Exhaust system idea

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77cruiser
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Re: Exhaust system idea

Post by 77cruiser » Tue Oct 03, 2017 6:37 pm

Can't remember what Pipemax said for mine, I think over 3" but I had Calvin set me up a rough draft, & I went with it.
I did 421 with a 2 1/4 choke & 2 1/2 col. It dynoed @ 530. I didn't test anything else on the dyno, so no proof if something else would be better.
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Re: Exhaust system idea

Post by cgarb » Tue Oct 03, 2017 7:08 pm

How would that work if you ran the two pipes off the collector into one larger box and then the single 4 out of that into the muffler? Would one larger box have any adverse effects?

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Re: Exhaust system idea

Post by MadBill » Tue Oct 03, 2017 9:49 pm

1989TransAm wrote:
Tue Oct 03, 2017 3:05 pm
I just sold my TransAm today as a guy made me an offer I could not refuse
Just a tad O/T, but: So if you won't be 1989 TransAm anymore, how will we know it's still you? #-o
As an aside, many years back I saw a white Trans Am with plates reading BLK BRD. Obviously not his first Firebird.. (I've had two TAs over the years. The first was promptly written off a block from my house after I sold it circa 1984. The tail lights of the second had been repainted to read F..I..R..E....B..I..R..D which I thought was kinda lame, so I went to the nearest scrap yard and found an unmolested pair...from my my previous TA!)
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Re: Exhaust system idea

Post by 1989TransAm » Wed Oct 04, 2017 12:04 am

I have used that name on a lot of forums so I will have to keep it. Also 1989GTA on some forums. :wink:

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Re: Exhaust system idea

Post by BOOT » Wed Oct 04, 2017 1:00 am

I'd y-pipe it sooner(your h-pipe effect) then because it's a truck go into one long terminator box(a larger tube but doesn't have to be super larger cause it's long w/e meets the required volume), taper down into your final size pipe, muffler at the end(if chambered use 12" of tailpipe min). I think dual 2.5 into 3.5 would be fine, larger would just be louder and more weight.
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Re: Exhaust system idea

Post by scubasteve231 » Wed Oct 04, 2017 1:32 pm

To address the collector/pipe size recommendations, I would agree that my engine would probably do best with 2.75, just based on what other people are saying about their engine/exhaust combos.

The headers I have use 3" pipe for the collectors and I have plenty of 3" pipe that I already own which is the reason I chose it, just a convenience/cost thing and the fact that it's pretty close to what I need. I also have quite a bit of 4" pipe on hand as well, and 4" inch pipe has just a little less cross sectional area than dual 3" which I thought would make sense after the pipes merge.

I did plan on buying the Pipemax program to hopefully optimize the collector length.

For those who have Pipemax, can it calculate a collector length based on a user set pipe size? Or does it simply spit out what it thinks is best and that's all you get?

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Re: Exhaust system idea

Post by scubasteve231 » Wed Oct 04, 2017 1:50 pm

In response to the recommendations for running a single large box, I agree that it would probably be the more efficient way to do things but I'm not so sure that will work with the layout of things underneath the truck. If there was a single large box it would have to live right behind the transmission crossmember. At that point I'm not sure if the collector lengths will be too long. I'll have to get under the truck with a tape measure and see if a single large box is possible.

The only con I can see is that a single large box is going to be 100% custom fabricated. It will probably have some flat sides too which will need reinforcing and deadening. My original plan is a little bit more simple, by using two huge mufflers as terminators and the pre-made Y pipe I get to save a lot of time buy using off the shelf parts. I guess the big question is, how much weight would a large single box save me and is that worth the extra hours of design/fabrication time. It could potentially save money in off the shelf parts but that may be eclipsed by the time spent on it.

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Re: Exhaust system idea

Post by ptuomov » Wed Oct 04, 2017 2:24 pm

scubasteve231 wrote:
Wed Oct 04, 2017 1:50 pm
In response to the recommendations for running a single large box, I agree that it would probably be the more efficient way to do things but I'm not so sure that will work with the layout of things underneath the truck. If there was a single large box it would have to live right behind the transmission crossmember. At that point I'm not sure if the collector lengths will be too long. I'll have to get under the truck with a tape measure and see if a single large box is possible.

The only con I can see is that a single large box is going to be 100% custom fabricated. It will probably have some flat sides too which will need reinforcing and deadening. My original plan is a little bit more simple, by using two huge mufflers as terminators and the pre-made Y pipe I get to save a lot of time buy using off the shelf parts. I guess the big question is, how much weight would a large single box save me and is that worth the extra hours of design/fabrication time. It could potentially save money in off the shelf parts but that may be eclipsed by the time spent on it.
Exhaust work in a street car or truck seems to be 90% packaging and 10% theory. At least by my experience.

As noted above by many people, combining the pulses at the right distance from the collector theoretically helps with power and sound. I think the easiest way to accomplish this in your setup is with a large-diameter H-pipe, so I'd agree with one of your replies. The H-pipe will quiet it down by 1-2 dB right off the bat and will give you a lot more flexibility with downstream configuration.

After the H-pipe, I'd step up the diameter for a while. I think that after that point, it's just minimizing the restriction. The other side of the coin is keeping it quiet and light, while fitting it under the truck.

I wouldn't fabricate a termination box or termination boxes. I'd just pick larges case volume perforated pipe packed mufflers like Borlas or Magnaflows that fit and put them where your drawing has the terminator boxes now. The combination of the H-pipe and the large case mufflers will terminate pulses well enough for me.

Here's one thing that I found interesting, relating to temperature. While I'd step up the cross-sectional area right after the H-pipe, then as the exhaust cools you can keep the velocity and thus pressure losses the same with smaller pipe diameter. So after dealing with the pulses you can gradually shrink the pipes. Here's my attempt to quantify the effect after reading some papers on the topic and coming up with an eyeball estimate of the temperature decay coefficient. I think (but don't know) that the temperature gap between exhaust gas and the ambient air decays by about 25% per every meter of exhaust pipe. This is an eyeballed estimate from a graph, but I am hoping it's ballpark:

80 Ambient temperature F
14.7 Ambient pressure psia
459.67 R at zero F
1400 Turbine outlet temperature F
3 Pipe length in m
2 Back pressure psig
0.75 Temperature decay coefficient per m
3.5 Turbine outlet pipe diameter OD inches
0.065 Pipe wall thickness inches

Distance from turbine outlet m, Gas temperature F, Gas pressure psig, Density ratio relative to turbine outlet, Constant velocity flow area sqin, Equiv. single pipe OD inches, Equiv. dual pipe OD, inches Speed of sound m/s, SoS ratio relative to turbine outlet
0.00 1400 2.000 1.000 17.84 4.90 3.50 644 1.00
0.50 1223 1.667 1.083 16.47 4.71 3.37 613 0.95
1.00 1070 1.333 1.167 15.28 4.54 3.25 584 0.91
1.50 937 1.000 1.251 14.26 4.39 3.14 558 0.87
2.00 823 0.667 1.335 13.37 4.26 3.05 535 0.83
2.50 723 0.333 1.415 12.60 4.14 2.96 514 0.80
3.00 637 0.000 1.493 11.95 4.03 2.89 494 0.77

What this sheet says is that if you need 3.5" pipe after the H-pipe, 2.25 meter (about 7.5 feet) further downstream you only need 3" pipe. So you might want to go very big, right after the H-pipe and run big first set of mufflers, and then bring it down to a single 4" muffler at the end. I agree with your plan of shrinking the cross-sectional area towards the rear, not just because pulses are combined but also because the exhaust gas has cooled.
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Re: Exhaust system idea

Post by travis » Wed Oct 04, 2017 3:22 pm

Another option...spintech can make you a muffler in about any configuration you want. I had them make me a dual 3" in/single 3" big case muffler for my suburban because i needed something relatively quiet and drone free that would fit into the existing muffler spot and connect to the existing dual 2.75" pipes. I couldn't be happier with it...it sounds really good and no resonance issues, and runs just as well as it did with no muffler

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Re: Exhaust system idea

Post by scubasteve231 » Wed Oct 04, 2017 4:25 pm

travis wrote:
Wed Oct 04, 2017 3:22 pm
Another option...spintech can make you a muffler in about any configuration you want. I had them make me a dual 3" in/single 3" big case muffler for my suburban because i needed something relatively quiet and drone free that would fit into the existing muffler spot and connect to the existing dual 2.75" pipes. I couldn't be happier with it...it sounds really good and no resonance issues, and runs just as well as it did with no muffler
That's actually a really good idea, I imagine it would be pretty easy for them to make me a large terminator box with the inlet/outlet configuration to suit my truck. I could even send them a CAD drawing of the dimensions that I needed. The only other factor would be the price they quote me and how that compares the other options that have been discussed so far.

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Re: Exhaust system idea

Post by travis » Wed Oct 04, 2017 4:32 pm

scubasteve231 wrote:
Wed Oct 04, 2017 4:25 pm
travis wrote:
Wed Oct 04, 2017 3:22 pm
Another option...spintech can make you a muffler in about any configuration you want. I had them make me a dual 3" in/single 3" out big case muffler for my suburban because i needed something relatively quiet and drone free that would fit into the existing muffler spot and connect to the existing dual 2.75" pipes. I couldn't be happier with it...it sounds really good and no resonance issues, and runs just as well as it did with no muffler
That's actually a really good idea, I imagine it would be pretty easy for them to make me a large terminator box with the inlet/outlet configuration to suit my truck. I could even send them a CAD drawing of the dimensions that I needed. The only other factor would be the price they quote me and how that compares the other options that have been discussed so far.
Mine was one of the more expensive options (8000 RV series)...it was about $220 including shipping. I gave them the dimensions I needed and it fit perfectly.

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Re: Exhaust system idea

Post by GARY C » Wed Oct 04, 2017 8:09 pm

I did this for my 79 400 4spd T/A because I had most of it and it would fit... It's 3" X dumped into 4" open pipe tapered down to 3" bullets tapered down to 2.5" tail pipes.
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Re: Exhaust system idea

Post by 77cruiser » Wed Oct 04, 2017 9:50 pm

GARY C wrote:
Wed Oct 04, 2017 8:09 pm
I did this for my 79 400 4spd T/A because I had most of it and it would fit... It's 3" X dumped into 4" open pipe tapered down to 3" bullets tapered down to 2.5" tail pipes.
Same but different. My 2 1/2" collector into 3" the rest of the way.
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Re: Exhaust system idea

Post by scubasteve231 » Mon Oct 09, 2017 1:09 pm

Since the replies have slowed down I'll go ahead and say thanks to everyone for the input. I think I understand better what the needs are in terms of overall design.

I do want to run an H pipe now. I'd like to talk about optimal placement. I know most cars place it where they can, usually after the transmission crossmember just because that's where it fits and it can't go anywhere else because of ground clearance. But most cars are slammed to the ground, my truck is not.

I've read a lot of theories about where to place the H pipe...paint/crayon line burning off, etc. Based on my understanding of the H pipe, I believe that the closer it is to the exhaust ports, the more of beneficial it will be. A lot of that is to help with the pressure spike of the double pulse in the firing order on each side, and the sooner you can reduce pressure from that double pulse, the better. For a V8 with headers, that place would be as far up in the header collectors as possible, just after the primary tube junction tapers down to the final collector tube size. Obviously I'll have to see if this will work with the real estate under the truck, but is my thinking about placement correct?

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Re: Exhaust system idea

Post by ptuomov » Mon Oct 09, 2017 1:26 pm

scubasteve231 wrote:
Mon Oct 09, 2017 1:09 pm
I've read a lot of theories about where to place the H pipe...paint/crayon line burning off, etc. Based on my understanding of the H pipe, I believe that the closer it is to the exhaust ports, the more of beneficial it will be. A lot of that is to help with the pressure spike of the double pulse in the firing order on each side, and the sooner you can reduce pressure from that double pulse, the better. For a V8 with headers, that place would be as far up in the header collectors as possible, just after the primary tube junction tapers down to the final collector tube size. Obviously I'll have to see if this will work with the real estate under the truck, but is my thinking about placement correct?
Pipemax is one program that suggests H-pipe locations. Those are related to the "optimal" collector lengths in the sense that the full-diameter cross-over looks a lot like collector end to the pulses. The same way the optimal collector length is not zero, the optimal location of the H-pipe isn't at the primary ends either. That all said, the optimal H-pipe location as prescribed by Pipemax is going to be pretty close to the header primary tube ends.
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