Lotus Elan

Cam Timing Problems Help!

PostPost by: mark030358 » Mon Dec 20, 2004 12:14 am

This is a long one Gents please bear with it,

I have a Big Valve engine with the correct (and standard cams). The specs are:-

Inlet opens 22 deg BTDC
Inlet closes 62 deg ABDC

Exhaust opens 62 deg BBDC
Exhaust closes 22 deg ATDC

This gives a MOP of 110 degrees.

Ok so far, I think....

Anyway I have had the block decked and hence need to retime the cams and have used the following method

Set engine to "true" TDC (use DTI)
Place DTI on number 4 exhaust tappet (always under tension due to rotation of the engine)
Rotate engine to remove slack and stop at TDC again
Now slowly rotate engine 'till tappet moves (opens) to 50 thou, read angle from timing disc.
Rotate engine 'till tappet starts to close and gets back to 50 thou point on DTI, read angle from disc.

The results at 50 thou were:-

Angle when opening 64.5 deg BBDC
Angle when closing 16 deg BTDC

therefore MOP = (64.5+180+16)/2 -16 = 114.25 therefore advanced by 4.25
Now here is the problem,
When I repeated the exercise I used 60 thou of lift and got the following results:-

The results at 60 thou were
Angle when opening 62.5 deg BBDC
Angle when closing 18 deg BTDC

therefore MOP = (62.5+180+18)/2 -18 = 112.25 therefore advanced by 2.25

When I repeated the exercise at 70 thou of lift I got the following results:-

The results at 70 thou were
Angle when opening 60.5 deg BBDC
Angle when closing 20 deg BTDC

therefore MOP = (60.5+180+20)/2 -20 = 110.25 therefore advanced by 0.25

I have not checked the inlet cam yet but I cannot understand the readings for the exhaust and I dont want to go any further. I have also tried to do the same at max lift of the cam (ie either side of the dwell) by the following method:-

Set engine to "true" TDC (use DTI)
Place DTI on number 4 exhaust tappet (always under tension due to rotation of the engine)
Rotate engine to remove slack and stop at TDC again
Now slowly rotate engine 'till tappet moves to max lift, read angle from timing disc.
Rotate engine 'till tappet just starts to close, read angle from disc.

The results at max lift were
Stop at max lift (on start of dwell) 132.5 deg BTDC
Start when closing (coming off dwell) 125.5 deg BTDC

therefore MOP = (132.5-125.5)/2 +125.5 = 129 therefore advanced by 19

Would appreciate some help
Thanks
MArk
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PostPost by: khamai » Mon Dec 20, 2004 3:08 am

Mark,
I refer to you an article written by Tim Engel on the GGLC web site

<a href='http://www.gglotus.org/ggtech/ggtech.htm' target='_blank'>http://www.gglotus.org/ggtech/ggtech.htm</a>

click on "Twin Cam" engine and then you see Tim's article about "Dialing In Cams".

I believe the article will shed some insight on what you're experiencing.

One comment - Besure you're turning the engine clockwise to do your measurement. This assures you've properly taken the slack out of the timing chain. You'll get erroneous readings if you go counterclockwise.

Kiyoshi
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PostPost by: type26owner » Mon Dec 20, 2004 3:28 am

(62+180+22)/2=132-62=70 ABDC=Exhaust
Angle when opening 64.5 deg BBDC
Angle when closing 16 deg BTDC

Nope. It's (64.5+164)/2=114.25
114.25-64.5=49.75 ABDC
70-49.75=20.25
It's 20.25 degrees off.

(22+180+62)/2=132-22=110 ATDC=Inlet

Suggest you use the 'left-hand rule' to keep the rotational direction of the crankshaft in focus.
-Keith
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PostPost by: rgh0 » Mon Dec 20, 2004 9:01 am

Mark

The correct timing for the standard big valve type D cam is

inlet opens 26 degrees BTDC and closes 66 degrees ABDC with a MOP of 110 degree ATDC

exhaust open 66 degrees BBDC closes 26 degrees ATDC, MOP of 110 degrees

The timing you quoted was for the standard engine type B cam. The 2 cams have the same MOP at 110 so the actual measuring technique to set at 110 degrees is the same.


If you have not done this operation before draw yourself lots of circular diagrams to understand the calculations. Also be very careful turning the engine if not confident the timing is nearly right as very easy to get the valves to hit each other or the piston and and the inlet valves are easily bent and its hard to feel the valves hitting each other as you turn the engine ( been there done that)

Rohan
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PostPost by: mark030358 » Mon Dec 20, 2004 9:06 am

Gents,
Thanks for the swift replies. Guess I'm a tooth out!

Cheers
Merry Xmas!!!! *8)

Mark
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