731 vs 885 head swap

[AlexKing]

there seems to be no definite answer.

my understanding is that using a 885 head will lower CR. some say that it isn't noticeable and some say it is really bad.

i also understand that the 731 head keeps the CR the same ( or close to) as the stock head but doesn't breath as well as the 885 head.

so my question is; if i have an eta and do not plan on doing forced induction, which head will giv me better performance assuming i do the motronic 1.3 swap and all other i parts?

[SmokeE30]

deck an 885 head to give you a better compression ratio

[TurboJake]

Port matched 731 for N/A
885 for boost.

[e30leigh]

what do you want to do?

if you want to chase HP of any kind then you need the 885. no questions.

if you want to freshen up the eta and give it a less restrictive flow to gain a bit more power on the cheep and leave it at that. then id go 731. they do tend to be cheeper then 885 heads.

the 885 has larger ports, larger valves, larger combustion chamber. the ports and valves give you the added flow. the larger CC gives you a slightly higher cc rating.... slightly but it can also lower your compression ratio slightly. however the added volmetric efficiency is increased largely.
think of the 200 eta head as a hollow swivel stick youve been tryn to breath through. the 731 head is a regular sized straw, and the 885 head is a garden hose.

think the basic theory of HP.... more in and out faster = HP. then think of what you want to do.

[e30trooper]

i decked my 885 head and i get the same comp #'s as you would with a oem b25.

My block is a USDM eta 2.7 with the dished pistons. Comp #'s were 155psi. :D
head was decked 0.10/inch.

[MR 325]

The 731 cylinder head isn't as special as most think. I'd much rather have a nice 885 head on top of the Eta bottom end. As said above by e30leigh the 885 head has larger valves and ports. Contrary to popular belief the 885's stock cam is also better than the 731's cam.

e30trooper, we're talking compression ratio not compression testing.

[KillaCam]

search, there is a lot of info out there but once you read most of it you can make the appropriate decision

[e30trooper]

The 731 cylinder head isn't as special as most think. I'd much rather have a nice 885 head on top of the Eta bottom end. As said above by e30leigh the 885 head has larger valves and ports. Contrary to popular belief the 885's stock cam is also better than the 731's cam.

e30trooper, we're talking compression ratio not compression testing.

I was making and example. Even with the loss of compression , after decking my engine still made avg #s of a stock b25. Thats mean with the swap it didnt lose much if any.

[AlexKing]

Thanks guys. i have searched but i couldn't find a definite answer and there are so many recommendations for both the heads.

anyone have any idea about how much decking the 885 would be?

[XW70]

731 keeps your C/R, and will breathe fine for a fairly stock engine other wise.
i also read later 731's have larger valves?

Im doing the 731 swap cause its what fell into my hands but for a built motor with all mods to take advantage theres no doubt the 885 is the way to go.

[sayed ali]

885 all the way

(Optional) Decking the 885 head to get better compression

[5Toes]

Yeah but it is a fine line on decking the 885 because you will smash valves... right??

Atleast on a b25 you will

[pantelones]

Contrary to popular belief the 885's stock cam is also better than the 731's cam.

http://www.e30tech.com/forum/showthread.php?t=108802

I have yet to read this site, but I will when I have a chance....

http://www.tildentechnologies.com/Cams/index.html

I personally believe that the 200 head is fine to work with. I don't like the idea of mismatching different piston and combustion chamber designs. And I also don't like the port design of the 885. The 200 and 731 are both designed to work with flat top pistons, and they allow for more room to shape the port with out adding material; you also can always put bigger valves in, but you can't put smaller ones in. I am also someone who is capable of doing this myself (I have access to machines) and I have the will to take the time necessary to do so.

You can deck the 885 and use an adjustable timing gear. I believe Pete Mchenry used to do this.

What ever floats your boat though. :up:

[e30leigh]

731 keeps your C/R, and will breathe fine for a fairly stock engine other wise.
i also read later 731's have larger valves?

Im doing the 731 swap cause its what fell into my hands but for a built motor with all mods to take advantage theres no doubt the 885 is the way to go.

who ever told you 731s have larger valves are retarded. i had an 885 and 731 on the bench. both genuine bmw heads. the 885 has larger valves. i think they are 2 or 3mm larger on 885's

as for the cam profile between the two.... added duration is lost as it doesnt have the lift with the 731 head. and the opposite with the 885 cam has good lift but not as much duration. so they would be closely matched. but id say the 885 cam would be better due to the larger valves open wider would let in more air then the loss of 10 degress of duration. but in a perfect would more duration on that 885 would be better with the same valve lift.

[Gary Horneck]

731 works great for me on my e motor.

http://www.r3vlimited.com/board/showthread.php?t=251796