Topic: Exhaust system

[bootguy]

I need to replace my exhaust system. the issue is looking at headers the down tubes on all that I have seen are 1.5" going to a 2.5" collector.  The muffler shop does not recomend an exhaust over 2" for the 2.5L engine.  Is there an issue going 1.5" to 2.5" then down to 2"?

[sharpxmen]

I need to replace my exhaust system. the issue is looking at headers the down tubes on all that I have seen are 1.5" going to a 2.5" collector.  The muffler shop does not recomend an exhaust over 2" for the 2.5L engine.  Is there an issue going 1.5" to 2.5" then down to 2"?

i remember that mine was 2-1/4'' but i could be wrong. Why did the shop say they don't recommend going over 2'' (don't tell me they sold you that bs about breathing better or back pressure) - you can go larger and you'll be fine.

[Jeffy]

i remember that mine was 2-1/4'' but i could be wrong. Why did the shop say they don't recommend going over 2'' (don't tell me they sold you that bs about breathing better or back pressure) - you can go larger and you'll be fine.

most aftermarket kits are 2-1/4", IIRC.  I forget if that's ID or OD.

[dwtaylorpdx]

It all depends on your engine. I have 2.25 all the way back, The standard header flange
can reduce the diameter nicely from the 2.5 to the 2.25, then I used a Matrix cat (Random Technologies)
and a Spintec muffler, I would not do the Spintec again, its too loud for a daily driver. Perfect for a trail rig.
I like it better than a flow master, (Has a different tone, lower note) I'm going to swap it for something quieter
as soon as I have some spare toy money....

The 2.5 has big ports, and a relatively long stroke, I don't understand why the muffler guys all tell us the
2.5L engine can't use the 2.5 exhaust then turn around and install a 1.5 to 2.5 header on a 305 chevy...
The cylinder volume, stroke etc are almost identical. and the headers should perform in a similar fashion...
Especially since our motor is redlined about 1000 rpm's higher for normal driving...

I get a little bit of exhaust blubbering and popping on a long downhill run when coasting if
I rest my foot on the gas pedal, nothing extraordinary... And I gained a full gear back on
the main hill in my commute, I don't fall off the power band as easy.

Once I rebuilt my engine (I put the header and exhaust on before my rebuild) I'm pretty sure I'm using
the pipe but without a side by side test I can't prove it...

Dave

[bootguy]

They did talk about back pressure.  I understand the 1.5" going to the collector, it is the 2.5" down to 2" that makes no sense.  It seems to me that if the collector is 2.5", I should be able to take that all the way back.  I am hoping to get a little more power, and maybe some help with the MPG.

[sharpxmen]

most aftermarket kits are 2-1/4", IIRC.  I forget if that's ID or OD.

OD

[dwtaylorpdx]

The collector naturally tapers, the after-pipe starts at the collector volume
and shrinks just a bit more, so with a 2.5 collector the taper to 2.25 is about right.

If you get too big of a tube the exhaust loses velocity, and begins to pulse
too much and reverses flow(reversion) if you go far enough. The reason a lot of imports get
away with large exhaust is variable valve timing. Or they go so big the exhaust
reversion breaks down. But on our motor with everything being pretty static it is what it is.

If you read up on Chev small blocks they often point out the best torque for
pickups is with 2" dual exhaust. 2.5 with a big cam and 3 if your going to make it really spin. (7500ish)

Its a lot of voodoo, but the tri-Y designs have the best torque vs HP curves,
even better if you have stepped headers..

There is no one answer to the question. Other than its unlikely that youll get any more
power with the extra 1/4 inch and all the parts cost more as the tube size goes up.

Dave

[bootguy]

I could now use some suggestions and schooling on headers. (not too proud to ask)

[dwtaylorpdx]

Your going to get opinions all over the map on this, here is mine... :)

Tri-Y headers are best for our application, they provide the best compromise for mid range power and top end.
Short tube headers help torque but fall off at upper rpm's
Long tube will let you rev, butt you'll give up a little pull at low to mid range rpm.
Stock manifold is not bad until it cracks. Probably equal to short tube headers.

The only source of the Tri-Y headers was Thorley, and after they went bankrupt
and got bought out they discontinued the 2.5 header. :(

There has been a lot of talk about  the Borla headers cracking.

The pacesetter headers IMHO are junk They have sucked on every
rig I've worked on with them....Others have had great luck, Shrug who knows...?

I paid like 300 bucks for the Thorley on my jeep and with 100 K miles on it its still solid.

Its very easy to over exhaust a side flow head, so there is not much point in getting too crazy with tube sizes...

Dave

[FourbangerYJ]

Zoomie's...wouldn't have to worry about a cracked collector. 

[stan98tj]

i run banks header and exhaust. it's 2.5" and stainless all the way through. im happy with it. the tone is nice and sound isnt too loud. call em directly at the warehouse in california and they may work out a $$ deal for ya.