Lotus Twin cam engine advice for rebuild

[Bunky Twincam]

Hello all,

I have an "L" block with a bore size of about 82.80mm.

I also have a head that has big valves, fast road cams and a set of twin 45 webers.

I am after piston advice, I want to do some car club events but still want to run on pump gas.

I have looked around and found a QED forged piston for about 90 Lbs, will this do the job or is there a better option?

I also found the Accralite forged piston 83.25mm, can someone please tell how far oversize I would be going if I use this piston?

I just don't want to take my close to standard block out too far on the first use.

I am running the standard crank and rods, however is there a better choice of rods and what difference will this make?

Any advice would be great.

Cheers

[europatek]

FWIW - Best improvement you can make is to ditch the crank and use the crossflow 711M. The extra stroke makes a world of difference. Keep the 125E rods and use a low comp height piston. I use Datsun "L" series pistons modified to suit. 83mm will give you 1670cc, 84 - 1720cc. Rod stroke ratio is at low end of scale but this combination does work very well. My Cortina has been like this for 20 years and I have a 711M crank ready for the Elan when the time comes. You can see my current rebuild post in the engine section.

[Bunky Twincam]

I have a 711M crank machined with new bearing ready to go, does this just go straight into the 1558 block?

Can you please give me some more info on the pistons, where they come from, who makes them and what mods need to be done, what sort of comp ratio do you have?

Thanks
Craig

[rgh0]

Hi Craig
For a road engine with more power and for occasional track day using a large capacity big bore and long stroke bottom end is a good starting point as this helps get the power without spending big money on expensive components to handle 9000 rpm. and you get great torque in the midrange which is what you really want in a road car.

The longer stroke 711m crank with the standard rods and Datsun pistons is a good combination and cheap way to achieve this. You can also get purpose made low compression height pistons for this application from the various Lotus parts suppliers (e.g. Dave Bean in the USA can supply them).

Taking your block to 83 mm would be the sensible next step and can be matched to the pistons available. You should target for a 10.5:1 compression ratio for running on Australian 98 Octane unleaded.

You also need to do some head work to get the 150 hp that's achievable with this bottom end. Bigger 1.625 inlet valves and 1.4 inch exhaust valves with longer stems, combined with a high lift short duration cam ( eg QED 420) and suitable springs and retainers and cam followers. The right dimensional combination to set this all up needs some careful measuring but it is not that hard to do. How much porting you do on the head it up to you and whether you have the skills to do yourself or can find someone who knows what's needed and can do it without ruining the head if they make a mistake. With the above combination and just cleaning up the ports you will get maybe 145hp doing some more serious porting on the head will get you 150+ hp at 6500 to 7000 rpm.

I don't know what cams or valves you have today but assuming you have a standard lift longer duration "fast road" cam you will be lucky to get over 130 to 135 hp if you don't do the head work.and cam change above

You also need to go to 34 or 36 mm carb chokes and suitable jetting and a bigger bore exhaust system. and probably also modify the advance curve. The carbs jetting and ignition timing is best determined based on a rolling road dyno session

cheers
Rohan

[Bunky Twincam]

Rohan,

The head is well ported and polished and has 1.625 inlet valves and 1.77 exhaust valves fitted.

I have a spec sheet on the cams but don't really understand it.

I was told by the company that dialed the cams for me that they were at the low end of a race cam.

Here are some readings from the cam chart.

Duration 291.5
Peak cam lift 0.37761

I am looking at running my standard crank and rods together with a set of Accralite 83.25 pistons.

I have a set of correctly jetted 45mm webers with 36mm chokes fitted and a Tony Law set of extractors.

I am not racing the escort, I just want to do some club gymkhanas, the odd hill climb and track day.

Does this combination sound OK?

I am not after max power and revs, just something around 135-145hp and about 6500 -7000 revs.

Do I need to do anything else? are there better products to do this job?

Cheers

[europatek]

Hi Craig, the pistons suit Datsun 1600, 240Z & K etc.... L16 and L24 are 83mm as standard. Oversize up to 0.060" is/was available. "L" series pistons are not as easy to find these days in Australia but you still can with a little research. I recently purchased a set of NOS ACL pistons at +0.060". Alternatively you can get them from the USA to suit a 240Z. You'll get six so will have two spare.
To use the Datsun pistons the small end bush needs to be reamed to suit the 21mm pin. The pistons crown needs the flycuts added and the height reduced slightly to suit the deck. The gudgeon pin is shortened and circlip grooves added. This way the pistons is fully floating like a twincam was originally. (Datsun are a press fit small end) It may sound like alot but it really quite simple.
Comp ratio will depend a little on how you finish the piston crown and if you use flat top or dished pistons. Most L series Datsun have a dish in the crown but some are flat top.
I concur with all that Rohan said in his post.
For your intended use I would recommend the 711M crank and Datsun piston conversion. Trust me the extra torque and power is so much better. 40's or 42's may be better also. 45's will give you more top end but you will sacrifice some low/mid range for it. In my experience no carb is correctly jetted until the engine is running and tested.