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
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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.
I agree!!! I always wanted to say that but knew I'd be bitch out by all the BBC guys.David Vizard wrote: ↑Sat Sep 29, 2018 10:49 amIt 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'd go to a 2.4 intake valve, maybe larger if it would fit. Even if it meant going with a smaller exhaust valve. I always thought the BBC exhaust port and valve was too big. Maybe 1.75 may have been more ideal.David Vizard wrote: ↑Sat Sep 29, 2018 12:09 pm OK you head porters out there. I have just been playing with an
Edelbrock big port head (started life at about 355-365 cc) I have ported one of each port looking only at the airflow gains with the intent to get the most that is practically possible. At this moment in time I have 452 cfm on the good port and 433 on the bad port at 1.150 valve lift. Good port vol is 384 cc and the bad 380 cc.
Valve is still 2.3.
Any comments on this before I throw in my $2 worth of thoughts?
DV
Dave,David Vizard wrote: ↑Fri Sep 28, 2018 2:28 am 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
Or maybe kept the "W", flat head design and just made better/lighter domed pistons for it and stood the ports up right more!! Imho, the old "W" engines had zero valve shrouding unless I am missing something!David Vizard wrote: ↑Sat Sep 29, 2018 10:49 amIt 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
David Vizard wrote: ↑Sat Sep 29, 2018 12:09 pm OK you head porters out there. I have just been playing with an
Edelbrock big port head (started life at about 355-365 cc) I have ported one of each port looking only at the airflow gains with the intent to get the most that is practically possible. At this moment in time I have 452 cfm on the good port and 433 on the bad port at 1.150 valve lift. Good port vol is 384 cc and the bad 380 cc.
Valve is still 2.3.
Any comments on this before I throw in my $2 worth of thoughts?
DV
Sorry pdq67,pdq67 wrote: ↑Sun Sep 30, 2018 3:20 am Dave,
Would you please do an old --206 BBC head as well as the old -215 head and the way laid back open chambered smog head.
I ask this because the old -206 head and the -215 head can be used to up the CR of the old P/U Truck 454 engines.
Up from like 7.8 or do to like 9.+ CR using the old .020" thick steel shim head gaskets.
pdq67
MTENGINES wrote: ↑Sun Sep 30, 2018 3:13 amDavid Vizard wrote: ↑Sat Sep 29, 2018 12:09 pm OK you head porters out there. I have just been playing with an
Edelbrock big port head (started life at about 355-365 cc) I have ported one of each port looking only at the airflow gains with the intent to get the most that is practically possible. At this moment in time I have 452 cfm on the good port and 433 on the bad port at 1.150 valve lift. Good port vol is 384 cc and the bad 380 cc.
Valve is still 2.3.
Any comments on this before I throw in my $2 worth of thoughts?
DV
What head?