Twincam rebuild

[Lucifer]

Hello all,
New here and greatly appreciate the collective wisdom shared. I am rebuilding an engine I rescued many years past from a fate worse than death.
No car, just the engine. But it's a start isn't it?
Anyway, I have got to the distributor, and it appears it is from a pre-crossflow i.e. pretty much useless.

So i need to source a 41189 to suit sprint cams.
Does anyone know what degree distributor cam the sprint cams should run?

Thanks and best wishes to all.
Lucifer

[Andy8421]

Lucifer,

To clarify your question, are you asking about the maximum advance stop that is part of the base of the 4 lobed cam that actuates the points where the bob weights fit?

stop.jpg (72.73 KiB) Viewed 2854 times

[Lucifer]

Thanks for your reply Andy, and for taking the time to check yours.

Yes, that is the one. The cam in the distributor, the one that operates the points, and sets the maximum advance.

The one which came with this engine has 18 degree advance, and in a 25D distributor (with vacuum advance).
I reverse calculated that to produce a maximum advance of 46 degrees plus whatever the vacuum advance unit does to the equation.

If I use ignition timing of 10 - 12 degrees static advance and 25 - 28 maximum advance for an engine with sprint (valve operating) cams, (thank you Rowan), (26 - 11)/2 gives a maximum 7.5 degrees or thereabouts. So 18 degrees is therefore well off.

If I have calculated correctly, and please feel free to correct me, a distributor cam of 8 degrees or thereabouts should be about right and toss the vacuum advance to make a 23D distributor.

I noticed yours Andy shows a 12 degree advance. That should produce a max advance of 33 degrees at the crank. Obviously yours is working well so there must be a degree of flexibility in the max advance.

Incidentally, the cams that were fitted to the engine were 41/73/65/40, 293 duration, .375" lift with flat top pistons.
If it ran at all, it must have been rough.

Bet wishes, Lucifer

[Andy8421]

Lucifer,

The picture I posted was an example, not necessarily correct for your application.

There is a lot of confusion about appropriate advance curves for the twincam engine, not helped by Lotus themselves confusing distributor advance with crankshaft advance in some of their publications. A well regarded twincam book by Miles Wilkins also has contradictory and confusing advance curve data. I suggest you search this forum for discussions about suitable advance curves (I use "site:lotuselan.net my_query_here" on Google as it seems to locate more relevant threads than the forum embedded search function).

A word of caution, Lucas distributors weren't precision devices when they were new, 50 years later most are worn out and unlikely to produce a reliable advance curve. If you want to stick with a mechanical advance system, I would recommend 'The Distributor Doctor' who will be able to supply a tested distributor to your specification. Personally I have gone down the 123Igntion route, as it has a reliable programable advance curve, and the bluetooth version also has an immobiliser built in.

https://123ignition.com/product/tuneplus-4-r-v-kent/

[2cams70]

Read here. I've posted a few times the Ford (not Lotus) specs. for the 41189 distributor. These seem to be the correct ones. If your engine does not have standard cam, compression ratio, etc. these may not apply.

viewtopic.php?f=38&t=47025&start=15

[The Veg]

Welcome Lucifer!

[Lucifer]

Thank you for your help and advice Andy and 2cams, and for your welcome veg.

I have given the distributor issue some thought, and checked out the after-market offerings. Certainly the 123 ignition system looks good, and interesting to read your feedback. I have previously read as many posts on the topic as I could find too.

One of the reasons I was hoping to use a Lucas distributor is that I have a Lumenition breakerless unit sitting on the shelf (new and unused, and to suit a 23/25D). If only I wasn't such a Scrooge.

I have sent an email to the distributor doctor and am awaiting a reply. It may end up being more cost effective to just go the 123 route.

Thanks again and best wishes,
Lucifer

[rgh0]

Thanks for your reply Andy, and for taking the time to check yours.


I noticed yours Andy shows a 12 degree advance. That should produce a max advance of 33 degrees at the crank. Obviously yours is working well so there must be a degree of flexibility in the max advance.

Incidentally, the cams that were fitted to the engine were 41/73/65/40, 293 duration, .375" lift with flat top pistons.
If it ran at all, it must have been rough.

Bet wishes, Lucifer

A high reving engine wilth a longer duration cam can tolerate more advance to around 32 degrees at around 4000 rpm. A standard Sprint engine needs less. Lots of variables affect what the curve should be such as fuel, compression ratio, cam duration, piston intruder (if it has one). If building a standard engine get a disributoer built to the standard curve. The are usually local distributor places that can do this. If building a moified engine you really need to determine the best curve on the dyno and then get the distributor built to this curve. The programmable 123 dizzy makes it easy to set up your own curve based on road testing which is its best fearure.

cheers
Rohan

[Lucifer]

Hi Rohan,
Thanks for your advice.
I plan to rebuild to sprint specifications, with the exception that this engine has a small valve head though I suspect it may have had a gentle port sometime in the past. Do you have any thoughts on running a sprint cam in a small valve engine?

The block and head are at the machine shop at present, and I will check the static and dynamic compressions when I get them back to unsure it will all work together. That makes the standard sprint jetting and ignition a good base.

Just about everything that moves is being replaced. Only the L block, crank, head, inlet valves and cases are to be retained. The rest is ... going, along with my bank balance!

Regards
Lucifer

[rgh0]

The slightly larger valve sizes in the Big Valve head are not really signficant. Most of the power increase came from higher compression, the Type D cam with a little higher lift and duration and the use of tubular extractors and electric radiator fan

cheers
Rohan