ZR1 stock mufflers

[WVZR1]

Has anyone just opened the non-functional tailpipes on the factory systems. It is mentioned to block an outlet on those aftermarket systems with dual tips to make a bit more tolerable at lower RPM levels. It would be nice to just liven' the system up a bit. A search of "exhaust" seems to cover every conceivable modification but this. Is this that foolish an idea?

 

[Hib Halverson]

Cutting a hole to make both pipes work will, indeed, liven up the sound of the exhaust, especially, the low rpm resonance.

It probably will not improve performance in a practical sense.

 

[WVZR1]

I've read a good many of your technical articles and I appreciate your time to respond to my post. If only I had an exhaust shop up my sleeve I'd be all set. It isn't exactly what I would like to do but seemed an inexpensive way to see what most of the available muffler only applications might accomplish. You DIDN'T mention if you thought it FOOLISH!

 

[ArtZR1991]

There have been discussions on this before, and the recommendation has generally been to NOT do it.

Apparently the LT5 exhaust was origionally designed with them open (hence the two tail-pipes per side), but it was found that closing one outlet per side helped low end torque a bit. This also affected the final calibration of the ECU. For some reason, probably the fact that this was late in the design process, and the design was otherwise fixed, the dual tips remained, and were never evolved to the wide single tip, like the LT1.

I have read this in books on the subject, and on posts on various forums in the past.

It makes sense to me, as back pressure has a great effect on low speed power. This is a very common design issue in camshaft choice in street hot rod engine choice, also. The LT5 was carefully calibrated to have a small amount (I used to know the number, perhaps someone else can help).

So the street drivability and "percieved" power may be adversly affected if you open up the outlet. I suspect that hi speed operation would not be affected either way.

Hope this is helpful.

 

[Hib Halverson]

Which book did you read that in?

The reason the 90-91s had two outlets was it looked cool and, at the time, C4 tradition (84-91) was two outlets. The ZR-1's were made square to differentiate it from the base car's which were round.

The exhaust on both L98 and LT5 was designed for one outlet. There was a small hole drilled in the "fake" outlet to allow some coloring of the pipe so it would not be obvious it was a fake.

GM also ran tests to see what happens if the phony outlet was made real. The result was high interior noise level at low rpms, ie:" the dreaded resonance.

As for the backpressure issue, if that were true then the installation of ANY aftermarket exhaust would noticeably reduce low end performance which....it does not.

and the design was otherwise fixed, the dual tips remained, and were never evolved to the wide single tip, like the LT1.

Hate to tell you this, but the 92-up ZR-1 had the wide, single outlet.

 

[Ken]

Hib, I recall reading somewhere, at some time, that that mufflers on Z-51 equippped cars had all four outlets as functional openings, while the non-Z-51 cars had only two functional outlets, with holes drilled as you describe to give the effect of functionality. Is this true?

I have tried for years to find the article, or book, in which I read that description, but to no avail.

Please enlighten me if you can.

 

[8388]

The real 2 hole outlets were on Z51 cars with a rating of 250 hp.
Vettes with a 240 horse rating had one real one fake outlet.

I believe it was this way from 88 thru 91.

 

[WVZR1]

I guess us "hillbillys" just know how to get something started. I had studied this a bit and assumed most everything Hib has passed on here. The opening up the exhaust was meant to be an inexpensive test of the resonance thing before wandering aimlessly around the aftermarket exhaust choices. There just seems to be to many choices for a system out there. It's down to this I guess "open it up" and maybe "plug it up".
Hib, one question here. Just open it up with a 2.50 hole saw?
Once again hope it doesn't sound foolish!

 

[ArtZR1991]

Don't worry Hib, you don't have to hate telling me this. I do not disagree with your version about how the dual tips came into being. My discussion was about how the second tip never came to be open. As to the text reference, and where I read it, I will copy and print the book reference if I remember where it was.

When I referred to the tips never being changed, I was referring to the fact that the ZR1 went into production with the duals. Did not mean that they never changed in the life of the production of the ZR1. I meant the comment only in the context of why they would be on the car and still not be meant to be opened up. That, after all, seemed the obvious question.

I stand by my statements about back pressure and low end torque and specifically with regard to camshaft design and exhaust selection. As to low end torque being adversly effected by reducing back pressure IN THIS EXAMPLE, I would not suggest the effect would be "noticable". But it is not necessarily a benefit, and can often be counter-productive. To be clear, I make this comment with reference to the exhaust tip being opened up. In camshaft selection, where the issue is similiar, the problem of "over camming" can be very noticable.

So I would stand by my recommendation that most owners/drivers NOT make this change on an otherwise stock LT5. I think it is example of how a seemingly simple, intuative change can be counter-intuative and counter-productive.

And in regard to WVZR1's original question, it would seem foolish to me.

Regards,

 

[ArtZR1991]

With WV's last post, it would seem that it would be an interesting experiment about resonance. I agree with Hib that resonance will likely be an issue.

WV, if you cut it open, be sure you have a plan to close it back, just in case you keep the original system and don't go to aftermarket.

Regards,

 

[ArtZR1991]

As a matter of pure luck, I found the source of my orininal belief that opening these holes would cost power. It is from the ZR 1 Net web site FAQ, and is from Jim Ingle (GM powertrain person?), and surprisingly, is right after one of Hib's very good contributions. In fact, he is probebly the most prolific contributors, and I read all of his writings. I recommend it to every one, at

http://www.zr1.net/ZR1_faq.html


It turns out that it was a styling issue, which I did not emphasize, because I was interested in the techinical issue (the styling debate is moot, since the cars in question -ours- already have the tips), and I was correct, and still stand by my original comments. There are other sources I have read as well, but this suffices, I think.

In summary, Hib is right, don't do it because you will not like the sound, and I was right, don't do it because it will cost power.

Here is the question and answer from ZR 1 Net (I will add that the emphasis is that this pertains to otherwise "stock" cars. This seems obvious, but I quess should be ststed.) :

******************************************
QUESTION: How does one pop out the opening to the non functioning exhaust outlet in an early stock ZR-1?

ANS: The production Corvette mufflers with two "tailpipes" on each side are actually designed as single tailpipe mufflers. The dummy tailpipes are the result of Styling's wet dream. We have told them many times that if they put two tailpipes on a muffler it will cost us power if we make them both active. While I'm sure this isn't obvious to someone outside the business, the reason is simple: we have to pass the same noise laws regardless what the exhaust system look like. If we give up volume inside the muffler to create an additional exhaust path, we have to restrict the flow internally to have the same noise level as measured by the required tests.
So, the pipes you see with "disks" blocking them are not blocked at all; they are strictly dummy tailpipes welded to the ends of the muffler. The small bleed holes in the muffler shell were a compromise to let a little bit of exhaust out so they appear to be working. (Note: this applies to all production Corvette mufflers, not just ZR1s.)
If you drill through the muffler shell at the base of the dummy pipes, you will effectively destroy the primary tuner which resides in that part of the muffler. you won't like the result. It will create a strong "boom" between 1200 and 1800 RPM that's a real headache producer. Further, you won't improve the performance.
Thanks to Jim Ingle for this info.
***********************************************


Added "EDIT":
In addition, please go to the next question in the list (if you visit the ZR1 Net FAQ). In that answer, Graham Behan directly addresses the point I made about the effect of high performance camshafts on low speed torque. This goes to the issue of flow, and applies to exhaust and intake systems as well. "More" is NOT always better! Thanks.....

 

[Ken]

Interesting. Thanks Art!

_ken

 

[ArtZR1991]

Thanks, Ken, I would love to hear from WVZR1, if he did it and what he found. I have seen this discussed before, and would love to get a first hand report from someone who actuall tried it and can tell us what he found out!

 

[WVZR1]

I've not done it yet and am just a bit hesitant. The car is in the paint shop for likely the next 2-3 weeks and it will take probably a week to trim it out. I need to decide first on a system if it doesn't give satisfactory results.
I'd be interested to know if anyone has done the 92-96 Magnaflow LT1 mufflers on a ZR1. Dimensionally they seem very close and wonder why if it is a fit they wouldn't advertise it as such.
It has been a very interesting thread and I'll stay "tuned". I'm somewhat amazed no one had previously done the mod!

 

[vettl83]

WV, something must be different or Cosa, Flowmaster, Borla, B&B Triflow, and Power Effects wouldn't offer specific systems for the LT1 and LT5. I can only assume that the systems are tuned and designed for the specific motors. If your going to go after market exhaust try to listen to the different systems on the cars before you make your decision. Sound is subjective to what one likes. I have headers, X-pipe and Flowmasters no cats and love it, others say it's too loud at wot. Hope this helps 84 CF 91 ZR-1

 

[WVZR1]

Well we're about to find out what we might end up with opening up the second exit on each muffler. I'd like some feed back on the alternative though.
I've purchased a low mile Corsa LT5 system for the car and I'll likely try the exit modification first. I'd like some input from maybe someone with this system on their car with original cats etc. I understand resonance is not a problem and they're somewhat quiet. It's not exactly what I was looking for but a start.
How loud are these?

 

[caninelver]

header question

My mechanic tells me that if I don't put over 5000 ml between yrly inspections I can legally get rid of the cat and he suggests I get a set of long headers-I think he said 2 1/2" and he would install them to the existing stock mufflers. I like the idea but dont know anything about headers. Any suggestions as to whether I should get ceramics,stainless,black,whatever. I dont race but I do have a lead foot. Also would headers damage the stock mufflers or should I get new mufflers at the same time. The ones that are on now are mis-matched and look bad. Thanks,

 

[Hib Halverson]

If you're talking about drilling the plug out of the stock mufflers and running that before you put on the Corsas, as I said before, there will be no practical performance gain but there will be an increase in interior noise, especially in the 1700 rpm range.

The Corsa system for the LT5 is one of the better aftermarket systems out there, however, my first choice is Flowmaster.

In fact, I posted a tech. article about building a killer Flowmaster system for modified LT5s. It's here on the CAC at http://corvetteactioncenter.com/kb/question.php?qstId=521