When is 4" exhaust not enough??

At what level do we NEED a bigger exhaust? What are the signs?

Would this decision be based on injector/turbo size or ????

I have kept straight-piped stock exhaust on my F350 based on EGT's not being an issue and because I can make 30psi with AC codes and a 38R. Would I be likely to see any gains from a larger exhaust on that truck?

the restriction in the exhaust is the turbine wheel of the turbo anyway, you dont need really anything bigger than the outlet of the turbo, and 4 inch is probably overkill in most cases

if im not mistaken,anything over 4" isnt needed unless youre way up in the HP ranges.

Or want a deeper sound

A 4" exhaust wont be your restriction for a long, long time. Like 1000+ hp.

my exhaust is only about 18" long but its just 4".

Too big of an exhaust can actually make the exhaust flow less than a smaller one, hot exhaust air isnt very dense so it flows easy, if the exhaust is too big the exhaust temps will cool off and start to slow down the exhaust flow

So your saying that if you go with to big of an exhaust trying to be careful that you can actually cause problems ?

SoCalPsd-Corey

When I went to have my truck dyno'd we aluminum taped a piece of 4" exhaust pipe bent to a 90 on my tail pipe to route exhaust outside. It was only like a foot long on each side. We taped the s'it out of it. It wasn't coming off!..... So we thought. During full throttle it was blown off with enough force to bend the pipe when it hit the wall 4' away, bounce back, hit my truck, and land 10' away. Now thatsI some pressure! I'm going with a 5" as close to the turbo as possible.

Probably not, but it becomes counter productive. 4" and 5" exhausts are cool, but bigger isn't always better. I don't think a 4" or 5" exhaust is going to hurt anything. But I would be willing to wager that a true 5" turbo back would flow less than a true 4" turbo back in most any PowerStroke application.

:doh: erased what i origiinally posted here because i reread the dzchey's post.

vanderchevy18 said:

When I went to have my truck dyno'd we aluminum taped a piece of 4" exhaust pipe bent to a 90 on my tail pipe to route exhaust outside. It was only like a foot long on each side. We taped the s'it out of it. It wasn't coming off!..... So we thought. During full throttle it was blown off with enough force to bend the pipe when it hit the wall 4' away, bounce back, hit my truck, and land 10' away. Now thatsI some pressure! I'm going with a 5" as close to the turbo as possible.

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You don't think it could have been a good thing that your exhaust was carrying that much velocity at the outlet?

You thinking wicking away heat and drawing from turbo rather than creating back pressure like I was thinking?

Possibly. I know that the hotter your exhaust and thus exhaust gasses stay, the better they'll flow.

If i remember correctly when i had my exhaust slipped off from the cat (thanks to a retread laying on the free way) and let it blow straight out i had around 21psi at 2500 and with the stack 5in from the cat Im at around 13psi at 2500.

Ill see if i can get the two print outs from my trans guy. I noticed these when he was looking at my truck two seperate times.

JAP said:

Possibly. I know that the hotter your exhaust and thus exhaust gasses stay, the better they'll flow.

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Of course though there is a point when the hot fast flowing gasses become counter productive because of egt and backpressure issues. That's what we're trying to find here. I will let y'all know what my findings are when I get the 5" pipe to 6" stack put on. I know I've always been told and believed that 4"exhaust was plenty, but after seeing that I'm of the opinion its not. I'll let you know exactly what differences there are if any at all.

I'll have the stack on in 2 weeks. Nothing else will change. I'll measure drive pressures, egts, and watch for smoke or haze qualities to change. Anything else?

Nope. Sounds like a reasonable test.

Mole?

This phone makes the weirdest corrections.

Ah. Yeah mine too. Sucks cause I'm such a stickler on spelling and grammar! I had a guy the other day email me from his phone and it changed the sentence from, "I'll send you the cylinders.....", to, "I'll send you the a$$holes....". LOLLOLLOL