Thread: Camshaft/compression ratio relationship

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A fellow on another forum wanted to know how static compression ratio relates to camshaft timing. I don't know of a mathematical formula, so I just posted what I do know and laid out how I go about getting in the ballpark with a new cam. Hopefully, this will be helpful to some of you on this board.

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. You don't make any compression at all until the intake valve closes.

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 intake closing 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. This chart was taken from the catalog of small block Chevy grinds with emphasis on the 350 cubic inch motor. Be aware that larger displacement motors will tolerate more camshaft and smaller displacement motors will be less tolerant of more camshaft. In other words, with the proper choice of cam for a given set of conditions in a 350, the same cam will be milder in a 406 and wilder in a 302.

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 moving down the intake runner into the cylinder has inertia. Once it's 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.....

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