10/12/2023 0 Comments Compression calculator bbcShowing degrees of rotation and piston position, as you can see the valve on the wider LSA closes 5 degrees later on the wider LSA and the piston can,t compress ANYTHING until both valves are closed, the tighter LSA allows the piston to compress almost a tenth of an inch more cylinder volume, you'll find the tighter lca usually makes more low and mid rpm power, and in many cases more peak power but has a slightly rougher idle and the power peaks faster in the rpm curve were not talking a huge change, maybe 7-8 hp/ft lbs but the tighter lca tends to lope more, idles rougher and be more responsive, keep in mind that if you had selected a cam with 5-7 degrees more duration on the wider lca the overlap would be similar, but the effective compression would be even worseĪs a general guide you want to get the valve open to well above the 1/4 of the valve diam. 25-.50 higher compression can be tolerated but theres No absolute real definite answer, depends on quench, cam LSA, cylinder head design, advance curve, fuel/air mixture, intake temperature, coolant temp, spark plug design, piston design, heat barrier coatings, combustion chamber surface texture, and a bunch of other parameters. The main advantage I see in aluminum heads is lighter weight and their much easier to repair when damagedĪn aluminum cylinder head allows heat transfer to the engine coolant at a significantly higher rate than an iron head, and generally about. Http///srweiss/tablehdc.htm Īluminum cylinder heads tend to allow you to run about 1/4-to-1/2 point more effective compression, IE, if iron heads get into detonation at 10:1 ALUMINUM might ALLOW YOU TO RUN 10.3-10.4:1 BEFORE GETTING INTO DETONATION, BUT ON THE PLUS SIDE AT LEAST IN THEORY IRON HEADS AT ANY GIVEN CPR WILL HAVE A SLIGHT ADVANTAGE IN HPīut in my real world testing the difference is much closer almost non-existent This is the ICL.ġ0.-Advance the cam with a button to where you want it to be.ġ1.-Repeat all steps to verify the cam is where you want it to be. It should be close to, but before, BDC.ĩ.-Add the 2 readings ATDC, then divide by 2. The quickest, and most accurate, way of finding ICL :ġ.-Install dial indicator on top of retainer.Ĥ.-Turn engine backwards to -.100 drop on indicator.ĥ.-Turn engine forwards until indicator reads -.050.ħ.-Continue turning engine over max valve lift until indicator reads -.050 down on closing side of intake.Ĩ.-Record this too as degrees ATDC. 050 LIFT figures, notice that the tighter LSA (LOBE SEPARATION ANGLE)cam CRANES 110921 has the intake close at 45 degrees ABDC while the wider lsa CRANES 114681closes the valves at 50.0 ABDC (the wider LSA results in the valve closing 5 degrees later on the pistons compression stroke, effectively reducing the effective compression ratio LOOK heres TWO cams WITH IDENTICAL DURATION EXCEPT FOR THE LSA,(LOBE SEPARATION ANGLES) assuming both cams are installed with identical LCA (LOBE CENTER LINE ANGLE)remember lobe center angles can be changed thru indexing the cam when degreeing it in, LSA is ground into the cam during manufacture, the tighter LSA of the crane 110921 builds a bit more cylinder pressure, AS THE VALVES CLOSE ABOUT 5 degrees earlier (ABDC AFTER BOTTOM DEAD CENTER)ġ10921 intake valve closed at 45 degrees after bottom dead centerġ14681 intake valve closed at 50 degrees after bottom dead centerĪnd results in slightly more torque over a NARROWER rpm band so its better with a manual transmission, the crane 114681 with its wider LSA tends to work better with an auto trans with its wider torque band but very slightly lower peak torque, the crane 110921 has more overlap and better savaging in the mid rpm band, but it idles rougher at low rpms and that overlap doesn,t help if you use nitrous That assumes durations and cam lobe designs are the same of course " A cam with 108 LSA will close the intake valve sooner on the compression stroke and create MORE cylinder pressure than a cam with 112 LSA. ( is that overlap with a tight LSA bleeds off compression) which has more overlap and will reduce your DCR due to greater overlap." Very common mis- conception, is that a cam which is ground on a 108 degree lobe center. But you can advance or retard the cam in relation to the crank when itâ s degreed-in." You cannot change Lobe Separation Angle because it is ground at the factory. This difference is how far advanced or retarded your cam was ground at the factory, 2-degrees in our example here. To find intake lobe advance/retard, simply subtract the Intake Lobe Centerline from the Lobe Separation Angle (i.e.: LSA 114 IC 112 = 2-degree advance). These are also the figures used to indicate how much the cam s intake lobe was ground advanced or retarded from the factory.
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