How large of a Muffler body will fit 67 GTO convertible
 

If this has already been asked I apologize-
I am running HO manifolds with down pipe to X to mufflers. How Large of a muffler will fit on a 2.5" system. I am looking at 18" muffler oval body with total length of 24" with offset 2.5" inlet and offset 2.5" outlet - reversible.
I have heard the larger the body - the better.
Let me know.
Thanks
Mark

I use a 20" oval muffler body (2-1/2" offset/offset) on my '64 A-body cars (same chassis/floor pan from '64 -'67) so an 18" muffler is a good fit.

B-man - Thank you - It is nice to know I can use as large as a 20" muffler. I am using a straight thru design (no chambers) to eliminate back pressure and to not lose performance. I want to use as large as possible to keep drone and excessive volume down as it is a convertible.
Considering one of these three based on muffler performance tests that I have read.

Dynomax ULTRA FLO™ WELDED - OFFSET / OFFSET
17749

Xlerator Performance Muffler Stainless Steel 2.5" O/O 4x9 Oval - XS1266

Jones MF1266

I was set on the Race Pros until I have received conflicting testimonials of unwanted drone and excessive noise. Some people really like the Race Pros, some don't like the at all. I believe the smaller 14" oval case will not dampen the volume enough.

What is the muffler you use that gives you a 20" case?

I thought I would ask here because when I research the larger mufflers the manufacturers claim they are not compatible with the 67 GTO.

Comments welcome.Thanks for your advice.

Mark

Dynomax 17749 is what I use.

Super Turbo, not Ultra Flo.

Subscribed, same car with same manifolds. Not ready for exhaust system quite yet but close. Body is off chassis right now so now is the time.

I would wait until the body is on the chassis with the rear bumper mounted. You need to start installing the system from the rear working forward to fit it right.

You also need the floor pans there so you can tuck the mufflers up into the right place and fit the pipes close to the floor pans.

Thanks for the advice b-man. I also thought it would be better to fit the exhaust with body off as I plan on welding all of the joints and it is much harder lying on my back.
Is there any way to fit it with the body off and get it right?
Right now my body is just sitting on the frame and I can easily lift and set back down. Deck lid is on but rear bumper is not mounted.
My thought was to get the chassis, exhaust and driveline fitted before mounting the body.

I have the 2.5" X pipe exhaust system from the HO manifolds to rear bumper - minus the mufflers. I know the muffler choice is critical for performance and comfort in driving.

I plan on having the RARE HO 2.5" exhaust manifolds ceramic coated inside and out by a local shop. My research tells me that they will maintain their finish longer and it will create for less heat under the hood.

What is the difference between the Super turbo and Ultra Flo on the Dynomax 1774 Muffler?

MagnaFlow #11266 Satin Stainless or #14366 Polished Stainless - These are 4" x 9" body and 18" long.

I wanted straight-thru design, and no drone, so I used their largest body muffler. It's 5" x 11" and 22" long. I'm using 3" offset/offset. Unfortunately, I haven't fired the motor yet, so can't comment on sound.

MagnaFlow #12577 Satin Stainless and #14577 Polished Stainless - These are 5" x 11" and 22" long.

https://www.magnaflow.com/products?partnumber=12577

They fit fine under my '65 chassis.

http://forums.maxperformanceinc.com/...ictureid=12933
http://forums.maxperformanceinc.com/...ictureid=12934
http://forums.maxperformanceinc.com/...ictureid=12936

A couple more comments...

Unless you are going to have tailpipes custom bent, you will want to decide which tailpipe you want to use and then order the muffler orientation to match. Some of the kits bend the tailpipe for a center outlet muffler. Some also only fit if the two mufflers are splayed to the outside at the rear slightly and not parallel to each other. Some also exit behind the tires (3" especially) and others run all the way to the bumper. I ultimately decided to use the Torque Technologies 3" tailpipes because they seemed to match the system I had pieced together from different manufacturers the closest. Even still, the exit angle of the final section is not parallel to the frame and I will have to do some tweaking to get the fit I want.

B-man is right that if you start at the back and work forward, you will likely get a better fit, but since I used such a mixture of brands, I assumed I would have to modify as I went, and started at the front. You can get the muffler to floorpan clearance right with the body off if you're careful with your measuring.

This is the process I'm planning as My body is still on the old rusty frame and I can take measurements as I need.

I'm one of the RacePro haters. I still have them on my Cutlass, but mainly because it is not yet ready to be driven on the road. I also just finished having a 3" system installed on my '73 T/A clone, and used the Borla ProXS mufflers (replacing 40 series Flowmasters).

I don't mind the RP's at idle. According to my ear, the RP's were quieter at idle than the FM's were, even though the Cutlass had significantly more HP.

I downloaded a dB "app" on my phone, I know those are not all that accurate, but it SOMEWHAT able to determine if something is louder than something else. I did not get that app until after the FM's were already removed, so no "measurement" of them.

I measured the Cutlass and the 'Bird at idle, and the dB's are basically the same. I also measured them at 2000rpm (at the back of each car), and again the sound level was virtually the same.

So the quantity of sound is basically equal, but the quality of the sound is very different. The RP's get raspier as more throttle is given, very raspy at WOT. The Borlas have a very smooth even tone, no "rasp" at all. To my ear, the Borla are MUCH more pleasant.

I try to get a video in the near future. I had the 'bird strapped the chassis dyno the other day and was going to record then, but the PC running the dyno software died... so maybe next week I'll get that done.

Thanks guys for your comments and advice. What I am understanding is that the straight through mufflers are the way to go for performance with the largest possible case that will fit to minimize volume and drone.- no performance loss due to back pressure with straight through configuration.

FWIW I dropped 12db at idle by going from the RacePros to Dynomax 17743s... http://www.dynomax.com/mufflers/supe...set-17879.html

Thanks all for your advice- I am all ears and I have been doing extensive research in an effort to only buy once.
I believe I will purchase the Dynomax 17749 with a 20" body - overall length is 24"- It should lessen the sound and drone even more than the Dynomax 17743 which has a smaller 14" body. I am even considering the magnaflows that are 22"but I don't think they make it in a 2.5" pipe.