Lotus Twin Cam Rebuild Part 2
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| Magazine Title: | Grassroots Motorsports |
|---|---|
| Published: | July 1990 |
| Author: | John Hagerman |
Contents |
It All Adds Up
Virtually no modifications are allowed on a Street Prepared cylinder head, so it's essential for the engine builder to pay close attention to all areas of head preparation. This attention to detail is the key to realizing a significant gain in horsepower from your engine rebuild. Since this is the case, it's always best to have the work performed by a shop that has extensive experience in reworking your particular brand of cylinder head. Therefore, we chose Lotus specialist Dave Bean Engineering of Santa Barbara, California, to do all of our head work.
Head Hunting
Over the years, Twin Cam heads were produced in a variety of configurations. The engine designers juggled four primary design areas to create these configurations: compression ratio, valve size, cam specification, and carburetor type. The most desirable head used in an American-market Europa was the 115 HP Big Valve, which was fitted to 1973 and 1974 models; since Street Prepared rules allow updating or backdating of engines, owners of the earlier 105 HP nit will benefit from the installation of the more powerful Big Valve engine. This is the unit we used for our project car.
Have Your Head Examined
All cylinder heads are not created equal. Many of the late-model TC heads were produced with the ports slightly out of alignment, causing a reduction in the flow through the ports. This misalignment would be eliminated if the head was to receive a full porting job for racing, but Street Prepared rules prevent such hacking. It therefore becomes advantageous to have several cylinder heads to examine and choose from. Admittedly, very few competitors have more than a single head on hand; however, shops specializing in Lotus will often have many available, and trades are often possible.
When examining a head, it's important to inspect for previous machine work; the head may have been modified beyond the legal limitations. The ports and combustion chambers must be original, and the head should not have been milled. The face of the head should be checked with a machined straight-edge to ensure that it's perfectly flat. The condition of the seats is unimportant, as these will be replaced; however, the seat area of the combustion chamber should be closely checked for cracks.
If you're not going to be running your Twin Cam in either the Stock or Street Prepared categories, you may not care that the head needs to be or has been milled; you may want the associated increase in compression. However, if the head shows signs of warpage or has been milled for reasons unknown, it is extremely important to check the line bore of the camshaft bearing saddles. When a head warps, it will be distorted on the top side as well-the side that supports the camshafts. This top-side warpage will cause the camshaft saddles to become misaligned, and the cams will bind in the bearings. This binding will cause at the very least a loss of power and accelerated bearing wear; at the worst, it will cause broken camshafts and chunks of stuff ricocheting inside the engine. If the line bore is found to be distorted, all is not lost; the cam bearing saddles can be linebored to restore the proper geometry (an expensive proposition, typically costing around $250).
Do not glass bead your aluminum head to clean it. While glass beading makes a beautiful finish and is an effective method of removing carbon from the combustion chambers, a portion of the glass beads will become imbedded in the aluminum. It will then be virtually impossible to remove the beads from the metal prior to assembling the engine. Once the engine becomes operational, the vibrations of the engine will dislodge the glass beads, allowing them to merrily circulate through the internal workings of the engine. The Street Prepared rules allow for the ports to be machined up to one inch in from the mounting faces of the intake and exhaust manifolds, allowing the manifolds and ports to be matched. On the Stromberg head, this rule allows for a significant improvement in the flow through the intake tracts. That's because the port walls feature a stepping which requires the intake charge to abruptly change its direction of flow-and fortunately, the step is within the first one inch of each of the intake tracts. This step can be legally removed by grinding and shaping the port walls so they blend in smoothly from the intake manifold.
The Valve Grinding Operation
The primary reason for reworking a cylinder head is, of course, establishing a proper seal between the valves and their seats. However, the manner in which the valve job is performed will have a significant impact on the power output of the engine.
When a valve job is performed without installing new seats, the seats will by necessity require grinding to establish the proper finish. This grinding causes the valves to become slightly recessed into the intake or exhaust ports, resulting in a loss of power. This power loss is caused primarily by the valve and seat geometry restricting flow of the intake and exhaust gases at low valve lift, and by an increase in the combustion chamber volume, which reduces the compression ratio. Therefore, if the seats require more than a light touch-up, they should be replaced to maximize the engine's power.
When performing the grinding operation, it's desirable to have a small contact area width between the valves and the seats, since a small contact width will improve flow at low valve lift. However, the valves rely on this seat contact for heat dissipation, and an insufficient contact area will cause the valve faces to overheat and burn. Dave Bean recommends a contact area width of 0.040 inches for the intake valves and 0.060 inches for the hotter-running exhaust valves (on a racing engine, Dave squeezes these down to 0.030 and 0.040 inches, respectively). To set the contact area width, the seat is cut immediately up-port from the contact area (the port diameter is enlarged directly underneath the valve head) until the contact width is reduced to the desired value. The location of the contact area on the valve face is also important. The distance from the contact area to the outer edge of the valve face is referred to as the margin. If the margin is excessive, flow past the valve will be reduced. If the margin is too small, the valve will be more susceptible to burning. Bean uses a margin of 0.040 inches on both the intake and exhaust valves.
When the valve seats are ground, the faces of the seats are cut at a 45° angle. To further improve the flow past the valves, additional grinding can be performed on the seats immediately up-port from the area of contact between the valves and seats; this is the process used to create the so-called multi-angle valve job. By grinding an additional two angles into the seat, with each successive angle being more parallel to the port than the previous, the profile of the seat is rounded in the area near the valve, and flow past the seat is thus promoted.
To maintain the thin line of contact between the valves and the seats, it is essential not to overdo the valve lapping operation. The purpose of lapping is to provide the optimum finish and fit between the valves and the seats, and only a minimal amount of lapping is required. When the areas of contact on the two components have a uniform gray appearance, the lapping process has been completed.
The Valve Sprints
Valve springs do "wear out". After years of being preloaded in service, the free length of springs tends to decrease. This decrease in the free length of the springs will cause a reduction in the spring preload on the valves. If the spring preload drops to around 45 pounds, the valves will tend to bounce on the seats at high revs. Since the actual rate of the spring will be unchanged, the preload could be restored by shimming under the springs. The springs on the Twin Cam head, however, have little room for shimming before the coils will bind at maximum lift (particularly if a high-lift cam is installed). If this is found to be the case (and it should always be checked), it will be necessary to replace the springs with new units. It's a good idea to replace the springs anyway, to eliminate the possibility of old springs breaking from fatigue.
Dave Bean recommends a spring preload of at least 55 pounds on a stock camshaft, with 65 pounds being preferred. With this preload, the stock springs are good to 7500 RPM when using the stock cam profile. For high-lift racing cams, Bean increases the preload to between 75 and 85 pounds. Preloads much higher than this will cause accelerated cam lobe wear, higher stresses on the valve train, and possible coil binding. While lightweight valve train components are neither needed nor allowed on a Street Prepared engine, they reduce the spring rate requirements demanded in a highly-modified racing engine. High-lift cams and an increased engine speed require the valve assemblies to accelerate and decelerate at much higher rates than when using the stock hardware. Lightweight valve assemblies allow the valves to closely follow the profile dictated by the camshaft, without requiring the valve train to have excessive valve spring loads.
The Valve Tappets
The previous owner of our Europa apparently believed that changing the oil was a pointless exercise. Upon teardown, our engine showed the results of this man's theory: worn parts everywhere. In addition to the lovely sight of gold main and rod bearings, the top end of the engine looked quite tired. Dirty oil wreaks havoc on the cam tappets and the tappet sleeves, causing the tappets to rattle around within the sleeves. The upper limit on the tappet-to-sleeve clearance is 0. 003 inches. Since we wanted to do the engine right, we replaced both the tappets and the tappet sleeves.
The Valve Guides
The valve guides should be replaced as part of every valve job. The Twin Cam engine was designed with the lightest valve train possible, which means short valves and therefore short valve guides. As guides wear, the valves are more susceptible to rocking during operation. This rocking causes an inconsistent seating of the valves and an attendant loss in power.
The best guides to use are those made of bronze alloys. In comparison with iron valve guides, bronze guides have a lower wear rate due to lower friction between the guide and valve stem, and they provide superior heat transfer. But the greatest advantage of bronze guides is the ability to run much tighter guide-to-stem clearances. This tighter clearance provides a more consistent seating of the valves, thereby providing an improvement in the sealing of the combustion chamber. When working with bronze guides, Bean hones them to size, running minimum clearances of slightly under 0.001 inches for the intakes and slightly over 0.001 inches for the hotter-running exhausts.
The Cam Bearings
Two widths of cam bearings are used on a TC head. As it turns out, the narrower bearing provides sufficient support to allow it to be used for each of the cam journals (with the exception of the frontmost bearing, which must have an oil feed hole). By installing the narrower bearing in each of the cam bearing saddles, rotational drag on the engine will be slightly reduced (remember, it all adds up!). If the bearings are bought as a rebuild set, you will receive the usual mix of narrow and wide bearings. However, Bean has the narrow bearings available individually, and he will gladly supply you with a sufficient number to complete the job.
The Carburetors
While the Big Valve was the most powerful engine fitted to a U.S.-spec Europa, it was equipped with the lowly Stromberg emission carburetors, rather than the much more desirable Weber or Dell'Orto carburetors available on the European versions. Without the emission controls, the European Big Valve produced a healthy 126 HP; obviously, the Stromberg carbs cost the engine a bunch of power.
Street Prepared rules allow the carburetors and intake manifolds of an engine to be changed; unfortunately, the two barrel Euro-carbs will not simply bolt onto a head that was originally equipped with Strombergs. Oddly enough, Lotus chose to cast a head with intake port runners that were designed specifically for single throat carburetors, rather than simply adapting the Strombergs to the original Weber-equipped head. And to make matters worse, a switch to a Weber head wouldn't be legal in Street Prepared, since no Weber-equipped Europas were ever imported into the U.S. What all this means is that you're pretty well stuck using two single-barrel carburetors for the induction, and the options are limited. Good results have been reported using 2 inch diameter S.U. carbs in place of the 1 3/4-inch Strombergs; however, we have had no experience with this set-up. We chose to stick with the Strombergs.
The rather poor throttle response of the original carb setup can be significantly improved by eliminating the intake crossover tube, which is used to heat the intake charge to reduce emissions. By removing the crossover tube and the secondary throttle butterflies, and capping all the holes produced by this operation, the intake charge from the carburetors is provided a much more direct shot into the combustion chamber.
The Exhaust System
Exhaust systems on Street Prepared cars may be replaced or modified at will, provided the noise is kept under control. Bean manufactures a "tri-Y" (4 into 2 into 1) tubular steel header, which is adapted to a low-restriction muffler. This header is sized to improve the powerband of lightly warmed-over engines, rather than being designed for the requirements of highrevving racing engines. This system also serves to cut a significant chunk of weight, saving several pounds over the stock system.