a 2.7i question

[evandael]

this is a pretty common question but i haven't read anything about what i'm going to ask.

as it turns out, i have a 2.5i bottom end, 2.7e bottom end, and 2.5i head/etc laying around. i may just put the 2.5 back together and scrap the 2.7, but i was wondering if i could build a motor with these parts and nothing custom save for two parts, which leads to the question:


if i piece together an engine (206.7mm deck height) with an e crank (40.5mm throw), e rods (130mm length), and i pistons (34.2mm compression height), that will give me a combined height of 204.7mm. math happens, and that leaves the pistons 2mm under the deck.


why can't i just deck the block 2mm, and run an adjustable timing gear to put some advance back into the cam timing (shorter deck height = cam closer to crank = cam timing retarded relative to TDC)?

wouldn't that effectively create the same quench, static compression ratio, and ptv clearance as a stock i engine?



i'm not too worried about the rod ratio (which isn't that bad @ 1.6) as i spend most of my driving between 2-5k rpms. i occasionally run it to the (stock, 6500rpm) redline, but i like a torquey midrange.


comments and advice please. engines are still a gas-powered mystery to me.

[digger]

heard of a couple people doing it on this forum with much better results that the other budget 2.7 versions. people seem hesitant to take that much off the block for whatever reason.

[evandael]

cool. this picture (from the sticky at the top of the forum) shows a decent amount of meat between the top of the deck the coolant jacket.






eyeballing it, it looks to be almost 7mm. 2mm shouldn't compromise the block, especially in that area, doubly so on a low-comp NA motor.

[Wanganstyle]

Why not just reassemble the 2.5L and call it a day?

easier/ cheaper/ less chance of fuck ups and in the end of the day the gains possible are almost moot.

[evandael]

pros: work at a machine shop, have all the parts, would be an educational hands-on experience, cheap torque

cons: i could fuck it up and be out like $300 for another 885 head, or the motor could suck



not a big list of cons, all things considered.

[Wanganstyle]

pros: work at a machine shop, have all the parts, would be an educational hands-on experience, cheap torque

cons: i could fuck it up and be out like $300 for another 885 head, or the motor could suck



not a big list of cons, all things considered.

if money/time/resource consumption is a moot point to you then go for it?

I swapped into my e30 a 2700cc 10.5:1 hardy and beck built 327S engine and honestly in the end of the day the stock 2500cc M20b25 was pretty much 98% as good; a m20b25 replaced the 2.7i engine and i really did not notice it. (same car had ETA 2.7, 2.7i hardy beck 327S, 2.5i m20b25)


Been there done that spent my own money and wont do it again.

[digger]

i would go for it. should be alot better at the bottom end and midrange and atleast as good peak power as a b25. set the piston to head at 1.0mm. if you plan on adding a mild cam might be worth deepening the piston reliefs a bit.

refresh the head while you are at it.

[Wanganstyle]

i would go for it. should be alot better at the bottom end and midrange and atleast as good peak power as a b25. set the piston to head at 1.0mm. if you plan on adding a mild cam might be worth deepening the piston reliefs a bit.

refresh the head while you are at it.

of course you would :)

digger to engine as wangan to diff.

:)

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[redlightpete]

You're getting 8% more displacement and compression for the cost of the decking and the adjustable cam gear. This is a lot more elegant than the common eta piston, low compression configuration.

[JinormusJ]

digger to engine as wangan to diff

:up:

Aussies doing big things

[Wanganstyle]

You're getting 8% more displacement and compression for the cost of the decking and the adjustable cam gear. This is a lot more elegant than the common eta piston, low compression configuration.

I had high comp pistons in my 2.7 327S engine

in the end of the day unless you are willing to spend on a cam and head upgrade the 2.5 /2.7 is almost moot. I did not want to mess with an aftermarket cam; the E30 I race now uses a 24V engine :)

Around town is better in the 2.7 high comp vs the m20b25 -I will admit.

if willing to do full monty (cam, head parts or work if desired, high compression via block/pistons) then the 2.7 theoretically is better. The budget will be more than a 24V swap; Stock BMW everything is always easier.

[redlightpete]

:up:

Aussies doing big things

In North America, "do it yourself" means changing your own oil. In Australia it means making your own pistons. Or at least that's my impression from watching movies and reading this forum.

[JinormusJ]

a 2.7i question

In North America, "do it yourself" means changing your own oil. In Australia it means making your own pistons. Or at least that's my impression from watching movies and reading this forum.

Changing your own oil is too hard out here in Murica. DIY applies to things like getting off the couch to change channels or having to physically put in cook-time in the microwave instead of hitting "chicken", "meat", or "popcorn"

[Madhatter]

problem we have in aus is we dream something up, then get told by about 5 different "experts" that it won't work when you want to have the part made/machined/modified. So you often end up with builds happening out of spite just to prove people wrong :D

[digger]

A higher comp 2.7 with stock head and cam will have a fair bit more area under the curve than the b25 and the torque increase at low rpm will be higher than the 8% change in displacement suggests. You need a good tune to make it worthwhile though.

If you want power at the topend you need to start getting into topend mods. The easiest and economical being a head refresh and cam upgrade to a 27x which is good allround performer.