Road cam question for head modified to big valve + status

[au-yt]

I am thinking along the same lines as you that they are D type. The Lift in the Waggotts sheet has total lift so with clearances it comes close to the actual lift I have. Added to that the engine runs well on sprint jetting with a tweek.
As I need to check the cam clearances its a good time to identify the actual duration.

[au-yt]

I've decided not to change the cams. I did however change the 123Ignition map and the engine is running very sweetly.
I have some good friend here with an ELAN in a similar state of tune. and this is the map. and I'm very happy with the result.

[2cams70]

That looks like a heck of a lot of advance at high RPM. Certainly a lot more than standard.
Good combustion generally means less advance required because the mixture burns faster.
From the factory the Lotus Twin Cam has much less ignition advance than the Ford Crossflow for example.
Also a problem Cosworth had with the Heron combustion chamber SCA engine - lots ignition advance required = poor combustion = poor efficiency

[au-yt]

One I have learnt is with new fuels, is that the flame speed is slower and the higher the octane the slower the burn. Most carb tuning places recommend 95 even with 11:1
Ive set the 02 around 12 lambda so its rich as the best stoichiometric ratio is 14.7 but its impossible to get that consistent with Weber's, fuel injection yes.

[2cams70]

I'd reckon that 95 Octane is marginal for any old engine designed to run on super leaded fuel. I'd be sticking to 98 with no ethanol. The lower the octane rating of the fuel the less the advance you can run before preignition or knocking occurs. Knocking at high RPM is dangerous because it's often goes unnoticed before piston damage occurs. High octane fuels burn more stably than low octane ones. You don't get combustion happening in pockets of mixture first. It ignites and burns more progressively than low octane fuels hence you can run more advance when using it.

[au-yt]

Interesting, I have a friend with a 1900CC Twin cam 11:1 big cams with 22,000Ks on it since build, running on 95 with the same advance and no damage.

[2cams70]

Anything is possible on an engine modified to that extent. You may get away with running too much advance at high RPM if you don't drive frequently for sustained periods at high RPM and high load but it's not ideal. The OEMs when setting the factory specification would have accounted for the full range of operating conditions and that's why they are more conservative. Having an ignition system with knock sensing is safest.

The factory ignition map for the Lotus Twin cam reaches maximum advance at only 2,400 crankshaft RPM for example

[mbell]

I believe the 123 numbers are total advanced, not advance from static*. So total max advance of 32 degrees, with ~20 degrees of advance from idle setting of 12 degrees.

The data I've seem suggest lotus used total advance in the low 30's range, exact value depending on cams/head/distributor/emissions setup etc.

So I am confused if people think 32 degrees total advance is now too much? Or the 123 data has been miss-understood to be 12 + 32 = 44 degrees total advanced?

*My understanding is you static time them at 0 degree then can set a low advance to ease starting.

[au-yt]

The 123 Ignition as I understand it is actual, unlike a mechanical distributor there is no preadvance and therefor the actual advance is actual.
From my early experience with EFI the way it works was explained to me, the initial setup with engine at TDC and the distributor set on the number one position, what you actual doing is setting the unit at an internal max advance and the processor retards the spark, and as the revs rise it reduces the retard.
This makes sense as the 123Ignition has some clever features
https://www.123ignition.com.au/

[greg40green]

Anyone got a pair of sprint cams for sale ( NOT reground ) .
Thanks ,