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cams, valve timing and ignition timing  Rating:  Rating
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 Posted: 05-25-2009 01:15 am
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fauxtog
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sooooooooo.  we have variable timing for the valves. supposedly 110 / 110, 115 / 115, and  ?100 / 110 ?
we have multiple cam grinds available like the stock one or the 104.
and we have multi distributors set ups like the 25D or the 45 with points or pertonix or....

I'm wondering what the performance benefit is for the different valve timing set ups. with the two different cams.  ie top end performance or low end torque or gas mileage or ease of driving.

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 

John H

btw i know this gets discussed and has been discussed but I never saw an answer to why 115 vs 110 especially with the higher lift cam and higher compression

ps any generous soul have a dellorto set up to spare?  ;)

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 Posted: 05-25-2009 02:37 am
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Jensen Healey
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The 104 cams are supposed to use 104 pulleys.

I'll let someone more knowledgeable comment on the cam timing.

Kurt

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 Posted: 05-25-2009 05:04 pm
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Esprit2
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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
    jeff@advanceddistributors.com  e-mail, from website
    http://www.advanceddistributors.com/  website

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

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 Posted: 05-25-2009 09:51 pm
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subwoofer
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Esprit2:

You obviously know these engines well, do you have any experience with upping the compression for the 2.0? SJ Sportscars can provide 11:1 pistons for the 2.0, which (I guess) would be good in concert with a pair of 104 cams.

--
Joachim

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 Posted: 05-26-2009 03:09 am
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Judson Manning
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John,

There really is very little emperical data (dyno runs) to differentiate the various combinations of cams, sprockets, carbs and ignition settings.  However, thanks to people like Tim the JHPS board contains a lot of information to help people piece together some ideas.  Others have been kind enough to share their data so we can examine the dubious 'should' and 'ought to' statments that are out there.

The data out there suggests the dual 104 cam combo on 104MOP sprockets produces extremely 'peaky' results (http://www.lotusbits.com) with not much usable power in the everyday driving range.  One could argue shifting to 115/115 timing would produce more low-end torque and drivability.   Get a set of 104MOP sprockets, then find us a dyno shop and let us know!

Instead of dropping $1500.00 on a set of Dellortos, how about putting a 107cam on that exhaust and using those 104MOP 'green dot' sprockets.  My personal experience with various combinations suggests it is hard to beat the Excel SE 104/107 set-up for power and drivability.

Judson

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 Posted: 05-26-2009 05:13 pm
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Joel
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I'm outta the JH game for a bit and only poke my head in here occasionally these days - but it's good to see ya back Tim!  I also subscribe to S1S2s3 forum. 

Cheers,

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 Posted: 05-26-2009 08:31 pm
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Esprit2
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Judson Manning wrote: (Snip)... The data out there suggests the dual 104 cam combo on 104MOP sprockets produces extremely 'peaky' results (http://www.lotusbits.com) with not much usable power in the everyday driving range.  One could argue shifting to 115/115 timing would produce more low-end torque and drivability.   Get a set of 104MOP sprockets, then find us a dyno shop and let us know!
Judson,

It's a big leap from minimal available data to a firm statement that 104 cams on 104 MOPs produce extremely 'peaky' results.   It's true, the power curve isn't as broad, or as strong below 3500 rpm as it is with a pair of 107 cams,  but a pair of 104's is not "extremely peaky".   They start to take off around 3500 rpm,  pull like a train from 4000 rpm on up,  and eagerly blow right past redline if you let them.   I know,  I do have a pair of 104's/ pair of DHLA45's/ 37 chokes on an otherwise stock Federal 907.   It doesn't even have the advantage of a desireable higher compression ratio.   It's not a trial to drive on the street,  but it does offer much improved top-end.

It's not 'cammy',  it idles smoothly,  and if you lug it down it will pull away smoothly under throttle without complaint.   Granted, you need to shift down if you want it to pull away smartly from "down low",  but it's not like driving a cammy "full race" hotrod small block that won't idle.

I know from experience.   I installed the 104's in 1995 and used the car to commute to work in congested metro traffic and it was never a problem.

I don't suggest 2 x 104's on 104 MOP's for anyone in search of a mild mannered gentleman's cruiser... use the 107's.   But it does bother me when folk who haven't lived with the 104's talk in all or nothing terms about how bad they are.   They aren't bad in traffic,  and what they offer to the more spirited driver on the open road can't be touched by 107's.   What does the owner want?

I also have a pair of DS2 cams on a 2.2 converted 907 hotrod.   They're hotter than the 104's,  but still trundle through traffic without complaint.

Last edited on 05-26-2009 08:57 pm by Esprit2

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 Posted: 05-26-2009 08:50 pm
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Esprit2
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subwoofer wrote: (Snip)... do you have any experience with upping the compression for the 2.0? SJ Sportscars can provide 11:1 pistons for the 2.0, which (I guess) would be good in concert with a pair of 104 cams.

--
Joachim


Joachim,

The 907's semi-hemi combustion chamber is very tolerant of higher compression on pump gas.   11:1 isn't a problem on the pump gas available in the USA.   Of course, a lot will depend upon the gas where you are.

If you're considering 104 cams or hotter,  then raise the compression to at least 10:1.   11:1 would be good if you wish to drive in a spriited manner,  but a waste of high octane fuel if you'll be trundling along in traffic most of the time.   Be sure you know what you want,  because it's not a simple change later.

I have a pair of 104 cams in a low compression 907,  and it works well.   I just know that it could work much better with higher compression...  the cam's true potential isn't realized when matched with the low compression.

If you're thinking 104 cams and Hi compression,  then also consider a pair of Dellorto DHLA45E's with 36 or 37mm chokes.   The 37's are more in keeping with the 104s and hi-compression fun mentallity.   But if you think the majority of your driving will be trundling around town,  then go with 36mm chokes.   They give up some top end, but the engine will be more commute-friendly.

Last edited on 12-10-2016 07:22 pm by Esprit2

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 Posted: 05-26-2009 08:55 pm
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Esprit2
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Joel wrote: (Snip)... it's good to see ya back Tim!  I also subscribe to S1S2s3 forum. 


Thanks Joel,

I just bopped in over the quiet holiday weekend and saw an interesting thread.   I'll fade away again as this thread fades... see ya over on the Lotus list.

Last edited on 05-26-2009 09:13 pm by Esprit2

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 Posted: 05-26-2009 09:06 pm
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subwoofer
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Esprit2 wrote:
The 907's combustion chamber is very tolerant of higher compression on pump gas.   11:1 isn't a problem on the pump gas available in the USA.   Of course, a lot will depend upon the gas where you are.

If you're considering 104 cams or hotter,  then raise the compression to at least 10:1.   11:1 would be good if you wish to drive in a spriited manner,  but a waste of high octane fuel if you'll be trundling along in traffic most of the time.   Be sure you know what you want,  because it's not a simple change later.


The cars will not be used for commuting, so that is not much of an issue. From what I can remember from the thermodynamics course I took way back when at university, the thermal efficiency will go up quite a bit with higher compression, so for cruising at highway speed, the upped compression should be good.

Regarding the pistons: to date I have only seen higher than 9.5:1 pistons for the 2.0 from one single source, and I can't help but think there must be a reason why, since 10.9:1 and 11:1 pistons are abundant for the 2.2. According to the source, they are actually lighter than the stock pistons, so weight should not be an issue.

As far as carbs go, I am about to order a set of 45mm Dell'Ortos.

--
Joachim

Last edited on 05-26-2009 09:08 pm by subwoofer

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 Posted: 05-26-2009 09:32 pm
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Esprit2
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Joachim,

If you have the option,  order the E version of the Dellortos... 40E or 45E.   The differences are in small details like fine pitch threads on the idle mixture screws.   The overall performance is unchanged,  but it's easier to fine tune the carbs.

Also, look for Lotus spec carbs.   Lotus worked very closely with Dellorto and many details in the carbs used on their engines were specifically tweaked for them.   The idle progression 6-hole pattern was specifically developed for the 9XX and makes a worthwhile difference.   Many of the details Lotus developed later became standard features across the Dellorto line,  but the Lotus progression holes remain Lotus-specific and never became Dellorto-generic.

The higher compression, all by itself, is good;  however, hi-compression also forces the need to burn more expensive hi-octane fuel.   So, from a cost efficiency basis,  is it still better?   If the goal is economy,  then HC may not be worth it.   If the goal is performance,  then yes, by all means go with hi-compression.   11:1 is a good street ratio.

JE Pistons offers high quality forged aluminum pistons for the 907 in just about any CR you could want.   I've not done a survey and don't know what other brands are readily available.   There isn't a structural reason why more pistons aren't available for the 907.   It's just demand.   It costs to develop any product,  and there just aren't enough 907's on the road to get every manufacturer excited about jumping into the market.

Consider installing the stroker 2.2 crank.   It's the best thing you can do for a 907.   Regardless of which crank you use,  use a cross-drilled version with the bearing shells that go with it.   If you stay with your existing 2.0 crank,  you can have it cross-drilled.   Then use the later bearing shell for the X-drilled crank configuration.

Last edited on 05-26-2009 09:36 pm by Esprit2

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 Posted: 05-28-2009 07:01 pm
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Judson Manning
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Tim,

We've had the opportunity to disagree in the past.  I do have the data with which to base my claims, and to date you haven't produced any.  While I do appreciate your personal experiences, they are after all subjective and wildly open to interpretation.  

To make the emphatic statement "The 104 cams require 104MOP pulleys (sprockets)" is about as ridiculous as your narcissistic "I know from experiece" comment.  The only way to answer John's question is to put the car on a dyno and adjust the timing until it produces the results he wants. 

When you can measure what you are speaking about and express it in numbers, you know something about it.   Your daily commuting ritual and your seat of the pants evaluations don't give us a single thing we can use. 

Just like the carb jetting disagreement years ago, when you have some actual data to counter my statements, feel free to criticize me all day long and defend the 104 to your heart's content.  

 Judson

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 Posted: 05-28-2009 09:27 pm
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Esprit2
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Judson Manning wrote: Tim,

We've had the opportunity to disagree in the past.


Judson,

Well, see there, we don't disagree about everything.

Judson Manning wrote:
Just like the carb jetting disagreement years ago, when you have some actual data to counter my statements, feel free to criticize me all day long and defend the 104 to your heart's content.


You were trying to apply Spec 9 DHLA45E jetting to 45D PowerJet carbs without recognizing the fundamental differences between the two types of designs.   And then you complained the result was rich and Spec 9 was no good (I believe you likened the result to fogging mosquitos).   Of course it was rich,  you were double fueling.   First line in any of the Lotus carb specs is the carb body to which the jetting is to be applied... ya can't ignore that.

The reason I defend 104's is because you blast them.   They're good cams for someone who is looking for a little performance;  but no, they're not right for someone wanting a relaxed cruiser.   Mild cams and timing are not the only solution for a 907.   Different horses for different courses... open your mind.

I left this forum a couple of years ago primarily because of the intolerant, territorial, myopic views of two people.   You were one of them.

After a long absence, I pop in for a visit and you're right there ready to be intolerant again.   I expected it, but was hopeful anyway.   Unfortunately, you quickly confirmed my worst expectations.

I'm outa here.   Anyone who wants an opinion other than Judson's knows where to find me.   See ya elsewhere.

Last edited on 05-29-2009 06:26 pm by Esprit2

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 Posted: 05-30-2009 06:41 am
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Judson Manning
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All -

John hinted at the beginnings of this little adolescent side-show "...i know this gets discussed...but I never saw an answer"

Like all good Americans, I will defend to my dying breath Tim's right to express his opinion.  However, answers and opinions are not synonymous, and you can't derive an answer from a philosophical discussion or revisionist history - only through experimentation and careful analysis of the resulting data.

For the record:  I didn't blast the 104 cam and I really don't think the 104 needs a White Knight to protect it, but that's just my opinion.  Contrary to his post, I do have experience with the 104, and no I wasn't trying to apply Spec 9 jettings to PowerJet carbs either. 

Getting back to the question:  To everyone out there with the means, please take your car to a local dyno shop, or organize a local dyno day with your local club and help us collect data.  The more data we have the better idea we have on answering these questions and putting an end to these adolescent distractions.

Judson

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 Posted: 06-01-2009 04:23 pm
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Mitch Ware
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Dynos are the best way. We just took and MGA up to the dyno shop and merely playing with jetting and timing realized a 23% increase in rear wheel horse power.

Happily this week we are going up to pick up a new engine dyno. That will really help out at the shop.

I guess now I'm going to have to build a new motor just to see what can be done. =)

Mitch #19670

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 Posted: 06-06-2009 04:55 am
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fauxtog
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Esprit

Thanks for the detailed response. 

Sorry for the late reply i thought the site would keep sending emails with each response so imagine my surprise when I logged in and saw the thread.

I guess now I need to verify the cam grind West Coast Cylinder heads did for me and the compression on the pistons from Delta. 

I can add this  timed balanced and mixture set right the engine has lots of speed in the 4000 to 7000 rpm  range,  and idles in traffic at 1100 rpm even with the pulleys at 115 /115   I'll try 110 /110  soon

thanks  Esprit and Judson.   play nice and share your toys

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 Posted: 01-19-2014 11:26 pm
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thatmak1
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I have a 1974 Jensen Healey with stock engine except Strombergs were replaced with a single Weber DGEV32/36 and electronic ignition. I recently purchased the car and trying to get it running smooth, idles rough but runs ok at over 2000 rpm's (or at least seems to). I have been reading quite a bit about timing and cams. My car has 110 mop intake and 115 mop exhaust (at least that's what is on the cam pulleys). Timing at idle (950 to 1000 rpm) is approx. 12 deg btdc.

What affect does this cam combination produce?

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 Posted: 01-29-2014 05:51 pm
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Barthol
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Hi,

I have been reading a lot of the topics concerning the cam timing, and there is a lot of valueble information on weather to go on 110/110 , 115/115 MOP
Also on cam types c,107,107 and cam sprockets.

I am a little confused about the meaning of these figures.
Normally I would set the ( messure) the cam timing in deg btdc and atdc for both open and closing.

Is there anyone who can explain the MOP figures and the cam sprockets numbers and how they compare to the opening and closing angle of the valves .

Cheers
Kim

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 Posted: 01-31-2014 09:16 am
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subwoofer
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MOP is also known as LCA, Lobe Centre Angle.

For 260 degree cams and 110/110 timing, you will have the following timing:

Ex: Open 60 degrees BBDC, close 20 degrees ATDC
In: Open 20 degrees BTDC, close 60 degrees ABDC
Overlap: 40 degrees

Same cams, 115/115 timing:
Ex: Open 65 degrees BBDC, close 15 degrees ATDC
In: Open 15 degrees BTDC, close 65 degrees ABDC
Overlap: 30 degrees

I am no expert, but large lobe separation angles are by some (like David Vizard) claimed to be a band-aid to make too-long-duration-for-the-purpose cams behave on the street. His claim is that the ideal lobe separation angle is an engine characteristic and that cam duration should be chosen for the desired overlap at this particular LSA (or (MOPex + MOPin)/2).

Since later 907s and 912s came with 104 degrees LSA, this could be considered to be a good choice. Vizard claims that too narrow LSA is preferable to too wide LSA, and that the ideal value varies in the interval 110 to 100 degrees depending on valve geometry and head flow.

So for streetability, that theory would indicate that you need short, high-lift cams at 104MOP. The 107 cams did just that, shortened duration and higher lift compared to C, D and E cams.
--
Joachim

Last edited on 01-31-2014 09:17 am by subwoofer

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 Posted: 02-09-2014 11:53 am
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roland11a
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Very usefull thread as i'm just at this point in my engine re-build.

Can somone shed some light on the cam belt sprocked retaining bolt and washer? The main list of torque setting in the workshop manual state 25 lbft however further on, in the actual section 56 lbft. is one initial and one final?
Also at this torque the thick retaining washer just bends/dishes where the centre hole of the sprocket stands proud from the end if the camshaft by about 1/8"
should there be a spacer behind?

Thanks

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