De-Cat or not De-Cat???

[Dominic]

I have a Cat and a de-cat pipe for my car, and have generally run with the cat in situ as I felt I had no noticeable performance gain from the de-cat pipe but a bit too much noise. I recently changed my manifold and downpipe for a larger bore stainless steel 4-2-1 set up, which seemed to release a little extra power, so I thought I would try the de-cat again. It did seem to help a little, but I really was not sure. Anyway, I always felt (as did the previous owners) that the car had more power than the 135 that it is supposed to have. Apparently the figure of 135 was quite conservative on Lotus's part, with 143ish being more realistic. So, I was interested to find out exactly what it is producing, and also what difference the cat made. I took it to the rolling road session on Saturday morning and did a run with the de-cat in first of all. It achieved 149 BHP and 144 torques. Then I fitted the cat, and back onto the rolling road; 146 BHP and 145 torques. The guys at ecotune commented that it was running a bit rich and was a little lumpy at the top end on the second run with the cat in, but I would suggest that is because I did not really get a chance to warm it all up before the run (should have been paying more attention) .Possibly with a warmer engine and hotter cat, the results might have been closer. Still, 3 fewer BHP and 1 extra torques, I doubt there will be any difference in performance. There is a lot less fumes with the Cat in, and the car attracts a bit less attention* being a bit quieter with the cat in situ so I think I will leave it on for just now.


*Says the guy who just fitted stripes

[campbell]

Fumes were a big deal for me and my cat has been in service for 15 out of 16 years of ownership

Great dyno results, btw.

[steve_weegie]

No flames to go with the stripes then Dom?

[Dominic]

No flames to go with the stripes then Dom?

Not unless I buy some stick on ones! ...did notice that with the de-cat pipe that the stripes are melting

[Dominic]

Fumes were a big deal for me and my cat has been in service for 15 out of 16 years of ownership

Great dyno results, btw.

Yea, I was pleased. Especially given that the car has done 90K miles... thought a few horses may have bolted by now. I like the state of tune it is at - 149 seems to be quite a sweet level, enough for spirited road runs, but not too much that the car is temperamental or peaky. A bit more for track would always be nice, but I s'pose I should just drive better.

Fumes are definitely an issue as the baby has already been out in the car, and I plan to take her out in it more often, especially now that she has a very cool seat.

[campbell]

Lol.

I get the distinct impression that if someone wants an S1 Elise with materially more than 150PS, and wants to avoid the requirement for regular rebuilds, then a Honda conversion is the sensible option.

For anything less, there's a 135 head, a PTP165, or a bit of Kiwirog...

Bravo.

[David]

It surprising how little difference a CAT makes. I did a back to back just to satisfy my curiosity.



I think the step in diameter is more significant than anything else. If designed right, it can help - on the Duratec it was slightly better with the cat, but compromised the pulse tuning at lower RPM.

[BigD]

pulse tuning

What's that then?

[tut]

Fingers on pulse with the left hand, whilst using the right hand to measure number of pumps per minute.

tut

[David]

[Ferg]

Fingers on pulse with the left hand, whilst using the right hand to measure number of pumps per minute.

tut

Is that a euphemism for a performance wank measure?! :lol

[BigD]

I was genuinely curious.

[Corranga]

Since my car decided that my cart should contain nothing and spat it all out, a decat seems to make some sense. I was thinking sports cat, but I'm worried that it'll just destroy one of those too

Glad the RR session went well

[steve_weegie]

^^ Redneck decat FTW!

My Saab did that halfway along the M8 a few years ago, after I turned the boost up a bit much. Went much better after that mind you

[David]

I was genuinely curious.

cut and paste . . .

As well as the gas physically flowing down the exhaust, there's a sound wave generated as the valve opens. By stepping up the diameter of the pipe, these sound waves can be made to reflect back and form a standing wave. If the low pressure part of the wave is arranged to be at the valve opening, it will help suck the exhaust gas out. The challenge is to get this to work over a large RPM range and is often achieved by having several points of refection, i.e. primary collector, secondary collector, and system.

Fitting a Cat will set up a refection but, as it is fairly well down the path, it is likely to be more noticeable at lower RPM. In my case it is out of tune at 4-5K RPM and causing a loss of power.

But my point was that a CAT does not effect power by restricting the flow, but more by altering the tune of the exhaust. By removing it, you could reduce the power or change the characteristics of the engine's response.