924Board.org

Guest · Posted: Mon Jun 11, 2001 12:26 am Post subject:

From: larso
Email: l_porsche@hotmail.com
Date: 11 Jun 2001
Time: 00:26:42

I am pondering, why did porsche make the 931 head "flow" real good, when intake flow is not important on a turbo car cuz the boost doesn't build up as fast with good intake flow, its important on NA cars of course, so what the hell is up with this "great flowing" head that porsche made for the 931, why didn't they just make deeper pistons and make the head flow about the same as the 924.
The 931 head on the 924 and the 924 head on the 931 makes more sense to me, with accompanying pistons of course.
Is the 931 head design just somethign stupid that was a waste of time and a marketing tactic? who needs flow in a turbo car on the intake side? we need it on NA cars not turbo cars, so whats up?

Guest · Posted: Mon Jun 11, 2001 1:16 am Post subject:

From: sp
Email: s2003r@yahoo.com
Date: 11 Jun 2001
Time: 01:16:37

i'll try to explain what i know about flow. first off, turbos, unlike superchargers, make boost until they either quit working or the wastegate saves them from death. so as long as you have the exhaust flow to push the turbine, it'll boost until it reaches the maximum that the wastegate or whatever controls. so if the turbo can force a lot of air through low flow heads, it can force even more air through high flow heads. and even if the preasure in the head is reduced, it is not the preasure that makes power, the goal is to get a greater volume of air into the cylinder. trust me on this, i'm a former automotive engineering major, and it's not former because i didn't know what i was doing.

Guest · Posted: Mon Jun 11, 2001 9:18 am Post subject:

From: Jon Furst
Email: jfurst81@hotmail.com
Date: 11 Jun 2001
Time: 09:18:15

In part, it was redesigned to move the spark plugs over to the intake side, as the exhaust manifold took up too much space to accomodate the plugs as well (plus that whole heat issue). My theory (completely unsubstantiated, mind you) is that since they had to recast the heads anyway, the decided to clean up the ports for better flow for the purposes of raising the peak power of the 931 to help silence the critics (the Lars' of the late '70's, hee hee) of the 924's low power, and to add a combustion chamber to the head and provide a more efficient quench area (for reducing headgasket failures due to detonation, a potentially huge problem for a turbocharged car).

Jon Furst
'81 931

Guest · Posted: Mon Jun 11, 2001 2:09 pm Post subject:

From: larso
Email: l_porsche@hotmail.com
Date: 11 Jun 2001
Time: 14:09:39

the 931 still gets really good low end acceleration, 0-30 in 2.4 seconds...so Jon maybe the camshaft is so choked that the head flow isn't gonna affect as much. They always say, if you change to headers, get a cam and a port and polish, so basically porsche left the emmisions freindly cam in, but changed the head, so it didn't affect low end that much.

SO all you power hungry 924 owners that have NA cars, GET a 931 head and a 270 grind cam and some pistons. OK?!?!?!?!?! OH YAH, and a port and polish EVERYWHERE on the intake side, i figure that gets you 150HP plus!

Guest · Posted: Mon Jun 11, 2001 2:22 pm Post subject:

From: larso
Email: l_porsche@hotmail.com
Date: 11 Jun 2001
Time: 14:22:18

Well, an automotive engineer, a schmotomotive bengineeer, sheesh, Turbo cars do not respond well to port and Polishes along with long duration high lift cams, they lag like nuts. Its not as simple as whatever you were doing in automotive engineering i guess.
YOu see the way turbos work, is that if there is not enough pressure build up at low rpms, you get such low boost that it is ineffective, the turbine has to be spinning at a certain rpm and up to have any effect on the air going into your LOW compression engine.
SO, with a port and polish and a very rounded off (high duration) and high lift cam (looks fat and tall at the same time), the INTAKE side will not build up enough pressure and resultingly, low speed, why? because when you have a more restrictive series of inlets (your pipes, your valves, etc.) AT low rpms, the air gets sucked in easier and faster. Try opening your mouth wide open, and suck in air, how much movement do you feel on your tongue? none?

Now close your mouth so that you could fit about 2 pencils through your lips, suck in air, how much movement do you feel on yoru tongue? i feel lots of pressure and speed. Now there is a certain LIMIT, such as too small won't work at all.
High duration cams are crap in turbo cars, however, POINTY and high lift cams will work well with turbo cars, i believe.

Guest · Posted: Mon Jun 11, 2001 3:10 pm Post subject:

From: sp
Email: s2003r@yahoo.com
Date: 11 Jun 2001
Time: 15:10:47

we all know that every engine, even N/A, lose low end power if the intake flows more than the engine can handle or even if the exhaust flows too much. like the idiot who puts two 650 cfm carbs on his 305 camaro, or the other idiot who puts big exhaust on his honda. but when everything matches up then it all works. i'll still agree with you that you may lose some low end. so that's why you've got to know what you want before you start doing everything in the world to your car. personally i don't care much about low end, i don't do a lot of hard take off driving. i like to drive around curvy roads, in which case i can surrender my low end and just use the trans to keep the revs up.

Guest · Posted: Mon Jun 11, 2001 3:49 pm Post subject:

From: sp
Email: s2003r@yahoo.com
Date: 11 Jun 2001
Time: 15:49:58

i just wanted to apoligize for my recent god complex. i've just had a bad week and felt like i needed to be defensive. i'm not even sure why i was defending the 931 head when i'm thinking about selling all of my cars and starting over.

Guest · Posted: Mon Jun 11, 2001 8:55 pm Post subject:

From: larso
Email: l_porsche@hotmail.com
Date: 11 Jun 2001
Time: 20:55:46

hey same here, parted out my 931.

I too like high end, but i also am losing interest in turbo cars so i don't really care anymore about the 931, id rather have an NA, turbos are too unrelaibal, they burn oil, they crack manifolds, they blow head gaskets, and aside from that ANY car can go fast with a turbo on it, a guy with a volvo 245 station wagon gets 0-60 in under 6.5 seconds with pretty stock set up but upped boost, porsche 931 turbo isn't fast for a turbo car IN MY OPINION>

Guest · Posted: Mon Jun 11, 2001 9:20 pm Post subject:

From: larso
Email: l_porsche@hotmail.com
Date: 11 Jun 2001
Time: 21:20:31

if you did put a 270 duration cam in a turbo car, i would say you would have to change your turbo, and change your valve springs, so that you caould actaully make use of the high end that you are sacrificing for low end....

Guest · Posted: Tue Jun 12, 2001 12:17 am Post subject:

From: sp
Email: s2003r@yahoo.com
Date: 12 Jun 2001
Time: 00:17:50

why did you part out, was it not worth selling as a whole car? i have a 931 to part out, another 931 good to go, a 78 924, and a 62 mg midget. i'm thinking of clearing the garage and getting something closer to a 928. what are your thoughts on the four valve 928?

Guest · Posted: Tue Jun 12, 2001 12:55 am Post subject:

From: larso
Email: l_porsche@hotmail.com
Date: 12 Jun 2001
Time: 00:55:59

lotsa people need parts but not many around my house want a porsche that looks like a volkswagon scirricco i guess....

:-P

928 4 valve....fast...high speed cruiser....cops...ticket....heavy....i dunno?