RFD

Here's the scoop as I just got done doing a "street / strip" work up of an LS7 port.
Low to mid lift flow improves an average of 6-7 cfm up to .600" lift where the stock port goes turbulent.
Porting allows the port to continue to flow up to .800" lift where it starts to level off at around 390 cfm.

In a live running engine, under dynamic conditions the turbulence we see on the flow bench at .600" lift
will happen much sooner as the air speed under running conditions is many times higher then our test air speed.

Air speed in the port is very good with average readings around 290fps and a high of 350fps over
the apex of the short turn.

here's today's flow numbers @ 28"

-------Stock intake ------- ported intake ----- difference ---- ported exhaust.
.100-------68------------------73.5-------------------5.5-------------67.1
.300-------217.4--------------224.1-----------------6.7-------------191.4
.500-------337.3--------------342.1-----------------4.8-------------260.8
.700-------331.9--------------387.9-----------------56.0-----------268.9
.800-------345.9--------------397.3-----------------51.4-----------272.1

Now a few notes about this, .. I used a 50 degree intake seat angle, ..
using a 45* might help the low numbers more but hurt the high numbers,
but I didn't get to try.

The exhaust still had the 45* seat, and exhaust ports seem to love 50*
seats, .. I'm going to try that to see if it helps it more.

Cheers gang, ..

Curtis

MadBill

Hmmm...
Flow bench results certainly speak loudly. On the other hand, there's been discussion on the Forum about the need for good low lift flow in the engine, to allow a late EVO to harness as much energy as possible while retaining adequate blowdown and good scavenging around TDC.
(FWIW, Dynomation usually calls for significantly less EVL than IVL and it almost always shows a considerable sag in the port velocity when the lift is highest, between the blowdown phase and the overlap period.)

Also, on the intake side there are proponents of using 30 degree seats to support low lift flow. David Vizard points out that at 0.050" lift, a 45 degree seat has a flow 'gap' of 0.035", vs. 0.043" for a 30 degree. To get the same curtain area, a 45 degree seat would need to be on a 2.48" valve to match that of a 2.02". Vizard uses a seat contour which I have confirmed has negligible impact on high lift flow compared to typical 45 degree cuts. He says good low lift flow allows a broader power band via shorter intake duration, as the good low lift capacity doesn't choke off the flow as badly when IVC is a little premature at high RPM.
Food for thought...

BrentB@TEA

30 degree seats don't work in the real world, Picking a cam or tuning on an engine that has excessive low lift flow is a nitemare that doesn't seem to pay off. Looks good on paper it just doesn't pan out in the real world.
JMO

Ferocity02

iTrader: (60)

So from the factory, would the LS7 heads like a low lift cam (.625" range) with a traditional split and lots of duration? Still trying to see how these heads are going to work. Thanks

RFD

Bill,

Interesting points, .. interesting post.
Man O man this looks like it's going to turn into one interesting discussion. ;-)

Yes there is a big discussion of low lift flow, .. and as we all know it's important.
But there are also other considerations, .. like blow-through during overlap.

In my dyno testing on engines like the DRCE or SB2.2 too much low lift
flow has netted less power, .. showed a good VE# on the dyno but some
of the VE# was going out the exhaust.

This is one reason the racing industry has migrated towards steeper
valve angles. ( there are other reasons as well )

Even in a cylinder head I build for a race series that limits valve lift to .347"
a 30* seat showed a flow gain but a loss of power. A 45* seat showed to
make the best power on the dyno and on the track, .. on a low lift engine!

I have found that the throat area and top and bottom angles on the valve job
play a much greater roll in the discharge coefficient and low lift numbers
then just the seat angle it's self. While curtain area size is important, ..
the efficiency of the curtain area or "flow window" seems to have a greater
impact on performance.

It's obvious to me there's more to the flow dynamics then just raw flow numbers.

case in point, .. the high lift turbulence seen in the LS7 head.
The air separated at a lift of around .550" to .600" on my bench.
In a dynamic situation I've had engines where the lift was as much as .100"
below this point and showed a power loss.

We had an engine with a set of 18* heads CnC ported by Mike Chapman, ..
VERY nice work, .. but the short turn separated at .800" lift.
The cam we used was only .680" lift. We reworked the heads and got the turbulence
out and didn't effect flow numbers in the lift range, .. the engine picked up
over 35 HP on the dyno.

Bill you said the flow bench results speak loudly, .. what else do you see?

Good conversation, stimulating.

Cheers,

Curtis

DAPSUPRSLO

iTrader: (14)

RFD, I think I understand the logic behind the head which stalls out at any lift costs power as the vacuum seen on the engine is much much greater which will cause what stalled out at 28" of depression to stall out much sooner at 100" or whatever depression is seen while operating on the motor. Now, that being said, if one is to run a much much more efficient intake setup which will not restrict the incoming air charge as much wouldn't this cause the intake port of the motor to see less depression then one with a more restrictive intake setup. Wouldn't this effectively reduce the problems that arise by running a head which eventually stalls out on flow at 28" of depression even with a cam that never produces gross lift past the stall point at 28"s. Just a question, you guys sound like the one's to ask though.

treyZ28

flow numbers aren't the half of it. dont start flow bench racing just yet

RFD

The simple a quick answer is NO, .. regardless of the intake the choke
or sepperation will still happen in the head.
The intake can make it happen sooner but rarely make it later.

I'll explain more later, .. right now it's late and I gotta get some sleep.

The biggest thing with these heads besides the flow numbers, ..
as the post above points out, .. is the velocity and cross sectional area, .
and wet flow.

There IS MUCH more to a cylinder head then CFM flow numbers!

Curtis

treyZ28

The modern internal combustion engine is way too dynamic to look at flow through a single port under a single, constant pressure differential and declare a winner.

The only thing flow numbers are good for is knowing if you fucked up or did better. IE: If your cross section remains the same and velocity numbers go down- you fucked up.

If you port them out and velocity goes up, good work. Its better than before. Not better than the head next to it, in front of it or on top of it- just better than before.

dont get me wrong, A 220cc heads, one flowing 120cfm at .550 lift intake side is clealry fucked. and one flowing 300cfm is probobly better. Its ok for general deductions and such- but flowbench racing is a no-no when its even remotely close.