Exhaust manifold flow

[2seater]

I finally got around to comparing a ported outlet exhaust manifold set to the stock system on the flowbench. All tests @ 28"wc. The test was done with the exhaust manifolds connected with the crossover and drawing air through the rear outlet. Each port was flowed individually and the O2 sensor was installed. The caveat is that the ported manifolds are also smoothed at each exhaust port which forms a more consistant bellmouth air inlet and the ported set I have is also ceramic coated inside and out.

The cfm comparison is as follows:

Stock-------Ported

#1=178 #1=222

#2=203 #2=249

#3=169 #3=184

#4=152 #4=203

#5=165 #5=201

#6=169 #6=197

The largest percentage increase was #4 which dumps into the log of the manifold at almost a right angle and the ragged outlet hole in the stock manifold log has the hole shifted almost all the way toward #6 which apparently causes a lot of turbulence for #4. When the hole in the log is opened up to the outlet diameter the #4 outlet becomes visible through the outlet. Wether the engine needs this easier breathing or not is an open question.

[Barney Eaton]

Good information. Since first seeing the crude and rough connection on the firewall side manifold it looked like cleaning it up would be a no brainer...... It does take some effort to remove and replace, but would be a good project for a cold winter day (if you have a heated garage)

[jonlabree]

It is a very simple job. Should take no more than 15 minutes.

Take a hole saw the same size as the inside diameter of the pipe and run it inside the pipe. You may remove a small portion of the thread for the O2 port but it is not enough to worry about.

[2seater]

It is a very simple job. Should take no more than 15 minutes.

Take a hole saw the same size as the inside diameter of the pipe and run it inside the pipe. You may remove a small portion of the thread for the O2 port but it is not enough to worry about.

Yep, I ground one with a die grinder and carbide burr, but the hole saw works better. The exhaust is a low grade stainless steel so use plenty of cutting oil and slow speed. The one you have is pretty well centered but the unmodified one I have is offset almost all the way to one side. The outlet could probably be made a lot more efficient if the stub pipe before the flange was shaped more like a funnel, wider where it connects to the log part, without changing the length or location of the connection to the exhaust system, but a lot of work for unknown gain?

[Vincent Vega]

Hole saw is by far the easiest way. See my avatar.

[padgett]

1988 "C"

[Ronnie]

Is enlarging the opening really needed for normal driving?

My experience when modifying motorcycle engines is that a slightly restrictive exhaust system will give the engine better low end torque than a free flowing exhaust. Of course the low end power comes at a price... a reduction in top end HP. In most Reattas top end power is rarely used.

I would like to hear what others think about the need for modifying the exhaust for Reattas that are daily drivers.

[Barney Eaton]

Maybe someone could make an insert to restrict it more and further increase the torque, 500 ft lb would be great, how much restriction does that take?

[padgett]

The whole collector outlet location is bad, have gas flowing in two different directions and making a right angle turn. Still more is better & anything you can do to open up the flow there should help.

[handmedownreatta]

Maybe someone could make an insert to restrict it more and further increase the torque, 500 ft lb would be great, how much restriction does that take?

lol

[Ronnie]

Quote:

<table border="0" cellpadding="6" cellspacing="0" width="100%"> <tbody><tr> <td class="alt2" style="border: 1px inset;"> Originally Posted by Barney Eaton

Maybe someone could make an insert to restrict it more and further increase the torque, 500 ft lb would be great, how much restriction does that take?

</td> </tr> </tbody></table>

lol

Before you guys laugh too much take the time to read excerpts of what John Lingenfelter had to say in his book about exhaust systems. I agree the 3800 stock exhaust is a poor exhaust system but making it bigger may not always be better for driving around town.

.

[jonlabree]

He is talking about pipe size not restrictions in the pipe, that is what we have.

Also if you take the headers off you will notice that they are welded on the inside, restricting them even more. I ported them to matich the heads and had them rewelded on the outside. MUCH better performance.

[Ronnie]

He is talking about pipe size not restrictions in the pipe, that is what we have.

Also if you take the headers off you will notice that they are welded on the inside, restricting them even more. I ported them to matich the heads and had them rewelded on the outside. MUCH better performance.

I agree with that but while some posters were busy loling and poking fun they failed to notice the question that I asked in my first sentence: "Is enlarging the opening really needed for normal driving? "

My comments after the question about restriction were aimed at cars running at 1500 rpm with the torque converter locked up not under hard acceleration conditions.

If I drove around with my foot deep in the throttle I could see where opening the exhaust would make a difference but not at 1500-1800 rpm.

[wws944]

Looking at those pictures, one wonders how it could possibly have gotten past the powertrain engineers? For three (or four?) model years?

It must have been intentional. In one of the marketing videos on the Reatta DVD set, it was mentioned that the 3800 had more HP than the Cadillac V8 engines of the same years. Just checked wikipedia, and this seems to be true. I wonder if there was some GM internal policy forcing the 3800 to be hacked a bit so it wouldn't make the the Caddy engines look as bad as they were?

[padgett]

Suspect it is also partly being engineered when the national speed limit was 55. Sombody just said it was cheaper to make this way and really did not matter.

To really be effective you would need to make a new rear manifold and bring the front crossover to a collector and flow t5the rear from and end to the same collector. Then tune the exhaust pulses (which are even on an even fire V-6) to reach the collector at the same time at a given engine state.

Frankly that rear manifold is terrible with pulses travelling in different directions at different times, colliding, then making a right angle turn with no radius. Free EGR.

[Barney Eaton]

Ronnie.... you are taking this too seriously. When you ask the question, about normal driving, you already knew the answer. How many of us drive normal, and what is normal anyway?

If I were to take seriously all the barbs that have come my way I would have folded the tent a long time ago.

Maybe it is time to fold the tent.... I'm at 6,000 posts and that is where the counter stuck.

Edited September 7, 2010 by Barney Eaton (see edit history)

[wws944]

...Maybe it is time to fold the tent.... I'm at 6,000 posts and that is where the counter stuck.

Keep at it Barney. Padgett is closing in on 10k...

[padgett]

Who listens ?

[Guest Mc_Reatta]

The tree that fell in the forest and didn't make a sound!

[padgett]

Define Sound. I believe that whether any one/thing hears it, if the environment is capable to transmitting vibrations, there is sound. Being hearing impaired I often see mouths moving with no discernable sound/speech. That does not mean it is not there.

It does mean that my world is very peaceful.

[2seater]

My intention is simply to present results that I find interesting and may be useful to others. There is truth in the fact that an exhaust system can be too large, or small, for a good balance between low end torque production and higher end performance. There are many articles out there with much research on the subject, David Vizard's research being one. I wanted to quantify a modification that many people have already done, not only on this forum, but the LeSabre and Bonneville guys too (probably other forums as well). One other item of some interest is catalytic converters. I removed mine many years ago but will be replaced and I tested the cheap 2.5" Walker universal converter. I have old data from the stock converter which I sacrificed some years ago. Again all tests are @ 28" w.c. except the last, which was @ 26.5" due to limitations of the small flow tube I have in place at the moment. The stock converter flowed 273 cfm, after gutting the cat., the flow rose to 323 cfm, but that is nowhere close to the 495 cfm flow through a 2.25" straight pipe of the same length. The 2.5" universal cat is somewhat shorter, and slightly larger inlet/outlet diameter, but it flows over 375 cfm, more than the gutted stock cat. I will rerun this last test with the larger flow tube used on the stock cat to see if there is a dramatic difference and report if there is.

Edit: Ran the test on the Walker cat. again using the same flow tube as the original testing of the stock cat. The correct comparison number for the new cat. is 355cfm.

Edited September 9, 2010 by 2seater
New data (see edit history)

[Guest 88reattacoupe]

Padgett I am going to define sound as, any vibrations in an atmosphere that vibrate our ear drums and end with an experience of hearing. By this definition , the tree did make vibrations. But, is it appropriate to say it made a sound, when the word sound is in need of a receiver. If there were no sentient beings who experienced the waves as sound, would the word sound make any sense ?

[padgett]

So "sound" is related to perception rather than source ? The ROE are needed even if there are none.

Meanwhile a 231 cid engine flows about 400 cfm at 6000 rpm but flow tests are measured in the context of a pressure drop. 28" means about a 1 psi drop. While a carburator needs a drop across the venturi to mix gas, FI does not so to make max power you need about a 600 cfm capacity, either with one 600 cfm inlet (about two inches) on a plenium or the same each for three, two or one cyl. (3.8 Jag had triple 2" SUs each feeding 2 cyl).

The reason is simple: an even firing six cyl engine has intake and exhaust pulses that are spaced around 720 degrees of crank rotation or 120 degrees effective (actually closer to 180 but as one ramps down the next is ramping up).

Now the intake is cold while the exhaust is hot and so takes up more space. OTOH the intake is being pulled in and is inheriently less efficient than the exhaust which is pushed out. And on the gripping hand a properly designed system will use overlap so the mass of the exhaust leaving the cyl will create a partial vacuum behind it to help pull the inake in and if the whole intake mass (see plemium) is already in motion that helps also.

Treat it as six one-cyl engines and you lose that advantage.

In other words:

a) for a normally asperated engine, you optomise toward getting the intake in and let the exhaust take care of itself. For a boosted engine, the reverse is true.

it is still an art form

Can say that the "C" 3800 is horribly restricted both in intake and exhaust but has a cam optimised for 2000 rpm operation (e.g. ded) so it really does not matter. "L" has noticably better flow at higher RPM.

V-8 is entirely different and is best treated as two four cyl engines. Unfortunately each "4" has two cyl on the left and two on the right. Some experiments have been done with "flat" cranks for a V-8 but the vibration is terrible. This is why if you take a V-8 with left and right duals, it gets MUCH quieter if you add a crossover ("H") pipe. That said, a 302 with 1000cfm intake and everything else optomised will wind to the moon.