So Cal Mark wrote:1.75? is this supposed to be a performance exhaust?
I did not ask for a performance exhaust, nor did he inform me of such. I just asked for some details to post here As far as performance goes, the exhaust comes out only at the end (which is not how it was before lol) and sounds great doing so.
So Cal Mark wrote:1.75? is this supposed to be a performance exhaust?
I did not ask for a performance exhaust, nor did he inform me of such. I just asked for some details to post here As far as performance goes, the exhaust comes out only at the end (which is not how it was before lol) and sounds great doing so.
This brings up something I have thought about for a while. If the outlet from the exhaust manifold or the inlet or outlet of the converter is 1.75", does it really matter what the diameter of the rest of the pipe is?
Don Avery
Dayton, OH/Sarasota,FL
1985 Pininfarina
2011 Eos
2012 Mercedes E550 Cabriolet
It depends on a lot of things. There are 'cat back' systems available for a lot newer cars and supposedly they add some power. Only dyno testing would tell the truth.
Csaba
'71 124 Spider, much modified
'17 124 Abarth, silver http://italiancarclub.com/csaba/
Co-owner of the best dang Fiat parts place in town
Roughly speaking (and depending on how you want to do all the numbers), 1.75" could flow enough for a 125hp motor. 2" could flow enough for about 160hp. Exhaust velocity is also a consideration.
vandor wrote:It depends on a lot of things. There are 'cat back' systems available for a lot newer cars and supposedly they add some power. Only dyno testing would tell the truth.
But is the "improvement" from an increase in the pipe diameter or less restrictive muffler? In theory I just don't see how a 2.25" inch exhaust is an improvement if it has even one choke point of 1.75" diameter. Once the gases have to slow down to fit through the smaller pipe, does it really matter how much larger the pipe afterward is? Which is why I have never purchased a header and 2.25" performance exhaust for my car. If the outlet of the header is 1.75", what is the point?
Don Avery
Dayton, OH/Sarasota,FL
1985 Pininfarina
2011 Eos
2012 Mercedes E550 Cabriolet
To be pedantic, the flow (velocity) will actually speed up through the restriction. The total volumetric flow rate through the system will depend on the sum of resistances along the whole chain, so in principal opening it up after the restrictive header could still be beneficial. As for the restriction, it will depend on how long that restrictive section is. Breathing through a 10" long straw feels a lot different than through a 1" one.
Good discussion about velocity through the tubes and restrictions. There is a ton more that goes into performance exhaust such as header design, shape, length. The width of the pipe from the header back or some other point such as CAT back. For a true exhaust performance gain I would think you need to go from the head back. So is there such a think as too much or too wide an exhaust... yes, there need to be some backpressure.
I had to install a resonator after taking the CAT out before the muffler. it was just too loud.
Volumetric efficiency of the engine plays an important role here.. IE how quick you can get air through the engine. Typically intake cfm on a street performance machine is lower than exhaust.. Otherwise low rpm response would be not so streetable.. Even with 1.75" header opening. A 2" or 2 ¼" pipe will flow better. Down side is it will be louder..
This is a really interesting thread. So it would appear whether the header is a 4-2-1 or a 4-1 the area of the collector will be less than the total area of each of the four pipes leading into it. If the collector is 1.75" then the area of a cross section is about 2.4 sq in. Divided by 4 would be .6 sq in. This would mean each of the 4 pipes would be a diameter of less than 1/2 in. That can't be right, so the collector does restrict the flow. How much? I have no idea what the inside diameter of each pipe is, but assume it is 1". That would mean the area of a cross section would be .785 sq in. Since there are 4, that would mean the total area of all four cross sections would be 3.14 sq in. So through the header the total area decreases by about 20%. This would be a gradual decrease, so I assume this acts as a merge collector to create back pressure. The question is after the pipe goes to a 2.25" pipe after the merge collector, is any additional decrease in area of of the cross section (say at the converter) really needed? For example, would a straight pipe provide the most horsepower? I'm probably missing something or making this too simple. I'm just trying to understand the mechanics of the exhaust system.
Last edited by davery on Thu Aug 06, 2015 1:49 pm, edited 1 time in total.
Don Avery
Dayton, OH/Sarasota,FL
1985 Pininfarina
2011 Eos
2012 Mercedes E550 Cabriolet
generally the tubes are at least 1.5" and the collector is 2.5". Our header steps from 1.5 to 1.75 per tube into a 2.5" collector. Dyno testing has proven a substantial hp and torque improvement with no other changes to the engine. At peak hp we had a 10% increase, peak torque was a 15% increase. What is really telling is the power and torque increase at 6000rpm. The stock hp and torque curves begin to drop at 6k, while our header/exhaust kept the curves nearly flat so that we had a 15% hp increase and 20% torque increase at 6k. This indicates the stock exhaust system doesn't flow enough at high rpm
TX82FIAT wrote:I had to install a resonator after taking the CAT out before the muffler. it was just too loud.
I haven't yet put a resonator on the 1800. Abarth Headers -> 2.25" -> Magnaflow backbox -> Twin pipes ..... it's a little on the loud side, and I suspect the boss is going to insist eventually.
On the 2000. Stock Manifold -> 1.75" -> Cat -> Resonator -> Magnaflow backbox -> Twin pipes ..... and it's a glorious note.
As far as performance goes, the exhaust comes out only at the end (which is not how it was before lol) and sounds great doing so.
