geeza

Calling on our resident experts!

Can anyone give me a decent explanation of the sound that comes from the engine/tailpipe on certain V8s during gear shifts etc. It's a kind of honking/suck back sort of noise.

I recently went for a spin in an original 289 Cobra, where it was just fantastic to be sat there listening to it in the open air.

Also, fans of the movie Bullitt starring Steve McQueen will hear lots of it during the chase scenes, coming from his Ford Mustang.

Is it something common only to Fords, or a wider phenomenon? And what causes it?

I can't want to hear what you guys come up with. There'salways something good on this board.

Cheers.

Peryn

I haven't heard it, but i've got a theory. Before you shift, you would normally be working in the higher RPM ranges. Thus, you will be putting out a higher volume of exhaust gasses than you would at a lower engine speed. Well, when you shift, the engine speed drops significantly. So the amount of exhaust you put out drops dramatically. You have this large flow "pulse" moveing through the exhaust before you shift, and a very lower pressure "pulse" after you shift. Between the two i imagine there is a sort of vacuum. Not a lot, but there is a vacuum between exhaust pulses. When that high pressure pulse clears the exhaust, air is now free to move from the outside to inside your exhaust pipes to fill that vacuum and meet up with the lower pressure pulse. That air moving in could be what your hearing.


Again, i haven't heard the sound, and really have no idea what causes it. This is merely speculation, that seems to make some degree of sense to me.

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Dont cry kid, It's not your fault you suck.

Tech

...well i've always said Ford sucks...











oh, come on, SOMEONE was going to say it

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~I swear sometimes i feel like i'm married to a child.
~You better watch who you're calling a child, Lois, cause if i'm a child than you know what that makes you? a pedophile. and i'll be damned if i'm going to stand here and be lectured by a pervert.

brandon11983

*tee hee hee*

You are a better man than I, Tech.

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WarWagon

Seems like it would be reasonable, but although the exhaust gas volume may be reduced, there is still no vaccum, as it's still a positive displacement of exhaust gas coming behind it. Also, that wouldn't be a v8 or ford-exclusive sound.

Can't say I'm familiar with the sound you speak of, but then again, you won't see me behind too many Fords.

geeza

There's no doubt that pressure changes do occur in an exhaust system. Witness what happens to performance and fuel economy when the exhaust becomes perforated. The thing sounds and runs like shit. The engine relies on acertain amount of back pressure from the exhaust gases that are trying to escape.

I like that theory from Peryn. The fact that the noise is at its peak during the actual act of shifting, ie where revs drop substantially before being picked up again (albeit at a lower level) by the next gear engaging would tend to validate the "Peryn theory".

Any advances on that fellas?

Moonduck

Is it loss of backpressure that causes the performance loss or loss of scavenging? If backpressure were beneficial, top-fuel dragsters would run mufflers and reducers. As it is, they run open, tuned pipes because less backpressure means better flow-through in the overall engine.

Scavenging is what occurs when an exhaust pulse travels down the exhaust pipes. The pulse is shot out of the cylinder during the exhaust cycle at a fairly high velocity. As it exits, it creates a low pressure area behind it, a vacuum if you will. In a tuned system designed for optimal scavenging, the pulses will be timed and pipes constructed so that a given pulse will pull, or "scavenge" the pulse behind it.

Using the example of motorcycle engines and their exhausts, take a look at 4 cylinder inlines and the various forms of performance (and street) exhaust patterns that you see (I'll be referring specifically to Honda engines such as the one in my CB750). You tend to have 4-into-1, 4-into-2, and 4-into-4.

4-into-4, frequently called drag pipes, have no scavenging built into them whatsoever. They are usually open, and short to enhance low-range rpm (pipe length usually being what is modified in the process by which pipes are "tuned"). These pipes are designed for optimal flow as the intake side of the engine is also tuned for optimal flow in these sorts of applications. Drag pipes can be streetable, but are usually loud and not the most efficient application over the broad-range of rpm needed on the street. Reducers are sometimes put into these pipes, but usually to reduce certain undesirable performance traits (such as pipes not running in the proper rpm, but looking great at the length they're at), and to reduce the serious noise produced by short, open pipes.

4-into-2 are commonly seen on street 4's. Collected exhausts such as this are usually designed for scavenging or for economy of manufacture. The idea is that these pairs fire at precisely opposite points in the engine cycles, 180 degrees from each other, and the pulses travel in such a timed fashion as to help pull the next pulse from the cylinder more quickly. The low-pressure area behind the preceding pulse is what allows this. The issue with 4-into-2 (and 4-into-1 as well) is runner length. For optimal performance you want your various exhausts pipes to be equal in length, this means that each cylinder will perform similarly at any given RPM range. If your collected system has 4 different run lengths, the you will have to tune your timing and intake side differently on each cylinder to compensate for the differences in performance at a given RPM. The desire for equal runner length is what inspires some of the spaghetti designs that you see in exhausts on high-end engines.

4-into-1, when properly designed, is the best for scavenging as you have each pulse being pulled out by the pulse before it. On the truly well-done designs, the pipes collect in a squarish output with the pipes stacked in pairs. The firing order will rotates along this block in one direction causing each pulse to tpull the pulse following it around the pipe stack. (Not really able to describe this well, sorry) There is some serious flow work needed at the collector to keep unwanted turbulence down. Truly hot exhaust makers put a cone with four concave runnels corresponding with each pipe, less solid makers simply cut a line in each pipe and flatten them against each other in a plus-sign look. The cone style produces better flow, less turbulence, and enhances scavenging by removing unneeded resistance to flow. The downside of a 4-into-1 is need for better mufflers. In putting all of your flow through one muffler, you need a big can, making the muffler more expensive to build. There are also, insofar as some apllications are concerned, aesthetic issues with 4-into-1

(I used motorcycles as an example as it is usually easier to visualize the type of designs common to bike exhausts, without getting into scary automotive designs that have crossovers on the #3 and #7 cylinders in their tuned 8-into-2 set-ups. Runner lengths get to be nightmarish on those engines, but scavenging can be that important in some performance applications)

Basically, a perforation kills any chance at low-pressure wash following a pulse. The perforation kills high-pressure and low-pressure conditions by allowing outside airpressure to equalize the pulse in mid-pipe. Gas pressure eithers enters or exits through the perforation depnding on where the pulse is in its' travel. This means that the engine, designed with this scavenging in mind, loses an efficiency factor, making it run like crap. Back pressure is basically bad as it prevents maximal flow, and this is why common performance mods on any internal combustion engine usually include opening up the exhaust for better flow.

Tuned open exhausts make great sense on limited RPM engines (racing applications and constant RPM designs like various small engine applications) where noise is not a problem. Tuned collected exhausts are better on others, especially if noise is an issue. Flow is always important though, regardless of exhaust theory.

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On topic for the thread, it's been years since I watched "Bullit", and I don't recall the "sucking" noise you mentioned. I probably just didn't notice it. "Bullit" is a fascinating movie, especially on the production side. Much as I like old Stangs, they went through something like 8 1968 Mustangs, and only ever needed the one Charger. It was that much tougher a car. The Stang also needed a number of performance mods to keep up with the Charger (they chose the 390 engine, as opposed to just dropping in a better mill, they went through a lot of rigamarole to hop it up. I would've gone one year back and gotten 67's with the 427 in 'em, rawr.) It is possible that the odd sucking you hear is from exhaust and engine mods performed on the Stang. I'll have to watch/listen to the movie again to see if I hear it.

A possible theory would be scavenging in reverse. In other words, an exhaust pulse exits the cylinder at high velocity and then the driver gets off the accelerator completely, the pulse is still moving quickly and producing vacuum/low-pressure behind it. As it exits the tailpipe, with no serious volume of pulses behind it, the temporary low-pressure area still exists and "sucks" fresh air back into the tailpipe to equalize pressure. Dunno how plausible that is, nor how different from other folks' explanations.

An excuse to watch "Bullitt" again? Aw, heck.... =)

Moonduck

As an aside, this is just exhaust theory as I understand it. Not busting anyone's chops here =)

BooRadley

Dead on moonduck. Good to see you are applying yourself !

About the backpressure/scavenging effects : My theory on that is , you need to close your exhaust valves for maximum compression. With a supercharged wide-open intake ( such as a dragster ), you already have more compression than can ever need and more pressure going out than can be dealt with, so the loss by low backpressure is much less than the performance gain by blowing flaming nitrous out...

Another thing: You see individual runner open headers only on drag racers. F1 cars have 8-2 systems or 8-4... I have never seen them running 8-8... Of course, I could be wrong, but I think that's pretty true. Nascar runs 8-2. WRC? 4-1

About the sucking noise : I'm willing to guess the car is being run without an airfilter, and as the secondary barrels of the 4 barrel carb kicks in, it creates the sucking sound from the immense amount of vaccuum caused by the engine as the engine draws huge CFM through the 2.5 inch hole of the aircleaner. I had an old pontiac which did that.

If you want to read the importance of scavenging, read something on two-stroke expansion chambers. two-strokes wouldn't run for shit without them

Moonduck

Thank you, sir. Knowing your background, I take such praise seriously.

My thoughts as well. The "scary applications" I was referring to above were the exhausts I've seen pics of from F1 cars. If there is honestly a more finely tuned and designed piece of automotive artwork than a serious F1 car, I am not aware of it. Those things are scary, and the attention to detail is phenomenal. They worry about tonths of horsepower.

Oh, dead on! Now that you mention that, it puts me in the mind of a friend's El Camino with a 454 in it running no ari cleaner (eventually needed a ring job and basically a top-end rebuild as a result) It made a sucking, gurgling noise when he romped it. Sounded cool if you didn't know how bad it was in the long run. Gasket kits and ring sets for a 454 ain't cheap at all.

No doubt there. Glad we're on the same page =)