Is my CR too low?

[Andy(UK)]

Having done this the wrong way round by buying my pistons based only on the CR guide on KB's website I have now calculated my compression ratio more accuractely and I find myself left with a CR that is (way) below the reccomended value by Edelbrock.

Ive got Edelbrock 6025 rpm heads with 2.02" intake valves, the Edelbrock RPM cam intake lift at valve 0.496" exhaust lift at valve 0.520".

Ive worked out that with my 71 302 (standard decks, will only be milled to bring into true) the CR with 60cc heads (2.022 intake valves) and KB115 6.5cc pistons will be 8.84:1. Edelbrock say I must have at least 9.5:1 for the RPM kit.

Heres what Ived used:

Compression Height 1.608in
Cylinder head volume 60.000cc, 3.66ci
piston head volume 6.500cc, 0.3965ci
Compressed Gasket thickness 0.047in
Gasket Volume by calculation 0.621
gasket bore 4.100 in
cylinder bore 4.030 in
deck clearance 0.016 in
stroke 3.000 in
rod length 5.090 in
Crank to deck clearance 8.206 in
Deck Volume 0.077
TDC Volume 4.754
Swept Volume 38.267
CR 8.84

Even if I bring the deck clearance to zero I only get to 9.1:1 and then Im concerned about valve clearances.

Have I totally cocked up!?! Will this engine be any good?

As an option I could sell my 115's and use the KB116's with the -2.6cc domes, that would bring me to 9.8:1 or higher depending upon the amount milled off of the decks to true the up.

Does anyone know if the KB116's will work with rpm kit on a 71 302 without causing valve clash.

You see this is what happens when someone with little knowledge tries to play a big boys game!

Many thanks for any help, I need it!



Andy

[nevermind65]

Find a .035" head gasket. That'll bring it up to 9.13, although I calculated the original to be 8.86.

Otherwise, you'll probably never know the difference unless you are blueprinting the engine.

[Andy(UK)]

Ive calmed down a bit now and worked out that if I change to a .039" Fel-Pro #1011-2 gasket and bring my deck height down to 8.2" from the standard 8.206 Ill have .002" deck clearance and a CR of 9.55:1.

Thats better!

Thanks

Andy

[69 Mach I]

God does not close a door in your face without opening up another. If your CR is that low, then you are a perfect candidate for some forced induction. God wants you to have a supercharger, so get right with Jesus and get yerself a blower.

[SCOBRAC]