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PostPosted: May 14, 2021, 5:13 pm 
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Hey guys- I'm working on a microcar with a bike engine, and there is NO room to run a 'proper' round exhaust under the car's fiberglass belly pan, which has me looking for an out-of-the-norm solution..

Right now I've got the exhaust dumping almost straight into the ground under the engine, but it's too loud and obnoxious up there, and I'd like to stretch the exhaust out to the back of the car (likely just past the rear axle center line).

An inch is a mile at every turn on this car; as she sits I've got about 4.5" clearance underneath (road to bellypan).. My exhaust is 1.75 (round) up to where it ends under the engine, and I'm wondering if I can run an under-car exhaust section of tailpipe that is rectangular @ 3" wide x 1" thick (.065 wall).. This is super low-profile stuff when it comes to exhaust, but I'm only feeding 800ccs through it.

The paper cutout pics below help to visually compare the tubing volume, and the potential ability to flow this exhaust. For what it's worth, this proposed 5 foot long rectangular section will be virtually perfectly straight- it's sole purpose is to put the noise/exhaust behind the driver- no sharp curves or bends..

For sure this is not ideal.. every solution is a compromise, and at this point I'm willing to make some compromises for a better driving experience. Any thoughts on this unconventional approach?

--ccrunner

*Aside: This car also has an exhaust 'dump/open header' valve that allows me to have a truly 'open' free-flowing exhaust.. My fuel controller will also let me run fuel map #2 on a switch if I want to run the car with open exhaust in anger.. the reality, though, is the dump valve is just a static/car show/ego thing that will rarely get used; it is an option though..


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1972 Honda N600 Restomod "ccrunner's N600 VFR800 repower"

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PostPosted: May 14, 2021, 6:09 pm 
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I think oval tubing would be better as you wouldn't get "dead" spots in the corners. But, finding oval in that size would be the same as finding rocking horse s&^$. I wonder if you could find a round tube that you could squish to the nominal dimensions of 3x1.


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PostPosted: May 14, 2021, 7:58 pm 
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It would flow about the same as a 1.95 round pipe.

We were talking about this a couple years ago during your pre-planning stage:

viewtopic.php?f=27&t=20127

Standard 11ga is a better idea to prevent the hot metal from trying to go round on you from exhaust pressure and help quiet the exhaust.
You could cut a few slots to weld in plates like a flowmaster for an integral muffler.

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PostPosted: May 14, 2021, 9:01 pm 
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my car was too noisy with the standard side exit exhaust and i had no room to get the 2-1/2" exhaust out the back so i made a transition piece to turn the muffler exit under the car and transition into two 1" x 2" tail pipes from rectangular pipe.
i mounted it to the bottom of the car with MG midget exhaust rubber mounts (cotton reels) my car is now supercharged and i have noticed no drop in performance with the exhaust, in fact i had to adjust the fuel in the PCM to account for the straight through exhaust system.
and now the exhaust noise is left behind.

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PostPosted: May 15, 2021, 9:52 pm 
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Have you seen the NASCAR flat exhaust designs?
Attachment:
NASCAR Flat Style Exhaust.jpg

Might that work for you?

Cheers,


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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

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PostPosted: May 15, 2021, 10:05 pm 
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Lonnie-S wrote:
Have you seen the NASCAR flat exhaust designs?


Wow, that is some fancy flow diverting geometry inside those spin trap mufflers, wasn't aware of those before today!


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PostPosted: May 18, 2021, 4:28 pm 
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Thanks for the thoughts guys... I'm going the route Miatav8 and John suggested some while ago and run a 1"x3" (.083 wall) rectangle exhaust pipe down the center under the belly pan.. If having it exit out the rear doesn't tame it enough, then I'll fab a smallish chamber muffler to help quiet things down.. I wanted to avoid an under-the-pan exhaust, but I think it's going to simply be what's needed to make this thing freeway tolerable.

--ccrunner

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1972 Honda N600 Restomod "ccrunner's N600 VFR800 repower"

1963 Volvo P1800 Restomod
http://locostusa.com/forums/viewtopic.php?f=36&t=16309

1959 Berkeley SE492 Restomod...
viewtopic.php?f=36&t=19397

"ccrunner's 1960 MGA coupe Restomod" found on MGExp.com


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PostPosted: May 20, 2021, 1:33 pm 
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Because the corners don't flow as much as the center of the pipe, there is a calculation for rectangular duct equivalence to round duct for equivalent pressure drop.

1x3 with .083 wall is .834 x 2.834 inside which works out to equivalent to a 1.61" diameter round pipe, so just a bit smaller area (15%) than the 1.75 pipe. So for the quiet exhaust, not bad. Pretty heavy wall for exhaust, but HREW is not probably available thinner.

You could squash some 3" exhaust tubing to a flattened oval and be pretty close on area, a bit lighter as well.


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PostPosted: May 20, 2021, 1:44 pm 
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Any reason not to split a tube length wise and add the center section back in with sheet metal?

I had to do this for my megaphone tail pipe:
Attachment:
IMG_20170211_102659668.jpg



You will probably have this issue with both the square tube or a fabricated design, but keep in mind that the flat walls will have a much lower natural frequency and will be more prone to crack over time. If you do a fabricated piece, it is much easier to add internal stiffeners. This was two versions before the one above:

Attachment:
IMG_3201.jpg


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PostPosted: May 20, 2021, 2:20 pm 
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Go with the rectangular pipe if it fits the available space, move on to the next problem.

Even Real Racing Cars(tm) have to make compromises in the exhaust system, even though it can cost them power.

The only problem you might see would be fatigue cracks from "oil canning." The exhaust pulses will try to blow the first few feet of the pipe from rectangular to round, and after long enough that can cause cracks. That's why "flat" mufflers are usually elliptical. Racing mufflers that were actual flat boxes solved the problem by drilling from top to bottom and welding posts in to keep the flat panels from ballooning. But, honestly, I doubt you'll have that problem. If you want to do some prevention anyway, space some 3/16" rods three or four inches apart in the middle of the first few feet of pipe.


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PostPosted: May 20, 2021, 3:27 pm 
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duratec7 wrote:
Because the corners don't flow as much as the center of the pipe, there is a calculation for rectangular duct equivalence to round duct for equivalent pressure drop.

1x3 with .083 wall is .834 x 2.834 inside which works out to equivalent to a 1.61" diameter round pipe, so just a bit smaller area (15%) than the 1.75 pipe. So for the quiet exhaust, not bad. Pretty heavy wall for exhaust, but HREW is not probably available thinner.

You could squash some 3" exhaust tubing to a flattened oval and be pretty close on area, a bit lighter as well.


I didn't consider the frictional loss, but the difference is only about 2.5% when you consider typical 18GA exhaust pipe wall thickness. A 1.75 pipe performs like a 1.65 duct and a 1x3 with 0.83 wall performs like a 1.61 duct.

Here is a way for you to make the 1x3 an integral, tunable (for back pressure and db) muffler that will also prevent ovaling.


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Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12


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PostPosted: May 24, 2021, 3:46 pm 
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5-24-21

Thank you guys for your thought and input on this :cheers:

Here are a few pics of where it is now... I went from 1.75" round to 1"x3" rectangle.. it sounds much better than before, and I may still go in and make a small 'chamber muffler' much like Miatav8 has drawn :cheers:

It's nice, and it's close to being what I want; still looking to shave a few more decibels.. so much nicer than it was before I did this 8)


--ccrunner


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Dean

1972 Honda N600 Restomod "ccrunner's N600 VFR800 repower"

1963 Volvo P1800 Restomod
http://locostusa.com/forums/viewtopic.php?f=36&t=16309

1959 Berkeley SE492 Restomod...
viewtopic.php?f=36&t=19397

"ccrunner's 1960 MGA coupe Restomod" found on MGExp.com


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PostPosted: May 25, 2021, 9:18 am 
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You did an excellent job on the transfer from round to rectangle. :cheers:
If you have the extra length at the rear exit, use a directional tip turned down at a 45*. Just cut the sides and bend the upper web down, so the pulses is angled toward the road surface, which will help break up the sound.
Davew


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PostPosted: May 25, 2021, 10:10 am 
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Yup, excellent design and execution. Congrats.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886


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