Lotus Elan

Pistons for High Compression Race Engines

PostPost by: SJ Lambert » Mon Oct 31, 2011 10:50 am

Hi All

Besides the Dave Bean listed piston 597-E-0835, a piston with Compression Height of 1.13", what other pistons are there available for a standard height block and standard stroke crank to run high compression with BD centre to centre length rods of 5.23"?

Cheers

James
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PostPost by: rgh0 » Mon Oct 31, 2011 11:25 am

Hi James
I use forged JE pistons with intruder dome to get what I want in terms of high comp ratio
I dont use BD rods but the slightly longer than standard twincam rods from Carillo that close up the piston below deck height in a standard build.

Never actually done the calcs on the DB low compression height piston in standard block / standard crank / BD rod but i presume from your comments that it actually fits. However to get a race compression ratio of around 12.5:1 you need an intruder around 4 to 6 mm in height above the piston deck which the DB piston does not have.

I am sure JE can make what you want - but - not sure why you want to use BD rod length in standard height block and stroke and all the hassle of fitting the gudgeon pin, the valve cut out and the rings all into the same narrow low compression height space and then trying to rev it all to 9000 rpm ? I have never been convinced of the supposed benefits of longer versus shorter rods over the difference between BD and twin cam rods and always been more worried about the mechanics of keeping the top on the piston.

cheers
Rohan
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PostPost by: SJ Lambert » Mon Oct 31, 2011 12:16 pm

G'day Rohan,

I'll redo my sums but am pretty sure it all checked out a while ago when I had the pen and paper out. I have a narrow pin crank and a set of Carillo rods on hand and was indeed worried that the intruder on the DB piston wasn't tall enough. Thanks for the advice, as the specific intruder height was the unknown for me in my calculations. Sounds like I should get JE to do a piston with a 6 or 6.5 mm intruder and then trim 'em as necessary? I think my cylinder head will be pretty meaty as far as overall thickness is concerned.

Our iron head motor for the Elfin 300 is on even longer rods with low compression height pistons that only have one compression ring and one oil ring. They're going OK so far. Larner continues to recomend the longest rods and shortest pistons in such applications, the BD rods that are on hand were actually destined for the 300 engine, but I decided that they're not long enough for it so as they're about the longest type that can realistically be put into a standard stroke Twink race motor they've got the nod for the job....

Is there a specific contact at JE that I should approach who's got the intel on Twin Cam pistons??

James
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Elfin Type 300 (Holbay S65 - 120E) mechanic

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PostPost by: rgh0 » Mon Oct 31, 2011 12:33 pm

I dont have a specific contact - bought their pistons from a variety of sources including them direct over the years. These days if I wanted a set I would go direct again.

With a 36 to 37 mm combustion chamber and normal sort of head gasket of around 0.8mm compressed and the piston deck around 0.5mm below deck height for adequate clearance at full revs you need around 7cc of intruder volume for 12.5 which equates to an intruder of around 4mm with normal big valve cut out in the piston. I have got a piston with a 6.25 mm intruder which seems to be a JE standard for a twincam racing engine and then machined it to the specific target i need for the engine build I am doing and I normally end up around 4.25mm. The 6.25 mm intruder piston probably works on a full thickness head and full height block standard rod length with its higher deck clearance rather than the slighly longer rod I use and Carillo make which is why they make it that way. These day most blocks nad head have been skimmed so need to take a little of the intruder.

Just need do lots of measurments and calculations !!!

cheers
Rohan
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PostPost by: cabc26b » Mon Oct 31, 2011 1:51 pm

FWIW -

We always end up trimming the pistons for valve to piston clearance. The last motor had a McCoy head, JE pistons with Carrillo rods (std length) we ended up going the opposite direction on compression ratio so we could run pump-gas if we had to. so we whittled the intruders and started with the thickest cometic gasket available. I think we ended up at 11.1 to 1.

George
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PostPost by: qedmotorsport » Thu Nov 10, 2011 10:18 am

We have pistons with an intruder height of 0.250", std compression height and std gudeon pin size:

http://qedmotorsport.co.uk/qed-shop/lot ... ious-sizes
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PostPost by: SJ Lambert » Sun Nov 13, 2011 2:59 am

CP also do a 1.10" high piston for the 5.230" c to c rods.
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PostPost by: bigvalvehead » Mon Nov 21, 2011 12:20 pm

Hi All

Pics below of pistons I had made by JE.

Comp height is 1.13 for use with BD length rods and std stroke.

The dome on these is higher than normal at .3 rather than .245 as I want to build a high compression engine for evaluation purposes running approx 13.2 C/R.

The engine will run n/j rods and std t/c stroke so will be 1594 cc

These pistons were ordered through Topend Performance in the USA who were very helpfull and delivery was very quick.

I'll post a dyno sheet when the engine has been built.

Cheers
Dave Hughes
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