David, The overlap period on the cam you have is 41 BTDC (inlet opening while exhaust is open) plus 40 ATDC (Inlet is open and exhaust closing) =81 degrees total overlap.
In N/A engines, by having the large overlap period you get better scavenging of unburnt gases and a fresher charge in the cylinder. In turbo engines cams usually have minimal overlap. Start of cylinder fill timing varies by a few degrees of advance with increased engine speed. Inertia and pressure of the moving pressurised inlet column helps overcome the residual gas pressure. This advance drops of as gas speeds increase, due to increased drag at higher velocities. The CA18DET uses (0, 60, Inlet) and (59, 9 Exhaust) timing giving an overlap duration of just 9 degrees. The grape racing article refers to engines looking "for higher boost with a restrictive turbine housing" so the figures they use are a bit conservative. Larger extractor exhaust systems to the turbo with a larger A/R housing gives good top end but often at the expense of bottom end. These engine tend to come on like a switch. All yee Hah! or nothing. Really horrible to drive anywhere except dragging. A good roller bearing turbo also helps reduce backpressure as the energy required to spool up and overcome internal friction is less. My thoughts any way, Let the flames and corrections rip! :-) Cheers, Feral Errol Get Serious tm<http://www.datrats.com.au/> -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of David Andrews Sent: Sunday, July 06, 2003 1:49 AM To: [EMAIL PROTECTED] Subject: Cam Timing Ok guys, I was discussing cam timing with andrew on #ozdat (austnet IRC) tonight which inspired me to go away and have a look at the specs for my L28 cam and see if i couldn't work out how its working. After drawing a pic i've decided that this cam is absolutely shithouse. Its a crow cam with the following specs: Adv duration: intake: 295 exhaust 298 Lobe separation: 108 Intake opens 41 deg BTDC, closes 75 deg ABDC Exhaust opens 78 deg BBDC, closes 40 deg ATDC Now according to my little diagram, this mean that of the 295 degrees in which the intake cam is open, only 62 of them are when the exhaust is closed. So for 232 degrees the intake charge is going straight out the exhaust!! What the?! After reading grape ape racing tech article on turbo camshafts, this cam sounds absolutely terrible for turbo's. http://www.grapeaperacing.com/GrapeApeRacing/tech/turbocams.cfm Black! Problem is, I can't think of why you would make a cam like this in the first place?! Am I on the wrong track or something? Dave --membersozdat------------------------------------------------------- OZDAT Mailing List Please Note:- Send (un)subscribe requests to [EMAIL PROTECTED] Send submissions to [EMAIL PROTECTED] No unauthorised redistribution of this email http://www.ozdat.com/ozdatonline/index.htm http://www.ozdat.com/ozdatonline/listindex.html http://www.mail-archive.com/[EMAIL PROTECTED]/ ---------------------------------------------------------------------
