A "floating pressure drop" type flowbench - By David Vizard

[AceR]

I won't be doing anything until the new year.. Xmas just takes up too much time!!!

[stef-1]

I know this is a very old thread but I just wanted to say something. Whilst the equipment and measuring side of the floating pressure drop bench leaves a little to be desired there is one aspect of this that has some merit. That is the premise of a variable test pressure for the valve lift (I don't mean random) or to normalise it, a variable test pressure for a L/D ratio. So you would test at very high test pressures at low L/D ratios, lowering the test pressure as the L/D ratio gets higher.
This would be very close to what happens in a real engine and due to the vacuum cleaner motors we mostly use it would be acheivable to pull higher depressions (within the structural and measurement constraints of the bench).
I am already maxing my bench at low lifts (most I can currently measure is 40" H2O - really need electronics) with some interesting results. I am planning to build a new bench next year based on what I have learned building and operating my current one. I will build this to take 100" depression at low L/D ratios. I want to also spend the time until then collecting as much cylinder and port pressure data as I can from various sources. I want to use this to standardise what pressure drops to use at particular L/D ratios. This should allow for a more realistic simulation of airflow in the cylinder head.
Results would have to be normalised somehow to get a measure of discharge coefficients, etc.

Any views on this?

[Tony]

Results would have to be normalised somehow to get a measure of discharge coefficients, etc.

If you are going to correct all the readings back to some standard test pressure so that various lift increments can be compared, why not just test at a fixed pressure and be done with it.

Testing at a hundred inches is fine in theory, but at around 514 Lbs per square foot of air pressure is going to require some pretty robust bench construction techniques.

[1960FL]

This would be very close to what happens in a real engine

Stef,

I think since this thread has been dead for a while and the need for those to come blow David’s horn has slowed what better time is there to resurrect a thread based on principles.

You make a statement that many have made and I personally struggle with it, maybe because I don’t under stand or maybe because I see it different than others most likely the first and my hard head.

For this discussion lets talk about SIICE (Spark ignition internal combustion engine) at WFO (Full throttle) in the Mild street build format. Now lets think about this engine running, we know at idle this engine (depending on cam) will have somewhere between 15 and 20 inches of mercury manifold vacuum and somewhere in the 1 to 5 inches of mercury at WFO under load. This difference is that between atmosphere and the intake. (not the combustion chamber) Though we can assume that the piston motion must be developing this depression as the valve is open to the intake during the intake cycle. This said the intake valve is seeing a Delta P of combustion chamber to intake manifold not atmosphere; so to get the air/fuel into the chamber at WFO we know the piston pumping demand must be greater than the WFO manifold vacuum or we would not have flow in the right direction.

So now lets talk low lift flow and the timing around it (please do not get me wrong I feel low lift flow is important) but I see three types of flow in the cam cycle and I base them on cam timing and piston motion.

1.) You have IO low lift (negative piston motion to stop positive piston draw) “Scavenging time frame”. I/E 10° BTDC to 10° ATDC say 0 to about .155” valve lift.
2.) You have positive piston motion, mid to peak lift to mid lift (slow to fast to slow Piston demand) Pumping or intake draw the time where the engine creates the vacuum that builds in the intake manifold and we measure against atmosphere. 11° to 170° ATDC. Max piston speed is near 90° +/_ (peak demand)
3.) Low lift to IC (positive piston motion to stop to negative piston motion) “The ramming effect or inertial effect time frame”. 10° BBDC to 35° ABDC.

So does the intake valve really see super high DP across it at low lift?? I am not sure! Piston wise I don’t think so, Yes I believe that a professionally tuned open header exhaust can develop large wave forms and some large narrow RPM band scavenging but again this is only a small portion of the intake cycle and truly the most complicated in respect to all the things at play her.

So my conclusion is that I believe that testing at higher depressions is the logical progression to standard testing at 10” or 28” but I also believe that a port that will flow properly across the whole L/D scale at high depression (40”, 60”, 100”) will work better than one tested at 10”.

As for Mr. Vizzard and his correlation to how this VD bench relates to engine operation my jury is still out why? Because the premise is based on Low Budget, Home shop and these fellas are not working on super tuned open exhaust race engines they are weekend warriors trying to hot rod Moms old Cortena or Fiesta. Thus any huge advancement that they may make at low lift based on the entire package could just as easily create far worse effects in the areas of reversion. And worse off as the depression across the port falls as lift rises the air speed too diminishes thus not showing the areas that could be creating high speed turbulence and killing the port.

In conclusion one might be better off building a fixture to cap the combustion chamber simulating the piston (full dome design) at TDC set both intake and exhaust valves at cam spec TDC lift then test pulling in both direction until you find a magic spot where it flows more pulling out the exhaust then it does pulling on the intake runner. This I do not think is very feasible with Mums vacuum and some vinyl tube!

These are just my thoughts and opinions and probably to much of the latter but I would love to see some good discussion on these theories.

Rick

[jfholm]

Here is some more food for thought on this subject. As close as I can tell on my engine with a hot street cam, nothing too radical and 6000 rpm when my intake valve opens I have a piston speed of around 1800 to 2000 fpm. When the intake is closing I have a piston speed of 5515 fpm. Exhaust opening I have 4686 fpm and when the exhaust is closing I have a piston speed of 3163 fpm.

We know that when the exhaust opens there will be very high pressure from blow down and very high velocities that are really hard to duplicate. I have read numerous articles that state it is very hard if not impossible to get the exhaust perfect on a flow bench. We do know that the exhaust will be drawing on the intake at overlap but by the time the intake is opening the main pressure for exhaust is gone. On my engine the fastest piston speed is around 74.3º ATDC and the fpm is 5717. My intake lobe centerline is 106º ATDC so at that point the piston speed is 4792 fpm.

Watch this video on an actual engine running paying attention to the amount of air/fuel coming in at low, middle and full lift. I think it answers a lot of questions we have been discussing

John

[ivanhoew]

john , when your inlet valve opens , is the piston still on its way up ?

[1960FL]

Robert Exactly what I am saying,

Look at this cam spec mild street HD cam for the intake side

IO 11 BTDC
IC 35 ABDC

and the lift at TDC is .138 "and the piston is stopped...." so yes we know and hope the exhaust is drawing harder than the 1 to 5" of merc (13 to 65” WC) pulling from the intake track...

My point is that on the intake side the pressure differential across the valve is not that huge in a mild full exhaust street engine probably more enertial effect from pumping and thus there should be more time spent developing methodology to test port and combustion chamber shapes to enhance scavenging and reduce reversion. But the DV Floating Depression bench is not the answer nor is making the intake side flow super great on the bench at 100” if you have no idea what going to happen when the reversion wave comes the other way???

Again I believe low lift flow is important and can be measurable when talking RAM effect, but for me Split overlap is a Black hole I am still trying to quantify with a valid test method. And it only matters in the positive light if that exhaust system is working to perfection…. Can you say PipeMax!

Rick

[1960FL]

John,

This is the same video that has been posted before and i thin Ed, or SSR noted it is missing the overlap sequence....

Wold love to see that part full race & mild street..

Rick

[jfholm]

John,

This is the same video that has been posted before and i thin Ed, or SSR noted it is missing the overlap sequence....

Wold love to see that part full race & mild street..

Rick

The intake cycle is there and that is what I was trying to show to support what you said Rick. If you note when the intake first opens you can notice that there is not a lot of flow and what flow there is does not seem to be very fast and then as the valve opens the flow really takes off once it is open a ways.. So I am with you in that steady state flow testing is probably the easiest and very valid. That is the way most pros do it.

Also on your Harley, being a Hemi, your problem with overlap is even greater. My understanding is it is very easy to get over scavenging in a Hemi. I think Hotz alluded to the overlap scavenging problem when he was talking about the different height seats. If your exhaust seat was higher than the intake then you may tend to have less overlap scavenging. Just a thought.

I always love to read your posts Rick as they sure make you think things over and not blindly assume stuff. This forum rocks.

John

[stef-1]

Interesting comments!

Results would have to be normalised somehow to get a measure of discharge coefficients, etc.

If you are going to correct all the readings back to some standard test pressure so that various lift increments can be compared, why not just test at a fixed pressure and be done with it.

Testing at a hundred inches is fine in theory, but at around 514 Lbs per square foot of air pressure is going to require some pretty robust bench construction techniques.

I don't think I will be converting back to another lower test pressure! I just need to sort out some means of comparing my results in a normalised format.

The bench will be different to a PTS or superflow type bench. It will be designed to cope with the stresses at hand. I may not be testing at 100". It will depend on the results of the operating pressures that I survey and develop into resonable test pressures.
I agree with the others that for older tech street engines and mild competition engines testing at normal test pressures is fine. But I feel that for competition engines and some new high performance production engines a varible test pressure is quite valid. Most OEMs and race engine manufactures do that anyway with CFD these days.
The exhaust blow down is something we cannot do on our benches. The only methods I know of measuring this is a proper blow down rig or CFD.