Removing Catalytic Converter?

[vesnyder]

I've recently purchases an '83 SC Cab, which has the catalytic converter. I registered the car with collector plates (Ohio) that allows me to remove the catalytic converter. Anyone have any experience with this? How much on a power increase can I expect? I should probably replace the exhaust at the same time - what exhaust system would you recommend?

Thanks

[makaio]

Since you do not have to deal with any smog issues, your best $ would be spent on the early style heat exchangers, or the SSI's. With a 2 in & 2 out muffler. With this set-up you will see about 20 more HP.

If you don't want to spend the $, remove your cat, and knock all the ceramic crap out of it, then put it back on. That doaes the same as a test pipe.

As far as mufflers go, I would suggest either a dansk, or a GHL. Both great mufflers.

[carnut169]

I have an 87 w/ a B&B stainless steel muffler and purchased a GHL stainless cat test pipe but found that the car felt slower w/o the backpressure that the cat provides. I would not recommend that set-up. Everyone seems to like the SSI set up, but it will cost you $600 (used ebay) to $3000 (new GHL w/ heat exc) for those equal length pipes.... my .02- spend your $$ on DE- you will be much faster than your neighbor w the SSI & no DE experience.

Our cars are already more that most of us can handle.
Good luck- Sean.

[Formerly Steve Wilkinson]

I removed the cat from my '83 SC and installed SSI headers (requires some new plumbing to/from the oil tank) and a Fabspeed dual-outlet muffler, as well as a considerable variety of other engine mods. Pretty loud...not what I'd recommend for a daily driver but it's way neat for a fun car.

Stephan

[rscupper]

Hey Sean, or anybody else - When you change the exhaust pressure, resonance, etc... by installing a different (not necessarily better) exhaust system shouldn't the car be re-tuned?

I ask as I am completing (finally!) a major project and had some trouble tuning my originally-selected cams with my headers, which affected/determined the distributor profile, etc.... My problems are pretty much solved, but I thought tuning 'around' a new exhaust system would be SOP.

Can you enlighten me?

[Jdub]

Purchase a test pipe right here for about $125.00. Get all needed gaskets as well, and all new nuts and bolts.

Go to your local muffler shop. Have them hack/flame off the old nuts and bolts and replace these with the fresh nuts and bolts (do NOT ask them to put on your "test" pipe). Now go home, wait till it cools a bit, unscrew all the nuts and put your test pipe in with fresh gaskets. You may have to rubber hammer the rusty joints betwixt exhangers/mufflers but this procedure ensures you can do the job without hassling with rusty nuts and bolts.

Jw

Yes, no and maybe. You have to measure any changes you have made to your car to see if they help or hurt its performance. And then adjust acording to what happened.

Also try to make only one change at a time so you know whats doing what to the car. IE Dont change the gearing exhaust lighter or real heavy driver, 6000 stall torque convertor, pull the turbo 350 out and install a powerglide etc. Or order a bunch of parts and then go to the track, thats the best way to see a big jump in performance (much lighter wallet) which means lighter car better performance.

Primary pipe length and diameter, collector design and length of the collector will all make differences in performance.
Now you may not be able to feal them but under controlled conditions you should be able to measure them.
I would keep an eye on the spark plugs to see if it is running richer or leaner, or any other indicators of fuel mixture issues.

I played around with collector length and saw differences in 60' times and 1/4 mile ET's and mph.
Which required changes in jetting for the carb.
The little time I have owned my 74 coupe it seems the best performance gains come from gearing changes in the trans axle.
Or adding the 3.2, I guess that helped also.

[masraum]

I would go with SSI's and a good muffler. They are proven performance. If you aren't the type to like a loud car then go with a single out muffler, stock would probably be fine. As far as hollowing out the original cat, I wouldn't. I used to be a proponent of that, but over the years I have decided that if you are going to go that route (to fool the inspectors) then you need to put a piece of pipe inside the cat otherwise the shape of the cat actually interferes with the flow through and can hurt performance.
Yes, it wouldn't be a bad idea to get the tune of the carbs checked after changing the exhaust radically. Probably less necessary with a comp controlled car since they can adapt *somewhat*, but since you don't have a comp controlled fuel and spark system...

[911nut]

Get rid of the cat with the test pipe. You will feel a difference at about 5000 rpm on up.
As an experiment, I knocked the ceramic out of my cat and re-installed it. Felt just as bad as the cat with ceramic. The cat shape (2.5 inch dia to 5 inch dia back to 2.5 inch dia) is the source of the backpressure.
Backpressure is the enemy. Cats increase bassckpressure.

[carnut169]

Do not remove the guts on the cat... it is VERY expensive to replace and you can get a test pipe for under $200.

Yes, it may have more power above 5,000rpm... but the majority of the time (if you drive on the street) you are not above 5k, and below 5k it does feel LESS powerful.

Backpressure in not the enemy. A little is needed to help pull the burnt gasses out of your engine. Perhaps a V8 needs less- but our cars do need some backpressure.

I think you can have a company like Autothority customize a chip for your car that takes advantage of mods... I am not sure you can just make tuning adjustments to take advantage of exhaust/ intake mods.

[911nut]

Carnut169,
If the cat is shot (as mine was) doesn't matter if you remove the ceramic or not.

Sometimes we all run to the redline with our cars so the added torque and power can be enjoyed. It doesn't hurt a 911 to run close to the redline;it's built to take it.

If 911 engines need "a little backpressure" to run properly then all race 911 would have mufflers, wouldn't they? Or maybe they'd run smaller exhaust valves instead of the biggest that they can get. Ideally you would want a vacuum in your exhaust system to aid in the removal of exhaust gases. If the 911 engine needs "a little backpressure" someone will have to convince me with theory and data to support it.

An '83 SC has a K-Jetronic continuous, mechanical fuel injection. The K-Jetronic is an amazingly flexible unit and will easily support more air flow through the engine without any mods. As a matter of fact, there are no mods that can be made to a K-Jetronic unless you want to play with the mixture settings or you want to disconnect the O2 sensor. Buying a chip, therefore, is not an option.

[carnut169]

Fellow nut-

The original post asked if he could expect a performance gain from removing the cat. I answered based on personal experience and from what I have read. Even Bruce Anderson says little to no performance gains can be expected from removing the cat. If the cat is bad, that is another story- but I don't think his is... he just said he is not required to have it.
Furthermore, I am not referring to 'race' cars. The vast majority of the time those of us who use the car on the street stay below 5,000 rpm. I agree that it will not hurt your engine to go above it, but come on- who drives that way all the time?? I remember reading a detailed post on how the exhaust system works... I have looked and looked, but cannot find it. Hopefully someone steps in and gives us a hand resolving this. I will say that I think that is why the ssi's work so well... the equal length pipes help the exhaust suck the burnt gasses out of the engine better than the unequal length pipes (backpressure).
Lastly, you are probably right about the 83. I have had an 86 and now have an 87, so I have little knowledge when it comes to the earlier cars. I did say "I think", and was not trying to come across as an authority.
Cheers!

[RarlyL8]

Removing the cat will not help much.
The cat is only part of what ails the stock SC exhaust system.

Equal length header pipes allow the cylinder engine pulses to occur individually at the collector. This scavanges the exhaust more efficiently.

Backpressure is important for street engines that are designed and tuned for it. Low end torque will be sacrificed when the exhaust is "uncorked".

[Jdub]

I have to agree with Rarly in that backpressure is an essential part of any exhaust setup. Consider the Yamaha two-strokes (or any two-stroke for that matter) and how critical the exhaust is on those reed valve setups.

I also have a '53 Triumph T100 alloy engine in a Norton featherbed frame (a "Triton") that has straight-through pipes that can be retrofitted with packing (heat resistant fiberglass-like material). I always play with the packing material to get the pressure just so. Else, I'd just leave it all out and worship the glorious racket that this old pushrod engine creates.

Jw

[rstoll]

Maybe my cat was bad, don't know. I gutted it a couple weekends ago and got a noticeable performance improvement. Especially above 5000 rpm. Car pulls harder all the way through the range. Gonna add a Bursch Carrera muffler next.

[masraum]

Exhausts do have to be "tuned" or matched to the engine, the rest of it's parts, and it's planned use. By going with less backpressure, larger diameter pipes, whatever, you can reduce low end torque while increasing highend hp. Less low end torwue would probably not be desired by most people who drive their cars regularly on the street. Race cars usually require more topend power so they will flow more air and therefore have more topend power while sacrificing low end power. The engine is an airpump and for it to work in it's most efficient manner it should have matched parts, induction, cams, exhaust, that flow similarly. Too big in any of those areas compared to the others will usually make the car slower in some way. I've read somewhere that exhaust backpressure with resonance (not muffler resonance) creates a scavenging effect that helps to suck the exhaust out of the cylinders. I wish I could remember where I read it or more details. Adding a race exhaust to a stock car without otherwise changing the car will likely hurt the cars power at some point in the operating range.
This is not to say that we should go for a freer flowing exhaust on 911's. Often our cars have some compromises built in to make the cars more marketable or legal, often a restrictive exhaust is one of them. Opening the exhaust up with a correctly engineered system should help in those cases.