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1965 GTO Hardtop 389 tri-power (WS), close ratio M21, 3.90 posi rear (ZK)
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Discussion Starter · #1 ·
I'm trying to select good (low end throat) sounding mufflers for my '65 GTO. It is a 389 tri-power car with close ratio M21 and 3.90 posi-rear. When I bought it, it had two glass packs (cherry bombs), which didn't sound bad - but I obviously want to find something better. The rebuild will be mostly back to original (Crane "blueprint series" cam, with very close to original lift, duration, overlap). Again, I want the low end throat and as much rumble as I might muster from the original set-up, but I'm looking for anything wild (I guess that will be limited by my choice of original engine specs). Suggestions?
 

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I'm trying to select good (low end throat) sounding mufflers for my '65 GTO. It is a 389 tri-power car with close ratio M21 and 3.90 posi-rear. When I bought it, it had two glass packs (cherry bombs), which didn't sound bad - but I obviously want to find something better. The rebuild will be mostly back to original (Crane "blueprint series" cam, with very close to original lift, duration, overlap). Again, I want the low end throat and as much rumble as I might muster from the original set-up, but I'm looking for anything wild (I guess that will be limited by my choice of original engine specs). Suggestions?
Here is something wild, chambered pipes. You can listen to a couple video's on their site as well. Never tried/had these, but they do sound good and have my attention when it comes time for some mufflers.

 

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I had great luck with Laminar HP-2’s,on my ‘64 Tri-pwr , I was back after that low-end Cherry old school-floor-boom sound , only with a newer greater improvedVortex-scavenging thru design, I got the deep-boom sound back ,no rasp, with nice exit flow breathability, for the Tri-389 engine, Avail in clean 304SS long and short versions also,

here’s an info Video on them (the HP is about Halfway thru the vid) and a few other design comparisons as well,
Also a pic below from when the installing of the two 18”black 409SS HPs was underway in mine
Font Parallel Rectangle Circle Diagram Cylinder Font Auto part Engineering Titanium Automotive tire Vehicle Hood Motor vehicle Automotive lighting
 

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1967 400/400
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Pay very close attention to the intermediate pipes (coming from either your headers or stock manifolds, to the muffler). My car has a modified 400 and 2.5 inch duals, but Pypes insisted I install and X pipe for the intermediates, so I did.

With their Race Pro, straight through mufflers (the 2nd loudest that they have), the car was WAY too quiet. I then put Cherry Bombs on it, still WAY too quiet. Then 2-chamber Flowmasters, and it's still too quiet.

I'm not talking that it's not insane loud, I'm saying that it's not loud at all! The Xpipe will make it sound like a Ferrari. It'll have a throaty, aggressive, rumble, but it will not sound like a GTO or a 396 Chevelle... It'll sound like a 2021 Corvette with stock mufflers.
 

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‘65 GTO, 400, 4spd, Tri-Power
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On my '65 GTO, 400, Tri-power, Im running Hooker headers (just barely fit), 2 1/2" pipes through Flowmaster 50 Series Delta flow mufflers. Its pretty loud, on the verge of being too loud, but it gets plenty of attention. Nice classic rumble at idle, lots of growl, and when those outer carbs kick in, it sounds pretty badass.
 

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1967 400/400
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On my '65 GTO, 400, Tri-power, Im running Hooker headers (just barely fit), 2 1/2" pipes through Flowmaster 50 Series Delta flow mufflers. Its pretty loud, on the verge of being too loud, but it gets plenty of attention. Nice classic rumble at idle, lots of growl, and when those outer carbs kick in, it sounds pretty badass.
Xpipe, Hpipe, or duals?
 

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‘65 GTO, 400, 4spd, Tri-Power
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Xpipe, Hpipe, or duals?
Hmm, I wonder if I may have just answered my question in the other thread re poor vacuum/stalling out at hard braking? Will a cross pipe on the exhaust help to create more vacuum? My vaccum gauge is supposed to show up tomorrow, so should have more info soon.
 

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1967 400/400
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straight back, through duals. I dont run any cross pipe, but I do get a lot of pops/sputters off throttle on decelleration, so wondering if that might help. Kinda like it though ;)
As I said, prepare to lose that GTO sound, if you add the X. My car sounds great, out back at idle and when I get on it, but inside and when cruising through the car show, its so quiet!!!!
 

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1967 400/400
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Hmm, I wonder if I may have just answered my question in the other thread re poor vacuum/stalling out at hard braking? Will a cross pipe on the exhaust help to create more vacuum? My vaccum gauge is supposed to show up tomorrow, so should have more info soon.
I suspect that you have a vacuum leak. This wasn't an issue before, so it has to be something that you changed. We'll know tomorrow
 

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‘65 GTO, 400, 4spd, Tri-Power
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I suspect that you have a vacuum leak. This wasn't an issue before, so it has to be something that you changed. We'll know tomorrow
I think so too. Maybe the tri-power is also not creating as much vaccum as the 4 barrell when its running just on the center carb/off throttle? Heres a good video I found explaining exhaust scavanging, and the pros/cons between H and X pipes...the comments also seem to back up what you are saying about sound, X pipe is higher pitched/quieter, H pipe has more rumble, straight pipes sounding closer to that stock GTO sound.

Best comment, lol! "sure if you want your American muscle car to sound like eurotrash I guess the x pipe does sound better to you. " :LOL:


Heres a short video of my car at idle after the first test run after the tri-power swap. Carbs are not tuned yet, idle is set way too high, and you might notice some steam starting to come out of the rad cap. I didnt notice that until the end of the video, when I quickly shut the car off just before the cap exploded. Luckily I was out of the way :)...

 

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I don't know, but I am running stock exhaust with Flowmaster mufflers on my 64 tripower and I think it sounds great.
 

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1967 400/400
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I think so too. Maybe the tri-power is also not creating as much vaccum as the 4 barrell when its running just on the center carb/off throttle? Heres a good video I found explaining exhaust scavanging, and the pros/cons between H and X pipes...the comments also seem to back up what you are saying about sound, X pipe is higher pitched/quieter, H pipe has more rumble, straight pipes sounding closer to that stock GTO sound.

Best comment, lol! "sure if you want your American muscle car to sound like eurotrash I guess the x pipe does sound better to you. " :LOL:


Heres a short video of my car at idle after the first test run after the tri-power swap. Carbs are not tuned yet, idle is set way too high, and you might notice some steam starting to come out of the rad cap. I didnt notice that until the end of the video, when I quickly shut the car off just before the cap exploded. Luckily I was out of the way :)...

Well, I don't want to hijack this thread, but if I understand you correct, you're just running the center carb on a six pack? Hit me up in the stalling thrread
 

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1965 GTO Hardtop 389 tri-power (WS), close ratio M21, 3.90 posi rear (ZK)
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41 Posts
Discussion Starter · #14 ·
A friend recommended Flowmaster 44 series. Any thoughts? Also, I've heard that pipes were different diameter on 4bbl cars vs. 3x2 set-up. Like 2" vs. 2.5", or difference in pipe diameters before and after mufflers (?). I'll have to look into this. I'm trying to stay "mostly" original, but I want it to sound good (not "wild", just nice throat & rumble).
 
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