Exhaust Flow

MadBill · Post by **MadBill** » Thu Dec 15, 2005 9:44 am

Is that a Philip H. Smith quote? I last read his stuff ~ 35 years ago, but the style sounds familiar. 2nd guess: A. Graham Bell.

Re a muffled car, I agree with 67RS: The best approach is to build an optimized open header system (cam included) and by correct use of 'empty volume' mufflers or Smith/Vizard-style expansion boxes, trick the engine into overlooking the presence of high flow mufflers downstream.

If you're stuck with significant backpressure however, I think it's the cam that needs tweaking more than the exhaust port. I could see substantially advancing the lobe: Earlier opening to get a head start on evacuating the gas before the BP builds and increases pumping losses on the exhaust stroke, and closing it early before BP causes much reversion into the chamber and intake port. Oh yeah: Maybe open the intake later for the same reason.
(Note to self: Look up the timing of the early (302 c.i.) vs. late (350 c.i.) Z28 cams. The early cam was said to be good for 20 HP more with open headers, but was at least 10 HP down with stock exhaust)

Mac1 · Post by **Mac1** » Thu Dec 15, 2005 4:42 pm

MadBill,

The author was Arthur P. Fraas, M.S. Test Engineer, Aircraft Engines.

Question:
If a 4.0" x 4.0" engine with 10 : 1 C.R . has 1 psi exhaust back-pressure at TDC, what percent of the intake stroke is used up to drop the 1 psi to zero psi?

2 psi to zero?

Pick a rpm.

Mac1 ....no clue

MadBill · Post by **MadBill** » Thu Dec 15, 2005 8:35 pm

No Fair! I'm not genned up on Aircraft Guys!
Since the turbo spelled the end of cutting-edge piston aircraft engines (bigger and bigger turbos, until the piston bits just disappeared altogether, leaving only the turbo aka 'jet'), there hasn't been too much written outside enthusiast sites. Which camp does Fraas belong to, ancient or enthusiast?
Re "1 psi to zero", we don't actually need the bore and stroke for the general solution. From the gas laws, and assuming constant temperature for simplicity: (P1 x V1) = (P2 x V2) So if P1 =(14.7 +1) and V1 ='y' and P2 = 14.7, then V2 = 15.7/14.7 x 'y', or 1.068y, or an increase of 0.068y.
If the CR is 10:1, then cylinder displacement = 9 times y , so 0.068 is 0.068/9 or about 0.75% of travel, in this case 4" x 0.0075 or about 0.030"
Man, I can't believe that Thermodynamics was my worst subject at school, but it's about the only one I've needed since! (hopefully the foregoing will withstand scrutiny and not prove these words..)

SteveS · Post by **SteveS** » Thu Dec 15, 2005 10:00 pm

Bill, I hope you are right because that is what I came up with as well.

Mac1 · Post by **Mac1** » Fri Dec 16, 2005 6:01 am

MadBill,

Camp Ancient. (Aircraft Power Plants, Fraas, 1943 Book) Oldie but Goodie!

Thanks for the Thermodynamics math lesson.
Love an example. I would never have figured it out.
So, with 5 psi back pressure to drop to zero at the begining of the Intake stroke, the piston would be down the bore .151" with a 4.0" stroke. Is this correct?
MadBill, SteveS
How come you Guys know this stuff so readily?
Thanks again,

Mac1

SteveS · Post by **SteveS** » Fri Dec 16, 2005 7:38 am

I became familiar with Boyle's law when I received my SCUBA certification.

Just kidding, well some at least. I backed into the chamber volume and simply applied the gas law to determine how much the incremental increase in Vol2 would be and it came out to .38 cubic inches........with a bore of 4in it has to have a depth of .03 in to give you the volume of .38 cubic inches. 15.7*5.6=14.7*5.98.

SWB · Post by **SWB** » Fri Dec 16, 2005 1:25 pm

Now referance that to crank degrees and calculate the loss in camshaft duration during the inlet cycle.

SWB

lil289 · Post by **lil289** » Fri Dec 16, 2005 1:42 pm

Thanks for the informative posts. OK, so big exhaust system after the headers is the way to go. That brings up another question. I want to build a mild street car that is also stealthy and quiet. When going from a 2-1/2" to 3" exhaust is there a significant increase in noise? I do not want the V8 version of the typical Honda with the "fart can" muffler, which, as someone mentioned before, only turns the acceletor pedal into a volume pedal.

By the way, what causes burbling and popping in the exhaust system? Can going to a 3" system cause this or is it totally unrelated?

Lastly, what is the best way to design a free-flowing yet quiet system? Are single exhaust systems just as good as duals if sized right? For instance, PHR ran an article about a single exhaust system install (they did it to maintain emissions legality) and claims that anything over a 3" single system is wasted on a 400 HP small block and that they have made 500 naturally aspirated HP with a single 3" exhaust and catalytic converter. I don't plan on using cats but was just wondering about the single exhaust as a option.

http://www.popularhotrodding.com/tech/0 ... index.html

SteveS · Post by **SteveS** » Fri Dec 16, 2005 3:04 pm

SWB wrote:Now referance that to crank degrees and calculate the loss in camshaft duration during the inlet cycle.

SWB

That would be the next question.......give me a rod length.

Mac1 · Post by **Mac1** » Fri Dec 16, 2005 4:25 pm

SteveS,

6.4" rod length, c-c.
L/r 3.2

1 psi = 8.7081 deg. crank rotation. Piston down bore .03022222".
5 psi = 19.604 deg. crank rotation. Piston down bore .15111111".
I think this is close. Please tell me it's so!

Mac1

SteveS · Post by **SteveS** » Fri Dec 16, 2005 7:57 pm

Mac1 wrote:SteveS,

6.4" rod length, c-c.
L/r 3.2

1 psi = 8.71 deg. crank rotation. Piston down bore .03022222".
5 psi = 19.604 deg. crank rotation. Piston down bore .15111111".
I think this is close. Please tell me it's so!

Mac1

Way to go Mac. Using Pipemax, I came up with 8.5 to 9.0 degrees of crank rotation to achieve a bore depth of .03" and 19.5 to 20.0 degrees to achieve a bore depth of .15"

MadBill · Post by **MadBill** » Fri Dec 16, 2005 9:03 pm

<By the way, what causes burbling and popping in the exhaust system? Can going to a 3" system cause this or is it totally unrelated? >
My belief is it's at least 90% due to exhaust leakage points pulling in fresh air during the negative pressure pulses, which then combusts with excess fuel.

SWB · Post by **SWB** » Sat Dec 17, 2005 9:18 pm

SteveS wrote:
Mac1 wrote:SteveS,

6.4" rod length, c-c.
L/r 3.2

1 psi = 8.71 deg. crank rotation. Piston down bore .03022222".
5 psi = 19.604 deg. crank rotation. Piston down bore .15111111".
I think this is close. Please tell me it's so!

Mac1
Way to go Mac. Using Pipemax, I came up with 8.5 to 9.0 degrees of crank rotation to achieve a bore depth of .03" and 19.5 to 20.0 degrees to achieve a bore depth of .15"

Now realize that back-pressure increases with RPM significantly and it's easy to see how much a bad exhaust system can hurt HP! I've seen cars that won't even REV quickly beyond a certain point, but that was an extreme case.

SWB

devilbrad · Post by **devilbrad** » Sun Dec 18, 2005 1:13 pm

Very interesting read, all though you guys went over my head (which always happens here!!) with the formulas!! I'm not really clear on what you are trying to determine?? Regardless.....I have some simple questions. I just did the math and my E/I ratio is as follows:
.200 = 90%
.300 = 81%
.400-.600 = 77%.
I've yet to choose a cam for this motor. A little background on motor...its a 318 Dodge Magnum, .030, 10.25:1, the above heads are Edelbrock aluminums for the Mag motors with some port work done 269/207 at .500, 28" readings I can post the rest of the numbers and any other info you guys need). I plan on spraying the motor with nitrous, its basically a track only motor using EFI. How do I use these E/I numbers to determine duration and LSA?? I'm having a lot of trouble finding someone to help me with cam choice on this motor. So far we are narrowing it down to a 232/244 at .050 536/544 on a 108 or 110 (maybe more for nitrous) LSA. This is using 1.6 rockers. I know its a bad idea to pick a cam based on heads, and a motor should be built around a cam design, but this is what I have to work with. Any input?? Idle vacuum is not a concern. I will be running full exhaust with 1 5/8" headers and 3" collectors.