So all the research and dialouge have boiled down to these conclusions.
To achieve the MAXIMUM benefit from LT headers then a system with adequeate flow AND proper length secondaries must be addressed. The plan is make the engine react as if the headers are open.
Secondary length is determined by the torque characteristics required for a given application. It is arguably the most important aspect of LT headers considering that primary diameter is fixed once you've bolted the headers on. Large diameter, short collectors favour top end. Small diameter, long collectors favour bottom end. A typically race cammed SBC that revs to 7500rpm can use a secondary of about 18 to 20". A heavy car with a tight convertor can use a secondary of 40" or more.
Flow can be based on the 2.2cfm/horse power rule. This applies to the diameter of the exhaust and the muffler(s).
(These facts are roughly quoted from David Vizard's SBC books).
If you run any kind of Y-pipe then the LT header effect, that is the tuned RPM scavenging, will be lost.
Open case mufflers will appear to the engine as open atmosphere. Mufflers like these attached to a properly spec'd secondary length will react like an open header.
Glass pack mufflers behave like an extension of the collectors and contribute to it's overall length.
3rd gen cars, particularly lowered ones, have little room to allow the mufflers to be placed close to the headers (40"+/- depending on the muffler type) so that option is not available.
Drag cars that typically sit up a little higher can take advantage of mid vehicle mounted mufflers.
Cut-outs at the collector and the ability to add extentions to achieve the desired tuned length may be the best way.
The attached photo is one of the cleanest installs I've seen. While you're giving up on the benefits of the LT headers with the full exhaust attached, once you uncork the headers (and you should block off the rest of the system) you have an open header. At this point you would need to provide some means of extending the secondary beyond the cut out as in this case it appears too short to provide the tuning boost down low in the RPM range.

 

Thanks for the info. I have been doing my reserch on a good exhaust system for a 3rd GEN, and I agree that is a very nice system.

 

I thought that someone might beg to differ on those points.
I guess the facts don't lie.
Any experiences to the contrary?

 

I suppose this is a new way of thinking about the short vs long debate. Typically I always thought of long being better simply because of the longer primaries and longer collector. But Y-pipes would negate those benefits. By how much, I don't know.

I don't know of anybody who has dynoed short with Y-pipe and then long with Y-pipe. There are quite a few high horsepower engines operating quite well with short headers and good 3"+ rear systems though. So I suspect we're probably not leaving as much power on the table by going with short headers after all.

 

The primary length of the header system , as I understand it, is responsible for establishing at what point the exhaust is most effecient or where the torque peak will be. Short pipes favour the top end (like a dragsters zoomies) and longer pipes will move the torque peak down the RPM scale.
I would expect that dyno comparisons between short and long headers WITH y-pipes would continue this trend.
What would be interesting to see is the results of testing between short and long tube designs with various collector/secondary lengths. We know that collector length will 'rock' the torque curve about the peak. Short secondaries build more torque above the peak and vice versa. I'm guessing it also holds true for shorty style headers.
Still, once you add the y-pipe the real benefit is gone. There are a lot of engines making tons of horsepower without completely effecient exhaust systems like you mentioned. It's just that I can't help thinking about how much R&D goes into a top flight Nascar engine exhaust and the Engine Masters guys put a lot a effort into their headers WITH mufflers.There's power there to be sure.
I think that on a 350hp small block with crappy exhaust (like my Hedman shorties, 2.25 y-pipe and a single cat) that there is another 10% or so waiting to be tapped. ( I hope more!)
I run 102 in the quarter now (13.4). This season I'm hoping to upgrade to either true duals with the mufflers in the choicest place (unlikely) or a cut out system and properly spec'd secondaries for track days.
I'm hoping for a couple of tenths.

 

the only experience i have with muffler flow comparisons on my L98 was between flowmaster 80 series catback and my new powerstick chambered tube catback. i realy didnt notice much if anything at the track between the systems. if anything my 1/8 mile mph went down alittle and top end stayed about the same.

so it would seem the increased flow of a more straight pipe lost low end torque and kept strong top end torque/hp, which is what you'd expect from all the exhaust theories.

i dont know the differences between y pipe and true duals tho. I have only seen 1 dyno result. I searched extensively on camaroz28.com which is a big site for LT1 crowd. mufflex 4inch system was dyno'd against a true dual 2.5 inch system. true dual made like 8-10whp more and lost some power in the 3500-4500 rpm range. it did suprisingly pickup some power below 3500 rpms. I cant find the thread at the moment tho