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#1065676 - 03/25/20 04:57 AM Exhaust question?  
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gcmonteSS Offline
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Nanaiimo British Columbia
I am new to the forum, and have a question about my 1987 Monte SS exhaust system. My car has not one, but two catalytic convertors, round, tapered at the ends, and about a foot long. The head pipes are stock, but after they enter and appear to merge at the square four bolt flange, there is a Y pipe on the other side of the flange, instead of a single cat, with the right side cat immediately behind the Y, and the left one about a foot behind it on the other pipe. All the connections are welded, and the system looks to have been on the car for awhile. I am the car's fourth owner, and suspect the second owner, who owned a muffler shop installed this set up, or perhaps Canadian spec cars had a different exhaust system? While not a true dual, I suspect it is more efficient that the single cat/pipe system. Does someone know if the head pipes, once they merge and go into the flange, continue internally as two individual pipes within the oval, or open up one big pipe? Thanks.

#1065687 - 03/25/20 09:39 PM Re: Exhaust question? [Re: gcmonteSS]  
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mmc427ss Online content
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That system on the car is one of the more unique ones I've heard about. Not sure why someone would remove the stock cat, which had a 4 bolt flange on both ends, and then bolt the rear exhaust Y pipe to the front Y pipe, then cut and weld in two bullet cats. Would have to assume a muffler shop job, they couldn't find a cheap replacement for the original SS cat so they welded in two bullets to satisfy someones emission requirements.

Here in the good old USA federal law says you can't modify a cat system from the cat forward. Ignoring that law is common practice in the majority of states. Not familiar with Canada laws.

That system in your car if it doesn't leak, meets what ever your emissions laws say, doesn't look like junk, leave it until the time come to junk it.

Try to preserve the cast iron exhaust manifolds, the Y pipe back to the 4 bolt flange, if you intend to stay stockish, no headers. There are few good choices for quality exhaust parts that fit and gain much power on the 305. From the 4 bolt flange back there are good exhaust systems available.
Bob

#1065688 - 03/25/20 09:58 PM Re: Exhaust question? [Re: gcmonteSS]  
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BadSS Offline
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Pearl, MS, 39208
What you described was owner installed and should be more efficient as long as the dual converters are higher flowing variety.

Below is a pretty good picture of the stock exhaust system.
[Linked Image]

The head pipes terminate at the beginning of the oval, "merging" into a larger single oval pipe.. That's not a bad thing as it acts a lot like "true duals" with an H-pipe. It would have been cool if they had done at least a little bit of protrusion into the oval to help with reversion, but didn't.

Would love to see a picture of your exhaust.

#1065691 - 03/26/20 01:15 AM Re: Exhaust question? [Re: gcmonteSS]  
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86ttop Offline
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The catconverter installed on the Monte Carlo SS was a monolithic one, very much free flowing and not that long ago, readily available from Walker. I replaced mine years ago with a Walker, direct fit, meaning it just bolted in place of the original one. Good luck with your car!!

Very nice underbelly there BADSS!!

Last edited by 86ttop; 03/26/20 01:16 AM.

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#1065692 - 03/26/20 01:52 AM Re: Exhaust question? [Re: 86ttop]  
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BadSS Offline
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Originally Posted by 86ttop
Very nice underbelly there BADSS!!


Should have made it more clear that was a photo I found on the net - but it does look nice for sure. Mine only looked like that for the 12 month warranty period - lol.

#1065714 - 03/27/20 06:06 AM Re: Exhaust question? [Re: gcmonteSS]  
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gcmonteSS Offline
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That photo was the one I saw recently online, of a Monte for sale, which indicated my system was modified. On my car, the original cat and the second square flange/bracket behind the stock cat are gone. The second non-stock Y pipe is welded to what was the original front cat flange, and splits immediately upon exiting the wide section behind the front Y pipe. So only about 4 inches of the system isn't true dual. I will probably remove the two bullet cats and replace them with straight pipe, as I have no worries about testing here. At least for now I don't have to change or mod the support to accommodate a pipe of the driver's side. The only engine mods would likely be a mild cam (Comp Cam 256 or 262), new valve springs and seals, and new steel timing gears and chain,so current exhaust will be adequate. Headers possible sometime later, which would then mean a proper true dual setup. The car only has 93000 km on it or 56K miles, so the original engine is strong, and the trans and diff have both just been rebuilt and beefed up.

#1065715 - 03/27/20 01:21 PM Re: Exhaust question? [Re: gcmonteSS]  
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BadSS Offline
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Pearl, MS, 39208
Long tubes, x-pipe and true 2.5" duals would help a lot. Stick with a good muffler like the MagnaFlow or Borla XS.

As far as cams, back in the mid to late 80s I probably installed 15 different cams in 305HO engines. By far the best overall cam back then with the stock stall speed was a high-efficiency Lunati 06107 which was a 255/265 205/215 .427/.453 on a 112 spread. Probably the next best overall back then was Comp's HE260 212/212 on a 110 spread. Cams with overlaps in this range were easy to tune and pulled plenty of vacuum to keep the computer "happy". I installed a number of larger cams and ended up refusing to install anything bigger than the Comp HE268 218/218 on a 110 unless they had or wanted me to install a higher speed torque converter. The HE268 (and slightly large/more overlap) in a 305 was a little tricky to tune with the CCC, but doable.

Years have past and while my favorites were relatively aggressive cams for the day,,, there are better grinds now. One of the newest cams I've put in a 305 was the Comp Xtreme 4x4 254/ 262 210/218 .447/.462 on a 111 spread. I'd have to say,, it bumped itself to the number one overall spot for a 305 without headers.

Good to see you're budgeting for better springs and timing chain - even with 56K miles on the car it's good insurance to change them while you're doing the cam.

HTH


Original owner of 1985 SS black hardtop w/gray interior. Frame-on restoration - 406 sleeper w/ heavily ported FIRST TPI. However the 1.875" headers, 3.5" duals, TH400, Ford 9", notched frame, and 10-point cage w/swing-out bars might tip you off it's not a lil' ole 305 TPI. Other mods: 2" drop, 18" wheels, f/r disk brakes, and custom stereo system.
#1065719 - 03/27/20 06:23 PM Re: Exhaust question? [Re: BadSS]  
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Richie Cat Offline
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Originally Posted by BadSS
Long tubes, x-pipe and true 2.5" duals would help a lot.


Why do you favore the X pipe ?
Just curious.
Thanks.


83 SC, 355 w/TPI
#1065724 - 03/27/20 06:40 PM Re: Exhaust question? [Re: gcmonteSS]  
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gcmonteSS Offline
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Nanaiimo British Columbia
Thanks for the cam suggestion, BadSS. I recently had to replace the mid to rear exhaust system, so will e sticking with the 21/4" pipes and Hush Thrush mufflers for the time being. While the mufflers aren't the most efficient, they strike a good balance between retaining the V8 sound and not droning on the highway. I'm just shy of 70, so the car is primarily a cruiser, and won't be seeing a dragstrip, but I still like to walk on it from time to time, which is why the transmission was rebuilt with a Trans Go stage 2 kit and other internal mods, as well as replacing the Auburn diff (bad spider gears) with an Eaton unit with carbon fiber clutches, The ring and pinion were still fine, so just replaced the carrier.

#1065725 - 03/27/20 07:23 PM Re: Exhaust question? [Re: Richie Cat]  
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BadSS Offline
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Originally Posted by Richie Cat
Originally Posted by BadSS
Long tubes, x-pipe and true 2.5" duals would help a lot.


Why do you favore the X pipe ?
Just curious.
Thanks.


I should have said X or H-pipe. Either is an advantage over none at all. H-pipes typically give more of an old time muscle car rumble while the X-pipes generate a higher pitch, especially at upper RPMs. Both give gains throughout the RPM band but H-pipes tend to provide a little more low end and X-pipes tend to add a little more top-end when compared. It really boils down to personal preference.

Where space permits (not enough room under a G-body), I prefer doing a long "terminator box" where the two head pipes join into one big empty box/muffler and two pipes exit it and goes into two mufflers near the rear of the car (I did this on my G8 GT). This is fairly easy to do on the newer Corvettes, G8 GTs (like mine), the Chevy SS (same as the G8) and late model Camaros.


Original owner of 1985 SS black hardtop w/gray interior. Frame-on restoration - 406 sleeper w/ heavily ported FIRST TPI. However the 1.875" headers, 3.5" duals, TH400, Ford 9", notched frame, and 10-point cage w/swing-out bars might tip you off it's not a lil' ole 305 TPI. Other mods: 2" drop, 18" wheels, f/r disk brakes, and custom stereo system.
#1065727 - 03/27/20 07:32 PM Re: Exhaust question? [Re: gcmonteSS]  
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BadSS Offline
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Pearl, MS, 39208
Originally Posted by gcmonteSS
Thanks for the cam suggestion, BadSS. I recently had to replace the mid to rear exhaust system, so will e sticking with the 21/4" pipes and Hush Thrush mufflers for the time being. While the mufflers aren't the most efficient, they strike a good balance between retaining the V8 sound and not droning on the highway. I'm just shy of 70, so the car is primarily a cruiser, and won't be seeing a dragstrip, but I still like to walk on it from time to time, which is why the transmission was rebuilt with a Trans Go stage 2 kit and other internal mods, as well as replacing the Auburn diff (bad spider gears) with an Eaton unit with carbon fiber clutches, The ring and pinion were still fine, so just replaced the carrier.


2.25" and those mufflers are efficient enough for a stock-ish with mild cam SS. There really isn't a "need" for you to go with a 2.5" system with MagnaFlows or Borlas - if more or less starting from scratch they wouldn't have cost much more than a 2.25" system and would give room to grow for future mods. Sounds like you're good to go though!!


Original owner of 1985 SS black hardtop w/gray interior. Frame-on restoration - 406 sleeper w/ heavily ported FIRST TPI. However the 1.875" headers, 3.5" duals, TH400, Ford 9", notched frame, and 10-point cage w/swing-out bars might tip you off it's not a lil' ole 305 TPI. Other mods: 2" drop, 18" wheels, f/r disk brakes, and custom stereo system.

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