|Twin Cam Rebuild|
Dialing in Cams
By Tim Engel
(Ed. Tim is a member of LOON - Lotus Owners Of the North. This article appeared in the LOON Tibune. Tim Engel resides in Mound, Montana and is on the Internet at <firstname.lastname@example.org>)
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:
1. An inexpensive dial indicator with a base, and various extensions.
2. A Degree Wheel and Adapter Hub (auto parts store, Ford Parts Dept.).
3. Stiff wire to make a pointer a hanger wire works well).
4. 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" down from TDC. With a positive stop, you obviously can't keep rotating the engine in it's normal direction. It will be necessary to bring the #1 piston up to the stop-bolt and take a degree reading. Then reverse direction one revolution and make the piston back up to the stop from the other direction and take a second reading. The effect is "similar" to taking dial indicator readings at the same height on the way up and on the way down.The problem with it is that:
A) Any clearances or slack in the engine components will effect the accuracy of the readings when you reverse the direction of rotation. With a tight, fresh engine the error should be minimal. With a worn engine it may be questionable. Even so, this method is probably more accurate than using the cast-in pointer.
B) This method only simplifies determining "TDC". You will still have to use the dial indicator on the cam lobes to measure MOP (after removing the stop-bolt -- two crank revolutions per cam rev). 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
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:
Exhaust MOP Calculation: [(26 + 180 + 66) / 2] - 26 = 110 ATDC [(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
Exhaust MOP Calculation [(16 + 180 + 48) / 2] - 16 = 106 ATDC
[(50 + 180 + 14) / 2] -14 = 108 BTDC 4 advanced - need to retard.
2 retarded - need to advance.
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.PART II
When you get around to degreeing your engine...1.) You will need a way to mount the dial indicator to the top of the head. The normal mounting accessory for a dial indicator is an articulated arm. The type that attach to any convenient, nearby bolt or stud are probably the easiest to use. Others use a magnetic base; however it isn't going to stick to an aluminum head. If that's what you have, you will need another way to attach it. For the Lotus/Ford Twin Cam engine, for example, make a small "U"-shaped tray/table out of plywood or lumber-core. Cut it large enough to bridge the spark plug valley and overlap the inner cam cover studs. The legs of the "U" only need to be about an inch wide to be strong enough. The bottom of the "U" should be wide enough to provide a mounting surface for the magnetic base... probably covering the two rear sparkplugs.
This configuration will provide both a solid mounting surface and easy access to the #1 spark plug hole. Set it in place on top of the cam cover studs and use a mallet to strike it smartly above each stud, making impressions on the bottom side. Flip it over and drill clearance holes for the studs, using the impressions as position guides. Remove the cam cover, set the board down over the studs such that it rests on the bearing caps and secure it with four washers and nuts. Clamp, bolt or screw the magnetic base to the table. Both the table and the indicator should be solid, as any movement will translate into bogus readings from the dial indicator. With a little creativity, a similar set up can be used for most engines.2.) The stem of the dial indicator is not long enough to reach through the head and touch the piston at TDC. Extensions are typically available, but I don't have one yet. I'm looking for an off-the-shelf item now, or I may have something beautifully machined or totally cluged. If you decide to buy your own indicator, inquire about extensions.
3.) The biggest problem degreeing the Twin Cam engine as installed in the Europa is going to be reading the degree wheel attached to the engine's front pulley. It's a two person job with someone lying under the car taking degree readings and someone else topside reading the dial indicator. I don't have a good alternative for you right now. If you have one, I'd like to hear it.
4.) To find TDC... a simpler, slightly less accurate alternative to the dial indicator is a bolt/jam-nut screwed in through the sparkplug hole. Put the piston at TDC. Run the bolt in until it touches the piston. Back the piston down out of the way. Screw the bolt in about another .200" or so and tighten the jam-nut. Now you have a positive stop for the piston at a fixed distance down from TDC. With a positive stop, you obviously can't keep rotating the engine in it's normal direction. It will be necessary to bring the #1 piston up to the stop-bolt and take a degree reading. Then reverse direction one revolution and take the piston back up to the stop from the other direction and take a second reading. The effect is "similar" to taking dial indicator readings at the same height on the way up and on the way down. The problem with it is that: A) Any clearances or slack in the engine components will effect the accuracy of the readings when you reverse the direction of rotation. With a tight, fresh engine the error should be minimal. With a worn engine it may be questionable. Even so, this method is probably more accurate than using the cast-in pointer.
B) This method only simplifies determining "TDC". You will still have to use the dial indicator on the cam lobes to measure MOP (after removing the stop-bolt -- two crank revolutions per cam rev). If the engine is still in the Europa, you're back to a two-person job again. Since you have to enlisted a helper anyway, you may as well do the whole thing the optimal way. The beauty of it is that:
C) This method is dead simple (turn it until it stops), requiring no special skills in the use of a dial indicator. You can achieve a slightly reduced level of accuracy easily with minimal chance of introducing any operator error. If you don't know what you're doing, but you insist upon doing it anyway, this may be the way to go.D) One person could "TDC" the Europa from under the car. Just use a wrench on the front pulley bolt to turn the engine back and forth while taking readings. Be sure to re-torque the pulley bolt when you're done.NOTE: .of course, you still have to deal with the dial indicator and a Europa helper to measure the cam's MOP.