Need help figuring compression ratio..

[TruckManiaMike]

hello again everybody! Trying to find compression ratio for my 440 to find the right cam to throw in it and I am befuddled. I thought I got all the parameters dialed in the calculator but must be wrong. I come up with a CR of 8.14:1 which should be 119.6psi but I compression tested the cylinder at 170psi. I have TRW l2388 flat top pistons with 1.926 CH and cylinders bored 30 over. Stock 906 heads should be 88-93cc.

I want to upgrade heads to aluminum Edelbrock but unsure what cc chamber to get and which cam to go with it for my street rod truck. Was thinking comp cams .484 nostalgia cam, comp website recommended 9.5:1 compression but does that matter? I’m new to all this so please learn me something!!

 

[Don Frelier]

.125 Deck Clearance would be 1/8" down in the hole at TDC.
It must be higher up based on your results (but if you haven't measured the heads I would use the higher 93 cc number for the head volume)

 

[TruckManiaMike]

I looked up the pistons to be an ultra low compression piston. Just roughly tried to measure CH with pistons out of motor from where the piston rings stopped scarring the motor to deck subtracted by the top of rings to top of piston and got .150 CH which dropped CR to 7.8:1….read in another forum .150CH Is accurate for my pistons

 

[TruckManiaMike]

I’m thinking my 906 heads must have been milled down to close the chamber up…

 

[RemCharger]

I’m thinking my 906 heads must have been milled down to close the chamber up…

Are the heads on the motor or off?

 

[TruckManiaMike]

They are off

 

[RemCharger]

Well then let's have a look

 

[TruckManiaMike]

Sunken valves so I’m replacing with aluminum.

 

[Don Frelier]

They would need to be milled a bunch to essentially turn them into closed chamber heads.
I cut mine .055" and got about 13 CC reduction.
Intake was milled .062 otherwise the intake wouldn't fit.

I'd get the smallest chamber they offer.

 

[RemCharger]

I think the e-streets are 75 cc. My daughter's boyfriend runs those on a factory bottom end in a 76 camper special with a 292 purple shaft and RPM intake. Runs 12.50s decent cheap combination

 

[1 Wild R/T]

Got a pair of calipers? Measure the bolt flange below the spark plugs.... from the head gasket surface to the bolt contact area is 1" on a stock head... So if it's .990 it's been cut .010

Without factoring in the Intake closed angle you can't calculate what the cylinder pressure should be... So 119.6 could easily be 140 or 110... But 170 either means you have dramatically less chamber volume that you think... Or your compression tester is faulty...

 

[TruckManiaMike]

Okay, I’ll just get the smallest CC edelbrocks and roll with it. But i think my heads were decked down from 88cc to maybe 75-80cc which would add up to the right CR to put out 170 psi @8.5:1 CR. So I’m hoping swapping to 75cc heads holds me right around where I’m at maybe a little more compression around 8.5-8.9:1 CR would that be bad to run with the nostalgia cam that seemingly recommends 9.5:1?

 

[TruckManiaMike]

New compression tester, and I got .772”

 

[Don Frelier]

Wrong direction from the gasket surface to where the bolt head sits.
Is the gasket still on there?

 

[1 Wild R/T]

Now try turn the contact points 90 degrees so your measuring the thickness of the head... Head gasket surface to bolt contact pad...

 

[Budnicks]

I think the e-streets are 75 cc. My daughter's boyfriend runs those on a factory bottom end in a 76 camper special with a 292 purple shaft and RPM intake. Runs 12.50s decent cheap combination

you can get the E-streets & now the Performer RPMs
both in 75cc chambers now,
in I know 84cc (when I got my last set RPMs the smallest CC was 84cc)
& possibly 88cc too
has been for a while, the E-streets are the same ports, cheaper parts used

Edelbrock, they own Trick Flow now too

RPM 75cc

Search results for: 'rpm big block chrysler 75cc cylinderhead hydraulic roller camshaft 60825' - Edelbrock, LLC.

Edelbrock is the most respected name in performance! Since 1938, Edelbrock has manufactured its core products like Carburetors, Cylinder Heads, Intake Manifolds in the USA for quality and performance.

www.edelbrock.com

 

[1 Wild R/T]

you can get the E-streets & now the Performer RPMs
both in 75cc chambers now,
in I know 84cc (when I got my last set RPMs the smallest CC was 84cc)
& possibly 88cc too
has been for a while, the E-streets are the same ports, cheaper parts used

Edelbrock, they own Trick Flow now too

RPM 75cc

Search results for: 'rpm big block chrysler 75cc cylinderhead hydraulic roller camshaft 60825' - Edelbrock, LLC.

Edelbrock is the most respected name in performance! Since 1938, Edelbrock has manufactured its core products like Carburetors, Cylinder Heads, Intake Manifolds in the USA for quality and performance.

www.edelbrock.com

88cc is the open chamber version...

 

[TruckManiaMike]

I got 1” exactly on all but the end bolt hole which was .995 but measure other head and all were 1”. So I’m off somewhere!

 

[zyzzyx]

The heads came out of the factory really close to 1.000. Your heads have NOT been cut! How far down from the deck are the pistons when
they are at TDC?

 

[RemCharger]

I got 1” exactly on all but the end bolt hole which was .995 but measure other head and all were 1”. So I’m off somewhere!

I'd say your compression tester more off than your caliper!! (.005 )