993MAN

Yes. I had a look.

Caveman

All it needs is elbow grease, lots of swearing, knuckles to be skinned, more swearing and a nice cup of tea. If you can undo a seized up bolt you can probably do this...

oliverjamesthomas

The question in my mind is why would you want to?

I put bypass pipes on mine and found they led to increased oil consumption, they resonate like hell and the car had a whopping great flatspot. I took them off on the advice of a certain notable engineer who has created some rather nice cylinder heads and the car started to behave again. Oil consumption was back to normal, flatspot was gone and I didn't need ear defenders whenever I drove it!

Oh, by the way - if you plan to take the car to track, forget it. My car is standard in the engine department and registered 104.2dB with the by-pass pipes on!

Regards

Oliver

Monique

IMO, back pressure provided by the Cat system is well understood by P engineers.

Unless you do a lot of track time and do not require the mid range grunt, you will be better off with standard cats. Hey... it is my opinion!

993inNC

Well not being an engineer, I suppose anything is possible, but increased oil consumption because of bypass pipes? Hadn't heard of that before. I'd love to hear from some of the engine rebuild gurus here.
flatspots are probably from valve float, which may indicate you have engine issues, not exhaust issues. And as far as resonating, all by pass pipes should do is amplify whatever mufflers you have, they don't make their own noise, they're just the conduit.
You obviously didn't have mine on your car you wouldn't have had those issues

oliverjamesthomas

Sorry, should have proof read that before posting.

The advice given was not in relation to reducing the increased oil consumption, but was in relation to the general performance of the engine and the benefits or otherwise of having them on the car.

The oil consumption issue was an observation of my own, although it has been borne out by a couple of other people that have taken the same route and experienced similar issues. I wouldn't wish to guess at an explanation for it, because I don't have one. Suffice to say, I was using around 1 1/4 litres of oil per thousand miles whereas I am now back to the more usual for my car, 3/4 litre per thousand.

The pipes I had on the car were the type FVD and alike supply, namely two totally separate pieces with the tapped mount for the sensor in one of them. As an aside, I was noise tested at a track yesterday and registered 98db using the standard system which is, I am sure you will agree, considerably reduced from the level registered using de-cat pipes.

Regards

Oliver

Bill Verburg

yes, it is very easy to do, but if the exhaust hasn't been apart from day 1 a torch will be very helpful.

cat bypass does not lead to increased oil consumption, the cat may mask oil consumption by burning it more completely

back pressure is not a design criteria to enhance hp or torque. Gas speed and acoustic resonance are. In a muffled system acoustics are moot, and the gas speed change can slightly affect the torque curve, but it's a give and take sort of thing. some spots are better others worse.

I have 2 sets of mufflers for use w/ my cat bypass, completly stock for most of the time including tracks w/ noise *****, and a set of RSRs for days that I am willing to live w/ more noise. Honestly I prefer the sound w/ bypass & stock mufflers. There is zero resonance w/ either set in the cabin. The RSRs are just much louder.

Caveman

God bless those RSRs...

TroyN

I did bypasses several months back and it's not difficult at all, and no need for a torch. There's lots of little connectors on the stock cat to disconnect but that was the worst of it. Last time on track I was able to outpull a 996 (never checked whether 3.4 or 3.6) on the straight, and the car does feel up on power too. Sound is awesome and no resonance.

993inNC

Wouldn't you say Bill that back pressure directly results in how fast the gas exits once past the valve? thereby effecting the torque? Its really the same idea, IMO. If there was no back pressure, the gas would exit at a much faster, unrestricted speed. Back pressure keeps the valve from floating at high rpm (with stock springs).
As I understand things, to much flow causes the gases to escape to fast which means that the fuel and exhasut is "pulled" from the head prematurely and results in a loss of power. Back pressure (whatever the value is) allows for the gases to regulate (hopefully equally) as they get past the valve.

Oliver's original post about flat spots may be a result of the gases exiting to quickly at the rpm he noticed the "spot", because of the pipes design or the mufflers he is using.

Bill Verburg

In a way yes, but the design criteria is not to get back pressure per se. The exhaust is a single state device that must accomodate the maximum flow volume at wot/max revs. As such it will compromise flow at lower throttle opening. There have been sporadic attempts over the years to build 2 state devices that change the exhaust to accomodate both low and high speed conditions, my first car had manually operated lakes pipes, Audi S4s had a 2 stage exhaust for a short while and I believe that maybe the new GT3s have something similar.

Any way a single state exhaust can only be optomized for a relatievly narrow rpm band, you get to choose where the optimum gas flow is, narrow pipes are better down low, bigger pipes are better up high, longer pipes are better down low, shorter pipes are better up high, you mix and match to suit. They are all a compromise and all will have differnt curves, the dips and valleys of the tirque/hp curves can be moved a little both up and down and in magnitude but will always be there.

The whole point of a cat by pass is to enhance flow at max revs, which it does.

valve float occurs when the valve springs can no longer control the mass of the valve in syncronization w/ the rest of the engine. i.e. the cam is no longer controlling valve events

993inNC

Very well said. I thought that b.pressure though (if correctly applied) helps in minimizing float. I realize the function of the springs are supposed to control it, but in addition w/pressure........? No?

Crimson Nape Racing

I have bypasses with RSR's and love the sound. Never any problems at any track (including Barber, and they're strict). You may need to take the bumper cover off to get at a couple of the bolts. I didn't, but it was a miracle that I got them off. You do smell more oil, but that's o.k. with me.

Bill Verburg

Try LRP someday

I also took pity on my neighbors, but since Harleys seem to be the rage these days , I may need to rethink that one

TRINITONY

How does a Harley compare to cat bypass with RSR? My car sounds so so quiet when driving next to a Harley.. And lately that's been quiet often.