Lotus Elan and Lotus Elan Plus 2 Forums

Well here I am again...this time trying to dial in the cams...have made a "dropstick" to go down the No1 plughole and marked 90 degrees before TDC and after TDC (moving the degree disc until equalised ) thereby getting an accurate TDC and BTC reading...so far I'm happy that this is the most accurate way to find TDC and BDC.

But marking the dropstick at TDC, 90 degrees, BDC and 90 degrees has left me with a dilemma,why is the vertical distance between BDC to 90 degrees 44.6mm and the vertical distance between 90 degrees and TDC 33mm

Shouldn't they be the same?

P.S. Stroke is 77.6mm





John

Hi John
If you draw the various triangles formed by the rod, the crank and the vertical axis up the bore you will see the triangles are a different shape from 0 degrees to 90 degrees compared to 90 to 180 degrees. The different shapes mean different amount of vertical movement over the first 90 degrees of the 180 degrees of crank rotation that forms a stroke of the piston compared to the second 90 degrees

I could solve for your rod length from your measurements but its late and I'm going to bed. Easier if you tell me your rod length and I can confirm your piston displacement measurement.

cheers
Rohan

Good night Rohan

Got me thinking (again) and yes,taking a 10cm crank and a 10 cm rod....at 90 degrees (bloody pythagoras) the vertical height will be 3.66mm not 5cm.

John

John,

Your method for finding TDC and BDC is basically good. A refinement not mentioned in your post is to approach 90-degrees BTDC and 90-degrees ATDC from BDC. This requires turning the crank one way to reach 90 BTDC, then reversing crank rotation to reach 90 ATDC. This refinement ensures the connecting rod is in compression and clearances are consistent for both measurements.

Bill

Bill

Correct, ( I was doing it all in the clockwise direction - as per setting camshafts - keeping the chain taut )but I wonder how much difference there would be?

will re-do tomorrow

John

John,
Another thing that throws you calcs out is that the gudgeon pin is not in the middle of the piston.
Cheers
John

John

Well you learn something new every day,or in my case plural...

Watch this space for my new toy...it's in the post.

John

There is a downloadable spreadsheet at

lotus-twincam-f39/line-twin-cam-porting-article-t16194.html

that should answer many of your questions. It is almost at the end of the post. (20070702 Engine.xls).

Have fun.

David
1968 36/7988