i stumbled across this article this morning and was trying to break it down in my head maybe someone can explain a bit further.
the main part of the articvle is this...

Here is a 4-year old quote from a great head porter and engine guy, Larry Meaux:



"Peak_HP = Flow_CFM * .257 * Number_of_Cylinders

This is the estimated potential Peak HP to expect.

You multiply 87 percent (.87) times cam's theoretical max lift , round off to nearest .050" in Flow Test, then see what CFM is at 28 inches of H2O.

example=> .700" Lift cam:
.700 Lift times .87 = .609" Lift
Flow head at .600" Lift , then take CFM at 28 inches and calculate HP potential with above formula.

.257 Factor = for beginning engine builders and engines near 10.0:1 Comp Ratio

.285 Factor = would be for Professional engine builders with wet sump pans, lightweight rotating assemblies, low tension great sealing rings, deep oil pans, etc. and excellent use of inertia/wave tuning with 9.5 to 11.5:1 Comp Ratios or 11.5 to 13.0:1 CR ranges without fully utilizing inertia/wave tuning effects

.300 to .310 Factor = Current ProStock Technology with dry sump, unlimited carburetion, High Comp Ratio, ultra lightweight rotating assembly, etc, and maximum use of inertia/wave tuning, etc, 14:1 to 17:1 Comp Ratios.

(usually no better than .3200 efficiency or no worse than .2980 eff %)"..

i understand the concept of it all for the most part, what i dont understand is how they can use this one set of numbers for all engines. i mean a 350 with stock bore at 4 inch compared to a 400 with stock bore of 4.125. the bigger piston will push more air making the flow numbers change correct?
when you are looking at the dozens of different heads to buy and flow numbers on them all. you cant just use that set of flow numbers and decide okay, this will do that and so on. by the way i noticed one thing about afr. they flow test with a .060 bore. compared to most others using a .030 bore. i thought that was kind of funny

 

IIRC AFR built a correlation from their data and the numbers I believe are very close to those as well. When it comes down to it, flow is flow. The numbers on the head are what the port in the head can support. So it is giving the max potential of that head. As far as bore size... it has an effect but the head is the limiting factor in this case so it assumes that. Its a pretty accurate correlation from what I've been told.

 

hmm yeah , i mean i have seen a couple video's on it now, one of them from you tube from a guy named vintage iron, he for sure sounded very knowledgable and was obviously working in a machine shop because he kept refering too "when we" flow heads here in shop.
pretty interesting stuff

 

Vintage iron is the guy on stuff like that. I think he even said dyno numbers are pretty spot on too.

 

If you flow the head correctly, you put the correct bore size on the flow bench. Especially with big valves, the edge of the port closest to the bore wall is going to get somewhat shrouded. The flow numbers would thus be somewhat different for the same head on different bore diameters.

 

yeah he did say that 84, so i am looking at the brodix ik 180 heads to replace the 113's. i could pretty much use the flow numbers on brodix web site using the calculations given there and be really close to what my wheel hp would be from what i understand that equations gives you the wheel hp not crank hp. from what i figured earlier doing the math is with a comp cam i have been looking at, extreme energy 282 hydraulic roller i should be right around 360 wheel hp. how would you figure the torque from that now? because i am fine with that number for hp.. i am getting confused because according to that it doesn't even matter what intake you use, carb or tpi..it goes against everything i always "thought" i knew i guess

here is what i have pretty much decided on as far as engine upgrades after months and months or reading and trying to put together a mid range torque engine. keep n mind, i aquired this car with the tpi gone.
1. comp cam extreme energy 282
2. edelbrock performer rpm air gap
3. brodix ik 180 heads
4. holley ultra hp 650 cfm
5. hooker headers without egr crap
6. pypes stainless steel x pipe true dual exhaust system

either have the heads milled down to keep the 58 cc chambers or use the thinest head gasket i can run safely too keep it as close to original compression ratio as possible.
now these heads from what i can tell only flow about 72% efficency for intake/exhaust so i will for sure want a cam with dual pattern around .20 more than intake to bump that % up some right? maybe even run a 1.6 on exhaust valve and 1.5 on intake?

 

I'd say 360 is the ball park. As far as torque... well thats a whole different story. You'd probably be looking on a 350 for it to be pretty square. 360/360 sounds reasonable. I seem to notice torque doesn't change so much as more so where the peak is imo... look at a lt1 vs a l98, torque is about the same just where in the rpm band it occurs changes

And the equation is within reason of course. You put a tiny intake and carb on it and it definitely won't make that power. But you put on something reasonably sized and it should be close.

 

You can't calculate torque as accurately because it varies more based on the bore and stroke combination, and not just the flow rate of air through the heads.

 

hmm yeah 84. that formula said something about 5250 didnt it? where hp and torque meet.. so you could probably expect higher torque numbers below the 5250 area and higher hp above it?

 

oh right, i forgot that part , ie. 400 is known for mountains of torque from its 3.75 stroke and stuff.... see i dealy my goal is to slowly get all of these parts together. this year.. then next winter. i plan on going with a scat l9000 rotating assembly and making the 400 i have tucked under the bench in my garage into a 406 with around 11-1 compression using all of the parts i buy this spring. i would get it done now but many things are in the way, new clutch set up for 400. machine shop time especially with this covid crap going on. i would imagine my 1 machine shop in town will be backed up for most of the summer now already.. but mainly. my wallet is only so fat and about to go on a big diet. story of my life though, i have all sorts of good plans. just me and my wallet dont see eye to eye usually