Cylinder Pressure = Horsepower

Dave Hughes



                                                          CYLINDER PRESSURE = HORSEPOWER


              When building a high performance engine, everyone understands that air flow is important in making power.  But what most don’t understand is the importance of cylinder pressure.  Not including cylinder pressure in the engine design can cost you 30% or more power loss, and that is a BIG deal.  More cylinder pressure means more power – but there are limits, especially with pump gas.

              Cranking cylinder pressure is measured by installing a pressure gauge in place of the spark plug and cranking the engine over several times.  (All the plugs must be removed and battery connected to a charger).

              The limits of the cylinder pressure are based on the octane of the fuel you are using.  Octane is a rating of how much cylinder pressure the fuel (usually gasoline) will tolerate without exploding.  Ideally, the fuel should burn – very rapidly – but not explode.  Exploding fuel is called detonation, and it is very destructive to the engine and parts and must be avoided.

              We build lots of pump gas engines and try to maximize the cylinder pressure in all cases.  When using pump premium, 92 octane, we shoot for 165# of cylinder pressure with cast iron heads and 200# with aluminum heads.  The reason the aluminum heads require more cranking cylinder pressure is because they draw heat (pressure) out of the combustion chamber so much more rapidly than the iron that you must start with a higher pressure.  Heat equals pressure, and pressure equals heat.  Cylinder pressure is the result of many factors, and this is the reason most shops, comic books and the internet do not talk about it.

              Factors that create cylinder pressure:

              Obviously, compression ratio plays a part of making cylinder pressure.  All things being equal, the higher the compression ratio, the higher the cylinder pressure, and the opposite is true too!

              Next is the camshaft, and now it’s going to get tough.  First, you must understand that the cam’s size is based on how long the valve is open, duration!  More specifically, the duration when checked @ .050” lift.  The cam size is not based on lift!  Repeat that 3 times, turn around and repeat it 3 more times and don’t ever mention it again!  The cam controls when the valves open and close.  Now here is the important point—the intake valve closing point.

              The compression stroke, which makes cylinder pressure, does not start at the bottom of the stroke like your 6th grade science teacher taught you.  It starts when the intake valve closes.  Therefore, as a cam gets bigger, longer duration, the intake valve would close later, with the piston closer to the top of the stroke, which would effectively make the compression stroke shorter, and that will reduce the cylinder pressure.  All things being equal, big cam = lower cylinder pressure, small cam = higher cylinder pressure.

              If you advance the cam, the intake valve closes sooner (earlier).  Piston is farther down from the top of the stroke, making the compression stroke longer, increasing  the cylinder pressure.

              If you spread the lobe separation angle (L.S.A.), the intake valve will close later, now making compression stroke shorter and resulting in lower cylinder pressure and vice-versa, if you close up the L.S.A.

              Now you know why no one wants to talk about cylinder pressure.  It is not simple or easy.  But to do the job right and get the most bang for your buck, you must include it in your build-up.

              And we’re not done yet. 

              Cylinder head material.  As noted earlier, the aluminum draws out the heat from the combustion chamber, much faster than the iron head, reducing the cylinder pressure.  So we design in more pressure to start with.  A general rule-of-thumb is that whatever the maximum compression you can use with the iron head, raise it at about 1.5/2.0 points or more when switching to aluminum heads.

              Get back to your seat young man – we’re not finished just yet.

              Altitude.  As it gets higher, the cylinder pressure drops.  This starts to really affect cylinder pressure around 1200’ – 1500’ altitude, and we have some racers @ 6000’.  And it really hurts them.

              Now you know why no one wants to talk about cylinder pressure.  It is not a subject that is easy or fun to discuss or understand.  Getting your local engine builder or some mail order shop to discuss it with you is impossible.  What frightens many customers is when we tell them they need 12:1 or 13:1 compression and want to use 92 octane with their combination.  They never heard of such things.  That is because the people they are talking to don’t understand about cylinder pressure.  We have customers running over 14:1 on 91 octane.  They are at 6000’ altitude, aluminum heads and a BIG cam.

              Note that in our Dyno Test #21, a 740 HP GEN 3 Hemi used pump gas and 13:1 c.r.@750ft altitude.

              When using race gas, the cylinder pressure is again critical, but compression ratios of up to 18:1 can be run with high octane gasoline or alcohol.  However, a similar problem exists here again with engines being built with low cylinder pressure and horsepressure being given away.

              But wait, who’s that standing by their phone ready to help customers order the right combination of parts and even discuss cylinder pressure?  It’s the friendly guys at HUGHES ENGINES!

              Now who’s your Daddy?  Pick up the phone, give us a call or email us, we can help you!

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