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Old 02-02-2013, 11:54 AM
ssmonty ssmonty is offline
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demarques 191,
I've building a 383 too with rpm heads(70cc), air gap intake, & scat crank/6.0 rods, & 5cc pistons.
Just thought I'd mention that the 64cc heads are going to give you over 11:1 CR, and with that cam(XR270) it would put your dynamic compression over 9:1 if you have a zero piston to deck and 0.39 gasket. At least with the CR calculator I prefe to use( a good article IMO, and link at the bottom of the page-
Many different opinions on dynamic compression out there. Many say not to run over 8.5:1 DC on pump gas, others say it has no revalence on a running engine. Some say not to go over 180psi cranking compression, some say 200psi is ok? I know that just because you have only 160psi with a starter, doesn't mean much in a engine with 11:1CR and a big cam at higher rpm as it can still detonate/destroy itself.
I know it depends on the combination of parts, temp.,fuel quality,rpm,ect... I have a 350 that has less than 9:1 CR w/cast iron heads, HE260 cam, and 14* initial ign. adv.. Most of the time it runs great, but a couple of times when the ambeint temp got around 100*F, and a good heat soak on the engine/bay, it gets very sensitive to throttle input or it will detonate and have severe pre-ignition symptoms(bucking/surging/stalling). I believe a bigger cam would probably bleed off enough cylinder pressure to make it run better, or back off the timing, but I dont want to lose the low-end throttle response I have for the 99 percent of the time it runs great.
Just saying you might want to look at your CR a little harder because the quality of gas is only going to get worse.
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demarques_191 (02-04-2013)