Valve and Cam Timing
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The character and performance of a piston engine is largely determined by it's valve timing. Variables like compression ratio, induction and porting are important links in the same chain, but, valve timing sets the personality.
At 6000 RPM, the choreography between opening and closing valves, reciprocating pistons and high velocity gases is a precision exercise. There's a fine line between an engine that sings and one that chugs... or self-destructs.
In single-cam, push-rod engines like the Ford Kent or Renault/Europa units, there isn't much that can get out of whack. Both the block's crank-to-cam distance and the timing chain's pitch are accurately controlled during manufacture.
In overhead cam engines, particularly with multiple cams and/or cylinder heads, there are a multitude of dimensions that can get out of spec during an overhaul. The crankshaft to cam distance is an amalgam of block height, head gasket thickness & crush, head height and (on 907/Turbo engines) cam carrier height. In production, every dimension has a tolerance and consecutive engines off the assembly line will have noticeable differences.
If the head, block or cam carriers are cut to true up warped surfaces or to increase compression (not a good way to do it), the distance between the cams and the crank will also be shortened. When the idler/tensioner pulls the new found slack out of the timing chain or belt, the cams will be rotated slightly out of time.
To properly time an engine's valve train, you will need:
- An inexpensive dial indicator with a base, and various extensions.
- A Degree Wheel and Adapter Hub (auto parts store, Ford Parts Dept.).
- Stiff wire to make a pointer (coat hanger wire works well).
- A means of making small adjustments to the cam timing.
For engines in which the cams are driven by dowel pins (Lotus Twin Cam Ford), offset dowel pins are available. Since you won't know what offset you need when you start the job, purchase an assortment of dowel pins before starting. Other engines like the Lotus 907 and Turbo require special two-piece pulleys that allow the outer rim to be indexed relative to the inner hub. That's about a $175 investment.
First find the TRUE Top Dead Center for the fully assembled engine. Set the number 1 piston to the factory indicated TDC position. Install the Degree Wheel on the front of the crankshaft. Grind a fine tapered point on a piece of heavy wire and bend up a pointer, using any nearby bolt to mount it. Install the Dial Indicator on the cylinder head such that the plunger (with appropriate extensions) passes through the spark plug hole and rests on the piston. The plunger should be depressed by about .300" to .400". Zero the Dial Indicator.
Using a wrench and the retaining nut for the pulley on the front of the crankshaft, take up all the slack in the timing system by turning the engine one full revolution in it's normal direction of travel. For the rest of the process, do not turn the engine backward, since slack in the drive system will introduce errors.
When the piston is at TDC, the crank/ rod journal is moving sideways relative to the cylinder bore. With bearing clearances and such, the crank can move quite a bit between the time the pistons stops going up and when it starts going down. Therefore, measurements are taken before and after TDC and averaged.
Noting the Dial Indicator, turn the engine through one more revolution. Approach TDC slowly and stop precisely .200" before TDC (It's not important where you stop, but stop accurately so you can repeat it again and again). Note the degree reading at
.200" before TDC. Rotate the engine to .200" after TDC and note the degree reading.
Rotate the engine one more revolution, this time watching the Degree Wheel. Stop precisely at the degree reading that is halfway between the two readings you just took. That should be TRUE TDC. Loosen the degree wheel (be careful not to disturb the crank) and turn it until zero aligns with the pointer. Re-tighten it.
Repeat the process. If there is still a difference in the degree readings either side of zero, halve that, reset the degree wheel and try again. When zero on the Degree Wheel is precisely half way between your .200" before and .200" after TDC readings, it is indicating the TRUE TDC of the engine. Permanently mark TDC somewhere on the engine for future reference (front cover/ pulley, flywheel/ block, etc.).
Now check the cam timing. Use a similar process to find the Maximum Opening Point or MOP for the cam(s). Compare the measured MOP in crankshaft degrees with the value specified for the cam. Any difference will have to be "dialed out", or "degreed" by using appropriate offset dowel pins or adjustable pulleys.
MOP is an English term. In the USA, the term is "Lobe Centers". They're equivalent so don't let them confuse you. Either way, the unit of measure is crankshaft degrees, not camshaft degrees. If the shop manual doesn't give the MOP for the cam, it can
be calculated from the cam's timing specs as follows:
Lotus 907 "C" Cam Specs Intake Duration Calculation
26 BTDC Intake Opens 26 Opens before Top Dead Center
66 ABDC Intake Closes 180 Between Top & Bottom Center
66 BBDC Exhaust Opens 66 Closes after Bottom Dead Center
26 ATDC Exhaust Closes 272 Duration (total open degrees)
The MOP occurs half way between opening and closing. In this case, 272 / 2 = 136 from the opening point. However, we want to measure from TDC, so subtract the 26 the valve opened before TDC...136 - 26 = 110 MOP.
For the exhaust cam, the numbers add up the same, except that the process ends 26 After TDC. Subtract the 26 to get back to TDC, and the MOP is still 110 . However, the difference is that the intake MOP is after TDC and the exhaust MOP is before TDC
Intake MOP Calculation: [(26 + 180 + 66) / 2] - 26 = 110 ATDC
Exhaust MOP Calculation: [(66 + 180 + 26) / 2] - 26 = 110 BTDC
Draw a quick graph that looks like a two humped camel centered on a vertical line. The vertical line in the center of the page is TDC. A horizontal line low on the page is zero lift. Starting on the zero lift line 1/2" to the left of the TDC line, draw a symmetric
bell shaped curve that sweeps up to the right, peaks to the right of the TDC line and returns to the zero lift line. Label that curve INTAKE. For the exhaust, draw a second bell shaped curve that's a mirror image of the first one, peaking to the left of the TDC line and ending 1/2" to the right of the TDC line. The two humped camel.
The distance from either peak to the TDC line is the MOP. Events start on the left of the graph and progress to the right. Advancing the cams will shift the curves to the left. Retarding the cams shifts the curves to the right. NOTE that to advance an inlet cam, you decrease the MOP. To advance the exhaust cam, you increase the MOP. Advancing or retarding both cams an equal amount does not change the overlap.
This is easy to screw-up when you're just playing with numbers in your head. When degreeing a DOHC engine, always draw the camel and label it with degrees for Valve Opens, MOP and Valve Closes. Mark both the specs from the workshop manual and
the measurements you take from the engine. It will help you avoid the dumb mistakes.
Street cams usually have a MOP of 110 to 115 . A high MOP moves the humps apart, reducing the overlap. This gives a strong low end and a weak top end. Racing cams usually have a MOP of 102 to 104 . A low MOP moves the humps closer together, increasing overlap. This gives a weak low end and a strong top end.
Low end torque can be improved (at the expense of high end power) by advancing both cams an equal amount... up to about 2 (careful, advancing the cams reduces valve to piston clearance). A 110 MOP becomes 108 for the intake and 112 for the
exhaust. Moving both cams like amounts in the same direction keeps the overlap the same. For single cam engines it's easier... just advance the cam about 2 . The intake and exhaust have no choice but to move together.
Move the Dial Indicator to one of the tappets, setting it up with the plunger as close to perpendicular to the tappet face as possible. With the valve fully closed (there should be a clearance gap between the valve and the cam), the plunger should be depressed a little more than the specified maximum lift of the cam. Zero the Dial Indicator.
It is difficult to tell exactly when the tappet starts moving down and stops moving up... like finding TDC at the piston. Therefore, watch the Dial Indicator and rotate the engine slowly until the tappet has moved about .050". Note the reading on the Degree
Wheel. Continue rotating the engine until the tappet has gone all the way down and returned to the .050" point. Again, note the reading on the Degree Wheel.
In the same way that the correct MOP for the cam was calculated from the cam's opening and closing specs, use the two readings to calculate the actual MOP. For the intake, the MOP is half the total movement indicated by the degree readings minus the number of degrees before TDC when the valve first reached the .050" open point.
If the first reading for the inlet cam was 16 BTDC and the second was 48 ATDC, then 16 + 180 + 48 = 244 . The midpoint is half the duration or 122 from the point where the valve opened... at 16 BTDC. The MOP, from TDC, is 122 - 16 = 106 .
For the "C" cam the MOP is supposed to be 110 , so the intake cam is advanced 4 .
Repeat the process for the exhaust valve. For the sake of illustration, assume the resulting readings show the exhaust valve to reach .050" of lift at 50 BBDC and 14 ATDC. This example still assumes the 907 "C" cam with a spec MOP of 110 .
Intake MOP Calculation: [(16 + 180 + 48) / 2] - 16 = 106 ATDC
Exhaust MOP Calculation: [(50 + 180 + 14) / 2] - 14 = 108 BTDC
4 advanced - need to retard. 2 retarded - need to advance.
Reduce Intake MOP >> Increase Exhaust MOP
Increase Intake MOP >> Reduce Exhaust MOP
(Think about the direction from TDC.)
Use offset dowels or adjustable pulleys to dial in the cams and then repeat the procedure to verify the results. For street engines, accuracy within 2 is adequate. For racing engines, the cams should be dialed in within 1 . 1 degree resolution is
possible with offset dowel pins. If you can't be perfect, try for a little advanced, since normal wear tends to retard cam timing.
For the Lotus-Ford Twin Cam, Cosworth probably produces the best offset dowels. They offer dowels in increments of .006" of cam offset. For the Lotus Twin Cam, .006" offset = 1/2 of Cam Degree change. But MOP is measured in Crankshaft Degrees. Since the cam turns half of crank speed, 1/2 at the cam is equal to 1 at the crank. In the example above, the intake cam needs to be retarded 4 at the crank. That's 2 at the cam. 2 is four 1/2 increments at .006" each, or a .024" offset dowel.
When using adjustable pulleys, follow the manufacturers instructions. Some use offset dowels to key the hub and rim together and the procedure is similar to that used on the Lotus Twin Cam. Others use a multitude of dowel holes or keyways.
In the early days of emissions controls, Lotus built some engines with mild street cams set at a low MOP. The increased overlap let some exhaust back into the chamber before the valve closed. This diluted the intake charge in much the same way as
Exhaust Gas Recirculation, but without the valve. The poor low end performance typical of a Low MOP/ High Overlap helps explain why Federal Lotus Twin Cams and especially 907's of the mid to late 70's couldn't peel a grape below 3500 RPM.
That said, the knowledgeable tuner could calculate the correct MOP for a given cam from the opening/ closing data given in the workshop manual and re-time the engine. For a little more low end torque, advance both cams 2 beyond the theoretical MOP.
Always check cam/ tappet and valve/ piston clearances. Amazing what you can do with a bit of knowledge.
Reprinted with authors permission.
<p>Originally printed in: LOON Tribune Article, Newsletter for the Lotus Owners Of the North