Here's the deal -

1981 US 4.5 (stock) that had stock manifolds, cat and Borla s/st exhaust with RMB (no rear muffler). Loved the sound but wanted a little more power.

I just finished installing 1985 tubular manifolds, modified Y pipe with guttted cat and new O2 sensor. I also fixed some oil leaks in the cam housing caps (front and rear) and replaced the TB while I was in there. I fired her up and at first (as expected), the sound was a little louder but not too obtrusive...(borderline for my taste). I took it out for a ride and as soon as I got on it, I just hated the sound. LOUD, coarse and lots of popping on decel. The engine seemed to be running fine, but the exhuast note would mask any minor issues. So...home I went to remove the RMB and re-install the Borla rear muffler.

Now, its too quiet and the decel popping is still there but sounds out of place on a "quiet" system. Also, at a steady cruise, I'm getting a funky, subdued sound (like the sound of a steady cylinder miss).. but there is no miss.

Could this sound be resonance from the hollow cat? I believe the popping is excess fuel which suprises me since I'm still running stock fuel pressure. Any thoughts on the popping or the resonance?

For those interested, objectively, I found NO real difference in performance. If I had to say there was a differnence, the only thing I could somewhat sense was a very, very slight reduction in sub 3000 RPM pull and a little more willingness to rev after 5500 RPM. If I had to give it a number, I'd say 1-2% at best...thats it. From 3000-5500 RPM there is no perceptable difference. All in all, not enough noticeable performance change in my book but the sound change is significant for the worst in my view.

I used to love how my car sounded! It was a sweet, snarly sound that was not too mellow and not harsh at all. The occasional exhaust pop added a little character without sounding out of place or like something was wrong. Now, the sound is less exotic, more lumpy cam, drag strip, rough with the RMB and "quiet car- running rough" with the Borla rear muffler on. Also, when you shut the car down, you hear lots of "tink","tink", "tinkin" from the tubular manifolds thermal expansion/contraction.

Anyone been down this road and can you anyone offer any suggestions?

 

Performance is not something you can measure on these cars without a dyno, so get thee to a dyno. If you like what you had, go back to it, the popping is a bit of both of the following: a) air pump valve pops on deceleration and b) 928's pop on deceleration, and yes it is unburnt fuel. Normal when you open her up for breathing.

To mitigate the loudness you could instal cats, which would take away about as much power as mufflers would.

 

John,

The sound issues are mostly due to the hollow cat. This is acting as a resonating chamber now rather than a muffler. I'm guessing that you hear the "slight miss" type sound at about 1800 - 2000rpm. I have the same issues. What's been reccommended to me is to replace the cat with a straight pipe, or install another cat. I've also been told that some have even cut the cat open installed straight pipe and welded cat back together to maintain a stock look.

 

A little pop on decel is a good thing. I love the way the Turbo burbles when letting off the gas. I am hoping the RMB will add a little burble to the GTS.

I agree, get the car on a dyno. That will tell you so much more.

 

ATTENTION: This is a slight hijacking in progress!

John-
Curious about your swap to the 85 manifolds. I have a set sitting in my garage that I want to get installed. Any real trick to getting the old ones off or the new one's on? Did you fab your own y-pipe? Luckily for me, here in Michigan we don't have inspection, so I can get away with a straight pipe in leu of the cat.

 

Anthony,

My GTS has stock manifolds and Cat. The exhaust is comprised of GT(smaller than stock GTS) resonators and an RMB. The sound is glorious. The sound on deceleration is exactly what you are looking for: burbling... I get a lot of thumbs up as I pass other cars. The sound is very close to the sound of the Ford Cobras.

Sorry for the hijack.

 

Thanks for the quick replies:

Heinrich,

I appreciate your point of view and agree that a dyno is the only way to truely measure results. Perhaps I should have better defined my expression of "no real difference in performance" as "performance that one could sense by driving the car" as opposed to "measureable performance" on a dyno. Even then, I would be surprised if dyno measurable performance showed any significant ( 5% or better in my view) over stock numbers (although I didn't do a baseline dyno run before so I guess it would be a mute point).

Hupp,

I think I get the sound over a broader RPM range but I haven't driven it enough to say for sure. I read all the posts about this and was going to weld in a straight pipe until I read a post where someone did the straight pipe ( inside a hollow cat for exhaust velocity reasons) and actually found his perfromance was worst than the hollow cat. There was no mention of sound improvement so I opted to leave the cat hollow... mistake on my part (I think).

Cobalt,

Yes, before this mod, the pop and burble sounded good and were appropriate. Now, with the quiet (muffled ) system, the pop doesn't sound right. Like an ACDC guitar riff in the middle of a christmas tune... it doesn't fit.

Andrew,

I guess everything is relative. I have a well equiped shop (no lift) and it took me some time. I would not attempt this without an Oxy/Acetalyne outfit. Getting the old manifolds off was the hardest part. I only broke 1 stud (before I began using the torch) but my car had 2 others that were already broken and were a bear to get out of the head. The drivers side was harder than the passenger side due to space. I already had the front of the engine apart which was nice becuase the P/S pump and alternator were not in the way. ( I'm not sure this can be done with them in place). Oddly, my new/used manifolds needed to have some holes enlarged on the head flanges or the studs wouldn't line up (very close, but some holes were smaller than others on the manifold, I made them all the same). On the passenger side, near the motor mount, there is a bracket that goes from the frame crossmember to the fender well that must be trimmed or it could come into contact with the tubular manifolds accordian thermal expansion ridges when the engine moves under hard acceleration. There was about 1/16-1/8" clearance at rest without the mod but I didn't want to chance having it hit and make noise or ruin the manifold so I just trimmed about 1/4" to be sure). And finally, the Y pipe does need to be modified. The passenger side leg of the "Y" needs to be extended by about 2". I purchased some 2-1/8" OD 304 stainless tube and sectioned it in ( another pain in the butt because where and how you cut is critical due to compound curves, the direction of extension and proper tacking must be done with everyhting socked down tight on the car.

All in all, it was quite alot of work for more negative and positve results in IMHO

 

My experience with modifying exhaust systems typically allows the engine to breath more freely thus the lump is flowing more. As more air is passing through the system there's a need for more fuel to keep the mixture at the same level as where it was before the mods.

Back in the days of carbs one would increase the main jet size and play with slides and needle positions.....mostly for acceleration. On decceleration the same happens in that the mixture must be richened by adjusting the pilot screw as your 'off throttle' the only fuel circuit thats working is the pilot through the old pilot jet and adjustment screw.

My question would be of the systems and sensors that monitor and control fuel for the engine. Are they in correct functional condition and if so can they handle modifications to the stock system or do they have to be upgraded also to 'manage' the increase in flow? I'm not an expert in fuel injection but have tons of experience with the old fashioned carbs...........not apples to apples comparison I know but a combustion engine works on all the same basic principals. My 2 pennies worth.

 

So to clarify the sound issue:

You have stock header manifolds, gutted cats, and no muffler?
Then you tried with the Borla back on?
Is there any intermediate muffler?
If you are running a straight open exhaust system, yeah it's going to be loud, poppy, and obnoxious...

You need at least one muffler of some kind to tone that stuff down.

If you do have some sort of intermediate muffler, then try a monza resonator tip instead of the RMB.
It's a cheap fix, and usually helps kill the raspy, poppy sounds.

If you don't have any intermediate muffler at all, and the Borla is too quiet for you,
try a flowmaster. It will quiet the car down, but keep some of the rumble.

 

John,
Did you go the 'S4' dual exhaust route with the tubular manifolds using a twin chamber gutted cat? - or adapt the 16v cat to the manifolds, and go single pipe?
If the former, the cat will likely sound like crap - for the resonance issues mentioned. I had a gutted cat on my S4 for a while - one that had straight pipes welded inside for exactly that reason. It is cut up now to have the chambers replaced with pipe alone, for the internal welds released creating a tin can alley chorus racket. If that is what you have, either fill or remove the chambers. I've since installed on the S4 a good cat , GT resonators - and soon will have a RMB arriving to replace a GT muffler: this will match 86_5Tiburons system noted above.
If you have retained the single pipe system on your 16v, here is what works beautifully on my '80: So simple - all OEM, except that the cat is rodded out. This early cat has no wacky resonance issues, and opens up a note which I'd love to capture on the S4. The 'GT' style resonator is key IMHO to mellowing the sound - and the huge factory muffler works well. Noise levels are aggressive, but not obnoxious - very pure ... The '85 manifolds would not likely change the characteristics much.

 

Malcolm,

I agree with you fundamentally. However what appears to have happened may be contrary to that view. By freeing up the exhaust, I would have expected a lean condition (more airflow by virtue of the free exhaust with no increase in fuel). Yet, the popping seems more frequent (and I thought indicative of a rich condition).

Bcdavis,

Close; original system was stock cast iron manifolds, stock y-pipe and cat, Borla sinlge pipe (cat back) system but with rear muffler removed and straight RMB in its place. (The standard Borla cat back system has a small intermediate muffler/resonator(??), then the larger primary muffler with dual tips... only the rear most primary muffler was removed on my old system) The new system made no changes to the Borla single pipe cat back system (other than trying it with and without the primary rear muffler/RMB... the intermediate has always been in the equation). The only real changes to the new system are the tubular '85 manifolds and the fact that the stock Y pipe was extended on one leg and that the cat was gutted. The new system is now too loud without the muffler, too quiet with it, and harsher/more pop prone than befor. I need to look into the Monza tip... any suggestions of where to look?


Garth,

I adapted the 16V single cat with Y pipe and utilized my existing Borla cat back system. I'm not 100% sure but I think the early hollow cat MAY have some wacky resonance issues... at least I hope thats what drastically changed the tone of my system. I also can't imaging the tubular manifolds effecting sound quality but something has... my hope is its the cat.

 

Man, I feel your pain. I was dissatisfied with my '85 years ago because it sounded a bit too timid. This was a looong time ago, before RMBs were around. I lopped the final muffler off. It was incredibly loud, I couldn't be believe that even with cats its was louder than my old GTO with headers and turbo mufflers! More importantly, the sound wasn't pleasant. I put some resonators on, and then put on a custom mid-muffler, that did nothing. Then i put on some round mufflers where the final was and it was OK. It lost a lot of bottom end when it was just the pipes, got a little better in the end.

On my '83, I put on an Ott X-pipe....really loud...then an 3.5" "Afterburner" system with a spintech muffler...HUGELY loud...put in a Vortex insert..minimal change...Put Car-Tech inserts in the X-pipe...little better. Still loud, popping and sounding like it was missing just as you described.

Went back to the drawing board. Installed 32V GT cats in place of the X-pipe. Figured they were designed for a larger displacement motor with greater output, so they ought to do well with my Euro 4.7L. Installed an Ansa midpipe because legend had it that it was just a hair larger diameter pipe than OEM, and then a replacement final muffler (Liezstritz- Devek - under a hundred bucks). I'm pretty happy with it now. Its loud, boisterous, can definately hear the engine over the fan noise when revving, but most of the annoying resonance is gone.

I have had those 32V manifolds in my basement for 3 years now too. Got the car up on jackstands once thinking of installing, and realized I just wasn't up to the task...wish I had a lift! But hey, since people are telling you to buy your own dyno.....

 

John-

Thanks for the info. I know it's a lot of work, but I think I need to do it. Can't wait for my wife to get back to work (she had June off) so I can work on this stuff at night after the kids go to bed.

 

I agree with hupp....I've seen(heard!) this on other cars where the owner hollowed out the cat.
PLUS - it does sit directly under the old ASSOMETER!

 

http://www.jcwhitney.com/autoparts/P...001736/c-10101

http://froogle.google.com/froogle?q=...iw=1062&tab=wf

I think these are an excellent addition to any 928 exhaust system.
Even if you still have to change something with the cats, or have to add a bigger intermediate muffler,
I still think having one of these tips really helps to smooth out the raspyness, pops, etc...

Also, I had headers on mine, and did not hear any heavy tinnyness or resonance.
So I think if you are hearing something like that, it's probably from your gutted cats...

I did the same thing, trying many configurations, many mufflers, etc, until I found something that sounds good.
If you want it loud, but not that insane loud, I would really recommend dropping a flowmaster muffler where
your intermediate lives, and run a Monza tip. I thought it was the perfect blend between too quiet, and too loud...