I'm getting ready to pull my engine out to freshen up the bottom end and make some performance changes. Before pulling the engine I thought I would check the cold cranking compression to get a baseline so I can compare before and after. I checked all of the cylinders I could get a gauge on (couldn't get the gauge on #4 and #8 due to header clearace). All cylinders read 170#-175#. The heads are 119cc and the piston has a 32cc dome. The head gasket is a 1017 FelPro with a bore of 4.540 and a compressed height of .041. Current cam is .655/.660 248/254@.050 108LSA.

Planned changes include cc'ing the heads to 113cc. I'm also installing a new cam .725 254/260@.050 also on a 108LSA.

I'm wondering what the cranking compression will be with the new setup? The compression should increase from 10.37:1 static to ~11:1. The cam is bigger by 6*. The extra compression should raise cranking compression by ~8lbs and the cam having a little more duration will decrease that a little. I'm guessing the cranking compression will be 180#-185# with the new setup. Does this sound about right?

Thanks.

Ken, I went from a 266/266 108lsa 106icl to a 273/281 107lsa 104icl and dropped 20lbs. Went from 225avg to 205avg. Seat duration went from 300 to 311 on the intake. Tested with motor cold and throttle blades wide open. HTH

Originally posted by JOHN WILSON:
Ken, I went from a 266/266 108lsa 106icl to a 273/281 107lsa 104icl and dropped 20lbs. Went from 225avg to 205avg. Seat duration went from 300 to 311 on the intake. Tested with motor cold and throttle blades wide open. HTH Thanks, John. I assume that was with no change in static compression?

no change, your off setting the comp with cam.

I made a similar move with my SBC a couple years ago and managed to keep cranking compresison pretty close.

I went from 282* adv, 236* @ 050, and 10.3:1 static to 294* adv, 250* @ 050, and 11.3:1 static. Pressure went from 215 to 220 psi.

I think you are right that the cranking pressure will increase by a few pounds.

Eric

Ken, when I changed heads I had to cut the intake reliefs .150 for the 23dg valve. This added ~1cc which dropped me from ~12.6 to ~12.45.

Ken - Assuming that the advertised duration is also 6 degrees difference, my calculations show a 7 psi gain in cranking compression.

edited

Someone has found a relationship between DCR and cranking compression:

<snip>....At cranking speed, the absolute cranking pressure (“CP”) is a function of the 1.2 power of the effective compression ratio (i.e., for 8-1 compression ratio, use 8^1.2) times atmospheric pressure (14.7 psi @ sea level, &c.). This adjustment (1.2 power) is a polytropic value used in preference to the traditional adiabatic value (1.4) for the ratio of variable heats for air and similar gases at the temperatures present. This compensates for the temperature rise caused by compression, as well as heat lost to the cylinder. 1.2 is not accurate in all cases, since the amount of heat lost will vary among engines based on design, size and materials used, but provides useful results for purposes of comparison....<snip>

Stolen from here: http://victorylibrary.com/mopar/cam-tech-c.htm

Got ya' thinkin', didn't I ?? :cool: ;)

:

BTW.....that cranking pressure(200lbs) is always contingent on having a power plant that breathes very well ;)

i'm surprised that the cranking pressure is that low too. my 454 with LS-7 pistons and 116cc heads and a .638/.663 260/270@.050 cam made 235psi cranking pressure. maybe i didn't check it the same way? someone mentioned "throttle blades wide open"? i did not check mine that way. how much difference does that make?

edited

it makes little or no difference, at least on mine. Mine cranks out 220lbs either way.

edited

how about 270 psi cranking compression on 468 bbc it has compression ratio ranging from 14.66 to 15.09 depending on what cylinder you want to look at ......yet there is only 5 psi between worse and best cylinder.......all with a doug herbert roller cam 256/268 @ .050 with .685" lift.

im lowering the compression some as we speak. it will now be 13.56 to 13.94 to 1 depending on what cylinder your looking at. might know soon to see what that does to the cranking compression......of course the psi test was on a cold motor with blades open bla bla bla.

ddeennis, why so much variation from cylinder to cylinder? That nearly half a point . . .

Ken,
Your guess of a wash as far as difference is pretty much right on.
I come up with a theoretical cranking compression of 169 now & 172 after or 3 lbs difference with a few assumptions like .015 deck, rocker ratios & lash ;)
Also, I come up with a compression difference of from 10.35 to 10.93 changing nothing but the cylinder head volume.
This is a 468 correct?

Thanks to everyone that replied.

Originally posted by Wolfplace:
Ken,
Your guess of a wash as far as difference is pretty much right on.
I come up with a theoretical cranking compression of 169 now & 172 after or 3 lbs difference with a few assumptions like .015 deck, rocker ratios & lash ;)
Also, I come up with a compression difference of from 10.35 to 10.93 changing nothing but the cylinder head volume.
This is a 468 correct? Thanks for the reply, Mike. The piston is .010 in the hole, rockers are 1.7 and the new cam will lash at .025. The engine is 460ci.

Originally posted by kjett:
</font><blockquote>quote:</font><hr />Originally posted by Wolfplace:
Ken,
Your guess of a wash as far as difference is pretty much right on.
I come up with a theoretical cranking compression of 169 now & 172 after or 3 lbs difference with a few assumptions like .015 deck, rocker ratios & lash ;)
Also, I come up with a compression difference of from 10.35 to 10.93 changing nothing but the cylinder head volume.
This is a 468 correct? Thanks for the reply, Mike. The piston is .010 in the hole, rockers are 1.7 and the new cam will lash at .025. The engine is 460ci. </font>[/QUOTE]=
175/10.33, -- 10.92/180 ;)

Originally posted by Wolfplace:
</font><blockquote>quote:</font><hr />Originally posted by kjett:
</font><blockquote>quote:</font><hr />Originally posted by Wolfplace:
Ken,
Your guess of a wash as far as difference is pretty much right on.
I come up with a theoretical cranking compression of 169 now & 172 after or 3 lbs difference with a few assumptions like .015 deck, rocker ratios & lash ;)
Also, I come up with a compression difference of from 10.35 to 10.93 changing nothing but the cylinder head volume.
This is a 468 correct? Thanks for the reply, Mike. The piston is .010 in the hole, rockers are 1.7 and the new cam will lash at .025. The engine is 460ci. </font>[/QUOTE]=
175/10.33, -- 10.92/180 ;) </font>[/QUOTE]Great! Thanks again, Mike. That's right about where I was hoping the compression would end up. While I've got your attention, any idea how much piston to valve clearance I will loose with the aforementioned changes? The old cam was .655/.660 248/254@.050. The new cam is .725 254/260@.050. With the old cam I had .147 and .155 clearance on intake and exhaust respectively. Of course I will mock everything up and measure it just like I did the last time. Just curious if I may have to have the pistons fly cut. If I do it won't be a huge deal since I'm installing new rods and will be having the entire rotating assembly rebalanced anyway.

++++++++I have edited this+++++++++++

I doubt you will have a problem as you aren't changing the duration much or centers at all.
I'd think you will lose about 40 thou (changed from .020).
I'd be kinda interested when you check it,, always like to keep those little tidbits tucked away where I can end up losing them anyway :confused:

====EDIT+++++++
After posting this I thought about it & given the new lift numbers you will probably change the p/v clearance more than my first estimate as it is obviously a much more aggressive lobe.
Soooo, my new guess is about double my first guess :D Like .040" which should still leave you pretty safe in my opinion.

My experence with compression guages has been that they are only good for checking a variance between cylinders. Absolute numbers can change between different brands of guages and actually vary a fair amount from 2 different guages of the same kind.

Originally posted by Mike Feudo:
My experence with compression guages has been that they are only good for checking a variance between cylinders. Absolute numbers can change between different brands of guages and actually vary a fair amount from 2 different guages of the same kind. =
Absolutely agree.

Also most software does not account for weather conditions & altitude.
All I am referring to is a theoretical cranking pressure numbers & the difference between to known combinations not actual real world numbers although Ken's cranking numbers come real close to the gee wiz what if numbers. ;)
And, the program I use uses .050 timing numbers along with an average cam lobe of a certain family like mild rollers vs aggressive & inverted ones so the actual numbers can vary somewhat between cams of different intensity.

And in the above example just changing the cam type from aggressive to inverted which assumes a much faster lobe the cranking compression goes from 180 to 189 as the program assumes less seat timing for the same .050 numbers

Ken....Sounds like a nice cam ........Oh wait thats just about the same as mine. graemlins/beers.gif graemlins/beers.gif I didn't get to the track at all this year....The back half took a few months and I just wanted to enjoy it.....although I did have a dream I ran a 10.41.... :D Good luck with it.....Scott

Originally posted by Eric68:
ddeennis, why so much variation from cylinder to cylinder? That nearly half a point . . . like any other unblueprinted engine compression varies from cylinder to cylinder because of deck heights that very from reworked rods and others that are not and factory tolerences that stack up against you. combustions chambers that cc out at different specs. unsquare blocks....so on.... unless the engine is blue printed for exact everything this is just how it is.

mine is mostly because of my deck heights very from .001 to .010 being the worst. i have some chambers that are not cc dead on to each other. and when you get this high up in compression the slightest cc makes a big different. .4 compression for a race engine like this i think at this degree isnt going to make jack squat difference not for the outlay of money and time to true everything up.

i may be wrong but im just not going to take the time or money to rework everything to make it dead exact.

i guess later on if im search of more power and i need to tear down the motor for what ever reason them i can true it up and gain a few more hp.

i have seen a few articals about blue printing loosening up factory tolerences and seeing gains of about 50 hp putting the engine back together with same parts and what not.

Originally posted by blaauboer:
Ken....Sounds like a nice cam ........Oh wait thats just about the same as mine. graemlins/beers.gif graemlins/beers.gif I didn't get to the track at all this year....The back half took a few months and I just wanted to enjoy it.....although I did have a dream I ran a 10.41.... :D Good luck with it.....Scott Scott, remember there is a lot more to a cam than the lift and specs at .050 ;) Your car turned out real nice, BTW. Oh yeah, did you see my new best? Check my signature. I hope to bust those numbers in the spring. 10.30@130 mph, 3760lbs N/A through tail pipes is the goal tongue.gif