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I know these questions would have been better asked a quarter century ago, but here they are anyway:
At peak power, what is the turbine outlet pressure in the stock 993 Turbo?
In creating that back pressure, which part is the tightest restriction? Is it the catalytic converter, the pipe between the cat and the muffler, or the muffler?
Relatedly, based on dyno experiments and acceleration logs, which will add to the power more: Eliminating the cats (replacing with a straight pipe) or completely bypassing the mufflers?
(I’ll respond here instead of your other thread)
I’m sure someone has done some logging at one point but as far as I recall, no one has posted any specific logs with different exhaust set ups. It’s been said that removing the cats do improve turbo response by shifting the curves and allow boost to come on 500 rpm’s sooner. The stock heat exchangers are supposed to be good flow but can be improved on by doing some ‘work on them’. Before and after :
Many have done cat delete/muffler delete etc. I think benefit is more for weight loss. Also for whatever reason, if I recall correctly the tubing dia is reduced (?from 2.75 to 2.5”) at some point (after cats?). Most who have built exhausts have done it with 3”.
(I’ll respond here instead of your other thread) I’m sure someone has done some logging at one point but as far as I recall, no one has posted any specific logs with different exhaust set ups. It’s been said that removing the cats do improve turbo response by shifting the curves and allow boost to come on 500 rpm’s sooner. The stock heat exchangers are supposed to be good flow but can be improved on by doing some ‘work on them’. Many have done cat delete/muffler delete etc. I think benefit is more for weight loss. Also for whatever reason, if I recall correctly the tubing dia is reduced (?from 2.75 to 2.5”) at some point (after cats?). Most who have built exhausts have done it with 3”.
Thank you, this is very useful information.
In terms of the headers (pipes that connect the exhaust ports to a collector and then to the turbine inlet), the factory item is usually very durable but the pipe size is usually larger than what I'd want it to be. The aftermarket turbo headers don't last and are usually even larger pipe size. Based on what I've read about 993 Turbo headers, the situation sounds exactly like that.
In terms of the turboback exhaust, I've been polling tuners who measure things like that and the peak power turbine outlet static pressure consensus seems to be 10 psi. I am going to make a SWAG that 50% of that is stock catalytic converters, 30% baffled muffler with small passages, and 20% small pipe sizes in between. Just a guess of the composition of the 10 psi based on general life experience, I would appreciate if someone would set me straight with some 933tt specific data.
What would be nice for sustained high rpm power on the autobahn would be to take down that 10psi turboback exhaust back pressure to 1.5psi (like John Kuhn did with my turbo 928 car, some details here: https://www.speed-talk.com/forum/vie...8faae9e4b76e6a), then install free-flowing turbo, and high-overlap long-duration cams. And maybe smaller diameter headers to bring up the low end power with minimal top-end penalty (see this dyno test for a four banger with high-overlap cams: http://morrisonfabrications.com/larg...old-dyno-test/).
My thinking is that the too small restrictive turbo-back exhaust doesn't really hurt turbo spool, because there's just not enough exhaust flow at low rpms to create much of back pressure. However, the cat and the pipe distance to the cat may create unfavorable resonances that cause the turbo to spool at slightly higher rpms compared to a straight downpipe of right tuned length. Just some theories.
Does anyone have the stock 993tt turbo-back exhaust pipe diameters recorded? The cat-pipe sections and the muffler inlet, outlet, and internals? Johnny come lately would appreciate the info!
Thank you again for all this the useful information!
Let me speculate a bit on this exhaust. The post-cat exhaust diameters of this exhaust go down sensibly as the exhaust gas is expected to cool. If we take the post-cat inside diameter to be about 53mm / 2.1in, then this by my rules of thumb would give about 0.025bar / 0.36psi back pressure at 204hp per side. This is not a lot. 3" OD 0.06" wall pipe between cats and mufflers would based on my SWAG lower this section's pressure loss to about 0.07bar / 0.1psi, which in my opinion would basically be waste of space that could be used better for other purposes for an otherwise stock car. Of course, 3" exhaust cars aren't otherwise stock. Right off the turbine outlet, the pipe could however be even larger because it turns and because the flow swirls out of the turbine, increasing the flow path and thus velocity.
The muffler flow loss would need to be measured, but given the computations about the pipe sizes, making that muffler internally a straight-thru design a la "gondomod" should make the muffler loss pretty small also. As it is constructed from the factory, there's likely a big loss from the pipe being cut in the second chamber.
I suspect that the big issue is the cat. First, almost all of the cat flow is in the center. Only about the inner 85% of the cat OD flows anything. (For estimates, see https://avestia.com/FFHMT2017_Procee.../FFHMT_135.pdf). Then the element itself has a high drag and only flows a fraction of the straight pipe.
Overall, the system looks consistently (i.e., well) engineered for stock power and desired sound level. To make the exhaust pressure loss much lower, one would need to either take up more volume, compromise on emissions (although a modern and expensive catalytic converter may help here), or accept a higher noise level.
Who produces the currently lowest back pressure catted and muffled exhaust system for 993tt that passes Euro 2 emissions tail pipe sniff test (idle and 2000 rpm)?
I attended a session with Jake Raby 11 or 12 years ago. Flat 6 pre/post dyno mods and I recall him saying many/most result in less, not more power. I asked him specifically about our cars and at the time he felt the stock system offered the most power. He would be very helpful in ensuring you get more power as well as more noise.
I attended a session with Jake Raby 11 or 12 years ago. Flat 6 pre/post dyno mods and I recall him saying many/most result in less, not more power. I asked him specifically about our cars and at the time he felt the stock system offered the most power. He would be very helpful in ensuring you get more power as well as more noise.
Who’s Jake Raby?
It’s my understanding and experience that 1990’s turbo cars have high back pressure at the turbine outlet with the stock systems at peak power. Replacing the turbo back exhaust and adjusting the ECU calibration usually yields nice gains with the nineties turbo cars. This is not so with newer performance car exhausts that often have an extra inch or more of pipe diameter at the turbine outlet, my new AMG cars being a good example.
I’ve been talking to some tuners and they say that the peak power turbine outlet exhaust back pressure is in the 7-10 psi range for 993 turbo. If that’s true, there’s big gains there not only in terms of power but also in terms of improved reliability from reducing that back pressure.
08-05-2022, 03:39 AM
Stock exhaust restrictions and back pressure
