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Old 03-09-2021, 02:08 PM   #1
JerryJS8
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Default Need advice on split 3" exhaust system for 740 16v Turbo

Evening fellow T-brickers!

I need ya'lls help as I consider a new arrangement for a 3" Exhaust system for my 16 valve turbo. What I want is a throaty idle but very quiet when cruising. Letting the motor rev out shouldn't make a fart can impression, rather just sound a bit more aggresive than the stock system. Main objective of my build is a fully stock looking oldtimer that does not boast, but only hints at high horsepower.

What I am currently doing is matching up a 3" cosworth T3 housing flange to a 5" 200 cell metal cat. After the cat I want to split the pipes from 76mm down to 60mm using a full-through silencer like this.



Afterwards I'd have two 60mm axle bends going parallel into a second silencer finished off with stock like twin s-pipes. Easier said with a picture than in words, so here's a quick rendering model of what I'm thinking.



Going with two smaller diameter over-axle bends will give me the most clearance to the axle, but I can't imagine how it'll all sound with split pipes from a single source. Will I just be losing definition of the exhaust impulses at idle and still just have a loud system, or will I be as quiet at speed as I hope and still have a throaty note at idle?

If we got any experienced exhaust builders in here, I'd really appreciate your input!!!
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Old 03-09-2021, 05:36 PM   #2
VB242
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I was thinking about doing this exact same thing until I stood under the car and looked at where it had to fit, I'm just going to use 3", but will be watching to see how yours turns out.
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Old 03-10-2021, 02:38 AM   #3
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Have you considered a low profile section over the axle? I can't find the page that had a photo of a low profile section around a bend, it was basically the same as this but using a 1" pipe bend for the outsides of the section. You just match the cross sectional area of the 3" pipe with the low profile section.
IMG-20210310-062845-398IMG-20210310-062848-203
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Old 03-10-2021, 04:06 AM   #4
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I would go 3" with big, oval supersprint mufflers. It fits quite well...
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Old 03-10-2021, 06:02 AM   #5
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@Sparkles Nope, ain't considered that. It's an option, but don't know if that'd be more work than installing a split system?

@MadDog Maybe I will end up doing that... but this piece is just soo nice to look at as well
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Old 03-10-2021, 07:24 AM   #6
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Hmmmm....

Last edited by JerryJS8; 03-11-2021 at 07:32 AM..
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Old 03-15-2021, 03:42 AM   #7
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Quote:
Originally Posted by Sparkles View Post
Have you considered a low profile section over the axle? I can't find the page that had a photo of a low profile section around a bend, it was basically the same as this but using a 1" pipe bend for the outsides of the section. You just match the cross sectional area of the 3" pipe with the low profile section.
IMG-20210310-062845-398IMG-20210310-062848-203
looks nice :-) what grade of steel did you use? 304 (1.4301)? or 321 (1.4541)?

stainless steel pipe
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Old 03-15-2021, 10:09 AM   #8
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I watched Season 5 Episode 60 of Engine Masters last night. Anyone building an exhaust would be well served by watching it.

They make it clear that dyno is not the same as a car. And of course the engine was a N/A 400 hp 383 Chevy, not a 4 cyl turbo.
Back to back tests of:
Dual exhaust 2.5 inch with turbo mufflers.
Dual exhaust 2.5 inch with straight through mufflers.
Single exhaust 3 inch with turbo muffler.
Single exhaust 3 inch with straight through muffler.

No surprise . . . Straight through mufflers killed turbo mufflers.

But the 3 inch single exhaust with one straight through muffler killed the dual exhaust with straight throughs. Single exhaust 3 inch and straight through was king.

https://www.motortrendondemand.com/d...xhaust/123988/
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Old 03-15-2021, 10:25 AM   #9
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I gotta get back on that series, way too many episodes in my backlog, including that one. Makes me happy to know that both my truck and wagon are running the right setup though, 3" single straight thru's.
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Old 03-16-2021, 08:42 AM   #10
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Quote:
Originally Posted by stylianou View Post
looks nice :-) what grade of steel did you use? 304 (1.4301)? or 321 (1.4541)?
Just mild steel. Also I fluffed the width of the low profile section and it only has the cross sectional area of a 2.5" pipe, it'll do for now.
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Old 03-18-2021, 01:50 PM   #11
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I've been thinking something similar for my 88 765.
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Old 03-18-2021, 04:17 PM   #12
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You could make one of the two sections a bit longer under the car to create an effect close to a Helmholtz resonator. Aim for 1/4 wavelength.

Single 3" over axle with split outlet, as my brother has on his 16VT 760 does have a drone at highway speed. Never bothered to eliminate the drone as its not on the road now.
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Old 03-21-2021, 02:14 PM   #13
JerryJS8
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Quote:
Originally Posted by dbarton View Post
I watched Season 5 Episode 60 of Engine Masters last night. Anyone building an exhaust would be well served by watching it.
That was definitely a fun video! Even interrupted my Netflix movie to cram it in

As they said in the video, dual 2.5" is a 38% increase in surface area over a single 3" pipe, which would make you think that the increased volume would increase output. But with dual 2.5" you have an increase of 66% in pipe curcumference, which results in 66% more inner diameter surface area dragging on those exhaust gases riding along the surface relative to the gases that have no contact to the pipe.

On the turbo 4-banger it'd be a 3" DP through the cat into the first muffler. After that I'm thinking about two outcomes.

1. Single 3" exhaust through single 3" straight through mufflers will allow gases to run with less contact to the inner diameter walls, reducing drag, increasing bottom end efficiency and maximum flow, but causing a droning sound at some frequencies when the engine does not produce enough volume to keep the gases moving at a constant speed until they exit the tailpipe.

2. Single 3" downpipe into a split first muffler with two 2.25" pipes over the axle results in a 12.5% increase in volume over a full 3" pipe, but also a 50% increase in pipe diameter. This will still allow enough volume to flow and not choke out the motor up top and it will (hopfully) result in enough inner-pipe-surface-to-gas-contact-drag at light engine loads to prevent droning. Another benefit would be inside the split mufflers you would have 50% more surface area where the perforated muffler piping can absorb even more noise compared to the BLAAAARRRGGGGG of a 3" straight through.

Since in my head the second scenario makes sense, it doesn't mean it reflects reality. What I want to avoid is droning at all cost, even if it means going with a smaller diameter exhaust or less efficient muffler design. I don't want to build system with 600+HP potential, just a comfortable one for 300HP at the wheel.

I had though about a helmholz resonator, but I watched my garage neighbors experiment themselves to death with their calculations and eventually just accept the drone...
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Old 03-22-2021, 08:41 AM   #14
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Don´t be afraid of Helmholz. I made it on my car and it worked. As I already had 2pcs metalic catalytic converters, a big muffler in the middle and end and had no other solution than change the mufflers or Helmholz...
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