I run AFR 195cc LT4's on my stock LT1 shortblock. They're fully ported, have a final port volume of 203cc, and flow 280cfm @ .6 lift. I run the heads with a 233/239 .569/.577 shelf Comp Cam and think of the car as streetable. Nevertheless, I've been told that my heads are "pretty big" for an LT1. AFR 210's start off at 203cc, and AFR cnc's them to ~210cc. I've been told AFR 210's are just "too big" for a 350ci motor and that AFR 220's are just out of the question.

Now think about LS style heads. My friend has some Patriot Performance LS6 heads that he did some touch-up on and they are pushing 230cc and about 325cfm. Lingenfelter ports LS6 heads out to somewhere in the neighborhood of 245cc. No one says 230cc is too big for stock cube LS1's. In fact, their "small" 5.3L heads end up fully ported coming out to about 210cc and flowing about what AFR 210cc LT4 heads flow.

So . . . does it make sense to compare LT1 and LS1 heads in terms of cc's or not? If so, why don't we see more AFR 220 and 227cc heads on stock cube LT1's?

 

in my opinion, anything over 200cc is too big for a 350ci(or 346) engine under 7000 rpm.

i know of a cammed LS1 fbody (stock heads) that out performs other fbodies with the same setup, even the same cam, but ported heads. and i think it's because he shifts at 6400 rpm, and the guys with the ported heads shift around 6600 (because they're scared to push the stock bottom end). but they're shifting too early.

 

you have to compare Apples -to- Apples =>

since the LS-6 intake port's CenterLine Length is "longer" than the AFR 210cc
intake port CenterLine Length....the LS-6 head will hold 7.2 CC more port volume "even" if both heads were same exact cross-sectional, shape, velocity
(which isn't really the case, but an example)

so you have to look at or say the LS-6 is really 245cc - minus 7.2 cc when comparing -VS- AFR210cc ,...it then becomes 237.8 cc -vs- 210 cc, (on equal terms of port CenterLine Lengths)

a 327.8 cc head flowing 325 cfm (2.717 avg cross-sect area) = 287.1 fps

a 210.0 cc head would have to flow at least 287.1 CFM (2.40 area) = 287.1

the 210 cc AFR has to flow at least 287.1 CFM to equal the average port velocity of the 237.8 cc (245cc) port volume LS-6 head

so anything above 287.1 CFM out of a pair of 210cc AFRs, and there is more port velocity in the AFRs, if not, there is less port velocity in the AFRs
--------------------------------------------------------------------

so do you think that "if" 2 heads have the same port velocity they will make the same HP/TQ , all else being equal ???

the LS-6 325 cfm 287 fps port velocity head is going to make more TQ/HP

than the AFR 210 287.1 cfm / 287 fps port velocity head

same port velocities , but different flow

325 CFM * .257 * 8 cylinders = 668.2 hp
287.1 CFM * .257 * 8 cyl = 590.3 hp
----------------------------------------------------------------------
5.000 " Port CenterLine Length =>
300 CFM port @ 2.400 Area = 300 FPS port velocity (616.8 hp)(197 cc vol)
500 CFM port @ 4.000 Area = 300 FPS port velocity (1028 hp)(328 cc vol)

the Cylinder head with the largest port volume and Flow will have to turn higher RPM (Piston Speed) to create the same inertia effect at the Intake valve closing point, but will ultimately make more HP
----------------------------------------------------------

now all that typing above was just theoretical Math ,
using calculated average cross-sectional areas, and calculated(not measured)
port velocities

in reality you have to use a Pitot Velocity Probe, and probe and port map the velocity inside the induction system path

its very possible to have 2 heads have identical 210 cc port volumes , same identical flow CFM numbers, but have "drastic" differences in port velocity in certain areas of the port.

i see this all the time with NHRA SuperStock heads
example=> 041x SBC 165.0 cc port
everyone in the USA has just 165.0cc to play with,
and you can have 10 pairs of #041x heads all at 165.0cc and have the very same exact Flow CFM numbers on both intake and exhaust sides ,
yet , those 10 pair of heads can all produce different HP/TQ,

i've personally seen 75 HP "LOSS" between 2 identical pairs of 041x heads,
same Flow numbers on Int/Exh, same 165 cc port volume CC's,...
the only differences showed up when you actually probed the head with Pitot Tube...you have to use a Pitot Tube to know the real port velocity !
---------------------------------------------------------------

LS-6 specsin ported shape, not stock)
Intake Roof= 6.480" Floor= 4.525" Port CenterLine Length= 5.503"
Exhaust Roof= 4.385" Floor= 2.400" Port CenterLine Length= 3.393"

AFR 210 cc specs: (in ported shape, not stock)
Intake Roof= 6.165" Floor= 4.500" Port CenterLine Length= 5.333"
Exhaust Roof= 3.865" Floor= 1.935" Port CenterLine Length= 2.900"

even if both heads had the exact same "average cross-sectional areas" ,
the LS-6 heads will always hold more Port Volume CC's on both
the intake and exhaust sides because the LS-6 ports are longer !
--------------------------------------------------------------------

another example=> NHRA ProStock
would anyone in their right mind recommend more than a pair of 360cc Darts
or anything more than 380cc port volume for a 500 cid BBC serious Bracket Race engine. yet ProStock heads have approx +100 more CC's port volume for 500 cid BBC (480cc), and make a ton more Torque and HP than any 360cc port vol Dart on a 500 cid engine

the 485cc port volume is also flowing about 555 CFM (4.228 avg area=315fps)

the 360cc Dart heads have to Flow = 472.7 CFM just to equal the same
port velocity....you're not going to find a pair of Dart 360cc heads that Flow a "real" 472.7 CFM on a legitimate reading FlowBench
maybe more like 380 to 400 cfm range instead, and thats
only equal to 266.6 fps port velocity for the Dart 360cc

so a Dart 360cc BBC head has a "lot less" port velocity than a 485 cc ProStock head

you shoudn't judge a Cylinder Head's potential port velocity on Port Volume CC's alone !!

 

Larry's entire post is excellent. Reread it a couple of times, and understand it.

Thanks, LM.

 

OldSStroker...

here's a few more Port CenterLine Lengths =>

i'll post more on my Website in future

LS-1 (ported shape)
Intake Roof= 6.400 Floor= 4.575 Length= 5.488
Exhaust Roof= 4.240 Floor= 2.390 Length= 3.315

GM # 12555690 LT-4 (ported shape)
Intake Roof= 6.285 Floor= 4.580 Length= 5.433
Exhaust Roof= 3.840 Floor= 1.915 Length= 2.878

GM #034 cast-iron Bowtie heads (ported shape)
Intake Roof= 6.200 Floor= 4.620 Length= 5.410

GM #292 cast-iron Turbo heads (ported shape)
Intake Roof= 6.090 Floor= 4.300 Length= 5.195

GM Dart-Buick alum 10 deg SBC (ported shape)
Intake Roof= 7.200 Floor= 4.770 Length= 5.985

GM Chevy 18 deg SBC alum (ported shape)
Intake Roof= 6.800 Floor= 4.370 Length= 5.585

ProAction/ProTopLine cast-iron 285cc 14deg (ported shape)
Intake Roof= 6.860 Floor= 4.475 Length= 5.668
Exhaust Roof= 4.430 Floor= 2.510 Length= 3.470

GM SBC Canted-valve alum (ported shape)
Intake Roof= 7.420 Floor= 4.570 Length= 5.995
Exhaust Roof= 4.410 Floor= 2.300 Length= 3.355

 

Thanks again, Larry. Bret will have good use for these.

 

So basically, the LS1 guys can run more port volume because the geometry of the runner is such that, for the the same total CFM, they maintain a higher velocity. In order to get LT4's to flow 325cfm, you might end up sacrificing port velocity, which in turn would mean that you'll have to spin the motor higher relative to an LS1 headed motor flowing the same amount of air? I assume higher port velocity has the effect of making larger heads (bigger port volume) more "streetable" ???

 

I'm not sure if they're "optimal" for a 350-355ci motor, but most fully ported LT4 heads end up with runner volumes in the 210-220cc range and they are very streetable. One thing to consider is that a 280-290cfm LT4 head is not going to do you much good with the stock (unported) LT4 intake.

 

The 3 1/2 inch runner length don't do anything for HP either,if you don't intend to turn it 8500RPM's.Another 1 1/2" runner length will do wonders for low in power and torque and driveability and make plenty at 7000RPM's.
Talk to Bret Bauer about his intake,NiiiiiiCE unit. Don't forget about your filter,TB and CAI.

 

The important thing is the flow number vs the cross sectional areas of the ports not the volume really. If you know the flow curve and how and where it backs up you have a good idea of what the head will do in terms of hp and rpm. The put your intake on and see what the heads do as well as sometimes they don't aways do what you think. Then you can add the runner tuning and cross section of the intake into your equation for predicting what you want to creat power in the rpm band that you have chosen. Just because you have good flow for a certain valve or intake port cross section doesn't mean the head might not still be too big for your application.

MAKE the engine your biotch, not the other way around. DECIDE what power you need to make to run the numbers you want to be happy with the engine and then go about creating that power with the right parts and the right combination. Larry comments are excellent but I am only shortening them down to say that without port length volume doesn't say anything and then it still doesn't say anything without a flow sheet. You can have heads that have lots of volume and won't turn any rpm if they don't flow anything. I can take volume out of several heads and pick up power way up high if the place they are big and tight is all wrong. The flow bench can see this when you turn it up and know what you are doing.

 

for another example, just take a look at Reher-Morrison's Engine selection
HP & TQ numbers on the various BBC engines they offer

their
509 cid = 805 HP & 675 TQ Brodix BB-2+

522 cid = 845 HP & 680 TQ Brodix 2X

540 cid = 855 HP & 680 TQ Brodix 2X

555 cid = 910 HP & 750 TQ Dart Pro 1

565 cid = 985 HP & 775 TQ Dart Big Chief 18 deg

finally,
572 cid = 1100 HP & 835 TQ

you have to go all the way to a 572 cid to make as much or more Torque,
than a 480-485 cc vol. ProStock Cyl Head on a 500 cid engine

the 480 to 485 CC ProStock heads are just about capable of making
800 to 830 Torque on 500 cid (1300 to 1360 HP ranges)

their
509 cid = 805 HP & 675 TQ Brodix BB-2+ 312 to 320 cc port volume
500 cid ProStock engine 1360 HP & 824 TQ with 485cc port volume heads

824 - 675 = 149 TQ difference & about a 2000 rpm difference in RPM range

and 555 HP difference also

the ProStock engine might still have more TQ & HP at 7000 RPMs than the
509 cid / 805 hp/ 675 tq engine

RPM------TQ------HP---ProStock curve
6800----672.1----870.2
6900----688.3----904.2
7000----704.0----938.4

 

and i always thought it was thee cross section that was the big deal

 

That is mostly what you want to know as when you know the port length then you really know how much "bigger" the ports really is in cross section.

 

yeah...the Cross-sectional is still the "big deal"

if you know the Port's CenterLine Length and the advertized Port Volume
then you can estimate the "average cross-sectional area"

once you calculate the average cross-sectional area with that method..its still just an estimate...the port velocity should really be measured with a Pitot Probe instead on a FlowBench, developing a Port velocity profile at various depths and port locations

and once you calculate the "average" Cross-sectional area by knowing the Port's CenterLine Length and Port's Volume CC's,..you then can calculate the Port Velocity in feet per second when you also know how many CFMs the Port flows.

Port Volume "should be" just the "result" of the correct Cross-sectional Area for the desired engine RPM range.(with a given port centerline length)

 

Bret,

all on a 4.000 Bore Flow Fixture at 28" inches test pressure

the following are for a pair of Legal castings # 10239906 for a Chevy SBC 377 cid legal size engine in a certain Class DragBoat that was slightly faster than MPH record near end of year 2000

1.940 int / 1.500 exh 4.000" Bore Flow Fixture @ 28 inches SF-600
as run shape, stock factory GM shape,.... as it came from Sweden
Lift---Intake---Exhaust
.200--121.4----98.2
.300--179.4----123.8
.400--219.2----137.5
.500--220.8----145.9

after legal 3 angle valve job and anything else you might get away with
Lift---Intake----Exhaust
.200--134.0----106.7
.300--187.3----141.7
.400--223.1----157.1
.450--234.6---------
.500--220.8----163.1

10239906 as cast ,
unported Port CenterLine Lengths= (170 CC Volume)

Intake Roof= 6.185 Floor= 4.770 Length= 5.478
Exhaust Roof= 3.865 Floor= 2.100 Length= 2.988

equates to an "average" Cross-sectional Area of 1.894 sq.inches
(if you use no Port Corner Radius correction factor)

220.8 CFM @ .500" Lift = 279.8 feet per second (theoretical)

but using Pitot Probe to actually measure Port Velocity
there are spots in Intake Ports above 350 fps, and some spots much lower than 279.8 fps