Dropping the Cat on a 3.0 911sc '83
11-25-2011, 11:52 PM
#1
Dropping the Cat on a 3.0 911sc '83
Dropped tha cat a few minutes ago and I've got a cat replacement pipe that is basically a straight pipe (with o2 bung).
I'm going to run it tomorrow to see if it does it's job without losing any low end.
Has anyone 'experimented' creating a bit of back pressure in this kind of set up? It would be easy to weld up a slight restriction device (large bumper washer opened up but smaller than the pipe diameter) in the area where it meets the muffler.
I've done this with lakes headers on hot rods and tuned the exhaust that way for solid low end and low restriction top end.
Anyone had any issues losing low end with the open pipe (after retuning)?
I'm going to run it tomorrow to see if it does it's job without losing any low end.
Has anyone 'experimented' creating a bit of back pressure in this kind of set up? It would be easy to weld up a slight restriction device (large bumper washer opened up but smaller than the pipe diameter) in the area where it meets the muffler.
I've done this with lakes headers on hot rods and tuned the exhaust that way for solid low end and low restriction top end.
Anyone had any issues losing low end with the open pipe (after retuning)?
11-26-2011, 12:17 AM
#2
Team Owner
Porsche uses high flow cats anyway and i will be really surprised if you notice any low end drop off . Im thinking you will be very pleasantly surprised.
11-26-2011, 11:18 AM
#5
Rennlist Member
I lost a noticeable amount of low end power when I installed my test pipe but felt a slight gain in top end HP...not much, though. The main reason I removed the cat was to gain better access to the lower rocker cover and reduce heat in that area. But for the heat issue I'd reinstall the cat just to get that torque back.
Also, with the M&K 2-out muffler I have, exhaust noise is an issue with the cat bypass in place. I'm not sure a pre-muffler would be much help in that regard as even with an otherwise 100% stock engine, the exhaust note is REALLY loud, especially in the upper RPM ranges. As in "arrest me!" loud.
Also, with the M&K 2-out muffler I have, exhaust noise is an issue with the cat bypass in place. I'm not sure a pre-muffler would be much help in that regard as even with an otherwise 100% stock engine, the exhaust note is REALLY loud, especially in the upper RPM ranges. As in "arrest me!" loud.
Last edited by jackb911; 11-28-2011 at 12:24 PM.
11-26-2011, 12:03 PM
#6
Rennlist Member
Well, this thread caused me to do some online research regarding the test pipe vs. pre-muffler issue. I found this:
What a difference! It looks like a M&K cat bypass is in my future. I was considering a Dansk unit but the M&K nicely muffles the high frequency harshness and costs considerably less than the Dansk.
What a difference! It looks like a M&K cat bypass is in my future. I was considering a Dansk unit but the M&K nicely muffles the high frequency harshness and costs considerably less than the Dansk.
Last edited by jackb911; 11-27-2011 at 02:05 PM.
11-26-2011, 04:49 PM
#7
First test run.
The car is freshly tuned about 600 miles ago with new air filter and idle set at 950. The temp today is in the low 70's w/low humidity.
The cat bypass is the Bursch 2 into 1 straight pipe with 02 bung.
I'm actually quit pleased as I expected a noticable drop in low end crispness and it is just a slight reduction. I've experienced real low end power loss messing with induction/exhaust experiments and this loss is def in the fine tuning area and hardly noticable.
At the top end it really sings...I was doing 120 mph before I 'woke up' and looked at the speedo and not the tach. Breathes much better.
Once fully warm ( I did an extensive 80 mile test drive, stop/go and ..well, up to 120)...my idle went from 950 to 1100...breathing better so I'll reset that now but want to have the mixture retested prior to adjusting the pipe.
Without an 02 analyzer I can't tell where I am in the rich/lean spectrum at what rpm so need to adjust that first.
That Bursch pipe is 2.40 " internally so I'm guessing a reduction of that .4 would be the max I'd try right now to create a bit of turbulance. I'll try a circumferance restrictor first (large washer opened to 2") and/or probably also try a suspended restrictor (small plug welded to a thin bar so that the plug sits in the center of flow...I've had good luck with those as at speed the hot gas normally follows the walls of the pipe if uninteruppted)....my 'scientific' approach is all hotrod home garage tuning but I do enjoy the experimenting.
BTW, I enjoy it because I have a lift and working is easy/fun rather than bust ***.
I'd say all in all you could run it as is because if you stay in the power band there is no issue...if you cruise at 2k you would notice it but only slightly.
Pat
The car is freshly tuned about 600 miles ago with new air filter and idle set at 950. The temp today is in the low 70's w/low humidity.
The cat bypass is the Bursch 2 into 1 straight pipe with 02 bung.
I'm actually quit pleased as I expected a noticable drop in low end crispness and it is just a slight reduction. I've experienced real low end power loss messing with induction/exhaust experiments and this loss is def in the fine tuning area and hardly noticable.
At the top end it really sings...I was doing 120 mph before I 'woke up' and looked at the speedo and not the tach. Breathes much better.
Once fully warm ( I did an extensive 80 mile test drive, stop/go and ..well, up to 120)...my idle went from 950 to 1100...breathing better so I'll reset that now but want to have the mixture retested prior to adjusting the pipe.
Without an 02 analyzer I can't tell where I am in the rich/lean spectrum at what rpm so need to adjust that first.
That Bursch pipe is 2.40 " internally so I'm guessing a reduction of that .4 would be the max I'd try right now to create a bit of turbulance. I'll try a circumferance restrictor first (large washer opened to 2") and/or probably also try a suspended restrictor (small plug welded to a thin bar so that the plug sits in the center of flow...I've had good luck with those as at speed the hot gas normally follows the walls of the pipe if uninteruppted)....my 'scientific' approach is all hotrod home garage tuning but I do enjoy the experimenting.
BTW, I enjoy it because I have a lift and working is easy/fun rather than bust ***.
I'd say all in all you could run it as is because if you stay in the power band there is no issue...if you cruise at 2k you would notice it but only slightly.
Pat
Trending Topics
11-27-2011, 08:08 PM
#8
I've been unscientifically messing with my mixture/idle and believe I can tune the slight low end torgue back in by dialing in the correct tune for the open pipe.
Back pressure is really not what most folks think. You don't need 'back pressure' to run well thru the rpm spectrum...it is only an issue if the car has been dialed in to deal with 'back pressure' as our cars are that have cats in them.
Note that racing sports cars that use all of their gears and rpm range do not have restrictive devices at all, just a tuned exhaust system.
So, I'm going to wrap the straight pipe with header tape (it'll keep it hotter inside the pipe and hotter gas moves) and then just retune.
Back pressure is really not what most folks think. You don't need 'back pressure' to run well thru the rpm spectrum...it is only an issue if the car has been dialed in to deal with 'back pressure' as our cars are that have cats in them.
Note that racing sports cars that use all of their gears and rpm range do not have restrictive devices at all, just a tuned exhaust system.
So, I'm going to wrap the straight pipe with header tape (it'll keep it hotter inside the pipe and hotter gas moves) and then just retune.
11-27-2011, 09:08 PM
#10
11-27-2011, 09:35 PM
#12
Burning Brakes
Experiments by the SAE have demonstrated conclusively that the Butt-Dyno CPU has an inherently positive bias with regards to the benefits of minor modifications.
11-27-2011, 09:50 PM
#13
Rennlist Member
If it is setup for one system, and you change-back pressure does have an effect.
11-27-2011, 11:20 PM
#14
Rennlist Member
The bypass pipe (GHL) definitely adversely affected the low RPM response on my very stock 3.2. Not a lot but I can feel the difference. The throttle response is not as crisp as when the cat was in place.
Interestingly, when I installed a Euro pre-muffler on my '79 SC back in the early '80's, I noticed no drop in low RPM performance. Could that be a function of CIS vs. Motronic?
Interesting theories, interesting thread!
