Noahs944

Calling all engine experts.
Gurus, I need your help please.
I am porting my 8 valve head (2.5 l, stock valve size).
I changed the angle of the short side radius pretty significantly. Was this a bad idea?
It is now "flatter" and less curved by maybe 10 degrees.

I changed if from the picture on the right to the picture on the left.
I thought it was a good idea, but have read since "don't touch the short side".

Essentially the shape of the ports hasn't changed with the exception of casting flaw removal & modifying the short side.

If I messed up, I will accept this & be humbled.

I have read countless threads of 944 head mods, but there isn't a lot of info that pertains to what I am asking here.

Van

I am not a head expert, and have never done porting myself, so take what I'm about to say with many grains of salt. But, I think you screwed up.

My understanding is, that good porting will help the flow of air at partial valve openings more than at full lift. Because of valve lift duration, the valve is at full lift for a very short amount of time. So, think of it this way: When the valve is just starting to open, the air needs to move in a sort of S-shaped curve to get from the walls of the intake passage, around the valve seat, and around the head of the valve. By flattening out that short radius, the air now needs to make a tighter bend than it did before. Air has mass, so the molecules moving closest to the wall (the boundary layer) might separate from the wall at the valve seat. This "separation" will create low pressure and eddy currents that will, in fact, reduce the flow over that part of the valve seat.

I seem to recall reading that actually building up the short radius so the air is flowing more in the direction of the valve stem actually gets a larger volume air into the cylinder.

Again, I have no evidence to back this up - consider it "fake news" or "alternative facts" to what might actually be seen on a flow bench.

Noahs944

Thank you Van. What you say makes sense. I hope it's not true, but it probably is (you are pretty smart)

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Also, the exhaust ports have "v" shaped casting .... errr "flaws" on the sides. The wide part of the "V" is close to the valve head (when valve is at rest on it's seat). The intake ports didn't have these.

Should they be removed with a port job or are they intentional?

mhariush

Sorry to hear that. And thanks Van for some insight into porting. This is way beyond my engineering skill set. But, I do have a spare 2.5L NA head that has what looks like minor bent valves. So if you need to start over... let me know.

jhowell371

Proceed carefully Grasshopper, bigger ports and runners don't always make more useable power. Smokey Yunick (Google him youngsters) perked up Ford's Boss 302 by actually reducing port and runner size for their Trans AM racing program.

T&T Racing

IMO, Smokey Yunick found that reducing port and runner size increased air/fuel mixture turbulence, resulting in a "nearly" perfect air/fuel mixture. This eliminated air/fuel stratification. A "perfect " burns uniformly and produces more power. I am assuming the carburetor throat diameters limited the volume of air intake and the intake manifold chamber is a poor mixing device.

The AFM is the limiting factor for air volume flow into the intake manifold of a Porsche 944 8V.


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V2Rocket

maybe...but does it really matter all that much when youre boosting it?
make the ports equal and run it.

Voith

It will probably change the breakaway point.



http://garage.grumpysperformance.com...-can-help.462/

This guy seems to know what he's talking about..

Per vers

As Spencer said. Turbo engines are not sensitive about porting. As long as there is a positive pressure in the intake port.

Noahs944

I've screwed up too many times. I've had to hire help.

PaulD_944S2

Not sure how you can get much work done with those assistants around!

Of course, with them around I don't think port radius of an 8 valve head matters!

Arominus

Find a flow bench and see how it does, you may have hurt it some, but the bench will be the only way to tell.

jhowell371

JMHO but seems to me that air flow is air flow. A boosted engine should appreciate a superior flowing port just as much as a N/A engine.

V2Rocket

You're right...airflow is airflow, and optimizing the shape of the head will benefit both NA and turbo engines.

However...the degree to which it may affect overall output may be negligible.
Noah is doing a sort of "budget turbo setup" so nothing is going to be "optimized" anyways...so may as well just run with it

Noahs944

Bite your tongue lad!

There will be optimization. I've decided to run the rebuilt head that's on Wilson now. I learned a lot from the porting experience. I have a "feel for it" now & appreciate the responses given.

Yes... Spencer "budget" is right... but I just paid for arguably the best stand alone system on the market & am buying a matched set (within 1%) of high flow modern injectors, and will be optimizing the system as best as possible. Budget, yes... but the budget for the conversion is enough to buy most people's 944 cars.