Performance exhaust upgrades

Here’s my thought provoking question for the weekend.
Just getting ready to pull the pin and do the Simply Performance upgrade. (No affiliation) .
As they’re out pf the UK (different emissions) I was going with the euro style downpipes and deleting the cats.
My question would be if I went totally catalytic converter free is it a given I should be changing out my ECU?
Thanks all in advance.

MY and engine might be helpful. On a 5.3 V12, I don’t see why deleting the cats would impact the ECU at all.

Thanks Kirby for saying that. I was sort of thinking the same.
I went to talk to my local Jaguar mechanic and he was suggesting it was necessary.
Thank you for your response.

On the US 5.3 v12 there is no ECU sensing post catalytic converter. So the presence of cats should be irrelevant.
You will need to install O2 sensor bungs in the euro down pipes. Easy to do if you can weld or know somebody who can.

Ummm. You might like to know exactly why he thought so. A bit of a yellow flag if he claims to be an expert. Just sayin.


I put in Simply Performance full exhaust system on my 1988. The ECU will function just fine as is. There are some theories that with the freer flow exhaust, you can alter the ECU to give you more power, but it will run fine without any changes.

They will come with no bungs for O2 sensors, and aftermarket seals for the exhaust manifold. I went ahead and bought OEM seals, maybe I wasted my money?

Wait until you get the exhaust system installed to your liking…took me several tweaks until I got it rattle free. Then bring it in for someone to weld in the bungs and install the O2 sensors. There is very little room for error and so the O2 sensors need to be just right, as well as reach. They will end up super close to the frame.

I found the stainless steel joints they included for the middle sections a bit weak. I ended up buying some much thicker heavier duty aluminum ones from NAPA. Everything else is top quality.

Also, I don’t know if it’s Simply Performance, or just my car, but the driver’s side (left) downpipe and exhaust is extremely close to the body, whereas the right side has more room. I’m currently about to try a trick someone else here did by putting an extra engine mount spacer on the right side, so the engine rotates slightly away from the left side, giving the left side a bit more room. You’ll see how 1/8" is very important!

If you change how the air goes in or how the exhaust goes out, then the current ECU may still work, but it won’t necessarily give you the same air:fuel ratio at all revs and all engine loads as you had before, otherwise there would be no performance change.

Whether it is a change for the better or worse I don’t know, but logic says either a different part of the ECU map will be in use than before the change, or it will be richer/leaner at different parts of the map. If it isn’t, then what is causing the performance change?

kind regards

It’s definitely more of a throttle response. Much quicker reacting power.

The ECU will respond to the O2 sensor if it’s richer/leaner.

Greg, if you have the Simply Performance exhaust, can you please post a short video on startup. I would really like to hear a how it sounds. Also a few pics of the install would be awesome too!
Thank you, and thank you all for your responses, I feel like a nervous kid in a candy store but this was always on my bucket to do list.

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Well in case Greg doesn’t have a video handy, I have the exact same exhaust on my '89 (5.3 V12).

It has quite a bass note, so small speakers might not do it justice. Just sayin’ :stuck_out_tongue_winking_eye:

Too big maybe, had to use a google drive link.

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Thank you Paul, sounds great, Daddy like!
It doesn’t sound over the top .
Do you notice anymore performance or less heat in the engine bay?

I noticed more performance - a little bit more power once big reverse flow mufflers were deleted. But biggest difference is throttle response is quicker. The power just comes faster.

Sound is the best. Wide open throttle, you hear the beast. But it’s not annoyingly loud. Just deep.

I’ll admit, its a real pain to hook up on jackstands by oneself. A lift would have been nice, and a helper. But in the end, I’m glad i did it. The rear end pipes for the mufflers look very sweet too.

Oh, and a bit easier changing oil filter with no big cat there!

Did you go from the downpipes back or did you do the headers as well?

I put in the Simply performance downpipes. No headers.

I second everything that Greg has said. Throttle response has a nice snap to it, and oh yes the sound.

One thing I will add, you can order the down pipes from Simply Performance with O2 bungs pre-welded now. Which is what I did. Although as Greg pointed out, there isn’t much room for error. The right side of mine was perfect. When installing the O2 sensors, the left hit some excess metal on the welded flange of the ‘frame’. That metal simply didn’t exist on the right side. 10 minutes with the die grinder took care of it.

The cats on my old exhaust were in really sad shape, broken apart internally. So I’m sure this wasn’t helping.

New exhaust on the floor prior to install. The car is in the air so I’ll take a pic later today of the system installed. As Greg said, bit fiddly installing but not the end of the world.


I deleted my cats on my 85 5.3 and straight piped it back to the rear resonators and had zero impact on ecu function.

Thanks all a lot of great info. I’m not sure I’m going to install them myself, I have a friend who runs an awesome custom exhaust shop. But a lot of good stuff I can pass the heads up on.
I wouldn’t mind seeing a picture of the underside with the carcar up in the air though.
Thank you all again.

Interesting where they put the O2 bung. I made the big mistake of putting it there BEFORE downpipes were installed. I guess the angle is important. You can see there is no clearance if it came out perpendicular. I ended up having to cap and put O2 bung a little further down.

There’s no room to “swing a cat” in there (no pun intended) as they would say.
These are good pictures to see.
One more question did you go with the original Simply Performance set up or I see they now offer the 21/4” tail pipe set up?

Not sure what the original set up is. Do they now offer two choices? I did buy the big tail pipes, they simply bolt onto the rear mufflers. They really look good on the car.

Oh, one issue with stainless steel, looks so good new. But as it ages, it turns yellowish/golden. Oh well.