I have a 1969 Camaro convertible with Doug Thorley 1 5/8 inch shorty headers with 2 1/2 inch mandrel bent stainless exhaust with H Pipe and Flowmaster Series 50 mufflers on my SB 383 motor. The engine is a crate motor with AFR heads that are port matched to the Edelbrock Victor Jr. that was dynoed at the Chevrolet dealer at 505 HP. However, I just spent the day at Westech here in SO CAL watching the exhaust system choke my rear wheel horsepower with 3.5 PSI of back pressure. I am looking for mufflers that will allow me to run with near zero back pressure, but keep me within the law. Any suggestions on mufflers that will allow my motor to realize the horsepower at the rear wheels?

Thanks,

Jeff

Magnaflows, thats what I use. Are you dead set against 3" exhaust? I think that might be part of your choke.
What size (diameter) collectors on your headers?

Mufflers are way over rated as power producing devices in your exhaust system. The real power is determined by the six feet of tubing closest to your cylinder heads.

I run dual Dynomax UltraFlo SS 17272, they are completely straight thru, you can see from end to end with no obstructions. Rated for >1000cfm flow
Bought them from RockAuto.com with 5% discount code, cheapest route. Definitely quiet enough to have a conversation, no resonating, tons of flow. Windows Media Video of my exhaust is here:
http://69camaro.nextmill.net/photos/camaro1.wmv

http://69camaro.nextmill.net/photos/exhaust/ultraflo_ss_17272_web.jpg

It is a convertible which makes it tough, and I have the car lowered which requires the shorty 1 5/8 header with the 2 1/2 collector which is going directly into the 2 1/2 pipes.

I have the Hotchkis X Plate which allows the larger pipes, but I am trying to not replace the whole exhaust, only the mufflers if possible?

Thanks,

Jeff

Thanks for the info. on the mufflers. I am going to install cutouts before the mufflers and run another dyno at Westech as soon as I get it back from Bowtie Overdrives as I smoked the 700R4 transmission.

Cruizer, Could you post a copy of the dyno sheet? You could also send it via pm.

Would be happy to post my dyno sheet, but not to sure how to do so??

Jeff

See if this helps.

The thumbnails you are used to seeing are most likely attachments posted by Team Members. The program lets them upload directly from their computer and creates the thumbnails automatically and inserts them into the post.

You can create thumbnails, but you need the thumbnail and and image both on the internet "somewhere".

You click this little guy , type in (or copy) the address to the thumbnail and then highlight it with your mouse and click and by entering the address to the larger picture it becomes a link.

I'd suggest not bothering with the thumbnail and just posting a direct link to the picture. Just copy/paste the web address to the picture and the forum software will make it a clickable link.

hope that explains it a bit more.

You can scan and copy to a disk,, then go to photobucket.com, set up a free account, then include it into your signature. Make sense? Lets see if someone else has an easier way.

There are quite a few guys on this site running 406's with 2.5" exhaust. Greg O is running bottom 11's with 2.5" h pipe and 30 series flows. I doubt a mandrel bent 2.5" system is hurting you power much at all.

Once you get a digital copy of the dyno sheet by scanning it, see the link in my sig on how to post pictures.

i was planning on taking my car to westech. what did the dyno day cost you?

Notice the modern, straight thru "glasspack" type mufflers here..

I figure a couple of short 2.5"er's like as shown right under the back seats where the stock resonators fit and then stack two real W/D/T, smooth perforated inner pipe glasspacks, one on top of the other where the stock transverse muffler fits and you should be good to go AND still have a 100 percent straight-thru exhaust system.. Just run the ends out stock

You probably can't install any more glasspack b/c I figure your Convert bottom brace is in the way or I'd suggest installling a couple in front of the short, resonator glasspacks....

And sorry to hear about you smoking the 700, probably time to step up to a built 200 jobber and be done with it.

Good luck with everything.

pdq67

Magnaflows or Dynomax turbos would work ok at 2 1/2", but a 3"
system would be better at a legit 500+hp engine.

Try to do 3" from the headers to a 3" X-pipe then 2 1/2" will be ok to some 2 1/2" muffs.

exhaust system choke my rear wheel horsepower with 3.5 PSI of back pressure.
Jeff

How much power at what rpm did it make?

Is 3.5 psi way to much back presure?

What is a reasonable amount of back presure?

How was the back presure measured? At what rpm?

When mine made 712 corrected rwhp, 676 raw (on Westech's dyno) it also had 1 5/8" headers and a 2.5" exhaust sys. They were long tubes with an X pipe and dumped at the axle (Flow 40's).
I later went to a 1 3/4" with 3" sys ,and picked up 2 mph in the 1/4.
Open header, with a 12" long, 3" collector extension, and 2#'s more boost, picked up 3 more mph. My car was also loosing weight (about 200#) during this time, so I really wonder how much difference the larger pipes make?


http://i12.photobucket.com/albums/a224/sc68z28/dyno_1.jpg

---Bill.

If you want no back pressure or very little,I ran Short 'Glasspack" for years on my camaro.Dualed all the way out its not that bad really and wasnt loud in the car or cruising at all.When you got on it now,it would come alive...but isnt that the point?

Just a follow up question and maybe a dumb one,but I noticed your profile says you're in Ca. Do you have to run converters on a car this old? I assume not but I know Ca. laws are quite different.

The Magnaflows and ultraflows (and others) are basically modern glasspacks, without the obnoxious snap crackle and pop (sorry PDQ ;) ). They are straight through design as well.
I agree with RS502, 2 1/2" mandrel pipes will work, but at a legit 500+ horse they are becoming a restriction IMO, especially if full length out the back.
No matter what you decide to do, don't underestimate the exhaust, I went from 11.70 @ 114.2 to 11.41 @ 116.5, and picked up over half a tenth in my 60' time, with no other changes but exhaust, I went from a full length 2 1/2" (not mandrel bent but decent bends still) with flowmasters and a crossover pipe, to 3" magnaflows a 3" Dr. Gas X, and dumped at the axle, all mandrel bends. Funniest thing is it's barely louder unless I'm really on it, and it sounds 100% better IMO.
Granted, the mandrel bent 2 1/2 is better, and would have helped mine out, but that likely would only account from some of my gain.

Do you have to run converters on a car this old? I assume not but I know Ca. laws are quite different.

No cats. 1972 maybe 73, 74 and earlier are not checked. Sort of smog exempt. We had a floating 30 year and older deal. Last I heard, that got suspended a while back. ---Bill.

Yes, I think Ahnaold doing a job on you Cali. guys!!

pdq67