2.7i Pros and Cons

[stormtrooper15]

I know the at least the rod bearings are bad because of severe knocking and shavings in the previous oil and oil filter. Adjusted values and knocking persisted. Used stethoscope on top and bottom of engine and sound is worse near the bottom around the oil pan.

[ak-]

it's only 0.2l of displacement - it doesn't really even make that much difference.

if you already have an eta, fine, slapping a B25 head on makes some sense. Pros: it's cheap.

Cons: If you started with a B25, putting in the B27 block is hardly worth the effort. I collected a ton of 2.7i dyno graphs and most of them made the same or less power as a stock B25. They peaked torque a little earlier but really didn't make much more torque (often less) either. The displacement gains are mostly lost in the fact that the pistons don't match the shape of the 885 combustion chamber and you lose almost a full point in compression ratio.

The lower CR for boost doesn't even really make any difference either because the B25 already has a fairly low CR and it works great for boost out of the box. it's going to make what, 10ft/lbs more at 15psi? what's an "assload" of torque? blunt? :p

I don't know Nando, when I went 2.7i (seta internal), from a 2.5i I made roughly 50wtq more on the same PSI (12psi) on the same tune for the first dyno pass before the operator raised the boost controller for some WOT tuning.
Now the car is making 53wtq more than whp at 15psi and the only things changed during the rebuild was an equal length manifold (not sure if it matters much on a 6cyl) and a bigger downpipe (from a 3" to a 3.5") because blunt said my pipe was too small for his liking.

[digger]

2.7/2.5 is an 8% increase, at a given low rpm you gain a higher % than this though.

[EatsHondas]

You do realize that motor in that video was probably 700+hp.....

The car was then wrecked I believe but the heart was saved and it turned into this.......

The Blue Monster

Little fun fact.

Hahaha not at the time no, I didn't!

But the fact that you can squeeze that power from that teeny displacement is incredible.

Gotta hand it to the Swedes.

[nando]

2.7/2.5 is an 8% increase, at a given low rpm you gain a higher % than this though.

I don't think so, it's not a straight displacement increase = more power, and you know that.

the 2.7i doesn't have the same VE as a 2.5. the fact is they both make basically the same peak torque, only the 2.7i does it at a lower RPM. So it has larger displacement, same torque, which means it has to have lower VE. An engine with lower VE N/A will also have lower VE with boost.

It's not going to make a lick of difference in the end. in fact, I'd put my money on the boosted B25.

[jalopi]

blunt said my pipe was too small for his liking.

heh. hehehehe

[IronFreak]

I don't think so, it's not a straight displacement increase = more power, and you know that.

the 2.7i doesn't have the same VE as a 2.5. the fact is they both make basically the same peak torque, only the 2.7i does it at a lower RPM. So it has larger displacement, same torque, which means it has to have lower VE. An engine with lower VE N/A will also have lower VE with boost.

It's not going to make a lick of difference in the end. in fact, I'd put my money on the boosted B25.

not all 2.7 were created equal. We don't know if OP has a E or ETA short block.

[nando]

True - but even the seta engines with 885 heads don't really end up with that much more power than a B25.

[stormtrooper15]

eta short block with seta pistons

[IronFreak]

True - but even the seta engines with 885 heads don't really end up with that much more power than a B25.

I'd agree with that. But from the position the OP is in, I think swapping in the 2.7 if its in good shape is a better route than rebuilding the 2.5.

[UgkNW*]

I was thinking about getting a 885 head for my 86 325e but after reading this....

It's pretty much pointless because the difference you would see you loose to the fact of compression.

[Regnar75]

^ you missed the point man, people here are discussing whether to go 2.7i or rebuild his cammed m20b25, I think it would be difficult to find anyone claiming the 2.7i build is not an improvement over an ETA, and even more difficult to find someone who has done the conversion and wants to go back.

just read the 2.7i thread, its stickied in the m20 section


unless i missed you checking the sarcasm button?

[digger]

You can't get any worse than a stock eta.......A 2.7i should make about the same power as a b25 if it's not high mileage and you fit the same induction as a b25 with a custom tune. Many m20 builds even big $$$ ones never get a custom tune and that's where many seem to lack. If you want more power you need breathing mods but the low compression means you need to limit how much duration you throw at it.

Also a small amount of compression doesn't make much difference at all, it's not like going from 8.8 to 11:1,people seem to get hung up on it

[crappycoco]

I'm in the same boat as OP. Bearings are shot - I also get the dreaded knocking. Took it to GAS in Berkeley, they told me the bearings are on their way out, but head is good.

So if I understand correctly:

- an ETA engine with an i head is bad, because the compression drops, due to inadequate pistons?

- a SETA engine with an i head is good, no compression drops or other problems? Do the 15lbs injectors keep up with the increase in displacement? Need to update the ecu map?

From what I've read up until now, the bottom end swap can cost you from 2-3k, but most of those estimates are based on the fact you're upgrading a SETA engine with an i head. What about our case? If you already have an b25 engine with everything, but just need to swap out the bottom end?


Edit: nvm, found this: http://www.e30tech.com/forum/showthread.php?t=48288

[nando]

I would just rebuild the 2.5 bottom end unless you happen to have a seta laying around. the compression is a little lower but not significantly. as far as the extra displacement, it's only .2 liters. the stock injectors would work fine.