Making a B27 as or more fun than a B25

[krazy]

Hi all,

I recently picked up an 86 325e from my mechanic which had a full 885 head swap on it. My mechanic told me the following parts were swapped in from a 885/B25 engine:

• cylinder head
• intake manifold
• exhaust manifolds
• throttle body
• engine harness
• computer
• crank sensor
• injectors

Everything else on the vehicle is bone stock ETA (except the Bilstein Sports). Bottom B27 block has ~93k miles, all the 885/B25 pieces have ~140k miles.

I have a few questions:

• In relative terms, how does the performance of this particular setup compare to a stock M20B25 - including the whp? (Sorry for asking this for the millionth time here, but it seems most of the existing advice is geared towards less thorough head swaps i.e. without all manifolds or throttle body, etc.)
• Based on what's already done, what are the next few performance upgrades I should do? And what will they get me?

My goal is to turn this into a fun driver, that's loosely comparable to a M20B25. Also, if it isn't even worth it to do any of this and I should just swap in rest of the B25 bottom end... that's a valid answer, too.

(For what it's worth, I don't want to boost it. It'll be my daily and I want to keep it going for a little while.)

Thanks!

[digger]

IMO you are better off fixing the foundations of the engine to use the B25 piston with the 885 head in a manner that has good compression ratio and combustion characteristics. Alternatively the eta piston with a nicely ported 731 head works just as well, but in your part of the world that is likely not a realistic option as those heads are much rarer.

There are a few options on this with a 2.7 or 2.8L using OEM parts. Its more involved than the 885 head onto the ETA bottom end but the engine will be better and respond better to “bolt on” parts. It’s quite cheap to do if you are handy and can do most of it yourself.

With what you have now I would at most try and get a proper chip for this setup, the foundations aren’t there to warrant spending too much time and money IMO. i did once see someone get about 160whp out of a similar setup but it had some bolt ons etc, consider than a fresh b25 gets about 150-155whp

It’s a good upgrade for the ETA but it will only get you so far unless you add a turbo.

[krazy]

Thanks for the feedback, digger! Very valuable, as always.

So you're basically saying I should upgrade to the B25 pistons, get a better chip, and that's about the most I can do on this engine? Any idea where that lands me for whp?

I haven't looked into it (or thought it through) but is getting the bottom half of a B25 and putting my 885 top half on it generally worth the value/$ and time?

I'm very familiar with the driving experience of a stock B25 but still new to the feeling of a '27i'... Any thoughts on how they compare and what the experience is like with each? (I imagine the low-end torque for a '27i' would be especially fun as a daily driver, for example?)

[squidmaster]

It really depends how involved you want to get and how much money you want to spend. Yes, the 885 head + accompanying components will be a huge boon to your low-revving, 100-120whp ETA, but it's not going to be a Hellcat, or even an e36 M3. The low-end torque from the larger displacement B27 is great fun for daily driving, and the added RPM range and HP from the B25 top-end makes it a fun drivers car as well. But if you want to break 170whp you will need to go forced induction, or upgrade internals along the lines of: camshaft, pistons, valves, port and polish, etc. (something like the Bimmerheads Sport Head). You may find that the 160-170whp with minor mods is actually a blast to drive around and take to the track or auto-x, and leave it at that. Many people do. The e30 chassis and suspension is a bit outdated, so you may deem it more beneficial to upgrade suspension components instead.

[krazy]

Thanks squidmaster! (Planning to look into your chips, when the time comes for this project.) That's more or less exactly what I was getting at.

[digger]

Thanks for the feedback, digger! Very valuable, as always.

So you're basically saying I should upgrade to the B25 pistons, get a better chip, and that's about the most I can do on this engine? Any idea where that lands me for whp?

I haven't looked into it (or thought it through) but is getting the bottom half of a B25 and putting my 885 top half on it generally worth the value/$ and time?

I'm very familiar with the driving experience of a stock B25 but still new to the feeling of a '27i'... Any thoughts on how they compare and what the experience is like with each? (I imagine the low-end torque for a '27i' would be especially fun as a daily driver, for example?)

with what you have you cant just add B25 pistons the stack height wont work correctly due to pistons being shorter height and you still get low compression etc

- you would need to deck the block 2mm (nominally) to retain the same piston position with respect to the deck as the B25) so this means full engine out and dissemble. then an adjustable cam gear will allow you to set the cam timing correctly. gives about ~9.2:1 CR IIRC.
-or you can machine the b25 pistons and use a thicker gasket with 135mm rods but ive not seen anyone do this method. if i was doing a budget 2.7L i would look into this more as you can bump the compression quite alot if you can find someone to machine the pistons correctly (basically offsetting the dome but retaining same shape profile.

as an alternate you can maximize the CR of what you have now but the pistons still wont match the head so it will only help you so much and the valve recesses in the eta pistons are very shallow

[mr2peak]

Honestly, just get a decent chip tuned for your current setup, that should get you more power than a normal m20b25. You say you want it comparable to a b25, you're already there!

Look over the rest of the car and see what else needs attention.

[nrubenstein]

Honestly, just get a decent chip tuned for your current setup, that should get you more power than a normal m20b25. You say you want it comparable to a b25, you're already there!

Look over the rest of the car and see what else needs attention.

This. Tearing the motor down again to optimize isn't going to gain you that much.