Difference between 2.5i & 2.5si engine

G

gonads

Member
Hi all,
A quick query, I was in the stealers today picking up a couple of bits and asked in passing about the difference between the two specs. The spares bloke had a look at the relative sets of part numbers and came to the conclusion that the difference was in the electronics/control side of things rather than physical differences in the induction/engine/exhaust etc. Does anyone know if is this correct?
 
Thats rubbish, they are completely different engines with the si engines (n52)cast in magnesium & running valvetronic valve lift technology as well as vanos (valve duration) which the m54 has-they are also 6 speed as opposed to 5.
regards
 
Not strictly true... As you can as a facelift (in the uk) get the n52 as a 2.5i and 2.5si.

From memory There was a thread a while back... Aside from mapping there were hardware changes too.
 
ok, which years of car are we talking about here? The earlier pre 06 cars used totally different engines to the later facelift cars.
regards
 
The N52 isn't cast in magnesium but it does use it. It was the lightest production straight six at the time, I don't know if that's been eclipsed since.

The N52 used an engine block which used magnesium and aluminium for the crankcase; magnesium is extremely light, but it is at greater risk of corrosion from water and it can creep under load at high temperatures. For this reason, pure magnesium or magnesium as a conventional alloy is not suited for dealing with the sorts of loads and forces that central engine components have to deal with. Because of this, BMW employed a magnesium alloy for the crankcase shell, while an aluminium inner block was used to counteract the disadvantages of the magnesium. The N52 used Alusil cylinder liners and featured Valvetronic and Dual VANOS.
 
aquazi said:
Not strictly true... As you can as a facelift (in the uk) get the n52 as a 2.5i and 2.5si.

From memory There was a thread a while back... Aside from mapping there were hardware changes too.
Click to expand...

This is correct, there re hardware differences, the manifold for one iirc
 
Hello,

I believe the difference between the two engines (providing you are referring to the N52 engined cars) is the map and the inlet manifold. The 2.5i has a single DISA unit on the outside of the manifold, whereas the 2.5si has two DISA units, one on the outside and one on the opposite side underneath the intake runners. I have an N52 engined 2.5i and have fitted a manifold from an si and had the car remapped. The wiring harness is identical as the connection for the second DISA unit is just blanked and clipped up. I am not sure on the exact power gain of mine as it had a VANOS issue when it was remapped which has since been fixed, but it does have a bit more shove.

Hope this is of some use.
 
Hi all,
Thanks for all the multiferous replies. The chap at the stealers was comparing cars of the same model year (post facelift) rather than pre and post. The reason for the original query was that I have a 2.5i (N52) and sometimes find it quite sluggish (especially when compared to a previous Honda S2000). I've been debating whether to try to upgrade the car in some way or swap it for a 3.0. I'm quite happy with the car, colour spec etc. but could do with a bit more urge, 170 odd bhp from a 2.5 six seems a bit mickey mouse these days. I think I'll have to investigate a new manifold and re map, anyone have any ideas of cost?
 
I only paid around £80 for the manifold off eBay. It came off a 325i with the N52 engine (I believe these always had the higher power output of approximately 218hp) and had the second DISA unit. I then transferred the other DISA unit and subsequent parts to the new manifold and fitted it myself with some new inlet gaskets (around £20 from BMW I think).

The remap was around the £380 mark I think but I cannot say exactly how successful it was as my Vanos solenoids were causing issues when it was on the rolling road.

Hope this helps.
 
The si also has the all important sport button :)

Personally if you want more power for less... Sell up and get a 3.0si... Or if you have the cash an M.
 
Thanks for all that, I think I'm arriving at the conclusion that a 3.0 or possibly an M is the way to go. I just have to get a house move sorted out first....
 
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