I ROCK

Well I'll post this again. It should be the case that routing tailpipes to low-pressure areas under the car will have a "supercharging" effect (although much more minor than an actual s/c) on the engine. Since the engine has to push the exhaust out against atomospheric pressure, routing the pipes to a lower-than-atmospheric pressure area reduces the amount of work the engine must do and frees up (creates?) more hp. Should I just grab a barometer and crawl under there or what? I'm also very interested in theories of exhaust scavenging. If I put cheap flowtech headers on my car (89 IROC LB9 auto 2.73 posi, no heavy mods) where would the best place be for H-pipe(s)? I know one should connect the left and right parts of the y-pipe right behind the headers, but where exactly? Exhaust gurus, please enlighten me!! Thanks.

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"We lost Dale. This isn't fun anymore"

R.I.P. #3

"The inevitability of death serves as the most clear evidence that to truly live, one must constantly court death, flaunt death, smack death around, and basically grab death by the throat and dare it to come too soon."

- Me

The ODB

BadSS has done much work with crossovers. Talk to him about that.

As far as flow only is concerned, the exhaust pipe is an orafice... therefore the pressure differential on each side will primarily dictate the flow (ignoring scavenging). You are right that a low pressure area at the exhaust exit would speed up flow out of the pipe to an extent.
I'm pretty positive that you will have little luck crawling under your car with a barometer. To really do any good you would have to create the low pressure area in the right spot.
For an efficient way to do this, just look at a carburetor booster as an example. The booster venturi creates a low pressure zone at the mouth of the fuel-feed tube. This is what meters fuel into the engine.
There is already low pressure at the rear bumper of your car and behind the rear tires. I would go to the inside, halfway behind your passenger-side tire to set up a venturi. Keep in mind that it will have to be very strong because of the wind drag on it.
To deal with reversion, BadSS has some easy mods you can do for that.

ODB

I ROCK

That sounds good. you mean route the pipe(s) up behind the bumper behind the pass. side tire? I will do some research on venturis...right when I was really getting into carbs someone smashed my LG4 to bits...

BadSS

Thanks ODB for the plug. I've tried a number of things with dual exhaust. Most of the cars I've worked on needs at least 3" dual pipes. I'll extend a 3" pipe off the colletor (usually a 3" collector), depending on the engine size and power range usually between 12" and 20" (depending on what the engine wants or the available space with what is to follow). I make a ring for the 3" pipe so a 3.5" pipe can slip over the 3" collector extension (us old farts call it a torque tube)and weld the 3.5" pipe to the ring that I place between 6 and 10" (no scientific reason just figure the longer (or closer to the header's collect ring the better) from the end of the "torque tube". I extend the 3.5" pipe about 6" past the end of the "torque tube" and use a flowmaster 3.5" to 3" cone so the rest of the exhaust can be finished with 3" pipe. These lengths are flexible in my mind thinking that some kind of reversion discouragement is better than none. The 3.5" systems are a little easier in that you don't have to use a 3.5" to 3" reducer. I certainly think the larger the ring and pipe used to discourage reversion the better this will work (like using 4" pipe and a 4" to 3" "cone"). With a street set up you only have so much to work with under the car,, and with speed bumps to consider. This same principle can be used on any pipe size. It works,, definately have to increase carb jetting and the idle quality is better. I've dropped cars over .25 seconds using the same mufflers. I must point out this is being done on longer duration cams than most will run in a "typical" EFI set up,, or most street cars as far as that goes (usually solid flat tappet or rollers in the 240 - 270 intake range @.050). The more power you're forcing through the pipes (and more cam overlap),, the better this seems to work. H-pipes work as well,, helping sound reduction more than power,, although most should benefit power wise from it. Generally speaking it seems I've gotten the best results with the H-pipe closer to the headers than the mufflers. I've seen multiple H-pipes (usually on Mustangs back in the early 90's - I think Flowmaster put out some Mustang specific info back then,, may still do) and while I haven't tested that myself, makes sense. I like the idea of an X pipe,, but have yet to actually try one. I've got some ideas working in my head that I'll be trying soon and will post after I get "real world" - in car results.

Anyway,, performace will vary depending on what your combination needs. An H pipe on my 3700 lb 85 SS with 3.5" dual Borlas cut it from a three run average of 11.43 to a three run average of 11.39 (Vericom times). At the track the car ran 11.372 @ 121.33 off the bottle, so my Vericom is accurate in this car.

I haven't done a lot of thinking about the typical 3rd gen "Y" to single pipe,, but I like the shared pipe principle. Seems like working on enlarging the "Y" pipe and using some "anti-reversion" principle right past the "Y" pipe connection would be pretty simple and still effective. I'll do some befores and afters,, IF I do any exhaust changes to the wife's IROC.

Hope that helps.

Metalifann

Just some simple physics to help you out. Faster air= lower pressure. You'll find the lowest pressure where you have the most turbulance under your car.
As for crawling under you car with a barometer, you'll have to mount the sensor underneath the car and drive at controled speed with controlled winds to find out exactly where the best place is. just crawling up underneath the car will yield nothing because the pressure will be the same everywhere; 101.3 kPa