How about a instead of a 3 inch turbo downpipe someone build a 3 inch up pipe right out through the hood.



Jason

 

Boomtube huh... Very interesting

 

Snorkle???

 

Has anyone put some math on this?

3" = about 7.065 sq."

4" = about 12.56 sq."

So about a 78% increase in area for the gases to exit?

 

thought i would revive this thread after a bit of curiosity reading yesterday.

you are all familiar with the STS turbo systems (rear mount turbos) and there is much doubt here (and across all the internets) about the real usefulness of such a system. the doubt is understandable, the chief reason being the sheer length of exhaust pipe leading up to the turbo (all the way at the back) causes the exhaust gases to cool considerably and also slow down, which could lead to a spool up measured with an hourglass.

however as a student of engineering i like to look into things deeper to see why and how things work and what can be done to fix them, and simply going to STS' website brought something up.

in this thread and others the topic of turbine pressure differential has come up. basically, you want as little pressure on the outlet of the turbo as you can relative to the turbine inlet, so that the gas on the pre-turbine side is encouraged to flow across the turbine and then out the outlet.

STS on their site (http://www.ststurbo.com/the_technology) claims this:
they raise a good point. with the turbo alllllll the way at the back, yes the pipes leading to the turbo are long and pose problems but the turbine outlet has only a few inches of pipe (less pipe length = less restriction from pipe walls) and with the tips sufficiently far out a venturi effect should help pull gases from the turbo.

nice sounding scientific example:


what are your thoughts on this? the cooled exhaust problem could be partially alleviated with ceramic coating or a SS pipe (or both?)...could this actually have some merit?

 

yes with a rear mount turbo its very easy to have only 1psi on the outlet side of the turbo where as you need 3" or bigger system to achaive this with a front mount system..
i think its certainly a swings and round abouts thing with rear mount turbos ...
I'm not convinced the they are as responsive I'm not convinced there ultra laggy either but as they are very convenient for turbo conversions i think they are great .. as they gas is cooler it seems logical to me to put a VGT from a diesil back there ( I will) .. any way different thread ..
as for 951 exhausts.
I always turn nice taper from 1mm past the turbine blades tapering out in the discharge of the tubine housing i have not made it 14 deg but i think it is always better than a 90 shoulder it had ..
and with the pipe i genrally go as big as a can fit and try to not have big steps in it .
so mine is 3" for about 12" to the starter motor then 3.5 into the tunnel then 4" down the middle
then 4" pressed to 3" high probably equal to 3.5 pat the rear cross member.

 

they seem to work well for the 928 guys who have very limited engine space as well. good numbers being put down on dynos. and 944s also have a ton of space where the muffler hangs and a turbo could be made to hang from the rubber mounts id think. also there is less suspension stuff in the way for us. intake could be put in one of the rear cubbies or the spare tire well...or up near the fuel filter with some ducting to the filter

see these pics...pretty sweet looking.

 

Or...you get the same low-outlet-pressure effect by dumping the exhaust from the turbo straight out the bottom of the engine bay, instead of running it alllll the way under the car to the back bumper. And you STILL keep the high pressure and temperature going into the turbine. AND when you build a rear diffuser, you don't have to route an exhaust pipe through it (I always hate that).

 

Keep in mind that the rear mount turbo systems are being marketed to cars with ‘normal’ compression V8s. there is enough initial grunt from the basic engine that you don’t feel the lag like you would in a smaller low compression engine that was built for turbocharging.

BTW – the picture of the turbo in the red car reminds me of the old prank of stuffing a potato up somebodies tailpipe – that set up would slice and dice the potato into French fries!

 

As Chris said, all of the rear mount sets I have ever seen installed and tested were on V8's so the NA side of the motor has instant tq so lag would not be an issue if even noticed. in the tv shows that have installed and tested they confirm no feel of lag, but is that no lag or just not "noticed" lag??

V2: Your points would certainly prove that we should all be running as big as exhaust as possible and as big a hot side.

"and with the pipe i genrally go as big as a can fit and try to not have big steps in it .
so mine is 3" for about 12" to the starter motor then 3.5 into the tunnel then 4" down the middle
then 4" <= Reverse velocity stack. LOL

Nice pic that shows how much difference there really is:

http://www.lindseyracing.com/LR/Part...ST4INCHBF.html

 

so define lag then ..
some one said its the delay or spool up time when your already above the above the boost threshold rpm.

some define it it as being being below the boost threshold rpm .

I kind of define it as a " inadequate torque" at any time . wich of corse can be cured with cube's and compression as wee see on most rear mount set ups

 

“lag” is that period of time that you are not getting the power from that bigger turbo that you just spent $2k on.

There is no ‘official’ definition, the common use of the term is the time it takes for power to come on after opening the throttle once in the powerband (above min spool up RPM).

 

As I have said in a few other posts, lag means you were in to high a gear when you hit the gas. lol

So how do we define "min spool up rpm" ?

I always judged lag as the time it takes once you are out of vac but not yet into boost. Ie the vac/bost guage is sitting on zero, you hit the gas and the time from 0 boost to X boost is lag.

 

My understand of lag always was that below a certain RPM, you can forget about spooling the turbo, so you downshift into an RPM range where it WILL spool, but then you wait for the lag while the shaft gets up to speed, THEN the power comes on. Essentially nonexistent in a modern engine with a relatively small turbo, compared to very noticeable and abrupt in an older, lower-compression engine with a relatively larger turbo.

 

Another "rear mount"