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"Lotus Twin Cam Engine Project"

      While much has been written about the Lotus/Ford Twin Cam engine, few will doubt its credibility as a high performance unit. From its inception at Nurburgring to its long held use in Lotus road cars, the myriad of race wins and road tests simply speak for themselves.

      While Lotus no doubt produced a capable engine, serious sports car enthusiasts are always left wanting more. It can be said that to their eye, a lot of power is good, more is better and too much is just right. Actually achieving “too much power” is not an easy task however and this is where many attempted projects fail, either through poor planning, poor execution, or both. This article will explore these concepts in detail, show-casing how all too often an attempt at “too much” gives “not enough”.

      This project all started with a conversation I had with Carl Matschke of Big Dreams Auto Restoration in Grants Pass Oregon. Carl was familiar with our work on MG cylinder heads and figured that our expertise with those types of engines could just as well apply to the Lotus Twin Cam. Since we both felt Lotus did a pretty good job from the get-go, it was agreed there was no use spending a lot of money for what may result in a temperamental engine or one with sub-par road manners. At the conclusion of the conversation, Carl decided to bring the head by for us to look over and see what (if anything) might need to be done with it.

      At first glance, some modifications were apparent. The valve size was definitely not stock and the head was converted from dual Stromberg carburetors back to Weber DCOE’s. While the Weber manifold job was done well, the valve size enlargement was questionable. In order to keep the larger sized valves from hitting each other at overlap, the previous modifier had obviously sunk the valve seats quite appreciably into the chamber, leaving a distinct ridge around their periphery. Adding to this were burn patterns indicating the engine had not been running well and after checking the actual valve size, I quickly began to feel rather unhappy about the whole thing.

      What I found were 1.70” inlet valves and 1.45” exhaust valves (For reference, stock inlet valve size is 1.53” and the stock exhaust valve size is 1.32”) and after removing them, I became distressed at the sight of the poorly executed “big valve” modification. While some attempt at proper integration of the larger valve seat size was made, my initial reaction (and that which I still stand by) is that it would have been best had they done nothing at all. We also found a broken valve spring, but in light of all the other butchery involved, that was the least of our worries. It was time to call Carl again and relay the bad news, this head needed help!

      In talking with the customer, it was learned that this engine had previously been modified and tuned by a “Lotus Guru” whose (then) business was located in the San Francisco Bay Area of California. This man had been responsible for the larger valves, as well as bigger cams, a .040” overbore, more compression, and larger than stock 45 DCOE Weber carburetors. With all these modifications, a dyno output of 130bhp (at the flywheel) was said to have been obtained. For reference, the early production versions of the 1558cc engine were factory rated at 105bhp. This knowledge gave us some clues about what had gone on, as well as some sort of baseline from which to begin our work.

      With both Carl and the customer’s blessings, we embarked on a mission to better match the cylinder head and induction system components to the intended use of the engine. Our goals were to get as much torque output from the engine as we could within a wide and street usable RPM range, make the engine run on available pump fuel and keep it reliable. Making a statement such as this would be easy, however actually achieving these goals would take a little more effort.

Getting Started:

      One easy place to start was with the carburetors and inlet manifold. While the head had been modified to accept a DCOE type manifold, that manifold had never been modified to accept the larger 45mm sized carburetors. Since the engine was not making full use of these larger sized carburetors anyway (even in it’s apparently, highly modified form), we felt a change to a smaller sized carburetor might be the best remedy.

      In fact, sizing the carburetors was an easy choice, as many charts relating carburetor size to horsepower output are available. Using one of these charts will show that a pair of 40 DCOE’s will run out of flow capability (become too small) at about the 150bhp mark. Since 130 horsepower is well within the capabilities of properly tuned 40 DCOE’s, going back to smaller carbs was an obvious choice. Had we been looking to optimize the engine for higher RPM horsepower outputs, our choices would have been different. With this simple choice made, it was time again to tackle the cylinder head.

      Our investigation of the cylinder head continued at this point with some baseline flow testing. We didn't so much care what it flowed at this point, but were really just curious to see how bad the situation actually was, and what we found was almost laughable, it was bad!

      Airflow evaluation with a project like this is not as simple as plunking the cylinder head down on the flow bench and sucking some air through it to “see what it does”. While that might be good enough for some shops, the actual CFM numbers we record are only the first step in finding out more information about how well the item under test is conveying air. The simplest of the three computer programs we use for port evaluation needs only test depression, CFM and some basic valve and throat size measurements to calculate a variety of useful outputs. This allows us to view the flow past the valves as efficiency rather than merely a CFM number. These efficiencies can then be related to any valve size or cylinder head we’ve ever tested. What might be an outstanding efficiency level for one make of cylinder head might be very poor for another, but knowing how one compares to the other in terms of efficiency gives us an idea of how far we are off from a known “ideal” and a goal to shoot for. Our initial testing showed the head was achieving very poor efficiency levels right off the bat and our soon to be cured port molds would help us understand why.

      Whenever a head undergoes serious development, we feel it is absolutely essential to use molds of the ports to evaluate their size and shape. In the case of the Lotus head, a distinct “hourglass” shape to the inlet port was immediately evident. This was due to the largely unmodified port having had a very large inlet valve installed without any consideration to the transitions from one cross sectional area to the next. While the first half of the port looked fine (owing to its simple inward tapering shape originally designed by Lotus) the second half was not so well constructed. The distinct outward tapering shape of the second half created a port whose over all dimensions mimicked the form of an ideal venturi, in reverse! This “reverse venturi” shape probably accounted for the inlet ports great ability to flow air the wrong way and their subsequent poor ability to flow air the right way (yes we test them both ways). Having a mold like this is of questionable value when all you intend to do is look at it however and our intents were to be a little more scientific than that.

      Our next step in the port evaluation process would involve slicing these molds in critical areas at 90 degree angles to the direction of airflow. These slices would then be transferred to graph paper and outlined. The number of squares within the outline of the port mold would indicate the cross sectional area of that mold slice in square inches. The cross sectional areas and their distances from the valve seat along the port centerline are then entered into the computer for further evaluation. Doing this allows us to evaluate the velocity and efficiency of each segment and help determine where the port needs to be larger or smaller, by how much and the effect on air pressure with these changes.

      The problem with the larger valves previously installed, was if the head were ported to match the valve size, the ports would become very large and poor low range output would be the result. Since we wanted to build torque at lower RPM’s, we needed to keep the ports smaller and thus match the valve size more appropriately to these smaller ports. Since nobody wanted to risk the head to welding it (with subsequent heat treatment and total re-machining of all critical surfaces), we chose the best available compromise, a 1.625” inlet valve and a 1.375” exhaust valve. While we would gladly have used even smaller valves, the previous attempt to “blend in” the seat inserts during the big valve modification left no material to do the job right and without welding, what you saw was what you got.

      The steps to take in reworking the head were thus:

1) Renew the seat inserts to allow the use of smaller valves with a better valve job.
2) Modify the port shape to lessen disruptive changes to port cross sectional area, as well as correct past inconsistencies in port modification.
3) Modify the combustion chamber to unshroud the valves at all critical valve lift points and promote greater combustion efficiency.
4) Perform a high performance valve job allowing maximum flow past the now smaller valves under the limitations already presented.


      We removed the seat inserts with a TIG welder before carefully measuring the seat pockets and ordering inserts of the correct size. Once the seats had arrived, the head was heated to 200 degrees Fahrenheight while the seats cooled on a block of dry ice. Doing this allows the head to expand and the seats to contract for ease of installation and a very secure fit once the temperatures even out.

      The next steps would involve hours of delicate port work. Starting with the exhaust side, the good thing about them was even though they had been poorly reworked in the past, the head porter had not gone too far by making them too large. Because we had just enough material left to get the shape we wanted, we were able to create a very effective exhaust port. Since all the ports were different, it was a time consuming job using templates and measuring instruments to check our work. The process was to grind a little and check, then grind some more and check again, all while being sure not to remove material from areas that were already made too close by the previous modifier. After the exhaust ports took shape, it was time to move on to the intake side

      The inlet manifold did not involve a tremendous amount of material removal, but still took some time as with the exhaust ports (I.E. grind and check). The inlet ports inside of the head itself however, were a bigger challenge. Getting enough cross sectional area past the guide without creating holes in the roof became the name of the game. In all Lotus Twin Cam heads, the roof area of the port right before the valve guide dips down a little. The reason for this seemingly obvious fault is because the inlet cam gallery and spring platform happen to be right above it. While moving the roof up would allow a much straighter port, it would also mean breaking into this area up above. In order to avoid disaster, we used a simple 1:1 thickness checker and proceeded with caution. To allow us to grind the port to the shape and cross sectional area we needed right from the start, it was a simple matter of using our port molds as a guide towards making the necessary templates and measurements for verification.

      Further in, the roof was blended as best possible into the long side of the port with what had been left from the previous modification work (where we would have filled it in if we could) and the short side was reshaped using another port mold derived template. All this resulted in a much more even change in cross sectional area, as well as a port large enough to take full advantage of the now smaller valve size.

      Modifying the chamber was fairly straightforward, as the previous modifications had not left us with too many options. We settled on a simple radius profile for deshrouding that would allow very good low and mid-lift flow without unduly sacrificing upper lift performance.

      The inlet valve job used very little top cut (due to the previous work rather than any ideal), a 45 degree seat, a 60 degree first undercut and a 75 degree second undercut which was then blended into the throat. The exhaust seat profile was a more complex 7 angle profile using 12-28-45-56-67-78-85 degree steps in that order of occurrence. We then blended the lower angles into the throat area to simulate a true radius shape.

      Back on the flow bench, we found our smaller inlet valves were now flowing an average 11.4cfm more air at all lifts than had the previous modifier’s ultra-large combination. The exhaust ports flowed remarkably better as well, with an average 14.8cfm gain. Since the port sizes had not increased by much and since the valve sizes had actually been reduced, these gains represented a dramatic increase in flow efficiency. While we knew the ports were not as good as they could have been were we given a fresh head to start with, our flow bench results indicated we had done well and it was time to take the engine to the dyno to really see just how much of an effect we had made.

Dyno Time, "Take one"

      The first run on the dyno did not prove as successful as we’d hoped. By the end of the first day we were easily making high end horsepower, but did not have good low end torque output. Additionally, the engine was very rough coming up on the cam in the lower RPM ranges (exactly what we didn’t set out to achieve) and this prompted us to make some cam timing changes. After making changes in various other areas as well (including carburetor calibration and ignition timing), we ended up leaving the session with mixed results.

      The good news from our first dyno session was that our carburetor choice had proved effective. The carbs responded well to every change we made and did not seem to be a great restriction when viewing manifold vacuum at higher RPM’s. We also made over 150bhp at 6,800 RPM (as high as we ever took the engine) and the curve had not yet peaked or fallen off. Since we had reduced both valve size and carburetor size, we felt this 20+ horsepower increase was very rewarding.

      The bad news, was we were going to need some different cams if our original goals were going to be met. While the upper RPM power numbers were great (and certainly a far sight better than before) the torque below 3,800rpm wasn’t good at all and we predicted the engine’s road manners would be poor as a result. Since high RPM horsepower was not what we set out to achieve in the first place, a reduction in cam duration was the obvious choice.

      After considering various options, a mild competition grind from Iskendarian (with about 12 degrees less duration and only slightly less valve lift) was chosen. Our initial inlet cam setting was on a 102 degree inlet center line with a 104 degree separation.

Dyno Time, "Take two"

      Our second trip to the dyno proved much more satisfying. Immediately our baseline pulls established over 15ft/lbs more torque at 3,500 RPM than we’d had previously. After spending time narrowing in on an ideal carburetor calibration and timing advance setting, we had increased that gap to over 20ft/lbs and the rough running we had experienced previously was nearly imperceptible. After lunch I opted to try some cam timing variations and gained even more low RPM torque at nearly no loss in upper RPM horsepower. The final runs showed a 32ft/lbs gain in torque over the previous cams and we had only lost about 10bhp at the very upper RPM ranges. In fact the Isky cams had proven to be much more effective from the beginning of our pulls all the way up around 5,300rpm where they began to lose out to the original speed grind installed by the previous mechanic.


      By utilizing smaller valves, smaller cams and smaller carburetors, we gained 12bhp and untold amounts of torque (the original dyno results were unfortunately not available). Reasons for this are as follows:

  • By using smaller valves and porting the cylinder head to match their size as best possible, we were able to provide the engine with the airflow potential it needed to make excellent torque and horsepower.
  • By using smaller carburetors we allowed them to function as designed for better atomization capabilities and proper metering.
  • By using smaller cams, we were able to “pivot” the torque curve around the peak torque point to emphasize the lower RPM ranges we originally targeted. Additionally, the early inlet closing point encouraged full use of the (now lower) 10.4:1 compression ratio, while still allowing safe running with available pump fuel.

      By now it should be obvious that good engine packages don’t just happen by accident, they come about through careful consideration of all the components and to their interactions together as a system. Without flowbench and dyno testing, this project could never have turned out as successfully as it did. Knowledge of the right items to use and thorough testing to verify that all components were working in harmony was essential and necessary.

      With everything said and done, we were very pleased with the increase in performance and the customer expressed great appreciation that we were able to spend the amount of time and effort we did with development and testing to ensure that the job was done right. While this article has only touched on the most notable points of the project, juggling the many details involved was no easy task. As with any project of it's kind, this job will only serve to build experience and become the baseline for future developments. If the results we attained this time are any indication, we'll be looking forward to the future!

Our first look of the cylinder head after an initial clean up revealed huge valves and poor workmanship.

After removing the valves, the poorly integrated valve seats and "unfinished" appearance are evident.

One last view of the poorly modified inlet ports. Note: drastic inconsistency of port shaping.

The completed inlet port mold ready to be sliced apart.

The port mold slices ready to be transferred first to graph paper and the resulting cross sections then to the computer.

Valve seats welded and ready to be removed. TIG welding shrinks the seat, eliminating the interference fit for easy removal.

The new seats in place after utilizing the shrink-fit method described in text.

A view down the previous exhaust ports. Note: drastically uneven modification to the short side turn.

Using a snap gauge to check port width past guides. Getting the sides of the inlet ports straight and true was the first operation.

The area within the inked outline is considered a "no grind" area during this operation of the inlet port work. We focused on the sides of this outline until the very end to avoid making holes in the casting.

We've only worked the sides and roof of the inlet port in this picture, the uneven past modification to the floor and short side turn is again quite evident.

Can something ugly become something nice? We think the revised combustion chambers did.

A view of the finished exhaust ports shows very precise shaping of the short side turn and our radiused seat profile.

Comparing this photo to the original, there is not much comparison. Note: final shaping around guide area and more even port contours.

View down the inlet port, including the manifold this time.

The before and after port molds side-by-side with the final shape at left.

The differences in inlet valve diameter, with the smaller valve at right being our choice.

All valvetrain hardware was sourced from Dave Bean Eng. in San Andreas California (www.davebean.com). Dave has been a big help with all our Lotus projects and supplied the goods seen here.

The last day on the dyno, success at last!

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