advertised head flow figures, any specific way of measuring on the bench?

[JIMP]

O.K. I was thinking if there is an easy answer to this, I ended up thinking that probably no, so I write down an experience, some of you maybe you're familiar with this but for me was a rather unpleasant "surprise". I had a set of handmade 044 heads and manifolds, on the bench they were tested flowed 157cfm at 0,6" lift and 25" height, I needed something bigger than this as I wanted to boost a little bit the power -for street use- , I ended getting some ready ported heads and manifolds from known builder, advertising at about 200cfm in the same conditions, so I thought that this should boost me there I was expecting (with acompanying changes in cam, comp, exhaust etc) which didn't happen in a clear way -only 7HP more but with slightly lower torque- so I was curius to see why, I dismantled the engine and decided to flow the heads, well in the same conditions with their match ported manifolds they made 164cfm! and the best I could do was to put some clay in the intake and that went us to 175cfm, so far away from what is advertised. Now my questions are
-is there any different way they use to flow the heads and avertise the figures? maybe with complete intake length including venturi, horns etc? because this cuased me finally to pick the wrong heads for the job
-is this a common behavior, meaning to advertise some "ideal" flow nrs, which in 90% of cases arent present, so I will have to deduct 15% each time from the advertised figures to be close to what I need?

maybe some easy answer from you, but anyway I would like to hear it..

Friendly

Dimitrios

[dragvw2180]

 Purchasing a big dollar part from someone you do not know is tough , anyone can claim anything . Mike McCarthy

[JIMP]

Mike it is not unknown, this is why I bought from there without hesitation, its very well known, my question remains, is this a common sales trick or something else is happening that I miss?

Friendly

Dimitrios

[andy198712]

i'd question them about and see if they offer to put it right?

[JIMP]

Andy I have already done that -first thing- and waiting for an answer, just wanted to hear some more opinions to be better informed before I discuss with them again,

Friendly

Dimitrios

[stoneloco808]

I remember shopping for cylinder heads and noticed the advertised numbers as well.  I recall a magazine did a review on a particular set of heads.  The article covered everthing from dimensions and actually testing it on a flow bench.  The article also mentioned the adveristised numbers as it was flowed at IIRC 28" and produced impressive numbers.  But for some reason the I like to call gentlemen standards of the community is to use 25".  The article posted the numbers using the 25".  They also did it with just the manifold and another was column featured the fancy clayed intake horn they made up to try and replicate the manufacturers clause in the advertisement.  I say try finding that advertisement and see if there was a clause in the flow chart and try to replicate that.

[wph]

You simply can not compare flow numbers between heads unless they are measured in a same bench.
Even if you think (as a head porter) that your flow numbers are valid, there's always a chance of calibration
error or a fault in your bench unless you know the math behind flow numbers, areas and port speeds.
Every now and then I make a huge improvement over my previous port- just to realise that I forgot to
screw in a spark plug. Building and maintaining a DIY flowbench has taught me to be sceptical if something
looks better than it should. Regarding hp figures vs. flow you need to test the total inlet tract to get reliable
estimations of engine performance.
The most common standard for test pressure is 28” of water column, 10” and 25” are also seen but IMO 
they are for benches which can not pull 28” of vacuum below test piece. If possible, tests should be done
with the highest possible test pressure and then converted back to "standard", if the numbers match you're
good to go. Radius around port/manifold entrance is also mandatory to get meaningful measurements,
personally I prefer machined piece over clay when ever possible.

[JIMP]

well as I thought initially
as stoneloco808 describes another story of overvalued figures, starting to make sense now.
wph I can really undesrtand some difference between flow measurements, but almost 15% difference is just not a mistake I think. Now if you cant just compare flow nrs in general why everyone tries to get them in 25" measurements where all heads can be compared? I dont think this is a valid option, also I was aware that I had spark plug installed, the springs were tough enough on the exhaust so as not to suck the valve and open alone and the flowbwnch was not handmade, my heads were handmade. The bench was capable to go well over 28" but we wanted to see on 25" to compare directly. As far as for the 10" measurement I would say that should be the only one to take into consideration if I would like to build an engine but not sell heads. Again I dont try to argue here or to blame someone, I try to understand how should I pick my heads next time seing the advertised figures, is it a common situation or I was just unlucky enough with those heads and builder this time? I mean if general rule so I will try and get next time the bigger one just to be sure

Friendly

Dimitrios

[TexasTom]

In my opinion, if you're searching for a particular number for a specific use/application/build, it would be most useful to have the specific pair of cylinder heads flowed by the supplier before shipment. As stated before, benches and especially setup can vary so if you want flow numbers for Your heads by the supplier, they must produce them (and/or backup advertised numbers).
From personal experience, the 25" pressure most closely estimates potential horsepower available from a cylinder head in this (our) application. For example, if the ports flow 200 cfm @ 25", the potential is there to make that number in HP from those heads, in a general sense. BUT, the variables are immense! Be reasonable ... a 1776 is not going to make the same number as a 2276 or 2347 with those heads. This is where proper parts matching is necessary!
Back to the flow numbers ... I personally feel the number should be measured with the manifold you're using and with a radius at the top: clay, metal, whatever. If you want to bolt everything together in the intake tract (carb/throttle body/air cleaner/etc) the number will be more accurate for Your particular application. Will it flow better with a larger venturi and how much? Stick it in there and find out!

Sorry your numbers didn't match!
TxT

[K-Roc]

Dimitrious, what size of valves are in the heads and what is the cc of the intake port?

What is the displacement of the motor you are using these on?  What rod length?

Also what cam and RPM range ?

i would like to run the numbers through Pipemax for you.

Cheers

[Torben Alstrup]

I think I know where you bought the heads. Unfortunately that is the average picture from that company. I have seen the same scenario many times. One of the "issues" is that this company flows at 28" where as most other flow street heads at 25" That will make a difference of about 5,5%.The next thing is exactly what you experience, wrong shape and too large ports for the valve size. Also, was the numbers with or without manifolds (?) that alone can make a difference of about 10 cfm. With or without clay at the inlet, which will make a variation of about 3-5 cfm. If your engine wasnt/isnt on the large side for the valve size it really hurts performance.

It is correct that flow numbers will vary a little from machine to machine, but the variation should be within 1,5% or the machines are not calibrated correct.

It is a tough way to learn it, but now you know.

T

[wph]

It is fundamental to know the test pressure which is used, here's a copy of SuperFlow manual,
found it from www.lindseyracing.com/. A higher test pressure can reveal flow quality problems
which do not occure with lower one, this is why a port should be tested with a highest test
pressure possible. Measured numbers can be converted to lower a standard if they match
(for example measured @25" matches measured values @46" converted back to @25")
Then there are your calculated average and measured port speeds to further stir up the pot...

 

[K-Roc]

I often have potential customers read this article when they ask me for flow numbers...

http://rehermorrison.com/tech-talk-70-airflow-fallacies-avoiding-the-pitfalls-of-the-flow-bench/

I have often wondered why 25" has become the standard for flowing VW heads, It seems that most of the V8 world uses 28"... Or higher..


Cheers.

[wph]

I often have potential customers read this article when they ask me for flow numbers...

http://rehermorrison.com/tech-talk-70-airflow-fallacies-avoiding-the-pitfalls-of-the-flow-bench/

Cheers.

Good reading

[K-Roc]

Google. "piston speed vs. cfm demand"   

[JIMP]

OK now that's a lot and interesting information here, I have to read all of them, so I was right when I asked him about telling me what method he uses for flow figures, probably I'll have to wait for his answer and make my conclusions. In the meanwhile dont you think it would be good if some of you who have or use flowbenches to make some practical suggestions of which method is "closer" to our engines, who knows, maybe we start speaking the same language finally! Again what was anoing was the fact that I was misleaded in some way to choose something I had already, I could have taken a bigger one if I knew from the begining, thanks a lot for your input and help on this

Friendly

Dimitrios

[Taylor]

Google. "piston speed vs. cfm demand"   

I had shit to do this week!!! Damn you. 

[K-Roc]

Google. "piston speed vs. cfm demand"   

I had shit to do this week!!! Damn you. 

Sorry Dude.

[BeetleBug]

What about the CB Strip Dominator heads that was on the flow bench during the JPM dyno day? Advertised flow above 300cfm. CB do not mention if they are flow tested in 25" or 28" of water. Neither if they used manifolds or not.

Can someone post the flow results?

-BB-

[kb]

CB Strip Dominators. With manifold and clay. 25". Flowed on a SuperFlow SF-600 iirc.
Advertised at over 300 CFM...

[JIMP]

O.K. we're heading somewhere now, I start to see the light!

Friendly

Dimitrios

[modnrod]

All I can add is this seems to be very prevalent in the VW industry, not just flow figures but advertised claims about anything and everything.

It sucks.

[Torben Alstrup]

Dominators are rated at 28". But that still only equals about 283 cfm. Did´nt see the test. Was it with or with out manifolds ?

T

[bang]

with manifolds. looks like cb does ther flow without spark plug 

[Torben Alstrup]

Hmm. That´s not fair.

[K-Roc]

18mm is .720" lift  that shows your last flow number on the list,

I didn't look very hard but did CB state that thier advertised flow number was at. .720" lift?

Perhaps CB measured at more lift?

[BeetleBug]

18mm is .720" lift  that shows your last flow number on the list,

I didn't look very hard but did CB state that thier advertised flow number was at. .720" lift?

Perhaps CB measured at more lift?

Exactly. So with all the above mentioned factors it might very well be that their claim of 300+cfm is actually not so far off.

267 @ 18mm + 2mm more is perhaps 280 cfm. Then add the 28" factor of 5% = 294cfm +  the marketing factor and everything falls into place.

-BB-

[bang]

asked geers and he is running up to 22mm lift

[Martin]

are the strip doms your testing 94mm bore or 4" bore?

I have a set of 4" Bore heads here that ive had tested on John's flow bench at 28"

[bang]

mine is 94mm 50x38