Hey guys,

I've got a 350 with 305 heads (small combustion chambers) on it and it should be about 9.5:1 - 10:1 compression ratio. When I do a compression test it has about 150 psi of compression. I've been told an atmosphere is about 14.7 psi at sea level, so is my reading of 150 psi about 10:1?

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84 Camaro Z28
350 4 bolt main with 305 heads
Engle Cam 210/216 @ 0.050
K&N Filter
Weiand Action Plus intake
Accel Supercoil
Suspension Techniques lowering springs
Energy Suspension bushings

 

You can't tell what the compression ratio is by the cranking compression, there are too many things that affect cranking compression like ring seal, and cam type(big cams usually lower cranking compression due to higher overlap).

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92 Camaro RS 5.0 5-Speed (Quasar Blue)
T-Tops
14" Open Air Cleaner
3.08 Posi-trac
Edelbrock TBI Intake
Crane cam
Ported & polished stock heads 3 angle valve job
HyperTech Chip
SLP Headers (Ceramic Coated)
UltraFlo cat-back exhaust

74 Dodge Dart Sport 360 (11.2 1/4 mile)

2000 Dodge 1500 Ram Sport

 

BK,

If you know the specifications of your engine, you can calculate the static compression.

This may help if you have MS Excel or Corel QuattroPro:

Engine Calculator.xls

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Later,
Vader
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"Let the bodies hit the floor!"
Adobe Acrobat Reader

 

If you take Vaders advice post what you come up with. Im curiuos yet lazy .
SSC

 

The only way you could measure your CR by the ratio of pressure readings is if you had some way to compress the gas n the cylinder, let it sit there for long enough to cool back down to ambient temp (remember, the act of compressing it raises its temp, which adds to the pressure) and then measure the pressure. Obviously this isn't practical.

Readings anywhere from 140 to 180 is pretty normal for a properly working engine. Consistency is about the only important thing about those readings.

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"So many Mustangs, so little time..."
ICON Motorsports