as discussed on the Internet Z car club.
[The Internet Z car club is an international mailing list with over 125 members and transmitted to a member's computer account over the internet.]
All information published here is the property of the original authors who are members of The IZCC and may not be reproduced for commercial purposes
Everything stated here is based on the experineces and opinions of the original authors. The authors and editor accept no responibility for any damages arising from use of this information.
Q:
Does anyone know why early Z-cars wreek of exhaust with the windows open?
A:
They all do that. There is a tremendous (well relatively speaking) turbulence in the wake of the car. You can see the turbulence in how the exhaust blackens the back of the car if you have a smoker. Exhaust gas seeps in around the deck lid, where hoses penetrate the deck floor and from any rubber plugs that may have rotted out. With the windows up, you get some positive pressurisation of the cockpit by virtue of the vent inputs being in front of the radiator. With the windows open, you lose that pressurisation and may even get some negative pressure from aspiration across the window opening. A few years ago I went on a campaign to seal all openings in the rear of my car. Silicone seal around every plug and hose penetration. made a major difference. Also make sure that the seals around the tail lightsare not compromised. The newer non-acidicstuff is nicer on the nose but not as strong in my opinion. Only thing I can really say is stay away from the toothpaste tubes. You pay 50 cents for the silicone and 2 bucks for the tube and theblisterpack.
Usage tip:To dramatically speed the cure, mist the silicone with water after it has been applied and is ready to set. Siliconehardening is triggered by airborne moisture so flooding it so tospeak, gives it all it needs.
Q:
Is there anything that I can do to get rid of the loud resonance in my exhaust system around 2000 RPM.
A:
The problem is the long pipe from the header to the muffler setsup a series of RPM resonances, audible to you as "boominess" orringing. These resonances return reflected exhaust pulses at allthe wrong times and make the engine feel real flat. At higher speed, the pipe length is so long and back pressure is rising so that theseeffects are largely gone. I've investigated this phenomena using apiezo pressure transducer and scope and am comfortable that myexplanation fits the observed data. A smaller pipe would probably changing the resonant frequency and raising the static pressure enough to mitigate the effects. My solution, developed after testing with open headers with a variety of collector lengths, is to terminate the header with a small resonator about 9" out from the tubing junction. This standard glasspack-type resonator acoustically terminates the collector so that it appears to beworking against conditions similar to being open. A second benefit of doing this is all booming and ringing are gone. The car soundsalmost stock until you wind it up. Then all I hear is the bit of residual header ring - very nice. I actually use a 3. 5" diameter tube for the exhaust, mainly because that's what I use on my turbo motors and am set up for it.
Q:
How does the higher back pressure prevent the reflected finite amplitude pulses from reaching the engine?Is the flow rate at highrevs great enough to mostly dissipate the reflections or does it havesomething to do with whether or not weak shock waves are generated?Also, what effect does the presence of a turbo have on all this?
A:
Good question. I'd guess it has to do with whether or not shock waves form. It might also have to do with the change intemperature with RPM changing the speed of sound enough to causethe effect. I know that's an issue in two stroke expansion chambers. Some racing bike tuners are now insulating the chambers to reducethis effect. I tend to get rushed for time on things like thisand end up noting the interesting data from the instruments and postulating a plausible theory but not doing a whole lot of work to back it up. yeah, I know, a bit sloppy but my work on creating a 36 hr day has not been going well of late. In this context a turbo does exactly the opposite of what is desired. Off-boost, the turbine has practically no effect on the acoustic energy being passed. That is, of course, when the issue of boominess arises. Under boost, the turbo is a quite adequate muffler by itself. I got in a pinch in my first turbo conversion way back in 74 andended up driving the car sans muffler and tailpipe for a couple of weeks. I learned to WOT it whenever I saw a cop. The car was actually quieter under boost than atidle. And ohhh that turbine whine!The resonator is available at most automotive parts stores that carry mufflers. It is also referred to as a bullet muffler. Usually about 3. 5 inches in diameter, tapered at the input and output. I can also tell you what does NOT work. Cherry bombs. What a piece ofshit!I got more pressure from a Cherry bomb with no pipe aft than I did from the stock system!The lack of matching to the inlets and the heavily louvred core really kills flow. Never tried any other brand. I put on headers and a turbo muffler and found the same resonance. I purchase a "standard glasspack" resonator (at a local NAPA) asyou talk about and installed it below the drivers seat (similar tothe stock setup) and it helped tremendously. I didn't notice a greatdrop in performance, but it did help my comfort level!
Q:
What is a good exhaust system for my Z? What size exhaust system should I use? What about a dual exhaust system in a Zcar?
A:
My experience has been that if you retain the stock cast iron manifold (not bad in its own right), any 2.5" exhaust with a turbo-corvair-style muffler will work fine. I started fabricating my own years ago because I could do it cheaper and I could make it a one piece all-welded affair. These performed as well as any store-bought system I ever tested.
On the other hand...
In talking to several seasoned street-machine guys, I've gleaned that a 2.5 system will always be loud. They've said that a 2.25" system will have damn close to the same backpressure and be much easier to live with noise-wise. Too bad no one makes a 2.25" system for these cars, I'd like to try it. On another note, I've had one of these 2.5" systems behind my 73 240z engine with stock exhaust manifold for years. The muffler eventually blew-out and I tried replacing it twice to get the same level of noise reduction the system originally had - with poor results. I tried a cheap (forget brand) turbo muffler and that a Sonic Turbo. Both were (are) much too loud for the performance of the car. I've seen better results on a friend's car with dual exhaust systems he's tried. He was using a dual 3-into-1 header and started out with 2. 5 system. Too loud. Then he built his own dual system. It ran better and was quieter. A dual system can give less backpressure and doesn't have the resonance problems of the 2.5 single system. I believe the 2.5 system has a problem with the resonant frequency of thetubing being too low due to the thin, large diameter of this pipe. Asmaller diameter pipe has higher resonant frequencies, which correspond toohigher rpm which you don't encounter around town (i. e. 2500-3000 rpm). Idon't have the equations handy, but I'll bet that the resonant frequency is more than a linear relation with pulse frequency (rpm). I'll bet it's a squared or cubed relation with pulse frequency. The point I'm getting to is that even going down to 2.25" pipe will raise the resonance frequency greatly while lowering the backpressure a small amount (with respect to that imposed by the muffler.)
Header installation hints:
I have a set of headers on my main Z now that use very thinflange metal. I have had no leakage problems, however, because I installed the rest of the system properly so that there is no stress onthe headers themselves. The key is to install a flex joint between theheader and exhaust pipe so that there can be motion between the headers and exhaust when the engine rotates on its mounts or when the frame twists. I have traditionally used a stainless steel bellows made for aircraft exhaust (check your local FBO) but lately many OEMs have been including stainless steel braided flex hose in their exhausts. These should now be available from your car parts store or a muffler shop. The second key is to support the forward end of the exhaust tube so it is rigidly mounted. On rubber, of course, unless you like lots of exhaust noise in the cabin. I generally mount to the rear transmission stay. Lastly, (and none of the commercial exhausts do this that I'm aware of) a small resonator as close as possible to the header/manifold exit as possible is necessary to break up the resonance of the long pipe thatwill cause a giant flat spot around 4000 RPM. I usually just prowl the local muffler emporium until I find a suitable OEM part. Typically from an older Caddy, it should be a glass-pack and have an inner diameter at least as large as the inlet. I have used a cherry-bomb-style muffler but they flow really badly. This resonator will completely kill the boominess headers on Zs are noted for.
Header and Exhaust recommendations:
If you drive mostly on the street (don't most of us? ), you might want to consider using slightly smaller diameter pipes than what some of the header folks are selling. (2. 5" is typical. . . ) I used 2. 5" pipes all the back from the header collector to a singleturbo muffler (the old hot rod standard) with 2. 5" inlet and outlets. The results were less than desired. It was noisy and there was a highly noticeable loss of low end power. When I reached 4, 000 rpms, it was great, but since most of my driving was limited to street, a few autocrosses and regular Sunday morning "banzai" runs, I came to regret running such a large pipe. When I was in the process of restoring my '76 280 (returning to "stock" condition) I opted for essentially the same setup exceptthat I used 2. 25" i. d. pipes and a comparable muffler. This setup wasmuch more pleasing for my use but alas, I didn't get a chance to fullyenjoy it as the car was destroyed in an accident (taxicab vs. Z; myZ, unfortunately, lost with irreparable damage to frame) but thecouple of months I ran it were great (comparatively speaking of course. )There will be dissenting opinions on this but it worked for me!:-) The headers that I purchase (not the high buck versions) werea bit thin in the weld at the flange. I had them back weldedat a welding shop. Most headers that I have seen are welded on the face (faces toward the head) and then ground flat. Some of thewelds, if they don't get real good penetration, can crack easily. By welding around the back side of the header flange, it will strengthen the header and also makes them somewhat quieter.
Q:
What does header wrap do for the headers?
A:
use a thermal header wrap (was from iPD, the Volvo guys) to quiet the "pinging resonance" of the headers that was so annoying. It also reduces the under the hood temperature, thus helping toeliminate vapour lock, carb/injector problems, etc. Highly recommended. One box with 50 Foot is enough to do the headers for a Z. Make sure you use the header wrap paint as well, to help preserve the wrapping.
Q:
Why are most of the back bolts on exhaust manifold broken? Have others out there experienced this? Is there a fix? Can it be patched somehow to reduce the noise, it's really bad cold and I can still hear it a little when hot? Would a third party manifold be the solution?
A:
The manifold warps with the ends bending up and it snaps the bolts on the ends. Thisis what I was told. I don't think that this could be resurfaced. I have ~150K miles on a 1982 280ZX (original owner). About 6-7yrs ago I started getting noise from the exhaust manifold that was very slight and went away when warmed up. It got worse a year ago and I had the exhaust manifold replaced ($700). A few weeks back the noise returned and I've again got busted nuts on the manifold. I took it back to my mechanic and he said that this was common with this design. Apparently the manifold warps bad enough to break off the nuts on either end. Take the manifold to a shop that does head surfacing and have it surfaced to remove any warpage. Replace the stud with a Grade 8 or better and (important) don't tighten it too tight. Just enoughto compress the exhaust gasket. The surfacing job should not cost you over about $40 and the stud no more than a buck.
Q:
Can you put the glasspack resonator in the driveshaft tunnel?
A:
Don't try putting the glasspack in the driveshaft tunnel with the 2. 5 system. I did and only was successful in further reducing my ground clearance and adding weight to the car. Any reduction in overall noise was minimal - not worth it. It just shifted the "resonance" up by a few hundred rpm, as far as I can tell.