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  #16 (permalink)  
Old 09-21-2005, 08:01 PM
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Quote:
Originally Posted by matt167
Basicly, what he means is just what I said about cylinder preassure. What the cylender preassure does is allow for a better and more compleate burn..
So why put headers on a street car?? Isn't it going to
defeat the purpose of the "required" back pressure?

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  #17 (permalink)  
Old 09-21-2005, 08:02 PM
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Backpressure is never good I belive. It's a pumping loss that the engine must use horsepower to overcome.

I think header primaries and collectors and H/X pipes are not sized to create pressure, but to create negative pressure through exhauts inertia,

Exhaust size, I think, is a matter of opinion and circumstances. I for instance have a light little truck with a high reving engine, but only 1.5" primary shorties dumping into 3" duals, with no crossover, When I swithched from the dynomax ultraflows glasspacks to the flowmaster delta 40's I noticed no change in power or torque, Even from no mufflers. But half the flow of the dynomax.

I won't put 3" on my heavier automatic GTO though.

In my case, If I did have backpressure at low rpm, my 72 deg's of overlap would make my truck run like crap.

You know how much horsepower you can lose from exhaust pumping losses on a purpose built nitrous engine?

Just my 2cent.
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  #18 (permalink)  
Old 09-21-2005, 08:45 PM
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Quote:
Originally Posted by smallblocker2
Backpressure is never good I belive. It's a pumping loss that the engine must use horsepower to overcome.

I think header primaries and collectors and H/X pipes are not sized to create pressure, but to create negative pressure through exhauts inertia,
Just my 2cent.
Smallblocker2... You win the cookie, you see the point of
my earlier post. Your engines don't "NEED" backpressure.
That is the point of installing headers, and it's what happens
inside the headers that counts, once the exhaust valve opens,
a column of fluid (Yes, Fliud) starts to move down the tube,
in a true equal length system, as the first pulse of fluid moves
into the collector it will create a low pressure area in the 3
other tubes, which will help pull the next cylinders pulse
along its tube, and so on... and if you've got enough overlap
in your camshaft, that low pressure will draw more air/fuel
into the cylinder. and it doesnt need to be running WFO for it
to happen. On my toy, it happens when it comes up on the
cam, which is about 2500rpm...

Have you ever read the hows and whys of
the flowmaster muffler?? It is a Hermholtz Resonator.
Engineers have been using the principle to build intake
manifolds for decades. Wanna see one?? look under the
hood of most modern cars...
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  #19 (permalink)  
Old 09-21-2005, 09:11 PM
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You're talking about how they use the open chamber in the inlet of the muffler as a plenum to revert the exhaust pulse wave, like a resonator box?

I still don't understand helmholtz resoators on intakes. I'm wishin' I could find and afford 4 weber ida's on a small chevy intake
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  #20 (permalink)  
Old 09-21-2005, 09:26 PM
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Quote:
Originally Posted by farna
bigun -- you're effectively running a single 3.5", as that's the "bottle neck" -- pressure will "back up" the two 3 inchers after a few seconds. Back pressure helps cylinder scavenging, so some is needed. The higher rpm an engine runs the less back pressure is needed.

A pair of 2.5" pipes isn't the same as a single 5". Do some simple math! Area of a circle is pi x radius (squared). So compute the surface area of a 2.5" pipe and multiply by two ( 2.5/2 [squared] x pi = 4.91 x 2 = 9.82 sq. in.). Then compare to the surface area of a 5" pipe (5/2 [squared] x pi = 19.64 sq. in.). A single 5" pipe is MORE THAN TWICE the size of two 2.5" pipes. In contrast a single 3" is 7 sq. in whereas two 2.5" is 9.82 sq. in, and a pair of 2.25" 7.95 sq. in. A pair of 2.25" pipes is no better than a single 3".

You're running LPG, which throws another wrench into the equation. LPG burns much cleaner than gasoline, but requires more volume to produce the same amount of power. Because it's cleaner and more complete burning, scavenging shouldn't be as big an issue as with gasoline. Another "problem" is how your engine is built. Is it a high compression engine with a cam leaning toward LPG or is it a dual fuel setup? If the engine is optimized for LPG that would be a critical factor in how much back pressure it should have. I'm not sure how much it should have, to be honest, or what operating range you typically run in.
Engine was rebuilt by previous owner to pull his 4 K LB construction trailer.I was told it was basically stock with an RV cam. I got it in trade because he was known for jack rabbit starts in granny gear with the trailer attached, He tore up two 4.10 ring gears in the rear diff. It is a mildy built AMC 360 with a stock cast iron two barrel. We used a duel fuel adapter on top of the stock 2150 carb, but it runs strictly on propane. Ignition is a Mallory mech. adv. electronic, coil is a stock as are the wires. Exhaust hedders are Hedman 4 into 1s. Tranny is a T18, transfer case is a D20 twin stick out of a scout. Front Diff is a D44 out of the J20 The rear is a D60 from the same source, gear's are 3.73 to 1 tires are 37X12.5X15.5s off a miltary Hummer. Total cost as the truck sits right now is about $2,000. Opperating range unknown as speed ometer is off and it does not have a tach as of yet. Oh to make more conversation the 3.5 is actually EMT. What can I say I am an electrician LOL. Complete build if any one is interested may be seen here.http://www.alaska4x4network.com/showthread.php?t=7778
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  #21 (permalink)  
Old 09-22-2005, 03:05 PM
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Quote:
Originally Posted by Canzus
So why put headers on a street car?? Isn't it going to
defeat the purpose of the "required" back pressure?
Let me put it this way, there can be too much which will hurt performance and too little will hurt performance in the low end, the headders are slightly bigger than the stock manifolds, say a 1 5/8" primaries headder on a street car, creates the perfect amount of backpreassure for a normal street vehicle and that is 1 big reason it is the most common size primarie for headders. you need more of the perfect balance of backpreassure and flow. And it also depends on the HP that the engine will make, I'm not saying that your gonna put a 2" exzaust system on a V8 that will pump out 600 horse, at that leval 1 3/4 primarie headders will be best along with at least a 2.5" pipe. but for the most part 2 1/4" is standard for most V8's and 2 1/2 is the other common size for most V8's. You get more torque in the low end with smaller systems, and you get more power in the top end with larger systems, so really it comes down to what you are going to be using the vehicle for. And it is the same with cams and carbs, with cams, there is a perfect size that will match what you will be doing with the engine and match the cams effective comp ratio, to your engine's comp ratio, with carbs, it is engine size and driving style, there is always that perfect CFM.
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  #22 (permalink)  
Old 09-22-2005, 03:28 PM
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Backpressure

To put in my two cents, One of the magazines did a comparison of exhausts 2in all the way to 3 in on a dyno. They found that once you got so big the power just stayed constant, but the db went up. In other words there was no penalty for too big except noise. I wish I knew the article so I could reference it. (I will search later tonight.)
On a personal note, I have worked in the auto industry for 10 years now and have always known one thing. The base engine designers / exhaust designers will give their right Knut for 1 inhg less back pressure!
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  #23 (permalink)  
Old 09-22-2005, 05:18 PM
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Quote:
Originally Posted by SmallBlock_guy
The base engine designers / exhaust designers will give their right Knut for 1 inhg less back pressure!
Oh, I have no doubt, but for Smallblocker2, check this article
out... http://autospeed.drive.com.au/cms/A_1969/article.html
It explains the helmholtz resonator as used on the modern engine...

I just realized that I spelt helmholtz wrong...
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  #24 (permalink)  
Old 09-22-2005, 08:47 PM
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I thought the helmholtz resonator was for producing power, not sound deadening. Doesn't it consist of tuned intake tract's, jioning a sized plenum, with 1 tuned inlet?

That is an interesting article. It explained the extra chambers I've seen on some Vortec engines inlet tubes.

That's why honda Vtec's have 2 sets of intake runners with butterflies in them right? To take advantage of low and high rpm induction pulse wave tuning?
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  #25 (permalink)  
Old 09-22-2005, 09:43 PM
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Okay, I have two ideas nobody is going to like...

IF we don't want backpressure of any sort why do we run headers instead of venting to atmosphere at the cylinder head?

Also, I believe that velocity plays a role of sorts...

We are trying to optimize flow, correct?
Therefore if we can create a sort of low pressure area behind a given exhaust pulse that just happens to coincide with another exhaust pulse then the first pulse helps to scavenge the second pulse. Is this not sound logic? anyway, If the pipe is too big (ie too large a collector) then you have no velocity with which to scavenge. I consider the exhaust pipe (downstream from the header) to be a mode for the exhaust to escape in a less deafening (and frequently safer) manner. I would give the motor just what it wants and nothing more in order to keep the velocity up as opposed to having it die off.

I am not saying I am right, I am just saying that what I think makes sense.
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  #26 (permalink)  
Old 09-22-2005, 09:45 PM
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Here is something for the exhaust back pressure discussion. A few years ago, I helped a friend with his Harley. He had just put 2" open pipes on his 1000cc sportster. I had 1 3/4" pipes on mine. His would idle for a minute or two, cough and then die. Mine idled fine. Everything about the 2 engines was the same except the exhaust. I suggest he add baffles to the pipes to slightly restrict the exhaust flow. He did and it ran great. In his application he had too much exhaust flow for the camshaft, cylinder head, carburetor combination and it leaned the mixture in the chamber to much for it to idle. Slightly restricting the flow made a huge difference. When you only have 2 cylinders, there is no room for error.
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  #27 (permalink)  
Old 09-23-2005, 04:15 PM
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Quote:
Originally Posted by smallblocker2
I thought the helmholtz resonator was for producing power, not sound deadening. Doesn't it consist of tuned intake tract's, jioning a sized plenum, with 1 tuned inlet?
Its original intention was to deaden sound, but some smart
guy figured out that sound waves move much the same way
air moves...

Quote:
Originally Posted by smallblocker2
That is an interesting article. It explained the extra chambers I've seen on some Vortec engines inlet tubes.

That's why honda Vtec's have 2 sets of intake runners with butterflies in them right? To take advantage of low and high rpm induction pulse wave tuning?
Yes, the Vtec uses them, as said, but the first domestic production vehicle to use the long/short runners was the
Ford Taurus SHO. Yeah, it's a Yamaha design, but it works
really well, as does the Duratec 2.5L/3.0L manifolds...
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  #28 (permalink)  
Old 09-23-2005, 04:17 PM
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I put on headers to create less than atmoshperic pressure, not more. And to vent the gasses away from my vehicle, not into the engine compartment.

If the sportster was retuned, would it have run?

All my toyota buddies tell me they NEED backpressure, because they lose power when they increase exh. flow, with no consideration to tuning the engine for it.

Hey canzus, The taurus has a yamaha motor in it?

Also, Why do they make those pipes with kickouts and removable plates to put behind the collectors. Is it for more power? or just for noise?

Last edited by smallblocker2; 09-23-2005 at 04:27 PM.
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  #29 (permalink)  
Old 09-23-2005, 04:51 PM
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Quote:
Originally Posted by smallblocker2
I put on headers to create less than atmoshperic pressure, not more. And to vent the gasses away from my vehicle, not into the engine compartment.

If the sportster was retuned, would it have run?

More than likely, everyone I know that have installed
open pipe on a Hog have had to spend a few hours/days
pulling their hair out to get them to run...


Quote:
Originally Posted by smallblocker2
Hey canzus, The taurus has a yamaha motor in it?
Yes, the Original Taurus SHO, DOHC 220HP, was a Yamaha
designed/built engine, then it became a Yamaha designed and
manufactured engine, but assembled by Ford US, then it
was a Yamaha design only, manufactured and built by Ford US
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  #30 (permalink)  
Old 09-23-2005, 07:04 PM
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Of course you want back pressure. That way you keep more of the burned combustion in the chamber to dillute the fresh fuel/air mixture and are able to benefit from the latent heat of evaporation from that hot exhaust. NOT

Header science, which I am no expert in and leave that to Stahl, Burns and and others, is not the same as exhaust science. The tuned length, diameter, collector shape and style will have MUCH more impact over how much power the engine makes than what size exhaust you stick behind it, unless it is WAY too small.

If you modify the exhaust system you have to tune the carb, simple as that. That's the reason one application will run better than the other, re: Harley and Toyota (EFI in that case). End of story.

If you want to run a 3" duals and it makes you happy, do it. It's noisy (I know I run one on the street) but it's your car and you should enjoy it the way you want.
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