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Discussion Starter #1
I looked it up a lil bit but i want to really learn how compression is figured and came up with and what factors to use and such. I know there are guys on here that know this stuff good. HELP?

Is 9.2:1 compression good for a procharged 2v?
 

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Proud American
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compression is calculated from the bore size, piston dish, how high the piston raises above the deck of the block, bore size of the head gasket, compressed head gasket thickness and the size of the chamber on the head....

9.2:1 seems to be just fine for procharged motor... I know that my motor which is 11.5:1 will do just fine on boost too, just depends on what fuel you want to run and how much boost....


I always use a calculator online to do my Compression ratio calculations, but if you look hard enough I am sure you can get the correct formula for it...
 

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Discussion Starter #3
my fuel will be 93 octane. Yeah i guess a formula or maybe something of that nature would help.
 

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Proud American
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93 with a procharger should be just fine....

Head Gasket space = (Bore ÷ 2)2 x 3.14 x Gasket thickness x 16.387064
Deck Height space = (Bore ÷ 2)2 x 3.14 x Deck Height x 16.387064
Compressed Volume = Head Gasket space + Deck Height space +
Piston Top volume + Combustion chamber volume
Uncompressed Volume = Compressed Volume + Displacement
Compression Ratio = Uncompressed volume ÷ Compressed volume
 

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93 with a procharger should be just fine....

Head Gasket space = (Bore ÷ 2)2 x 3.14 x Gasket thickness x 16.387064
Deck Height space = (Bore ÷ 2)2 x 3.14 x Deck Height x 16.387064
Compressed Volume = Head Gasket space + Deck Height space +
Piston Top volume + Combustion chamber volume
Uncompressed Volume = Compressed Volume + Displacement
Compression Ratio = Uncompressed volume ÷ Compressed volume
Yeah, that looks easier, lol. Nope, nevermind, still have a headache.
 

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Discussion Starter #8
hmmmm... ima read up on it a good bit. Good info guys. As long as the procharger will go good with this compression then ill be happy. How do i find out what i need to type in on the calculator? Where do i get the info of the cobra shortblock and livernois stage 2 heads to plug it into the calculator?
 

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Here's an idea. Measure how much oil it takes to fill the cylinder when it is at the bottom of the stroke with both valves closed (with a cyringe(sp?)), then measure the amount it takes to fill it with oil at top dead center with both valves closed. Take the first measurement and divide it by the second one. There you go, uncompressed volume divided by compressed volume. Ha ha, don't try that at home, or on your own car, lol.
 

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specs on the 03 Cobra Motor:

Engine
Displacement 4.6L (4V) (281 CID)
Number of cylinders 8
Bore 90.2 mm (3.55 in)
Stroke 90.0 mm (3.54 in)
Firing order 1-3-7-2-6-5-4-8
Oil pressure 138-310 kPa
Oil capacity 6 ± 0.25 a
Compression ratio 8.5:1
Cylinder Head and Valve Train
Cylinder head gasket surface flatness 0.10 mm (0.004 inch) max. overall
Combustion chamber volume 52.6 ± 0.5 cm
Valve arrangement (front to rear) b
Intake (left hand): S-P-S-P-S-P-S-P
Valve arrangement (front to rear)
Exhaust (left hand): E-E-E-E-E-E-E-E
Valve arrangement (front to rear)
Intake (right hand): P-S-P-S-P-S-P-S
Valve arrangement (front to rear)
Exhaust (right hand): E-E-E-E-E-E-E-E
Valve guide bore diameter —
Valve stem diameter—intake 6.975-6.995 mm (0.2754-0.2746 inch)
Valve stem diameter—exhaust 6.949-6.970 mm (0.2744-0.2736 inch)
Valve stem-to-guide clearance—intake 0.020-0.069 mm (0.00078-0.00272 inch)
Valve stem-to-guide clearance—exhaust 0.046-0.094 mm (0.0018-0.0037 in)
Valve head diameter—intake 37 mm (1.46 inch)
Valve head diameter—exhaust 30 mm (1.18 inch)
Valve face runout 0.05 mm (0.002 in)
Valve face angle 45.5 degrees
Valve seat width—intake 1.8-2.2 mm (0.071-0.086 inch)
Valve seat width—exhaust 1.8-2.2 mm (0.071-0.086 inch)
Valve seat runout 0.05 mm (0.002 inch)
Valve seat angle 45 degrees
Valve spring free length—intake 42.16 mm (1.6598 inch)
Valve spring free length—exhaust 42.16 mm (1.6598 inch)
Valve spring squareness —
Valve spring compression pressure—intake 711.47 N @ 26.19 mm (159.9 lb-ft @ 1.031 inch)
Valve spring compression pressure—exhaust 711.47 N @ 26.19 mm (159.9 lb-ft @ 1.031 inch)
Valve spring installed height—intake —
Valve spring installed height—exhaust —
Valve spring installed pressure—intake 289.1 N @ 36.14 mm (64.99 lb-ft @ 1.4228 inch)
Valve spring installed pressure—exhaust 289.1 N @ 36.14 mm (64.99 lb-ft @ 1.4228 inch)
Roller follower ratio 1.8:1
Hydraulic Lash Adjuster
Diameter 16.000-15.988 mm (0.6299-0.6294 inch)
Clearance-to-bore 0.018-0.069 mm (0.000709-0.002717 inch)
Service limit 0.016 mm (1.0006299 inch)
Hydraulic leakdown rate c 5-25 seconds
Collapsed lash adjuster gap 0.80-1.20 mm (0.0315-0.0472 inch)
Camshaft
Theoretical valve lift @ 0 lash—intake (primary and secondary) 10.0 mm (0.3937 inch)
Theoretical valve lift @ 0 lash—exhaust 10.0 mm (0.3937 inch)
Lobe lift —
Allowable lobe lift loss —
Journal diameter 26.962-26.936 mm (1.0615-1.0605 inch)
Camshaft journal bore inside diameter —
Camshaft journal-to bearing clearance 0.025-0.076 mm (0.00098-0.002992 inch)
Runout —
End play 0.025-0.165 mm (0.00098-0.00649 inch)
Cylinder Block
Cylinder bore diameter 90.2-90.239 mm
Cylinder bore maximum taper 0.016 mm
Cylinder bore maximum out-of-round 0.016 mm
Main bearing bore diameter 72.402-72.422 mm
Head gasket surface flatness 0.15 mm (0.006 in) max. overall
Crankshaft
Main bearing journal diameter 67.493 mm
Main bearing journal maximum taper 0.05 mm
Main bearing journal maximum out-of round 0.05 mm
Main bearing journal-to-cylinder block clearance 0.023-0.055 mm
Connecting rod journal diameter 56.866-56.886 mm
Connecting rod journal maximum taper —
Connecting rod journal maximum out-of-round —
Crankshaft maximum end play 0.130-0.301 mm
Thrust bearing journal diameter 67.493 mm
Thrust bearing journal maximum out-of round 0.05 mm
Thrust bearing journal maximum taper 0.05 mm
Thrust bearing journal length 17.725-17.775 mm
Piston and Connecting Rod
Piston diameter 90.180-90.191 mm
Piston-to-cylinder bore clearance -0.010/+0.026 mm
Piston ring end gap — compression (top) 0.30 mm
Piston ring end gap — compression (bottom) 0.50 mm
Piston ring end gap — compression (oil ring) 0.65 mm
Piston ring groove width — compression (top) 1.53-1.549 mm
Piston ring groove width — compression (bottom) 1.519-1.539 mm
Piston ring groove width — oil ring 3.031-3.055 mm
Piston ring width — compression (top) 1.47-1.49 mm
Piston ring width — compression (bottom) 1.47-1.49 mm
Piston ring width — oil ring 2.854-2.984 mm
Piston ring-to groove clearance — compression (top) 0.04-0.079 mm
Piston ring-to groove clearance — compression (bottom) 0.029-0.069 mm
Piston ring-to groove clearance — oil ring 0.047-0.201 mm
Piston pin bore diameter 22.0042-21.998 mm
Piston pin diameter 21.991-29.994 mm
Piston pin length 61.60-62.03 mm
Piston pin-to-piston fit 0.0058-0.0132 mm
Connecting rod-to-pin clearance 0.018-0.033 mm
Connecting rod pin bore diameter 22.012-22.024 mm
Connecting rod length 150.7 mm
Connecting rod maximum allowed bend 0.038 mm per 25 mm
Connecting rod maximum allowed twist 0.050 mm per 25 mm
Connecting rod bearing bore diameter 56.866-56.886 mm
Connecting rod bearing-to-crankshaft clearance 0.027-0.069 mm
Connecting rod side clearance 0.15-0.45 mm

a With installation of a new filter.
b P=Primary, S=Secondary, E=Exhaust
c Time necessary for plunger to leak down 1.6 mm of travel with 222 N force and leak down fluid in tappet.

Coolant Type
Motorcraft Premium Engine Coolant VC-4-A (in Oregon VC-5, in Canada CXC-10) (green color) ESE-M97B44-A
Motorcraft Premium Gold Engine Coolant VC-7-A in (Oregon VC-7-B) (yellow color) WSS-M97B51-A1
Radiator Cap Pressure Test Specifications 110 kPa (16 psi)
Thermostat Opening Temperatures
Starts to open (Cobra) 80-83°C (175-182F°)
Fully open 104°C (219F°)

Generator
4.6L, 4V 4G HO 120 amps @ 6,000 pulley rpm (2.87:1 ratio) internal regulator

Engine Ignition
Ignition timing 10 degrees BTDC ± 2 degrees non-adjustable
Firing order 1-3-7-2-6-5-4-8
Spark plug gap mm (in) 1.32-1.42 (0.052-0.056)
Spark plug type AWSF-32EE
 

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Proud American
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11,288 Posts
Baker... use a deck hieght of .015 (Thats what I always use when i dont know for sure) and I use a .004 gasket thickness with a modular motor.

and a livernois head is going to be be 54cc+ a few from the porting... I am in the middle of doing homework, so i am not going to run the numbers, but you get the picture...

with a -17 cc piston ( stock PI piston) you will probably be right around 9.2-9.6:1
 

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Discussion Starter #12
Hey matt... ima put these heads on a 03 cobra shortblock. Not my pi pistons. So what you say with that setup.
 

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Baker... use a deck hieght of .015 (Thats what I always use when i dont know for sure) and I use a .004 gasket thickness with a modular motor.

and a livernois head is going to be be 54cc+ a few from the porting... I am in the middle of doing homework, so i am not going to run the numbers, but you get the picture...

with a -17 cc piston ( stock PI piston) you will probably be right around 9.2-9.6:1
Use -18 for 03 Cobra short block if thats what you are still thinking about. -17 with your 2v block like Matt said.
 

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Proud American
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I take back my orginal estimates of mid 9s, and am going to fall back down to high 8s.... 8.8-9.0 : 1 seems more reasonable... which is plenty safe with the blower....

but run the numbers... I just dont have the time...
 

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You need to get the info on the heads because if they are 54cc then the compression ratio would be less than the 52cc stock Cobra heads. Stock compression is 8.5.
 

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Discussion Starter #17
im cheap! lol. forged shortblock new is just too much! I found a descent deal on a cobra shortblock so im going to rock the cobra shortblock with my livernois heads.
 

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I'm cool..
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Why dont you build the engine yourself. It would be even cheaper than buying a forged cobra short block. Unless you got the short block for under 2K?
 

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Here's an idea. Measure how much oil it takes to fill the cylinder when it is at the bottom of the stroke with both valves closed (with a cyringe(sp?)), then measure the amount it takes to fill it with oil at top dead center with both valves closed. Take the first measurement and divide it by the second one. There you go, uncompressed volume divided by compressed volume. Ha ha, don't try that at home, or on your own car, lol.
+1. If the heads are off it will be easier to measure. Compression and boost go hand in hand for a given octane. More compression and more boost will make more power, but will need race gas. Less compression (mid 8s) will handle more boost and make more power on pump gas at the expense of a drop in off boost performance and lower mpg.
 

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+1. If the heads are off it will be easier to measure. Compression and boost go hand in hand for a given octane. More compression and more boost will make more power, but will need race gas. Less compression (mid 8s) will handle more boost and make more power on pump gas at the expense of a drop in off boost performance and lower mpg.
So which do you recommend? A higher cr with less boost or a lower cr with more boost? A higher CR would be nice for a DD car...
 
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