Camshaft Alternatives for N/A Engine

[drdoom]

Thought I'd solicit insights on cam profiles for the N/A engines. Anyone have experience with running non-standard cams?

[rgh0]

No direct experience (yet) but LotusBits recommend a Kent L14 cam profile if going beyond the last of the standard cams used in the 180 hp 2.2L 

 

http://www.lotusbits.com/upgrades_engine.html

 

cheers

Rohan

[MrDangerUS]

I'm investigating a feasibility of the similar modification for my friend who has 1978  2.0l car.  I'll post my findings, promise.

Meanwhile, I found something interesting...

I did not know that S300 valves may fit 907 (912?) head...

I think Gorgio's is 912

 

Giorgio said (see below) that he was able to use S300 valves without changing seats, just a machining job.  That's very exciting! It means a significant improvement in engine breathing !

 

What is your opinion?

 

*****

I found this statement in Gorgio's thread  https://www.thelotusforums.com/forums/topic/53461-rebuilding-upgrading-an-912-engine/?page=2

 

 

• 
• Basic Account
• • 1.8k
• Name: Giorgio Taricco
• Car: Lotus Esprit S2.2 1981 RHD from UK
• Location: Torino
• Joined April 5, 2007

• Author

 

REBUILDING & UPGRADING AN 912 ENGINE

 

 

 

Posted November 24, 2020

I have fitted 300S valves, the biggest one you can put without changing the seats, just machining job with the correct angle, I have bought the In&Ex valves from Garry Kemp.

[drdoom]

Hello John,

Interesting topic, the valve sizes. I've no empirical data but am inclined to regard the 907/912 as tending toward over valved from the get-go. At a GGLC tech session back in the '90's our tutor ( Charlie Rockwell, IIRC ) who'd begun some of the earliest independent software analysis of engine building held forth that the 9xx series engines were cam limited, whereas the eponymous Twincam was more valve and port limited. As has been often stated the 9xx engine really lights up the lower end of the power curve when stroked for a mere 10% displacement increase, something I suspect illustrates why the 907 at 2 litres is unable to energize its intake flow until wound up considerably. The late Tony Rudd in discussion at our local club event in '86 advised that the 907 ports/valves were good to 8500 rpm as stock. So I'm not confident there's a lot to be gained in upsizing the valves on these engines, though the modest size increment to S300 intake should remain suitable for a street car if done well.

Your thoughts?   

[MrDangerUS]

In my opinion, the "104" cam and larger intake valves should help quite a bit.

Two "107" cams can give you max. 44° overlap with 104/104 MOP pulleys.

But, 104/107 cam combo with 104/110MOP pulleys will deliver a more favorable 48° overlap.

Garry Kemp offers an optimized intake valve, which measures 36.75mm and fits onto the standard valve seat insert (the seat needs re-cutting to accept the extra 1.25mm diameter).  These valves are an improvement over the original S300 valve, which had not a particularly good head shape.  Garry's valves are 0.25mm larger than a S300 but more importantly, the valve head shape was developed on the flow bench and flows better than the factory S300 valve!  It is one piece stainless steel with a waisted stem.  Also, they are cheaper than an S300 valve.

In addition, one may increase the final drive ratio to move the engine work point up the torque curve.

Also, this thread has some meat:  

 

[drdoom]

To what extent is overlap the critical factor in this? As I've gathered over time it is the inlet closing point which determines RPM at which peak will be achieved, of course overall output comes down to area under the lift curves where cam profiles are concerned. I've tinkered extensively with an engine sim program and it was in that where the 9xx series N/A engine seemed to benefit or be indifferent to overlap figures lower than created with 104/104 MOP. Garry has advised 106 as likely optimal for a tuned 2.2 street engine, and offers 2 grinds of his own developed specifically for the 9xx. Back to the overlap topic, it was fiddling there in the software that brought forth the 107 cams on 110 MOP notion. When I ran that configuration on Federal, Domestic or tuned spec either 2.0 or 2.2 capacity it was superior to the C cam at any MOP. I've yet to attend to the cylinder head but may well turn to Garry for a solution.

Good thought on the final drive - I brought over 2 trans prepared by Harry Martens, for me OEM final drive with reversed teeth and for a pal one with the 4.88 final drive as he wishes to pursue 2.0 capacity to the bitter end.

Cheers 

[MrDangerUS]

Wow, new grinds? I'm all ears! What are their specs?

[drdoom]

Don't know how new they are, they were mentioned in an email some time ago. GK demurred when I inquired as to specs back then, so I'm as curious as you, John. When I get onto it once again I'll press Garry for some description of powerband, plus lift figures. From what the learned types in the business have stated online it seems cam timing figures are rather dubious when comparing one outfit's product to that of another. If you get ahead on this do share what GK relates, will you?

Cheers

[MrDangerUS]

Of course.  Meanwhile enjoy some numbers from my file:

............ ......... ..      Kent   Lotus HC OE Turbo D.Smith  QED  QED      QED    QED  MITS9  MITS9  Lotus HC 
INT. Cam ID .....     L14       104/107  107/107       DS2     Q420      Q420     Q420      Q420    BP255   BP270  104/107 104/107

Cam#valve/type 4/DHOC 4/DHOC     4/DHOC    4/DHOC   4/DHOC  4/DOHC 4/DOHC 4/DOHC 2/SHOC 2/SHOC   4/DOHC  4/DOHC
Int. Total Lift         .415"    .410"      .378"   .415"    .420”    .420    .420     .420     .425”    .433”   .410”   .410”

In. Opens, BTDC      35°      32°         22°     36°       38.5°   40.5    38.5       36       28°       24°      30       30 
In. Closes, ABDC     67°      60°         50°      64°       66.5°   64.5     66.5     64       64°       60°      62       62 

In. MOP    ATDC     106°    104°        104°   104°       104°    102    104      104     108°     108°     106     106 

Int. Duration          282°    272°       252°    280°      285      285     285       285      272°     264°   272    272   

 

107 cam Exhaust

EX. Total Lift         .378      .378        .378    .378     .378     .378     .378      378     .378      .378     .378  .378

EX. Opens, BBDC     50        50           50       50         50       50       52        56        50         50         54      56

Ex. Closes, ATDC     22         22           22      22         22       22       20        16        22         22        18      16

Ex. MOP   BTDC      104       104        104     104       104     104     106       110      104       104     108     110

Ex. Duration           252       252        252     252       252     252      252      252      252       252     252    252

Overlap                   57        54           44       58        60.5     62.5     58.5      54.5      50         46        48     46

LSA                       105       104         104    104      104       103      105     107      106       106      107   108

Cam advance                                                                   0         +1       +1       +3                                   +1      +2