Open headers on a 350, will it cause problems?

[roger97338]

You do know this is a Chevy forum, right?



(Posted in good-natured fun.)

[HAULIN IT]

Well guys, I'm going to have to ask: "How much backpressure should we want to have? 1 pound, 6 pounds...is there a basis so we know how much to want, maybe the bigger the engine the more we would want, Right? I guess I'll have to make a tap for a gauge so we can be sure to get just the right amount once I find out what that number is...I wouldn't want to have too little"  

[chevy80c10]

rich dont get me wrong. i love american trucks/cars but im just a chevy guy lol and ford is "blue oval bad" but i do give credit where its due, just wont admit that ford are better than chevys ;-)

i second that

[Irish_Alley]

http://www.veryuseful.com/mustang/tech/engine/exhaustScavenging.pdf
dont know if this is usful im kind of confused after reading it. at one point it says even 1.5psi will hurt performance (high rpms) but then it says "Building a system to divert the flow into a smaller system can help bolster lower rpm power, just as
with today dual runner intake manifolds, but you'll never find a dual runner intake on any engine that's
targeting the greatest performance potential possible. I should also add that such systems are
inefficient from a standpoint of weight and surface area."
i think the main ideal about this is a stock engine would like back pressure while a performance motor will hate it.


2. Calculations and Comments by Dave Stadulis of SMSP Exhausts Relating Flywheel HP to Exhaust CrossSectional Area (Diameter):
quote:
Here are the numbers for 16g tubing:
OD (in.)....ID (in.)...Area....%Increase......HP.......HP/in^2
2.25........2.120.......3.53.....0%...............200.......56.66
2.50........2.370.......4.41.....25%..............275.......62.34
2.75........2.620.......5.39.....22%..............318.......59.00
3.00........2.870.......6.47.....20%..............400.......61.83
OD is exhaust outer diameter, ID is inner diameter, Area is tube cross-sectional area, % Increase is
increase from the prior OD, HP is Flywheel hp, and HP/in^2 is hp per square inch cross-sectional
area.
For the 2.75 in. tube, I assumed 59 HP per square inch of flow area, I used Larry's numbers for the
others....you are talking HP at the crank :
2-1/4" for up to 200HP @ the crank, 2-1/2" for 275HP, 2-3/4 for 320HP...
or 60HP (at the crank) per square inch of (cross-sectional) flow area.
This 60HP/in^2 is to get you in the general vicinity. It also is based on the inside diameter of the
tubing not the OD (i.e. 2" in your example). The ID for 2' 16g tubing is 1.87" and this will yield a limit of
165 crank HP. 2-1/4" 16g (212 HP), 2-1/2" (265 HP). Now you can get different sized tubing such as 2-
1/8" and 2-3/8" to fine-tune a vehicle but you can't get cats and mufflers in those sizes so you should
go up a size when building an exhaust in those cases.



so there is a limit on the size of piping you want for hp, so...... why not just run 3" on a 200hp engine?

[rich weyand]

Note those horsepower numbers are per pipe, so 2.25" dual exhaust (assuming mandrell bent) is good for 400 hp.

I've seen a more pessimistic number of 185 hp per 2.25" pipe: http://www.exhaustvideos.com/faq/how-to-calculate-muffler-size-pipe-diameter/

Also note that 5100 * (350/2) / (12*12*12) = 517 cfm for a 350 with a (not possible) volumetric efficiency of 1.0.

So a naturally-aspirated 350 can't use more than 500 cfm for carb/throttle body, and 2.25" dual exhaust is good to 370+ hp.

[Irish_Alley]

make sense, but take into affect of a cat. how would you determine this? i mean i experienced it my self with out a cat i lost power? but i like this
http://www.popularhotrodding.com/enginemasters/articles/hardcore/0505em_exh/
it goes threw the who exhaust except the cat. which my only thought is the cat will help low output/exhaust volume engines, but high outputs it hurts and maybe mid performance.

[rich weyand]

Not sure about cats, but I can tell you that putting expander tips reduce horsepower by increasing back pressure.  BTDT.

I took the expander tips (on it when I bought it) off my 1978, and horsepower and torque increased noticeably.  The expansion slows down the exhaust and creates turbulence, which has the effect of putting a restrictor on the pipe.  In other words, it's poser stuff, a cosmetic effect to make it look like your vehicles needs the exhaust size because of its horsepower.

Similarly the people who put big rims on cars.  Big rims on performance cars are to provide enough room for large discs and calipers.  Big rims, though, increase unsprung weight and negatively impact suspension and handling performance.  Putting big rims on small brakes is more poser stuff.

Expander tips and big rims over small tips say "I don't really have a performance car -- in fact I just made performance worse -- but I want it to look like I have a performance car."

[HAULIN IT]

http://www.veryuseful.com/mustang/tech/engine/exhaustScavenging.pdf
dont know if this is usful im kind of confused after reading it. at one point it says even 1.5psi will hurt performance (high rpms) but then it says "Building a system to divert the flow into a smaller system can help bolster lower rpm power
i think the main ideal about this is a stock engine would like back pressure while a performance motor will hate it.

Irish, I guess you missed my "Eye Roll"smiley? I was kidding...the ideal amount of backpressure for an internal combustion engine is zero. I actually do have an oil pressure gauge I use to measure backpressure...if any shows up on my performance stuff, I'm pulling mufflers, ect. until the source is found.
 You are blurring/confusing backpressure & velocity, maybe scavenging...possibly a couple other physics related terms.
Backpressure is caused from a restriction at a point past the exhaust valve which causes some amount of burnt gases to remain in the combustion chamber. How or why in the world would we ever need/want that?
Velocity is speed...how fast the, in this case, exhaust gets away from the exhaust valve to allow a full amount of new, fuel filled air to enter. Done properly, it can help pull the intake charge into the combustion chamber (scavenging). The demo derby car with the guy putting zoomie pipes through the hood comes to mind...he has ruined all of the scavenging effects of a multi-cylinder exhaust system, probably made them out of too big of pipe killing the velocity & now it runs lean...wonder why it seems like it take so much throttle to make it move, but its not because he took away the backpressure!

Things to go "funny" when backpressure is reduced from its prior state depending on how/why it had backpressure in the first place. Namely an engine can become VERY lean in fuel mixture since more air out, allows/helps more air in but you "forgot"  to add the ever important fuel to the mix. Now your engine runs poorly...oh, & the extra heat (think cutting torch...adds oxygen to cut) can effect the exhaust valves. Hey, there is the first wives tale again!

The chart you posted is a minimum for eliminating backpressure/ideal size for a given engine power not to encourage "Lets use the small pipe to make backpressure so it becomes the correct amount"
You misunderstood what the author was getting at...keep velocity up using the smaller pipe to a point that you dont introduce backpressure in the operating range. There are MANY compromises in the making of something with an engine...noise, pollution, standards, ect. ect. all enter into this mix. Cats were not installed to boost power on underperforming  cars...they helped MAKE the under performing cars. Lets not get started on EGR valves!

So to say that your catalytic converter HELPED make power because it introduced the "right" amount of backpressure is just downright silly! You had other things in your tune wrong from the result of no cat that had they been corrected, would have resulted in MORE power, assuming you actually did experience a better running engine with a cat in place.  Spend some time researching on backpressure with an open mind & you will find this isnt just my opinion, but fact. Lorne 

   

[rich weyand]

Note that some small engines, especially two-strokers, require a certain amount of backpressure.  My college roommate's Yamaha factory drag bike had a 3/4" straight-leg exhaust that was a couple feet long, and the length and resulting backpressure was tuned to the porting.

That having been said, for large four-stroke engines, I agree that less is more when it comes to backpressure.  Then properly tune to the flow rate.

When in doubt, consult aviation.  If backpressure helped power, they would do it regardless of cost.  Aviation piston engines run open shorty exhausts just far enough from the manifold to clear the engine cowling.

[Irish_Alley]

yeah lorne about the rolling eyes "you will lose performance due to lack of back pressure
Speaking of Wives tails..." i didnt see them. but its cool i just was confused on why it hurt my performance but then i did ask if a proper tuned engine would help. but i know im combining the multiple physic terms with my question. so let me get this straight, you combustion chamber starts at 0 then your headers dont equate to back pressure they help with scavenging ( but too big will kill scavenging) at a certain  point you cross that line between scavenging and back pressure when you go to small/restrictive you will create back pressure which will hurt scavenging then if you open it up too much to git rid of back pressure, you will also kill the scavenging. so this is where im confused lets say the deal about exhaust piping, on a h/o engine you want bigger pipe but not to big. on a non performance you want big but not to big. but at the same time you dont want to run too small. im thinking of a garden hose this would be velocity too small and you would have so much water pressure it would shoot out, too big and you would have the water fall out and drop right on the ground. you want something in between. to give you some  back pressure but higher velocity. is this the same ideal with exhaust pipe?