I'm trying to understand what cranking compression can tell me. My BBC has 230psi after about 5 pumps. This was done cold with the lash at .021 Cam is set/measured as follows:
int @.05 open 33 BTDC, close 72 ABDC max lift .775
exh @.05 open 85 BBDC, close 33 ATDC max lift .740

Calculated Static CR is 14.38:1

Any help is appreciated graemlins/waving.gif

It's telling you there is a small chance it may not run on pump gas :D :D

I don't know, my cranking pressure is 225 psi, calculated CR is 11.2:1 and it runs on Sunoco premium just fine.

Originally posted by Epistuff:
I don't know, my cranking pressure is 225 psi, calculated CR is 11.2:1 and it runs on Sunoco premium just fine. =
Well that's a bit of an exception.
11.0 tuned properly & built right is not a problem (unless you live in CA with 91 premium :( ) but 225 cranking is a little high for my liking.
Now build yourself a 14.0+ 230psi one & tell me how good it runs on premium anything ;)
BTW, I have 3 different guages & they are not within 30 psi on the same engine.
The digital one that reads the lowest seems the most believable.

Hey Mike please try and smarten me up some here. :D I have only used CC for street eng to look for differences across the board. Is 230psi about what you would expect for a 14.3:1? From what I have heard the CC has a lot to do with the cam and valve lash??? Do any of your engine dyno software packages tell anything more? I have heard a great deal of BS about this over the years. :confused:

I know I never mentioned this, but on this 555 I use a Fel Pro 1057 head gasket with no issues to date. I spray paint them with Aluminum paint (old habit) and torque ARP bolts down in three steps to 60ft-lbs with moly. Remember I spray a light 175hp NO for the 3 to 4 sec. Last motor I had O ringed with SS wire and copper gasket and it was always a pain in the butt with water leaks. Fel Pro makes a very good head gasket for sure. There O ring in the gasket actually embeds in the head and helps a lot I think. It's no fluke as I have about 500 runs on this engine with that set up. Just figured I'd give you another data point. ;) As I'm sure no rational eng builder would try using gaskets with 14+:1 and NO... :cool:

BTW I picked up a new set of those Summit Pro line (Detroit power) +250 BBC rods.

That 230 psi is your dcr or dynamic compression ratio as opposed to your 14.38 static if I am correct. It tells you if you camshaft is building or bleeding cylinder pressure, what type of is most likely required and probably many other things that I cannot think of. Standard air pressure is 14.7 psi and your scr is 14.38 so if your cam did not build or bleed any compression the dcr would measure at 211 psi. The 230 means the cam is building cylinder pressure. Higher cylinder pressure shows that is pushing the piston down with more force hence more power. I said so much I think I lost myself...wolfplace take over

Mike,
I think my exception comes from the fact that I run closed chamber heads, a stick and 4.10 gears. Seems to be a combo that is not prone to detonation. I have run 91 octane at times without an issue, but I usually run the 93 or 94 Sunoco. Now, a 14:1 motor,.....I would probably never find out because my battery and starter would never turn it over smile.gif

Got digging around and found a cool web based calculator. Man the WWW is unreal... :D It says it should be 232psi with the measured 72 ABDC. I did not have the throttle body open graemlins/clonk.gif so that was worth about 5 psi. Still crawling but very interested.... :confused:

cc calc (http://www.rbracing-rsr.com/comprAdvHD.htm)

Tossed in a 505 I have, but have never run dyno ed or measured CC. Using this calculator, it says 258psi with cam advanced 4 deg and intake close at 63 ABDC. This engine is 4.350 bore, 4.25 stroke 6.535 rod in short deck :D dart 109.5 Chamber and Weisco 48.4 CC dome. Piston in the hole 20 and ,038 gasket. 14.75: CR I need to get this one to the dyno. :D

Paul.
You are right, cranking compression is determined from when the intake valve closes.
The higher the static compression the higher the cranking compresson will be the sooner you close the intake valve.
Now there is a lot of other things that will effect it to a certain extent, like Barometric pressure,( a big one!) valve lash, cam intensity, engine temp, air temp, humidity & probably a lot of other little things I missed as well.
& of course,,, who's gauge you are using :D
I have a compression ratio program that ESTIMATES cranking compression from static, cam timing etc. but have found it less than accurate in real life as it uses .050 cam timing with different types of cams for inputs so it assumes a lot.
230 on a 14.0 engine is not unusual. It could be a bunch higher or lower depending primarily on cam timing.
In my opinion, you are using the gauge for what it is best at, seeing if the number makes sense for what you have & if all the cylinders are close to the same.

Just like Jim's (epistuff) deal on here, most engines are normally not going to like 225 cranking on pump gas but his does as he has a good combo. Lower gears, probably excellent quench & an efficient combustion chamber.
Lot's of variables :confused:

Here's another thought to screw up the works,,
You can build an engine with say 13.0 & overcam the crap out of it & on paper the dcr will tell you it will run on 91 octane.
The problem comes when that engine gets up in the rpm & becomes efficient & starts actually seeing that 13.0. Now that dcr becomes less of an issue as you are starting to keep pressure in the cylinder & it will detonate.
It also works the other way.
We do restricted engines you just could not put a load on at low rpm in most cases.
Small cam hi compresson deals like 13.5+ with a cam of say 240 or so @ .050 & at low speeds they will detonate themselves to death if you ain't careful but from say 3500-4000 up they will never see that 13.5 compression as the intake is too small to let enough air in to fill the cylinders :(
Is this enough confusion yet??

Doggy,
I sort of see what you are doing but it doesn't work that way. If you were compressing 14.7psi by 14.3 I suppose it would come out to 211 but someone else will have to answer that as I ain't an engineer,,hell, some days I have enough trouble doin' the machinist thing :D
Anyway, you are only compressing what is in the cylinder from the time the intake valve closes & this can be completly different depending on a number of things but at cranking speed most of them are listed above I think :confused:

Maybe one of our resident engineers will stop in & expand on this topic ;)


Paul,
Just looked at your last post & thanks for the link.
I will have to play with it & see how it comes out on some different engines

Mike..that calculator looks at the valve timing at .050...just thought I would pass that on

That calculator seems pretty close. It predicts 185 psi for my combo, and I've recently measured 180 psi, cold engine, throttle body closed. Note: removing the carburetor made no difference whatsoever.

You can't estimate cylinder pressure by multiplying atmospheric pressure (14.7 psi at sea level) by the mechanical compression ratio. Cranking compression is always higher because as the air is compressed it increases in temperature, which increases pressure. That is why my 7.8:1 DCR engine makes 180 psi cranking compression and not 7.8 X 14.7 = 115.

my 468 has right on the money 13 to 1 compression with very very open chambered heads..lol...and with a doug herbert roller cam part #B6H 256/268 @ .050 and .683/.685 lift. lash set at .024 it cranks out 270 psi. with the plugs pulled and the carb wired open with a fully charged battery.

i run 36 degrees total on it locked out. and drive around town with 91 octane fuel. but i run sunoco race fuel at the track.