BPU exhaust 2.5 or 3?

Probably a silly question but as most people recommend a 2.5 inch exhaust for JDMs when going BPU because it has to be restricted anywaycan you then open out to a 3 inch catback? Is there a converter or whatever as there are very few 2.5 catback systems available for a Supe. Also does a catback exhaust remove one of the cats, or is it literally from the cats back so two separate decat ipes would be needed?

Many questions

I wouldn't recommend a 2.5" exhaust although I have a 2.5" second de-cat for a few years.

A cat back is from the back of the second cat, therefore two de-cats are required. As an interim you can have a second decat and cat back exhaust and get little more performance while you sort, first decat and fuel cut defender and boost controller and Restrictor ring

Basically the more free flowing (and like for like a 3" will be more) exhaust you fit the more your boost will rise on a J spec. If you have removed the stock fuel cut by fitting a fuel cut defender (FCD) then you can reach dangerously high boost levels.

Cat back are indeed as they sound from the cat back so x2 decats usually fitted.

If you fit say x2 decats and a freeflowing 3" exhaust you will then need a restrictor ring, normally positioned inbetween the rear most cat and cat back exhaust. (if you had a 2.5" exhaust and left the second cat you might hover around 1 bar but so many variables you have to sort of suck it and see)

If you upgrade the exhaust without a FCD then the stock setup will save you from over boosting (1 bar limit) as people want to run over 1 bar generally for bpu then you need to remove the restriction but then you have to be mindful of over boosting which can be catastrophic for the engine/turbo's. It hasn't happened for a while on here because the steps are so well documented but it only takes one hard pull with the wrong setup to cause serious problems.

Have you got a boost gauge installed yet?

I'm just going to buy the full list of stuff together as a one-er and get it all fitted, I think this is the best way forward as all the pieces complement each other and I dont want Supe off the road any longer than necessary. Do you have a JDM then? It seems to be accepted that there's no power to be gained from 2.5 to 3 inch exhaust on JDMs as ours need restrictor rings but a full 3 inch system is what I initially wanted and the price difference isnt too bad. And finally surely the standard exhaust system is smaller by definition so do I need an adaptor to fit a 3inch onto the manifold of the car which I assume is the only original part of the exhaust staying with BPU? I'm guessing the manifold comes out at 2.5?

Yes I have a boost gauge already fitted but it seems that a very easy plug and play BPU on a JDM is to go 3 inch all through and 1 bar restrictor then? So do I need an adaptor to fit the 3 inch to the manifold? I've seen BPU kits available but if you just buy the parts separate its about half the price so I am just shopping around all at once. Assuming the above is the case and 3 inch with RR gets between 1 and 1.2 boost I take it you then use the Boost controller (which can only increase boost) to sneak it up to as near 1.2 as possible safelyworking with the restricotr ring? If not what is the Boost controllers role as if the RR puts a ceiling on the boost how can it possibly help?

I have HKS Hi power silent , 3.5 inch and restrictor ring , before that the car had 4 inch Sard unit , how it was engineered it had the flow of a 4 inch but used one 2.8 into two 2.5/ inch pipes back into 120mm tip

Yes I have a boost gauge already fitted but it seems that a very easy plug and play BPU on a JDM is to go 3 inch all through and 1 bar restrictor then? So do I need an adaptor to fit the 3 inch to the manifold? I've seen BPU kits available but if you just buy the parts separate its about half the price so I am just shopping around all at once. Assuming the above is the case and 3 inch with RR gets between 1 and 1.2 boost I take it you then use the Boost controller (which can only increase boost) to sneak it up to as near 1.2 as possible safelyworking with the restricotr ring? If not what is the Boost controllers role as if the RR puts a ceiling on the boost how can it possibly help?

Ok if you buy a 2.5 or 3" catback it will bolt up to a stock cat no problem, just doing this will probably raise the boost on a JDM car.

You 'could' then fit a boost controller and up the boost (note a boost controller can only increase boost not cap it) I say could as you don't want to do this with cat's in as the increase in boost but with natural restrictions still in place (the cats) is potentially very bad due to high exhaust gas temperatures (extreme cases you can melt stuff!)

So the done thing is to decat (some only do the one closest to the engine but both is most popular) to remove the stock restriction and then fit back in a restrictor ring to limit boost to the level you want.

I fitted a ring, noted the boost, ground some out refitted etc until I got 1.15 bar (needs a good hard pull to test - I was lucky in that I got the car with a boost controller fitted but it's just used as a boost gauge now, but has a peak readout and overboost warning which certainly is handy - but new they are a lot of money just to have those features!?)

Some might fit the ring get say 1.1bar then use a boost controller to have a 1.2 or even 1.3 setting, I personally don't see the point of variable settings and am happy with a consistent boost (in the lower bpu band).

My thinking is the flow is the most I can have for the boost setting I have, ie those increasing the boost on a 1.1 restricted setup will be putting things under slightly more pressure/temperature than if they had the natural restrictions removed to give the 1.2 or 1.3 (but if you did remove those natural restrictions there is no way back as to reiterate the boost controller can't cap boost, so if the lack of restriction allows for 1.3 bar that's what you'll get regardless of boost controller settings).

So personally on a JDM car I would hold off the boost controller initially and see if you ultimately want a variety of settings instead of a fixed slightly conservative level. Start slowly and build up, 1.0bar will feel a good chunk better.

So you could just research a nice exhaust you like the sound/tone of, fit that and see what the boost does, then FCD, decats, fuel pump, plugs etc and restrictor ring

Even with 2.5 inch bore de-cat pipes and free flowing exhaust of similar bore you are very likely goig to need a restrictor ring to control boost as the stock wastegate can't flow enough diverted gasses to control boost safely, as it was designed for a certain back pressure. So going bigger than 2.5 inches is really cosmetic / nice warm feeling talking at the bar sort of thing. Only when you see the 2.5 causing unacceptable back pressure (which can be measured with a boost (or any suitable pressure) gauge plumbed anywhere into the exhaust just after the turbos, so easy to know rather than wonder), do you need huge exhaust diameters.

So Chris you are saying there is no real benefit to 3 inch pipes on a JDm and I'm going to end up with the same 400hp ish mark with either?

yes that's what he's saying, so base exhaust on look (tail pipe exit straight or to the side) ground clearance and sound.

So Chris you are saying there is no real benefit to 3 inch pipes on a JDm and I'm going to end up with the same 400hp ish mark with either?

yep, just measure the back pressure, a nipple to take a bit of copper brake pipe welded on the first de-cat pipe near the turbo end, coiled up for three turns about three inches in diameter to disperse the heat, then hose to a pressure gauge in the car (through the window if you like, for a temporary test). If you get a centre zero vac / pressure gauge you can then measure air filter restriction too, and conclude the stock air box and paper element is superb... Anything else is guesswork.

I ran 3” decats, 4” cat back on my bpu i ran a 63mm restrictor ring 1.35 bar 431.3bhp, my previous setup was same decats, 3” cat back, 1bar boost, made 397.1bhp, the bigger exhaust defo helped with the extra boost/heat, turbos are still as they were 3yrs ago before i made the Resrictor ring larger

Bigger exhaust the better imo

I already have a fully installed boost gauge in the car though will that not be the way forward to tweaking?

Yes to a point, what Chris is talking about (I think!) is if you wanted to prove for yourself that it makes no odds you can do that in the way he describes, I suspect only a handful of people have such a thing, 99% of bpu's won't!

Your only issue with a boost gauge plumbed in is you need to be careful going in with too high a boost as you can't really rely on the gauge to show instant boost nor you to back off earlier enough, but that said most restrictor rings (I got mine from Chris) err on the side of caution ie smaller and will need likely need some opening out to get back to the boost level you want.

Yes I see and I totally agree, however for the money I reckon stick a 3 inch on in case I want to go even further in the future as for the money theres not much in it, I do totally accept Chris and your help though it all makes very good common sense. I was going to get a 1 bar ring from Whiffbitz but if Chris has a better option I'm happy to buy from you Chris? This is not all theoretical btw, I will be buying the whole kit in the next 2-3 weeks including a FMIC (I know you dont strictly need one but probably a good idea) and a Greddy boost controller just to make sure.

As a final question in regards to the boost controller, lets say I put the standard 1 bar ring in, can I actually adjust boost with that ring in with the greddy to 1.2 without all the faff of disassembling the exhaust and altering it? I mean surely this is the function of them and I can then safely control it from within the car permanently at 1.2 or whatever? I know they are dear but I was always going to get one thinking this is their purpose in BPU?

Yes , you want to mechanically restrict the car to a safe level i.e. 1 bar and increase with a boost controller

The tech section has very good in dept explanation of BPU

http://www.mkivsupra.net/vbb/showthread.php?94705-BPU-(Basic-Performance-Upgrade)

Even with 2.5 inch bore de-cat pipes and free flowing exhaust of similar bore you are very likely goig to need a restrictor ring to control boost as the stock wastegate can't flow enough diverted gasses to control boost safely, as it was designed for a certain back pressure. So going bigger than 2.5 inches is really cosmetic / nice warm feeling talking at the bar sort of thing. Only when you see the 2.5 causing unacceptable back pressure (which can be measured with a boost (or any suitable pressure) gauge plumbed anywhere into the exhaust just after the turbos, so easy to know rather than wonder), do you need huge exhaust diameters.

Sorry Chris & OP , off topic

Is it worth corner balancing a road car? KW suspension Variant 3 , OE arms

Do you need adjustable drop links to do it?