Hi Guys, here is the deal. Mild cam, reusing it for my 72 ragtop(few options automatic, 3.08 posi). 455, 10.25 to 1 true compression (but I am at 3500 feet, density altitude hits 6000 some days). Put cam in and was degreeing it, but have a question. Straight up install gets intake open @5BTDC, closes @35 ABDC, intake lobe center measures and calculates to 105 lobe center. Retard two degrees on sprocket gets intake open @1 BTDC, closes @39ABDC, intake lobe center calculates 109 but measured 108 (must be measurement error) So which way do I go? In previous engine it was straight up, even tried the old chain set and it measured same as the new one. Car ran good but seemed kinda slow up top and had an overheating problem. Could that have contributed?? Cam has less than 5000 miles on it.... Open to opinions here as to which way to go. Engine will be stock other than compression and cam. Cam card wants 108 lobe center to advance cam 4 degrees...so it would seem that the 2 degree retard position is the way to go... Some companies do cam cards diferently so want to make sure I am not making a mistake here....its one of those things you do every couple years and can never remember! ou: Thanks for any insight.... Later Tim PS even ran the spellchecker this time!
If you are correct on your readings with the 109 lobe center, I would leave it there. That's a pretty small cam, so advancing it would make it weaker at higher rpm. Jim Burek
There must be a typo in that line. Simple answer to your question: DCR Go here to find out about it and get the calculator: http://webpages.charter.net/darrell1/dynamiccr.htm If you have too much compression and you close your intake valve too early, your particular gas won't support the resulting excessive cylinder pressure. I'm assuming you have 93 octane. Fooling around with the numbers to get 10.25 SCR, with the ICL at 108 deg and the IVC at 63 deg, I get a DCR of 8.28, which should be ok for 93 oct. and iron heads. Putting the ICL at 105 with a resulting 60 IVC, DCR is 8.46, which IMO is too high for 93 and iron. I would retard it. Did you have a problem with detonation before? What is your quench distance?
Hi guys, thanks for the quick rsponses...appreciate it. I guess I should expand on whats going on here.... This engine was going to be my aluminum headed monster. It has sealed power .030 forged aluminum pistons in it, plan was 10.25 to 1 with the aluminum heads and 288-94H and should be no worries...that was to go in the hardtop and put the hardtop engine (422hp) into the ragtop. Right after I had block machined and set up, those cool new .038 forged pistons came out, and with the cost overruns (HUGE!) on the ragtop, I decided I would now use this block with stock small valve heads and the GSCA cam, since it worked fine before, even though its a little small, and build another block up later for the aluminum heads (ie, trying to save money). I was worried about the high compression with a small cam, thus this post. I did not have detonation before, that was with 9.5 to 1 or so, but at 2200 feet altitude. Deck height is at .010, heads shaved .002 for cleanup. Will be running fel-pro blue head gaskets... I have tried two timing chains and both (straight up) opened the intake at 4 or 5 degrees BTDC. I'm assuming the cam is off slightly. If I'm that close to detonation, perhaps I should buy the 288-94H and put it in there. I have to tell you though, with a four year every nut, bolt and wire restoration, firing up a new engine with an untested ignition system, untested fuel system,untested carb, untested electrical system could be hairy...at least with this cam its already run in, so that takes some of pressure off.... Thanks for the help!! later Tim EDIT: That compression ratio was calculated with the aluminum head chamber size, I am now using stock 70 heads so the SCR may drop slightly....but have not measured the heads yet...
