WETTE VETTE

I see a lot of talk about cams, compression, heads, etc. but never see much about cranking cylinder pressures. Can cranking cylinder pressures be a clue to where a motor combo is possibly off? For example a motor with 9:1 compression and slightly too large a cam may produce lower than desired cranking pressures. The opposite can be true if too small of cam is run with too much compression or CI. Advancing the cam raises cranking pressures and increases low end power while retarding does the opposite. What about a motors ability to run pumped gas? Is there any correlation to cranking cylinder pressure and what octane of fuel is required?

Craig

articfriends

iTrader: (1)

In my humble opinion,yes.On different street car motors i have under cammed and over cammed different compression motors. Example 1-pontiac 467 ci,cam was around 228/238 at .050,motor had around 11.2 compression,cylinder pressure was 230 psi cranking,needed 100plus octane to drive without detonation,changed cam to 244/254 at .050,had no problem then running 94 octane,cranking compresion dropped to 195-20. Example 2- a friend of mine put a big ass ultradyne cam in a smallblock-355 10-1,cam was like 255-260 at .050,wouldn't idle under 1800,had no throttle response,only had 150 psi cranking compression,ran worse on race gas,advanced cam 2 degres,had 160 psi,advanced it 4 degrees,had 170 psi,advanced 6 degrees-had 180 THEN before starting it we checked valve clearence,had less than .050 valve clearence,pulled cam out,put one in the mid 240's at .050,had 175 psi straight up,fired up and ran great and with a little carb tweaking would idle at 1000rpm's. Advanancing or retarding a cam much more than 2 degrees usually has a neg effect on one end of the powerband and indicates the need for more or less cam. On the chassis dyno at baker engineering we dynoed a smallblock in a circle track car that had a cloyes hex adjust timing chain alomg with a removable center on cam cover,we started out with cam advanced about 1 degree from what ultradyne reccommended(i always try to put 1/2 -1 degree extra in with the thought that under a load and with slight stretch the cam timing will be dead on). Made several pulls including advancing and retarding the cam about 4 degrees,advanced 4 degrees we picked up about 12 ft lbs of torque on bottom end and gave up double that on top and car seemed to idle slightly better(but this was a race car so idle quality wasn't top priority). retarding the cam did the same thing,we saw 5or 6 hp near peak but gave up torque from upper midrange on down,idled terrible. So when it comes to boats i guess i'd say unless you've built a octane monster and are trying to now get it to run on pump gas you don't want to get hog wild with the cam and end up with a bravo killing barely able to idle combination,yeah,it sounds good with a big lope and dry headers but guys do this and end up with their peak at 6800 rp'ms and lack of midrange torque at what should be a good cruising rpm. We were on burt lake in northern michigan a couple years ago,a guy had a beautiful 43 ft black thunder with 800 sc's with big daddy cam's,this thing sounded awesome idling until we followed him thru the no-wake zone up the river and we seen it stall 100 times as he tried to put it in and out of gear to keep it running . Talking to him he told us he was limited to running on just that lake (instead of being able to go up river to next lake and out to lk huron/ lk michigan where a boat like that is at home) unless he wanted to put plugs in it after both no-wake zones,Smitty

WETTE VETTE

I have a similar story to those with a 12.6:1 iron head 496. With a 267/276 solid roller ground on a 112+4 the motor created 250 lbs. of cranking pressure. It idled well and had good low to mid range power, but seemed to lack on top. I retarded the cam 6 degrees and lost 20 PSI during cranking, and picked up 100 RPM on top with a minor loss on the bottom end. Then the cam was then re-ground to 278/284 and it produced 180 PSI cranking installed 112+4. This cam was too large to idle well and the low end was a bit soggy. From 3500 and up it was a rocket with a 300 RPM gain on top. From the original grind to the re-grind the motors peak RPM with the same prop went from 6000 to 6400 RPM with only the cam grind tweaked. I was younger then and idle quality was less important than the peak number on the radar gun!!! I ran that motor on 110 octane.
Valve lash can also effect cranking pressures much like adv./ret the cam.

mcollinstn

"too large" and "too small" apply to cams whose efficient rpm range for a particular combination are outside of the desired range. The cranking compression is a byproduct of the combination, not a value that is adjusted by changing cams.

Match your compression ratio, flow values, intended fuel, and desired rpm range to your cam. Then your cranking compression IS what it IS.

Keep in mind that a "too large" cam will bleed cylinder pressure through overlap at low revs. This reduces cranking compression. BUT at higher rpm, the overlap stops BLEEDING pressure, and actually starts allowing the inertia of the system OVERFILL the combustion chambers. This will cause the "effective" compression ratio to go thru the roof at high revs and your motor will eat itself.

WETTE VETTE

Hence the need for high octane fuel when using high compression!!