731 question: b20 cam vs. b23 cam

[E30 Wagen]

I have a 731 head from a '84 320i which I had inspected and rebuilt, still with original lifters and cam which the machinist said were in great condition. I'm getting ready to put it on my rebuilt eta block (still at the machine shop), but now I'm realizing I made the stupid mistake of never researching cam differences between the 320i and 323i. Turns out they have different part numbers, respectively 11311289228 (superceded by 11319058572) and 11311278866 (superceded by 11319058577).

Anybody know if there is a major difference between the two? Should I reconsider using the 320i cam and getting a 323i or even using a 325i? I was thinking about sticking with the motronic 1.0 and getting a chip from mark d, but now I think I should just go straight to 1.3 since I have all the parts. I'd appreciate any advice, thanks!

[Hooffenstein HD]

I know the late model (motronic 1.3 era) B20 cam is pretty much the same as a late model 325i "K" cam with 6 degrees more advance. I'm not too sure if the early one has been measured.

If a stock cam is the only option, I'd probably be looking for a B25 K cam. I'd say to go find the B20 L cam but I know they were never sold in the USA, so wouldn't be worth the hassle.

http://www.bmwforums.com.au/showthread.php/39849-M20-Cam-Profiles-revisited

[E30 Wagen]

^ Any idea what the difference between the early and later cams in the 320i is? Build date for the car my head is from is 5/84.

[Hooffenstein HD]

^ Any idea what the difference between the early and later cams in the 320i is? Build date for the car my head is from is 5/84.

Not too sure. I would think it was when Motronic was introduced in late 1987 for the series 2 E30's but can't be too sure. What date is the ETK listing the cam supersession?

[E30 Wagen]

http://www.realoem.com/bmw/enUS/showparts?id=1131-EUR-05-1984-E30-BMW-320i&diagId=11_4366

So it looks like they used 11319058572 until 9/87 then switched to the later cam (11311716139), which I'm guessing is the B20 L cam.

I'm not sure when 11319058572 superceded 11311289228. It looks like 8/85?

http://www.realoem.com/bmw/enUS/part?id=1131-EUR-05-1984-E30-BMW-320i&mg=11&sg=25&diagId=11_4366&q=11311289228

So I've likely got the early version of the B20 cam. Man, I really hope it's worth just leaving it in because I don't feel like taking it all apart again. And now I'm kind of confused about what the best fuel injection system to use is.

[Hooffenstein HD]

So I've likely got the early version of the B20 cam. Man, I really hope it's worth just leaving it in because I don't feel like taking it all apart again. And now I'm kind of confused about what the best fuel injection system to use is.

Looks like the supersession for the B20 cam coincides with the Motronic/Series 2 E30.

I imagine the B23 cam switched at the same time they changed ECU and ignition module on the 323i. They switched to a #308 ECU and a siemens module in 1985.

I would just throw it in and see how it goes. I doubt the difference is really all that much, might not even be noticeable on the butt dyno. I know my 2.7i falls on its face hard at 5,5000rpm and that's with the Motronic B20 cam/731.

[E30 Wagen]

I know my 2.7i falls on its face hard at 5,5000rpm and that's with the Motronic B20 cam/731.

Hm, that's not too encouraging. Almost thinking I'd rather have a standard 2.5 but I feel like I've spent too much now to not do anything with this 731.

I wish it wasn't so impossible to find more information on this.

[raudonis]

Difference between factory m20 cams is miniscule. There is a small table with part numbers and lobe specs at some german e30 forum, just google "e30 m20 nockenwelle". What is your compression? If low, your best option is schrick 272.

[raudonis]

Here it is:

Also felow forum member Digger did some measurements and found pretty much the same

[E30 Wagen]

^ cool, thanks!

[Hooffenstein HD]

Hm, that's not too encouraging. Almost thinking I'd rather have a standard 2.5 but I feel like I've spent too much now to not do anything with this 731.

My B27i in an 1125kg car(weighed on a scrap yard's scales with an overfull fuel tank) will still stomp a stock 325i to 100km/h by a couple of car lengths and the mid range is absolutely incredible. I'd take the 2.7 every time.

I should mention that I'm running a 731 intake manifold, with the TB hole bored out for a B25 TB. I've recently picked up an 885 manifold which will fit right on, seems like the 885 manifold is port matched to a 731 head from factory and actually too small for an 885 head. I've got the valves out of a spare 731 head, bolted the manifold on and ran my fingers all over the port entry and can't feel any ridge whatsoever. That should hopefully pick up the top end.

If I did things again, I would have had the machine shop install 42mm intake valves and clean up the area behind the valve guide because it really needs some improvement on the 731.

Raudonis, thanks for posting that cam specs graph. Saved.

[E30 Wagen]

So I've made up my mind then that the smartest thing to do is to use a b25 cam, intake, and motronic 1.3. I'm not sure that enlarging the valves is worth the time and expense but I will look into having that done also. I only want to build this engine once

[digger]

any of the "i" cams will be much of a muchness. probably deciding factor will be finding one that isnt scored

the 731 will hardly make more power than a b25 if at all unless you get the head ported.

[E30 Wagen]

so, you're saying a b25 cam is kind of overkill...?

[digger]

so, you're saying a b25 cam is kind of overkill...?

overkill is not the right word, one way or another you probably wont notice the difference over a 320i cam is what i meant.

you want a ported 731 with 27x cam to make hp improvements