which pistons

[tdafforn]

Hi all,
Now I have got my 711m block I need to think about pistons..
I have a big valve head with normal big valve cams.
looking at the usual suspects I have found:
1 ) forged pistons from Accralite..117.50 each in VAT (Ouch!)
Only trouble I am worried about is that they claim they give a CR of 11.6:1. Isn't this a bit high? These piston remove the need for skimming the 711m block
2 ) OE pistons, cast and 9.5:1 and cost 99.50 each
3) Cosworth equivalent forged at 690.00 a set giving a CR of 12.0:1

I also heard that Miles Wilkins has some cheap lotus originals, but apparently they are not suitable for BV? I don't quite understand this as according to the elan ref. books I have there wasn't a difference between BV and non-BV pistons.

So my question is which should I go for? The car is for spirited road and perhaps the odd track day. I also use it to commute.


thanks for any help
Tim

[types26/36]

I have fitted the standard pistons to engines that had big valves (bigger then standard big valve) and had no problems, it will of course depend on the cam lift/timing and also how deep the valve seats are in the head. Whatever pistons you use it would be adviseable to do a dummy build and check the valve to piston clearence with modelers clay, if the clearence is not enough
(work on a minimum of 60 to 80 thou) then you would have to have the valve pockets cut deeper.

[rgh0]

Tim

There are a few variables you need to consider in piston choice

1. Maximum Revs used - if sticking with a standard bottom end and a 6500 rpm limit standard cast pistons are fine. If building a high rev steel bottom end then you need quality forged pistons.

2. For road use with normal unleaded fuels you probably want a maximum of around 10.5:1 compression ratio anything above this needs racing fuels especially after running for a while with coke build up in the combustion chambers increasing compression ratio.

The actual compression ratio you will achieve with each piston type need detailed calculation as the quoted comp ratios are a rough guide at best. The suppliers should be able to quote the piston volume above the piston deck. This may be postive with a high domed piston or negative with a standard flat topped piston with large cutouts. You then need to combine this with details of piston deck clearance to top of block when assembled, gasket thickness and combustion chamber size to detemine the actual compression ratio in you engine build. Given your head has probably had some metal removed and the larger capacity of the 711 stroke you will probably get close to 10.5:1 with standard pistons and a thin head gasket. The higher compression forged pistons may require some material machined off their domes and / or thicker head gaskets to bring the compression ratio down to an acceptable level.

3. Cut outs in twin cam piston are required to get adequate clearance with the valves. These do not need to be large provided deck heights are correct and you are running a standard Lotus cam. High lift long duration cams require bigger cutouts. Heads that have had a lot shaved off them bring the valves closer to the pistons and also need bigger cutouts. Valve size itself is not all that critical in determining cutout size. The smaller cutout C type Lotus pistons were used on later engines so I dont see why they would not fit a big valve motor provided everthing else was OK in terms of clearances. The parts manual lists the earlier A type piston with larger cutouts and later C type piston with smaller cutouts as suitable for all cars. I would always check piston to valve clearance wih a dummy assemble on any non standard engine build and nobody actaully builds a bog standard twin cam these days.


regards
Rohan

[chrishewett]

Tim,
My 711m is in for a rebuild at the moment. It has very big valves, as big as you can go. The cams are L2. The pistons need replacing and my engine man is using Miles Wilkins std pistons and machining the valve pockets. This is a big saving even with labour as these pistons are ?10 each.
Chris

[Foxie]

Tim,
When I was putting my 711 motor together, I used the Accralite pistons, and had the domes decked, also was able to order slightly shorter forged rods from QED.

Sean Murray