Underpowered N52 2.5i - upgraded to 2.5Si

[Zedebee]

There are many warnings on this forum to avoid the facelift 2.5i as being underpowered, so not being a speed freak, I bought one.

First impressions are that it is the smoothest and quietest Z4 that I have owned and, if I hadn’t been spoilt by previous M54 3.0i and N52 3.0Si variants, it is plenty powerful enough for this type of car.

I suspect most of the “missing power” is at the top end, which is not where my automatic spend most of its time. The 2.5i delivers 177bhp at 5,800rpm compared to the 2.5Si’s 218bhp at 6,500rpm.

So what’s the difference? It seems to be all about the intake manifold and ECU tuning. The 2.5i has only one DISA valve (not the same as the US spec 3.0i, which doesn’t have any), whereas the 2.5Si has two. The intake manifolds themselves have different part numbers (as do the 3.0i and 3.0Si), but all the associated hardware, including the gaskets, is common across all four engine designations.

A common modification in the US is to replace the 3.0i manifold with the 3.0Si manifold for a bolt-on power increase, with further increases available by tuning the ECU.

It should be possible then, to release power in the 2.5i by simply swapping out the manifold for one with 2 x DISAs from the 2.5Si or 3.0Si. Online sources suggest 15-20bhp, with an ECU tune required to release full 2.5Si power.

Unsurprisingly, I’m on the look out for a manifold with part number 755 9523 or 755 9525 (common across many N52 applications) if anybody has one and is interested in whether this power gain is a reality.

What about the N52B25 fitted to the E89 23i? This produces 204bhp at 6,400bhp. It has exactly the same manifold and DISA configuration as the 2.5Si, but is down on power by about 15bhp. I can only assume this is to do with the ECU tune, unless anyone knows any different. Anyone out there flashed theirs with a 2.5Si tune?

Constructive thoughts and advice welcomed.

 

[Mr Tidy]

It's an interesting topic so I hope you find the right manifold.

I noticed the E89 30i is also a bit down on power compared to the 3.0Si - 254 v 265bhp. But I seem to remember reading E89s can be run on 95 RON fuel whereas a minimum of 98 RON is recommended whenever possible for the 3.0Si, so maybe the mapping is different?

 

[Street]

I have a 2.5si with a poorly engine. Looking very likely that I'll break it for parts if you're looking for an intake manifold
https://www.facebook.com/marketplace/item/1382033143027974

 

[GenZed]

I have a 2.5si with a poorly engine. Looking very likely that I'll break it for parts if you're looking for an intake manifold
https://www.facebook.com/marketplace/item/1382033143027974

I have a good engine 115k
but you where i am

 

[Zedebee]

So, having acquired a 2.5Si manifold, I can confirm that the only difference is the 2.5i is missing the second DISA and has a blank where the aperture for it should be. Therefore, it should be a straight swap.

Like the first DISA, the second DISA is electrically operated and should be connected to the DME. Happily, the loom appears to be present on the 2.5i and is clipped to the underside of the manifold just next to where the second DISA should be, but the plug connector is blank where it should be a 3-pin female same as the first DISA.

I need to get hold of a connector, then swap the manifolds. A number of US threads suggest this should make a difference straight away, although I don’t know why it should. Both DISAs are “open” by default, so having the second DISA in open position should be no different to the 2.5i, unless and until the DME is programmed to control it.

My understanding is that the second DISA is closed at high revs to shorten the intake path, giving higher torque and therefore more power.

Given no other differences, it should therefore be possible to flash the 2.5Si map onto the DME to release full Si power.

Any insights from the forum are entirely welcome (including who can flash the 2.5Si map for me).

 

[Zedebee]

Result! The blank plug connector for the second DISA is in fact just a blanking plug over the actual plug. That means I can swap the manifolds over right away

 

[B21]

Maybe a bijouette supercharger aka Patriot66?

 

[RMB]

Watching with interest. I did do some research on this before but felt it perhaps too expensive for me as the manifold and new DISA valves plus a map required. Keep us posted.
This could be a service you provide and I can pop back up again!

Edit, mine is faster than I expected and revs well and sounds great for half the price of a 3.0si. I can enjoy reving it without too much risk of hitting big speeds or getting in trouble with the law quickly, unlike my M240i. That said a little more power would be welcome.

 

[enuff_zed]

So, having acquired a 2.5Si manifold, I can confirm that the only difference is the 2.5i is missing the second DISA and has a blank where the aperture for it should be. Therefore, it should be a straight swap.

Like the first DISA, the second DISA is electrically operated and should be connected to the DME. Happily, the loom appears to be present on the 2.5i and is clipped to the underside of the manifold just next to where the second DISA should be, but the plug connector is blank where it should be a 3-pin female same as the first DISA.

I need to get hold of a connector, then swap the manifolds. A number of US threads suggest this should make a difference straight away, although I don’t know why it should. Both DISAs are “open” by default, so having the second DISA in open position should be no different to the 2.5i, unless and until the DME is programmed to control it.

My understanding is that the second DISA is closed at high revs to shorten the intake path, giving higher torque and therefore more power.

Given no other differences, it should therefore be possible to flash the 2.5Si map onto the DME to release full Si power.

Any insights from the forum are entirely welcome (including who can flash the 2.5Si map for me).

Can you not get the DME from Street and fit that with the EWS coded out?

 

[raymond.harper]

Old thread here does mention the SI has higher compression. https://z4-forum.com/forum/viewtopic.php?t=135135

 

[Zedebee]

Can you not get the DME from Street and fit that with the EWS coded out?

I’ll enquire. [ref]Street[/ref]?

 

[Zedebee]

Old thread here does mention the SI has higher compression. https://z4-forum.com/forum/viewtopic.php?t=135135

Not true. If you study RealOEM closely, there is a low compression version of the 2.5i that has the same manifold as the 2.5Si and the second DISA, presumably to keep it on power parity with the regular 2.5i. Every other source I have found says the regular 2.5i and 2.5Si share the same internals and compression.

 

[Zedebee]

The old manifold is off, but it’s a pig of a job. There are a lot of connectors to undo. The electrical ones are no problem, but the pipes are a horror. I broke the top crank case breather, but fortunately I have another one which came with the new manifold. Then I thought I had everything off, but there was still one more pipe. How I wish I had [ref]enuff_zed[/ref]’s legendary tiny hands. It took me, my daughter (small hands) and her boyfriend to get it off.

 

[enuff_zed]

May sound like unnecessary expense, but what state is the starter motor in while you have easy access to it?

 

[Zedebee]

May sound like unnecessary expense, but what state is the starter motor in while you have easy access to it?

Good call. It’s super easy to get at now, but has been working fine. I’ll check it out in the morning. The only problem is I really want to get the car back together and not wait another week.

 

[Zedebee]

Watching with interest. I did do some research on this before but felt it perhaps too expensive for me as the manifold and new DISA valves plus a map required. Keep us posted.
This could be a service you provide and I can pop back up again!

Edit, mine is faster than I expected and revs well and sounds great for half the price of a 3.0si. I can enjoy reving it without too much risk of hitting big speeds or getting in trouble with the law quickly, unlike my M240i. That said a little more power would be welcome.

I’m with you. I went for drive round the twisties today and found I had all the power I needed. Fitting a sound generator has made it sound much more like a sports car, and automatics have a “sport mode” which is activated by pushing the lever to the left (same as the sport button would do).

I picked up a 2.5Si manifold with all the trimmings including both DISAs for £60, so it’s a low cost exercise so far. It was advertised as off an E89, but it had E46 painted on it by the breaker. Don’t waste your time fishing for a E85/6 3.0Si manifold, they’re exactly the same (at least, I haven't identified any differences) but cost around £250.

 

[pvr]

Interesting read

 

[RMB]

[ref]Zedebee[/ref], did you find by the part number? 11617559525. I have seen one on eBay for £40 ish but without the DISA valves. https://ebay.us/m/SdWeQs
I’m not sure if it would have to be that one specifically or if a 3.0si one would work.
Did you buy streets one he mentioned?

[ref]Street[/ref], did you sell your intake and disa valves?

 

[Zedebee]

[ref]Zedebee[/ref], did you find by the part number? 11617559525. I have seen one on eBay for £40 ish but without the DISA valves. https://ebay.us/m/SdWeQs
I’m not sure if it would have to be that one specifically or if a 3.0si one would work.
Did you buy streets one he mentioned?

I didn’t buy Street’s because I found one elsewhere and he wasn’t quite breaking the car yet.

755 9525 (2.5Si) definitely fits. 755 9523 (3.0Si) should fit, but I haven’t tried it. I don’t know why you’d pay extra for the 3.0Si manifold though.

The only thing you’ll need apart from the manifold is the second DISA, everything else (including the first DISA) you can reuse. No extra pipes, no extra connectors required.

Try to get a manifold with DISAs if you can, because they seem to be quite expensive on their own.

 

[RMB]

[ref]Zedebee[/ref], thanks, well hopefully I may be able to pick up Streets then.
Cheers I hadn’t realised the existing disa …114 can be reused and I would only need disa 11617560538.
Just seen that on realoem.
And sorry I just re read the thread and saw you sourced yours from a breaker not street.
Looking forward to hearing about the drive.