Speedtoys

That spring valve..I dunno. Neat..but...

When that $1.50 in engineerin' fails, that's how I see it at least..


I would like MSDS headers this summer.

SMTCapeCod

I have the CVTs with the internal valve on my Volvo. Completely different application but...they have a decent performance tone when closed, compared with stock. Pretty subtle WOT but the Volvo has a lack of displacement and a turbo that dampens the sound.

I have had some rattling, episodic not constant. From what I've read the earlier versions were prone to that and the manufacturer allowed exchanges until they updated to an updated design..basically some material along the edge of the butterfly plate.

Its an appealing idea to have it both ways...but in the end you're putting the spring tension against all that flow you wanted to have with the cams in the first place.

There's also a Patriot (brand) version on Summit racing. Somehow its
http://www.summitracing.com/parts/PTE-H3200/

*Someone* with a fun-around car should install a pair of the electric dumps to the kickdown or WOT circuit....

LEADASS

O.K., Stupid question. Why wouldn't you just use a Y instead of the X with a Y? If the X seems to be the source of drone...? I'm really struggling to determine the best route. I was thinking Headers- X pipe- hi flow cats then true dual back. Problem is early '86 car means no true dual. I was then thinking headers to Y pipe single cat (3") and single back. I'm a bit lost now.

Speedtoys

The X is the source..its the engine cadence in. With true duals, the cadence is broken up by the uneven pulses down each pipe...no harmonic.

Resonator pipe. Gone.

This is what you hear: (the red line)

SMTCapeCod

I think your "Y" into single has merit- the two into one has been proven to deliver some good power in different setups. Bear in mind that the diameter of the pipe also factors into total noise though, 3" and up is tough to hush at all!
Is the battery box different between 86 and 86.5? If not, I don't see any impediment to duals- if desiring stockish look get an 86.5 final muffler and then S4, GT or custom mids. If you don't care about stockish, some of the compact aftermarket mufflers can be fit in the rear pumpkin space...if the battery trays aren't different.

Speedtoys

No..you want the resonance pipes to be 1/4 wave.

Not half.

You want the echo FROM the pipe to be 1/2 wave.

hb4

Right, the resonating section of the (main) exhaust is half wave, and the blind pipe is quarter wavelength to make an out of phase half wave echo.

I measured a peak at 300Hz but as Jim pointed out, that was probably a harmonic. The resonance at 2200 that I'm hearing gives a ~150Hz frequency or a little over about 10.5 ft wavelength.

A Helmholtz resonator can also provide an out of phase half wavelength, but consists of a small, short entry tube and a larger cavity (works like a soda bottle). I figure a 1-1/2" dia neck, 4" long with a 4" dia cavity, 14" long would do it and might fit next to a glasspac type muffler where the resonator would normally sit. Now I just need to summon the courage to try it, unless someone can figure out how to fit a 2' resonance tube on each run of the two-pipe setup.

andy-gts

In the straight section from the cats back to the primary mufflers louis ott placed 4 in glass packs and cleared up alot of drone from what I remember. anyone else tried upstream instead of down stream to change the drone

Speedtoys

150hz...

the 1/4 wave of this is only 55cm at standard temp.

Where is 10.5 feet coming from?


It will be longer at higher exhaust temperatures, and my 140hz was about 32" at 500d F.

hb4

I think we're saying pretty much the same thing. 32" is quarter wavelength, 64" is half, and full wavelength is 128" or 10.67'.

Looking back, I figured 2200RPM gave 146.7Hz and 31.07" quarter wavelength at 500deg exhaust.

Oli928

Agreed...I couldn't stand (sound) of my '89 auto w/x-pipe, cat delete, factory mids and RMB. I put the rear muffler back in and gained performance (butt-meter told me so) and got rid of rasp at 2,200-2,500. If you can live without extra burble "RMB only" provides, this was a good set-up for me. FYI Borla "S" type mids now.






Yes, right mid Borla touches trans plate...since corrected. Black paint on welds is engine paint (might have been overkill against future brown monster)

terry gt

E mailed the tech dept at dynomax today re their VT mufflers . Described my system and the drone issue . They assured me the that their VT mufflers would eliminate the drone and also provide full flow through power . So I think I will try a pair installed in the mid position . they are 4.5 x9.75 and 14" body , offset in / center out . Its a shame to redo the system Colin did such a nice job , but the drone is got to go.

SMTCapeCod

^^ I've been looking forward to someone installing them on 928. I hope you are able to post after the fact impressions and sound/vid files. One other piece of advice though- take a moment to clean it with alcohol and get a good coat of primer/exhaust paint on it or high heat powdercoat if you would like it to look decent into the future. I'm about a year in and there is evident/abundant surface corrosion on mine.

Corsa mufflers are billed as anti-drone as well, but I think they may have gone to solely cat-backs instead of universal fitments.

Craftsmanship on Colin's was impressive, someone might get a nice add-on if you elect to sell it and it meets their needs.

antlee928

Agreed. I did Roger's X-pipe and hi flow cats using standard cat back system and sound was marginally enhanced but still not there. I did note an enhanced V8 rumble (resonance) coming from nrear the front of the car and put this down to the X-pipe. I happened upon a free RMB (oval exit pipe) - not sure of origin of manufacture. Installed and found the sound waaaay to loud. I took it for a highway run and after about 20 mins turned around for home and changed back the standard rear muffler. Found my config with the RMB cause me severe headache and neck muscle tension. BTW, that rear muffler certainly works well in the sound damping department. After having read many exhaust reports and mods, I am thinking the best way forward to achieve some better exhaust sound is probably to go with the 928 GT rear muffler. My understanding it is smaller and of a differing design. Can anyone confirm this. BTW, if anyone has a GT rear muffler they are happy to part with then let me know. I am in Australia though.

Cheers

Stromius

Thanks to Bill (hb4) and Thomas (supercedar) today, I am experimenting with an X pipe, hi flow cats and a Black Sea R&D prototype rear muffler I bought 2 years ago from Constantine.

Originally I got it to get rid of resonance I kept getting in my stock 88 setup + RMB. At 2100-2200 RPM, I got the raspy resonance that didn't sound so great. The Constantine rear muffler (CRM?) got rid of the resonance and tuned down the LOUD exhaust note plus gave a little more torque I always thought due to the small, custom spec'd muffler compared to the RMB.

This is the muffler (right one)

My 88 has the small (CS/GT) resonantors along with Vibrant 7100 hi flow cats. I am not getting any of that tell tale resonance at 2100 rpm. I have tried this on the highway in 4th and 5th, uphill in town in 2nd and still NO resonance. It's definately a bit louder of course with the X pipe but still tolerable. I may try to experiment with the larger resonators someday but for now, very grateful to the local help and going to enjoy the extra hp!

My last SMOG check ever for this car was yesterday so I considered this as a graduation present of sorts.