Author Topic: Porting HP gain??? (Read 6894 times)

lqbanotxano · « **on:** January 23, 2010, 10:07:20 PM »

There is a well reputed person in England that does a lot of work on Fiat/Lancia heads. His name is Guy Croft. He wrote a book on preparing Fiat/Lancia motor for race, road or street applications. His name is Guy Croft. I used his book to prepare my 1980 Fiat Spider head ( I had that car 22 year sold it 3 years ago).His specialty is �heads� He also works on other make heads such as Toyota, Peugeot, VW, Spitfire, Opel etc. He did only one 4cylinder BMW head many years ago but will be doing others soon. I sent him an E-MAIL asking him questions about the M42 head. He replied asking for any detailed pictures of the M42 head. I sent him some I found on this site. He responded with pictures & instructions of a modified Peugeot head. This head is very similar in design to the M42 head. You can read his instructions (with detailed pictures) on how to prepare this head, at
http://www.guy-croft.com/viewtopic.php?f=4&t=2122
We can do this port work ourselves & the valve & valve seat work can be done by a qualified head/valve shop. It should not be that expensive to do most of the work yourself.
Does anybody know what gains can be had from the M42 head with this type of port work?
Does anybody have any experience with any sort of port/valve �massaging��not extensive race preparation for BMW?

P.S. this work is usually done to the intake only.

Ivan in Sugar Land, Texas

dwtaylorpdx · « **Reply #1 on:** January 24, 2010, 03:14:44 AM »

Speaking very much in generalities...

Porting is a mild multiplier in the total engine package, porting a head is usually a percentage gain based on the rest of the engine work, on a totally stock motor porting wont be a ginormous increase, but on a cammed balanced blueprinted motor you'd see more benefit from it.

There is also a lot to porting that is experience, while Guy makes it look easy, its not always the obvious moves that make any given engine perform better.

Head work is composed of steps, each one adding a bit, a 4 or 5 cut valve job and lowering the guides and beveling the guides can get you 90% of the gain. So its not always worth the effort to start in grinding the intake or exhaust passages. Basic De-burring is obvious, and de-flashing the casting can help make the head less prone to cracks. But changing the passage shape is often the road to ruin as far as power goes, unless you have CFD
analysis of the port and manifold and can pre-check the work. Very few hand port jobs compete if there is a CNC ported head available.

Port matching, its another anomaly, most engines make the best power with the manifold port just slightly smaller than the head port if the matching is going to cause a ballooned area, or a ramp to another surface.

Given the level of engineering in most BMW engines, I'd focus on a good valve job, good pistons, a top notch assembly job (Which is not as obvious as many think...) Guys who build hot motors for professional race teams etc are going to make more power than most garage mechanics. Its all about practice and knowing the ground your walking on... Not to say there are not a few out there, but I work with a lot of amateur racers and few can actually run all season and win championships with their own motor.

Valve train is another neglected area and again you could figure it out but the pro who does it all the time knows the cocktail that works as far as springs and all the moving bits.

Well there's my long winded opinion...

Dave

flyinglizard · « **Reply #2 on:** January 24, 2010, 06:31:30 PM »

After looking at this head,some things are obvious. The port volume, relative to throttle body size, is huge. The port angle is very good, the valve size is adequate.
Just by folllowing the basics of flow engineering; Narrow/shorten, the huge valve guide body. Remove any hard edges, esp on the outside wall. Fuel and air have mass and the fuel bits get caught by the edges. Dont touch the inner radius.
A slight mismatch at the exhaust port to header is usually a good thing. Do a search for "anti reversion" port matching. I use about 030 in.
Remove the hard edges inside the combustion chamber. Use a head gasket for a guide. You want the air to not "see" any edges in it's travel. Try to visualise the airflow into and out of the chamber. It runs down the far side of the bore, bounces off of the piston as it does a 180, gets compressed as it is still rolling over, up to TDC, and gets lit. There are some areas of sharp edges around the chamber, hindering this flow, just a little. IMHO.
Some time spent on the small ,smooth round edges will show some improvement, flow wise. The thottle size looks to be the power restriction, on a stockish M 42 engine.
MM/Protech

Torque · « **Reply #3 on:** January 24, 2010, 07:14:25 PM »

Quote from: flyinglizard;85542

After looking at this head,some things are obvious. The port volume, relative to throttle body size, is huge. The port angle is very good, the valve size is adequate.
Just by folllowing the basics of flow engineering; Narrow/shorten, the huge valve guide body. Remove any hard edges, esp on the outside wall. Fuel and air have mass and the fuel bits get caught by the edges. Dont touch the inner radius.
A slight mismatch at the exhaust port to header is usually a good thing. Do a search for "anti reversion" port matching. I use about 030 in.
Remove the hard edges inside the combustion chamber. Use a head gasket for a guide. You want the air to not "see" any edges in it's travel. Try to visualise the airflow into and out of the chamber. It runs down the far side of the bore, bounces off of the piston as it does a 180, gets compressed as it is still rolling over, up to TDC, and gets lit. There are some areas of sharp edges around the chamber, hindering this flow, just a little. IMHO.
Some time spent on the small ,smooth round edges will show some improvement, flow wise. The thottle size looks to be the power restriction, on a stockish M 42 engine.
MM/Protech

Honestly the TB flows like the Amazon compared to how restrictive the AFM is. Doing anything past the AFM, without doing something about it first is (IMO of course) pointless.

flyinglizard · « **Reply #4 on:** January 24, 2010, 10:58:58 PM »

True. The ports can pass so much more air than the AFM.

dinu.negrean · « **Reply #5 on:** April 27, 2010, 05:14:30 PM »

And what exactly can we do with the AFM if that is what you think is holding us back?

1991 E30 M42 · « **Reply #6 on:** April 27, 2010, 07:27:38 PM »

the afm is not holding us back, it flows enough air to support over 200hp, the head and the cams are what holds these engines back

bmwman91 · « **Reply #7 on:** April 27, 2010, 07:56:51 PM »

Take it from me, the AFM is not a restriction on an M42, unless you are bored, stroked & have ultra-light valve train components so you can rev over 8000RPM. I have been working with MAF conversions for the past 4 years, and the M42 is not being restricted by it. Intuitively it might seem like a spring-loaded door would be an obvious impediment to air flow, but it is not. Air resistance is quantized in terms of pressure drop (psi, force / area). The flapper door has a surface area of ~4sq in, and it takes less than 0.3lb to open it all the way. 0.5lb/4sq in = 0.075 psi drop, and that is probably still pretty conservative.

Also it should be noted that a port / valve job alone is not of benefit. You will need to tune the fuel & ignition maps on a dyno with a wide-band setup to fully utilize the changes.

deekay · « **Reply #8 on:** April 28, 2010, 12:39:09 AM »

well, i noticed a pretty big difference when i ditched the AFM for an Alpha-N.

and i noticed a pretty big difference in the other direction when the A-N failed on me and i had to go back to the AFM.

admittedly the power loss was all above 6K rpm, but it just doesn't wind out to the rev limiter like it used to. ditching 150lbs of weight and driven accessories made up for it.

take that as you will, it's my experience in working on my car.