Port CFM for Simulations

[GARY C]

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

Is the sim software set up for CFM of the head or head/intake?

[skinny z]

There is a data point for minimum port CFM. It can either be an input and I would enter my flow bench results or it can be a result and other data will determine the minimum CFM.
But seeing as it's listed as port flow it could either with or without a manifold I suppose as there's no distinction.
I may have to approach the software developer for an outright answer.

[Stan Weiss]

OH if you use 254 cfm and it picks a cam and then you use lets say 247 cfm and it picks a cam. Just how much difference is there in seat-to-seat duration and 0.050" duration between the 2 cams?

Stan

[skinny z]

OH if you use 254 cfm and it picks a cam and then you use lets say 247 cfm and it picks a cam. Just how much difference is there in seat-to-seat duration and 0.050" duration between the 2 cams?

Stan

10:1 383. Single pattern spec.

254 CFM: 284/284/106 LSA/102 ICL. Peal HP RPM 5610.
247 CFM. 279/279/106/102. Peak HP RPM 5370.
5 degrees in the overlap as you can see.
The .050" value isn't specified. That's a question for another day too. (Endurance vs intensity)

[GARY C]

There is a data point for minimum port CFM. It can either be an input and I would enter my flow bench results or it can be a result and other data will determine the minimum CFM.
But seeing as it's listed as port flow it could either with or without a manifold I suppose as there's no distinction.
I may have to approach the software developer for an outright answer.

If it's like most programs the min/max is still based on head flow alone, I have not seen any that take into account the intake although I think they should and there are probably some that do but maybe more advanced then what I have dealt with.

[digger]

It can be a bit flawed to enter head cfm curve and intake geometry as though they are not coupled together. If the alignment is good it’s ok. but if the port is pointing at the ceiling and the manifold is to it’s not going to be correct

[skinny z]

Simulations aside, that will be something that has to be addressed. When this was first assembled as a 350 with the same heads and intake, I couldn't get a good look at the port / manifold alignment. I'll need a much smaller bore scope as the two plane intake doesn't allow a lot of room for a camera.

[steve cowan]

There is a data point for minimum port CFM. It can either be an input and I would enter my flow bench results or it can be a result and other data will determine the minimum CFM.
But seeing as it's listed as port flow it could either with or without a manifold I suppose as there's no distinction.
I may have to approach the software developer for an outright answer.

If it's like most programs the min/max is still based on head flow alone, I have not seen any that take into account the intake although I think they should and there are probably some that do but maybe more advanced then what I have dealt with.

on Engine Pro you put a flow % for intake manifold,EG - 90 % IS 10% flow loss with intake bolted on.
Then it calculates total runner lengths,min csa,Max csa,plenum volume etc,these are starting points/ recommendations for the application.
Pretty sure pipemax does the same.

[steve cowan]

I have said this before and tested for myself over 12 months at the track.
In a 23 deg head where the Min Csa is the push rod pinch it will stop you dead no matter what you do,if it is to small for the application.
And I am talking stock location not epoxied up raised runner type deal,you can oversize the intake to help slow the airspeed in that area but only a bandaid solution in my opinion.

[skinny z]

...on Engine Pro you put a flow % for intake manifold,EG - 90 % IS 10% flow loss with intake bolted on.
Then it calculates total runner lengths,min csa,Max csa,plenum volume etc,these are starting points/ recommendations for the application.
Pretty sure pipemax does the same.

This particular program doesn't offer that flexibility.

I have said this before and tested for myself over 12 months at the track.
In a 23 deg head where the Min Csa is the push rod pinch it will stop you dead no matter what you do,if it is to small for the application.
And I am talking stock location not epoxied up raised runner type deal,you can oversize the intake to help slow the airspeed in that area but only a bandaid solution in my opinion.

This isn't a question of how good the head is or what can be done. This is as good as it gets, with the exception of ensuring that the head/manifold interface is streamlined
While I haven't measured (but I will), the science says the MCA is less than 2 sq. in. That's extrapolated from the 255 CFM peak.

Somewhere, someplace I've the information regarding CFM vs port cross section. I think against a standard 1 sq in aperture. But I've no idea where I buried it.

[steve cowan]

skinny,
If you don't have pipemax i suggest you buy it.
It gives you camshaft intensity changes for your set up
You can get lost for days on Larry's program.
There is a couple of DV engines filed away there that are in line for what you are looking at.

[skinny z]

I do have an older version of Pipe Max. 10 years old or more. It's my go to header calculator. I've never looked at it much beyond that but I'm thinking maybe I should.
Thanks.

[Stan Weiss]

This is from a program which is still in alpha testing. Hopefully I got all of your information entered correctly. The only difference between the 2 runs is one used your flow numbers and for the other all flow numbers were adjusted and 254 became 247. Both are for peak HP at 5610 RPM.

Stan

skinny_z_254.gif

skinny_z_247.gif

[skinny z]

Those results are very interesting. And it appears your program may be a step up from the one I'm using.

(I'm hesitant to name it outright because the last time I did, my thread ended up in the David Vizard forum. I've decided, seeing as the man has all but vanished from Speed Talk, I've let my Hot Pass subscription expire and no longer have the privilege of posting there).

Anyway, I see your results have largely left the overlap the same. Also the LSA has tightened up compared to my results.
But that there is little to distinguish the 254 from 247 spec poses other questions.
As for peak HP RPM, I'm supposing that's an entry point in your program? I have that option as well but the numbers really get skewed then. I think because mine is centred around a 10.5 CR and once outside of that, it gets weird. At 10:1, some of the results are counter-intuitive although DV gave me an explanation as to why that is.
Thanks for that Stan.

[skinny z]

This is from a program which is still in alpha testing. Hopefully I got all of your information entered correctly.

FTR: My results for that particular simulation run was 383, 10:1, 2.02/1.60 valves, 1.6 rocker. I deliberately inputted 280 advertised and the program responded with a minimum 254 CFM port flow. Along with LSA, ICL and suggested minimum lift.
I've done the reverse and entered my flow of 254 and the duration changes from the previous test.
Hence the question asked in post #1.