2.5" vs 3" exhaust... any benefit to the bigger size? (1 Viewer)

[AaKnight]

Im running a stock 1fz-fe, and it's time to replace my muffler. I'm considering going to 3" pipe all the way out.

Was wondering if 3" would be beneficial at all, or just a waste of time and money.

It's just that as I'm shopping for big can mufflers, I notice the 3" and 2.5" aren't much difference in price.

with my lift it appears i have room between the tub and frame rail to fit a slightly thicker pipe, so SHOULD I go bigger?? Or will I regret it?

 

[Marco Lau]

unless you are running Force Induction, having larger exhaust will most likely reduce back pressure and resulting in a slight lost in Torque and maybe a hair increase in peak hp.

 

[AaKnight]

That's what I was thinking someone might say. thanks.

 

[LandCruiserPhil]

Larger exhaust does nothing until you replace the most restricted part first, the Y pipe.

Magnaflow makes a nice over the counter fix but you can spend more for the best flow

 

[sxp]

It's probably not needed, but it won't hurt anything either. The back pressure thing has been proven to be bunk. Engines don't require back pressure.

 

[Pascoscout]

Getting ready to redo my exhaust too. I went with 2.5" as that's what matches the magna flow y pipe. Can't see a reason to go any bigger with a stock motor.

 

[IronCladLou]

Another thing to keep in mind is that Q=AV. If the flow rate, Q, doesn't change (i.e. you haven't modified the engine to flow more gas like using forced induction, etc.), then increasing the area, A, by using larger exhaust piping only slows down the velocity, V. So...going larger just means the exhaust gas will just be traveling slower. That is not a way to "improve" performance. If Toyota could have gotten more power out of the 1FZ-FE by simply going to larger exhaust piping, they would have done it.

 

[Oscar Witte]

The only difference between the two you're going to notice is a slight difference with thetone of the exaust.

 

[Heckraiser]

Another thing to keep in mind is that Q=AV. If the flow rate, Q, doesn't change (i.e. you haven't modified the engine to flow more gas like using forced induction, etc.), then increasing the area, A, by using larger exhaust piping only slows down the velocity, V. So...going larger just means the exhaust gas will just be traveling slower. That is not a way to "improve" performance. If Toyota could have gotten more power out of the 1FZ-FE by simply going to larger exhaust piping, they would have done it.

Came in to say this. Larger diameter = slower exhaust velocity, which reduces the engines ability to pump it out, reducing scavenging effect, which will impact performance. To put that another way, there is a "sweet spot" for exhaust diameter for any given application that best balances overall flow vs. exhaust velocity. Too small or too big can negatively impact performance. Someone else probably knows how to calculate this for a specific engine and usage.

You can throw that theory out the window if you've got a turbo (although bigger exhaust can create some tuning issues specific to the 1fz-fe computer).

I doubt it would make too much of a difference, but I'd stick to 2.5" for a naturally aspirated 1fz.

 

[Dave 2000]

The back pressure is BS when it comes to power, any extra to be gained is in the headers, one cylinders exhaust 'pulse' travelling down the header helps draw inlet air into the chamber during valve overlap. Narrow but longer headers = more torque, large bore headers less torque but ultimately maximum horsepower.

There may well be gains but as per @LandCruiserPhil you have to drive the gases through the header junction first, but there will be more psychological horsepower available.

Regards

Dave

 

[VuickB6]

Here is a detailed explanation for those that care to learn.

 

[brian]

3" is stupid loud. has nasty drone as well.
i replaced a crapped out oem system on my 93 with a full custom 3'', short of oe style down pipes in the 95-97 inline cat style.
it really woke up the engine, but it really turned up volume

 

[scottryana]

Well kind of. His videos are usually decent for laymen, but he is missing out on a large portion of what is scavenging. I would suggest doing a little more research on how exhaust gases reverb back into the combustion chamber without significant exhaust speed.

Here is a detailed explanation for those that care to learn.

 

[AaKnight]

@scottryana, can you elaborate a bit?

 

[scottryana]

A quick example is how long tube headers can work better than short tube headers, because there is a longer period of time the exhaust gas is kept in the smaller diameter runner before the expansion to the merge. The tricky thing is that the length of runner has a to be tuned by rpm. This is not a high performance engine, you will be fine with 2.5 or 3" But I was just pointing out the video above is ok, but not the end all be all of exhaust tuning. The thing people get confused is back pressure vs velocity. You do need velocity, you don't need pressure.

@scottryana, can you elaborate a bit?

 

[mudgudgeon]

Larger exhaust does nothing until you replace the most restricted part first, the Y pipe.

Do these have the flattened pipe where the exhaust crosses over the frame at the rear diff?
The diesel 80s have the pipe crushed to around half it's normal diameter in a space where there's room for 3" pipe.

 

[AaKnight]

ha!!! I thought I banged it flat running without a back pipe hanger. maybe it's supposed to be flat up there. It's pretty hefty tubing.

If that's factory.... wow. it looks so restricted up there.

That flat area + the rattling muffler is why I was looking into replacing the whole back exhaust.

 

[dogfishlake]

If anything, and without forced induction, keep your flow concerns around the front portion of the system. As the gasses cool, the volume carrying capacity of the pipe increases dramatically. If you do a custom system, you might be fine with a 3 inch to 2.5 inch step down system . Just more to ponder.

I grew to love the sound of a roaring 6 so every time I roll on my 80 with the 3" (boosted) exhaust, I love the music.

 

[AaKnight]

well actually the p.o. already has 3" cats, so I'll go 2.5 from there on out. ( that's exactly what my muffler guy told me to do. But part of me suspected he just didn't want to try to have to fight to fit 3 inch pipe over that rail. lol. )

I might have to look at the y-pipe sometime tho. it might have already been addressed for me, because both headers dump their smaller pipes into a 3" and they stay 3" till the cat exit to the muffler ....a project for a different day tho.

I appreciate the mud perspective, I don't exactly trust most shops when it comes to my cruiser.

 

[LandCruiserPhil]

Do these have the flattened pipe where the exhaust crosses over the frame at the rear diff?
The diesel 80s have the pipe crushed to around half it's normal diameter in a space where there's room for 3" pipe.

Same flat spot on our gasser.