Necked Down Exhaust ?????

Super XR7

Registered User
X Pipe Value??

I am working on building my exhaust system, and I was buying some U bend stainless steel piping from S&S Headers in Arizona when one of the techs told me that I should have a very short necked down area immediatly after the collector of 2". He stated that it would scavage the collector and add mid range performance. Is this correct?? I have the mid length Super Coupe Performance headers along with p&p heads, mild crane cam (.509/509 lift and 208 duration), 42 lb injectors, 76mm MAF, 1:1.73 roller rockers, rebuilt block by Robinson Racing, made a new raised top, buying MP 85mm TB, MP inlet plenum, rebuilt 95 SC and now working on exhaust. (anyone wanting a set of drawings of my downtubes, am willing to send via email)

Any comments on the necked down area?? Pro or con.

Thanks,
Mike Tuck
Super XR7
 
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Is he aware your application is a positive displacement supercharger?

That sounds like something you would do for a N/A application.
 
Can't figure out where you are talking about neck down area.
I've heard an X pipe can scavenge.....provided the pipe length is correct to have one cylinder pushing exhaust through the X at the right time that the other needs it drawn out.

http://shop.store.yahoo.com/zipproducts-c4c5/exhaust-performance-y-pipes--x-pipes.html

My setup has shorty headers direct 2 X 2 1/2 pipes to magnaflow resonator and a neck down into the stock y pipe. Tried Y pipe off, Y pipe on without rear mufflers, and ultimately put mufflers with stock Y pipe back on cause I saw no difference in performance....but that was at WOT.

Note superbike exhausts have great headers but ultimately neck down to the exit area....which evidently does not severly hamper the flow.

Also previously posted (http://www.qsl.net/ya5t/exhaust3.htm) says don't necessarily believe big exhaust pipes mean better system...under the paragraph SETTING FUNCTIONAL BOUNDARIES....
"....this tells us we can, under the right circumstances, have a tailpipe of smaller proportions than the collector without losing power."

Also another good quote on this I thought...regarding X pipes or H pipes...
" The reductions (in noise) amount to a minumum of one decibel to a maximum of 3.....as for power increases....in about 60% of the cases I have tried (mostly with balanced pipes), the engine responded by delivering up to about 12 hp more with 5 to 8 being the norm....I have never seen a reduction in power".

Others have posted that the X pipe can be used to replace the middle muffler (so called resonator) and still be tolerable.....or use a 2 in one out high flow cat in place of resonator. Still probably a good idea to include the X pipe if you can afford it or possibly make one.
 
Scavenging is the process of using pressure waves in the exhaust pulses to assist the extraction of exhaust gasses from the combustion chambers. When properly tuned, and exhaust sustem can use the pressure waves to create a vacuum just at the exhaust valve at the precise moment the exhaust valve opens, assisting in removing the maximum amount of exhaust gases.

So without doing some engineering calculations, any mans guess is purely a guess as every car has a slightly different setup. Long tube headers are claimed to be tuned so that the pressure wave created by the collector area helps to scavange the cylinders.

In a car like ours that has a positive displacement supercharger, exhaust scavanging has almost no impact on performance during WOT because during the overlap period, the pressurized intake mixture forces any remaining inert exhaust gases out the exhaust valve. This is because the intake is coming in way above atmospheric pressure.

So at best, scavenging will help you with part throttle engine efficiency. But when you look at raw performance numbers, it shouldn't have that big of an impact on our cars. They key on our cars is to have limited back pressure so that at WOT the inlet pressure will always be dramatically higher than the exhaust pressure. That will help ensure that the entire volume of the cylinder is filled with combustable components.

In theory that is.
 
Thanks for the info. I told the tech that the engine was supercharged but that did not change his thoughts. Will carry on with out any necked down area, but will look at the x pipe, though there doesnt seem to be any room without eliminating the resonator. I do not want a loud exhaust, but more sound than stock.

Thanks
Mike Tuck
Super XR7
 
Yes, the stock resonator has to go, and the stock cats of course.
If you look through the one that SCCOA offers, the magnaflow, it is completely cutout to be a Y inside......it is straightthrough, and it seems to work on my car, but it is bulky and hangs low.

An X in it's place is infinitely better..... or a high flow cat of slightly less overall size than the magnaflow is OK if you have to have cats.

Now regarding the theory that exhaust improvements on a supercharged engine doesn't make a big difference, this simply hasn't been the case with what's reported on these threads.

You make your first huge performance improvement with exhaust.
Any poor man's supercoupe project gets huge gains by simply bypassing the stock cats and resonator, possibly up to a second improvement. This stock setup is so bad the engine is literally suffocated when it goes to make power.

I then went from that to headers and got noticeably more top end. Perhaps beyond that, the fact that its supercharged doesn't require the most perfect exhaust tuning or setup could be true.

But if anyone is really clever and has the time, what this car really needs, and what would be a HUGE seller (I've tried to talk several SC parts people now into offering this) is the "bypass system" consisting of connecting from stock manifolds to stock back Y pipe with:

#1 - A realistically sized and cleverly designed straight flow through resonator that hangs good. Perhaps a box design like what was in that article is possible.

#2 - X pipe and/or high flow cat retrofit in this

Once someone comes up with this, which is only about 5 ft long and should be marketable for about $200 to $250, I'd buy a few in a second.
 
vettewars said:
Now regarding the theory that exhaust improvements on a supercharged engine doesn't make a big difference, this simply hasn't been the case with what's reported on these threads.


Note that in my theorizing I did not say that exhaust upgrades do not benefit our cars. I was specifically adressing the type of exhaust modifications that are often recommended to improve exhaust scavenging.

Forced induction motors respond quite well to improved exhaust systems. Especially systems designed to decrease back pressure.
 
Actually I was trying to say to a large extent you're right.
It's for that reason that I believe that the availability of such an affordable "center section" can and should be the most potent performance "part" for the SC.

Just too dam bad after all these years.....we're still bascially confused about the exhaust situation, with many paying dearly for high priced exhausts, when such a low cost center section is probably 90% of the answer.
 
Guys I'm trying to get information on the best possible set-up for my SC. I'm presently rebuilding her from about the ground up. I want to do it right the first time so that I don't have to do it again.

I keep hearing about the benefits of the 2-1-2 exhaust like supercoupeperformance offers, and also the benefits of the true dual exhaust. I keep getting so many different opinions that my head is spinning.

Which is better from a sound/ performance standpoint. I'm not looking for the rice sound, looking for a nice rumble. I'm also going to have somewhere close to 400 HP by the time I'm done (I hope).

Which will be better for me. Also I am going to put on the high-flow cats as I'm not sure where the job market will take me after graduation. Right now I don't have to deal with E-Check, but I may in the future.

Thanks guys.

Josh
 
joshbea6 said:
Guys I'm trying to get information on the best possible set-up for my SC. I'm presently rebuilding her from about the ground up. I want to do it right the first time so that I don't have to do it again.


In general, there is no inherent benefit to a true dual exhaust when compared to a 2-1-2 or a 2-1-1 in the discussion of performance. What you are trying to achieve is the optimal exit velocity of the exhaust gases. This is controled by the net volume of the exhaust pipes.

A 3.5" single exhaust pipe has the same cross sectional area as two 2.5" exhaust pipes so either would provide the same result.

Why true duals are popular with the V-8 crowd is mostly the sound, but also to get a sufficient cross sectional area for the larger horsepower outputs, a single large pipe is much more expensive than two smaller pipes.

For our application, there is limited space through or around the fuel tank, so a single larger pipe has been more popular than true dual exhaust. Simply because it's easier to route.

For sound quality, it really depends on what you want to hear. In general, V-6 motors don't sound good with a true dual where there is no H or X pipe installed. This is simply because the way the engines fire can sound odd. By using an X pipe, or a resonator, the two banks are combined into one sound source that can either remain 2, or be merged into 1. Also, reports by a few people have been that their exhaust has had some drone to it when using shorter mufflers. Shorter being whatever muffler seems to fit the stock location well. But then a few people moved to longer (19" long I believe) mufflers and have claimed the drone they were hearing went away.

So ideal is likely what is easiest for you to work with.

For your HP goals, I would look at what George and others have done, with long tube headers, 2.5" duals to high flow cats then either an X pipe and continue the 2.5" to the rear into 19" borla or equivalent mufflers, or into a resonator and out to a 3.5" single to the rear, then back into 2.5" and 2 19" borla or equivalent mufflers.

Check out this webpage and the pictures for what one guy has done:
http://www.thundercoupe.com/
 
I have modified by exhaust design to incorporate a X pipe. Downside is that I had go to the next shortest magnaflow resonator (18" to 14"). With the shorter resonator, does anyone think it might sound ricey or drone at highway speeds??

Mike Tuck
Super XR7
 

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Boost and exhaust

94 5 spd.
I recently replaced the stock res with a single in single out (2 1/2 - 2 1/2) maganflow res. (stock exhaust and car otherwise).
The resonator mod. alone didn't feel much different.
I then put on a 5% pulley.

My car performed much worse after the pulley mod, big time dotonation at low rpm, then missing at higher rpm. It sucked!!!, and the car was running great before.

I put the stock pulley back on, the car runs great again (lucky!).
I then replaced the stock 3-cat system with 2 1/4 downtubes and 2 high flow cats. to a y-pipe that feeds my single in/out res. (the rest of the piping and mufflers are stock).
It's a huge Improvement !!! (and my old cats looded fine).

It sounds deeper and a bit louder, no rice at all and thats my biggest fear.
Performance wise, no loss of torque, more mid range, and a little more top end.
I'm so happy with it now, I'm afraid to put the 5% pulley back on...
Let us know how it goes.

Cheers
MB
 
Mike,

A system like the SCP dual 2-1/2" into a single 3"and back into dual 2-1/2" will flow plenty to make 400 HP at the crank. Even with the high flow cats my engine is making close to 500 HP at the crank with the SCP exhaust system and Kooks mid length headers.

I wanted to but couldn't go true dual without changing gas tanks. So far the exhaust isn't holding me back and I'm happy with the sound, kind of a V8 rumble.

David
 
X Pipe Value??

Hey guys, do you think the X pipe would be worth it? With the X pipe I would have to go with the shorter resonator, without it I can go with the longer one. In our cars would the X pipe make a significant diff? (>10hp).

Mike Tuck
 
my single-chamber resonator

Originally posted by vettewars
Yes, the stock resonator has to go, and the stock cats of course.
If you look through the one that SCCOA offers, the magnaflow, it is completely cutout to be a Y inside......it is straightthrough, and it seems to work on my car, but it is bulky and hangs low.
(snip)
But if anyone is really clever and has the time, what this car really needs, and what would be a HUGE seller (I've tried to talk several SC parts people now into offering this) is the "bypass system" consisting of connecting from stock manifolds to stock back Y pipe with:

#1 - A realistically sized and cleverly designed straight flow through resonator that hangs good. Perhaps a box design like what was in that article is possible.

#2 - X pipe and/or high flow cat retrofit in this

Once someone comes up with this, which is only about 5 ft long and should be marketable for about $200 to $250, I'd buy a few in a second.
I have a stock SC resonator case with a pair of 2-1/2 inlets and a single 3" outlet, it's about 40-50% completed but I might re-do it with a pair of 2-1/4 inlets instead. Yes, the 2-1/2 inlets I have in it at this time are perforated.

'bird
(master ghetto-rigger)
 
Neat graphic, mike, how'd you make that?

To get back to original post for a moment,
here is a good illustration of the merge collectors
and transitions the tubing tech was referring to.

http://www.burnsstainless.com/MergeCollectors/mergecollectors.html

Burns offers 3-1 merges, but the collector alone is $200 /ea.

Kooks uses these hi-tech CAD-designed merges
in some applications, but not ours.

Kinda disappointing, for the pricetag;
but maybe of little consequence in our SCs,
as Mike8675309 says.
 
Exhuast created in Solidworks.

I noticed that George Davenport's exhaust is duels from headers into a X pipe then duel back into rear Borla mufflers. Has anyone heard this system? Wonder how loud it is, any droning (especially at highway speeds) and generally how does it sound??

Also working on replacing front brakes. Yesterday I got both front spindles off a 97 Thunderbird. Today I sandblasted them, measured the size and coordinates (off original spindles) of the ABS pickup mounting features, this will allow me to remount the original ABS pickups in the new spindles. Can share data if anyone wants it.

Mike Tuck
SuperXR7
 
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