For those who think bigger is better in the exhaust department I can across some infromation from a forum that is hosted by Holley that owns Hooker.

"The best way to describe exhaust on an internal combustion engine is breathing through a straw while under water. If you use a very small diameter straw, you can not force the air in and out due to the high pressure needed. If you go with a straw 5 inches in diameter, your lungs will not have enough power to push the exhaled air out of the tube and eventually you will use up your air supply. If you get a straw that is around the right size that your lungs have enough power to move the column of air and not so small that you are restricting your lungs, you will survive."

Probably one of the most simplistic but most effective ways I have ever heard an exhaust system explained.

The full article can be found on the topic itle "exhaust system literature" in the general topic section listed http://holley.iserviceassistant.com/...sp?id_Tenant=1

 

But, the analogy fails because you have two straws,
one to breathe in and one to breathe out. There is no
penalty for making either one bigger, unless you are
depending on inhale / exhale resonant pulse effects
to jam more air into your lungs or suck it out as you
hyperventilate underwater (making great traction).

Of course there's no benefit to either straw being much
larger than your trachea either- that, corresponding
roughly to your head && cam flow numbers.

 

The analogy is wrong since the air doesn't go in and out of the same tube on an engine. From the flow numbers that I've seen just one mandrel bend in a 3" pipe decreases flow. If you figure there's a total of about 360-450 degrees of turns in just a simple true duals with dumps like this
|.|
.X
|.|
(.)
plus any restriction even straight thru mufflers put into the sytem, you realize that even though dual 3" pipe technically should be too large, it's not.

 

Goods points...but don't you think that you reach a point at which the size of the exhaust becomes overkill? Maybe three inch duals is the perfect size for our cars. Do you really think you would gain much more with a 3 1/2 inch, 4 inch or larger dual system? I don't believe so. In fact, some people on this board may argue that you may even hurt your torque and horsepower numbers as you go larger and loose the pulsing effect that is created by the firing order of the cylinders.

I would love to see dyno results of a very simple straight back system that dumps before the rear axle using various sizes of tubing.

 

I think there definately is a limit for every combination. For most 3" duals should be fine, but for other that have 700-900hp they may need dual 3.5" or dual 4" with mufflers to flow enough.

 

here is some information along the same lines.

a prostock head exhaust port flows only about 55 to 60 percent of what the intake port flows.(most peaple believe the number should be 75 percent)

on the pro stock trucks that were 358 cubic inches anmd made 900 hp they ran a primary tube of 1 7/8,s in most cases.

so your right bigger isnt always better,but there is alot more to it than just this of course.

 

I have seen many write ups on the sizes. Comes down to this. Small pipes scavage better and flow more velocity and have more TQ down low. Bigger pipes do the opposite. If you have a big cam and go over 6k the larger pipes will help you and the smaller pipes get turbulant above 5k. If your staying with a stock cam ect, larger pipes will rob your low end AND you wont benefit up high because you wont go over 6k. You have to figure what your application is, like with EVERY mod you do.

 

P/S cars also run no street-type muffled exhaust system, they have straightline carbed intake manifolds, extreme compression, camshafts designed for the E/I ratios... et al....

The comparison bears nothing to a street car...

Flow heads with everything... TB-intake-headers-mufflers etc... then see how important base head flow figures, with a 70%+ E/I really are...

Ed

 

I run 1 7/8 primary headers, 3" tube from there back to the cutout on my stock motor.

See Sig, it runs O.K.



Good to see you poking around Ed!


John (ex NMRA t-shirt salesman )

 

Sure hasn't hurt my numbers with a puny cam

 

ed there wasnt necessarily supposed to be a comparison.it was just a statement on bigger isnt always better.

like i said later in the post there is more to it than that.

but you do get alot of peaple that like to over kill a setup.

 

If you had a straw that was 5 feet around, you could stick your whole head in it and breath normally... Just saying

 

I don't think a large exhaust (from the collectors back) hurts anything. I think you can have primarys that are too large because they won't scavenge well. Two different issues being discussed here.

 

I gotcha Roger....

It's just that, this type of comparision is always throw out by the Ford guys. Especially when flow numbers of different heads are compared. Can't tell you how many times the E/I of the AFR heads is brought up by TFS owners, claiming they flow too much on the exhaust. It's just that the TW's don't flow enough to compete!

That's why I mentioned the P/S combinations and data really doesn't apply to street cars... Peace guy... Just speakin' out...

Ed

 

My opinion on this issue is that bigger isn't always better too. My 2.5" duals are good for my setup. Once I hit about 500 hp (ain't gonna happen), I might need 3" exhaust.

 

 

I worked with one of the contestants in this years Engine Masters Challenge. I'm amazed at the power lost and gained with header tube size and length. A number of header designs were tested and evaluated and it was discovered that exhaust headers made swings of over 80ft/lbs @ 3000rpm on a 470ci engine.
Ironically the header combination that produced the best torque average were two different size primarys.
Sure opened my eyes to the "tuneability" of exhaust systems. There's definitely no "one for all" header/exhaust system.............
Nice topic,

Richard

 

That's a wild one Richard...

My old NHRA big block Chevy Stocker had some Jack Davis headers on it.
They were triple stepped on four tubes and double stepped on the other four.
1-3/4 to 1-7/8 to 2 inch and 1-7/8 to 2 inch on the others. He did it to compensate for the four "good" intake runners and the four "poor" intake runners the OEM BBC heads had...

I can't say whether or not they worked better than a set of straight two inch primaries but who am I to argue with the chief header designer of Hooker Headers..

Ed

 

It may be that some highend engines have certain cylinders tuned for one rpm and others tuned for a different rpm. It depends on the engine's useful rpm range I suspect.

Intuitively you might think primary pipe diameter would correlate well with hp per cylinder: bigger pipes for a cylinder producing 100 hp per cylinder (more flow) than one producing 75 hp per cylinder (lesss flow), but that's not always the case even in world class engines. Curious.

There is some evidence that primary pipe diameter (not just length) has an effect on where torque peak occurs.

Many PS and Nextel Cup engines use 4 into 2 into 1 (4-2-1) or "Tri-Y" headers. How come we don't on our street cars? It's very effective for most all NA applications, and has been for over 70 years.

Exhaust systems are perhaps the least understood of all engine systems.

If you have to have an exhaust mantra, don't make it "bigger is better", make it "flow counts and tuning helps" or something like that.

My $.02