M20 Cylinder Heads. Is There A Difference?

[Beastly_Bavarian_Beauty]

Just a quick query guys. Is there a difference between the M20 2000cc cylinder head and the M20 2500cc cylinder head? Or is it just purely all bottom end displacement? Are there any different types/variations, etc?

Thanks in advance.

[Ant]

totally diff dude

M20 B20 uses flat topped pistons with c/chamber to suit 200/731 head casting

M20 B25 uses domed pistons with c/.chamber to suit 885 head casting only.

you cannot mix the two types BTW, there are more variations but thats the simple answer hth

[DelaneyG]

The B20 has got domed pistons, the B25 head has larger valves
Im going to put the B25 head on a B20 block jus need to find a doner

[Jhonno]

2.5 head wont work on a 2.0 bottom end

[DelaneyG]

it does wait an see

[Jhonno]

the 320i head has ports which are too large for the 320i as it is.. a 325i head isnt gonna be a worthwhile mod

[Simon13]

2.5 has bigger valves and much bigger ports too

[DelaneyG]

Why wont it be a worthwhile mod?

[Simon13]

because the pistons and head chamber shape are different.

for a start the 2,5 is designed to go with a piston with a bigger bore size than the 2.0 slugs

[DelaneyG]

and what have the ports go to do with it? the bigger the ports the better, more flow. do you mean the combustion chambers?? if yes 500cc isnt gona make mucn of a difference theres only 4mm difference between the bore on the B20 and B25

[DelaneyG]

Itl take out standard 2.5,s when its done and tuned ones depending on whats been done to em

[Simon13]

ok the 320 slugs will hit the valves in a 2.5 head when u build it. It just won't work without alot of modification and all for about 10bhp

just stick a 325i throttle body and inlet manifold on your 2.0 and be done with it!

[Simon13]

no pun intended just a funny phase! the verb to "chib"

[Turbo-Brown]

and what have the ports go to do with it? the bigger the ports the better, more flow.

Not strictly speaking true! Sound nice when you say it like that, but go too big (as I've done) and the air speed in the inlet tract drops which will cost you low end torque and probably increase your fuel consumption and emissions to boot

[Beastly_Bavarian_Beauty]

OK... here's one for you then. The inlet manifold... is there a difference between the M20 2.0 inlet manifold and the M20 2.5 inlet manifold? Is there anything in it or do the two heads (although different) share the same manifold? Or are they different? If so what... and to what degree? Very interesting.

[Turbo-Brown]

Yeah, pretty sure the inlet runners on the 2.5 manifold are a reasonable amount bigger than the 2.0 manifold.

They're cosmetically different too.

[Beastly_Bavarian_Beauty]

So... the ports (runners) on the 2.5 inlet are bigger than the 2.0 inlet? And how are they different cosmetically?

[Turbo-Brown]

I believe so, the 2.0 manifold is more angular to look at whereas the runners on the 2.5 manifold sweep more gently from the plenum to the head...if that makes sense

[Beastly_Bavarian_Beauty]

So... for instance... what would you say this one was? Know it's out of a 5-series... but is it a 2.0 or a 2.5 M20 variant?

http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... :IT&ih=014

[Cloggy Saint]

that's a 2.0

[Beastly_Bavarian_Beauty]

Has anyone ever heard of or tried putting a 2.0 inlet onto a 2.5 engine/head? What are the effects of doing so? I'm guessing that you would lose high-end power, but gain low-end torque. Or is it a no pointer?

[Cloggy Saint]

I'm guessing you mean 2.5 inlet on a 2.0 head. To make this work properly you'd need to port the head to match the 2.5 manifold. I think you'd also need the 2.5 afm and fpr. I think Argos may have done this recently?

[maxfield]

Dont forget port the throttle body

[Cloggy Saint]

If you're using the 2.5 inlet then there's no need!

[maxfield]

Why as i thought the 325 TB was bigger than 320 and wouldn't work properly

[Cloggy Saint]

It is bigger, but i thought he wanted to know the effect of putting a 2.5 manifold on a 2.0 head

[DelaneyG]

ok the 320 slugs will hit the valves in a 2.5 head when u build it. It just won't work without alot of modification and all for about 10bhp

just stick a 325i throttle body and inlet manifold on your 2.0 and be done with it!

they wont cos its practicly the same as the 2.0 head but you work out what the gap needs to be before you torque down the head

[DelaneyG]

an itl be more than a 10 bhp gain when its all done

[Beastly_Bavarian_Beauty]

I actually meant what would happen putting either the 2.0 inlet onto the 2.5 bottom end, head and throttle body. Or a 2.0 inlet and throttle body onto a 2.5 bottom end and head. Increase in torque (low-end), but decrease in power (high-end)?

[DelaneyG]

the problem with doing it the other way round is the 2.5 bottom end requires more air an faster flow for the larger capacity

[maxfield]

I'd guess you woudl lose power all through the rev range as there is no advantage of a smaller inlet.

Datourer you've confused me

[DelaneyG]

pretty much, the idea is to get as much cold air in the engine as possible

[Beastly_Bavarian_Beauty]

If a 2.0 inlet was ported and polished around the port openings at and the throttle body cut-out and used with a 2.5 throttle body would that just be like a 2.5 inlet? By porting and polishing an inlet don't yu just port and polish the port openings and as far up the ports as possible (as surely porting along the whole lebgth of the ports is surely impossible). What's the usual strategy when porting and polishing an inlet?

[Cloggy Saint]

I'd guess you woudl lose power all through the rev range as there is no advantage of a smaller inlet.

Datourer you've confused me

yeah sorry, i joined the thread half way through & got the wrong end of the stick