Help me determine my compression ratio Options

[Cevan]

So I've started my engine build and have the engine mocked up to measure the deck height. My preliminary measurements are around .030. I've got to do some juggling around of components to get them more equal to each other. Here's what I've got:

head volume: 58.2 CCs
bore: 96mm
stroke 71mm

Based on the cam I'm using here's what I get for different deck heights:

.040 static C/R 8.84:1 dynamic C/R 7.76:1
.050 8.63:1 7.57:1
.055 8.53:1 7.48:1
.060 8.43:1 7.4:1

The engine is going in a 914 (obviously) that will be used for spirited street driving. I have 93 octane gas available here. The car will be driven between sealevel and 1000'. I'm using webers with SSI H/Es and a Bursch muffler.

[Cevan]

So after refining and practicing my deck height measuring procedure, I get deck heights between .026 and .028. I think that's close enough.

If I use the .008 shim that comes in my gasket set, I'll have a C/R of 9:1. Will my final deck height of .034 be a problem? I'm using 2.0 heads that Len Hoffman minimally flycut.

[ChrisFoley]

Stock rods, 6000 rpm max?
I'd do it.

[ME733]

.............034ths inch is way too CLOSE as a deck height...P/H...piston to head clearance....you can assemble it that way, but if you EVER overrev it you will flat spot the rod bearings.....look at you own compression ratio calculations.....AT .040 ths the compression ratio is 8.84...at...050 ths deck height =8.63.....Do you really think that a 0.21 CR change on a street engine will make ANY significant horsepower change ???. IT will not......you can improve the engines H.P. output, revability,and longevity, by taking (a lot) of time to precisely balance all the rotating components., ensureing port matching of heads, manafolds,carburators,.stacks,.and getting a really good set of dual valve springs with proper seat pressure...and I highly recommend .050ths inch piston to head clearance for a street engine...unless you have a lot of experience in assembling engines...and take the time to clay check EACH piston to head clearance measurement....using .040 ths inch as a minimum....then use .050ths as you will have a safety margin. I am Intrested in how you have established a dynaminic C.R.on an unassembled non- running engine.......................

[Dave_Darling]

There is a formula that takes valve opening times into account for dynamic CR. It doesn't take into account chamber shape or any of a zillion other variables, but at least it's a number... (IMG:style_emoticons/default/wink.gif)

The conventional wisdom I've heard is to shoot for .040" clearance between the pistons and the heads. There are people who run less, but they really REALLY know what they're doing... The advice most of them have for the Rest Of Us seems to be .040".

--DD

[Cevan]

.............034ths inch is way too CLOSE as a deck height...P/H...piston to head clearance....you can assemble it that way, but if you EVER overrev it you will flat spot the rod bearings.....look at you own compression ratio calculations.....AT .040 ths the compression ratio is 8.84...at...050 ths deck height =8.63.....Do you really think that a 0.21 CR change on a street engine will make ANY significant horsepower change ???. IT will not......you can improve the engines H.P. output, revability,and longevity, by taking (a lot) of time to precisely balance all the rotating components., ensureing port matching of heads, manafolds,carburators,.stacks,.and getting a really good set of dual valve springs with proper seat pressure...and I highly recommend .050ths inch piston to head clearance for a street engine...unless you have a lot of experience in assembling engines...and take the time to clay check EACH piston to head clearance measurement....using .040 ths inch as a minimum....then use .050ths as you will have a safety margin. I am Intrested in how you have established a dynaminic C.R.on an unassembled non- running engine.......................

Whoa there cowboy. I'm not making any assumptions here. And I didn't say or infer that a .21 change in CR would or wouldn't make any significant difference in horsepower. If you read my title, I'm asking for help in setting my compression ratio and deck height. So I appreciate your input.

As for the dynamic C/R, I used the compression ratio calculator from the website that makes Keith Black pistons (the ones I'm using) as Dave D mentioned.

[Cevan]

There is a formula that takes valve opening times into account for dynamic CR. It doesn't take into account chamber shape or any of a zillion other variables, but at least it's a number... (IMG:style_emoticons/default/wink.gif)

The conventional wisdom I've heard is to shoot for .040" clearance between the pistons and the heads. There are people who run less, but they really REALLY know what they're doing... The advice most of them have for the Rest Of Us seems to be .040".

--DD

If I use my lower number (.026), then I would need a .014 shim set. The best I could find were shims that come in sizes every .010, so a .020 shim gets me to .046.

[ME733]

There is a formula that takes valve opening times into account for dynamic CR. It doesn't take into account chamber shape or any of a zillion other variables, but at least it's a number... (IMG:style_emoticons/default/wink.gif)

The conventional wisdom I've heard is to shoot for .040" clearance between the pistons and the heads. There are people who run less, but they really REALLY know what they're doing... The advice most of them have for the Rest Of Us seems to be .040".

--DD

If I use my lower number (.026), then I would need a .014 shim set. The best I could find were shims that come in sizes every .010, so a .020 shim gets me to .046.

.....................well there you have it...Use .020ths of shimms...to achieve .046ths inch of P/H clearance....you will be safe even from a 7500rpm missed shift.(unless the valves float).