How do I calculate compression or guess

[greymouser7]

Some data liberated from krazykuda's post!

How do I calculate for compression with speed pro Pistons & J heads?

Am looking at the H116CP40 Speed Pro Pistons,
stock stroke 3.58?" Crankshaft,
(LA 360)

& a

nominal? Deck height? 9.6" or
9.58?"

Head gasket volume??? 12.4cc?
(I know this varies, but have no other data-what gaskets are recommended?)

H116CP40
Comp Dist 1.66"
Piston Dome= +5cc

Not sure of swept volume for the cylinder-> 752cc?

4.04" bore
Area of cylinder? 12.819 cc?

-for stock J heads & another set of J heads milled .040"

""The #3418915 'J-Heads'

'J-Head' Combustion Chambers are typically at 68.0 - 72.0 CC's.

Also, the Intake Port with the 2.02" Valves has measured as much as 3 CC's less than the
'X-Heads' {157 to 160 CC's}.

But, the Exhaust Ports on the 'J-Head" have measured as much as 3 CC's larger than the
'X-Heads" {73 to 70 CC's}.

Port wise, they are nearly identical, with a very minor variation in Port Volume.

Small Block Cylinder Heads

CC's / Port Volume / Size / Flow

1968 - 1971 {340} Casting #2531984 'X-Heads'

Open-Shaped Combustion Chamber

Factory CC Level..........................65.0 to 69.0 CC's
Recommended Minimum CC's......63.5 CC's

Intake Gasket Port Size..............1.16" x 2.27"
Exhaust Gasket Port Size............1.25" x 1.75"

Intake Valve..............................2.02"
Exhaust Valve............................1.60"

Intake Bowl Size........................1.75" {87% of Valve}

Intake Port Size........................160 CC
Exhaust Port Size.......................70 CC

Intake Flow.............................215 CFM @ .450" Lift........221 CFM @ .500" Lift
Exhaust Flow...........................140 CFM @ .450" Lift........142 CFM @ .500" Lift

Intake-to-Exhaust Flow....................65.5%..............................64.1%..............""""... "The notable "X" and "J" heads (named for an X or J cast into the head near the spark plug hole) fall into the large port category, as do cylinder heads with "915" as the last three digits of the casting number."


Recommended head gaskets?

Street driven cars.
One motor gets the
crane 693902 H-260-2
1500-4500 RPM lobe sep 112
Intake Duration @.050"Lift 204
Exhaust [email protected]"Lift 216
ADV Intake Duration 260
ADV Exhaust Duration 272

The other gets the Elgin P/N CL3003+103+VS9010-16

• Valve Lift (INT/EXH): .429/.444
• ADV Duration (INT/EXH): 279/289
• Duration @ .050" (INT/EXH): 209/220
• Centerline (degrees): 114
• Power Range (RPM): 1200-5200

Same as OE Purple #2899206 340 275hp cam

Not sure if the cam specs really matter but some of you guys have made comments about running too small of a cam with higher compression ratios.

 

[txstang84]

Krazykuda had a calculator which detailed it very well in a sticky but I don't see it, unless it's in another forum

Edit: just noticed you mentioned karl...

 

[yellow rose]

You can't just put numbers into a formula. You have to measure everything

 

[RustyRatRod]

You can't just put numbers into a formula. You have to measure everything

What he said ^^^^^^^

 

[greymouser7]

You can't just put numbers into a formula. You have to measure everything

So measure the heads before buying the pistons?

Is the deck height measurement critical for this? Some one said they cast the LA blocks pretty tall (does that mean more than 9.6?
Do they all have have the decks machined?)

 

[txstang84]

You can't just put numbers into a formula. You have to measure everything

I agree...hence the reason I mentioned his formula for calculating it...which involves a good deal of measuring.

 

[AJ/FormS]

Well, you could "ball-park it", to help with piston selection. Then when the pistons are finally in the block, you can work out the rest.
Or you could mock it up with what you got,get an in-the-hole measurement for that piston, then pull it out and measure the compression distance. A little math can then get you close to the actual block deck height.Close to, but not exact.
But I just gotta ask; What are you up to?
And why/how did you come to choosing those cams?Are you trying to work backwards to a particular Dcr number?.

 

[greymouser7]

Well, you could "ball-park it", to help with piston selection. Then when the pistons are finally in the block, you can work out the rest.
Or you could mock it up with what you got,get an in-the-hole measurement for that piston, then pull it out and measure the compression distance. A little math can then get you close to the actual block deck height.Close to, but not exact.
But I just gotta ask; What are you up to?

Code enforcement is cracking down again. I surprised him cause my two rolling chassis are registered, licensed, insured & now he wants 'running' -ok.

But every time I get to a running 360 it sells before I can buy it.

I have rebuilt j heads

& a deal for another set of assembled J heads -.040" milled.

Was going to put together two lowish buck 360's for 2 rolling chassis to finish out my hobby.


I Have: water pumps, timing chains, intakes, 273 rockers, a few block-cores, I think Rusty still has my distributors & I got to get him 2kits for the weights, 1 new eddy carb, rocker shafts (I need to know about them & 273 rockers), 2X headers, overhaul gasket sets, bolts, alternators, cam shafts-springs-lifters-retainers, valve covers, -some but not everything to complete & the budget just got better this 1 June to try & finish this up.

I have never done this before though. I have the books, & still need a few parts.

I am getting ready for deployment & would like to get this knocked out before I go to end the drama.

 

[AJ/FormS]

I'm not sure rushing one engine together is a good idea, much less two.
Have you considered dropping a couple of teeners in them to satisfy the man?
Or possibly crate engines.
FWIW: I'm not keen on those camspecs at all; but I have no idea what you are up to.

 

[krazykuda]

You can't just put numbers into a formula. You have to measure everything

Yes, you have to make a cover plate and measure the volume of the heads and how much volume is above the piston at TDC. Then we can plug them into formulas or calculators to find the compression.

You need to measure your parts to see what exactly you are running to get an accurate compression reading...

 

[moper]

If this is to satisfy Big Brother - I'd grab a couple of running 318s for now. Anything really that will bolt in unless you have the cash to complete the two builds simultaneously. In regard to the pistons - Any open chamber head will work fine with them. Even one milled a bit. I'd expect a set milled .040 to be in the 70-71cc range. I have never, ever seen a set of factory original heads that was at the spec of 72ccs when measured. All were larger, some by 4-5ccs. Regardless, the pistons will be below deck in any block. Below deck by a bit (IMO) in a stock, non-blueprinted block. So if it's a concern of too much static compression - you really have no concern. The other deal is the gaskets. You have choices from .028 or so through .050+ depending on the brand. So again - you have no worries. Order them up and get the engines built if you have the means.

 

[nm9stheham]

You indeed have my sympathies with the local ordinance; I am building a garage just to satisfy the same crappy issue. The golf course neighbor who sicked them on me and another gearhead nearby had better watch their step.... the next gang of drunk golfers who come onto our property may generate a call to the sheriff's office and a letter to the zoning board about the golf course being a public nuisance.

With the right range of numbers, you can get pretty close. Look up the Pat Kelly calculator (that I like) and use the 3rd tab.
- Nominal deck heights of an LA block are 9.599" or 9.600". Magnum block is 9.585". With these numbers and the compression height of your piston, a 6.123" rod length, and a 3.58" stroke for the 360, you can work out the deck height. For the pistons you mention, I compute a deck height of -.028".
- Nominal head combustion chamber volume for the 360 head: I'd use 72 or 74 cc for a ball park number.
- Nominal Felpro head gasket thickness is .051". Compute the volume from that and a 4.17" gasket hole diameter, or plug the number into the calculator.
- Piston eye brow volume is the 5 cc's listed for that piston.

For comparison for your results, I get 8.77 SCR from the above mentioned program, your pistons choice, 74 cc combustion chambers, and standard Felpro gaskets. That is a good compression ratio, but not a high one IMHO for a street engine.