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Old 07-23-2009, 04:28 PM
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350 150 Cranking Pressure

the engine have flat top vortec head 0.015 steel gasket 12-243-2 camshaft
226-234 duration lobe centerline is 111 and intake centerline 107 i only have 150 psi cranking pressure i think its low i would like to know if i give 4 degree avance on camshaft it will climb the compression or do i need a smaller cam

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Old 07-23-2009, 04:54 PM
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Compression check

When checking cylinder head compression, I take all the spark plugs out, use a remote starter push button, squirt a little oil in each cylinder, disconnect power to coil and turn engine over atleast 3 times watching pressure gauge needle climb at every revolution and watch for any leak down. Any cam should allow valves to completely close long enough to give a reading of over 200 psi for each cylinder (if you've built a 9:1 compression ratio engine).
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Old 07-23-2009, 05:11 PM
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i have checked without the spark plug but i dont put oil in it the engine have running yesterday
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Old 07-23-2009, 05:17 PM
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Low compression reading

The squirt of oil is to assure rings are sealing. Any engine worth building should have atleast 180 lbs in each cylinder and be within 10 lbs of each other.
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Old 07-24-2009, 12:03 AM
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What about checking it at WOT? That's what I've been taught.
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Old 07-24-2009, 01:42 AM
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Quote:
Originally Posted by roundhousedog
Any engine worth building should have atleast 180 lbs in each cylinder and be within 10 lbs of each other.
While I agree that all cylinders should be close to each other, I disagree with the 180 psi statement and will refer you to this quote from Crane Cams.......

"How does Cylinder Pressure relate to the octane rating of today's unleaded fuel?

In very basic terms, the more cylinder pressure we make the more power the engine will produce. But look out for the fuel! Today's pump gas is too volatile and cannot tolerate high compression ratio (above 10.5:1) and high cylinder pressure (above approximately 165 PSI) without risking detonation. Fuel octane boosters or expensive racing gasoline will be necessary if too much cylinder pressure is generated."

I will further agree that a motor can be built which will have cylinder pressures exceeding 200 psi that will run on pump gas, but the squish must be optimized, all sharp edges must be rounded, small "current technology" chambers, centrally-located spark plug, plug of the proper heat range, correct fuel/air mixture, correct spark timing and advance, no EGR, no PCV, cold air in, no heat crossover, yada, yada, yada.

Most of those who come on this forum looking for advice don't have a clue about how to set the squish on a motor and in fact may never have heard of squish. Without that basic knowledge and the rest of what I have enclosed in this post about running a motor on pump gas, the average home builder is somewhere between lost and don't have a clue.

Therefore, for you to make a broad and sweeping statement that any engine worth building should have 180 psi is, in my opinion, irresponsible.

Chevrolet327, warm the motor, pull all the plugs, disable the hot line to the coil, wire the throttle blades wide open and have at it again.
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Old 07-24-2009, 06:19 AM
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Quote:
Originally Posted by chevrolet327
the engine have flat top vortec head 0.015 steel gasket 12-243-2 camshaft
226-234 duration lobe centerline is 111 and intake centerline 107 i only have 150 psi cranking pressure i think its low i would like to know if i give 4 degree avance on camshaft it will climb the compression or do i need a smaller cam
I wouldn't worry about it as long as the engine is running as it should- the gage could be reading low, too, for that matter.

It's the deviation between cylinders that's more important, IMO. The cam timing can have a bearing on your compression readings, a leak down test can shed some light on whether or not there's excessive leakage anywhere.

Remember to warm the engine and hold the throttle open for the test, as TI said.
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Old 07-24-2009, 03:17 PM
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Quote:
Originally Posted by chevrolet327
the engine have flat top vortec head 0.015 steel gasket 12-243-2 camshaft
226-234 duration lobe centerline is 111 and intake centerline 107 i only have 150 psi cranking pressure i think its low i would like to know if i give 4 degree avance on camshaft it will climb the compression or do i need a smaller cam
I'm not sure why your concerned about cranking pressure. Given your cam specs this isn't too bad. What's going on is that there are two measures of compression that being the Static Compression Ratio (SCR) and the Dynamic Compression Ratio (DCR). DCR is what the gauge is measuring, but they have at best a loose relationship thru the cam timing and port velocity events.

1) The SCR is simply a mathematical relationship of all of a cylinder's volumes (piston displacement plus, deck clearance, plus gasket, plus combustion chamber, plus or minus any dish, relief, or dome on the piston) divided by the piston displacement. This hasn't much to do with gauge pressure.

2) The DCR is gauge pressure and is way more complicated as the true operating cylinder pressure is governed by what's called Cylinder Trap Pressure (CTP) or Volumetric Efficiency (VE). This is a measure of mixture density against the SCR which is cylinder gauge pressure. The DCR is sensitive to the SCR times the intake valves closing event in degrees After Bottom Dead Center (ABDC or just BDC) and dynamic operating events that change with throttle position and RPM. The most stable event is intake closing in degrees because those degrees don't change with RPM so it is the foundation of DCR calculators such as that found at the Kieth Black Piston web site. But keep in mind that as RPMs go up those degrees go by faster so in real time the event gets shorter.

The tricky part is what happens to the DCR as an effect of RPM on port velocity and throttle position on manifold vacuum. When cranking with a late closing intake, there is little in the way of port velocity or mixture density, this allows a rising piston to push the incoming mixture back out past the intake valve at these low speed events and will make an otherwise high SCR engine's compression appear to be low. An earlier closing intake of a milder cam would raise the cranking pressure, but can get you into other problems at higher RPMs by resulting in excessive dynamic compression pressure for the available fuel octane.

As RPMs and the throttle opening is increased both the mixture velocity and
density is increased. At a point closing in on the torque peak, the combination of velocity and density overcomes the reverse force of the rising piston and the mixture is rammed into the cylinder increasing the trap density (VE) when the valve finally closes. This causes the compression pressure to be higher than what would be expected from the SCR. This effect is what makes the high speed internal combustion engine a high speed engine. This allows the engine to continue breathing at high RPMs when in actual time, not degrees, the cam events are becoming shorter, so to keep gaining RPMs the engine needs to get more in and out in less time and this ram effect against the rising piston is what does it, unless you resort to a supercharger.

If you go to the KB Piston site and use their calculator you will see that your static compression ratio is higher than your dynamic ratio. The dynamic in their calculator has compensation for the intake closing event and the dynamic ratio will be lower than the static, however, it does not compensate for how the dynamic changes with engine speed and throttle opening which increases the DCR with RPMs and throttle opening. This is probably a pretty good guess at what to expect for cranking pressure. So take the (DCR times 14.7 psi) plus (14.7 psi) (times an adjustment for altitude if you live in the mountains). The answer is Absolute Cranking Pressure and should be damned close to what you're reading on the compression gauge. Keep in mind that at cranking speed things aren't happening very fast which gives time for pressure to bleed around the rings this may well show a gauge cranking pressure about 15% lower than computed. Oiling the rings will seal them up, which of course is closer to events when the engine is running and the rings are swimming in oil. But still this isn't anywhere close to the milli-seconds between events of a running engine. So expect number that are in the ballpark, but not in the glove close.

Bogie
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Old 07-24-2009, 04:14 PM
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This should help to illustrate the relationship between rpm's, volumetric efficiency and brake mean effective pressure (not directly attributable, but indicative of cylinder pressure)......

This is a 355 Chevy, AFR 195 heads, RPM intake, 850 carb, 1 3/4" headers, 10.5:1 SCR, solid flat tappet cam 248 @0.050" 0.560" valve lift............

RPM VE BMEP
1000 47.1 74.9
1500 57.1 111.5
2000 67.4 138.9
2500 70.2 141.6
3000 76.4 152.5
3500 85.6 173.3
4000 94.5 191.1
4500 101.5 203.6
5000 107.1 211.2
5500 107.7 206.5
6000 107.3 200.0
6500 103.0 185.1
7000 100.2 170.8
7500 96.5 156.2
8000 91.6 132.9

On the DynoSim, this motor made 540 hp @6500 and 500 ft/lbs @5000. You can see that the maximum brake mean effective pressure coincides with the torque peak (the sweet spot in the combination).

Last edited by techinspector1; 07-24-2009 at 04:20 PM.
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Old 07-24-2009, 05:01 PM
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150 is extremely common and is the most common on stock engines and is the actual correct spec

you want all of the readings to be no greater than 20% difference from highest to lowest readins,
according to the repair manuals anyways
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