Lotus Elan

Twin Cam Engine

PostPost by: J J DIKKE » Sun May 27, 2018 9:23 am

Hello All
I am a brand new member of the group and am refreshing my memory from long ago of owning Lotus cars!!

Would any of the knowledgeable people assist with the following.

I intend to re build a Lotus twin cam engine using the crossflow 711M tall block (probably my AX block). By preference I would leave the block at its maximum original height (undecked) as this helps to preserve the structural integrity and general strength within the block. An added advantage would obviously be the increase in cc which I also wish to achieve.

My intention I think is to use an 82mm stroke steel crank (if I can find one), in conjunction with the appropriate length rods and forged pistons. Am I correct in thinking that because the cylinder head would be higher on the engine, due to a full height undecked block, would I need an adjustment to the brass tensioner pivot? If so is there such a thing available anywhere to purchase or if not how I would I achieve the relevant modification on a standard size tensioner pivot?

Also, because the block would remain at full height (undecked) would I need a timing chain even longer than the extended 122 link chain?

Thank you in advance for any helpful advice or guidance you can offer from the obviously knowledgable people on the site.
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PostPost by: types26/36 » Sun May 27, 2018 7:01 pm

J J DIKKE wrote:I intend to re build a Lotus twin cam engine using the crossflow 711M tall block (probably my AX block). By preference I would leave the block at its maximum original height (undecked) as this helps to preserve the structural integrity and general strength within the block. An added advantage would obviously be the increase in cc which I also wish to achieve.

My intention I think is to use an 82mm stroke steel crank (if I can find one), in conjunction with the appropriate length rods and forged pistons. Am I correct in thinking that because the cylinder head would be higher on the engine, due to a full height undecked block, would I need an adjustment to the brass tensioner pivot? If so is there such a thing available anywhere to purchase or if not how I would I achieve the relevant modification on a standard size tensioner pivot?

Also, because the block would remain at full height (undecked) would I need a timing chain even longer than the extended 122 link chain?

Thank you in advance for any helpful advice or guidance you can offer from the obviously knowledgable people on the site.

I built a couple of tall block TC's a few years ago but changing the block height will not increase or decrease the cc of the engine, it is the bore to stroke ratio that determines the capacity.
I never bothered with altering the tensioner and I never had a problem although I used the std 77.62 crank with a 122 like chain, I know some people have modified the tensioner and you probably will have to with a 82mm crank, a thread can be found here:
lotus-twincam-f39/timing-chain-for-tall-block-t21006.html
Yes the engine will be taller then std TC and if fitting to an Elan bonnet clearance can be an issue, also the 711/AX block is wider and needs the Elan exhaust side mounting spacers reduced in length.
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PostPost by: Foxie » Sun May 27, 2018 9:59 pm

I was about to reply when Brian's post went up with the link to the thread, in which I see I had already posted on this topic in 2010 !

So I will only add that I did not need to modify the tensioner for an 82mm crank in a 711 block. I lowered the engine to stop the cam cover touching the bonnet by drilling new bolt holes the Spyder engine mounts. :)

I used QED 106038 Steel 5.05" rods and Omega pistons with the dome machined off flush with the piston top.

BTW, you will have to mark and grind off the high spots in the block/ big ends when fitting an 82mm crank.
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PostPost by: J J DIKKE » Mon May 28, 2018 8:22 am

Hello all,
Thank you thus far for your replies and information, may I explain further. The engine mountings and any possible bodywork alterations are not a problem, my questions are based only around the engine block.

I am an amateur at this although with some knowledge and my main focus is around the following items.
This is what I would like to achieve.

1. I wish to use an 82mm stroke steel crankshaft (where do I find one)

2. Most importantly as I intend to use only my 711M or my AX block I would like to leave whichever block at its full standard height (as it came from Ford) to preserve its strength and to give me the longest stroke therefore largest cc possible.

3. I probably intend to use 85mm forged pistons which I know are available.

4. Using the 82mm crankshaft and 85mm pistons I would have a considerably larger cc engine than standard but I do not have the formula to calculate what cc this would provide for me, perhaps one of you knowledgeable people could confirm what cc this would provide my engine with.

5. My main wish, as I said earlier is to retain whichever tall block I choose to use and not to have it decked.

6. I have purchased new Burtons Timing covers, modular water pump and all the associated parts to fit this and after looking at it quickly it fits perfectly with the top of my undecked/standard height AX/711M blocks.

Thank you all once again, if anybody is able to shed any light on my queries.
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PostPost by: types26/36 » Mon May 28, 2018 8:35 am

Using 85mm pistons with 82mm crank would give you 1861cc if I did my calculations correctly :roll:
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PostPost by: nmauduit » Mon May 28, 2018 8:54 am

a quick word on the block height : depending you your car, closing the bonnet may become an issue, you may want to check your clearance before and check how much you can add (plus how much you can lower the engine if need be).

In case you want to try other values, the formula for calculating the engine displacement is swept volume x number of cylinders
swept volume if area of the piston x stroke
area of the piston is pi x piston radius x piston radius

here it gives

3.14 * 4.25 * 4.25 * 8.2 * 4 = 1861 cm3 as said above
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PostPost by: rgh0 » Mon May 28, 2018 9:34 am

An 85 mm bore in an AX block is not a given. You need to ultrasonically test whatever block you use and offset bore to maximize whats possible. You want a minimum wall thickness of around 2.5 mm over any significant area. You also need to check the exact location of wall thickness remaining for the oil cross drilling between. If you really want a big engine (and have plenty of money) buy one of the special blocks with siamesed bores and no cross drilling. You also need to look at the head cooling passages as they get very close to the gasket fire rings in a big bore engine and the may need welding up and redrilling.

I am aware that longer pivot arms for the adjuster have been made to compensate for the fact that the arm pivot point moves up with the head versus the adjusting plunger location in the front cover this moves the plunger close to the edge of the pivot which may give you a problem or may not depending on exactly how much the pivot arm swings with wear on the chain and sprockets

I have never heard of anyone needing longer than a 122 link chain but I have never measured if its sufficient for a full height 1600 block personally.
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PostPost by: billwill » Mon May 28, 2018 9:42 am

See also, this article in the Wiki (rather hard to find).


http://www.lotuselan.net/wiki/Torquing_ ... am_Rebuild
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PostPost by: J J DIKKE » Mon May 28, 2018 11:25 am

Thank you all,

Where would I buy one of the “special blocks” with siamesed bores and what would be the maximum bore size that I could take this too?

Thank you. I hope to continue to receive information which may prove useful.

PS
Once again, I confirm that engine mountings or bodywork adaptions are not an issue.
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PostPost by: rgh0 » Mon May 28, 2018 11:34 am

QED make a special new block with siamesed bores in cast Iron. https://qedmotorsport.co.uk/qed-shop/lo ... nder-block

Alloy blocks can be source from a variety of sources but I understand www.wilcoxengines.co.uk/twincam.htm can supply.

Something like 2.1 L is possible with maximum conceivable bore and stroke and unlimited money

cheers
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PostPost by: Foxie » Mon May 28, 2018 12:19 pm

My 711 block, which I ran for 14 years, went porous after the last rebore to 83.5.

I invested in a new QED block (not a tall block) and used the 82mm crank and Dave Bean 85mm lo compression height pistons.

The only problem I encountered was that the valve pockets in the Dave Bean pistons are infinitesimaly smaller than the 42mm valves. The engine turned freely by hand, but dropped a valve after several competitions, as it could be seen that the edge of the valve was contacting the side of the pocket. Very fortunately the dropped valve immediately jammed in the port, all it needed was a new seat.

I opened out the pockets and replaced all the valves, and I did a full season last year with no problems. (That included driving the car to all the events, and of course driving home again !) :)
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