BBC output overview.

[David Vizard]

A basic CFM flow chart is often as far as most head porters go when gleaning info from a flow bench test. Well here we are going to the next step and analyze what can be found out about the heads characteristics when we look at the SAE discharge co-efficient of the intake and exhaust valves.

Firstly let me say that the decimal number of the SAE figure is simply another way of quoting it's efficiency. For instance 0.82 would be 82%. and so on.

Take a look at the graph below. Again the thick lines are those for the 322 weird port head. Note at low lift the intake flow efficiency gets as high as 85%. Compare that to a good multi angle conventional seat job of 72% and you can see that this high efficiency seat is allowing the 2.3 inch valve being used to simulate a conventional seat valve of as much as 2.6 plus inches. Part of the reason it is that at low lift the head seat and the seat on the valve form quite an effective venturi and as such there is pressure recovery across the seat. By the time the valve gets to some 0.200 lift most if not all of this effect is gone but the form used is still above average so the 0.250 lift point still has what you might expect from a bigger valve.

But I am straying off the real subject here. Note the vertical red and blue lines on the graph. These are the 0.25D lift values for the ex and in. respectively.Note how the CD increases after the valves 0.25 lift point. The steeper the increase here is an indication of how effectively the port bias is functioning. If the CD levels out without climbing up it is a sure fire sign that the port has insufficient bias in the bowl. So right there you know virtually for sure the next move is to increase the port bias.

[Warp Speed]

Whatcha selling?

At the moment I am dispensing a free cure for ignorance!
DV

What happened to the "Civility" you preached to me?

[Stan Weiss]

And so it begins AGAIN.

DRIP

DRIP

DRIP

Building to a flood and screwing up another thread.

Stan

[Warp Speed]

And so it begins AGAIN.

DRIP

DRIP

DRIP

Building to a flood and screwing up another thread.

Stan

DV needs to practice what he preaches!
He could have said nothing, but then it would be hard to play the victim I guess.

Remember the friend or foe button everyone talks about. He could use it, and save a bunch of drama........

[Stan Weiss]

And so it begins AGAIN.

DRIP

DRIP

DRIP

Building to a flood and screwing up another thread.

Stan

DV needs to practice what he preaches!
He could have said nothing, but then it would be hard to play the victim I guess.

Remember the friend or foe button everyone talks about. He could use it, and save a bunch of drama........

Jay,
It seems some people just follow David around, just to start something. I do not have the time I used to and so do not read as many threads as I once did. So I do not know how much technical information he posts, but JMO Headguy seems to be one of these people.

Stan

[David Vizard]

Whatcha selling?

At the moment I am dispensing a free cure for ignorance!
DV

What happened to the "Civility" you preached to me?

Warp,

I don't believe I was talking to you so you should not be offended. I am doing a straight forward tech post and I get a comment like 'Whatchya selling' which serves no useful educational purpose.I went three years without retaliating and not in one instance did I start the 'uncivility'.
DV

[David Vizard]

Warp,
One more comment on your post, --He could have said nothing, but then it would be hard to play the victim I guess.

Wrong guess- I am not the victim here. When I am pushed I tend to become the aggressor.

Hopefully we have that little bit of unnecessary diversion out of the way which should leave me more time to focus on tech stuff for BBC.

DV

[stokerboats]

Please continue with the technical BBC info. I am personally hungry for useful tips. Thanks!!

[JoePorting]

I think the best approach to personal attacks is just to ignore them. If you respond to them then you risk having two or three pages of non sense which makes everyone look bad.

[David Vizard]

I think the best approach to personal attacks is just to ignore them. If you respond to them then you risk having two or three pages of non sense which makes everyone look bad.

Joe, I totally agree but having spent the best part of thee years 'turning the other cheek' it does begin to wear thin.

DV

[David Vizard]

I believe I had got started on the subject of port bias and the inflection point in the CD at 0.250 valve lift. The point I was about to address is that understanding how port bias can factor in here can be instrumental toward achieving high lift flow efficiency. In fact bias can be he key ingredient for making even the worst ex. port flow efficiently.

A little illustration that harks back to my days hopping up Mini Cooper engines (BL's not BMW) The ex. each end of the head is virtually a square port which takes a right angle turn at the valve and has only about 3/8th of an inch to the valve seat. A more badly conceived port would indeed be difficult to find. After years of flogging these ports on the flow bench I became resigned to the fact that, for what it was, a CD closely approaching 0.7 was pretty good all things considered. My good friend David Anton of APT in Riverside CA, who also specializes in A Series engines, got onto a big winter development spree and much to my amazement he produced a port with a huge bias (about 50% of the valve diameter) and this SOB ran to 82% efficiency at about .35 D lift.

Just so you are able to appreciate the magnitude of this that is about as good as cup car heads were some 20 years ago and blows the typical well ported aftermarket performance head into the weeds.

About 15 years ago I had an aftermarket aluminum SBF head to port and the exhaust did not respond until I applied a hefty amount of bias to it. Will look for a shot of such as soon as I can spare a little more time.
DV

[Orr89rocz]

How did the power on dyno or track respond to the increased bias as you call it? Very interesting

[David Vizard]

How did the power on dyno or track respond to the increased bias as you call it? Very interesting

Our engines were so often the ones too beat. On the drag strip my daughters 1380 mini was totally untouchable by any size NA mini. She shut down all pro built mini's and on one occasion this included the factory's 8 port aluminum headed, fuel injected, titanium rodded so call super mini by such a big margin in qualifying that the pro driver driving it refused to to race her in the afternoon.

I don't know if this exactly answers your question but whatever was being done was beating the other guys.
DV

[Zmechanic]

Been mulling this over more. In the context of port bias, that runner entry makes more sense now. And the lack of velocity gradients does too. It's almost like a way to trick the port into behaving as though it were biased and canted over like a DRCE port.

[David Vizard]

Been mulling this over more. In the context of port bias, that runner entry makes more sense now. And the lack of velocity gradients does too. It's almost like a way to trick the port into behaving as though it were biased and canted over like a DRCE port.

I guess that is sort of one way of looking at it.
DV