Unofficialbmw.com Forum Index Unofficialbmw.com
The UnofficialBMW BB, answers for your BMW questions.
 
 FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   RegisterRegister 
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 

Back pressure and valves

 
Post new topic   Reply to topic    Unofficialbmw.com Forum Index -> Racing

Support Unofficialbmw.com :: Download Free Healthy Habits iPhone app!

View previous topic :: View next topic  
Author Message
edjack



Joined: 04 Feb 2002
Posts: 3887
Location: San Jose, CA

PostPosted: Wed Sep 26, 2007 6:47 pm    Post subject: Back pressure and valves Reply with quote

On another forum, the subject of back pressure came up. I stated that, since the engine is an air pump, one needs to minimize or eliminate back pressure in order to free up power. This was in alignment with everything I've learned about cars.

I was corrected: the poster said that, at very least, one needs back pressure to protect the exhaust valve stems.

Anyone in the racing community ever hear this? And does it apply to street machines as well?

And, do the racers "build in" back pressure to make the engine run well?

And is there any quantitative evidence to back it up?
_________________


Ed BMW CCA member since 1987 (Nr. 62319).
'97 540i 6 speed Aspensilber over Aubergine interior.
Back to top
View user's profile Send private message Send e-mail

See all UnofficialBMW Ads
Chuck Baader



Joined: 21 Aug 2004
Posts: 95
Location: Birmingham, Alabama

PostPosted: Fri Sep 28, 2007 11:28 am    Post subject: Reply with quote

I think you have found an urban myth. If exhaust and intake ports are designed correctly (as in all BMW motors), any back pressure on the system inhibits the intake process. My race motor, M20B27 has full open exhaust and makes the most power that way. Hope this helps. Chuck
_________________
SCCA #36 E30, ITA
BMWCCA #36 E30, MP
Back to top
View user's profile Send private message Send e-mail
edjack



Joined: 04 Feb 2002
Posts: 3887
Location: San Jose, CA

PostPosted: Fri Sep 28, 2007 11:47 am    Post subject: Reply with quote

I agree. The myth is hard to kill.
_________________


Ed BMW CCA member since 1987 (Nr. 62319).
'97 540i 6 speed Aspensilber over Aubergine interior.
Back to top
View user's profile Send private message Send e-mail
idanity



Joined: 21 May 2005
Posts: 3096
Location: N.W.U.S.A.

PostPosted: Fri Feb 29, 2008 10:13 am    Post subject: Reply with quote

i was told (when i was 11 or at least before i ever drove) that there is enough back pressure just to have exhaust pipes. and the only time you will NOT have back pressure is if you ever removed your exhaust.


or like some hot-rodders, have tiny exhaust (like 9inches straight through the hood, or out the fender), and they have enough backpressure. but just barely i guess.

i remember now, when i saw the cars for the 1st time, i wondered why have a pipe at all, and someone explained it w/out any pipes, the side of the head may get burnt w/all the open atmosphere and it needs a surrounded bit of exhale on one side to keep the motion stable. *(airflow stable), and not to cool the valves too fast or at least keep them at a steady temp.


its been a long time, and i hope i was able to help w/something.
_________________
..........
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic    Unofficialbmw.com Forum Index -> Racing All times are GMT - 5 Hours
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum



Powered by phpBB © 2001, 2005 phpBB Group