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Intercoolers and Catless downpipes


NewNiceMrMe
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It's my understanding that anything post-92 has to have a cat. If it has one and it's subsequently removed then it's an MOT failure unless you know a man.

And we all know how I feel about cats.

Edit: s'pose it depends if the downpipes are pre-cats, then it might be ok and I can't imagine your regular MOT tester getting into that level of detail.

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It'd be on a car that is very much after 1992.

Thing is, I'm exploring a few options with the Z4 and I appear to have found something to sort the ride out (new suspension :roflmao:).

The car isn't going to drop much more money that it has and as it is already putting out 372 horses it makes for a very quick machine. So, if I can get the suspension resolved (which is now proven as I've been in touch with someone who feels exactly the same as I do about the ride and has got it sorted with different suspension and claims the handling is transformed as the car is also much less skittish) - then I may have a car that is worth keeping.

So I explored some other options. I have looked at adding a Milltek but basically been told if I'm looking to take the car to serious power levels that heat is the main enemy. I've then been told the BMW exhaust system on the Z4 is excellent from the CAT back for cars running below 20psi (which mine isn't anywhere near of course). Because of that I'm told that if I really want to unleash the power and turn the car into a highly responsive and powerful one that I should consider installing a high efficiency intercooler and CATless downpipes.

The cars warranty is up in a month and I've nothing to lose if I spend a few grand on turning it into something very special, maybe a bit more if I tinker with the brakes too (although to be fair they're actually very impressive).

It's just an option I'm looking at. It looks very likely that a 400+ bhp Z4 is easily do-able. Okay I'm not far off that now, but it's not just about raw power.

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Do you need to go cat free?

I had a full Milltek (down pipes through to silencers) on my B6 S4. It had high flow cats. With a big old V8 and it remapped and the high flow cats I was a bit concerned ..... but then found at the first MOT my emissions were about 10% of the allowed limit.

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Cooling system/intercooler is of no interest to the MOT tester, but lack of cats 'should' fail an MOT, however, most sensible testers will 'miss' the lack of cats, or, you can simply have the cats put back on for the test, although thats a bit of a faf.

You could simply have the existing cats gutted, then they still look like they are there, as the check is only visual it will pass.

Don't Milltek do sports cats?

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I'm being advised that Milltek isn't good enough if I want serious power and that the CAT-less downpipes are the only way of giving a true release of power coupled with excellent response. I just wondered on the general view because the people recommending this are very well thought of in the tuning industry, especially where BMW are concerned.

My request was quite simple - on top of the remap, what do they recommend for a real release of the true potential of the 35i engine (which is known to have a lot of power 'reserved' by the OEM state of tune). They've said that whilst the Milltek (which they can fit) is the choice of many that it isn't their recommendation if you're really looking to get the responsiveness and delivery of power that the 35i can give.

Bear in mind this isn't just downpipes too - they're talking about the fitment of a high efficiency intercooler system too.

From what I've read, 410 bhp is easily acheiveable without any undue stress on the engine if I was to follow this route.

There is a Z4 35i in North America running 450 bhp with a verified 0-62 mph time of 4.0s. That might keep be interested. Maybe. :roflmao:

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As a side note, changing the suspension will, I'm sure, dramatically alter the ride, but it won't affect what is by your description, a fundamentally flawed chassis.

Things like the glove box flying open etc, the chassis won't be any stronger or less susceptible to flexing, it'll still transmit anything the suspension doesn't soak up straight through to your spine, do they recommend fitment of any strut braces or anything to stiffen the chassis up?

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Ref the CAT situation.

The latest MOT Changes list: [h=3]Exhaust[/h]A catalytic convertor fitted as original equipment but missing will be a reason for failure.

So you will need to swap them in/out or find someone that is willing to loose their MOT licence to ignore it.

You would probably also fail any roadside checks, but tbh in the last 15 years, I have only seen this being done once.

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Roadside checks do not include a visual inspection of the exhaust, only an emissions check via a probe in the exhaust, which you would still pass.

I've never, ever seen a roadside emissions check, although obviously they do exists for the general public, we occasionally have them sprung on the cab community though, but again, not in the last 8 years or so.

Not something I'd worry about in all honesty.

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They do roadside checks upon exiting the Tyne Tunnel on occasion. It tends to be HGV's and commercial vehicles more than anything but they're not just checking for red diesel.

I've seen them with what I'd call older vehicles, normal cars, a good few times.

Probably when they see a car that 'just looks like it might fail' an MOT or is in a really tatty condition? Over full with people and those people looking a bit shifty.

Your Z probably wouldn't be pulled in, as it'll look clean, new and in a good, looked after condition. However, the shifty bit - might get you pulled! :grin: As might the sound from your new exhausts..... :eek:

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Like others have mentioned, the only way of passing the MOT (without bunging them a brown envelope) is to use 'sports-cats'. These still have restrictions, but like everything in the world it's a compromise. I'd personally not want the agro in swapping the OEM cat on and off each year, it used to be a ball-ache on bike silencers, and that was only 4 bolts!

FMIC (front mounted intercooler) will make no MOT difference, it would only effect your OEM warranty, but like you say this is coming to a close soon anyway.

I'm starting to get into the 135i now and although quite comfy, there are issues with the ride. Mainly I think these are down to the run-flats and the compromise they've had to make suspension wise to compensate for them. I think re-springing will be on the cards along with loosing the RFT's. Will be very interested in your mods should you keep the Zed, the 35i engine is a peach for sure and VERY capable.

What brakes are fitted to the Zed? Is it the BMW Performance ones like on the 135? If so, some good pads would be enough to bring them up to spec

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What brakes are fitted to the Zed? Is it the BMW Performance ones like on the 135? If so, some good pads would be enough to bring them up to spec

I can't complain about the brakes one bit, they're excellent. They cope with the uprated performance without issue, but I'd probably change to fast road pads if I go down the intercooler/downpipe route. Brembo do a big performance kit for the car - but at £3,500 for the fronts alone...it won't be happening!

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Brembo do a big performance kit for the car - but at £3,500 for the fronts alone...it won't be happening!

Wow! That's a LOAD of money for just the fronts :eek: On the Z4M I replaced the OEM pads for Pagid RS29's and it made a massive difference at Spa, they didn't destroy the discs too. Only problem was round town they made the car sound like a squealing London bus!

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Yup, exactly. They're huge 4 pot calipers though and do look stunning - but you'd want them to for that price!

I hate the squeaky pads syndrome. I had that in my E46 M3 Cab and ended up taking the pads out. I did a fast road pad/uprated braided hose change on that. They were Pagid's too and the squeaked like hell. They were also a poor fit though so I'm not sure I'd choose Pagid's again.

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Your cat will certainly fail an MOT without CATs and the Milltek exhaust only removed the secondary cats.

Regarding parts and suspension - give us a shout before you do anything as we're Milltek, KW, Bilstein dealers. And AP Racing. And we do an intercooler for the x35i. :D

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