Cam Card ??

[BlueRidge1]

I recently rebuilt the engine in our 62 bn7 tricarb.
The cam was reground to the following specs per the card .
( I was told it is the same duration and lift as some 6 cylinder chevrolets) Going clock wise it reads as follows
top center
Exhaust Closes 28 degrees
exhaust opens 67
bottom center
intake closes 64
lobe centers 108
intake opens 31
intake lift 425 lash .018 rocker ratio 1.42
exhaust lift 425 lash .018 rocker ratio 1.42
cam model HA 406.8
What effect will this have on the ignition timing.
Currently I just turned the distributer till it starts ok but I would like to know were it needs to be to time it with a light if possible!!Thanks Bob

 

[Dave Russell]

Hi Bob,
You didn't say what the valve lash clearances were for the specified timing events. For cam timing checks, the clearance is often set to .050" lash. From your numbers, I suspect that the specified timing events are at the .018" lash settings. This would nominally be a 275 degree duration cam.

The IC event will have the single largest effect on engine characteristics. At lower rpm, the engine can't begin to compress the charge until the intake valve closes some 64 degrees after the piston has already started up on the compression stroke. As rpm & the resultant inertia effects of cylinder filling increase to overcome the rising piston's reverse pumping, cylinder filling will be more complete & compression pressures will rise.

The usual ignition timing requirement is that the distributor needs more initial advance, the centrifugal primary advance needs to come in sooner, the centrifugal secondary total advance possibly needs to be reduced. In crankshaft degrees, something like 12 degrees static, 30 degrees at 2,500 rpm for static plus primary centrifugal, & 38 degrees at 5,000 rpm for static plus primary centrifugal plus secondary centrifugal. Vacuum advance disconnected for these measurments.

The centrifugal advance springs & the advance stop can be altered to give any desired timing characteristics.

Just setting initial advance greater with no other distributor changes will help low & mid range power but often results in over advancing at the higher rpm, not good.
D

 

[BlueRidge1]

Dave, Ignition timing specs for a stock mark 2 are as follows.
static- 10 btdc ( is static at resting position)
strobscapic 15 btdc ( does this mean with a timing light?)600 rpm
max advance 35 btdc we will say 5000 rpm

Am I correct to understand I should change to the following.

Static 12 btdc
strobscapic 17 at 600 rpm
Max advance -should I reduce this a couple degrees below the listed 35? If soo do I achive this adjustment with the vacuum control adjustig nut and a strobe light?
Or is this one of those deals were it would be best to take it to my not so local authorized Austin Healey Specialist?

 

[Dave Russell]

[ QUOTE ]
Dave, Ignition timing specs for a stock mark 2 are as follows.
static- 10 btdc ( is static at resting position)
stroboscopic 15 btdc ( does this mean with a timing light?) 600 rpm
max advance 35 btdc we will say 5000 rpm

Am I correct to understand I should change to the following.

Static 12 btdc
stroboscopic 17 at 600 rpm
Max advance -should I reduce this a couple degrees below the listed 35? If so do I achive this adjustment with the vacuum control adjustig nut and a strobe light?
Or is this one of those deals were it would be best to take it to my not so local authorized Austin Healey Specialist?

[/ QUOTE ]
There are two centrifugal advance springs. The primary spring is weaker & controls initial advance up to about 2500 engine rpm. The secondary spring is stronger but has slack built into the anchor so that it does not immediately come into play. Once the slack is taken up in the secondary spring anchor, the combination of the two springs controls the advance. This gives a fast initial advance up to around 2,500 rpm & a slower advance above this rpm. There is a mechanical stop that limits maximum centrifugal advance. This combination might give something like 15 degrees at 600 rpm - static plus centrifugal, 28 degrees at 2500 rpm, & 38 degrees at 5,000 rpm.

The specs for the Tri Carb that I looked at show a static of 12 degrees & 15 degrees at 600 rpm.
Static would be at resting position where the points just open.
600 rpm would be with a timing light.

The maximum timing depends somewhat on altitude as this has a considerable effect on cylinder pressures. The higher the altitude the less the cylinder pressures & the more advance that can be used. Assuming that the distributor is calibrated reasonably well for your engine, I would just set strobe timing at 5,000 rpm for 35 to 38 degrees maximum advance (vacuum advance disconnected) & let the rest of the numbers fall where they may. If you get pinging under load, go more toward the 35 degree setting. Whenever in doubt, it's better to be slightly retarded for engine safety. Slightly rich fuel mixtures can give some extra protection against detonation. Over advanced timing will provide little power gain but a lot of engine damage. It's possible that with your cam, the primary advance could be quickened, by using a weaker primary spring to give about four degrees more advance between 600 & 2500 rpm. Maybe not worth the bother though.

Most likely, you can just set timing with a strobe to 12 to 15 degrees at 600 rpm & do a check for 35 to 38 degrees at 5,000 rpm & be pretty close. The most important number is at 5,000 rpm since you would be unlikely to hear detonation under load at this rpm. You can make minor adjustments with the knob, probably plus or minus three degrees. Greater adjustment would be done by turning the distributor body.

A perfect calibration for your particular engine & cam would need to be determined on a chassis dyno, or by extensive road testing. The maximum advance range of the distributor & the timing curve would then need to be calibrated on an old fashioned distributor machine. (Adjust primary & secondary springs plus the advance stop.)

Attached is a graph of distributor advance for a higher compression engine with more radical cam, LeMans vs the lower compression engine with less radical cam to illustrate the differences.
D

 

[BlueRidge1]

Dave Thanks for the info I'll get a light on it and see what the timing is currently set at. Concerning the adjusting knob I have had the distributer completly apart to clean and grease (i think I put it back close to the way it was) I just wondered If there was a reference point to set it at intially or does't it really matter much?
One more question if I may- My car is a early tri carb and has the small diameter balance pipe . Should I do something to increase the capacity of this tube? could this be achived by increaseing the length of the coupling hoses to the manifolds?

 

[tahoe healey]

Thanks Dave. I had noticed that it runs much better up here(6500 ft)with a bit of advance (3 degrees) but I never thought it was due to altitude. And it will be easy to read-just with the advance/retard screw when I come off the hill.

 

[Dave Russell]

Blueridge.
I would just set the vernier timing adjustment to the center of it's travel to start with. It has limited range so large settings would have to be done by turning the distributor body.

I doubt if the diameter of the balance pipe would make much difference.

TH,
I believe that the embossed marks on the side of the vacuum advance represent one degree of advance change. It usually requires about 8 clicks of the knob to move one degree. I think three degrees would require about 24 clicks. Different advance units had different markings. The housing is marked for A & R, advance & retard knob directions.
D

 

[BlueRidge1]

Thanks Dave ,I wish you lived in my neighborhood. Bob