I've been reading this board for about 2 years(just joined last nite) and by far the most numerous suggestion for Ex. Dia. is 2 1/2 ". On a stock 240 this has 237% the volume of the stock system; the factory gets 150hp (gross) from this system with a restrictive muffler.....if we multiply that by 237% we get 355hp and maybe more with a less restrictive muffler which most owners do. I would bet not everybody gets those numbers.So my question is ; is that large a diameter needed especially when most of the hp figures that I have seen posted on this and other boards is in the 200-250 range( we are talking non turbo applications here) Using this logic ( it's probbaly flawed) a 2" system would be in the 275hp range. Am I way off base here or not?
That was logical thinking, but that will just allow the exhaust to flow 237% more. Which could be more efficient for the engine, or could be less efficient, depending on the way your engine is tuned. All horsepower is not determined by exhaust.
2.5" is overkill for a NA engine, even for a hot 2.8L.
2" is big enough for a stock 2.4L and I'm running 2.25" on a pretty hot 2.8L and seriously doubt it's causing a restriction. I went from 2" to 2.25" with the stock 2.8L and didn't notice -any- difference in performance. I went up in pipe size as the old system was rusted out and was planning to hotrod the engine. Can't say if it would have made a difference after the engine mods...
Rice boys put big -tailpipes- on cars which is the LAST part of the exhaust that needs to be large so just because "everybody" uses a certain size, doesn't mean they put any thought into it. I'd think 2.25 from the header back to a resonator halfway back and 2 inch from there on out would work fine for most Zcars.
I chose my words poorly, (support) was the focus of my post.I realize putting on larger Ex. doesn't give you more hp. If the factory 1 5/8 can support 150 hp. then a 2 " having 51.5% more area should flow about 51.5% more which should support 227 hp. Humm .... I see my 1st example of 275hp. was wrong, 227 is the better figure. I think Steves280 has it right with the comments of his experience. Please note I'm not trying to dispute anyone just wondering that if anyone thought about this the same way and might come up with the same conclusion.
280zxreigns, a 3" has 341% more area than stock,so it may support 511 hp, using the logic of this discussion. I'm sure in the real world things are a little different; but I don't think its as far off as some think.
My 280z came stock with 2 in and i used similar logic as you and i assumed 2in was good for 130rwhp. I came up with single 2.5in supporting 203.5 rwhp. And 2.25 supporting 164 rwhp, you could acheive 160-170 rwhp range realistically so 2.25 is about all you need. Any more than 170rwhp will take some serrious work. 2.5 is more availble and i wont be resstricted by the pipe so thats what i chose.
You also have to remember that the factory exhaust is crush bent. At the bends, the diameter is much smaller.
When I first got my car, it still had the original muffler. By then, it was all clogged up with deposits so I just replaced it with piping of the same size but now mandrel bent. Doing that made a big difference!
Now I am planning to get 2.25 piping with a resonator and muffler.
Why? I want my exhaust note to be a little bit lower.
With the money I use for that, maybe I should just save up for a cam . . .