Static comp. ratio vs. Cranking pressure???
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Elder User
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Static comp. ratio vs. Cranking pressure???
Was wondering if there is any direct relation between the two. I know that the camshaft comes into play here. I don't know my exact deck clearance but the top of the piston was almost dead even with the top of the bore (is that good)?, When I pull the engine back out I want to try to figure out the deck clearance. Engine is a 390 bored .060 over, forged speed-pro flat-tops with 4 valve reliefs, C-8AEH/D2TEAA head combo with polished combustion chambers, chambers supposedly average at 69cc. Current cam is the Edelbrock Performer-Plus (it's to small for this setup) with 194/204 duration @.050 460/480 lift. The engine has 198 to 200 psi. cranking pressure in each cylinder. Had a guy tell me that that is probably close to 10.5:1 (is he blowing smoke) for all I know I guess it could range anywhere from 10.1:1 to 11.5:1 maybe higher hell if I know. The early 429's had like 180 psi cranking pressure didn't they?, and they are supposedly at like 11.3:1 Just thought it would make for some good conversation as I have been curious about this for a while.
Post Fiend
Joined: Mar 2004
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From: Union, Washington
My 1969 428SCJ had 210psi average cranking and was an honest 10.7 to 1 computed compression ratio. I rebuilt it myself so I did rely on someone elses word. I would think with you cam and pressures that it was close to 10 to 1 at least.
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And isn't your cranking compression more of an indicator of what kind of fuel you are going to be able to get away with than static comp. ratio? I am thinking of having a custom ground hyd. flat-tappet cam cut for my engine. Was thinking of something under 230 degrees of duration a .050" a little over I would consider. I haven't decided yet but was thinking Lazer Cams, Clay Smith, Crane, Comp. What do you guys think.
Post Fiend
Joined: Mar 2004
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From: Union, Washington
If you are gonna run this thing on the street you need to watch duration unless you are gonna run some deep rearend gears. I ran a Crane with a 312 duration but had a .525" lift. This was barely streetable in a 3200 lbs car with 4.30 gears. Trucks need to keep the torque at low rpms. Run a 230 cam but use lots of lift. Doesn't hurt the low end torque but lets the motor really breath at higher rpms. My old truck, 1968 F-250 390- 4 barrel with duel exhaust, 3.73 gears and a C-6, ran a custom cam(a friend did it for me) which was considered an RV cam at 252 with a .500" lift. Really made the 4 barrel work but still didn't destroy the lowend torque. The truck with the camper on and the race boat in tow would out run my partners 460 with 4.11 gears with his camper and no trailer, over the mountain passes around here. I don't see why you wouldn't choose a regular manufactures cam and follow his recommendations(the smart and safest way to go). I had knowledgable help with a lot of years of doing the selection of cams.
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If your cam has more lift than your heads can flow, will it hurt the performance at all? I may wait on a custom cam until I put some better heads on among other mods.
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Post Fiend
Joined: Mar 2004
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From: Union, Washington
Any intake combo has at least one limiting factor(part). Something will always be a restrictor. I had a racing outboard that I did a port job on, no help. Bigger carbs, again no help. Opened up and tuned the exhaust, no improvement. Found out from an expert, a factory design engineer gone out on his own, and he said the reeds(same as the valves in a 4 stroke) were the problem. Installed some custom reed cages and, wow! 30 more ponies on a dyno and 6 mph on the water. The bigger cam won't hurt if the valves, intake ports or heads won't flow enough. However, if you open the valves farther with the same duration you flow more air because the valve has to open at a faster rate, moving quicker otherwise. Also you would be amazed how well even a stock intake system will flow, better than most people think. I had a 1969 428SCJ with stock carb, 735CFM, that computed as good to 6000 RPM, just where the Cobrajet C6 shifted. After a cam change to a higher lift(.481" stock and .525" aftermarket) but same duration the motor turned 7000 rpms and was 2 mph faster(higher HP) and .015 seconds(more torque) quicker the quarter mile.
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