BBC output overview.

[David Vizard]

It’s been awhile since I last posted. Had some pretty major surgery in the last 3 months including a big time hip replacement for a truly destroyed hip joint. That’s what running can do for you. Between 1980 and 2000 I put on the best part of 40,000 miles of running. Did my last run just after my 70th birthday – only about 3 miles but my hip joint was sure giving me a bad time. Unfortunately I assumed it was arthritis so the fix was delayed by almost 5 years. There are a bunch of other, lesser things, but each put me further back on my work schedule. My Ford book is currently stalled as Is my Race Engine Challenge build. (got the crank & rods, heads are provisionally ported and looking really good but I don’t think it’s the end of the trail yet on heads.)

On top of that I have about 10 model heads to do for digitizing for clients that actually pay money. What with the almost certain arrival of big medical bills I am having to plow on with these model heads.
I also have a bunch of dyno tests (about 450) that are (or rather were) slated for Volume 3 of my BBC books. I will be 76 next birthday and with the stack of work on hand I think I will have to forgo the production of BBC performance Vol 3. That said I don’t like the thought of all the BBC knowledge from the tests done over the last 3 years going to waste so my plan is to start threads like this on both the generalities of hopping up a BBC as well as some of the particulars in detail Also plan to do some video’s on the subject.


So, withstanding the nath sayers, here we go.

Those who have taken the time to look at the power curves of some of the BBC I have posted will have noticed that the torque per cube figures are significantly higher than normal.
The keys to achieving this is in cam selection and head design.
Here is what needs to be taken care of as far as heads (and we are talking 24 degree one here).

• No matter which way you cut it the BBC’s cube capability far outweighs the breathing capability of even the best heads. To match a SBC’s 2.02/1.6 valve size for a 350 inch short block a 454 would need a valve combination of 2.62/2.06. The biggest valve that is usually seen in a 24 degree BBC head is 2.3 or (sometimes 2.35)
• The consequence of insufficient valve size is compounded by the fact that a 454 is readily starched to as much as 650 inches with 712 being on if you want to deal with the extra hassles involve.
• Because of the relatively shallow angle of the intake port to the valve axis we find the best output is achieved with heads that are bigger in bowl size in relation to the size of the main body of the port than is so often the case with SBC configurations.
• The result of point 3 is that we need to develop a seat and bowl combination that flows well from right off the seat to very high valve lifts, that is up to about 40 -45% of the intake valves diameter.
• Although the bowl needs to be a lager than normal volume the port feeding it needs to be the minimum size to get the job done so as to achieve the highest specific port energy. This means big ports are out and big bowls are in.

So where, in terms of head ports, does this leave us?
In simple terms It means having to cut seats that will give as high a coefficient of discharge (CD)as possible. Somehow, we must make that 2.3-inch intake valve appear to the cylinder as being much bigger than it really is. Seat design is the dominant factor to a lift value of some 18 -20% of the intake valves diameter. That works out to be 414 – 460 thousandths lift. Before 0.46 lift its all about the seat design - after that it is all about bowl and port form.
With an effective seat form established we must now make the absolute most of any high lift potential the casting being reworked may have. As far as the bowl work is concerned putting enough port bias into it will greatly help the need for good high lift flow. As far as the main body of the port is now concerned we must shape it to a form that best fills the bowl in an efficient manner.


Just for the record the best 300-320 cc port heads tried on the 572 GM update test engine wed did a couple of years back at Terry Walters shop in Roanoke was the AFR 320’s. Though down a little from the Darts flow (# 2 in our tests) they did exhibit the highest specific port energy. The shots of the head I am showing here out flowed and out energied the AFR’s by a goodly margin.

Take a look at the head shots.
Note the entrance to the port. Its shape is dictated by the #1 rule I give to my seminar attendees. IE ‘Cut the port where the air wants to go not where you think it should’.
The other shot shows the bowl shape used. It does not show very clear here but the port wall on the cylinder wall side is biased about 4 degrees inwards toward the cylinder center line. This was a big part of pumping the flow to the 420 cfm on the bad port (332 cc) and 432 (323 cc)on the good port.
Most heads with this port volume don’t make it past about 385 CFM on the same bench as used here (mine)
Also note the valve seat inserts. At this moment in time they are a full radius. The seat will be just one cut on this radius to produce a 0.060 wide seat.

Next post I will go into more detail with the flow curve illustrations and some dyno numbers.

DV

[F-BIRD'88]

Nice to see that you are back at it.

[Geoff2]

Keep on postin.......

[Orr89rocz]

To match a SBC’s 2.02/1.6 valve size for a 350 inch short block a 454 would need a valve combination of 2.62/2.06. The biggest valve that is usually seen in a 24 degree BBC head is 2.3 or (sometimes 2.35)

Curious as to how you derived that. Simply cubic inch / valve diameter? Valve diamter to cubic inch ratio really an important factor? Shouldnt it be more about valve area/curtain area in relation to bore size and/or engine cfm demand? 0.25” bore size increase shouldnt need a .60” valve diameter change i wouldnt think

[GuysMonteSS]

Hello David,I was wondering if you could comment on how well this design BBC head from CID would compare to yours ??
https://www.cidheads.com/products/copy- ... -pair-bare
Thanks !!
Guy

[David Vizard]

To match a SBC’s 2.02/1.6 valve size for a 350 inch short block a 454 would need a valve combination of 2.62/2.06. The biggest valve that is usually seen in a 24 degree BBC head is 2.3 or (sometimes 2.35)

Curious as to how you derived that. Simply cubic inch / valve diameter? Valve diamter to cubic inch ratio really an important factor? Shouldnt it be more about valve area/curtain area in relation to bore size and/or engine cfm demand? 0.25” bore size increase shouldnt need a .60” valve diameter change i wouldnt think

Curtain area Vs Cid
DV

[David Vizard]

Hello David,I was wondering if you could comment on how well this design BBC head from CID would compare to yours ??
https://www.cidheads.com/products/copy- ... -pair-bare
Thanks !!
Guy

This is a new one on me. I had no idea JK was doing this.

If their flow figures are right my head outflows this by quite a margin and, I am guessing, from a smaller port. More than that I cannot say.
DV

[Zmechanic]

Hey, I'll admit that's an intriguing looking port entry. How much, if any, velocity gradients do you see around that entry profile?

[David Vizard]

Hey, I'll admit that's an intriguing looking port entry. How much, if any, velocity gradients do you see around that entry profile?

I don't have a publication quality illustration of the velocity pattern done yet. What I can tell you is that the exploratory tests for such showed the new port to be significantly better. That is in itself not a surprise since the velocity mapping data was used to generate to port in the first place.
DV

[RevTheory]

There's some pretty interesting work in the chamber as well. I like it

[David Vizard]

There's some pretty interesting work in the chamber as well. I like it

Thanks Rev.
What is not so obvious is the area 0.200 before and after the exhaust valve seat which is cut specifically to optimize supersonic flow during blow down.
On a Nitrous test engine I did for testing Comps nitrous cams that move on the Dart heads not originally done for this flow mode increase the added power from the Nitrous from about 280 hp to 320.

DV

[Erland Cox]

To match a SBC’s 2.02/1.6 valve size for a 350 inch short block a 454 would need a valve combination of 2.62/2.06. The biggest valve that is usually seen in a 24 degree BBC head is 2.3 or (sometimes 2.35)

Curious as to how you derived that. Simply cubic inch / valve diameter? Valve diamter to cubic inch ratio really an important factor? Shouldnt it be more about valve area/curtain area in relation to bore size and/or engine cfm demand? 0.25” bore size increase shouldnt need a .60” valve diameter change i wouldnt think

Curtain area Vs Cid
DV

A 454 has 30% more volume than a 350.
A 2,62 valve has 68% more area than a 2,02 valve.
A bigger valve needs more lift than a smaller valve to reach the same L/D so you can not compare diameters or circumference.
A 2,3" valve in a 454 is equialent with a 2,02 valve in a 350.
One big valve can never make a 2 valve engine behave as a 4 valve engine.
The 4 valve engines superioroty is not in the valve area but in that a smaller valve needs less lift to get to a high L/D.

Glad to see you back David and I hope you will get well and stay well.

Erland

[David Vizard]

Erland,

Here is a graph of the curtain area per cube displacement. 2.62 only beats out the 2.02 in the 350 above .25 D. At all other points below the .25 D for the 2.02 it is exactly equivalent.


DV

[ns158sl]

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

[cjperformance]

David, Good on you for getting back on, not every one agrees with each others theories or practices and arguements concur, but this is what drives folk on to test their own and others theories or practices and hopefully lead to improvement and or better understanding.
Good luck healing up from your Op's and keep on posting and wear a thick skin.
Cheers,