Oddjob

I ran my 944S w/o cat and with a 944T muffler (straight through muffler, less restrictive than an n/a muffler). No chance did that car lose power by opening up the exhaust. But it was loud, and not a "cool" loud - not a good setup for a street car.

M758

8 valve 944 spec cars typically gain hp AND torque after deleting the Cat.

82-T/A

Well, like I said, you'll lose low-end power. You will always gain power in the upper rpm simply by virtue of what you're doing. At higher rpms, you'll want more air / exhaust flow, simple as that.

Eliminating a catalytic converter has the same effect as installing a larger throttle body on a car. You will see an improvement in the upper rpms where the improved air flow is needed, but you WILL lose power in the lower rpms.
Like I said though, for a STREET car (daily driver), you'll want to keep the cat, but go with a high-flow unit.

This is just how the combustion engine works. It's a fine line in tuning the exhaust to get it to where you want it for your needs. A track car, of course you're always going to be in the upper rpms. But on a street, you're going to want to keep that additional torque in the low-end since most of your driving will be from stop-lights and stop signs.

Basically, you'll see MORE benefit from a daily driver / street car by switching to a High-Flow catalytic converter, than eliminating it all together.

We can both agree though that the stock cat is very restrictive.



Todd,
2006 Pontiac Solstice
2002 Ford Crown Victoria LX-P74
1987 Pontiac Fiero SE / V6
1984 Porsche 944
1981 Pontiac TransAm WS6
1973 Volkswagen Type-2 Transporter

ausgeflippt951

True true; this is simply due to the fact that the cat is so restrictive that practically anything helps it.

This is why I compared no cat to a hi-flow unit.

And 82-T/A, the losses you have been talking about are so small that what losses MIGHT be present in deleting a hi-flow cat won't even be noticeable. It's the placebo effect in it's purest form.

Delete the cat and run a premuff. nuff said.

attyjeff

no cat here just a straight pipe,

82-T/A

I beg to differ... it's the placebo effect in it's purest form when someone thinks there isn't a loss in low-end power simply because they're so excited by the additional sound it makes, and the boost in power in the mid-range that they don't even realize they've lost power in the low-end.

There is absolutely nothing inherently special about an NA Porsche engine, especially a 924/944, that every other combustion engine on this planet doesn't also benefit or suffer from.

A high-flow cat isn't a universal hi-flow cat. You have to buy a high-flow cat that is optimized for your specific exhaust flow / piping. The 2.5 NA Porsche motor, being what it is, could benefit from a high-flow cat that flows around the 450-475cfm range. The stock cat is probably something like 350cfm. Likewise, an S2 which has half a liter more displacement and significantly more intake and exhuast volume due to the additional valve area... would probably benefit from something in the range of 575-600cfm.

I don't want to be mean here, I know it makes sense that the most flow possible would be ideal... but it is for this reason that every Honda Civic owner has a huge exhuast tip on the back of their car, a massive spoiler on the back of their front wheel drive car, and extremely negative camber.

Simply saying that no cat at all is better than any cat is just not true. I mean, lets look at the facts here. The entire exhuast system was designed (although under restriction from CAFE and emissions standards) as a complete package. It had to meet certain emissiosn levels, and certain sound levels. The exhuast piping that was chosen for the NA car was designed specifically to be used WITH the catalytic converter that it comes with. If you were going to completely eliminate the catalytic converter, then you probably wouldn't need exhuast piping that was as large as it is. Perhaps 1/8th inch smaller or something to that effect.


Every performance part on a car has it's pros, and cons. You have to give a little here, to get a little there. The larger the throttle body, the better the flow in the upper rpms (up to the point where it meets the ID of the intake manifold). But at the same time, the greater the loss in the lower rpms you'll see. You have to give up lower rpm power, for increased upper rpm power. That's just how it works. Some of the ways that engine manufacturers have been able to overcome things like this are with variable valve timing (to try to keep the engine in it's peak operating range with one cam or cam set ground specifically for the low mid, and the other for the mid high), or by supercharging and turbo charging.

Going with a smaller throttle body will typically give you a bit more torque off the line, at the expensive of upper rpm power. Obviously going to the extremes of either side are obsurd. But do you see what I'm trying to get at? I've already mentioned that Porsche did the best the could under the restrictions that were present. The exhuast piping was designed to work with all of the components. Simply deleting the cat will give the average driver (someone who uses ALL rpms of the motor) a disadvantage. You want to to OPTIMIZE the power of the engine so that it meets your needs. Anyone on here that says they DON'T care about off the line performance is either British, or just plain lying. We all love low-end torque and blowing away the car next to us at the stop light, just as much as we do screaming into the upper rpms. You want to OPTIMIZE your exhaust. Eliminating the cat is NOT optimizing the exhuast. Will it give you more power than having a stock cat on? Yes, absolutely... but you can improve that even further by getting a high-flow catalytic converter that puts the engine's flow directly in it's sweet spot... the exact cfm cat that you need in order to be RIGHT at the point where you've gained the most upper-rpm power you can, with little to NO loss at all in the lower rpms. I can guarantee to you that putting in a straight pipe does not put the motor in the sweet spot unless the car has forced induction.


Todd,
2006 Pontiac Solstice
2002 Ford Crown Victoria LX-P74
1987 Pontiac Fiero SE / V6
1984 Porsche 944
1981 Pontiac TransAm WS6
1973 Volkswagen Type-2 Transporter

ausgeflippt951

I don't disagree really with a thing you said above; in fact, much of it were things that I hit on on the previous page. Also, you are not wrong that eliminating the cat will show a decrease in torque over a cat --- but only if it's "tuned" (so-to-speak) for the rest of the system. The cat that we've got on our cars are ****ty, restrictive monsters that were not really "tuned" for the system as a whole. They do absolutely no good performance-wise.

That said, here's a good article I found [Cat removal article ] that showed that you DO actually gain a smidge for a testpipe but the gains are minimal at best. Given that they only netted 5hp and 3ft-lb for a testpipe over the stock cat on their civic, any "losses" are again, minimal.

Moral of the story, swap your stock cat w/ a Random or Magnaflow unit and net about the same power/torque. I guarantee you will not be able to feel a fraction of a ft-lb of difference in the car down low. Not even Schumi could do that.

thewitchdokta

i have an 87 951 and my cat is nearing the end of its life, and as opposed to replacing it i was thinking of removing it. does that affect the performance all that much? does anyone have any real data on that?