Exhaust system with turbo?

67mustangat16

Well-known member
First off I'm making my own dual side exit exhaust, but I'm wondering what size pipe sounds about right. I'm using the stock exhaust manifold. I was thinking 2 and a quarter inch pipe. What mufflers would be good and so on? Any info would be helpful.
 
This topic is somewhat "mysterious", here is why.

In most states you are not legally required to 'muffle' exhaust on turbo cars because the turbine chops up the exhaust note long before it gets to a muffler. In ND I could run straight out the back/side no muffler required.

In regards to exhaust size, a single 2.25 on a 250 would be sufficient, anything else is overkill.

About your dual side sump exhaust,unless you're running 2 small turbos, having dual exhaust after the turbine (exhaust side) is 100% for show and serves no real purpose.

Intercooler and carb plumbing, I would say no larger than your turbos output diameter. And I would recommend that you choose an engine size (200/250/300) and base the final size off you maximum theoretical boost pressure and how much cfm your engine will need at your predetermined max RPM.

No help at all am I?
-ron
 
That sounds like good advice to me, but I would note that ideal intake plumbing size is fairly scientific - you're balancing flow and velocity - and that *generally* speaking it's going to be bigger than the turbo's compressor outlet... given that you're using a T3 or T4-ish turbo. I'd match the intake plumbing size to the turbo's INLET size, which will probably be 2.5" to 2.75" versus a ~2" outlet on many turbos.

A 2.25" exhaust should be good to around 250hp on a turbocharged car, and 2.5" probably wouldn't be overkill. Remember that the downpipe - the section of exhaust right off the turbo - is the most critical section, and going huge here (3" is not a bad idea!) and then narrowing down is a reasonable thing to do. Turbos work on differentials, so you want to create as big of one as possible between the exhaust manifold and downpipe. After a few feet of big fat downpipe, the exhaust will cool and contract and going back to a smaller pipe is gonna be okay.
 
I'm using a 200 six and wouldn't a big downpipe into a dual 2" exhaust make much less back pressure than one with a single 2"? And didn't know that about the mufflers thankyou. And I'm using a t3/t4 to4b turbo. And thats what I needed to know about the intercooler and intake piping as big as the inlet size. Thanks
 
67mustangat16":2va8bff8 said:
I'm using a 200 six and wouldn't a big downpipe into a dual 2" exhaust make much less back pressure than one with a single 2"?
It is just extra work. Why you making me to the maths..?

using the internets..
http://www.mathgoodies.com/lessons/vol2 ... _area.html

Area of a circle= (Pi)(r)(r)
So..
a 3" pipe = 7.065 square inches
a 2.5" pipe = (3.14)(1.25)(1.25) = 4.90square inches
a 2.25 pipe = (3.14)(1.125)(1.125) = 3.975 square inches
a 2.0" pipe = (3.14)(1)(1) = 3.14 square inches.

Personally, I'd just go with a single 2.5" all the way from the turbo to the exhaust tip, and if you want the look of a dual tip system, have it welded on at the very end. Otherwise you have a lot of tubing to bend and get looking nice for not a whole lot (or any) performance gain.

And after that, I got curious to see what size tubing should go from the pressure side of the turbo to the carb/Throttle Body..

A T3/T4 with a .60 AR (estimated)
I use more power of the internets...
http://www.falcon6handbook.com/compcalculator.asp
VE = 80% (from their default settings)
Randomly chose a turbo calculator, using their defaults where specified and the VE from the falcon performance handbook.
http://www.rbracing-rsr.com/calcboost.html
I took a screenshot, slapped it into mspaint, saved as a JPG, resized it to fit the forum rules and viola!
200TurboCFM.jpg
It says we need 13lbs of boost on a 200 to make 300hp, and that will require 408CFM of air movement.

Then back to the googles for more potentially faulty internet research and I turn up this page (completely unverified, but sure sounds good)
Okay this is a quote from a book called maximum boost. There is a certain air velocity that shouldn't be reach. It shouldn't reach 450 feet per second b/c that will cause a restriction.

To calculate cfm you multiply desired bhp by 1.5. Then you use the formula velocity=airflow/area of tube

For example a 400bhp car will have 600 cfm and we are going to use a 2.5 inch diameter piping. So we use the formula velocity = [600cfm/pie{2.5/2}^2


So velocity equals 293 ft/sec. This means we are under 450 feet and we are safe from restrictions. Any tubes bigger then 2.5 will cause lag.
More maths.. grr
oki, lets use his digits to see where we need to be...
300*1.5 = 450 CFM (pretty close to the 408 calculated earlier)

Now using that velocity formula, I'll try random (common) pipe sizes
2.0": 450/3.14 =143.312ft/sec
2.25" : 450cfm/3.975 = 113.35ft/sec (3.97 calculated earlier from pipe diameter square inches)
2.5" : 450/4.90 = 98.4ft/sec
3.0" : 450/7/065 = 63.7ft/sec

And more potentially 'research'
http://honda-tech.com/showthread.php?t=1202591
turncoat":2va8bff8 said:
Okay then, let's see how much the charge pipe size (and thus volume) affects lag time:

Every foot of 2" x 0.065" tube will hold 0.095 CF.

If you can flow 400 CFM from 0 to 15 psig (please excuse the simplification of the transient, it's just a quick estimate), you can pressurize the charge pipe from 0 to 15 psig in 0.014 seconds. (Note this is not spool time, just the time it takes to fill a pipe given a certain flow which is what we are trying to isolate)

Change to 3" charge piping, your volume increases to 0.22 CF and for the same scenario, the time to fill increases to 0.033 sec..

So the question is can you feel the extra 0.019 sec it takes to get to 15 psig when you increase your charge pipe size from 2" to 3"? Not likely.

Please don't pick at that the calculations were massively simplified, it was just to verify the hunch that charge pipe volume is not a very sensitive factor in the common sizes.

The problem with the first post is velocities are calculated for a given pipe size and flow, which is relatively insignificant. What you want is pressure drop as the main component in sizing the charge pipe. Use the link in my previous post to calculate pressure drop
So basically he says it doesn't matter, use whatever comes along the cheapest or looks the best under your hood.

There are some good books on this subject, quite a long time ago I bought/read Corky Bell's Maximum Boost book, but I have yet to install anything... so if anybody will less potentially bogus info wants to slap me upside the head, I'm good with that..
 
One brief addition:
The pressure drop (i.e. the backpressure generated by the flow) in a pipe is proportional to 1/(D^4) so small changes in diameter make big changes in backpressure. Going too big on the diameter drops the exhaust velocity and can actually hurt performance at lower RPM; going too small can strangle things very quickly. At 300 HP, I'd guess that a single 3" all the way back would be about right. If you insist on the extra work of duals even though you gain nothing from it that could be a pair of 2" or 2.25" pipes.
 
CoupeBoy":ssq9lk7r said:
I'm using a 200 six and wouldn't a big downpipe into a dual 2" exhaust make much less back pressure than one with a single 2"?
It is just extra work. Why you making me to the maths..?[/quote]

Epic post! :)

I agree that a single exhaust is the way to go. Dumping into a turbo, going through a single downpipe, and then breaking back into two pipes which need to travel down the length of car seems like a lot of work for no gain.
 
This is one of the area where bigger is better and the biggest pipe you can work with is a good idea. That would probably be 3" from the turbo to as far back as you can get it. If you run it all the way to the back of the car, you will likely find that you will not need a muffler as the turbo does a great job of muffling the sound by its design.

IF you do decide you need to run a muffler and again, I doubt you will, do NOT use a chambered "Flowmaster" type design. The baffles and acoustics they use to muffle exhaust noise are counterproductive in a turbo exhaust system. You want the exhaust to exit quickly and efficiently after the turbine and not bounce around making cool noises in the process. 8)

A good old fashioned 25 dollar fiberglass muffler would be swell IF you feel you need it.
 
Yes I realize that running a dual all the way back would be pretty much useless. But I was talking about a side exit exhaust.(and yes because I like the look)
Thanks guys I learn more everytime I come on here.
 
Having side dump exhaust from a car that inherantly has a single side exhaust outlet will present its own challenges. You will have to cross over from the passenger side to the driver side somewhere, I would recommend right in front of the inspection cover on the transmission bellhousing. And then run it the rest of the way back.

In my brain for my '67 if I ever get around to working on it (darn life keeps getting in the way) I envisioned either a single 2.5" all the way to the rear after the turbo with no muffler, or a high flow muffler of some sort.

Or feeding it out just the passenger side through a NASCAR looking boom pipe...

FWIW, the other day I showed my wife a picture of a '70 Mustang with
70's style Side Pipes She says she liked them :roll: (ain't gunna happen)
 
CoupeBoy":8s8yfyzp said:
Having side dump exhaust from a car that inherantly has a single side exhaust outlet will present its own challenges. You will have to cross over from the passenger side to the driver side somewhere, I would recommend right in front of the inspection cover on the transmission bellhousing. And then run it the rest of the way back.

In my brain for my '67 if I ever get around to working on it (darn life keeps getting in the way) I envisioned either a single 2.5" all the way to the rear after the turbo with no muffler, or a high flow muffler of some sort.

Or feeding it out just the passenger side through a NASCAR looking boom pipe...

FWIW, the other day I showed my wife a picture of a '70 Mustang with
70's style Side Pipes She says she liked them :roll: (ain't gunna happen)
Well, one thing to remember with that style exhaust is to not dump it infront of the rear wheels, because then it covers the lift point and torque box on one side. (Previous owner did that on my car and it's a big pain in the butt. :banghead: ) looks cool though.
 
Turbo is completely different than naturally/aspirated (n/a)
N/a you need back pressure. turbo it is the enemy. Less back pressure=faster spool and boost .

That said 2 1/2" will work up to 250hp. It's all in what you want.
 
Codfish777":2yxy1pkc said:
Turbo is completely different than naturally/aspirated (n/a)
N/a you need back pressure. turbo it is the enemy. Less back pressure=faster spool and boost .

That said 2 1/2" will work up to 250hp. It's all in what you want.

an N/A motor DOES NOT need or want back pressure.

this is common non-sense that is spread around the internet.
 
country fried 6":3uxoq62d said:
Codfish777":3uxoq62d said:
Turbo is completely different than naturally/aspirated (n/a)
N/a you need back pressure. turbo it is the enemy. Less back pressure=faster spool and boost .

That said 2 1/2" will work up to 250hp. It's all in what you want.

an N/A motor DOES NOT need or want back pressure.

this is common non-sense that is spread around the internet.[/quote]

Yes it does, unless you are just running a drag strip. Your normal daily driver will be poochy untill wide open throttle with no back pressure. It is not a miss conception. I have bought imports that teeny boppers throw a 3 inch exhaust on a stock 4 banger. First thing I do is junk the exhaust and put a 2 1/2 exhaust with a real muffler. Power increases greatly.
I have been there and done that. Bigger is not always better.

But a turbo engine doesn't need back pressure


If you don't believe me. Throw straight pipes on your naturally/ aspirated car. drive it. Then put a flow master on it.
Tell us which one is faster for street driving.

I know which is faster.......I haven't just read forums...I have actually built several cars.

Hope this helps.
 
bigger is not always better, your right there. but, the reason for that is that too big of a pipe causes back pressure. when the pipe is over sized the flow through it is slow and the exhaust cools too much before it exits the pipe, creating a plug-like situation until exhaust volume is substantialy increased. your endorsement of flowmaster mufflers should prove this to yourself. the very idea that flowmaster promotes is that their mufflers increase scavenging, a negative pressure in the exhaust. now if your idea that "no backpressure is a bad thing" were true, then a negative pressure should be REALY bad. my current truck has a magnaflow muffler, essentialy a straight pipe, there was no loss in power. my last truck has a flowmaster, it worked equally well. I have built a few myself, and not one ran better with the factory restrictive muffler than with a properly size aftermarket or custom.
 
Y'all are confused on the back pressure thing. It's not like a garden hose because the exhaust pulses. An incorrectly sized exhaust costs power because the timing of the vacuum pulse that travels back up the system from the end when a high pressure pulse exits is incorrect. Ideally, that vacuum pulse will arrive at the exhaust valve in time to empty the chamber and even suck a bit of intake, depending on overlap. Incorrectly timed it will actually shove exhaust back into the chamber, causing all sorts of maladies. So, pipe sizing is a problem not only of diameter, but of length. That said, if you take a car than ran decently with a 2" exhaust and replace that with a 3" exhaust of equal length then you will indeed probably create a dog. Plus, your call will probably sound stupid (e.g. fart can on a Honda)
 
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