Lotus Elan

Cam Timing (once again!)

PostPost by: rgh0 » Mon Jul 03, 2017 11:47 am

Hi Vince

I have lost track of what cams your fitting. If SE or Sprint cams then there is not much reason to move from the standard Lotus settings as Torque improvement is marginal. They are fairly conservative cam duration's to begin with and playing with MOPs for torque versus power really only gets relevant with longer duration cams.

I guess the convention of referring to timing around TDC came because the relationship to TDC is what is important when considering inlet and exhaust timing events so that are the key numbers to remember, especially in a single cam engine when the relative timing is machined into the cam profile.

If your analysing an engines thermodynamic efficiency performance by doing 4 stroke cycle pressure diagrams you do it over 0 to 360 degrees and then from 360 back to zero so the pressure loop connects over the full 720 crank degrees of the 4 stroke cycle . You tend to do what makes the analysis easy to comprehend.

If looking at cam timing on a crank rotation basis thats fine just be careful of your starting point. If you start at TDC of compression and ignition cycle then exhaust MOP occurs at around 250 after TDC and then inlet MOP occurs at around 470 degrees after TDC. ( think I have that right doing it in my head)

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Rohan
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PostPost by: vincereynard » Mon Jul 03, 2017 7:38 pm

Rohan,

Thanks for the erudite reply.

I'm really leaning towards a bit of advance firstly because it is all new and any initial wear will tend to retard and because if in doubt ...etc I would agree that the engine probably would not notice a couple of degrees anyway!

Vince
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PostPost by: rgh0 » Mon Jul 03, 2017 8:35 pm

Yes a degree or two of advance with a new assembly will help compensate for chain stretch and sprocket wear so worth doing for that alone.

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PostPost by: oldchieft » Mon Jul 03, 2017 8:50 pm

Hi Rohan, could you answer a question?

Some where in the past I recall that a couple of degrees advance would give a bit more low torque, and a couple of degrees retard would give a bit more top end power.

Am I mis-rembering? or is a bit of a old engine builder myth?

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PostPost by: rgh0 » Tue Jul 04, 2017 11:10 am

I suspect this "advance for more torque" comes from single cam engines with a fixed amount of overlap where you have to change the timing of the cam events the same for inlet and exhaust and in the same direction. I have never really investigated what movements typically effect a single cam engine but it may be typically true that advancing the timing increase torque a little

In a twin cam engine reducing the overlap by advancing the exhaust and retarding the inlet timing events versus the crank will normally increase low rpm torque by increasing effective compression ratio at low rpm and reduce top end power by limiting breathing at high rpm due to the reduced overlap . You dont need to make these changes symmetrical or in the same direction as you do in a single cam engine so you have more freedom to change how you time each cam.

For example -- Advancing the exhaust while it reduces overlap around TDC for a particular duration cam also makes the exhaust valve timings earlier which blows down cylinder pressure earlier reducing torque so there are multiple factors at play and it depends on which factor dominates in a particular rev range

I have seen various recommendations on Lotus twin cam timing with regards adjusting it for increased torque versus power and I have played with a few of them over the years. However these have always been around timing race cams not road cams they often conflict for similar cams from different suppliers. I have never seen any recommendations I would trust on adjusting road cams on a Lotus twin cam for torque rather than power apart from the general direction of reducing overlaps typically increases torque.

The recommendations for 104 cam which is a higher 0.405 inch valve lift 300 degree cam in the Dave Bean manual for example are timing of MOP at 102 for race and 106 for the street for both cams. This advance of both cams by 4 degrees is aimed at maximum power on the track rather than driveable torque on the street. But this cam is really a race cam not a street cam to begin with.

DB for his 112 cam which is lower 0.375 valve lift 300 degree cam is 110 MOP for both and to advance the inlet by 5 degrees to 105 MOP for increased mid range torque. I am not sure why this works as it increases overlap but in the mid range this may be beneficial. This seems to conflict with the advice on the 104 cam which has similar timing !

John McCoy makes various timing recommendation many of which he considers tuning secrets so I cant discuss in detail here but in general he will advance both inlet and exhausts for maximum power though he does not time symmetrically or change the timing by the same amount.

I have experimented with running a shorter duration exhaust cam in conjunction with a long duration inlet and this will also increase torque while minimizing loss of top end power. The lotus twin cam exhaust flows to well compared to the Inlet and thus can tolerate a more restrictive exhaust cam without loss of power. This sort of combination is also recommended in various race configurations by both McCoy and Bean ( see the bean comments in his manual on the 116 versus 111 cam)

So in summary- A long way to say to many variables to make a general recommendation for a road car that i have found and on road engines I tune to the Lotus MOP specs for their cams, allowing a little for wear, so time to err on the advance side to compensate when building from new.

cheers
Rohan
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PostPost by: vincereynard » Wed Jul 05, 2017 10:46 am

Chancer wrote:Good on you!

Power to the people!


I received a refund this morning!

(Not without them still maintaining that they are vernier :? )
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PostPost by: nmauduit » Wed Jul 05, 2017 11:47 am

vincereynard wrote:
Chancer wrote:Good on you!

Power to the people!


I received a refund this morning!

(Not without them still maintaining that they are vernier :? )


In my experience they are good, reliable people - one of the few I would still trust...

There may have been a semantic issue on what should be deemed "vernier" and whet is merely "continuously adjustable" regarding the part in question - in any case, your experience on the use of such a part is of interest - please keep us posted on how you eventually achieve the timing you wish.
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PostPost by: vincereynard » Thu Jul 06, 2017 10:25 pm

nmauduit wrote: your experience on the use of such a part is of interest - please keep us posted on how you eventually achieve the timing you wish.


Well you did ask! No 1 advice would be to get a decent DTI gauge and not cheap junk of Ebay! Both the ones I bought were inconsistent and the magnetic base arms allowed too much flex.

Anyway by far the best way I found for understanding the ATDC / BBDC / BTDC confusion was to ignore it, treat each as a separate cycle and measure everything simply from TDC. (Much as Rohan mentions above.)

Therefore Input MoP = 110 and Exhaust = 250. (360 - 110)
Measured was - 111 and 254
A touch of advance - 109 and 249
Dowels required 2 and 5

Please excuse the oily scrawl.

p1050322.jpg and
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