Comp Ratio over 15:1 on ProStock - Darrin or Anyone ?

[maxracesoftware]

this was Posted by someone on another Board =>

>>why is it that small block comp drag race motors seem to usually respond to 16-1 & 17-1 compression ratios yet some competitive 500 c.i. pro stock motors I know of don't seem to net any gains past 15-1 to 15.5-1?<<<

[Darin Morgan]

this was Posted by someone on another Board =>

>>why is it that small block comp drag race motors seem to usually respond to 16-1 & 17-1 compression ratios yet some competitive 500 c.i. pro stock motors I know of don't seem to net any gains past 15-1 to 15.5-1?<<<

I can explain that in six words.

Volumetric efficiency and dynamic compression ratio.

Ok, I cant do it in six words so here it is.

The Pro stock engines are only 15 to 1 static, but the dynamic compression ratio is about 17.5 to 18 to 1. The small blocks you are talking about probably have 16.5 to 1 but there dynamic is still 17.5 to 1. Why? because you fill the cylinder less so you have to compress it more. We fill the cylinder A LOT more so we need to compress it less in order to extract the same amount of energy. Some of our engines only have 14.8 to 1 because they are so efficient at trapping the air ( high VE ). When you say competitive comp engines what engines are you talking about? Single carb or restricted induction system engines of course need a couple points more compression to make up for the decreased VE.. The 358cid Pro Stock truck engines where only 14.5 to 1 and they made 968 HP . With the cam technology we have now there is no doubt in my mind that they could produce 985 to 990 HP now at 14 to 1. Its been a couple of years since I did one of those so I am speculating.
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Darin Morgan
R&D-Cylinder Head Dept.
Reher-Morrison Racing Engines
1120 Enterprise Place
Arlington Texas 76001
817-467-7171
FAX-468-3147
Visit our web site at http://www.rehermorrison.com

[maxracesoftware]

>>I can explain that in six words.
Volumetric efficiency and dynamic compression ratio. <<<


Darin...Thanks for the explanation !!

Your explanation was exactly what i was thinking...but i don't have any real Dyno Ve% data from a current ProStock Engine

my Computer Program is calculating 130+ VE ,thats about 10 PerCent higher .... but can't really determine how much of the 130 Ve is going out the Exhaust port during OverLap or ring blowby CFM....not enough instrumentation on my Dyno yet to do this correctly

18 cr / 14.8 cr = 121.6 ve gain ?

121.6 Ve gain = 3.178 psi , or approx 88 inches water, or 620 mixture velocity in feet per second, at the Intake valve Closing point

means the "last" 221 cc of port volume moving at 620 fps just before and at intake valve closing point

which means intake port might be atleast 111.8 inches H2O or 700+ fps at max piston speed , slightly before or after.

.54 to .63 of speed of sound (Sonic Choke)

The ProStock intake port is handling the Air Velocity FPS very efficient !!

a live engine between RPM points of Peak Torque and HP
operating at 88 to 120 inches of water

700 fps or so peak port velocity would approx equal 350 fps as an average FPS in the Induction System , which equals = 28 inches of water on FlowBench

to simulate a real engine , the Cyl Heads might need to be flow tested between 80 to 120 inches on a FlowBench to get a more accurate idea of wet flow and dry flow results ???

28 inches = 238.6 mph + over short turn radius

111.8 inches = 477.3 mph over short turn radius (minimum velocity)

Flow Testing a Cylinder Head "Dry" at 238.6 mph (28")does a decent job of relating to a Live Engine
but,
Flow Testing the same Cylinder Head at
477.3 mph (111.8") in the Intake Port/Short Turn areas both "Wet & Dry"
has got to show some unexpected results ??
especially with Wet Flow , more Fuel Inertia over the Short Turn

[ozrace]

An observation:-
The 3.178 psi, or 88 inches H20, is the result of the kinetic energy in the air stream being stopped by the closed Intake Valve.
This is different to the differential pressure across the valve seat when the valve is open - which is what you would want to simulate in a Flow Test.
Differential Pressure across the valve seat is a result of the pressure in the intake tract vs pressure in the cylinder, and will change constantly during the period of Intake Valve opening.
Patrick Hale from RSA software has a program that gives a differential pressure reading for a given engine design at various valve lifts - I don't know how accurate it is, but his other software is good, so it's what I use to try get a test pressure to shoot for.

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[ozrace]

BTW - I think you are spot on in saying that a test pressure close to actual operating pressures seen in the engine is important, especially for wet flow testing.
I wonder if Darin or anyone else here has had the chance to try different pressures with a wet flow rig ?

I am currently building my own flow bench - after 2 years, it's nearly finished, and will be running within days.
It will be able to pull 60+ inches with ease, and I have designed it so that I can easily upgrade it to higher pressures (100-120") with a simple add-on.
I am intending to do wet flow testing at high pressures, but I don't know how this will affect visibility of the wet test, etc.

[maxracesoftware]

>>>An observation:-
The 3.178 psi, or 88 inches H20, is the result of the kinetic energy in the air stream being stopped by the closed Intake Valve.
This is different to the differential pressure across the valve seat when the valve is open - which is what you would want to simulate in a Flow Test. <<

the Pressure Differential and the Tuned Sound Wave moving thru the mixture is probably causing atleast the last 221 CC's to be moving at 620 fps to get 121.6 Ve% or a chance at turning an 14.8:1 CR into a dynamic 18:1 CR engine ....This last 221 CCs has to make it inside before the Intake valve closes, when it does, it should give 3.178 psi above atmospheric pressure inside the cylinder at the IVC point, and 121.6 Ve% approx

where is the last 221 CC's located ??
just before the Short Turn Apex towards/including the Bowl Area and Valve Job area ...the most important parts of the Port, besides the Total Induction Lengths/Cross-Sectional Areas,..where the Valve Job/Curtain Area, Chamber walls have the last chance to influence Flow

it might not be possible to make a Steady-State FlowBench make a Port go into Actual Sonic Choke at .55 Speed of Sound (613.8 FPS/ 86 Inches)
With a FlowBench, you don't have a Sound Wave moving back and forth,
and the effects of Induction Length
the FlowBench is a "Steady-State Device"
and the Engine is a "Batch-Processor Device"
being a Batch-Processor, Inertia /lag times will cause greater pressure differentials in a Live engine causing Port to go into Sonic Choke as early as .55 Speed of Sound , especially with a Fuel like gasoline or methanol
inside the Port with its Inertia influences and its Temperature/Cooling effects, affecting the Speed of Sound inside the Port

1116 FPS = 284.2 Inches of Water, Speed of Sound in Air at 59F/Sea Level

the Speed of Sound = ZERO in a vacuum


>>It will be able to pull 60+ inches with ease, and I have designed it so that I can easily upgrade it to higher pressures (100-120") with a simple add-on.
I am intending to do wet flow testing at high pressures, but I don't know how this will affect visibility of the wet test, etc.<< - OZRACE


OZRACE ... could you Post some Pics or some Flow Test results when you get some Flow data ?

[ozrace]

I will get some pics of the Flow bench up as soon as I can (a day or 2).

As I understand it, ports don't actually go into sonic choke at .55 Mach - but at this point (approx.) we reach the trade off where the energy required to move the air through the port becomes higher than the power increase (cylinder filling) that comes from higher velocity.

I have seen video of an orifice on a Flow bench at sonic air speed - amazing.
The flow stops increasing as soon as the speed of sound is reached, and sits completely steady even though the depression is wound higher and higher.

[maxracesoftware]

Post from another website ;

Quote =>

In the gas industry laboratory where I used to work, where precision gas flow measurement is required for revenue purposes, positive displacement flow meters are always used. These either use a bellows that fills and empties, or a partly submerged turbine where inverted buckets on a paddle wheel fill and empty as the thing slowly rotates. These are highly accurate and have low pressure drop and work right down to zero flow.

For laboratory (high) air flow references we used sonic nozzles. What you do is have an accurate calibrated nozzle, and increase the depression across it until the flow goes sonic. Any further increase in pressure differential across the nozzle gives no further increase in flow beyond that point.

You connect up the sonic nozzle to a monster vacuum pump and open the valve. You hear the air rushing into the hole with a mighty roar. As you open the valve further the roar gets louder, then suddenly it goes dead silent ! ! As the air rushing in gets to the speed of sound, no sound can escape back out. Fascinating stuff.

You then know exactly how much air volume is rushing down that hole, and you can put your test piece in front of it and know the flow will not change from the exact sonic nozzle calibration figure as long as it remains sonic.

[bill jones]

-Larry,,, do you have an explanation for what happens when you are doing this sonic AIR testing and you introduce a vaporized spray of liquid into the that air that is flowing thru the nozzles?
-Introducing the liquid in a volume equivelant to the same ratio as you would use for a race engine.
-We all know that engines need fuel but we usually only talk about the airflow like the fuel is non-existent.

[ozrace]

Bill
You would not normally see sonic flow on a Race engine.
The aim is to keep maximum velocities to around .55 x this speed.
I have heard claims that localized areas may go sonic on a running engine, and also on 2 strokes.

[bill jones]

-OK, so what happens when you mix the fluid in there?

[maxracesoftware]

-OK, so what happens when you mix the fluid in there?

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Bill heres a Link to Mondello's site for WetFlow Bench

http://www.mondello.com/Pages/Catalog/mondelloCFM.htm

there were a few Pictures of wetflow on his site

[bill jones]

-I'm asking about when flowing thru the sonic nozzle.

[ozrace]

Bill, I don't know that there are many sonic benches around, and probably none being used for airflow work on Race engines - so I don't know if anyone here would be able to answer that for you.

[ozrace]

Larry

A couple of pics of the bench as promised:-

http://niglish.homestead.com/files/tn_P ... 19__1_.jpg

http://niglish.homestead.com/files/tn_Picture_001.jpg

It has 12 vacuum motors inside that will pull a depression of around 80", but the 2 discs you see on the back in the second photo are covers that will allow me to bolt on extra motors without going inside if need be. There are motors available now that will pull 140".