Port CFM for Simulations

[skinny z]

Hi All

I'm running some software simulations for engine development.
I've a set of ported Gen 1 SBC heads with a peak intake of ~254 CFM (@.500")
That was on a bench with a 4.03 fixture bore.
My question is what would be a reasonable value to use as my input criteria once there's an intake manifold in place? The manifold at this stage is an RPM Air Gap. I suppose too that the carburetor could be considered. In this case it's a 750 CFM Barry Grant with vacuum secondaries.

Thanks in advance.
Complete head flow chart below.

RHS AMS.xlsx

[Stan Weiss]

I just enter the flow numbers and lift figures into the simulation software. I then pick / adjust the intake manifold to what will be used.

Stan

[skinny z]

Thanks for that Stan.
So, you're saying you don't take a few points of the port cfm with the inclusion of a manifold?
Seems to me that there are some flow bench stories here about flowing heads with intakes on. But I don't recall the results.

This particular cam profile software doesn't allow for any data regarding the manifold itself. Just minimum peak port CFM required relative to RPM and CID, etc. Or vice versa.

[steve cowan]

If you bolt airgap to a head and test on the bench the flow loss is terrible, if you look at porting a airgap I say good luck as I would not bother.
I swapped from a as cast airgap with open spacer to a as cast victor jnr, not port matched on either just fit as best as possible and the vic was worth 2 tenths and close to 2 mph in the quarter,this is on a mild 383 sbc approx 430hp.
Airgap is great in a mild application for the street where rpm is low.
Look at any decent induction tract on a 23 deg sbc- look at what goes into a half decent intake manifold there is a lot of work because line of sight is crap on 23 deg head and that's the challenge.
Forget the simulation stuff,grab a pair of dividers,ruler ,sharpie pen,calculator, some pitot tubes and a bench will help and get busy.
Remember I said a while back about time running away tic,tic,tic

[skinny z]

Yes sir. Time is ticking but this simulation stuff is how I pass the time. It may be a while yet but the wheels are in motion at least.

Anyway, that the Air Gap intake won't allow the full potential of the port, as you've stated, is the info I was inquiring about. Not sure how I'll incorporate that into the program though.
On the physical side, I do know there's a considerable mismatch is the port shape and size between the head and Air Gap. Something I will address this time around.

Question. What RPMs was that 383 you're referencing?

This build will re-use those heads. As they are. That kind of limits the peak RPM HP to less than 6000. I've thought about the Vic Jr as an option but that would be after it's up and running and I can get a gauge on the performances. But at less than 6000, I'm not sure how much benefit there might with going to an open plenum. Not that I wouldn't try it.

But I hear what you're saying.

Now back to the simulator.

[steve cowan]

I found shift points around 6000 to be the best and I track tested from 5800-7500 rpm
The airgap was quicker in the 1/8 mile and vic jnr was quicker and faster after the 1/8 mile.
For sub 6000rpm the airgap is good.
My 383 was/ is still CSA limited in the intake port.avg csa of 2.05" 182 cc
With a port matched vic jnr the engine peaked approx 6300rpm.
The heads and intake I am reworking again at the moment as I want peak a couple hundred rpm higher say around 6500 rpm.

[skinny z]

Small heads here too for a 383. What started out as an advertised 170cc runner has since been lightly ported ( mostly SSR and some bowl work) so maybe approaching 180.
With the measured CFM, it all points to RPMs less than 6000. So there's that Air Gap vs Vic Jr thing.

And then what of the Air Gap killing head flow? I'm going to continue to simulate using the flow bench data. I can't say how it will affect cam selection or anything else that follows.

I will measure the MCA when I get them off the 355 for an inspection. Just for information purposes if nothing else.
It may well be that they end up on the shelf if the last miles put on them wore them out. I hope not because I'm bound and determined not to spend another nickel on these things.
New heads will set this project back even further ( approaching 2 years and counting). Didn't someone say we'll all die of old age before this lump ever gets built?

Thanks as always.

[PRH]

Let’s say we’re testing a 572” hot street motor. 10:1, .650 lift solid roller cam, ported heads, “1050” 4150 carb.

We’re going to test two manifolds on the dyno. The heads flow 350@.650 lift with a radius plate.
With a race type single plane the head flows about 340.
With a large dual plane the head flows about 310.

How much difference in power between the two manifolds when the motor is tested on the dyno?

[skinny z]

80. At peak. Although have you outpaced the capacity of the dual plane so peak head flow is no longer the deciding factor?

[skinny z]

But the question then is, do you change the cam spec between manifolds (I'd venture a guess and say yes) or even from the baseline port flow numbers?

[PRH]

The actual difference was 28.1hp.

Tested back to back.

715.9 for the single plane, 687.8 for the dual plane.

Peak hp @100rpm apart.

Peak Tq 6.4ft/lbs difference.

[skinny z]

So, does imply that one can input the head flow data into a simulation program and not have to discount the fact that there's an intake reducing the overall flow potential of the heads?
It's certainly easier in the virtual world if that were the case.

If we were to take your example and put it more into the context of what I'm building, could we more or less say that the results would be, on a percentage basis, the same? That context being your everyday average 475 HP 383 that makes peak HP at 5500 RPM.

[PRH]

All I’m saying is........ sometimes the manifold attached to the head hurts the flow numbers on the bench more than it hurts the hp numbers on the dyno, or the performance in the car.

Also, the higher flowing one, even if it makes more power, may just not be well suited for the application(like a ported SV vs an EPS Performer on a motor going in a tow truck).

[RDY4WAR]

I have PT Engine Analyzer and wonder this myself when inputting intake specs. I like to just goof around with it with numerous different combinations of heads, cams, intakes, etc... There's no way to really know how much a particular intake will restrict the flow of a particular head when bolted to it on the flowbench without actually putting it on the flowbench. I just take a wild guess at it and hope it doesn't throw off the results too much.

[ozyfordman]

I just flowed an Assault air gap on a Vortec head that flowed 256cfm@ .5”. Worst port through the manifold went 230, best 240. I hope this helps