scarecrow

ok i heard that with straight pipe if i ran a 3 inch exhaust off the turbo and then somewhere under the truck bump up to 4 inch that my turbo will spool up faster as appose to running all 4 inch..... is there any truth to this???????????

RowJ

Never tried it but don't see how it would work.
3" will produce more back pressure than full 4" system. Increase in back pressure will not increase turbo spool up....just the reverse! Not a lot at spoolup, but....

RJ

piperca

While looking for a 4" exhaust, I heard something similar to what Scarecrow heard, quote:

"This has a 3-1/2" into 4" downpipe to allow for a small amount of backpressure which allows your turbo to spool up faster at lower RPM's than other systems on the market."

This system had the 3-1/2" section attaching to the turbo and running for about 2-3" before flaring out to 4".

Good question, I'd be interested in hearing the response too.

RowJ

I have tried that...DPP 4" system for a stock turbo comes that way. Don't think you have much choice, based on size of turbo outlet...but I still don't buy the increase in spoolup.

RJ

BigBlue

Are yall talking about that little coupling at the top of a 4" down pipe that connects to the turbo? If so, that's just an adapter to adapt the 4" pipe to the 3-3 1/2" outlet on the turbo housing. It's not for spool up.

CTD NUT

A lot of R&D from many different groups has been done on stepped exhaust technology.......the theory is to start the tubing small and slowly step it up in size over the length of the exhaust which promotes high velocity and good scavenging at the begining and slowly drops the velocity and picks up the flow in an attempt to eliminate unwanted back pressure down stream............does it work?....I'm not sure.......clearly some applications are working better than others........lots of drag racers are using stepped headers and several SCORE race trucks are using a fully stepped exhaust.........

There would be no benefit in the case of our CTD's to run a down pipe smaller than the turbine outlet..........and I don't know of, or have heard of, anyone posting any type of power gain or spool up speed with a system with a 3.5" down pipe over a 4" down pipe system.......

The trouble I have with attempting to get good scavenging at the turbine outlet with our CTD's is that since we don't use a header to truely promote good scavenging at the exhaust ports, we already have poor exhaust velocity as it enters the turbo........attempting to increase scaveging capabilties at the outlet is not much good until you can increase it at the inlet..........I have only ever seen one true exhaust header for our trucks.......I'm not sure why more people don't use them or make them...........getting exhaust out fast enough is always a big challenge for super charged or turbo charged motors!

WildBill

Stepped exhaust works on naturally aspirated engines and maybe even supercharged engines, but on a turbo diesel it wouldn't have the same effect. The size exhaust needed can be based on your power level. On gas engines with turbos, you can actually go to large on the exhaust system which will cause turbo lag and boost creep, which may be where somebody got the idea of quicker spool up with a smaller downpipe.

Forrest Nearing

what he said

scarecrow

thanks for the replys..................

PourinDiesel

Some back pressure does help turbo spool up, but after that it may become a restriction with increased CFM and Fueling.
Hence bigger housing take longer for the turbo to spool up (14cm,16cm18cm you get the picture)
Smaller exhaust will help high end stuff, but 5" will hurt some bottom end setups.

RowJ

posted by PourinDiesel
"Some back pressure does help turbo spool up,"
_____________________________________________
Care to explain how this can work? How can exhaust pressure build up in an enclosed turbine housing possibly increase flow over a turbine wheel ?
Yes bigger housing will slow spool up, but only because there is slower flow rate over the turbine wheel. Not the same thing.
No matter what the exhaust flow rate is over a turbine wheel....backing it up with a restriction on the exhaust side wont increase the flow. Can only decrease it..and decreased flow rate = slower spoolup, IMO!

RJ

CTD NUT

I agree.........pressure is the measured resistance to flow..........so, the higher the back pressure, the less potential flow you should expect.........when back pressure gets too high you can also experience reversion.........this is where exhaust housing inlet and turbine wheel drive pressures can exceed exhaust port pressures - not good...........not to mention back pressure kills any chance of scavenging in the manifold...........these are not 2 strokes, we want the absolute highest exhaust gas velocities at 0 pressure.........0 pressure means 0 resistance to flow..........exhaust systems that have exceptional scavenging characteristics can even create a vacuum............bigger exhaust housings slow spool up but lower drive pressures so if a 12cm housing and 16cm were to flow the same amount of air, the 16cm would do so at a lower drive pressure.......lower drive pressures will mean higher velocities and better scavenging with reduced back pressure which equates to more, cooler boost.