View single post by Esprit2
 Posted: 05-25-2009 05:04 pm
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Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 337
John H,

There was only one factory installed cam used in the Jensen-Healey...  the C-cam.   The various different valve timing values given in the manual are just a matter of timing that one cam to different MOP's (Maximum Opening Points).

The C-cam was designed to run at 110 IN / 110 EX MOP.   If you want your original equipment engine to run the best overall,  time both cams to 110 MOP.

When the original engines were going through pre-production testing,  they met the emissions standards of the day.   However, the new USA Federal emissions standards had been announced and were going to get more strict in 1974,  so JH/ Lotus chose to meet that coming standard from the beginning.   The test engines had passed the coming '74 Fed. standard,  but met the HC limit by such a narrow margin that it left no room for error or wear.   So they chose to tweak the engine a bit more before production.

Ignition timing and carburetion remained unchanged,  but both cams were re-timed to 115 MOP.   That retarded the intake 5°,  advanced the exhaust 5°,  for a total decrease in overlap of 10°  (from 52° to 42°).   That reduced the HC to a comfortable level,  but also reduced horsepower (about -10 Hp) and torque.   The engine was cleaner but weaker.   That's the set-up that went into the original J-H.

For 1975, the Fed's tighten the standard again.   Lotus backed way off on the ignition timing, leaned out the carbs and changed the cam timing to 100 IN / 110 EX.   The new cam timing went the other way,  "increasing the overlap" by 10° compared to 110/110, to 62°.   The extra overlap allowed the intake and exhaust gasses to co-mingle a bit longer.   The effect was like EGR (Exhaust Gas Recirculation), reducing the oxides of nitrogen.   The retarded ignition timing and lean carb mixture changes took care of the rest.

All together,  the changes pretty much castrated the 907,  and it didn't make enough torque to peal a grape.   One small upside of the 100/110 timing was that greater overlap favored high end horsepower;  unfortunately it did that at the further expense of low end torque.   As a result, they were able to advertise the same peak horsepower, and as we all know, horsepower sells cars.   However, the resulting "Torqueless Wonder" was not pleasant to drive in traffic, as it had to be rev'd hard to perform.

110 IN / 110 EX is the correct, as-designed C-cam timing.   100 and 115 are both emissions solutions that hurt performance in favor of lower emissions.   Again,  the 110/110 set up passed 1974 Federal emissions standards,  just not by a wide enough margin to make factory decision makers comfortable.   It's not a "dirty" setting,  and the engine runs the way it was meant to run..

If you have an original 100/110 engine,  re-time the cams to 110/110 MOP,  set the static ignition timing to around 14° BTDC,  and adjust the carb mixture for best running instead of setting it to produce a specified CO level at the tail pipe.

The 100° (rounded, really 97°),  110°  and 115°  MOP's are all present on the same, stock pulley.   Not all MOP's are marked on all part number versions of the pulley;  but dimensionally and functionally it's the same pulley.   The different MOP's are just a matter of how it's installed, flipped front to back, and indexed.

The stock pulleys canNOT be adjusted to produce the 104° MOP !!  New, dedicated pulleys are required for the 104° MOP.

Note that the 100° MOP, blue dot, single-MOP pulley is 100° MOP as marked; however, the blue dot on the 110/100 dual-MOP pulley is really 97° MOP. The math doesn't work out for 110° and 100° to both exist on one pulley, so Lotus cheated a bit with 97° MOP for the convenience of having both Red and Blue timing dots on one pulley. If you're just talking about it in conversation, then "100 MOP" or "blue-dot" works. However, if you're really timing the cams and trying to figure out the math of what's going on, then remember to use 97° as the working MOP for the blue dot on the dual-MOP, 110/100, red/blue pulley.

The 104 cam is a good street performance cam,  if street "performance" is what you want.   It will make more horsepower, and the engine will happily rev to redline and beyond.   But all that high end performance comes at a price in that the low end torque and drivability will suffer.   NOT A LOT, IMHO, but if you're one of those who is not pleased with the 907's low end performance,  then installing a pair of 104 cams will be a small step in the wrong direction.   I have a pair of 104's in an otherwise near-stock 907,  and I love 'em.   Different strokes for different folks.

The 104 cams require 104 MOP pulleys.   Don't run them at 110° MOP or you won't get what you paid for.   It's not wrong in a way that will bend valves or do something else that's potentially expensive to repair.   It's just not going to produce the good top end results you paid for... and it won't be the cam's fault.

The 104's high lift (0.410") will cause the stock valve springs to go coil-bound.   It's necessary to either cut the valve spring pocket a little deeper into the head,  or better yet, install a set of special, thin-wire valve springs.   Do not attempt to run a 104 cam with stock valve springs... something WILL break.

The "Three C's"  (Cams, Carbs, Compression) should always be changed in concert.    In order for a change to one to really produce it's greatest benefit,  the other's must also be changed appropriately at the same time.   Putting 104 cams on a stock, low compression 907 isn't going to produce the optimum results that give the 104 cams their performance reputation.    With a pair of 104 cams,  the compression should be 10:1 or higher,  and the carbs should be re-jetted.   Strombergs don't necessarily have to be replaced by Dellortos,  but new needles and jets should be installed.   Of course, Dellortos are the better companions for the 104 carbs.

If you install a pair of 104 cams in an otherwise stock Federal J-H 907,  you will get an increase in top end horsepower;  but you probably won't be as over-whelmed as you had hoped to be.   You’ll just end up being one more un-informed voice talking negative about the 104 cam when they really didn't do the cam justice during the installation.   Do it right, or don't complain later.

The 107 cams also require 104° MOP pulleys.   Starting from the stock C-cam as a benchmark,  the 107 goes the other way, milder compared to the 104 and stock cams.   The 104 has "hotter" valve timing, while the 107 has "milder" valve timing.   The 107 has 20° less duration;  however, the MOP change from 110 to 104 keeps the overlap in the same ball park.

The milder timing produces more low to mid-range torque, which really helps drivability around town.   Normally,  that would come at the expense of high end horsepower,  however, the 107 can also has:
1) significantly more lift, which helps the engine breathe better across the rev range, and...
2) very aggressive opening and closing ramps that result in more 'full open' time than with some 'hotter' cams.

The breathing gain due to greater lift, plus the aggressive way the valves are 'hammered' open and closed, offsets some of the expected top end power loss due to mild valve timing'. As a result, the upper rev range doesn't suffer 'as much' with this cam that favors low-end torque.

Peak horsepower is about the same,  but the improvement in low-end and mid-range torque is what makes the 107-cam engine 'feel' more powerful in commuter traffic.   There's a difference between power and torque, and many folk confuse them;  but the net effect is that the 107 helps the 907 feel stronger by making more low end torque.

In reality,  the 107 starts to run out of breath above 5500 rpm.   It will pull to redline, but not with the enthusiasm of the 104 cam. In contrast, a 104 cam will roar right up to redline with gusto, and scream right past it if you don't lift your right foot.

A pair of 107s was used in the 2.2 liter 912LC (Low Compression).   In that 'basic' 912 engine, the stroker crank really gives the engine's torque curve a significant boost.   Every 907 should get a 2.2 crank... IMHO, it's the best thing you can do for it.   Much of the 107 cam's reputation comes from the 91's stellar performance.   But much of the 912's torque really comes from the stroker crank.

The 107 cam will also help boost the 2.0 litre 907's low-end torque curve,  but don't get your expectations up to 912 levels if you keep the 2.0 crank.   IMHO,  the 107's reputation as the cure for all that ails the 907 is blown out of proportion and taken out of context from the 912 application.   It's beneficial for a commuter 907,  just don't get your expectations out of proportion.

One advantage the 107's milder timing gives it is that it's compatible with the stock 907 engine's lower compression (remember the three C's).   While a 104 cam really needs higher compression to perform at it's best, the 107 cam is at home with 8.4:1.   Plug-n-play.

The difference between the Lucas 23/25 and 43/45 distributors isn't significant, in terms of performance.   It's evolution,  new design,  new tooling.   The 43/45 is an improvement in terms of durability and more consistent timing.   And the larger diameter distributor cap spreads the terminals out a bit further, which makes it more resistant to arc'ing between terminals with higher voltage coils.   But in terms of timing and power output, there's no big advantage to either one.   If an older 23/25 distributor fails,  I would suggest putting your money into a 43/45 upgrade.   But don't switch expecting more horsepower.

The Pertronix Ignitor II is too large to fit in the 23/25, but it will fit in the 43/45. Being able to use the improved Ignitor II could be good justification for upgrading to the later distributors.

From a timing stand point,  the mechanical advance curve should be 16-18°,  all in by 2500 engine rpm (distributor rpm = 1/2 engine rpm).   Then set the static ignition timing to 12-16° BTDC, depending upon what local petrol will support.

John H wrote:  I'm running the 104 grind on a higher compression piston 10:1 i think with stromberg carbs set  the valve timing at  115 / 115  and ign timing at 10 deg BTDC   recently switched from the 45 dist with points to my 25D with pertonix *~*~*

115/115 MOP is NOT appropriate for the 104 cams.   Compared to where the cam was designed to run, 115 retards the intake 11°, advances the exhaust 11°, and reduces the overlap 22°.   You're applying emissions MOP timing to a performance cam...  apples and pomegranate.

At the very least, re-set the existing pulleys to 110/110 MOP, which is 'less wrong'.   But to do it right, purchase a set of 104 MOP pulleys and time the 104 cams properly.   You have the higher compression to support the cams,  now time them properly.

Mechanically,  the 43/45 series distributors are superior to the 23/25 series... it was just design evolution, and newer was better.   But this many years down the road,  it may well be a matter of which one is still in better 'used' condition.

Either series distributor will benefit from an electronics "points replacer" ignition system.   If you were starting from scratch and both distributors were in good condition,  I'd suggest going with the 45 and Pertronix (not that I'm pushing Pertronix...  but you mentioned it).   But your 25D is installed,  so if it's in good condition,  then there's no reason to replace it.

If either distributor needs work,  then send it to Jeff Schlemmer at Advance Distributors.   He can rebuild it to better than new condition,  re-curve the advance to anything you want,  and install an electronic ignition.   Good work, reasonable prices.

Jeff Schlemmer,
Advanced Distributors
1149 Quincy Street
Shakopee, MN 55379
(612)-804-5543  e-mail, from website  website

Last edited on 08-30-2017 03:59 pm by Esprit2