Mach 5

I have a '86 951 with a recently rebuilt stock K26-6 turbo. Given that the exhaust outlet on this turbo is 1 15/16", how does installing a 3" exhaust after the downpipe really benefit this application? It would seem like the flow is limited by the turbo exhaust outlet and downpipe. I need to replace the exhaust, just curious if/how a 3" is going to impact a motor with a K26-6.

Thanks.
Roger

333pg333

You're right to an extent although the stock exhaust often collapses inside further reducing the aperture significantly even for a stock turbo. Weight is another reason.

V2Rocket

pressure differential vs flow.

inlet to the turbine doesn't change size but the exhaust dump after the turbine outlet does, in this scenario.

in order to get W exhaust flow (at pressure X pre-turbine) through post-turbo pipe size Y there will be Z "back pressure" or "pressure loss", resistance to that flow due to friction.

if exhaust flow is still W (and pre-turbine pressure still X), but pipe size after the turbine is now 1.5Y there will be correspondingly (maybe not linearly) less Z "backpressure". so the turbine's job is easier as the exhaust gas flows "across" the turbine more eagerly.

pressure in vs pressure out drives the turbine, a bigger pipe on the outlet makes pressure out lower even if pressure in stays the same.

the result you feel is faster spooling and perhaps more power because the turbine isn't working so hard (which makes heat).

333pg333

....and that. :-)

PaulD_944S2

Hmm, Professor V2 Rocket!!

Good lesson!

Mach 5

Thanks v2rocket. I can understand where decreasing pressure after the turbo allows more exhaust gas to flow through the turbo. But still not clear how attaching a 3" pipe to a 2" pipe accomplishes this

Roger

V2Rocket

its because there's 12 more feet of pipe after that 2" pipe that runs to the back of the car.

if you didn't care about noise, emissions or the paint on your hood you could just unbolt the exhaust and downpipe and run the turbo with an open, exposed turbine dumping to atmosphere inside your engine bay...the exhaust system fulfills all those requirements though but due to its necessary length has to be increased in diameter (cross sectional flow area, really) to not have too much flow loss.

imagine you were shopping for extension cords and you need a cord that can carry 15 amps to power your saw. you could happily use a 14 gauge wire for a 10ft cord but if you needed a 50ft cord youd really need to step down to 12 or even 10 gauge wire.

for a given diameter (cross section) there is a a certain amount of resistance (electrical or gas flow) per unit of length. with enough length the resistance grows unacceptable. the saw still only uses 15 amps, but you have to upsize your wire (or pipe) to carry the same load further.

951Dreams

And this is why we keep you around Spencer!!!

No really, you have a great way of explaining stuff that just makes sense!! I had wondered this myself, and now I know!!

Plus I'm dense, so I need your hooked on phonics version.

blade7

I've tried open exhausts on a couple of turbo cars and it moved the torque to the right very noticeably,fine on a track, on the road not so good....

F18Rep

Been there, done that. Loved it, then 6 weeks later - hated it. Removed it. Sold it. Lost money on it. ...Bruce

Mach 5

OK. Very helpful. I think I'll go ahead and order the Lindsey 3" with Magnaflow muffler. Seems like a safe bet.

Thanks.
Roger

lart951

I have one lindsey exhaust for sale on eBay

Mach 5

Yes, I did see that, but it's a 4", which may be overkill...and I think you show "local pickup only", which would be a bit of a drive from Chicago!

Roger

lart951

Is a 3" till cat back section then it becomes a 4", i can ship you will pay actual shipping pm me if interested. With these cars unless you have an unmolested low miles specimen, once you start modifying it never ends, once you start with chips, maf, larger turbo larger exhaust is needed. On a stock turbo gains are very modest but once you get a larger turbo the upgraded exhaust is a must.

divil

I have the 3" Lindsey exhaust + magnaflow muffler in an otherwise stock 89 Turbo (including stock chip).

It didn't give the big improvement in spool up time that many people told me to expect, but there was a noticeable bump in the mid range torque. I really felt it at the track. Any weight saving is trivial for a stock street car imo.

I honestly don't know how much of the improvement was form the increased diameter and how much was form just getting rid of the 25 year old cat. My guess is it's mostly the cat because my downpipe is still 2.5". In hindsight, I should have just chopped out the cat and welded in a straight 2.5" pipe, and saved the $1000.

Noise-wise, it's louder and a little rougher sounding, but not bad. Definitely fine for a street car.