Measuring Static CR

[rb77413]

Ok since the Newport is headed to the body shop shortly, I want to spice up the 383. When I had it built, The guy doesn't quite remember the exact CR. It is bored .030. It runs fine on 92. I believe it has roughly 9.5:1 although I am expecting to see 9:1. Is there any way to measure the CR without taking the heads off? Is there a formula that can figure the CR given a compression test along with cam specs. The cam is a Lunati Street Master Grind #07301 .477 lift @ 225^ @ .050 (275 adv. duration). I want to put a bigger cam. Some where about the .507 lift w/ 235 @ .050 285(adv) The spec says it needs 2500 stall (Already have) and 10:1. It currently has an Edelbrock performer 383 on it. I am going to get the Performer RPM with this cam, if the CR will support the extra duration. What do you all think? It has 3.23's for now. Probably going to get 3.91's. or 3.55's. I need a little cruisability Thanks for any help.


Bob

[cuda66273]

The only way to get the true static compression is to pull the heads, and even then it's a task.

You have to CC the cylinder head combustion chamber.

Measure how far in the hole your pistons are.

Calculate the compressed thickness of the head gasket.

Measure the exact stroke

Then you get to do all the math.....

Once you get all that done then you need to calculate cylinder pressure created from valve and cam timing.

As you build more compression you have a few other things to address....Crank case pressure, dressing the tops of the pistons, removing burrs and sand texture from combustion chambers, calculating intake and exhaust velocity, quench, volumetric efficiency and on and on.....

Enjoy
Don

[451Mopar]

If the engine is on an engine stand, you can cc the engine through the spark plug hole. Rotate the engine so the sparkplug hole is the highest point. Put the engine at TDC and fill the chamber (while measuring the amount) with oil. When the chamber is full write the volume of oil it took to fill, this is the volume above the piston.
If you know the bore and stroke of the engine you can calculate the piston swept volume, or you can rotate the engine 180 degrees to BDC and continue to measure and fill the chamber.

Take the pistons swept area found in the second step and add the volume above the piston (at TDC) from the first step, then divide by the volume in the first step to get the compression ratio.

Before you get a bigger cam, make sure there is good piston to valve clearance.

[rb77413]

This can be done with checking springs right? Dunno the clearance but my Mopar engines book has them. This nengine has just under 3K miles on it. Also tossing around the idea of Aluminum head if the CR isnt too low. Dont want Al head if the engine is 9:1. Would be like 8;1. right? I heard tothat Al heads drop CR 1 point to compensate for the rapid heat dissipation of the AL. 10:1 is eqiv to 9:1 for octane requirements per say. Thanks again.

Bob

[mr.b]

use a compressiom guage on the cylinder. anything over 170 psi you may start to experience pinging esp. with iron heads. go to www.prestage.com to help if you have and spec. on the engine. cam profiles can vary compression ratios. if the engine is together then the best way to know exactly what you have is to use a compression guage.

[jelsr]

Any cam with .500 or so lift. Check piston/valve clearance, and valve spring retainer to guide clearance. This is not a contact sport! The retainer can contact the top of the guide and cause problems just as piston/valve contact can. I would check some other cam sites on compression requirements that express their views as PSI rather than ratio. Hughes Engines gives the figures as "XXX PSI minimum compression" for each of their cams, for example. You can then compare Dur@050 & Lift figures to arrive at PSI requirements VS CR ratio. Most will also give operating range. (X RPM to X RPM). Top end power is gained at the expense of low end power and driveability. Good luck.

[CrazyMoparGirl]

Just to clue you all in. When we had the engine rebuilt the mains were line honed. So the timing is a little sloppy. We decided that we were going to use a gear drive. However the only quiet gear drive available, that we have found, is for 3 bolt cams. Hence why we are being so specific with our cam selection and are questioning ways of checking static C.R.