The V8 Owners Forum -

Reason for the post above.

The cylinder has a finite capacity and lets assume its 100% filled by the valve opening and the cylinder head flow capability - i.e. 100% volumetic efficiency.

Now bear with me whilst I get my old unit head on and convert for a 3.9L

Cylinder capacity = 244 CI = .141 Cubic foot. divide by 8 = .017625 Cubic feet per cylinder

The engine has maximum power at 6000 RPM and there are 4 cylinder fills per RPM

6000 x 4 x .017625 = 423 Cubic feet per minute - now thats with a perfect 100% Volumetric efficiency on a 4.0L engine.

A good engine is 90% Vol Efficient - which is nice and easy 0.9 x 423 = 380CFM

Now the key is the relationship between the atmospheric pressure that you have stuffing that air into the engine and the rating at which that carb flow is taken.

Andrew

Darkspeed wrote:Hello Kev'

What is "one cycle" that you have in your calculation and how did you come to choose it?

Andrew

Inlet valve, seat to seat

Kevin.

Or another way to look at it. . .
you have a head that flows 175 cu ft/ min at full lift
you have a cam that is open for 240 degrees of every 360, every other rotation of the engine.
total flow from all ports is 175 X 240/360 X8 /2 = 466 cuft/ min, now even if you have a good mechanical roller cam you would do very well to have the cam fully open for 50% of that time, so assume a linear ramp up and down and you need to reduce that figure by 25% and you are looking at 350 cuft/min. If it is a hydraulic flat tappet cam then you want to reduce that figure by about 40% ie. 280 cuft/min.
Well that is how I work out cams, heads and engine capacities when I'm doing it
As correctly pointed out time to watch testing conditions manifolds and carbs give so much less of a restriction to flow that most flow benches cannot flow enough air to measure them so either convert with a fudge factor or state different test conditions.
Best regards
Mike

Right then - to balance the units you need to factor time as the measurement is Volume/Time
I thought maybe the cycle was a time period

For your example 85CFM would be at 9,645 RPM - which would need a Holley 680 or maybe bigger

That or its one mother of a capacity stretch.

Andrew

CastleMGBV8 wrote:

Darkspeed wrote:Hello Kev'

What is "one cycle" that you have in your calculation and how did you come to choose it?

Andrew

Inlet valve, seat to seat

Kevin.

I was using a 4,6 277Ci engine as an example but appear to have got my maths wrong.

Kevin.

CastleMGBV8 wrote:Bear,

You had me going there for a moment, the current NASCAR engine specs are for the carb to be restricted to 830CFM.

http://www.hotrod.com/techarticles/hrdp ... to_06.html

My post was not suggesting that the carb was strangling the engine just that the flow was sufficient to provide a certain power level whilst the flow figures for heads would suggest a much greater flow potential and higher power output which does not appear to be the case, just one of the things my aged brain can't quite understand.

Kevin.

No really the national races have to run a 390! heres a nice vid of one on youtube. In the NASCAR cup races they are allowed to use the bigger carb, the engine stays the same.

http://www.youtube.com/watch?v=qP3DKyoIInA

Hi All

Just wanted to ask what the design flaw was that makes my flow bench read 20% low? It was accepted at degree level and phd level, only argument was whether 59.6% or 63% effeiciency for sharp edged orifice flowing from one large space (BS definition of large space) to another large space.
The most important thing is whether the figures are repeatable day in day out year on year. Some proprietory flow benchs actually tell you to try and do all mods in one day dur to variations, not much use to me as our research and rtesting spans 25 years!
Of interest my flow bench flows exactly the same as Weslakes bench, test holes flowed within 0.25% on my bench and on his. His must have been exactly 20% out too

Peter

Hello Peter,

Nice to have someone like yourself pay us a visit and I hope you can drop by regularly to pass on a bit a wisdom.

The comment on your bench regarding its accuracy was from here.

Go Fast News post

Actually whilst your here can you answer a question on a calculation in your book please.
There is a calculation on valve and port flow and calculating the port diameter that takes the valve diameter less the seat width x 0.707
giving 41.1 for a throat of a 42mm valve - I was wondering why as we are dealing with a diameter we do not subtract 2 x the seat width?

I am working from memory here as I do not have the book in front of me so please forgive if I am not getting it absolutely right.

All the best

Andrew

burgesstuning wrote:Hi All

Just wanted to ask what the design flaw was that makes my flow bench read 20% low? It was accepted at degree level and phd level, only argument was whether 59.6% or 63% effeiciency for sharp edged orifice flowing from one large space (BS definition of large space) to another large space.
The most important thing is whether the figures are repeatable day in day out year on year. Some proprietory flow benchs actually tell you to try and do all mods in one day dur to variations, not much use to me as our research and rtesting spans 25 years!
Of interest my flow bench flows exactly the same as Weslakes bench, test holes flowed within 0.25% on my bench and on his. His must have been exactly 20% out too

Peter

Hiya

I think it must have been Dave Baker who wrote the thread.

I spent 3 years building the bench and testing re-testing, modifying etc etc until I got consistent results. I have added pressure tappings all over the bench and had as many as as six extra manometers, these let me measure drops and recoveries at all points. The figures were so close to those achieved with just two manometers I just use two manometers. It is very easy to fall into the trap of thinking the numbers are more than what they are, a simple, quantifiable way of making a before and after comparison. We get the same with rolling road bhps and whose rollers are right and whose are wrong. as long as the same flow bench is used and the same rolling road or dyno then improvements or losses can be logged. At the end of the day, 'when the flag drops the bulls**t stops' as they say and we have a long history of Championships under our belts to show we must be heading in the right direction! Please note I say Championships, not a few lap records or race wins.

Re the maths...which page of the book? I don't keep a copy in front of me

Peter

Is it just converting the angle to the diametric. (cos cut angle?)

Hi Dave

The throat would be narrower than reality if you just took 2 x seat width off the valve diameter, you would need a zero degree valve seat to give OD - 2 x seat width. The maths looks a little shaky to me I think it should be valve width - (seat width x 2 x .707) which gives nearer 42 mm for throat, I reckon we only allowed for one side of the seat when we were doing the maths for the book!

Did you build your own flow bench?

Peter

Sorry Peter, Dave and all, ignore the 2 x seat width, my post was a bit hurried and how you list it below is how I see the calculation should be and that the x2 part had been missed in the book which is what I was trying to get at.
I was wondering if I was missing something with the calculation on using one seat width x factor in the calculation.

All this is on the assumption that we are using a single 45 degree seat and not a 3 angle seat

With a 3 angle would the calculation also have to take into account the 60 degree cut as well. X2

I have built my own bench - See separate thread in general discussion.

burgesstuning wrote:Hi Dave

The throat would be narrower than reality if you just took 2 x seat width off the valve diameter, you would need a zero degree valve seat to give OD - 2 x seat width. The maths looks a little shaky to me I think it should be valve width - (seat width x 2 x .707) which gives nearer 42 mm for throat, I reckon we only allowed for one side of the seat when we were doing the maths for the book!

Did you build your own flow bench?

Peter

Hi Dave

It is amazing , even after 14 years since we wrote the book we still come across oddities!

The easiest way to do the work is the way you are, flow testing what is there, playing with it and see what happens, the maths is there for rules of thumb which go out the window when you are restricted to what is placed in front of you. If you need to just measure the valve throat physically.

Re the Merlin heads...in theory (remember that theory often goes out the window) the heads should, if properly developed, flow less with the valves out. Valves are capable of steering air flow, especially the more downdraught the port. Another thing we have found is that very large valves do not behave like valves up to around 43mm...big valves need exaggerated seats ( wider than looks correct) and fairly narrow valve throats, valve shaped are odd too, bell shapes and tulips get interesting, especially sort of bell shaped.

Have you worked out the theoretical max port flow for the Merlins? From piccies I would guess at 85-89%ish for efficiency of shape. This is where you need to be confident with the flow bench figures.

With regard to the accuracy of my bench, when we test sharp edged holes of any size we get the predicted air flow if we assume the efficiency of a sharp edged orifice ( thin plate) is 60% we certainly don't get 20% out. Mind you as long as the flow bench figures are repeatable then the bench must work correctly.

Peter

Thanks Peter,

On the book and the calculations I have just been absorbing every formula available and stuffing them into a spreadsheet so that I can look at theoretical flows for particular parts that I have.

At least you know someone out there is taking the time to read the book thoroughly

As you say there is nothing like the practical approach and having a bench to play with has certainly opened my eyes

On the Merlin heads I really needed more time to play I could only do so much in the couple of hours thanks for your thoughts and I will see what the theory says . I would like to hear your thoughts on the filled in area of the inlet port basically giving a wall to the inlet flow from any manifold - albeit a small wall and the flow divertor on the floor of the port.

Cheers

Andrew

Hi Dave

have you any pictures of the heads I have never seen any, hust a few pics on the net when I looked today.

Peter