Moparts Tech Archive
Basic BB OE head porting 101
From: so burlington,vt usa
Registered: Aug 2000
i thought id put a slightly different
spin on it.
when the other articles on porting the BB Mopar OE heads were done, the Edelbrock BB Mopar heads
didnt exist yet. so, im going to start off with some tests with the OE heads in stock form, then
do a decent valve job, then back cut the valves, then add a little bowl porting and short turn
work. i'll get the heads into the 240's on the intake side before i remachine the seats for the
2.14/1.81 valves. then i'll reblend the bowls to the larger seat diameters.
then some more serious porting will be done to see just how much reworking is required to get
a stock set of castings to flow what a set of E heads flow "out of the box".
since most E heads will break 280cfm in stock form, thats the number i'll be shooting for as
for sure, the amount of money the porting alone would cost to get OE iron to break 280cfm will be most of what a complete set
of brand new E heads would set you back.
there are still some instances where a mildly reworked set of OE castings is a cost effective
alternative to buying a set of $1275 heads, and as im going through the various stages of
reworking this head, i'll give some approximate costs for getting a pair of heads to that point.
you'll see that it doesnt take long at all to spend $600-800 or more on OE heads, and they wont
flow anywhere near the higher lift numbers of the E heads.
however, $600-800 is still a fair amount away from $1275, and if its really a budget oriented
type of build, the extra $500-600 may just not be available, or may need to be spent on other
aspects of the project.
anyway....here is where we start with this little series of tests....with an untouched 902 head.
this is the only "orphan" unmolested head i have, so thats whats going to be used. this head
has the same port configuration as the 346 and 452 head.
stock head, with OE valve job and valves:
the first logical step is to do a good "competition style" valve job. the intake got a 3 angle
cut, 30/45/60, with a 75 deg throat cut as well. the .060 bottom angle was left .100 wide.
the exhaust got a 40/45/radius bottom, and a slight cut with the 75deg throat cutter to remove
the ridge left from the bottom of the radius cutter.
the object with the first few steps here is to not lose flow at any point below .500 lift.
since the first few steps would be geared more towards a milder build up, which would use a
milder cam, its important to try and have an improvement in flow throughout the lift curve.
comp valve job, OE valves, no back cut:
as you can see...this gave us a slight improvement on the intake side, and was worth nothing
at this point on the exhaust side.
the radiused cutter and the 75deg throat cut made the edge in the short turn of the exhaust
port even more pronounced than in stock form. i actually thought id lose a couple of cfm
at this stage, so to end up even was okay by me.
i also tried a back cut OE valve on the comp valve job. on OE BB Mopar heads its usually
beneficial, and was in this case as well. at the higher lifts, the exhaust actually lost
a little bit, but the overall gain throughout the lift curve was worth the trade.
comp valve job, OE valves with 30deg back cut:
a nice gain on the intake side through .550, and on the exhaust through .350, then even til
.450, and down just over 1cfm at .500.
for the most part, this work is just to bring the heads back into shape.
a competiton valve job with back cut valves and a throat cut at my shop is $245.
a "valve job" means different things at different shops.
what you would get for that is:
disassemble and jet wash heads, bead blast heads and valves, clean springs, retainers and locks, grind valves, machine seats, throat cut, finish grind seats, lap valves, back cut valves, final rinse all parts and heads, assemble heads, set up springs.
any parts are extra.
on 99% of the old heads you get, they need all new guides($120 for all 16 installed, bronze), and need to be resurfaced as well($66/pr up to .015 for iron).
that would be $431 if you needed all the guides and had them resurfaced. for a performance build you would probably want positive valve seals, and to machine the heads for them is $20/pr, and the seals are about $25.
that puts the total at $476 for unported OE heads with your old valves, springs, and retainers.
i told you...it adds up fast.
now its time for a little grinding. the first step here will just be to blend the machined
throat cut into the bowl, and put a slight radius on the short turn to eliminate the edge.
this was a lesson in "less is more".
i removed a portion of the hump in the intake port, and did a mild
polish of the bowl. however, during the polishing process i ended up
making the ID of the bowl just under the valve seat slightly larger.
this ended up killing off some valuable low lift flow from the
if these heads were going to retain the OE valve sizes, and were
going on a motor that was going to use a cam with less than .500
lift, then this would have been a step backwards.
the higher lifts did show a slight improvement.
the exhaust work showed more promise, and was an improvement from .200
so, here is where im at now:
the exhaust is actually pretty good for the minimal amount of
work i did on it. with the bigger valve and a bit more porting it will
easily go over 200cfm.
the intake is still a far cry from the target 280cfm, and it will
take some serious porting and the bigger valve to get it there.
the next step is to machine the seats to accept the 2.14/1.81 valves.
i will test the head without blending the bowl into the larger seat
first. since the head already has some bowl work done, it will flow
better than if the big valves were installed and no porting was done,
however i'll be suprised if the result is much different from where
its at now.
with a back cut on the new valves, some(if not all) the low lift flow
should come back.
the test from an OE back cut valve to an aftermarket oversize valve
isnt that good of a comparison for the two different sizes. the shape
of the valve heads are quite a bit different, which does have an
affect on the flow. a better test for the difference in the valve
diameters would be to test the OE size vs the larger size with valves
of the same manufacturer and series.
i dont have any aftermarket OE sized valves on hand, and dont want to
wait for some to show up to move ahead to the next stage.
that info would just be a little bonus, but isnt really pertinent for
what the final outcome will be.
this is about 40mins total grinding time to this point, so that would be 160mins per head, 5 1/3hrs per pair. this would be about $240 for this amount of work.
although the extra 15mins i spent on the intake was a waste of time for this level of modification, it does start to come into play as the quest for bigger numbers goes on.
i was curious if removing that hump was worth anything at this point in the game...and its really not.
basic BB OE head porting part 2-bigger valves
okay, to save everyone from having to go back to the previous post to
see where we left off, i am putting the last set of numbers here.
to recap, this is pretty much what you'd get by doing a bowl porting
job using the MP templates with a competition style valve job, and
back cut OE sized valves.
the next step was to machine the seats out to accept a 2.14/1.81 valve
combination. since this is a somewhat budget oriented test, im using
the Ferrea 5000 series, which are some of the more "affordable"
stainless steel valves.
this first set of numbers is just after enlarging the valve seats,
and using the same 75deg bowl cutter i used to do the original seat
since the bowls had already been blended into the seats at the stock
valve size, there was much less of a ridge left from this operation
(as compared to when i did this in the unmodified bowls for the stock
there was a bit more of a ridge left in the exhaust port than in the
intake, and having any kind of edge on the exhaust short turn will almost
always kill off some flow...even if the valve opening is bigger.
this is with the valves "out of the box", no back cut:
as you can see, a slight improvement on the intake side at high lifts,
and not a lot of help on the exhaust with no back cut, and the ridge
in the short turn.
add a back cut to both valves and you get this:
a nice gain in the low lifts on both valves, but no help up top.
then i went back and re-blended the bowl work out to the larger
seats, being a little more careful on the intake side this time
to make sure i kept the bottom angle as wide as it was to start
this was worth basically nothing on the intake side, but now the
exhaust port is really starting to take off.
for the intake port, from this point on the gains are going to be
much harder to come by, and this is where the money starts to add
since i usually open the chambers up to the size of a 1009 gasket
where they are adjacent to the valves, that will be the next step.
after that i will do some minor streamlining of the intake guide,
and remove some of the bulge in the exhaust port on the outside
wall at the port exit. i will also finish polishing the entire
exhaust port. the exhaust port would then be at a point that most people equate to "stage II" porting.
the intake would still need some more work to get to that point.
i dont have a lot of time to answer the questions at the moment, but will later on today or tonite.
however...here is some more info as the head progresses:
as with most heads(and castings in general), there is some core shift
on this head.
when i went scribed out the line for the 1009 gasket, i could see that
i would be making a pretty nice cut on the exhaust side of the chamber,
but barely taking anything out of the intake side. of course the
neighboring cylinder would be the exact opposite(big cut next to the intake, almost nothing next to the exhaust).
so, after the cut i didnt bother reflowing the intake port since the
amount of material i removed was basically nothing.
that being the case i went ahead to the next step of the intake port
rework, which was to narrow down the guide boss.
this is the first step ive taken in the direction of fully porting the
there is a slight increase of low lift flow, but thats due to the
unshrouding cut. the cutter i use for that operation leaves a 15deg
angle from the chamber wall to the top of the valve job. this basically
creates a 5 angle valve job.
the original 3 angle cut, 30/45/60, the 75deg bowl cut, and now the
15deg top angle.
i took quite a lot of material off the guide boss, and opened up the
area on the inside wall next to the guide boss a lot as well.
i probably wont have to make this area any wider than it is now to achieve my 280cfm
goal, but may end up going back and taking more out of the roof in this
area to gain some more cross section at the short turn.
the exhaust port itself got no extra porting for this step, and all of
the gains here come from the unshrouding work done in the chamber.
a nice improvement on the exhaust everywhere...except at .350 lift, and
a nice gain on the intake right up through as well.
compared to the stock head and valves, at .500 lift this level of work
is showing a gain of 38.6cfm on the intake side, and 43.8cfm on the
at .500 lift intake to exhaust ratio is 78%.
believe it or not, in a moderate bracket type combination(11:1cr, big
solid cam, 850DP, single plane intake, 2" headers) heads with these
kinds of flow numbers can make 550hp fairly easily.
while it is probably a little premature for this stage of the game,
im going to go ahead and open the pushrod pinch area, and bring the
port opening out to the size of a valley pan gasket. this will probably
do little (or nothing) for me at this point, but i want to know thats
not what will be holding me back as the rest of the port starts getting
bigger. also, on a motor in the 550hp+ range, the added cross section
there would probably be a good thing.
for comparison, the average width at the pushrod bulge of an E head is around 1.060. on this port im working on its .980.
also, the E head has the port opening CNC'd to the gasket size already.
i will be going to those dimensions to start with, 1.060 width, and std gasket opening size.
as i said before, the next step for the exhaust is to bring it to what
most would call a "stage II" level of porting.
then, to get much more than what that will flow starts to require a disproportionate amount of time grinding metal vs the gain in flow.
now...on to the next step:
well....im finally starting to hone in on the flow numbers the
out of the box E heads typically flow.
apparently the pushrod pinch was a bit of a problem as the valve
started to get to lifts over .500. the only extra work on the intake
port from this step and the previous step is the work in the pinch area
and enlarging the port opening to the size of a valley pan gasket.
i was suprised at how much of a gain there was at .600 lift.
the exhaust got finished up to the std stage II type of port job, and
is actually very close to the Ebrock exhaust port at this point(slightly better actually).
i will admit however, that for what ever reason, this is a very good
exhaust port for the amount of work ive done to it. ive had numerous
BB Mopar heads that didnt flow this much with even a little more work
than what this one got.
from what ive seen of my own porting, and heads that other shops and
individuals have done, this is not "typical" of a stock head with this
level of prep.
anyway...heres where im at now:
.600-270.7/213.4-(248.1 with 2" flow tube)
compared to a "good" 906/915 head, the flow in the 450-550 range is a
little low, but since the height of the short turn is so much lower
on the later castings, there isnt really anyway of getting the same
kinds of results in that range, without possibly going to an even bigger
valve, or doing like Dulcich did, and use a bigger valve with a flatter
on the other hand, its also harder to get a 906/915 head to flow 280cfm
and not get into turbulence at high lifts because of that extra height.
the next step on the intake is to raise the roof near the guide boss a
little, remove the remainder of the hump on the outside wall/short
turn, scallop out a little of the bowl on the outside wall, and detail
the short turn a little more.
im also going to take some material off the length of the port walls
to add a little volume to the runner.
then, i will polish the entire runner, and hope ive reached, or am very near, 280cfm.
well, the intake port is now fully ported and polished, and it looks
like this is about how much work needs to be done to get a set of OE
castings to be on par with a set of out of the box Ebrocks.
i was hoping to get to 280cfm by .600 lift, but didnt quite make it.
its about .625 lift where i got to the 280cfm mark.
i havent cc'd the intake runner yet, but im guessing its in the 195-200cc
so, how does a fully ported iron head with big valves really compare??
well...lets take a look:
so, on the intake side the ported oe head has a slight advantage at the bottom of the lift curve, but gives it up at .400 at which point the E head starts to shine.
on the exhaust side, it appears it isnt that hard to get an oe head to
equal or slightly better the E head port as they come delivered.
what should be fairly clear however, is that if you have a "good" set of
ported OE heads, and simply bolt on a set of out of the box Ebrocks, dont
be looking for big gains in performance(as a few members have found out).
remember, the dyno test i did comparing the Ebrocks vs ported OE iron was done
using fully ported Ebrocks vs fully ported OE iron. and in that test,
using the same induction system on both sets of heads, there was only a
gain of about 20hp with the E heads. and in that test, the E heads had an
additional 1/2 point of CR over the OE iron.
if i had to point a finger on why there wasnt more of a difference in
that test, id say it was because of the similar port cross sections
between the two sets of heads.
the bottom line is that heads with port volumes in the 190-230cc range
are really rather small for a 448ci motor.
the displacement simply uses up the available area, and the port reaches
now i have to go back and see how long it takes to duplicate this work
without all the stops and starts so i can get a more accurate account
of the time involved.
then, i can get a better idea on where the cut off point is on the cost
effectiveness of reworking the OE heads.
Registered: Jul 2001
posted 10-27-2002 01:42 AM
Lots of good straight honest info in these threads. Those are very good numbers for a late style intake port. I want to add that it takes quite a bit of experience on these head to get those levels, and few shops get them to work that well despite various commercial claims. Did you take out the Huber Hump on the floor of the straight wall (just adjacent to the short turn)? If not, and this is an experimental head, wack it out completely (floor and wall) and see if it works for you. I've done this with some success, allowing for a nicer short turn form.
Like you pointed out, I've found the early intake port (915/906) with the taller short turn can flow more with a max-effort port, but it has to be just right and takes a huge amount of work with those giant guidebosses and that upright square stock short turn. It will be interesting to see how your 906 port job will compare.