Using a 2.5 inch exhaust on a KA-T??

[vadem]

I found a sweet deal on an HKS Hyper exaust only problem is its the na version which is 2.5 inches. I know 3 inch is better, but seriously, does it make that much difference at say 8 lbs, or about 250 rwhp? Does anyone have any expirence with this? The stock piping is like 1.8 inch, so 2.5 is quite an improvment. Also 400hp supras use mostly 3 inch exhausts, and they boost like 18 lbs. Your thoughts? Later

[littlemilla3]

You should be fine. Many DSMers use 2.5" if they want to keep it quiet and it suites them well. There will not be a big difference between 2.5" and 3" if you are only pushing 8lbs and 250 whp.

[TrunkMonkey]

i'd hold out for a 3" exhaust.

trust me, there is a huge difference. ask ADAMHU.

-demetrius

[andrave]

there isn't that big of a difference, I'd get the 2.5 if the price is right. it will be quieter, and you will lose a few theoretical HP against a bigger 3", but not a whole lot.

[C-Kwik]

It's not a few. When I went from a 60mm to 80mm exhaust, I noticed a significant increase in turbo response and power. Turbos rely so much on the pressure differential across the turbine that any help to decrease pressure after the turbo, tends to make a drastic difference.

[MikeMurphy]

Also note that 3 inches is MUCH larger than a 2.5 inch exhaust pipe, thus the noticable difference.

[andrave]

I'd like to see dyno charts to back that up, I'm guessing you wouldn't see more than a 15 hp increase at most, probably less, especially across most of the power band.A high quality mandrel bent 2.5 with a well built muffler (like the HKS) surely can't be that restrictive....

[C-Kwik]

I don't have dyno charts, but have seen plenty. But consider this:

pi*r^2

1.25^2*3.14 = 4.9

1.5^2*3.14 = 7.06

The difference in the area of the cross section of the pipes is about 2 square inches. That's pretty significant. Turbos like big exhausts. With an additional 2 square inches of room for the exhaust to sqeeze by the pipe, it makes it a lot easier on the motor to provide enough air to spool the turbo since the exhaust manifold pressure can be lower.

[TrunkMonkey]

I'd like to see dyno charts to back that up, I'm guessing you wouldn't see more than a 15 hp increase at most, probably less, especially across most of the power band.

here. some people argue that the gain was from removing the cat, but the only times i've seen a cat cause that much restriction is on big boost/high horsepower applications. i've seen people not gain anything from removing their cat, but there always a significant gain with a larger exhaust.

A high quality mandrel bent 2.5 with a well built muffler (like the HKS) surely can't be that restrictive....

compared to a 3", yes it is. save the 2.5" for n/a.

boost and backpressure just don't mix.

-demetrius

[hannibal]

To echo waht C-Kwik said, going from 2.5in to 3in is a 44% increase in cross section.The big block cat he was using has to be more restrictive than a straight pipe so I believe some of his gains came from using a test pipe.In another thread about charge pipe size, someone mentioned that hot pipe should be/could be bigger than the cold pipe to due the fact that hot air is less dense and can take advatage of the extra size.I think this would apply to exhaust tubing as well. With 44% more area, I believe those 23hp and 36lbs-ft gains.

Jay

[andrave]

yeah but that dude didn't say he was using mandrel bends on the 2.5", and he was using a ****ty ractive can...and on a turbo car removing the cat is known to make a big difference...

[C-Kwik]

He didn't say he wasn't either. And I believe the Ractive mufflers use a straight through design which should not have much of a difference, if at all, to any other straight through muffler of that size in terms of flow.

Yes, removing the cat can make a large difference. I did notice a difference in mine when I put a cat on to pass smog(loss of power). However, in the 2.5 inch piping run, he notes later that the cat was 2/3rds hollow. Not exactly the greatest formula for smoothness, but the cat would not be a huge restriction to airflow overall in that kind of condition.

Also, the 23 HP gain is only the gain in peak power. If you go to his website he posts his dynocharts. At about 4250 RPM he is up about 35 HP. Since his 2.5" run is on a graph and the 3" is on a chart, it's hard to see what the actual peak gain is, but 35 is a lot of extra HP.

http://www.turbo240sx.ca/dynorun1.jpght ... o%...2.jpg

[MarkEmark]

I'm running a 2.5" exhaust on my car right now (ka-t, 9 psi)--and if I had the choice again, I'd definitely get a larger exhaust. I'm not so sure about 3", but definitely at least 2.75". But, I don't have $500 to drop in a new exhaust; that's what happens when you go from N/A to F/I.

I'm running a catco high-flow cat. My downpipes are crush-bent and incredibly restrictive (I am currently in the process of re-fabricating them). The first section of my downpipe actually had a smaller (very very marginally) inner diameter than the diameter of the opening for the turbine!!!!

The crappy, restrictive design of the downpipes + the catalytic converter were causing quite a bit of back pressure in mine and were causing the boost to go from 9 psi at 5k to 6.5 psi at 6k--The engine was literally being choked at that high RPM.

Then I disconnected the exhaust except for the FIRST downpipe, and the car felt absolutely beautiful. Full 9 psi up to 6k rpm. It felt substantially faster with 200 pounds more weight and no exhaust (my brother/more gas was in the car) than when I ran it before, same conditions, with just me and less gas.

As I said, I'm getting the downpipe sections re-made into TRUE 2.5" piping with TRUE mandrel bends. Then, I'm having a straight-pipe manufactured to replace the cat (I'll just put the cat back on for emissions). If I'm still dissatisfied with how it runs with the straight pipe/rest of the exhaust (which is a 2.5" mandrel bent tanabe), then I'll just disconnect everything except for my two downpipes, at least until I get pulled over for excessive noise :pface

I'm also going to experiment with running the car with just the downpipes/catalytic converter and then nothing else--hopefully the cat will quiet things up a bit.

In any case, listen to me and wait for something a bit bigger!

[vadem]

Cool, im glad I asked this question, anway the hks is $200 which is a super deal, and its in very good condition, but I think ill pass it up, and get the Megan racing full 3in off ebay, they can be had for 250-275 + shipping Ive heard they are nice quality. Thanks doods.

Vadem

[ca240]

no he's not...good catch man...it's actually 30.4% increase in cross sectioanl area...but still that's a significant amount...later

[C-Kwik]

Actually it depends on perspective.

2.5" piping has a cross-sectional area that is 30.4% of 3" piping. But 3" is a 44% increase OVER 2.5" piping(or 144% of 2.5" piping). It depends on which pipe size you are using as the reference.

4.9/7.06 = 69.4%

7.06/4.9 = 144.0%

[daniel240]

sorry to threadjack but my turbo outlet is like 2.25in. i am making a down pipe that is 2.25-2.5'' would a 3'' help the rest of the way back or would it need to be 3'' from the turbo back?

[hannibal]

...going from 2.5in to 3in is a 44% increase in cross section.

Whew! I thought my calculator was broken!!A 3in diameter pipe has 44% more cross section than a 2.5in diamter pipe.ORA 2.5in diameter pipe has 30.6% less cross section than a 3in diameter pipe.

Dont know turbos but I can do math!

Jay

[Jonny 290]

i'm curious about the post a couple above, too. I've got a GT flange on my turbo, and the outlet can't be bigger than 2.5", but I want to move up to a 3" downpipe and exhaust eventually. Can we use a reducer welded to the turbo output flange to bump the size up? Will it help?

[TrunkMonkey]

Can we use a reducer welded to the turbo output flange to bump the size up? Will it help?

yes.

you can also use a 2.5" downpipe and run a 3" exhaust.

-demetrius

[MarkEmark]

Yeah, it's weird--on my turbo, I don't think it's physically possible to have anything bigger than a 2.5" downpipe...(just the first section, of course). My turbo outlet is 2.25" as well, and the flange that bolts on to it will not work with anything bigger than 2.5" (because the pipe will then cover the screw holes). That's why I'm confused to see how all of these stock T25 turbos (which are smaller than mine) on a stock sr20det can take a 3" downpipe..I don't see how the 3" end bolts up to a turbo that I'm sure can't have a larger outlet than mine (T3 Super 60, .48 a/r, .60 compressor)

Needless to say, going from what I have right now to 2.5" mandrel bent downpipes will be leagues better...

http://www.eniety.com/pichost/pic/123-2340_IMG.JPG

http://www.eniety.com/pichost/pic/123-2343_IMG.JPG

Not to mention, I won't be running a catalytic converter, and if I still deem everything too restrictive, I'm going to run just the downpipe/a straight pipe section, no rest of the exhaust (assuming it's not too loud)

Sorry for the thread-hijacking lol.

[TrunkMonkey]

your second pic isn't working. *edit*nevermind.

please don't tell me that that hideous thing under that 2.5" pipe in the first pic was your downpipe.

-demetrius

[andrave]

the T25 turbos use 3" downpipe cause they use a bolt on turbo elbow with a 3 bolt outlet that can bolt up to the downpipe. The downpipe doesn't bolt straight up to the turbo, but to the elbow (which is cast).I'm running 2.25" off each of my T25's into a single 3" collector, and from there to my race pipe.

[MarkEmark]

your second pic isn't working. *edit*nevermind.

please don't tell me that that hideous thing under that 2.5" pipe in the first pic was your downpipe.

-demetrius

yep, I'm embarrassed of it. At least I can't take credit for it; it wasn't me who made it. I guess you guys can see why I was having such drastic boost drop-off.

::hangs head in shame::

[huguetpj]

Yeah, it's weird--on my turbo, I don't think it's physically possible to have anything bigger than a 2.5" downpipe...(just the first section, of course). My turbo outlet is 2.25" as well, and the flange that bolts on to it will not work with anything bigger than 2.5" (because the pipe will then cover the screw holes). That's why I'm confused to see how all of these stock T25 turbos (which are smaller than mine) on a stock sr20det can take a 3" downpipe..I don't see how the 3" end bolts up to a turbo that I'm sure can't have a larger outlet than mine (T3 Super 60, .48 a/r, .60 compressor)

Maybe they do what I did... made a pipe section, soldered to the turbo exhaust flange I had made, go from 2.5" to 3" and then solder that to the 3" downpipe.

http://katcostarica.240sxonlin...2.JPG

[andrave]

soldering? solder requires heat to be less than 800 degrees F... I don't know how hot exhaust get but I'd think at the turbo it would be pretty warm. Whats an average EGT reading?

[huguetpj]

Ooops... wrong translation. I meant to say welded... actually MIG welded.