aeronautica86

because high flow cats are easier

Oddjob

Ive pulled cats out of 4 different 944s (1 '87 S, 1 '87 Turbo, and 2 '89 Turbos). All of them benefitted noticeably by removing the cat, even one of the '89s that only had 25k miles on it at the time (so it was not clogged/plugged).

I never found any rpm range that lost power after removing the cat. But I did notice the most improvement at high speeds, over 100mph in 4th, the turbos would definitely pull a lot harder.

Here are some acceleration tests that I ran with one '89 Turbo in 3rd and 4th gear (car had about 75k miles on it):

___________________4-6k rpm(3rd)____60-80mph(4th)____70-90mph(4th)
stock chips w/ cat---------5.6sec--------------5.2sec----------------5.1sec
stock chips w/o cat-------5.2sec--------------4.6sec----------------4.4sec
APE stg I w/ cat-----------5.2sec-------------5.1sec----------------4.9sec
APE stg I w/o cat---------4.7sec-------------4.5sec-----------------4.1sec

I got a bigger improvement from removing the cat, than from swapping to Autothority Stage I chips (which are claimed to be a 15bhp upgrade).

I did find that using a straight pipe or test pipe is better than gutting an existing cat. The flow through a straight pipe is better (less back pressure) than flow that expands into the empty cat cavity then is restricted back down to 2.5"

User 41221

Ahmet, do you have dyno proof that removing the cat hurts performance? I'd really like to see it. I've pulled more than one cat off a 951 and have yet to get a slower car out of it, and can back this up with some timed laps on a track. I've seen Karls dyno sheets, his midrange torque was better sans cat as well as peak hp/tq# increases he saw. I could also quote some *extremely* experienced professional wrenches who know FAR more about 951's than anyone here, who routinely will suggest removal of a cat (followed closely by a wastegate swap) as THE first mod to do on these cars.

This isn't "seat of the pants POWAH" bs, these are facts. If you are losing power after removing your cat, perhaps you need to look at you A/F ratio...

Regards,

ehall

So, jy, did you get all of that?
back pressure doesn't ensure bosst, it prevents it.
wrong side of the car genius.

TheRealLefty

Just added a 3" B&B stainless cat bypass, got good price from CDOC a few months ago. I'm a fan of backpressure for NA cars and even went so far as to have a stock 2 1/4 custom cat bypass to Flowmaster exhaust fabricated for my autocross car to keep the bottom end torque intact.

But in both theory and observation I don't think there's an upside to anything that adds restriction coming out of the turbo. What backpressure is in the system logically exists from the header through the crossover and then into the turbo. Once trapped in the spinning turbo, the sooner it goes away the better, IMO.

The cat that I removed from the 89 951 did not appear to be blocked or obstructed, but I really like the performance of the car better with the 3" bypass. It seems to be more linear in power as the RPM's climb and the boost rises (instead of the unmanageable Star Wars rush that sends you scrambling for the next gear). I'm controlled at 15 psi with a DP Tial, MBC, OE chips and Adjustable FPR and the car pulls through the top of second and third very, very well with the new pipe. I would not argue that a tad of midrange torque might be missing coming through first gear, but it is super sweet thereafter and much smoother at high rpms in every gear.

I think if you can get by the pollution police, this is a positive mod for any 951, street, track or combo use. Oh yeah, it sounds very cool, too, more whoosh and growl than before.

PorscheDoc

First Dyno:


After cutting out the cat, no other mods during this period. Temperature was 86 degrees IIRC.


Keep in mind the car is chipped, but it shows what a clogged cat can do to performance. And, i do have access to a 951 exhaust with a Cat, and I am the type of guy that will slap it on, and then dyno it to see the numbers......Time is my only issue.

Todd951968

I've done a backside dyno on my car with the full stock exhaust, a SFR test tube with sport CAT, and a no-CAT SFR test tube.


My '87 turbo is equipped with guru chips, fpr 15psi, and a SFR test tube. I had the original cat/downtube installed for emissions and, wow, what a dramatic difference (much slower). I couldn't take the stock exhaust off fast enough.

I bought the SFR hi-flo CAT to go with their mid section piping. It feels stronger than the stock CAT but not near the power of the no-CAT setup. The hi-flo cat is a nice compromise if you want more power but want to remain legal on the street. It also does away with the fumes I notice when I run with no CAT.

I'm saving for the full CAT-BACK system with the hopes it will improve power and sounds in both the cat and non-cat set up.

Ahmet

sh, I never claimed that getting rid of the cat would hurt performance, it wouldn't. All that I'm saying is I haven't seen a large gain unlike others here claim. I've also yet to come across a clogged one.
The 86 car I speak of in particular was a basket case when I purchased it, including high oil consumption and rich running (boost leaks), despite this, the cat was fine. All that I'm saying is, the gain from removing the cat is small, and I may care more about emissions than most of the rennlist public, but I don't see a good reason not to go with a replacement cat if you really believe yours is clogged. Like I said, a clogged Porsche cat is quite rare indeed from my experience.
Ahmet

User 41221

Yep, you are right, it was jy951... sorry!

The cat on my current '87 951 was very clogged at 90k miles, by the way. The heat build up at that you could feel coming through the torque tube tunnel was pretty significant. Once the cat bypass was installed, it now remains cool. The car also spools better now.

For what its worth, I have no issue with suggesting that a high flow cat be installed instead of a bypass. I like breathing clean air, too, altho in my experience, a properly tuned 951 with a bypass pipe will run cleaner than most emmission regulations require. A cat bypass obviously won't pass a visual inspection, of course.

Regards,

Trucho-951

In retrospect, I should have left my old CAT in place... was looking to restore that loss of power, but had heard that removing that CAT increased noise, and I like my Porsche quiet, so..
I used a Magnaflow 4 in. round x 16 in. long straight thru stainless steel muffler (Magnaflow part #14416) to replace the old CAT in my 86 951. Spool up was much sooner, my power band starts earlier around 3,000 rpm as opposed to 3,500 with the old CAT, and is almost as quiet as with CAT. However, I was suprised to find out that my 19 yr. old CAT was not clogged as I had suspected.

So I kept looking elsewhere for the source of my loss of power, and found I restored much more repsonsiveness by cleaning out the insides of the Intercooler with gasoline to wash out all those 19 yrs. of oily grime. Now, my car feels like new again, my BUTT DYNO tells me the top end of the powerband 4,500 rpm to 6,000 is much stronger. Next year, when smog time comes around, I'll be happily swapping back in the CAT.

H2Opumpr

I have swapped out the cat and a gutted cat back and forth at least three times for smog. There is a power difference for sure, but I think it must be noted that the car when set up to boost 15 psi without the cat will boost to only 13.5 psi with the cat due to the increased restriction. This is without altering my boost controller setting. So if I adjust my boost contoller to compensate, it is similar in power with and without the cat. So yes there is less power, but most likely because I dropped 1.5 psi out of the mix. The drivability of the car is better WITH the cat, than without. What do I mean by that, more torque in the lower revs. My car has a k26/8 so I need help in the bottom end area during street driving. In the end I suppose it depends on where you want your power.

IanS

Catalytic conversion is an exothermic reaction, so assuming the cat is performing its function, it will likely be radiating a lot of heat.

Trucho, good point about the intercooler. I haven't cleaned mine out yet, but everyone says it makes a huge difference.

SD Porsche Fan

Cleaning out the intercooler is on my winter project list.

I've heard a lot of different opinions on these cats getting clogged. I have only 77,000 miles and was only getting about 7 psi boost with the cat. I also experienced A LOT of heat from the torque tube tunnel. I would never run the heat even if it was cold outside. After I cut the cat out it was pretty obvious that it was in bad shape. With the bypass in place I'm getting about 11.5 psi boost and cabin temps are much more comfortable.

So why would some cats function for 17+ years and over 100K and others clog up to the point where they effect performance with considerably fewer miles?

jy951

You've answered part of the equation. Unfortunately, most listers on these forums are NOT professional mechanics (incld myself), so everyone falls for the "cat delete" option for STREET CARS. I fell for this idea too, unitl I removed the CAT from my street car and noticed considerable loss of mid-range power.

I spoke with several (local East Coast base) Porsche techs about this and found out that the factory actually uses the exhaust system to help optimize the turbo performance. By completely DELETING the CAT, you've changed the characteristics of the turbo set-up, yes, spool up will be faster, but now peak power will be moved HIGHER in the rpm range. You must think of these things as COMPLETE SYSTEMS (that interact) and NOT JUST SEPARATE PARTS (exhaust, cat, turbo, intake)

In fact, Audi does the same thing (as Porsche - they do share a history) with the S4 Bi-turbo (I also own this car). Audi OPTIMIZES turbo spool-up by using SMALLER turbos and 4 cats (2 main/2 pre-cats). Most S4 guys eliminate the pre-cats but KEEP the main cats. The combination of smaller turbos and the slight restriction in the exhaust allows for most of the boost to 'come on' EARLIER (1800 -2200 rpm) in the power range.

Like I said, CAT DELETE is great for track cars, because it enables POWAH higher in the rpm range (this is where the CAT becomes a liability), but on STREET cars, you tend to lose MID-RANGE torque which is noticable when you try to merge onto a highway (which I experienced). STREET CARS will rarely stay at the high end of the power range long enough (most ppl will get a ticket!)to justify a CAT DELETE over a 'HIGH FLOW' cat.

Based on my research, the comment from Ahmet about any (exhaust) restriction is bad for a turbo car IS NOT SUPPORTED by knowledgable Porsche techs.

jy951

I've read many of your posts on this board. Unfortunately, in much of them - you spew URBAN LEGEND regarding performance. Like I said BEFORE, a HIGH FLOW CAT on a STREET CAR assists in MID RANGE torque. Yes, HIGH END performance will be assisted through the elimination of a CAT in the exhaust system. THAT's WHY TRACK CARS ARE USUALLY CAT-LESS - duh!

If any 951 street car wants the quick spool up of a CAT-LESS set-up (but not lose significant MID-RANGE) than a HIGH FLOW cat is the best compromise. That was what I said in MY ORIGINAL POST!

So, who would I believe about turbo performance, a local Porsche/Audi tech wrenches 951s and S4s on a daily basis, or YOU, a guy on the Internet whose response to my original post (on this topic) make me question your overall knowledge of this topic.

For the record, I don't wrench cars for a living, my post was based on my PERSONAL experience of removing the CAT on my 951 and mid range performance was horrible! (I have chips, DP wastegate, etc.) In fact, another poster (read the complete thread) said he experienced the EXACT SAME THING!