Editing Camshaft/Compression Ratio relationships

This article is taken from a thread where the Scandanavian OP was asking the following.... "How to Calculate wath Cinda Compression ratio a cam needs to work optimally and soo on?? This would Bee Really Nice To Know??? *Here is my reply..... *The final word in cam selection should always be determined by the grinder for your specific application. It's a free service offered by all *cam manufacturers. *OK, now with that disclaimer in place, I will attempt to help you see how the SCR (static compression ratio) plays out with the cam timing. There may be a mathematical *formula, but I'm not aware of it, so I just use what information I have available to me and try to figure it out from there. We will not get into DCR (dynamic compression ratio) here. That's a whole other subject for another time. *The main point to consider is the intake closing point after bottom dead center. That's what controls the amount of cylinder pressure the *motor will make. *Here is a list of camshafts taken from the Crane catalog that I refer to when I'm trying to get in the ballpark for matching the SCR to the *IC point, taking into consideration the lobe separation angle and useful range of the camshaft. Again, this will only put you in the *ballpark and you should always lean on the manufacturer for a final decision. *First is the SCR range of the motor, then the intake and exhaust duration @0.050" tappet lift, then the lobe separation angle, then the *intake closing point and finally the operating range. Now please, don't anyone take this as the final word on camshaft choice. I only posted *it to help the OP understand the relationships and this is a very short list of the thousands and thousands of cam grinds that are available *to you. *7.50-8.50........184/194....104....16....500-4000 *7.75-8.75........194/204....104....21....800-4400 *8.00-9.50........204/204....110....27....1000-4600 *8.00-9.50........206/218....112....30....1200-5200 *8.50-10.00......210/210....110....30....1400-5000 *8.50-10.00......212/218....114....35....1600-5400 *8.75-10.00......216/216....110....33....1600-5400 *8.75-10.00......218/218....106....31....1800-5400 *8.75-10.75......218/230....112....36....1800-5800 *9.00-10.50......228/232....107....37....2800-6000 *9.00-10.50......232/232....107....39....3000-6000 *9.50-10.75......222/222....110....36....2000-5800 *9.50-10.75......224/230....110....37....2200-6400 *9.50-11.00......226/226....106....34....2400-6200 *9.50-11.00......228/228....112....41....2800-6200 *9.50-11.50......228/228....106....36....2800-6400 *10.00-11.00....232/236....106....38....3000-6400 *10.00-11.00....236/236....107....41....3200-6200 *10.00-11.00....236/242....112....40....3200-6600 *10.00-11.50....228/234....106....36....2800-6400 *10.00-11.50....230/230....106....37....3000-6400 *10.25-12.00....236/244....110....43....3000-6800 *10.50-12.00....238/238....106....41....3200-6400 *10.50-12.00....244/252....106....44....3400-7000 *10.50-12.00....246/246....106....45....3400-7000 *Notice that the range of operation (3400-3600 rpm's) remains constant as the grind gets wilder. It simply moves up the scale with more *duration and requires more static compression ratio to maintain the same cylinder pressure. The slug of air/fuel mixture running down the *intake runner into the cylinder has inertia. Once it is moving, it tends to continue to move. Using this phenomenon to advantage, we can *hold the intake valve open even after the piston has gone to bottom dead center, reversed its direction and is heading back up the bore. *This is where the balancing act takes place, leaving the intake open so that the intake charge continues to pack the cylinder, even with the *piston coming up the bore, but closing it soon enough so that the intake slug is not pushed back up the intake tract by the ascending *piston. If we push the charge back up the tract by leaving the valve open too long, we get reversion at the carburetor throat. The venturis *see air flow coming both ways and don't know whether to **** or go blind, so we get a rump-rump idle. It is the sound of a motor being very, *very inefficient because the rpm's at idle are out of the operating range of the cam. Once the revs get up into the cam's range, the motor *will smooth out and become efficient. *Cams ground with a narrow lobe separation angle (LSA), such as 104/106 will tend to build power quickly in the lower rpm range and then peter out higher up. *A wider lobe separation angle such as 112/114 will give up power at the bottom of the range for power higher up in the rpm's. Most cam grinders will use a middle of the range 110/112 for street motors. The wider angles would be helpful with Nitrous Oxide or a blower motor on alcohol (blower motor on gasoline works best at 110) or an EFI motor. *A rule of thumb that I use is that you need to begin considering a looser torque converter when the operating range of the camshaft hits *1400 on the low part of the range. A looser converter will make any cam more driveable, even a stock cam. You won't have to stand on the *brake at stop lights and the motor gets up into a useful range of rpm's more quickly. For instance, I'd be thinkin' of using a 2000 stall *converter with a stock cam just to make the car more driveable. *While we're on cams, I think most everyone knows that more cam requires more initial spark timing at the crank. Barry Grant has a list of *cam duration/spark timing here that you fellows should probably copy down somewhere because Barry is out of business and we don't know how *long this site will be up..... *http://barrygrant.com/demon/default.aspx?page=5

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