Cranking compression?

Robert Kane · Post by **Robert Kane** » Sun May 15, 2005 5:04 pm

Is this a accuate means of determining installed cam position, or just another old timers backyard method?
How many of you serious builders on here are doing it? Darin? Larry?
Thanks.
Robert

Post by **maxracesoftware** » Sun May 15, 2005 6:05 pm

Is this a accuate means of determining installed cam position, or just another old timers backyard method?
How many of you serious builders on here are doing it? Darin? Larry?
Thanks.
Robert

i had a couple of guys try that on my Dyno
and i played around with that once,
basically it didn't work, it just times the intake closing point for best Cylinder pressure at "starter RPM",
but you are interested in gains between peak TQ RPM and Peak HP RPM, or where you want your gains to occur ?

chevyracer · Post by **chevyracer** » Sun May 15, 2005 6:53 pm

It sure seems like a lot of people target the highest cranking compression on the dyno or racetrack. I think it really depends on how hard you have the engine cammed vs. static compression etc. I have never A-B-A'd it on a dragstrip to see if it will et the fastest with the most CC. It seems to me just because you have the most cylinder pressure (say a cam advanced +4 deg or so) doesn't mean that you will have the most dynamic pressures onced the engine is spooled up in the range that your engine runs in. Testing cranking compression will tell you the pressures with a 400 RPM starter motor but what about at 7000 RPM with overlap, ram filling etc. Just because the engine has more cylinder pressure being generated with a starter motor does that necessarily mean it will as well with engine running? I don't know? Lot's of intelligent engine guys on this board with the theory behind it

Bill Jones will be a great one for this subject and his knowlege and theory.

Should be a great topic!

Post by **maxracesoftware** » Sun May 15, 2005 8:35 pm

you could install a OEM cam in Reher-Morrison-Darin's 500 ProStock engine and have approx 388.6 cranking psi,
but it wouldn't win any races !!
just have a ton of cranking psi at starter RPM

SWB · Post by **SWB** » Sun May 15, 2005 9:55 pm

I like to set my lash to .050! Gives awesome compression! Little hard on the valvetrain, but that's what you got to do to win...

LOL

SWB

let r eat · Post by **let r eat** » Sun May 15, 2005 10:18 pm

excellent topic. Alot of old schoolers love the cranking compression rule of thumb on setting cam position. Alot of people I know that do things this way have won lots of races. Very interested to hear the theories against or for this method. My interest is this:

Is there any relation between cranking compression and the proper valve event timing at specific rpms? For instance.........an engine running peak hp @ 7000 rpms with 500 peak hp should have XXX cranking compression?

Post by **maxracesoftware** » Sun May 15, 2005 11:13 pm

some of my old Dyno Data on my Dragster's 355 cid Engine

http://www.maxracesoftware.com/MRH_Dragster_BR_2.jpg

355 cid made 720 HP @ 8500 483 TQ @ 6800
ran 7.50's 181.459 MPH best at 1420 Lbs. A1-Converter 7"

13.67:1 CR Duffy's Pistons gasports .043,.062,.187

Cranking Comp Test with Jesel Belt Drive

Cam on 106.5 CL (Zero Jesel Mark)
.010 Lash = 197 psi
.020=210
.030=225
.040=232

Cam on 100.5 CL ( 6 degrees more advanced)
.010=218
.020=226
.030=238
.040=245

Cam on 112.5 CL ( 6 more degrees retarded )
.010=176
.020=181
.030=204
.040=228

Engine made the most average HP/TQ and best Peak HP , Peak TQ with that particular Engine Combo with Cam installed on 106.5 int CL
and .030 Lash
ran the fastest down the DragStrip like the Dyno results showed.

it cranked more psi at 6 deg advanced and .040 lash
but made worse TQ/HP.