Liberator21

I was playing around with Desktop Dyno 2003, using my engine combination with different heads and such. This version allows you to change how the cam is installed (straight up, advanced, retarded). Changing this, changed the horsepower numbers considerably. This brings up my question. Because Comp Cams camshafts have 4 degrees advance ground into them, when you use a comp cams cam file (included in program), do you have to set the "installed" option to +4, or is it already assumed in the cam file? When you put any cam file in, the default setting in the program is "0". I ask this because like I said it changes the characteristics of the simulation considerably. Retarding the cam makes big improvements in upper rpm horsepower numbers, and doesn't seem to hurt the lower rpm numbers (HP & TQ) all that much. I didn't think it was going to do that. I know advancing the cam helps bottom end, and retarding it helps upper rpm, but it seems to make much bigger changes to the upper rpm range. Changing the cam by 4 degrees (advanced) took 30 HP away from the peak at 5000 rpm. Any comments?

articfriends

iTrader: (1)

Desktop dyno is a fun program ,BUT,don't bet the farm on its numbers. The settings for cylinder heads(stock,perf,lightly ported,etc-however they ask it,it's been a while since i looked at it)leave alot of room for error. Lapouttre racing(a custom headporter in bay city michigan) had a computer program that had you enter actual flow #'s,header tube dia,primary length etc,and he dyno's ALOT of motors,he says barring no ignition problems or fuel delivery problems it was within 2% of what you actually see on the dyno,but he payed 700-800$ for it. He also says the typical guy comes up with some generous #'s on desktop dyno that are off as much as 75 hp (usually in your favor)I dyno'ed several circle track cars at baker engineering on their chasis dyno,the best thing is to pick a cam thats as close to your combo as possible(hp and tq peak),install it at the intake center line spec they reccomend +1 degree to account for chain stretch under a load and run it. On the chassis dyno we advanced and retarded a ultradyne cam (using a cloyes hex adjust and a quick change cover) between pulls and sure enough we saw on average of 10-12 hp horsepower drop on top or bottom of spectrum from 4 degrees cam timing advanced or retarded,and it also shifted the peak 100 rpm's up or down. Have fun with it,don't pick a cam thats so big for your project engine that you create reversion problems with your marine exhaust just because it looks good on a computer program either,Smitty

Liberator21

Thanks for the info Smitty. I was just playing with it, I know it's basically a toy, but I was just wondering why such large differences with small cam timing changes. My engine's already built, I was just playing with changes to my basic setup to see what happens.