Comparing Head Flow Numbers

[Bret Young]

Okay guys.

I need some assistance from some people that know more about head flow numbers than myself.

Looking at two different heads with comparable flow numbers, but one suppliers heads are flowed at 28" of water (DRD) where the other suppliers heads are flowed at 25" of water (CB). I was just wondering if there was a formula where I could put these on an equal playing field to see how they compare if flowed at the same rate.

Here are the numbers.

DRD (28" water)
.100 - 54.4 CFM
.200 - 104.1 CFM
.300 - 145.5 CFM
.400 - 171.5 CFM
.500 - 184.5 CFM
.600 - 194.2 CFM

CB (25" water)
.100 - 56 CFM
.200 - 97 CFM
.300 - 130 CFM
.400 - 154 CFM
.500 - 168 CFM
.600 - 174 CFM

Will changing the inches of water that flow benches are using make a significant change in the results? Just trying to see how these two would compare if they were tested on an even playing field.

Any incite from those that understand this better is greatly appreciated.

Thanks!

[Bret Young]

Did some digging on Google after my post using different keywords trying to find additional information.

Here is a thread that I found. You mathematic guru's can verify if this is correct or not:

http://www.speedtalk.com/forum/viewtopic.php?p=122973

If the 25" CFM x 1.0583 = corrected flow numbers at 28" then this would be the corrected numbers.

DRD (28" water)
.100 - 54.4 CFM
.200 - 104.1 CFM
.300 - 145.5 CFM
.400 - 171.5 CFM
.500 - 184.5 CFM
.600 - 194.2 CFM

CB (Corrected at 28" water)
.100 - 58.96 CFM
.200 - 102.7 CFM
.300 - 137.6 CFM
.400 - 163 CFM
.500 - 177.8 CFM
.600 - 184.1 CFM

CB (25" water)
.100 - 56 CFM
.200 - 97 CFM
.300 - 130 CFM
.400 - 154 CFM
.500 - 168 CFM
.600 - 174 CFM


So this shows that the DRD head flows a little more than the CB heads that I currently have on hand. I wonder how much the additional 8-10 CFM that we see across the board will translate in output.....hopefully not much.

[helowrench]

roughly 5-8 hp based on a 2276 size engine that I was doing a bunch of research.
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unless they are done on the same bench, it's like comparing dyno #s IMO.

Were both done with intakes on or off? Carb on or off? Radiused entry on or not? Were they the same intakes (doubtful)? Ported or non-ported intakes?

The beauty of the "same bench/dyno" is that normally an operator will do things the same. If they are done by different places, you have a lot of questions to have answered to find out if the #s are comparable or not.
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[Bret Young]

[mark tucker]

Ive repeated flow bench numbers extreamly closely from what I was told they were. but the operator can fudge/alter whatever depeing on thier integrity and who is selling what or paying for what. but I would think both the drd and thwe cb heads are what they say they are but are flowed at diferent rates for selling or whatever. or possiably the bench used would not support that flow rate.just use the conversion factor , it should be good.

if you are flowing heads @ 28 and get a set that will not pull 28" then you go to 25" and retry.then to 20' if they will not pull the 25" a head that wont pull 28" will outflow the head that does pull 28". dont wory about what the actual flow#is when iot's all converted so you can compar it's good to go with. but !!!! a humongus port can out flow a small port but not perform well due to low airspeed/port velosity. A propely sized&shaped port for the size engine is better than a big higher flowing...in most cases, but pure racing can be a little different depending on many things.weight,gear ratio,camshaft ,cr, etc. in short get the best head for the app. I like the cb super pros,good sized and shaped port killer flow,high velosity,and the mini wedgeport too just a little smaller port. but great flow & velosity also. choosing the right head can be a headache.there are lots of varaition's recomendations,prices,and buddys thought&gotta haves. but remember in the end it's yours.choose wisely.

[miniman82]

Short answer: If you aren't using a cam that takes advantage of the high-lift airflow on either set of heads, you're missing the point. You need a cam and rocker combo that's going to lift the valve rapidly as close to .600" as fast as possible, which probably means a 1.4:1 rocker with an FK series cam. That also means your HP goal is pretty far up the rev range as well, so as long as that's what you're looking for you'll be fine.

If the parts you choose are well matched, 200 CFM will net you very close to 200 whp. But for a street engine, my preference will always be TQ and power under the curve.
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[Quokka42]

The logic behind that fudge factor was poor, figures produced at different test pressures will not correlate linearly like that. This is one reason there is not a standard - you can often make your heads appear better by "testing," or claiming to test at a higher vacuum. Of course different benches and methods produce different results as John pointed out. Retailers will usually pick whatever makes their heads look the best from all these options, then if they still aren't good enough will often just fake a couple of figures.

A flow bench can be a useful tool in flowing heads for R&D but these days marketing people are too good at BS for valid comparisons to be made between manufacturers.
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and then there is the issue of when a "higher flow" head makes less power. It's NOT just airflow #s.

Note that a flow bench is a statically environment (steady state flow), but in an engine the valve is opening/closing many times a second, and the piston is moving up/down a lot too. Is this a static environment like the flow bench? Hardly. The 2 are completely different.

That's not to say that a head with higher #s won't make more power, but it is absolutely no guarantee of it. Buying solely by flow #s is a mistake, a very common one, but still a mistake.

Flow bench #s do not account for reversion, nor combustion characteristics for example.
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[Quokka42]

True, and do not show port entry angle and velocity, swirl and a number of other factors. Graphing the supplied figures, though they are few in number, sometimes shows interesting anomalies, which are either freak effects occurring at particular air velocities, or fudged figures. Ideally you want to map your heads with your manifolds once you get them in case they need any changes to suit your setup - a rather simple flow bench setup is OK for this, as you don't need actual numbers.
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[Dan Ruddock]

Port efficiency is the key. As Mark pointed out air speed is as important as flow. With so many things in engineering the word ratio comes into play. Air flow vs air speed.

Air speed is what overcomes cam duration and is what keeps filling the cylinder even after the piston is on it's way back up because of inertia which is air speed driven.

It's all about balance which gives you the widest most useful power band.

Dan

[helowrench]

[Quokka42]

Port speed is a term that is often tossed about, but rarely understood. Velocity of the gases varies greatly throughout the port and the maximum speed is a limiting factor in flow, though the velocity and direction of flow is important as it passes the valve.

You really can't determine any of this from flow numbers.
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[mark tucker]

well if you have the cc of a port and the flow, and one set of heads has 80 cc ports and flows200 cfm@ 25" and another set that has a runner cc of 70 cc and flows 200cfm @ 25" it's kinda easy to figure out the one with the air going faster and port size in the intakes is also a factor in power&torque output and also rpm ranges. and to farther complicate this a combo that works great on one size motor may not be the hot ticket on another size motor.killer heads dont always make killer motor.and sometimes it's not a lot of cc's that can make them perform the way they should, this also go's for camshafts. a 2000 cc engine is a lot diferent than a 2300 cc engine. choose the parts for the app and the size engine your building. thats where cb's multie head line up shines, they have heads for pretty much every size cc engine you could ever want/need. I havent counted but I think they have oh so many head varaitions&castings it could give you fits trying to figure out just what you need/want. not just 3 stages of heads to fit all.just like condoms,you need one that fits just right.