# What is the pressure that is asserted on a piston ...

• posted on March 14, 2005, 7:17 pm
I'm trying trying figure out how much force is produced by a gas explosion in a piston during a normal firing. The air/fuel is correct and the spark and everything else is optimum. What is the usual pressure that is being
created to push down the piston? Say on a 350.
Thanks, David

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on March 14, 2005, 10:54 pm
I forgot the formula for the calculation of max cyl pressure. I think you will find it in the 700-750psi area. This is taken from an old pressure/time graph I made on a 350. This would be the max pressure and decreases rapidly as the piston goes down.

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on March 14, 2005, 11:00 pm
Upwards of 1,000 psi and higher
--
'79 Chevy K-10

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on March 15, 2005, 8:18 am
guessing i would say 8.5 lbs per square inch, (compresion ratio) area = pie x radius squared or 3.14 x ( .5x 4 ) squared = about 12.5 square inches x 8.5 psi = about 106.25 lbs. close enough for goverment work as they said in the GOOD OLD DAYS
old john

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on March 15, 2005, 11:52 am
Interesting calculations there, old John, but it fails to take into account the force of the fuel/air explosion. The formula for max cyl pressure is a thermodynamic formula that I don't recall. I have long since got rid of all my related text books so I can't look it up for you. This question brings up memories of my youth mathematically tearing down and rebuilding engines.

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on March 16, 2005, 1:32 am
that calculation is partly correct for doing a cylinder compression test only. not the explosive burning of fuel air in the cylinder. there's a bunch of thermodynamics involved with this number, but I think 1000psi is a good ballpark # assuming a good cylinder, rings, etc.
FWIW: a missing metric for calculating the pressure in the compressed cylinder (top dead center) is that atmospheric air pressure is approximately 14.7 psi. so 14.7psi * 9:1 compression ratio is approximately 132 psi air pressure in the cylinder.

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on March 24, 2005, 11:05 am
As a rule of thumb.... the cylinder pressures are 10x the static compression ratio
--
'79 Chevy K-10

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on March 24, 2005, 1:30 pm
What they've been trying to figure out is the pressure immediately after ignition of the fuel/air mixture (which decreases with the volume increase as the piston moves away from the head). A few guys came up with 800 to 1000 psi using different formulae.
Mad Dog, ever consider a cold air intake for the 79 K10 (may be just a couple drops in the bucket with your NO2 setup)?
Regards, Franko

explosion
spark

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on March 27, 2005, 4:42 am
That formula is for the cylinder pressure at TDC before the piston begins to move down in the bore.
Yes, i do have plans to fabricate a air cleaner bonnet that seals with the hood for use with a cowl induction scoop.
I'm on the fringe of detonation now and have to replace the head gaskets with steel shims in a effort to minimize the piston/head clearance "quench zone". I'm shooting for .030" quench area (don't try this at home). Redline is set at 6000 rpm so she will live fine with that clearance. This will bring the compression up over 10-1 but I'm pretty sure the motor will appreciate it by being able to crank the initial timing up a few degrees.
--
'79 Chevy K-10

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on March 28, 2005, 3:34 am
Will the scoop be facing forward or the windshield?
Have you considered a water-injection system (very fine mist) to address your detonation? They have NA and turbo models.
Regards, Franko

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on March 28, 2005, 7:32 am
Cowl induction scoop....
No, but i have considered alcohol
--
'79 Chevy K-10

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on March 29, 2005, 3:36 am
Oh, yeah, sorry (cowl induction scoop), duh... Am considering cutting a hole in my hood and installing a scoop but more for ventilation, especially in summer. I've found two or three models that should keep water out of the engine compartment even in a torrential downpour.
Been doing some reading on water injection systems -- some folks use and SWEAR BY injecting H2O2, that is, off the shelf hydrogen peroxide (less than 4% concentration), instead of water or water/alcohol. They also add alcohol to the H2O2 for subfreezing temperatures (50:50) with mixed results (sorry for the pun).
Am looking around for a single-stage system, low-pressure (60 psi pump), for injecting water or H2O2 into the high-pressure side of the turbo on our 6.5TD. Besides being less than 10% the cost of an intercooler, I like the idea of lower EGTs when it kicks in.
Ahhh, toys for the boys.

the

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on April 7, 2005, 12:35 am

How could you handle H2O2? It's highly reactive and rapidly decomposes into H20. It's reactive enough to ignite a piece of wood if you spill it. I used to work around it. Cheers, Roy

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on April 7, 2005, 1:25 am
wrote:

reread the post.... I've never seen a bottle of drug store peroxide ignite a piece of wood before.
-Bret

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on April 8, 2005, 9:33 am

Off the shelf "peroxide" is not H2O2. It is 3% H2O2 mixed with water. If you leave the bottle open for a little while it will be 100% water (H2O). Cheers, Roy

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on April 7, 2005, 2:06 am
Hello Roy,
Are you referring to H2O2 in concentrations good enough for rocket fuel (60% and higher)?
Also, I haven't heard of bleach blondes being set on fire (ummm, in a literal sense, that is).
Interesting application, though. I'll be installing a single-stage water injection system and will definitely try the off-the-shelf H2O2 but am not equipped to accurately/objectively measure performance improvements. Boys will be boys...
Regards, Franko

(sorry
into
used

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on March 24, 2005, 8:58 pm
Thanks, Now I have 2 rules of the thumb. LOL just kidding if you know what the other one is.
D.L. Man

explosion
spark