Timing the twin cam

[Grumblebuns]

This is a topic that I've been ignoring longer than I should have due to ignorance and lack of experience. I'm trying to figure out where I should set my timing. Obviously I have it in the ballpark since it runs OK.

First question is what is "static timing"? Is it timing at idle?

I took a timing light to my TCS after the morning run and addling around 1100-1200 RPM, I'm roughly 25-30 degrees. I'm assuming that the variation is called scatter. So anybody know where I'm at? I'm thinking that I'm pretty far advanced at idle since timing will advance with increasing RPM.

Second question is which direction do you rotate the distributor to retard the timing?

Joji Tokumoto

[TCS4605R]

Joji,

Static timing is done with the engine OFF.  Connect the power lead of a 12 volt lamp to the low tension lead on the distributor and the ground lead of the 12 volt lamp to the engine block or a good ground point.  Rotate the engine until the #1 piston is a Top Dead Center (TDC) on the compression stroke, then back the crank off counter clockwise about 30 degrees.  Take all of the plug wires off so the next procedure does not let the engine start.  The next procedure, switch the engine ON and put the car in 1st gear and slowly rock the car forward.  When the 12 volt lamp comes on, the distributor points have just opened - this is your static timing point.  Look down the hole in the transaxle bell housing and you should see the edge of the flywheel with timing marks on it - read the timing - could be 5, 10 or 15 degrees before TDC.  The Lotus shop manual says 5 degrees before TDC for Zenith-Stromberg carbs and 12 degrees before TDC for Dellorto or Weber carbs.  Static timing will get you in the 'ballpark' for ignition timing - the best way is with a timing light.

My engine has Webers carbs and 'L' cams - I use a timing light and set the timing to 30 degrees before TDC (if my feeble memory is correct) with the engine turning at 3,000 RPM.  A Zenith-Stromberg owner could chime in here with the degrees before TDC and engine speed they use for timing.

Tom
74 TCS - 4605R

[jbcollier]

No need to disconnect the plug wires, simply remove the wire from the points at the coil.  Removing the spark plugs, however, will make it much easier to turn the engine over in small increments.

At this stage in the game, who the heck knows what distributer or cam you're running.  So go back to basics:

Set the timing at low idle (600 to 800) to 15° BTDC.  Rev the engine slowly up while observing the timing marks.  Go at least to 5K rpm, briefly mind!

- What was the maximum advance recorded?

- What is the maximum advance minus the initial advance of 15°.  This gives you the total advance of the distributor.

Basically, you want no more than 38° BTDC at high rpm if you tend to drive the car moderately.  If you drive the car hard and spend a great deal of time flat out at high rpm -- say up a mountain pass, flat out -- then I would reduce the maximum advance to 34° BTDC.

Let your low idle advance fall where it may.  Anywhere from 5° to 15° is ok.  Anything more or less than that may give you trouble.

Best practice is to send your distributor to someone with a distributor machine and have them set up the advance curve to match your cam/carb combination.  All of the above still follows though.

[BDA]

It is a little odd that static timing is specified. Dave Bean gave me instructions to initially set the timing statically - ~5^o BTDC - just so it would start so the final timing could be set (27 - 30^o BTDC @ 6000 rpm). I think the real timing is really the same as Tom's because I think all the advance is in by 3000 rpm). But the workshop manual doesn't specify a dynamic timing value so I would be inclined to leave it at the static spec but John has it really right (as usual). It looks like there is one distributor for Webers and one for Strombergs so I assume the difference in timing spec is because of the a different spark advance mechanism.

[jbcollier]

Dunno, if the Bean says 27° to 30° max for a TC, that's what I would do!

[blasterdad]

Second question is which direction do you rotate the distributor to retard the timing?

Joji Tokumoto

Turning the distributor into the direction of rotation of the rotor will advance the timing,
turning the distributor away from the direction of rotation of the rotor will retard the timing.
Think of it like this, if you were able to move the numbers on the face of a clock, by moving 12 to the left, the second hand would reach it sooner, to the right later...

[BDA]

Dunno, if the Bean says 27° to 30° max for a TC, that's what I would do!

I have a BDA, John, so my timing could be different from yours. It is interesting that Tom uses about the same timing. I suspect, but don't know, that different distributor advance mechanisms are a large part of any differences in timing.

If your experience tells you to go to 38^o I'm sure that's a good setting for the TC with that distributor. Your earlier advice on setting the timing sounds about right to me.

[jbcollier]

I thought 27 - 30 was a bit conservative for a TC.  A BDA is an entirely different kettle of fish -- much more efficient combustion so not as much advance is required.

[blasterdad]

I thought 27 - 30 was a bit conservative for a TC.  A BDA is an entirely different kettle of fish -- much more efficient combustion so not as much advance is required.

Please excuse my ignorance, but what is a BDA?, (besides our beloved member from NC).

[jbcollier]

http://www.oocities.org/marty7_nz/bdaengine-index.html

[BDA]

It's also basically the next iteration of the twin cam. It uses essentially the same block (it might differ in deck height) and it was developed when the twin cam was becoming less effective in rally cars.

John, of course, is correct that the combustion chamber is smaller and more efficient, but I would have thought that the determining factor for a street car would be the advance mechanism or curve of the distributor since the revs are lower.

(besides our beloved member from NC)

 

[jbcollier]

Total advance required is based on the amount of time needed to burn the compressed mixture.  Old fashioned, hemi head designs with high compression, have a long thin combustion chamber that really slows down combustion.  Maximum timing advance could be as high as almost 50 degrees!  This is not efficient and results in tremendous amounts of heat being transferred to the piston and head.  For those reasons, most competition engines were dual plugged.

As an example I have a Ducati 250 desmo engine with between 12 and 13 to 1 compression.  With the stock single plug, the maximum timing is set to 40ish degrees.  Twin plugging reduces the maximum timing to 32 degrees.

Advance between idle and maximum advance is based on top end and cam design and is best set up on a dyno -- by someone who knows what they're doing (i.e. not me).

[Grumblebuns]

Thanks for all of the good info. I'll get to trying to figure this out in the next couple of days. I'm sure I'll have more questions later. In my earlier days of ownership, I took the Europa to a mechanic for tune ups, never did it myself. thus my lack of knowledge.

Joji Tokumoto

[BDA]

Advance between idle and maximum advance is based on top end and cam design and is best set up on a dyno -- by someone who knows what they're doing (i.e. not me).

Obviously not me either! Thanks for the good information, John.

My apologies for maybe taking us into the weeds from Joji's rather simple question. Hopefully some besides me found it enlightening.

[blasterdad]

Advance between idle and maximum advance is based on top end and cam design and is best set up on a dyno -- by someone who knows what they're doing (i.e. not me).

Obviously not me either! Thanks for the good information, John.

My apologies for maybe taking us into the weeds from Joji's rather simple question. Hopefully some besides me found it enlightening.

I have, thanks guys!
Only problem is now that I know what a BDA is, I want one!    (OK, four!).