BBC output overview.

[David Vizard]

David, I asked before if the bias made the area larger before the SSR.
Bowl area larger than are over the SSR?

Erland

Erland,

You should see what's going on when I mold it. That should be about 3 days.
DV

[David Vizard]

As you can see from the previous graph the high lift flow can go up considerably. But other than showing the flow has gone up it does not of itself say much more.

I have heard, may times, even from very experienced head porters, that a flow bench can only tell you so much. The inference being that you get flow numbers and as such they alone do not directly determine what the modified head(s) will actually do for power. While true of itself just how much the flow bench reveals is determined entirely on how good the observer is at data mining. Comments such as this are usually heard from porters who go little or no further than looking at the flow numbers. By stopping at that point the potential of the flow bench becomes extremely curtailed. That said let's move on to analyze the efficiency of the exhaust port in question. To do that we pull up the graph of the SAE discharge co-efficient (CD) as per below.


First take note of the vertical red line. This is the 0.25 D lift point which for the 1.88 valve concerned is 1.88 x 0.25 = 0.470. This is the 'inflection point' for this diameter of valve.

Before we go any further I should stress the point that the characteristics I am about to discuss are strictly for two valve heads with vertical of near vertical valves such as a typical BBC, SBC & Ford, Chrysler etc. Also typical pro-stock and cup car heads fall into this group. What does not is Hemi 2 valve heads and pent roof 4 valve heads.

First note the red line for the no-bias port. Note how, when it reaches the inflection point of 0.470 lift, the flow virtually stagnates. This 'flat lining' after the inflection point is a typical characteristic of a port with insufficient bias.If your heads ports exhibit this type of CD curve then it is a sure sign that bias is needed.

Now check out the port curve in blue. Note how, after passing the inflection point the discharge co-efficient starts to climb right back up. Just how far up can we pull this efficient increase? On the Pro-Stock heads I have worked on I have managed to see better than 100% but there is another story attached to how that can come about. Suffice to say that on more regular parallel valve casting in the top street ranks have shown as much as 88%.

Right, what I am showing here is that there is far more port defining data that can be gleaned, if you know what it is too look for from the flow figures. If you happen to suspect that this thread is going to show that flow bench results are by many, very much denigrated then you suspicions are right on.

This SAE CD graph is the first step toward getting better head porting results and getting them much faster.


Next step is to look at the before and after port energy for more revealing data.
DV

[Erland Cox]

For the graph above, how do the 2 flow tests look?
To get a better understanding.

Erland

[David Vizard]

For the graph above, how do the 2 flow tests look?
To get a better understanding.

Erland

Erland,

The flow test for the graph above is the last but one on page 7.
DV

[David Vizard]

doesn't the canted valves have some sort of play in this? Larger lift=less valve shroud?

It does but the effect is very small in the case of the BBC. The classic BBC 'porcupine' head is not an optimum design. GM would have done better by just scaling up a SBC head.
DV

I agree!!! I always wanted to say that but knew I'd be bitch out by all the BBC guys.

Joe,

A point of reference here - telling it the way it really is from actual data does not universally win you friends ---------------- and that's a certainty!!!
DV

[user-30257]

It does but the effect is very small in the case of the BBC. The classic BBC 'porcupine' head is not an optimum design. GM would have done better by just scaling up a SBC head.
DV

I agree!!! I always wanted to say that but knew I'd be bitch out by all the BBC guys.

Joe,

A point of reference here - telling it the way it really is from actual data does not universally win you friends ---------------- and that's a certainty!!!
DV

Then explain to me why a Big Block Mopar with B1 heads is down huge to a Predator head?

[F-BIRD'88]

GM would have done better by just scaling up a SBC head

If this was true then the aftermarket would have filled this shoe a long time ago. They haven't.

[David Vizard]

I agree!!! I always wanted to say that but knew I'd be bitch out by all the BBC guys.

Joe,

A point of reference here - telling it the way it really is from actual data does not universally win you friends ---------------- and that's a certainty!!!
DV

Then explain to me why a Big Block Mopar with B1 heads is down huge to a Predator head?

How on earth did we get to this from my original statement???????????
DV

[David Vizard]

GM would have done better by just scaling up a SBC head

If this was true then the aftermarket would have filled this shoe a long time ago. They haven't.

Sorry, F-bird - the numbers don't support your assumption.
DV

[Frankshaft]

GM would have done better by just scaling up a SBC head

If this was true then the aftermarket would have filled this shoe a long time ago. They haven't.

Sorry, F-bird - the numbers don't support your assumption.
DV

What numbers? Let's see them.

[user-30257]

Joe,

A point of reference here - telling it the way it really is from actual data does not universally win you friends ---------------- and that's a certainty!!!
DV

Then explain to me why a Big Block Mopar with B1 heads is down huge to a Predator head?

How on earth did we get to this from my original statement???????????
DV

Well then how does a mini copper relate to a big block chevy..

[Frankshaft]

Joe,

A point of reference here - telling it the way it really is from actual data does not universally win you friends ---------------- and that's a certainty!!!
DV

Then explain to me why a Big Block Mopar with B1 heads is down huge to a Predator head?

How on earth did we get to this from my original statement???????????
DV

Then if this is the case, how does a mini Cooper have anything to do with this thread then? The b1 Mopar example vs the predator head is a perfect example actually. A big block mopar is basically a scaled up sbc head. With a better valve angle to boot.

[F-BIRD'88]

When a inline sbc type cylinder head becomes even competitive in pro stock let me know.

Build two 427 cid engines .one bbc and one with any sbc head. With inline valves. See which one runs best.

[David Vizard]

When a inline sbc type cylinder head becomes even competitive in pro stock let me know.

Build two 427 cid engines .one bbc and one with any sbc head. With inline valves. See which one runs best.

When prostock engines start coming off
GM production lines let me know!
DV

[Stan Weiss]

I agree!!! I always wanted to say that but knew I'd be bitch out by all the BBC guys.

Joe,

A point of reference here - telling it the way it really is from actual data does not universally win you friends ---------------- and that's a certainty!!!
DV

Then explain to me why a Big Block Mopar with B1 heads is down huge to a Predator head?

Getting old, memory is not what it used to be. Didn't the B1 win a few Pro Stock championships?

Stan