How To Figure Compression Ratio

[MOPARMAGA]

The 255 cc stealth vs the 240 cc on the trick flow. Might sound better but the trickflow ports are a better designed port. The combustion chambers on trickflow heads are much better than stealth heads and the trickflow can be ported to max wedge, good luck with that in a stealth port.
Trickflow heads also come with higher quality parts such as valves, seats and springs out of the box

 

[MOPARMAGA]

With that much stroke, you need the dished pistons for pump gas. A flattop might be as much as 12 to 1.
You could check a compression ratio calculator (I like the one at Wallace racing calculators) and see if you have enough info. I realize MOST of the info you'd have to use would be guess-work. Probably most important spec would be the dish volume, kit info might have that for you.
Comment above is correct. To get an ACCURATE comp ratio, everything has to be actually measured.
(You can possibly get an idea how much the heads have been shaved, maybe. The outer row of head bolt holes are 1" thick stock. If you can measure them now, and get, say, .950, head has probably been shaved about .050.)
And there is NO direct correlation between cranking pressure and compression ratio. WAY too many variables.

12.9 with flat tops in my 470 with 65cc chambers

 

[33IMP]

12.9 with flat tops in my 470 with 65cc chambers

Yabbut! That's a tiny chamber for a big block mopar! Probably 20+ less than his iron heads, lol.

 

[MOPARMAGA]

Yabbut! That's a tiny chamber for a big block mopar! Probably 20+ less than his iron heads, lol.

Agreed, it was 11.6 at 78 cc's

 

[33IMP]

Agreed, it was 11.6 at 78 cc's

Even that's a bit much for pump gas... without a GIANT camshaft, lol.

 

[MOPARMAGA]

Love the car, especially the license plate frame. I remember Hines.

That's interesting

 

[33IMP]

I was under the impression that the whistle device, at least the one used by NHRA, just confirmed displacement.

 

[Dave999]

if its a high static CR engine running pump gas and has iron heads id guess the cam was chosen paying particular attention to dynamic compression ratio. i.e everything that Rusty said in his post mashed into an easy to use figure that can be related to something we all understand which is static CR dynamic CR is a tool not the be all and end all but it can shed some light.

i.e its not the the static cr that makes it ping its how much compressing is done after both valves are properly closed and the speed of the piston doing the compressing

dynamic CR is like static CR but takes into account rod length and stroke hence the position of the piston and its speed, and the seat to seat time of the valves

with the right cam you can run high static CR motors on the street BUT the kinda cam we are talking about will tend to rob some of the low RPM torque
this might not be an issue with the right gears and converter but it would be obvious in a manual car unless you move to port on port induction which can help grab some of it back.

id be inclined to say don't worry, someone put some time and effort into that combo and it works


Dave

 

[Killer6]

We have a circle track close by. Maybe I'll get lucky?? Thanks!

First of all, welcome to FABO!!!
Second, that Dart is Sleeperiffic Badassness!!
Last, if the cam is a hydraulic, it can skew the cranking compression results & barely even get You in the ballpark. Cranking speed, oil pressure while cranking, engine temp, design bleed-down rate of the lifters, all affect what You'll see cranking PSI-wise. Most track engines using that whistler are almost 100% solid cams, so keep that in mind.

P.S. I was working on a couple's '70 440-6 'cuda, They shared that a station owner told them He let His buddy dump his chemical leftovers....including the thinner from cleaning out the paint guns etc., in His tanks....needless to say They never bought fuel there again..

 

[92b]

I was under the impression that the whistle device, at least the one used by NHRA, just confirmed displacement.

A pump is for displacement. A whistler is for compression ratio.