been doing a bit of reading tonight on exhaust...basically seeing a lot of different setups, but haven't found a good explanation on "why" things were done, and what the power/EGT advantages/disadvantages will be...

It seems the stock downpipe is 3", and there are a couple 4" DP's available...

I've seen some run a 3" DP, then 4" pipe, then 5" stacks...

I just bought my 350 on Saturday, and the owner gave me two 5" stainless stacks that he never installed...

I figured I'd replace the muffler with a straight pipe, and then turn it up into the bed right behind the cab, and split it to the stacks...

should the straight pipe be 3"? 4"? 5"?

do they make 3"-5" adapters, or will I have to go with 4" in the middle?

any real harm in leaving the 3" DP?

Thanks.
Greg

 

no harm leaving it 3" if you got a 4" DP you might lower EGTs 50* .my sons 90 has 3" all the way back to the bed then turns up to 4" then 5" up to the stacks. heres a link for parts if you need, good people to deal with.






http://www.airflo.com/

 

thanks for the input, and the link...but I've got a local source that's gonna take care of me...

me and a friend are doing stacks at the same time, and his girlfriend's brother works at a local big-rig shop, and can get us the 4" pipe and elbows for dirt

Greg

 

I am no scientist/engineer; but, in my way of thinking, exhaust tubing need be no larger than cylinder bore, so long as it isn't excessively long.

Only one cylinder at a time is pumping exhaust into the system; and, it is not possible for more exhaust volume to be present than the volume of bore times stroke.

Of course, I could be wrong.

 

in "theory" I'd have to agree with you as far as volume/flow...

the EGT would be the difference, though...

Greg

 

This is how I did mine ('95 1 ton): stock downpipe and exh pipe to where the cat was, 3.5"-4" adapter, 4" pipe on back, over the passenger frame rail, up thru the bed to a 4"-5" adapter to a 5" stack. It's really not too hard to do.
I even bought some heavy steel strapping from a hardware store w/ pre-punched holes along its length for using as hangers underneath. The stuff is just over 1/16" thick and a good 1" wide... comes in 3' lengths.

 

There is actually more air in there than just bore x stroke. The air is compressed coming into the cylinder and stays compressed until it gets out of the head. The gases then expand from there, and after it gets past the turbo housing, the air expands even more and the velocity slows down as the air cools. Which makes it harder to push out of the pipe. So larger pipe is definately a benefit for getting air out.

 

I posted in your other post saying something about a 4" DP. I got mine from here: http://www.sourceautomotive.biz
Rip was awesome to deal with! I purchased the DP with adapters, and he even took the time to mark where the 3-4 adapter needed to be cut!
I paid 170 for the assy. and shipping. It took around 3 hours for the cutting, welding, and measuring aspect, and 2 minutes to put it in, I swear Cummins had a dodge sittin next to the engine when they designed it, it fits PERFECT! just make sure you take your time when you do it otherwise the stock bellhousing mount wont work, (the cuts, and welding for the adapter, and the old flange need to be almost perfect) and make sure you have a 4 inch clamp to mount it using the bellhousing mount.
A larger DP will aid in spooling, and EGT's. A stock DP can only handle so much air, so if its getting backpressure on the turbo this will only slow it down, so the larger DP will give it that much more room for air.

 

Hey Jeremy, nice stack! Wonder where the twin to that one is?

 

and keep in mind that the difference in pressure before and after the turbo is what makes it work. i think .

 

You would be correct if there was no combustion involved, but the expansion of the gasses necessitate larger pipe to keep backpressure down. We are lucky with diesel because it's not still burning in the exhaust like a gasser!

 

Yes, that's exactly right. When the gas expands it's temp drops with the pressure. This creates a greater differential across the turbine which makes it spin faster.

I would be wary of going smaller down the line however because you could setup an accoustic reflection which puts pressure pulses backward and hurts performance at some RPM. The turbo mitigates this effect some of course.

Edwin

 

Then where do the occassional flames from my stacks come from??

 

uhhhmmm,,,,,,,,,chili?

 

Beans? I meant generally, not of a fire breather like yours!
Generally, exhaust temps are WAY lower on a diesel because of this.