM
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Quote:
Originally Posted by gow589
If you run open exhaust the cam timing is adjusted to compensate.
i'm installing cam w/ less ex dur because i think my intake is the bottle neck.
lil more low end torque won't hurt my feelings.
i'm to wimpy to take advantage of a 6000rpm wot.
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Old 02-22-2011, 05:34 PM
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Quote:
Originally Posted by 001mustang
i'm installing cam w/ less ex dur because i think my intake is the bottle neck.
lil more low end torque won't hurt my feelings.
i'm to wimpy to take advantage of a 6000rpm wot.
I like a wide tq band. I put that one together looking for durability and driveability. I limited rpm to 5500 rpm. It is only 500hp, 534lb tq but it made 450ftlbs at 2000 rpm. I have another block I may work the other end of the rpm and power.

If you aren't playing your not having fun.
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Old 02-22-2011, 06:26 PM
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Quote:
Originally Posted by gow589
I like a wide tq band. I put that one together looking for durability and driveability. I limited rpm to 5500 rpm. It is only 500hp, 534lb tq but it made 450ftlbs at 2000 rpm. I have another block I may work the other end of the rpm and power.

If you aren't playing your not having fun.
sorry to hear you only have 500hp

how's your off idle torque?
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  #28 (permalink)  
Old 02-22-2011, 06:38 PM
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Quote:
Originally Posted by 001mustang
sorry to hear you only have 500hp

how's your off idle torque?
Not sure. I have not had it in the car yet. The car is 2900-3000lbs. I am about to screw up the mix. I am putting a set of down draft webbers with short runners. It is the wrong set up for the way the engine is set up but....as they say too bad. I will be welding up a new intake for it soon. We'll see how bad that screws up the mix.
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Old 02-22-2011, 06:41 PM
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Quote:
Originally Posted by dinger
With what I have read, this is mainly a 2 stroke issue. http://en.wikipedia.org/wiki/Kadenacy_effect

Yes exhaust is very critical on a two stroke and back pressure is indeed necessary but a four stroke engine is a whole 'nother animal! By the time the exhaust valve starts to open cylinder pressure has already peaked, or at least the power stroke is in a declining situation, (this is not a statement of fact but rather they way I understand it, right or wrong) and by that time it is exhaust scavenging that should be desirable. It would seem to me that at the instant the valve starts to open clearing the chamber of exhaust gases would be the main concern and that is what has me puzzled by all this, how can holding the gases in the cylinder at this point contribute to power?

Quote:
Originally Posted by gow589
If you pull the exhaust manifolds off a car you suddenly have a car that won't run worth a crap, won't idle, spews fire and black smoke (fuel) out the chambers. This has nothing to do with cyl timing.
True but it has little or nothing to do with back pressure either, place a set of short NON RESTRICTIVE pipes on the ports and it will run just fine. There is a variety of reasons an engine will hardly run in that condition but I don't think back pressure would be an issue one way or the other in that situation.
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  #30 (permalink)  
Old 02-22-2011, 07:09 PM
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Quote:
Originally Posted by oldred
... It would seem to me that at the instant the valve starts to open clearing the chamber of exhaust gases would be the main concern and that is what has me puzzled by all this, how can holding the gases in the cylinder at this point contribute to power? ............

i would think early pulling of the partially combusted gases would make headers hot as a downside and help w/ global warming as a plus...uhhh...unless your not ready for spring time.

power...hmmm...off idle, mid range, or wot...w/o variable valve timing...lots to squeeze between da ears.
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