Before reading please take the following planned engine modifications in to consideration-
L03, Caprice injectors, custom tuned chip, LT1 cam, Compcam valvesprings, Edelbrock Performer RPM intake, ported stock "swirl-port" heads, cat delete, A/C delete, smog delete


Right now I have a set of Hedman 1 5/8" x 3" headers and I already know I'm going to dump them by the door(s). What I can't decide is how to dump them; I could:

1. Buy collector adapters, replace all the stock crinkle bends with mandrel bends, attach a single 2.5" Thrush glasspack(perforated tube) with an adapter

2. Buy the Hedman Y-pipe (3" inlet, 2.5" outlet) and attach a single 2.5" Thrush glasspack, exiting under the passenger door

3. Fab my own downpipes from 3" tubing and exit under both doors with Purple Hornies (3" flanged inlet, 2.25" outlet, spiral louvered)

4. Same as above, just with 2.5" downpipes to 2.5" Thrush glasspacks

5. Same as 4, just with 2.25" Thrush glasspack

6. Buy the Hedman Y-pipe, replace 2.5" tube with 3" tube, run through single flanged Purple Horny (3" inlet, 3"outlet, spiral louvered)

I figure that no matter what it's going to cost me around $150 for the whole setup, with the exception of solution #1, so price isn't too much of a concern. I'm simply not sure how much room will be too much (low exhaust velocity= loss of low end tq, of course), especially since this will mainly be street-driven. I know that I need approximately 2.2cfm flow per hp and have referenced this chart a few times http://www.ls1tech.com/forums/genera...-mufflers.html , but I am not sure what the numbers would be like without all the restriction of a standard-length tube(or if this was simply done with sections on a bench). To me, options 3, 4 and 6 look the best.. and in order of cost from lowest to highest: 4, 3, 6.

What do you guys think?

 

My 84 has exactly the same set up as #2 except that it was a custom made y-pipe and it seemed to flow pretty well. It dumped right at the end of the stock location of the cat.

 

i had dumped true dual glasspacks on my caprice i lost alot of low end torque and mpgs but it did sound good.

 

That's exactly what I was worried about it. Between your response and 91camaro, option number 2 just started looking like a winner

 

Then again i had a purple hornie on my c1500 single exhaust and it didnt lose anything sounded pretty damn good i have a couple of exhaust vids on my youtube channel youtube.com/youngerprice the cherry bomb extreme is the most recent check out the 305 c1500 to that thing has straights on it with headers and some other goodies.Im now running true dual thrush welds these mufflers sound x10 times better then the glasspacks plus it has way more low end torque i loved glasspacks until i putted them on this car i guess it have been ok with tailpipes i probably wouldnt have lost as much torque.Bottem is a glasspack is a performance muffler if your putting them on a daily driver you wont like them.You wont like putting gas it ever day either.

 

Actually for the most part, glasspacks flow worse than performance mufflers. The Hooker Max Flow is definitely one of the best flowing mufflers, but for my purposes it would be truly overkill. The dual thrush welds probably give you more low-end torque because they are probably more restrictive than the dual glasspacks (the extra pressure causes the exhaust to flow faster in order to get the same amount of gases out). Due to the louvers in the Purple Hornies and their spiraled pattern (which allegedly increases flow speed- probably is more due to the factor I mentioned before), you didn't lose anything.

Lots of physics behind this stuff- I'd break it all down and solve the equations for flow if I weren't already super busy with school.

 

I normally dump in the bathroom..

 

Increasing exhaust flow speed does increase low end torque provided itw within reason, but only when you have open pipes. Baffles that block the flow just rob power. For a while, I had a 2.25" 4-cyl. exhaust with replacement edelbrock headers for the TBI cars on my car way back when, and it did make a lot of low end torque, and was whisper quiet. The low end torque was due to the high flow speed in the exhaust increasing the scavanging ability of the headers.

This is not a performance option, though. The effect only exists over a narrow RPM band. Once the flow speed gets beyond a certain point, you begin to loose a lot of power from the frictional losses.

With a motor like this, the best option would be to get (or fab) a full length exhaust. Open headers will just hurt the low end where the motor performs best, and make excessive amounts of noise. There are some cheap 2.5" and 3" mandrel systems out there. The ubiquitous flowmaster force II is the cheapest one available, and has a 3" I-pipe.

 

You make complete sense until I get to your third section. Scientifically, it doesn't work out at all.
Headers dumped anywhere before the axle won't affect performance negatively if they are properly sized to permit the right amount of exhaust flow and speed. A 3" mandrel bent cat-back would probably do the same thing (or worse) as a 2.5" Y-pipe due to the extra friction created in the exhaust tube from all the bends. Sure, the friction is negated by the increased bore size, but then the exhaust gasses take up more room, decreasing the scavenging ability of the overall exhaust system.

As I'm thinking about it now, probably the best-flowing option for my dumped exhaust would be a custom system like so:

2.5" pipes coming off the headers down to an X (not Y) and exiting out of 2.25" pipes w/perforated glasspacks under each door

 

The scavenging is a function of the momentum of the exhaust pulses exiting the headers, and traveling into the exhaust system. When the exhaust exits, the momentum of the pulses continues to try and pull out any remaining gasses and air that slips by the intake valve if theres overlap due to the negative pressure generated. This also allows the engine to recoup some of the lost work from expelling the exhaust gasses. With a stock engine and no overlap, this is where the gains mainly come from.


If your running equal length long-tubes, you can dump them as the primaries will do most of the work for you. If your doing shorties, then you should run at least a partial exhaust.

Ive measured the backpressure with shorties, 3" force II catback, and a hooker maxflow muffler. https://www.thirdgen.org/forums/exha...kpressure.html The negative pressure from the pulses is fairly small, and above a certian RPM and load with a full length exhaust, theyre effectively nil. You can get some improvement with an open exhaust, but having the exhaust leak from hell under the floor isnt worth the few extra horsepower, at least not in my view.

 

Thats going to be really tight under the car. The smaller pipes will help buy some room, but still itll be speed-bump fodder since it has to hang under the DS and floorpan. Bringing them to a true y-pipe and going to a single in the stock location will be more effective.

Another option is to skip the x-pipe and just do 2 2.25" or 2.5" pipes down each side of the DS. This will get some more ground clearance.

 

X2. How I love to crop dust everywhere I go....spread the love!

On a stinky on topic side note, get used to smelling like exhaust if you dump anywhere in front of your seat. Mine dump just under the front seats rear mounts. I call it "Catless Maro- for men”.

 

Hahah that's great. I could use a little of that cologne...
I'm going wait around for a nice discount code from summit or streetside auto and then I'll get the Hedman pipes and hook up a Thrush glasspack to it (right behind a hollowed out Cat, just in case I get pulled over and the cop does a visual).

 

Should I get attach a turndown to the glasspack- would the exhaust coming from the pipe affect anything under the car?

 

The turndown will help direct sound more than anything

 

Then I won't bother with it. I'm not going to be driving int much until January anyway, and that's enough time to find a cheap mandrel bent turndown. I couldn't see using any of that crimped crap for any part of an exhaust system no matter how small

 

My Y-Pipe just came in from Hedman, and damn am I happy to finally have in.

Much to my surprise they included a 3" adapter so that I can install a 3" cat-back. Should I install a 3" Glasspack instead of a 2.5"?
Keep in mind, this is going to be a Vortec headed 305

Edit: Disregard- that was kind of a dumb question. Vortec power obviously means that I'll need all the flow I can get so 3" it is. Stand by for a sound clip within the next few days