1993 Litton Corse I - Strato's Replica

Yes and No - gotta love that sort of answer

It will become a bit clearer as we progress but the essence is this.

You hold a fixed depression within the bench - now what the hell is that? - knowing that mental illness is not a matter to take lightly I shall avoid any comments in poor taste - I could go a little more toward allowing a fat chick to sit in there - but again that's just avoiding making an explanation that is clear and easy to understand.

With airflow, and many other measurements, we need some some fixed references to be able make the measurements meaningful and after a good few years of experimentation the industry as a whole came together and decided that a good figure for depression was 28" That is quite a miserable state or quite a fat chick so others use 10" of depression and then use a conversion factor......WTF is all that about I hear you mumble!

Nature hates a Vacuum - a bit like Mrs Darkspeed in fact - Hence no issue if I rob the motors from them - and she, Mother nature, will do everything to fill it in (vacuum)if one occurs and she does this by trying to stuff said vacuum with air. What we do in our little flow bench is create a vacuum so we start off with the top of the bench completely sealed - no air flow - If there was a head on a fixture above the hole in the bench it would have the valve closed. We run the motors to achieve our fixed datum vacuum of 28".

So there we are, valve closed no flow and a vacuum held in the bench at 28" - I shall explain that value and what it represents a little later. If I don't remind me.

Our measurement of flow across our orifice at this stage is zero as we have the same pressure on both sides of the orifice. If we then open the valve in our cylinder head good old mother nature see's the vacuum we have made and stuffs a load of air through the gap to try and fill it. On it way to the motors that are creating that vacuum that air has to pass through our calibrated orifice and on it's way through we get to measure it. YAY!

Now - We need to maintain a vacuum of 28" at all times so that we can have a fixed measurement datum, what we did by letting the air in through the inlet valve in the head is allow that pressure to be reduced - just a little - and we need to compensate for that by turning the speed up on the motors until we get our 28" back. We always need to have a that fixed depression in the bench no matter how far we open the valve in the head. It's that fixed orifice plate and fixed vacuum/depression on one side of it that allows us to accurately and repeatedly measure the flow. Each time we open the valve a little the flow will increase (hopefully), and we will need to wind the motors up to compensate for that loss in vacuum in response to that increase in flow.

No doubt someone will mention floating depression benches - not interested it's not what I built and it's a subject for another thread if you so wish. I wanted a vernier gauge not a stick with notches on it.

No problem at all georgeb- and you are correct with your assumptions about when you stop. With cylinderhead development you generally try and stop before you reach the limits of the casting by finding a water way !! - With carbs - absolutely correct they rely on a pressure drop and pressure differentials within the body of the device to create further pressure differentials across jets and without that functionality no fuel can get drawn into the airstream - That, like cylinderhead porting and modification is an entire forum let alone thread on it's own so we will wind back a little from those subjects.

The common ground with both is that eventually can get no further improvement in flow because you hit some sort of physical restriction that stops you - Once you have reached that point this is seen by no further increase in flow at that fixed 28" pressure differential or as you say for some reason the flow actually reduces.

All I know about trains and demands was written by Rev W Awdry.


I will get on with the information on the build of it and then progress to the relevance of all this to this actual thread - There is one, honest.

The pressure has to be constant - and 28" was also found to provide a good velocity or air in the port to be able to make accurate tests on what sort of flow was present within the port. If the pressure was not constant the reading of flow would be going up and down with that pressure. Having constants and standards to make comparisons against is the basis of all measurement.

I do love some of the old standards like grains and the history behind them.





Porosity is a pain in the .... finding an inclusion that goes through into a water way is one of those testing moments especially on the last port of a head.

If you have gone through all the trouble of building a measuring device you really need to be able to test it's accuracy - no point having a ruler if it's calibrated differently to everyone else's ruler - These differences in units eventually end in issues and disappointment. I especially love the Mars Orbiter mission where an error was made with Metric and Imperial.

What you need for repeatability is the ability to calibrate the bench against a datum and PTS produced a set of calibration plates derived from what were known as pass around plates. These were a set of plates that builders used to test the accuracy of benches and these original plates were then standardised into calibration plates.









These plates have since been tested on other manufacturers plates to confirm accuracy across not only PTS benches but many other makes.

Originally I ran the bench and calibrated it from a standard set of calibration figures and did not have it calibrated - when using the bench as a comparator and not an absolute measure calibration it is not 100% important but it's always nice to know that the figures you are reading actually have a place in the real world. When I eventually purchased a set of plates the bench was less than 2% out.

This is all so far over my head that it's ridiculous but I'm still finding it very interesting and look forward to the comparison between the two Webers as I have a 32/36 on my crossflow and know where there's a 40DFA I could borrow.

I have a 32/36 DGAV that I started to modify in the same way, then someone gave me a 38/38 DGAS which I still haven't fitted. I'm currently running a 32/32 DGV which I don't think has anywhere near enough flow for a lightly modified pinto but I drive like a grandpa so never really need the very high revs.

Useless information time - outside of this forum at least- but with a little relevance to this thread.

Ever wondered what a 390, 450, 600 etc that Holley use for sizing 4 barrels actually means ?

Well the figure that Holley put on the box is the air flow in CFM at a pressure differential of 1.5" Hg - Hg is Mercury and as it's a pretty heavy doohickey it takes a bit of suck to get it moving 1.5" up a tube - its the same as about of pressure required to move water 20.4" up a tube.

Note - 2 Barrel Holleys are tested at 3"Hg so do not compare the two a 390 two barrel is not the same as a 390 4 barrel.

converting Hg to Wg and flow at 20.4" to flow at 28" is actually quite straight forward.

My flowbench operating program has a conversion built it - the picture below of me testing a modified 38DGAS shows three sets of figures.



Depression - This is the actual measured pressure that the carb is seeing from the bench.
Flow - This is the measured flowrate of air through the carb at that depression
Ignore the lower right hand figure that channel is not being used.

Up in the corner is a comparative flow figure that I have set to give an airflow reading that would be equivalent to 28" - This allows you to do a lot of testing at lower(and quieter) flows and you just need to whip it up to 28" actual to do a double check. At lots of benches just do not have the capacity to test over 10"wg.

Back to the subject

A 390 Holley flows 390 CFM at 20.4" wg which at 28" is - 457CFM

If we want to compare our 400CFM 40 DFA Weber in Holley 4 barrel numbers that is 341CFM

A 390 two barrel Holley - flows 323CFM at 28" - about the same as a stock 32/36 DGAV

Hopefully this has also highlighted that without that crucial "wg figure - flow figures for anything are absolutely meaningless.

28" is pretty much what I gather in terms of what an engine can actually "suck". I'm no expert in these things although it's a compromise especially with carbs - you need a equal airflow through a restriction to allow fuel to be drawn through and still atomise it reasonably well. Bigger holes might make more power at the peak figure but at the expense of driveability for road usage yet you don't want the engine to be strangled - still a dark art to me

A most fascinating read though - I'm really enjoying reading your findings.

Huge subject and I just mess about mostly making noise but you do get to find out some interesting things

The reason I am testing a couple of valves is that the tiny back cut on the valve on the left improved low lift flow 15% at one lift point. - You will find that unmodified valve in the greater majority of Stage 3 Rover V8 heads - Peak flow may well be very close, but all the way to .300" this modification will flow more. Another manufacturer who has a slightly different shape valve (almost like my mod ) is not used very often as their valve is that bit more expensive..



Must really get on with that SU/Stron set-up - after I mow the Lawn